摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THERMODYNAMICS APPLIED TO THE IRON- CARBON SYSTEM. BY F. H. JEFFERY. Received 14th December, 193 I. In the volume of the Transactions for December, 1931,l there appeared two papers by Yap, Chu-Phay and also a paper by the author dealing with this subject.Yap, Chu-Phay often differs from the author alike in method and in results. The principal differences are :- ( I ) He considers that the liquid phase in equilibrium with austenite IS a solution of Fe,C in iron, the author finds that it is a solution of C in iron. (2) He considers that austenite is a solid solution of Fe,C in yFe below 1020' but of C in yFe above this temperature; the author finds no such transition. (3) For the liquidus from 1487" to 1130' he used the work of Ruer and Goerens and he drew the corresponding solidus rectilinearly, the author used that of Gutowski for both liquidus and solidus; for the austenite-cementite boundary he used results which he had derived from the dilatometric data of Sato, the author used the work of Tschischewsky and Schulgin.(0 If the liquid solution in equilibrium with austenite is a solution of Fe,C in iron, and austenite is a solid solution of C in yFe, the reaction taking place when solid is formed from the liquid is Fe,C -+ 3Fe + C v0 log ( I - n) + v1 log n + yo' log ( I - n') + Y: log n' = log K where n is the molal fraction of Fe,C in the liquid solution and n' the molal fraction of C in the solid solution also Yo = 0, v1 = - I, vo' = 3, vl' = I and 3 - A - log K = - 38 R 02' for dilute solutions, and if A, the molal heat of the total reaction in- volved in the formation of the solid solution from the liquid solution, be sensibly constant for the range of concentration and temperature examined, the equation can be integrated giving A R8 log n' + 3 log ( I - n') - log n = - + constant, Trans.Far. SOC., 27, 777, 1331. 9s99 F. H. JEFFERY hence the sum of the logarithmic terms is a linear function of I/S pro- vided the molecular constitutions have been selected so as to satisfy the phase boundaries. If the liquid solution is a solution of C in iron and austenite a solid solution of C in yFe (ii) where $2, n' are the molal fractions of C. With restrictions similar to those given above + constant. x Re log ( I - 12') - log (I - n) = - An equation of the same form as that of (ii) is applicable to such a system as a liquid solution and a solid solution of Fe,C in iron as solvent in equilibrium with each other, were such to exist, but it is not applicable to a system in which the solute in the liquid solution is different from the solute in the solid solution : and any equation derived from it or any calculation based on it is equally inapplicable to such a system.Yap, Chu-Phay appears to have used an equation of the form of (ii) and to have deduced from i t the existence of the system to which equation (i) applies. Yap, Chu-Phay considers that the liquid phase above the isothermal at 1487" is a solution of C in iron, but that below this isothermal the liquid is a solution of Fe,C in iron. This leads to the remarkable con- clusion that the compound Fe,C in liquid solution with one of its con- sti tucnt elements, iron abruptly, and completely, dissociztes at the temperature 1487". (2) Yap, Chu-Phay considers that the austenite-cementite boundary is very nearly rectilinear ; that the eutectoid isothermal is a t 720° C., the eutectoid composition being 0.80 per cent.carbon; and that this boundary meets the eutectic isothermal a t 1-68 per cent. carbon. The author in his paper showed that for the austenite-cementite boundary, if austenite is a solid solution of C in yFe, an equation of the form is applicable ; but that if austenite is z solid solution of Fe,C in yFe the form of the equation must be Yap, Chu-Phay appears to have used an equation of the form (iv) for his line from 720' to 1130'. He considers that there is a " point-saillant " on this boundary line at IOZOO, and that above 1020' the constitution of austenite is C in yFe, but below, Fe,C in yFe.'I 00 THERMODYNAMICS AND IRON-CARBON SYSTEM The author has calculated the molal fraction of carbon in the alloys The results are as follows :- which Yap, Chu-Phay has used for his line.720' C. 780 856 870 925 968 1070 I 130 0.80 0.92 1.07 1.24 1-33 1-55 1.68 1-12 3-61 x 1o-I 4-14 4'79 5-00 5'52 5-90 6.82 7'36 It will be found that if the temperature be plotted as a function of n the points will lie very well on a straight line through the second point and the last. If austenite is a solid solution of C in yFe an equation of the form of equation (iv) is inapplicable to the boundary above IO~O', while if austenite is a solution of Fe,C in yFe from 720' to 1020' it must be so up to 1130" in the absence of a transition line. This latter result is incom- patible with the result which Yap, Chu-Phay obtained by his method of considering the solidus and liquidus from 1130' to 1487" unless i t be assumed that there is a transition line at 1 1 3 0 ~ ; but of this there is no proof.There is no indication of a " point-saillant " a t 1020~. (3) Undoubtedly the choice of correct phase boundaries from which to form the thermal equilibrium diagram is of fundamental importance. Such a choice is difficult for the iron-carbon system, the results of various investigators not being consistent with each other. If the boundaries are substantially correct, thermodynamic equations can be found which satisfy the boundary conditions : and the molecular constitutions of the various phases so found are compatible with each other. In the author's work ten boundaries were involved, they gave consistent results : eight of these were involved in obtaining the constitution of austenite. The question arises whether if one set of boundaries gives consistent results it would be possible to choose another set from the work of other investigators which would give equally consistent results. The author thinks that the greater the number of boundaries involved the less prob- able i t is that a second consistent set could be found. Nonetheless in- asmuch as the thermodynamic equations apply to dilute solutions, i t is quite possible that further experimental work might make modifications of curvature at concentrations beyond those to which the equations can be applied, though not thereby automatically destroying the validity of the results based on these equations : i t is possible that such may prove to be the case for the solidus and liquidus between I 130' and 1487". The Goldsmiths' Metallurgical Laboratory, Cambridge.

ISSN:0014-7672    

DOI:10.1039/TF9322800098

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE BROMINE-BROMIDE-TRIBROMIDE EQUILIBRIUM. B Y ROBERT OWEN GRIFFITH, ANDREW MCKEOWX, AND ALBERT GORDON WINN. Received I 5th December, I 93 I . In connection with the study of the thermal reaction between bromine and oxalates (see the following paper), a rather more detailed knowledge of the equilibrium constant (K3) of the equilibrium Br, + Br- + Bra- than that hitherto available was desired.Previous work on this equilibrium may briefly be summarised. I t was first studied by Jakowkin,' using the well-known partition method with carbon tetra- chloride as the second phase. Later, Worley investigated the equili- brium by allowing two aqueous solutions, the one containing bromine, the other bromine and potassium bromide, to come to equilibrium via the vapour phase, and then analysing the two solutions. Again, Jones and Hartmann 3 combining Jakowkin's method with measurements of electrical conductivity, carried out an elaborate study of the tribromide equilibrium at 0'. In their calculations of the equilibrium constant they made allowance for (a) the hydrolysis of bromine and (b) the formation of pentabromide (Br5-). In each of these investigations most of the data refer to solutions of high bromine content, in which, owing to allowance having to be made for pentabromide formation and also to possible deviations from the laws of dilute solution, the interpretation of results becomes needlessly complex and to some extent uncertain. Lewis and Storch showed that the partial pressures of bromine above solutions of bromine in carbon tetrachloride are proportional to con- centration up to about o.zgM.(This corresponds to a concentration of free bromine in an aqueous layer in equilibrium with the carbon tetra- chloride layer of about o-oIM.) Above this concentration deviations from Henry's Law are to be expected. They also determined the value of K3 in presence of HBr a t 25' by the Jakowkin method.In measuring the partition coefficient of bromine between water and carbon tetra- chloride they made the aqueous phase N/IW with respect to H,SO, in order to cut down hydrolysis of the bromine. Lewis and Randall5 have recalculated some of Jakowkin's results obtained with KBr and weak bromine solutions, and Linhart has attempted to correct Worley's data (relating to solutions strong in bromine) by making allowance for pentabromide formation. Finally, Sherrill and hard 7 have carried out a series of measurements of K3 in the presence of HBr. They found that the values of this equilibrium constant (defined as K, = [Bra] [B!-]/[Br3.-]) decreases with increasing concentration of bromide, a variation which 1 Jakowkin, 2.physihd. Clem., 18,583, 1895 ; m, 19, 1896. 2 Worley, J.C.S., 87, 1107, 1905. 9 Jones and Hartmann, Trans. Amev. Electrochem. SOC., 30, 295, 1916. Lewis and Storch, J . Anaev. Chem. Soc., 39, 2544, 1917. Lewis and Randall, J . Amer. Chem. SOC., 38, 2348, 1916. * Linhart, J . Amer. Clem. Soc., 40, 158, 1918. 7 Shemll and Izard, J . Amer. Chem. SOC., 50, 1665, 1928. I01 8I 02 THE BROMINE-BROMIDE-TRIBROMIDE EQUILIBRIUM they were the first to note. investigations are summarised. In Table I. the results of these previous TABLE I.-VALUES OF THE EQUILIBRIUM CONSTANT K,. Author. Jones and Hartmann . Lewis and Storch . . Jakowkin (calculated by Lewis and Worley (calculated by Linhak) : Worley Shemll and izard' 1 Randall) . Bromide. KBr HBr KBr KBr KBr HBr Temperature.O0 2 9 25" 32.5" 2 5 O 26-5' K3. 0.05 I 0.06 I 0.062 0.063 0.065 0.057-0.060 We have carried out fresh determinations of the equilibrium a t the two temperatures 16.5" and 21-5', using HBr, NaRr, KRr and LiBr, varying the concentration of electrolyte between 0.03 and 0.75 N and keeping the concentration of free bromine below O*OIM. Experimental. The method of Jakowkin was adopted, with carbon tetrachloride as the second solvent. Two preparations of bromine were used, the one from Kahlbaurn, the other sample purified by the method of A. Scott.s Also two preparations of CC1, were employed. The first of these was an A.R. grade sample which had been specially purified for photochemical work by refluxing with pure chlorine in an all-glass apparatus, washing with Na,CO, solution, drying over Na,CO, and then over Na, and finally fractionating.The second preparation, of the same origin, was subjected to similar treatment, with the difference that the refluxing was carried out with bromine instead of with chlorine and that the final drying with Na was omitted. The bromides were Kahlbaum or Merck preparations and were found to be free from impurities which react with bromine. In all cases at least forty-eight hours was allowed for attainment of equilibrium between the aqueous phase and the carbon tetrachloride phase. The bromine content of each layer was determined by pipetting off samples, running into excess of sodium arsenite solution, and hack titrating with iodine. In determining the distribution coefficient of bromine between CCI, and water in the absence of added bromides, the method of Lewis and Storch for limiting the hydrolysis of bromine in the aqueous phase was used, ie., the aqueous phase was made h7/1000 with respect to H,SO,. Table 11.gives the results of distribution measurements using this pro- cedure. The effect of hydrolysis of bromine in the aqueous layer is shown in the results of Table III., relating to experiments with much weaker bromine solutions in absence of added acid. In these and the following tables, concentrations are expressed in moles per litre, and D represents the ratio concentration of Rr, in CCl, concentration of Br, in H,O' Scott, J.C.S., 103, 847, 1913.R. 0. GRIFFITH, A. McKEOWN, AND A. G. WINN 103 m J C C * , - .iBr&,o * . D . . TABLE 11. t = 21.5~ t = 16.5" 7 7 /-----J+.--.- 7 -________- 0,1880 0.1881 0.1775 0.370 0.1843 0-1838 0.00682 0.00682 0.00646 0.01328 0-00700 0~00700 27.6 27.6 27.5 27.8 26-3 26-3 CBr21cc,, * [Br21Hz0 . D . t = 16.5' 0'1272 0.0748 0.03755 0.02360 0.01653 0.004950 0.002939 0.001490 0-000985 0~000698 25.8 25'5 25.2 24.0 23'7104 THE BROMINE-BKOMIDE-TRIBROMIDE EQUILIBRIUM 1Wio 7h- 0.109 that the ratio (log,, y)/p, where p is the ionic strength of the solution, is approximately constant for a given non-electrolyte and a given salt. Clearly, what is required in the present instance is the effect of bromides on the activity of bromine, but, on account of the formation Ratios. - 0.2 0.4 FIG. I. 0' 6 P 4 of tribromide ion this cannot be determined directly.Yet, from Randall and Failey's data an approximately correct estimate of this effect may be obtained. From these authors' tables we extract the figures given in Table IV. TABLE IV.-ACTIVITY COEFFICIENTS OF NON-ELECTROLYTES IN SALT SOLUTIONS. (AFTER RANDALL AND FAILEY.) Non-electrolyte. salt. Na,SO, Na,SO, NaBr KBr Na,SO, NaBr KBr We have supplemented these data by determining the effect of Na2S04 on the activity of Bra, by finding the partition coefficient a t 21.5' between CCl, and an aqueous solution of Na,SO, of concentration M/6 \p = 0. j). In this case we find D = 31.3 ; hence Y B ~ ~ in M/6 Na,S04 = 31*3/27*5 =R. 0. GRIFFITH, A. McKEOWN, AND A. G. WINN 105 1.021 1.008 1.134 and (log yBrJ/p = 0.109. given in Table IV., we infer that : Combining this figure with the ratios For Br, in NaRr, (log y)/p = @.I09/1*24 = 0.088.For Br2 in KBr, (log y)/p = 0*1og/1.92 = 0.051.* These estimates are probably reasonably accurate; i t is not so easy, however, to evaluate the effects of HBr and especially of LiBr on the 0.4527 0.1767 TABLE V.-VALUES OF THE EQUILIBRIUM CONSTANT K,. 1.070 1.035 1-014 1.014 I -007 0.4900 0'2409 0.09346 0*08050 0.04508 0.4903 0-1912 0.04633 0.009678 0~008810 0.003665 CBr - = I*. 0.7500 0.5000 0.5000 0.2500 0.1017 0.100g 0'1000 O'IOOO 0.06727 0.03683 0' 1000 0'1000 0.4627 0.04627 0.1851 0.7500 0.5000 0-0500 0'2000 CBr2. 0.05 150 0.0391 I 0.0 I 074 0-01059 0.04618 0.03123 0.01497 0.01266 0-oogg I g 0.005356 0.008375 0.003467 0-01 127 0'01 109 0-009961 0-05020 0.01047 0.01092 0.00760 KBr Solutions at 21-5'.0.003782 0.004283 0'001 129 0~002060 0'02022 0.01289 0.00575 I 0.003 7 78 0-002004 0.003989 0.002 I 35 0'004792 1.105 1.070 1.070 1.035 1.014 1.014 1'014 re014 1.014 1.014 1.009 1.005 0.7023 0-4651 0.4904 0-2415 0'07404 0.08166 0.09248 0.09303 0.09386 0.09665 0.06288 0.03550 0.04772 0.00961 I 0.008530 0.02596 0.01834 0.0092 I 9 0.00 7 8 68 0.006141 0.003352 0.004386 0-001332 0.03483 HBr Solutions at 21.5'. 0-0 I 004 0~008401 0'004233 15~. 0.04630 0.009328 0.008369 0.003409 0.003899 0.001 142 0~004191 0.00255 I 0.0593 0.060 I 0.0573 0.0584 1.160 1.107 1.043 1-01 I LiBr Solutions at 21.5'. 0-5000 0.0500 0'2000 0.01078 0.0075 9 7 0'01120 0.009574 0.008550 0.003409 0~001206 0.002650 0.004188 0.061 8 0.0593 0.0572 KRr Solutions at 16-5".O.OOgg52 0.009078 0.006540 O.OIg50 0*004920 0.001 I 18 0'002 I02 0.003850 0.01327 0.005880 0.01 107 0.01 I 18 0-0103g 0.03277 0~01080 0.0550 0.0557 0.0550 0.0548* 0.0539 0-5000 0.2500 0-0500 0'1000 0'1000 NaBr Solutions at I 5.5'. o-oroSI O*OII39 0.007980 1.107 1.043 1-01 I o~oo1132 0.002 5 80 0.0043 I5 0.5000 0.0500 0'2000 0.0574 0.0560 0,0548 *Justification of this procedure follows from the derivation of Debye and McAuley (Physzkal. Z., 26, 22, 1925) of the effect of electrolytes on the activity coefficients of non-electrolytes. See also S h e d and Izard ( J . Amer. Chews. SOC., 53, 1667, 1g31), who have applied a correction analogous to that attempted here to their data on the equilibrium Q, + C1- =+ Cl,-.106 THE BROMINE-BROMIDE-TIBROMIDE EQUILIBRIUM activity of Br,.From Randall and Failey's data, i t is found that (log y)/p for non-electrolytes in HC1 is in general about one-fifth that in NaC1. Assuming the same ratio for HBr and NaRr, we obtain (log y ) / p = 0.018 for the effect of HBr on 7 ~ ~ ~ . For Br, in LiRr, the admittedly uncertain value (log y ) / p = 0.042 was chosen. In Table V. are summarised our determinations of K, in presence of HRr, NaRr, KBr and LiBr a t 2 1 . 5 ~ and 16.5". The values of y~~~ used in each case are given. The values of D employed were 27.5 for the experiments a t 21.5" and 26.3 a t 16*5", except in the three asterisked cases, relating to experiments with a somewhat high content of free bromine. For these the value of D used was 27-8 a t 21.5" and 26.6 a t 16.5". The values of K , given in the above Table are shown plotted against the ionic strength (p) in Fig.I. Discussion of Results. The following points call for comment :- (a) From the results with 0.1 N KBr solutions a t 2 1 * 5 O , i t is seen that a good constant K, is obtained with EBr2 varying nine-fold, no correction for pentabromide formation having been applied. It would appear that for solutions of low bromine content such as here employed pentabromide formation is not appreciable ; further, it is likely that previous estimates of its extent (in stronger Br, solutions) are too high, In solutions of NaBr, KBr, and (probably) LiBr the constant at first increases, passes through a maximum in the neighbourhood 0-4-0.6 N , and then decreases. The salt effect is greatest in the case of LiBr, least for KBr.This is in agreement with the results of Sherill and Izard, though the magnitude of the fall we ob- tain is somewhat less than theirs. (d) The values of K3 here presented are slightly less than those of previous workers. This is due to the correction made here for the activity coefficient of bromine. (e) The rise of K, with increasing p in the cases of the alkali halides is somewhat unexpected. Thus with NaBr at 2 1 * 5 O , K3 rises from 0.0568 a t p = o to 0.0601 at p = 0.5, a rise of about 508 per cent. Since, however, over the same range YBrp increases by about 10.7 per cent., (b) K3 varies with ionic strength. (c) With HBr, K , falls with increasing p. cent. greater at p = 0.5 than a t p = 0. Since by definition aBrp ' aBr- is constant, this means that the activity coefficient of the tribromide ion is approximately 16 per cent.greater than that of the bromide ion when the cation is Na+ and the ionic strength 0.5. That this result is not occasioned by an incorrect assumption regarding the activity of bromine in solutions of NaBr niay be demonstrated as follows. The constant K3 was determined a t 21.5" for a solution (A) which was o.05M with respect to NaBr and 0.15M with respect to Na,SO,, ie., with a total ionic strength p = 0.5 ; the value obtained was K3 = 0 . ~ 5 7 5 . In the calculation the value Y B ~ ~ = 1-134 was that obtained (see above) in M / 6 Na$O, solution €or which also p = 0.5. Combining these values of K3 and yBq, it follows that in solution (A) the ratio Y B ~ ~ - / Y B ~ - is ap- proximately 15 per cent. greater than iii a solution of zero ionic strength, in good agreement with the result for O*SM NaBr solution. This con- aBr3-R. 0. GRIFFITH, A. McKEOWN, AND A. G. WINN 107 elusion is not in agreement with that reached by Lewis and Randall," who postulated that the activity coefficient of tribromide ion is less than that of the bromide ion. We wish to thank the Department of Scientific and Industrial Research for a grant to one of us (A. G. W.) while this work was in progress. We are also indebted to Imperial Chemical Industries, Ltd., for a grant defraying part of the cost of this investigation. ildusprait Laboratory of Physical and Eleclvochemistry, University of Liverpool. 11 Lewis and Randall, Thermodynamics, p. 520.

ISSN:0014-7672    

DOI:10.1039/TF9322800101

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions.Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.

ISSN:0014-7672    

DOI:10.1039/TF9322800107

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.

ISSN:0014-7672    

DOI:10.1039/TF9322800126

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE ADSORPTION OF GASES BY SOLIDS. A GENERAL DISCUSSION. THE FIFTY-SIXTH GENERAL DISCUSSION organised by the ii-4RADAY SOCIETY was devoted to “THE ADSORPTION OF GASES BY SOLIDS.” The meeting was held in the Lecture Theatre of the Biochemical Laboratory of the University of Oxford on Tuesday and Wednesday, 12th and 13th January, 1932, the President of the Society (Dr. Robert L. &fond) occupying the chair. The subject was discussed in three sessions, following a General Introduction by Professor H.S. Taylor of Princeton, as follows :- Part I. Experimental Methods. The Introduction to this Section was given by Professor E. K. Rideal, Cambridge. Part 11. Kinetics and Energetics was discussed on the afternoon of 12th January and in the morning of the 13th January. The Intro- ductory Paper was written by Professor H. Freundlich, Berlin. Part Ill. Theories of Adsorption was discussed on the afternoon of 13th January, the Introductory Paper being written by Professor M. Polanyi, Berlin. On Monday evening, the I I th January, the President entertained the overseas members and visitors to dinner a t the Dorchester Hotel, London. In addition to the President’s English guests, the following were welcomed to England : Professor Arthur F.Benton (Virginia), Professor R. Marshall Chadwell (Massachusetts), Professor H. Jermain Creighton (Swarthmore), Dr. €3. Dohse (Ludwigshafen a. Rh.), Professor G. Drucker (Leipig), Dr. A. Farkas (Frankfurt a. M.), Dr. E. Huckel (Stuttgart), Professor A. Magnus (Frankfurt a. M.), Dr. C. Schuster (Ludwigshafen a. R.), and Professor H. S. Taylor (Princeton). During the evening letters of greeting to absent colleagues were signed by all present and despatched. By the courtesy of the Master and Fellows of Balliol College, Oxford, the main body of members and visitors were given accommodation in that College for the period of the meeting, though many of those attending stayed with friends in Oxford. An informal dinner of the Society was held in the Hall on the evening of 12th January, when the Master of Ealliol honoured the Society by presiding; roo members and visitors were present.The meetings were held in the Biochemical Theatre by the courtesy of Professor R. A. Peters and Sir Charles Sherrington; through the kindness of Professor Peters and his staff, tea was provided during the afternoon sessions in adjoining rooms. 129 I 0GENERAL DISCUSSION At the opening session (at which there were about 170 present) the President welcomed particularly the overseas members and guests who had come to Oxford, and introduced them personally t o the meet- ing. He expressed the regret which all felt a t the absence of Professors Bonhoeff er, Freundlich, Mark, Polanyi, and Volmer, who had been prevented a t the last moment from coming to England.The papers had been issued in advance to those present, and were all taken as read. The authors were invited to open the discussion on their papers as they were reached in the fulfilment of the programme. As a result of this procedure a lively Discussion (which is reported in the succeeding pages) arose on the majority of the contributions. In accordance with the wishes of the meeting, voiced by Professor Donnan, Professor J. E. Lennard-Jones was invited to summarise his paper from Section 111. during the early part of the discussion of Section I., in order that those present might bear that paper in mind throughout the Discussion. Professor Lennard- Jones' paper is, however, printed in the succeeding pages under Section 111. (Theories of Adsorption). At the conclusion of the Meeting votes of thanks, moved by the President, were accorded with acclamation to the Overseas Guests for their presence, to the authors of papers and to contributors to the Discussion, to the Master and Fellows of Balliol, to Sir Charles Sherrington and to Professor Peters and his staff, and to the Organising Committee, and in particular to Professor Taylor, on whose initiative the meeting arose, and to Mr. 0. Gatty, who had made arrangements for hospitality in OLxford. Finally, Professor Taylor thanked the President, first on behalf of the overseas guests for the munificent hospitality which he had accorded them, and then on behalf of all present for his sympathetic and business-like conduct of the meeting from the chair.

ISSN:0014-7672    

DOI:10.1039/TF9322800129

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE ADSORPTION OF GASES BY SOLIDS. A GENERAL DISCUSSION. GENERAL INTRODUCTION. BY HUGH S. TAYLOR. Recemed 24th December, I 93 I.It is eminently fitting a t a time when so much attention is being paid to the fundamental contributions of Faraday to electrical science that the Society which bears its honoured name should also devote some thought and discussion to a field of knowledge in which Faraday displayed the same qualities of genius and pre-vision that have given him so pre-eminent a position among the leaders of scientific thought. Faraday’s contributions to the problem of adsorption were as con- spicuously in advance of his time as were his discoveries in other fields of work. They are apt to be overlooked in modern presentations of the subject. And yet Faraday realised from his studies of reactions of hydrogen and oxygen a t the surface of platina plates that the phenomena are “ dependent upon the natural conditions of gaseous elasticity com- bined with the exertion of that attractive force possessed by many bodies, especially those which are solid, in an eminent degree and probably belonging to all; by which they are drawn into associa- tion more or less close, without a t the same time undergoing chemical combination, though often assuming the condition of adhesion ; and which occasionally leads, under very favourable circumstances, as in the present instance, to the combination of bodies simultaneously subjected to this attraction.” His studies of the inhibitory action of other gases on the process “ the very power which causes the combination of oxygen and hy- drogen is competent under the usual casual exposure of platina, to condense extraneous matters upon its surface, which soiling it, take away for the time its power of combining with oxygen and hydrogen, by preventing their contact with it,” are the pioneering efforts in the field of specific adsorption a t surfaces which have reached their culmination in the Langmuir concept of uni- molecular adsorption and the consequent modem interpretation of chemical reactions a t surfaces.Faraday’s emphasis on the ‘’ superficial actions of matter ” be- came overlaid by the considerations of “ porosity ” in connection with the properties of adsorbent materials. There emerged the hypothesis 131 leading to his observation thatGENERAL INTRODUCTION of capillary condensation which accounted for the observed adsorption in terms of liquid condensed in pores of the solid owing to the lowering of vapour pressure caused by surface tension effects.Adsorption a t plane surfaces should be impossible on this basis, nor should gases be adsorbed a t temperatures above the critical temperatures. Neverthe- less in spite of much argument to the contrary, this “ pore ” concept still persists in certain quarters. It undoubtedly is of importance with saturated vapours as the gas phase. For adsorption on plane surfaces two rival points have struggled for supremacy. Since the time of de Saussure (1814) it has been assumed that adsorption occurred in thick compressedfilms the existence of which was to be associated with long range forces of attraction extending outwards from the solid surface with progressively diminishing potential. It was by diffusion through such thick layers of adsorbed gas that Bodenstein, Fink, Stock, and others assumed that reacting gases in heterogeneous gas reactions reached a reaction surface on which the velocity of interaction was relatively rapid.Langmuir, in 191 5, emphasising the unsaturated nature of surface atoms in a solid lattice (the Braggs) and, a t the same time, the extremely short range of inter-atomic and molecular forces simultaneously provided adsorption with the “ unimolecular adsorption layer” and the heterogeneous reactions with a kinetics based on the extent to which a surface is bare or covered with unimolecular layers of adsorbed gas. So far as kinetics is concerned, acceptance of the Langmuir concept may be characterised as universal.As to adsorption, one can summarise the situation by saying that the thick compressed film has during the last decade become progressively thinner until now the tendency is to reinterpret the ideas of the compressed film in terms of the unimolecular 1ayer.l Adsorption and Concentration. The connection between extent of adsorption and the concentration (pressure) of the gaseous adsorbate a t constant temperature is one of the most characteristic marks of adsorption. The relationship is fre- quently expressed by the equation, x/m = kplI*, where x is the amount adsorbed by m grams of adsorbent, k is a con- stant, p the pressure of gas, and n a number which may vary from n = I to n = a. Over a limited pressure range a value for n may be found which adequately expresses the experimental results.Over a large pressure range this is not so, in which case an equation derived by Langmuir for a uniform surface on the basis of his concept of uni- molecular adsorption may often be substituted. This equation takes the form x - k’bp - - -- m I + & where k’ and b are constants, the other symbols having the significance given above. This equation will be seen to yield a value of n = I for low values of p(bp < I ) and n = w for large pressures, in formal agree- ment with the classical expression above. It therefore indicates an adsorption proportional to the pressure for low pressures and an in-H. S. TAYLOR I33 dependence of pressure (saturation of the surface) a t sufficiently high pressures. These equations are characteristic of adsorption in that, of the other possible processes that might occur, the one, solution, is governed by the relation x/m = kp otherwise known as Henry's Law ; the other, compound formation between solid and gas, requires that below a cer- tain pressure at a given temperature, no interaction occurs whilst, at this pressure, reaction goes to completion without further change in pressure.The continuous variation of the adsorption with pressure and the continuous variation of the exponent of the pressure with in- creased adsorption constitute, therefore, together, a decisive criterion of adsorption. Non-Uniform Surfaces. The Langmuir isotherm given represents an equilibrium between condensation of gas molecules on a uniform surface and their evapora- tion from the surface.The rate of condensation per gram of surface is obviously proportional to the pressure and the bare surface ( I - a) + dx/dt = k d ( I - a). The evaporation is proportional to the fraction of the surface of the area covered (a) - dxldt = k . p + dx/dt = - dxldt J E I ~ ( I - 0) = kzu At equilibrium or whence or since u = k . x/m, k'bp x/m = - 1 + bp' The evaporation process varies exponentially with temperature, where a is a constant and A is the heat of vaporisation (adsorption) which is constant for a uniform surface. For a non-uniform suKface in which, on different fractional areas al, 0, . . . an the heats of vaporisa- tion are respectively, A1, A, . . . An the number of molecules leaving the surface per unit time is a more complex quantity of the form - dx/dt = a1ale-~~~IIRT + a2~,e-A~/IIRT + .. . + a,u,,e-AnlRT. The implications of such an equation can be understood if one thinks of a surface composite of two types of equal areas but with different 9 Provided no change in molecular state occurs in solution. If association occur then x/m = kpn where 12 is the degree of association. If dissociation occur (e.g., of a diatomic molecule into atoms) x/m = k p l / " where n is the number of dissociation products from one molecule of gas. This last equation differs from the adsorption equation in that n would not vary continuously, as in adsorption, up to large values with increasing saturation. For dissociation in solution be- coming progressively less with saturation n would decrease to a final value of unity.1 34 CENERAL INTRODUCTION heats of vaporisation A, and &.Since the areas are equal the con- densation process will occur at the same rate on each area. The evapora- tion processes will be however different and given by the two expressions a,u,e-AdRT and a2u2e-”dRT. There is good reason to believe that a, = a, hence the rates of evapora- tion from the two equal areas covered will be in the ratio e--hdRT/e- AdRT = e- h- Ad/RT- Let us assume for example that (4 - h,)/RT = 5, that is to say the heats of evaporation differ by 3000 calories when T = 300” K. Then the ratio of the rates of evaporation from equal areas covered will be e - 6 : I or I : 150 approximately. That is to say, a t a gas pressure p at which the area with the larger heat of vaporisation is covered the second area will on the average be covered to the extent of I part in 150 that is, it will be relatively bare.The isotherm would be discontinuous, showing a t the pressure in question a region of small change in adsorption with increase in pressure until the first area was covered followed by a second section at higher pressures corresponding to the filling of the area with lower heat of vaporisation. For more complex non-uniform surfaces the isotherm would reveal correspondingly more discontinuities. It is worth discussing whether the recent results of Allmand and Burrage3 and of Benton and White* are not entirely to be attributed to the operation of such factors, and it is a problem for further experimental enquiry whether such discontinuous isotherms are not to be quite generally found.Benton and White4 have formu- lated a specialised interpretation of such discontinuities. An alterna- tive discussion of this point is given by Polanyi in this Discussion. The measurements of heats of adsorption a t various surfaces reveal quite generally the non-uniformity of the adsorbing surface. The initial areas covered are in general associated with high heats of ad- sorption. A progressive decrease in heat of adsorption occurs with increase of area covered. For a non-uniform surface, with sufficiently accurate measurements, this decrease should be stepwise. This of necessity involves discontinuity in the isotherms when examined over sufficiently small pressure ranges. The more varied the surface the less pronounced will be the discontinuities, since with minor differences in heats of adsorption (vaporisation), the less active areas will begin to fill with gas before the more active areas are completely filled.In this question of the heterogeneity of surfaces the decisive contribu- tions from studies of “ poisoning ” in chemical reactions occurring a t surfaces must not be forgotten. They have served to emphasise not only the theoretical but practical importance of the non-uniform surface. Mobility of Adsorbed Oases. The brilliant experimental investigations of Volmer and his colla- borators served to show that an adsorption layer on a surface might be extremely mobile and that the transfer of matter via such a mobile adsorbed film might be as much as 18,000 times greater than that occurring a Proc.Roy. SOL, 130A, 6x0, 1931. ‘ J . Am. Chem. SOC., 53, 2807, 3301, 1931.H. S. TAYLOR I35 through free evaporation into the air. This mobility effectively pro- vides a mechanism whereby the areas of a surface having the highest heats of adsorption become covered with adsorbed gases before the sur- faces of lesser activity.b The saturation of such areas does not need to be attained by condensation processes alone but may be assisted by this two-dimensional motion parallel to the surface. It is a probable assumption that the mobility of adsorbed gases will be correspondingly less on the areas of stronger binding energies. Indeed, there is reason to believe that the two-dimensional motion may be limited to adsorp- tion processes involving only the van der Waals' forces and that polar binding to the surface entirely suppresses any lateral motion.Ex- perimental examination of this question of mobility in relation to the nature of the binding forces is an important current problem. In this respect a recent study by Kautskyg of hydrogenation of niethylene blue a t platinum areas sparingly distributed over surfaces of silica and thoria gels, is of great interest. Polar adsorption of the dye- stuff by the thoria restricted its access to the platinum centres and so slowed down hydrogenation in comparison with the velocity when silica was used as the platinum support. Silica does not adsorb the dyestuff in a polar manner. The conclusion is compelling that the material hydrogenated reached the platinum from the solution, not by lateral diffusion along the surface of the support-material when adsorbed by polar forces.Nature of the Adsorption Process. In what has preceded it has been tacitly assumed that only one ad- sorption process is occurring with a given gas and a given surface. Evi- dence is accumulating rapidly that this is not necessarily universally true. Even in the older literature there is sporadic evidence that more than one type of binding may be involved in an association of gas and surface. Dewar showed that the adsorption of oxygen on charcoal at liquid air temperatures involved a heat of binding of 3744 calories and that the oxygen was readily recoverable by pumping. At o O C . , Keyes and Marshall found a heat of adsorption of 72,000 calories for the first gas adsorbed and a difficult recovery mainly as oxides of carbon. Langmuir found that carbon monoxide was adsorbed by platinum and readily removed a t liquid air temperatures, but was relatively irre- versibly adsorbed a t room temperatures and higher.Langmuir also found that the amount of oxygen adsorbed a t liquid air temperatures by platinum was only one-tenth of that a t room temperatures and that adsorbed in the higher temperature range is only removable by reaction with other gases. Benton and White have shown similar behaviour with hydrogen on copper and nickel, the low temperature adsorption being small and involving low binding energy, the high temperatures being greater and involving high heats of adsorption. The same is true for carbon monoxide on palladium (Taylor and McKinney) for hydrogen and for carbon monoxide on zinc chromium oxides (Garner and Kingman), manganous and manganous chromium oxides (Taylor For a summary of this work see Volmer, this Discussion ; Taylor, Tredise of PhysicaZ Chemistry, Vol.2, pages 1063, 1074, Macmillan & Co., London ; D. van Nostrand Co., New York; McBain, Sorption of Gases and Vapows by Solids. Chap. XI., Routledge, London, 1931. See also Bangham and Fakoury, J . Chem. SOC., 1324, 1931. Rer. d . clrem. Ges., 64, 2446, 1931.1 36 GENERAL INTRODUCTION and Williamson), zinc oxide (Taylor and Sickmann). In these latter cases the low temperature heats of adsorption are of the order 1-2 Kg. Cals. while the high temperature binding is about 20 Kg.Cals.' Benton also in his communication to the present meeting lists other cases of this kind, of which a striking case is that of carbon monoxide on copper. At liquid air temperatures the adsorptions of nitrogen and carbon mon- oxide are of the same order of magnitude but a t - 78" and 0" the adsorp- tions of the latter are one or two orders of magnitude higher than those of nitrogen. Activated Adsorption. About a year ago the present writer8 called attention to the ex- perimental fact that these high temperature adsorptions of gases with high binding energies were also processes occurring with measurable velocities. Furthermore, the velocities increased with increase of temperature in a manner familiar to students of chemical kinetics, the adsorption process occurring as though it involved an activation energy.For this reason this type of adsorption was designated "activated adsorption." It was shown that the concept of activation energy for such adsorption a t once provided an explanation for the absence of such adsorptions a t low temperatures, the temperature a t which the activated adsorption becomes manifest being a function of the magnitude of the activation energy and characteristic for a particular surface-gas mixture. Thus, with hydrogen and the metals copper and nickel, activated ad- sorption sets in around liquid air temperatures. With zinc oxide, manganous oxide, and mixtures of these with chromium oxide activated adsorption of hydrogen becomes measurable in the interval O"-IOO~ C. With alumina and glass such activated adsorption of hydrogen is not manifest until temperatures in the neighbourhood of 400° C.are reached. In another paper in this symposium the concordance between activated adsorption of hydrogen and the capacity of surfaces to induce the para- hydrogen conversion is emphasised. This extraordinarily simple generalisation of actual experimental observations seems to have aroused a quite unreasonable amount of opposition. Actually the concept of activated adsorption only general- ises a number of independent and hitherto isolated experimental ob- servations. By some the time-process has been interpreted to be ex- clusively one of solution of the gas in the metaL9 Benton has,8 in his communication to the symposium, shown that this is certainly not wholly tenable although no one will deny that some solution of these gases does occur in these metals.What the present writer would em- phasise, however, is that for all cases where available data for the It is unfortunate that, on the basis of data by Garner and Kingman it is being assumed that these adsorptions of hydrogen and carbon monoxide by oxides such as zinc chromium oxide involve interaction with the oxides a t high temperatures with production of carbon dioxide or water. On the surfaces of zinc chromium oxide free from adsorbed oxygen both hydrogen and carbon monoxide can be adsorbed with high binding energies (20 Kg. Cals.) but recovered unchanged by evacuation a t higher temperatures as hydrogen and carbon monoxide respectively. Moreover these reversible adsorptions obey all the criteria of adsorption, e g ., obedience to the Langrnuir isotherm (cf. Taylor and Williamson, " Adsorption of H, on the MnO-Cr,Os a t 305" and 444" C.," J . Am. Chem. SOC., 53, 2174, 1931). 8 J . Am. Chem. SOC., 53, 5298, 1930; 53, 578, 1931. OSteacie, J . physic. Chem., 35, 2115 1931 ; Ward, Proc. Roy. SOC., 133A, 506, 533,1931.H. S. TAYLOR I 3 7 classical investigations of solubility of gases in metals (e.g. those of Sieverts) overlap the determinations of activated adsorption, the solu- bilities in question in no single case amount to 5 per cent. of the measured adsorptions. This is a result diametrically opposite to that reached by Steacie in this regard. Well-investigated data in this matter are cited in the discussion of Ward's paper in the present discussion.The available data on the variation of activated adsorption with pres- sure also obey excellently the expression for the adsorption isotherm, a criterion for surface phenomena which we have already discussed. Did a solution phenomenon obey such a pressure relation the applica- bility of the Nernst Distribution Law to such a system would yield a peculiar and hitherto unknown distribution ratio. The obedience of the isotherms to an adsorption formula is equally strong evidence against this time process of adsorption being a compound formation involving the whole mass. A restriction of compound formation to the surface areas would produce the observed isotherms. A distinction between strong adsorption and surface compound formation is, however, merely a matter of expression and involves no fundamental difference in principle.The experimental evidence with respect to velocity of adsorption is now sufficiently good that a decision can be reached as to the method whereby the adsorbed gas receives the activation energy required for the activated adsorption. Thus, data of Taylor and Sickmann on the adsorption of hydrogen by zinc oxide a t 184" C. indicate that, on 20 grams of the oxide, 7.7 C.C. of hydrogen measured a t N.T.P. were adsorbed in one hour, the activation energy of the process (deduced from the temperature coefficient in the range 132" to 184" C.) amounting to some 13,500 calories per mol. It is quite evident from a simple cal- culation that this adsorption is not to be accounted for on the assumption that adsorption occurs when gas molecules with an energy equal to 13,500 calories per mol.strike the surface. Kinetic theory teaches that the number of such molecules striking the surface per unit area per second is approximately where iV/v is the number of molecules per c.c., W the root mean square velocity of the molecules. For the case in point, this equation gives a value of & x 3 x 1019 x 2.5 x 1oS x e-15 x GO x 60 molecules per sq. cm. per hour or approximately 2 x 1021 molecules per square cm. per hour. This is some ten times the amount actually taken up by the whole zinc oxide surface in the time interval in question. One must conclude, therefore, that the molecules receiving the necessary activa- tion energy are first adsorbed on the surface and obtain the energy required from the surface.An enquiry into the manner in which such energy can be communication has recently been initiated by Herzfield and Mrs. Mayer.lO a(N/v)We - 135WRT Nature of Adsorption Forces in Normal and Activated Adsorption. The development of the quantum-mechanical method of approach to physical and chemical problems has recently been extended by London to the problem of adsorption.ll London has shown that by considera- 10 2. physik. Chem., Bodenstein Festband, 1931. 11 Ibid., IIB, 222, 1930.I 38 GENERAL INTRODUCTION tion of the van der Waals’ forces alone, it is possible to derive, quantum- mechanically, values for the heats of adsorption of non-specific adsorbates on charcoal surfaces in good agreement with experimental results.For adsorptions with activation energies it is probable that the calculations involve not only the coulombic forces but also the non- classical interchange energies between atoms of adsorbent and adsorbate. The approach to the calculation of activation energies in such cases will probably follow the procedure used so successfully by Polanyi and Eyring in their treatment of the activation energies of chemical reactions.18 An attempt to interpret activated adsorption along these lines is a t present in progress in Princeton by Mr. A. Sherman under the guidance of Dr. Eyring. The problem under study is the calculation of the activation energy of hydrogen adsorption on charcoal surfaces. Only the preliminary results of such study are at present available, but these point to interesting conclusions with real significance not only in adsorption studies but also in respect to reactions a t surfaces. Sherman’s preliminary calculations indicate that the activated adsorp- tion of hydrogen on a graphite surface with the normal carbon-carbon distance of 1-54 A. would require an activation energy of about 50 Kg. Cals. owing to the operation of repulsive forces between the carbon and the hydrogen. If the carbon-carbon distance were very large, a large activation energy would also be required, although the repulsive energies between carbon and hydrogen would be small, because of the energy necessary to separate the hydrogen atoms to large distances. Between these, Sherman finds an “ ideal ” carbon-carbon distance such that neither controlling factor in the two previous cases is important with the result that the energy necessary to bring a hydrogen molecule up to the C-C pair is small. Actually with a C-C distance of about 2-4 A. the activation energy has decreased to a value of about 5 Kg. Cals. While this must be regarded as only a preliminary survey of this interesting field it must be evident that its further exploration promises to be full of interest not only in the new field of activated adsorption which has just been opened up but also in the realm of reactions a t surfaces where the evi- dence is already definite and compelling that the adsorptions involved in chemical reactions are activated in type. The University, Man clz ester. l * Z . physik. Chem., 12B, 279, 1931 ; J . Am. Chem. SOL, 53, 2537, 1931.

ISSN:0014-7672    

DOI:10.1039/TF9322800131

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13. ON THE ADSORPTION OF GASES. SECTION I. EXPERIMENTAL METHODS. INTRODUCTORY PAPER TO SECTION I. BY ERIC K. RIDEAL. Received I 9th December, I93 I . In an introductory paper it is impossible even to give a sketch of the enormous field embraced under the discussion. I have accordingly attempted to give in briefest outline only a short account of a few of the problems connected with the experimental investigation. We may note as of primary importance, the determination of the specific surface of the adsorbent. In logical sequence we must examine the various methods which have been employed for the examination of the final distribution of the gas which has been sorbed, i e . , between the surface and the bulk phase respectively.Examination of the rates of attainment of equilibrium involve the measurement both of the critical energy increments and the reaction velocities of the various processes taking place when a solid is exposed to a gas. Finally, we must mention the experimental methods which have been devised to examine in more detail the process of formation of surface phases. The Specific Surface. Of fundamental importance is the nature and extent of the surface phase of the solid. With the realisation of the somewhat porous structure of the surface layers of most solids, the evaluation of the accessible area rather than the true specific area has been more important. Some five methods have been employed for this purpose. These may be briefly enumerated as follows :- ( I ) Comparison of the rate of solution in some reagent with that of a uniform surface of similar material, which is a method originally employed by Schmidt,l W ~ l f f , ~ Durau and more recently by Schwab and Rudolph,* for evaluation of the specific surface of reduced nickel.Lack of uniformity in the rate of solution is the chief objection to this method. ( 2 ) By determination of the Newtonian " temper " colours developed by metals when attacked by reactive gases. From a knowledge of both the film thickness determined optically and either the increase in weight of the metal or the volume of gas which has been taken up by the ISchmidt, 2. PhysiR. Chcm., I I ~ A , 236, 1925. 'Wolff, 2. angew. Chem., 35, 138, 1922. 8 Durau, 2. Physik, 38, 419, 1926. 4 Schwab and Rudolph, Z.physik. Chem., I ~ B , 427, 1931. I39I 40 INTRODUCTORY PAPER TO SECTION I metal, the specific surface can be evaluated. This method has been employed both by Dunn5 and by Constable.6 Uniformity of film thick- ness and uniformity of composition of the film is, as observed by not readily obtainable, and all irregularities in the surface texture must be assumed to possess a substrate thickness a t least equal to that of the resulting film. (3) The change in electrode potential caused by the passage of a definite electric flux 6.010~ coulombs per sq. cm. for a 100 mv. change in interface potential across a liquid metal interface, permits of the evaluation of the specific surface. This method was developed by Bowden and Rideal,8 and has been used by Bowdens and by Volmer and Erdly Griz.lo A theoretical treatment of the method on the basis of the wave mechanics has recently been given by Gurney.ll (4) The fact that the conditions of reversible adsorptive equilibrium are quickly established between the surface of an insoluble salt such as lead sulphate and a solution containing isotopic ions such as the radio- active thorium B, has been utilised by Paneth and his co-workersl* to evaluate the specific surface of such salts, and by Hahn.13 ( 5 ) A number of investigators have attempted to evaluate specific surfaces from experimental data on the adsorption of solutes which are readily quantitatively determined in small quantities.It is assumed in all cases that the saturation maximum corresponds with a close packed unimolecular layer.Dyes estimatable by colorimetric methods have been employed by Paneth and Radu,l* by Bancroft and Barnett,16 and by Bancelin.ls Metallic ions such as silver determined by electropotentio- metric methods have been utilised by Euler.17 The inapplicability of the former, even when not colloidal, to fine grained or porous materials, and of the latter to interfaces which in general possess a potential difference, e.g., mercury in a Lippmann electrometer to the exact deter- mination of the specific surface, is clearly evident ; the method although simple is very restricted in scope and the accuracy attainable very problematical. After the evaluation of the specific surface of the solid the next prob- lem which arises in the experimental investigation is the examination of the detailed molecular structure of the surface, this is necessary since the characteristics of the adsorption isotherms as well as the thermal changc-; appear to be dependent on the structure.It cannot be said that much progress has been made in this direction. We may note the at- tempts which have been made for the evaluation of the mean grain size when crystalline, utilising Debye's method of X-ray examination by Astbury and Clark,l* and by FeichtknechtlQ and the more recent ex- 5Dunn, P.R.S., IOIA, 203, 1926. Constable, P.R.S., 117A, 376, 192s ; 119, 196, 1928. Evans and Bannister, P.R.S., 125A, 370, 1929. Bowden and Rideal, P.R.S., 120A, 63, 1928. Bowden, P.R.S., 125A, 446, 1929. lo Volmer and Erdly Griz, 2. physik. Chem., 150A, 203, 1930.IlGurney, P.R.S., I ~ A , 137, 1931. 12 Paneth, 2. physik. Chem., I O I , 445, 480, 1922 ; Ber., 578, 1215, 1924. l3 Hahn, 2. $&ysik. Chem., 144A, 161, 1929. l4 Paneth and Radu, Ber., 57B, 1221, 1924. l6 Bancroft and Barnett, COX Symp., 6, 73, 1928. l6 Bancelin, J . Ckim. Physique, 22, 522, 1925. l7 Euler, 2. Elektrochem., 28, 2, 1922. l8 Astbury and Clark, J.A.C.S., 47, 2261, 1925. Is Feichtknecht, 2. Elektrochem., 35, 142, 1929.E. K. RIDEAL periments of G. P. Thomson,20 in interpreting the diffraction patterns obtained with high speed electrons. The existence of a network postulated by Smekal2I and Lennard Jones and Dent,22 has been demonstrated by optical methods by Z ~ i c k y . ~ ~ The problem of the surface structure is evidently capable of being attacked in an alternative manner not by analysis but by synthesis.By laying down a fresh surface in vacuo either as small aggregates in cathodic spluttering or molecularly by evaporisation, its properties and the change of these properties on ageing and with time have been the subject of several investigations. We may mention the work of Estermann 24 on the growth of crystals of silver and cadmium deposited by molecular rays of these metals. Optical methods for the examination of the deposits have been developed by Reinders and Hamburger,25 whilst the numerous observations on the change in electrical conductivity with age have been the subject of theoretical discussion by Frenke126 and Ehrenberg and The adsorptive powers of deposited films for various gases has been the subject of detailed investigations by Frankenburger and his co- workers.% The experiments of Bastow29 on the sorption of nitrogen and hydrogen by deposited platinum are of interest in this connection.The Surface Phase. If a metal surface be exposed to a gas and sorption takes place, it is important to consider what experimental methods can be devised to dis- cover whether the gas is retained on the surface of the metal or no. A number of such methods have in fact been employed and a t least one of them has been found to be capable of yielding quantitative, as compared with qualitative, results as to the existence, extent and nature of the surface phase. We may briefly review these methods :- (a) At high temperatures the thermionic emission from a clean surface is given by Richardson's equation i = AT2e-+IKT.The work function + is very considerably modified by the presence of foreign atoms in the surface phase. For dilute films this modification may be expressed in the form 4' = + + a0 where 9 is the fraction of the surface covered with foreign atoms, a being either positive or negative. For surface concentrations of less than 10 per cent. in the case of caesium and for closely packed films, this simple expression is no longer valid. The method has been developed as a weapon for the examination of sur- face film structure, more especially by Langmuir, Becker, and Kingd~n.~O Whilst the method of examination by thermionic emission is naturally restricted to relatively high temperatures, the change in potential a t a gas liquid interface caused by the insertion of a film can readily be measured at ordinary temperature^,^^ and it has been found possible zoG.P. Thomson. P.R.S., 133, I, 1931. 21 Smekal, Physik. Z., 26, 700. 1925 ; 45, 869, 1927. 28 Lennard Jones and Dent, P.R.S., I ~ I A , 247, 192s. a3 Zwicky, Proc. Nut. Acad. Scz., 15, 253, rg2g. 24 Estermann, 2. physik. Chem., 106, 403, 1923 ; 2. Physzk., 33, 320, 1925. 2SReinders and Hamburger, Rec. Trav. Chzm. Pays Bas, 50, 357, 475, 1931. a7 Ehrenberg and Honl, 2. Physik, 68, 289, 1931. 28 Frankenburger, 2. Elektrochem., 35, 920, 1929, et seq. 2Q Bastow, J.C.S., 193~. 30 Langmuir, Becker, and Kingdon, see Schottky, Handbuch expt. Physik. ; 31 Schulman and Rideal, P.R.S., 130A, 259. 1931. Frenkel, Physical Rev., 36, 1604.1930. Dushman, Reviews of Modem Physics. 2, 381, 1930.142 INTRODUCTORY PAPER TO SECTION I (unpublished work by H. Whalley) to extend this method to metal gas interfaces. ( b ) The presence or absence of a surface phase can be demonstrated by the nature of the diffraction pattern obtained on reflection of electrons of relatively low velocities from metallic surfaces, a method due to Davisson and Germer,32 and extended very considerably by R ~ p p . ~ ~ The necessity for high vacua precludes the methods from becoming a general one for the determination of the conditions of equilibrium. It may be noted in passing that the difficulties of obtaining a metal sur- face really free from gas is clearly demonstrated by this method of examination. (c) Many attempts have been made to obtain information to the existence or absence of a surface film from an examination of the long wave threshold value of the photo-electric emission.Those of Ives 34 may be cited as an example of the method in its application to films of the alkali metals. The difficulties inherent in the method are great, and although as yet its general applicability must be considered restricted, it is a method worth further serious attention. (d) The most suitable optical method at present available for the detection of a unimolecular film is that based upon the observations of Jamin35 that plane polarised light when reflected from the surface of water was slightly elliptically polarised. The theoretical treatment by Drude (Theory of Optics) revealed on reflection of plane polarised light both the existence of a change in amplitude and in phase of the two component beams when there existed a transition layer or film of refractive index different from either medium.By suitable measure- ments it should be possible to obtain not only the film thickness and the refractive index but also the dispersion of the film forming material. I t is possible that with the development of the theory of molecular scatter- ing, the method may be capable of giving information on the electrical properties of the adsorbed phase. The most exhaustive examination of a surface phase by this method has been carried out by Tronstad on oxide films on metals. In all cases his films were relatively thick. Unimolecular films have been detected by numerous investigators and making assumptions which in some cases a t least are somewhat un- warranted, molecular dimensions have been calculated from the observations.The work of Sissingh and Haak36 on mercury surfaces, of Ives and Johnson 37 on thin films of rubidium, of Frazer 38 on alcohol on glass, and of Bouchet,S9 may be mentioned as representative of the experimental methods employed. (e) There are two thermal methods of investigation which appear to be capable of more precise development. The amount of energy carried away from a wire at a temperature T, by the impingement of molecules of a pure gas at a temperature TI on its surface is given by Knudsen’s (T2 - T1) where a is the accommodation co- equation in the form Q = Ka dc 39 Davisson and Germer, Physical Relr., 2, 35, 705, 1927.83 Rupp, Ann. Physik, 5 , 453, 1930. Ives, Physical Rev., 34, 117, 1929. 86 Jamin, JOUY. de Chem., 31, 165, 1851. *a Sissingh and Haak, P.R. Acad. Sci. Amst., 21, 678, 1919. ST Ives and Johnson, Proc. Opt. SOC., 15, 374, 1927. 88 Frazer, Physical Rev., 33, 47, 1929. 89 Bouchet, C.R., 185, 200, 1927 ; and J . Physique, 1931.E. K. RIDEAL '43 efficient. The accommodation coefficient a is dependent on the nature of the surface. In the presence of an adsorbed layer of a foreign gas the value of a rises. The experimental work of Hughes and Bevan,m Chapman and Ha11,O F a r k a ~ , ~ ~ and especially the investigations of Roberts 43 may be cited as applications of the method. Both the monochromatic as well as the total thermal emissivity of a surface is dependent on the nature of the surface.We may cite the values of the latter for (bright) platinum and oxidised platinum (black ?) from the I.C.T. Temperature. Pt 400' 600' 4.66 7-50 PtO. 8.6 11'0 Van Praagh and Rideal 44 found that the temperature of a tungsten wire maintained in vacuo varied for identical energy consumptions from 44" C. to 80" C., the latter in presence of a film of tungstic iodide on the surface computed to be ten molecules in thickness. A film this thick- ness still possessed the characteristic dissociation pressure of a bulk phase. The total emissivity is accordingly less for this film coated sur- face than for the bare metal. It is possible that the change in emissivity caused by the presence of a unimolecular film is and that the applicability of the method is restricted to thicker films.The Bulk Phase. The methods of examination available apart from those involving the measurement of velocities. for the presence of the sorbed gas in the bulk phase of the adsorbent are somewhat less general than those described above for the detection in the surface phase, Whilst analy- tical methods present no difficulty, the distribution and state of the gas or vapour are not so readily discerned. We may divide the possible forms of distribution into the following : compound formation, lattice distribution, distribution in the slip planes and between the crystallites, distribution in micro-capillaries or channels. The increase in specific volume suffered by substances as varied as charcoal, metals such as palladium, platinum and iron, zeolites and clays, when sorption occurs is readily m e a ~ u r a b l e , ~ ~ but on account of the slow rate of attainment of true equilibrium the exact reversibility of the dimensional increase is difficult to demonstrate.Further expansion on sorption of gases has been demonstrated in the case of both platinum and palladium by means of the X-rays,47 but as is well known the phase diagrams for these systems are still in an unsatisfactory state. A few measurements have likewise been made of the change in electric resistance of wires on sorption and desorption of a g a ~ . ~ 8 Hughes and Bevan, P.R.S., I 17A, 102, 1928. 4 1 Chapman and Hall, i&id., 126, 478, 1929. 42 Farkas, 2. physih. Chem., 1931.49 Roberts, P.R.S., 129A, 146, 19.30. 44 Van Praagh and Rideal, P.R.S., 134, 400, 1931. 45 L O G . cit. See also Langmuir, J.A.C.S., 38, 2271, 1916. 46 See McBain, The Sorption of'Gases by Solids. 47 Handwalt, Physical Rev., 33, 444, 1929 ; Osawa, Tohuku Imp. Univ., I, 14, UI Sieverts, 2. Metallurg. 3, 37, 1913, 45, 1925.144 INTRODUCTORY PAPER TO SECTION I Rates of Sorption. Measurements of the rate of establishment of sorption equilibrium in themselves give but little information, unless the rates of the various processes taking place can be separately evaluated. It would appear that surface adsorption equilibrium is almost instantaneously attained. The adsorbed gas may now undergo further operations involving thermal changes; it may undergo reaction with the substrate to form a chemi-adsorptive compound, such a process requires an energy of activation and involves a thermal change. This phenomenon appears similar to, if not identical with, Taylor’s hypothesis of acti- vated adsorption.The adsorbed gas may penetrate into the sub- strate either along the intergranular boundaries or slip planes or possibly in the finer dimensions postulated by Smekal, or actually penetrate through the space lattice of the crystalline solid. It has been fairly definitely established that both these latter processes mimic ordinary chemical reactions, in that energies of activation are required for the diffusion processes and that these differ for the intergranular or slip plane and lattice diffusim processes respectively. Furthermore, the rate a t which both these processes occur follows the ordinary Fick diffusion law in the sense that the rate of passage from the surface to the interior and in the opposite sense on desorption is dependent on the concentration gradient from the interior to the surface, the concentration a t the surface being proportional to the amount of gas actually superficially adsorbed which is by no means necessarily proportional to the pressure of the gas.We may cite the experimental work of Langmuir49 and Cla~sing,~o on the migration of thorium through tungsten ; of Dunn,51 F e i ~ h t n e c h t , ~ ~ and Wilkins and Ridea153 on the diffusion of oxygen through copper oxide, and of Ward 64 on the diffusion of hydrogen through copper, as examples of direct measurement of rates of such diffusive processes and the determination of the critical energy increments.Measurements of the rates and critical energy increments of the other possible reaction of the adsorbed gas, namely, conversion into a chemi- adsorptive compound, have generally been under conditions when the possibly important processes of intergranular and lattice diffusion have been ignored, nevertheless the existence of this transformation has been clearly demonstrated although the values given for the critical energy increments based upon the supposed rates may be somewhat inexact. We may cite the work of Garner and his co-workers on the adsorption of oxygen by charcoal,55 and of Taylor and his colleagues on the ad- sorption of hydrogen by oxides. We may note in passing that since the rate of the intergranular dif- fusion into a fully evacuated solid is governed by the amount of gas adsorbed on the surface and the texture of the surface, ( i t ? ., the number and size of the fissures), this rate may be very considerably affected by the presence of any substance sufficiently strongly adsorbed, which not only reduces the amount of gas adsorbed on the surface but may also reduce the number and size of the fissure entrances, this phenomenon is 51Dunn, P.R.S., III, 210, 1926. 49 Langmuir, Physic. Rev., 22, 357, 1923. 50 Clausing, Physica, 7, 193, 1927. 62 Feichtnecht, 2. Elektrochem, 35, 142, 1929. 53 Wilkins and Rideal, P.R.S., 128, 394, 1930. “Ward, P.R.S., 133, 523, 1931. 66 Garner, J.C.S., 125, 1288, 1924 ; 2451, 1927 ; 2870, 1928 : Nature, 124, 4’9, Ig29.E.I(. RIDEAL I45 exemplified in the work of Maxted on the influence of hydrogen sulphide on the rate of sorption of hydrogen by platinum.56 Rates of Surface Migration. We have already referred to the measurement of the rates of inter- granular and lattice diffusion processes involved in the phenomenon of gaseous sorption and to the fact that adsorbed molecules may undergo reaction with the substrate to form a chemi-adsorptive compound. The chemi-adsorptive compound is presumably rigidly held on to the sub- strate surface and can only undergo lattice diffusion a t elevated tem- peratures. We must esamine the experimental methods for determining whether the adsorbed molecules are held in the sense that they perform oscillations within a small compass on the surface or whether they can migrate over the surface. Direct evidence for the lateral mobility of molecules moving over a substrate of the same material and of molecules moving over glass have been provided by actual visual and micro-balance observations by Volmer and his c o - ~ o r k e r s .~ ~ The movement of metallic atoms deposited on glass and on metal surfaces from beams of molecular rays has been directly observed by Estermann, by Stern,5s and by C o c k ~ r o f t . ~ ~ Whilst the numerous ex- amples of mobility a t relatively low temperatures provided by the phenomenon of sintering are well known, Becker’s observations on the migration of the alkali metals and of the alkaline earths over platinum by observation of the change in the thermionic emission, likewise provide us with direct evidence on this point.The method of measurement of the change in the gas solid interfacial potential men- tioned above is likewise capable of providing similar information as well as on the rates of formation and evaporation of surface phases at low temperatures. A study of kinetics of the oxidation of copper61 provides us with indirect evidence for the lateral mobility of oxygen over copper oxide. The hypothesis of lateral mobility of adsorbed gases provides us with a very useful mechanism for the interpretation of surface actions,62 al- though if the activation process for a surface catalytic chemical reaction involves the activation of a chemi-adsorptive compound which by some is supposed to be the only species capable of entering into a subsequent chemical reaction, the hypothesis of lateral mobility of a t least one of any two surface reacting gases must be given up. A very brief consideration of the observed effect of temperature on these lateral velocities show that in general the rate of movement is not only far less than in the free gas phase a t corresponding temperatures but, furthermore, possesses a t least in the cases carefully examined on relatively clean and dry surfaces (e.g., the experiments of Becker) a definite exponential temperature coefficient indicative of an energy of activation being requisite for the process of lateral diffusion, i.e., in passing from point to point in a non-uniform atomic field.I t is possible that this energy of activation for lateral diffusion may prove to be identical with the ct seq.68 Stern, 2. fihysik. Chem., 106, 397, 1923 ; and 39, 774, 1926. sB Cockcroft, P.R.S., I 19A, 1928. 6o Becker, Trans. Amer. Electrochem. Soc., 1929. 61 Wilkins, P.R.S., 178A, 407, 1930. 68 See especially Schwab, Katalyse. I 1146 INTRODUCTORY PAPER TO SECTION I value obtained for the process of diffusion in the intergrannlar boundaries and fissures of the solid. We may make an approximate calculation for this energy of activation of surface diffusion from Wilkins' experi- ments (Zoc. cit.) on the oxidation of copper. The rate of oxidation as determined by the rate of change of pressure at 500' K. at 300 mm. pressure was found to be 1'74 mm./sec. The area of the foil was 2 sq.cm. and the volume of the system about 100 C.C. Hence the rate of loss of oxygen into the interior was 0.115 C.C. (at N.T.P.) per sq. cm. per sec. At this pressure by the Herz Knudsen equation a t least 3'103 C.C. would strike the surface per second. If free lateral mobility were possible for all atoms striking and the accommodation coefficient were unity on the oxide coated surface, i t is clear that the limiting pressure observed, viz., 300 mm. would be much less. If only a fraction e - of the condensing moiec:iles acquire sufficient lateral mobility to migrate to a fissure we obtain = 0*115/3-10~ whence E = 12,400 cals. per gm. mol. for the energy of activation for surface migration, a value not widely different to 9500 cals. for the energy of activation of the intergranular diffusion process.In Davisson and Germer's experiments 32 on adsorbed hydrogen films on nickeI, the apparent " melting-point" of the film was found to be 400° K. If a t this temperature the number of hydrogen atoms moving from point to point becomes so large that the electron diffraction pattern becomes too weak for observation we obtain the small value of E = Ca 1000 calories. The occlusion rate of hydrogen through nickel has been measured by Sieverts 63 who noted a rapid sorption above 200' C. It is an interesting speculation how far an atom can travel when once liberated in this manner, but the evidence provided by molecular ray experiments appears to indicate that the range of action when in the liberated state is small and that the molecule suffers rapid readhesion.On Energy Exchange between Gas and Solid on Adsorption. We have mentioned the experimental methods which have been devised to test the hypothesis that molecules impinging on a surface may condense and then undergo chemi-adsorptive reaction or pass into the interior via the intergranular channels or through the lattice. When condensation does not occur the impinging molecule is reflected. The life of a molecule on the surface may thus be veqf short or very long; we may summarise the various experimental methods which have been devised to examine the result of collision of a molecule with a surface and the effect of increasing the lifetime of the molecule on the surface, ie., in altering the relative magnitudes of 4 and kT. For extremely short lifetimes comparable with the lattice frequency (10 - l3 secs.) the interaction must be regarded as governed by the laws of wave mechanics. If the atoms are arranged in a regular lattice a certain proportion then are reflected or diffracted and for them there is no interchange of energy a t all.We are indebted to Stern and K n a ~ e r , ~ ~ Estermann and Stern,= and Johnson for confirmation of the ap- plicability of de Broglie's wave equation by the method of molecular rays.67 In this case, as shown by Roberts,68 the accommodation co- 6* Sieverts, 2. Elektrochsm., 16, 707, 1910. 62 Stem and Knauer, 2. Physik, 53, 779, 1929. 6s Estermann and Stem, 2. Physik, 61, 114, 1930. 66 Johnson, J . Franklin Inst., a%, 308, 1928 ; 207, 635,1929 ; 210, 145, 1930. *' Molecular Rays, 1931, Fraser, Cambridge Univ.Press. Roberts, P.R.S., 1a9A, r46, 1930.E. K. RIDEAL I 4 7 efficients are small and the extent of this energy interchange is thus very small; an approximate theory of this case has been developed by Jackson. When the lifetime increases in this manner and the energy interchange becomes more pronounced the reflection becomes more and more diffuse as shown by the experiments of L a n g r n ~ i r , ~ ~ Knudsen, '* and Wood.71 With molecular rays the conditions for condensation can readily be examined and Frenkel's 72 analysis tested. At the critical stream densities and temperatures the lives are naturally much greater than those obtained for specular reflection. W e r t e n ~ t e i n , ~ ~ by observation of the rate of condensation of mercury vapour on glass, obtained ca.10 - 13 seconds a t 300" K. Cockcroft 74 obtained I O - ~ seconds a t 250' K. for cadmium on copper. These lifetimes are all maximum values. When condensation occurs not only has thermal equilibrium with the surface taken place but the condensed atoms have reacted with one another t o form a condensate. We may regard this phenomenon as the formation of a chemi-adsorptive compound with the substrate followed by activation and reaction with a neighbour. The importance of the lifetime on the surface as a factor in facilitating the transfer of energy is thus evident. The question of energy transfer in relationship to time of contact is clearly important in chemi-adsorptive reactions as the rate of transfer of kinetic energy of translation, of rotational energy and of internal vibrational energy may not be identical.That differences exist in the rate of transfer for the different kinds of energy in simple molecular collisions is apparent from the recent work of Kneser on carbon dioxide where a relaxation period of as long as I O - ~ seconds was found for the transfer of the vibrational energy, whilst Herzfeld and Rice,75 observed as a mean time for the transfer of rotational energy I O - ~ seconds. We may note in this connection that Rice and Byck 76 obtained no signs of decomposition a t a platinum target a t 1600~ C. of impinging beams of acetone or dimethyl mercury. In the latter case, if equipartition had been attained, with an energy of activation of 35,000 cals./gm. mol. over 6 per cent. decomposition should have been attained. According to Taylor's values the energy of activation for chemi-adsorption of hydrogen on a metal is about 12,000 cal./gm. mol. Taking hydrogen a t roo mm. pressure 103 C.C. strike I sq. cm. of the surface per second according to the Herz Knudsen equation. If a fraction e - E/RT of these underwent the chemi-adsorptive reaction, reaction would be complete in about I O - ~ seconds. If, on the other hand, the reaction be supposed to take place after adsorption and we consider N molecules adsorbed per sq. cm. a t any time, the rate at which these enter into reaction will be Nve - E/RT where I/V = T is this period of interchange. If we insert the ordinary vibration frequency T = 10 - l3 in the above equation we obtain as rate of reaction LV . 4-10s, i.e., an extremely rapid rate of reaction A value of v = I O ~ or T = I O - ~ seconds gives us a rate equal to 0.42, which if the surface is b u t sparsely covered is a measurable rate of reaction. The same point may be demonstrated in another way. Langmuir, Proc. Nat. Acad. Sci., 141, 1917. 70 Knudsen, Ann. Physik, 48, 1115, 1915. Wood, Phil. Mag.. 30, 300, 1915, 72 Frenkel, 2. PhysiR, 26, 117, 1924. 73 Wertenstein, J . Physique, 4, 281, 1923. 74 Cockcroft, P.R.S., I r9A, 1928. 75 Herzfeld and Rice, Physical Rev., 31, 691, 1928. 76 Rice and Byck..P.R.S., 50, 132, 1931.

ISSN:0014-7672    

DOI:10.1039/TF9322800139

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No.13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE USE OF THERMIONICS IN THE STUDY OF ADSORPTION OF VAPOURS AND GASES. BY JOSEPH A. BECKER (Bell Telephone Laboratories, New York). Received 3rd December, 193 I. The object of this paper is to point out a relatively new but very powerful tool in the study of adsorption phenomena. This tool is thermionic emission. While it is not applicable to all surfaces and all kinds of adsorbents, it makes up for these deficiencies by the insight i t gives us into the nature of adsorption processes.The results obtained thus far from this tool have come largely as by-products of the study of thermionic emission of electrons. When the primary emphasis is placed upon the study of adsorption itself, one can confidently expect an abundant yield. This is quite apparent when one surveys the results obtained thus far. These results apply when strongly electropositive metals such as caesium, barium, or thorium or strongly electro-negative gases such as oxygen are adsorbed on surfaces of metals such as tungsten, molybdenum, or platinum. In these cases : the adsorbed particles exist either as adions (adsorbed ions) or adatoms; the ratio of adions to adatoms decreases as the surface concentration increases ; the rate of evapora- tion of adions or of adatoms varies rapidly-perhaps exponentially- with the surface concentration; as the concentration of adions plus adatoms increases the rate of evaporation of adions decreases while that for adatoms increases; the heat of adsorption or the energy required to remove an individual atom from the surface depends upon the con- centration of similar adatoms and is greatly affected by the presence of other kinds of adatoms ; the mean adsorption time or mean life of an adsorbed particle is a function not only of the temperature but also of the concentration; particles adsorbed in a given region of the surface behave like a two-dimensional gas and migrate to other regions-there is some indication that this may happen only above some critical tem- perature analogous to the melting-point in solids.The key to these results is the fact that the thermionic electron emis- sion from tungsten and similar metals varies by very large factors and in a characteristic manner when small amounts of electropositive metals are adsorbed on the surface. Fig. I illustrates this fact for caesium, barium, and thorium on tungsten a t 1100’ K. It shows the logarithm of the emission current in amperes per cm.2 vs. f, the fraction of the sur- face covered. They differ chiefly in the height at the maximum or so-called “ optimum.” This optimum occurs when the tungsten is covered with a single layer of adsorbed partic1es.l When the surface is covered with two or three layers, the emission current has the vdue characteristic of the adsorbed material in bulk.For any value off greater than zero, the current with caesium is greater than that with barium, and this in turn is greater than PhysicaZ Rev., 28, 341-361, 1926. 148 The three curves are quite similar to one another.J. A. BECKER =49 that with thorium. Between ,f = o and f = 0.8, the shape of each curve is such that its slope a t any f is proportional to (log il - log i,) where the subscripts refer to the value off. The ratio of the current a t the optimum to the current from clean tungsten, i.e. 2 depends on the temperature ; as the temperature decreases, this ratio increases. Thus, i 20 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f FRACTION OF SURFACE COV&RED.FIG. I. a 20 for caesium on tungsten log 4 = 13.3 a t I 100' K. while it is approximately 19 at 800" K. The similarity in the general shape of these curves is all the more striking since they can be obtained in three distinct ways, depending upon the method by which the material is deposited on the tungsten surface. In the case of caesium, the tungsten is exposed to caesiumvapour. Caesium atoms arrive at the tungsten surface a t a rate determined by the vapour pressure which in turn is usually determined by the tempera- ture of bulk caesium in the tube. The barium is deposited on the tungstenADSORPTION OF VAPOURS AND GASES 1 50 by flashing a primary barium-alloy filament a t a fixed temperature for a fixed time.2 Each flash deposits a small but definite amount of barium on the tungsten.Thorium can be deposited in this second way. I t can also be made to arrive a t the tungsten surface by diffusion from the interior of thoriated tungsten along the grain boundaries,* by glowing the tungsten at an activating temperature for various lengths of time, These three methods may be designated as the vapour, evaporation, and diffusion methods. The details of the use of these methods are fully described in the original articles, to which references are given. From the log i vs. f curves of Fig. I, it is possible to determine how the “work function” of the surface varies with f. While the exact definition of the work function is quite complex, it will be sufficient for the purposes a t hand to think of it as the work an electron must do to escape from the metal surface.It is customarily expressed in equivalent volts, i.e. the potential difference in volts through which an electron would have to fall from rest in order to be able to do this amount of work. This work function W in volts is obtained by solving Richardson’s Equation in its logarithmic form, e log i = log A + 2 log T - w- 2.3kT . (1) For our purposes A may be treated as a universal constant whose value is 60 amps./cm.20K2. is numeri- cally equal to 11,600. From Fig. I and equation (I) we can determine W for any value of f. Fig. 2 is a plot of this relationship for caesium, barium, and thorium on tungsten for T = 1100 OK. The work function for tungsten is very materially reduced when its surface is more or less covered with electro- positive materials.The effectiveness of the adsorbed material, in re- ducing W is large a t first, but decreases more and more until the surface is covered with a complete layer. Additional layers increase W until a t two or three lavers, W approaches the value characteristic of the adsorbed material in bulk. A careful study of the curves reveals that the minimum value of W is approximately equal to the resonance potential of the adsorbed particles in vapour form ; and that - is proportional to (W - R) from f = o to f = 0.8, where R is again approximately equal to the resonance potential or the minimum value of W. In the attempt to explain these facts, we can learn a good deal about the nature of the adsorbed particles. It might a t first sight appear that the enhanced emission of Fig.I is to be ascribed to electrons emitted from the adsorbed particles. While this may be true for f larger than about 2, it cannot be true for the first layer. If it were true, the excess current above that from clean tungsten should be directly proportional to f and should increase steadily to the value characteristic of the adsorbed material. Instead of this, experiment shows that log i, rather than i, increases linearly with f for small values of f, and that the emission passes through a pronounced maximum. Another hypothesis which is capable of explaining these as well as If W is expressed in volts and T in OK, dW df Physical Rev., 34, 1323-1351, 1929. Langmuir, PhysicaE Rev., 22, 357-398, 1923. Clawing, Physica, 7, 193, 1927.J.A. BECKER 1.51 other observed facts is the so-called " adion grid theory " An adion is defined as an adsorbed particle whose valence electron no longer rotates about its own nucleus but has been absorbed by the underlying metal. This definition is simi1a.r to that for an ion in a compound such as caesium chloride. That both adions and adatoms may exist on one and the same surface a t the same time was shown in a previous paper.5 Of course, any particular particle may be an adatom a t one instant and an adion a t a later instant and vice versa. 4.6 4.4 4.2 4.0 3.8 v) 5 3.6 0 > Z ; 3.4 0 I- 2 3.2 3 Y g 3.0 3 3 2.8 II 2.6 2.4 2.2 2.0 I .8 1.6 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 f = FRACTION OF SURFP.CE COVERED FIG. 2. These adions act like a positively charged open meshed grid placed very close to the tungsten surface.Such a grid and its negative image produce zero field a t some distance beyond the grid and an average field of 4770 = 4neN, between it and the tungsten, provided that the size of the grid is very large compared to its distance from the surface. 6 Trans. Am. Electvochem. SOC., 55, 153-175~ 1929.152 ADSORPTION OF VAPOURS AND GASES Q is the charge on the grid per cm.a of surface. N , is the number of positive ions per cm.2 If 1 is the radius of the adion in cm., then the potential difference between any point in the metal and the point outside a t some distance from the surface is 4mN,I. This means that the work an electron must do to escape has been reduced by 300.4 n . 4-77 . IO-~ON, . I volts. Hence AW = Wo - W = 1.80. IO-'N,Z . * (2) This equation can be used to determine values of N,. For any particular f, AW can be read off from Fig. 2 . The radius of the adion, I, can be found in tables obtained from X-ray data or tables based on quantum theory calculation^.^ The only unknown is N,. It is then possible to determine P, the percentage of adsorbed particles that are adions, since * (3) p = - ND N . where N is the total number of adsorbed particles. related to f, since N is very simply xr f V f=- Nl (4) where N , is the value of N for f = I. An approximate value of N , can be computed from the known or estimated values of atomic diameters, d, and the type of surface packing. From simple geometrical considera- tions, it follows that for loose packing I N l = S .( 5 4 while for close packing Since close packing appears more probable than loose packing, the calculations that are to follow will be based on close packing. It should also be realised that in the derivation of equations (5a) and (5b) it is assumed that the number of particles in the monatomic layer is not affected by the size and arrangement of the underlying atoms. This may not be true, or it may be true only if the size of the adions is larger than the atomic size of the base metal, or it may be true only a t low temperatures. In any event, it will be a rather simple process to recompute the data when more reliable values of Nl are available for barium and thorium. For caesium, the value of Nl has been obtained by direct observation. Table I gives the values of P, the percentage of adsorbed particles that are ions, for various values off.The values of I and d , used in computing P, are also given. In some cases, the values of I have been obtained by interpolation. It is also desirable to know how the tendency to form adions varies as f increases. As a measure of this tendency, we may take dN,I which represents the ratio of the increment in the number of adions to a small dN Goldschmidt, Trans. Faraday SOC., 25, 253-283, 1929. ?Pading, Jour. Am. Chem. SOL, 49, 765, 1927.J. A. BECKER 0'2. 48 27 12 I 5 3 0'4. 0'6. 0.8. 1'0. ----- 40 33 28 23 23 20 17 14 9.5 7-9 6.6 5-4 TABLE I. I = radius of singly charged ion. N, = number per cm.2 when f = I. d = diameter of neutral atom.f = fraction of surface covered. 1 I 1 1 increment in the total number of adsorbed particles. ( 2 ) and (4) and the experimental fact that From equations -- dW _ - aN,(W-R) . df where a is constant : it follows that a dND = ( W - R ) . dN 1.8 x I O - ~ (7) This means that the tendency to form adions is directly proportional to the amount by which the work function of the surface exceeds the re- sonance potential of the adsorbed particles. The proportionality con- stant depends upon the nature of the adsorbed particles. On the basis of the adion grid theory, i t is easy to see why the ten- dency to form adions must decrease as f or N increases. For as N in- creases, N , will increase. As a result, the adsorption field, which helps electrons away from the surface, increases.Consequently, more elec- trons leave the underlying metal to neutralise the adions, and fewer electrons leave the adatoms and enter the underlying metal. Evaporation of Positive Ions and of Atoms. The adsorption fields greatly affect the rate of evaporation of positive ions. Since these fields are in a direction to help electrons out of the surface, they should make it more difficult for positive ions to escape. Experiments with caesium on tungsten fully confirm this prediction. In fact, they show that in a region fromf= 0.01 to 0.20, the more caesium there is on the surface, the less evaporates in any given time, and any caesium that does evaporate comes off as ions. For very small values of f, the ion evaporation rate increases with the concentration.It comes to a maximum when the surface is only about I per cent. covered. These experiments show that the forces on an adion, due to neighbouring adsorbed ions and atoms, are appreciable even when the average separa- tion is 10 atom diameters. At first sight, one might expect E,, the rate of evaporation of neutral atoms to be unaffected by the adsorption fields and consequently directly proportional tof. Actually, the evaporation rate a t a given temperature increases rapidly with the surface con- centration. As a first approximation, E, increases exponentially with f. Such evaporation curves for caesium on oxidised tungsten a t various temperatures were given in a previous p~blication.~ For caesium, barium, But this is not the case.I54 ADSORPTION OF VAPOURS AND GASES thorium and oxygen on tungsten, similar curves, though not so accurate nor so complete, have been obtained. The interpretation to be put on these curves is that while the adatom as a whole is neutral, the forces on the various parts of the atom are not zero.This is undoubtedly due to the fact that the adsorption fields vary so rapidly with the distance from the surface. In these fields, the net force on an atom is much the same as on a negatively charged particle, since the atom evaporation curves are similar to the electron evaporation curves. The technique employed to obtain such evaporation curves depends upon the adsorbed material. For caesium on tungsten,l the evapora- tion rate E at a given temperature T and a givenf, is equal to the arrival rate whenever equilibrium prevails.The arrival rate can readily be obtained from t.he saturation positive ion emissi0n.l f is obtained by suddenly decreasing the temperature of the surface t o a sufficiently low value and noting the time that elapses before the electron emission reaches its optimum value. During this time, every atom that strikes the surface sticks to it. At the optimum emission, the surface is covered with a monatomic layer. The observed time, multiplied by the arrival rate, gives the number of atoms that were added before a monatomic layer was reached. The difference between this number and N,, the number corresponding to a monatomic layer, gives the number N which were present a t the prevailing T and arrival rate. In this way, a single point on an E vs.f curve is obtained. To get other points, the process is repeated for other surface temperatures and for other arrival rates. on tungsten, the process is as follows : A definite amount of barium or thorium is deposited on the tungsten. The electron emission from this surface is then determined at a testing temperature so low that the amount on the surface is not thereby altered. From a log i vs. f curve similar to Fig. I , f is obtained. The surface is then heated for a short time to a temperature at which evaporation takes place. The emission is determined a t the testing temperature. From this, a new value of f i s obtained. The decrease inf, divided by the time in seconds and multiplied by N,, gives E in atoms per cm.2 per second for the mean value off.If this process is continued, E can be determined for other values off, and a complete E vs. f curve obtained. The whole process is then repeated a t another temperature at which evaporation takes place. These evaporation ciirves allow us to draw conclusions as to the way in which the “ heat of adsorption ” or work necessary to remove an in- dividual atom or ion depends upon the concentration. If this heat or work did not depend upon f, then the rate of evaporation should be directly proportional to f. Since the rate decreases with increasing f for ions, but increases more rapidly than proportional to f for atoms, we conclude that as the surface concentration increases, the heat of adsorp- tion increases for ions, but decreases for atoms. If a complete family of accurate E vs.f curves at various values of T were available, numerical values of the heat of adsorption could be For barium or thorium obtained for various values off from the slope of a plot of log E vs. 2- T’ Another interesting quantity that can readily be obtained from a family of E vs. f curves is r , the mean life of an adatom, or the average 8 See also Andrews, Physical Rev., 33, 454, 1929.J. A. BECKER I55 number of seconds an atom spends on the surface under various con- ditions. It can be shown that T is very simply related to E andf, namely that t, is the time required to deposit one layer if every atom sticks and the arrival rate is equal to E. Table II., taken from a paper on (‘The Life History of Adsorbed Atoms and Ions,” gives values of T for various values off and T.The large values of 7 are rather surprising. This table also emphasises the dependence of r on f as well as on T. TABLE II.-MEAN LENGTH OF LIFE OF CAESIUM ATOMS. On a Tungsten Surface Partially Covered with Oxygen (Cs on OW) Nl = 4.0 x 1014 atoms/sq. cm. Fraction of Surface Covered: f. 0.8 0.8 0.8 0.8 0.80 0.85 0.90 0.95 I ‘00 0’2 0’4 0-6 0.8 1’0 1’2 Temperature To K. Xme to Form One Layer: f l sec. 830 I33 60 22 830 350 I35 59 28 I33 I33 I33 I33 I33 I33 Mean Life, r sec. 665 106 :: 2: 665 300 I21 27 53 79 I 06 I33 I 60 Surface Migration. The thermionic emission characteristics are useful in studyin, sur- face migration. This tech- nique apparently is well adapted for a quantitative investigation of the factors involved in the movement of adsorbed atoms.Briefly this technique is exemplified by depositing barium on one side only of a flat tungsten ribbon. The thermionic cur- rent emitted from each side of the ribbon is measured under standard testing con- ditions and is used to deter- mine the amount of barium on each side. The ribbon is then flashed a t a temperature BARIUM SOURCE I TUNGSTEN RIBBON I PLATE 2 FIG. 3. PLATE 1156 ADSORPTION OF VAPOURS AND GASES between 900" K. and I 100" K. for a short period of time. The emissions are redetermined under standard testing conditions. This flashing and testing is repeated until the emissions from both sides are not altered by flashing. Fig. 3 is a simplified diagram of the tube used, while Fig. 4 is an idealised curve of the result.The first part of Fig. 4 shows how the logarithm of the current from the front side (il) and from the back side (iz) changes as barium is deposited on the front side of the tungsten ribbon. The deposition is discontinued when f = 0.80 on one side. Subsidiary tests show that during this deposition and testing no barium reaches the back side of the ribbon. For the second part of Fig. 4, the tungsten ribbon is flashed a t about 1000' K. Periodically the flashing is interrupted, while i, and i, are determined under the standard testing conditions. As a result of this flashing, the emission from the front side decreases rapidly a t first and then more and more slowly; while the I2 0 emission from the back side increases very rapidly at first and then more and more slowly.When the total time of flashing has continued €or one or two hours, the emissions from the front and back sides are equal and do not change appreciably with further flashing. The value of this emission corresponds to f = 0.40 whereas originally f for the front side was 0.80 and for the back side was 0.00. Furthermore at any stage of the flashing the sum of the f values for the front and back sides is 0.80. In other words, the material that was originally on the front side redistributes itself until it is uniform ; the back side gains what the front side loses. In actual practice, the current observed on plate I is nearly equal to il ; but the current to plate 2 consists not only of iz but also of elec- trons which come from plate I as a result of reflection or secondary emission.The result of this is that the measured current to plate 2 is never less than I per cent. of the current to plate I. When iz is muchJ. A. BECKER I5 7 smaller than i, this is serious, but as i, approaches i, the errors become negligible. Furthermore the difficulty can be greatly lessened by in- serting appropriate shields in the tube. Very likely if the currents were measured by means of a properly designed Faraday cage arrange- ment, it could be eliminated entirely. Diffusion. Still another application of thermionics to surface phenomena is in the study of diffusion from the surface into the interior and vice versa. An example of this is the diffusion of thorium to the surface of thoriated t u n g ~ t e n . ~ Another example is the diffusion of barium and oxygen in the case of oxide coated filaments.2 As far as they go these studies suggest the following picture: There is a certain amount of work in- volved when an atom is transferred from the interior of a body to its silrface.This work depends upon the surface concentration; i t may also depend upon the temperature. For each volume concentration there exists a surface concentration in equilibrium with it. If the surface concentration is much less than the equilibrium amount, every atom that reaches the surface stays there. If the surface concentration ex- ceeds the equilibrium value, its rate of decrease is rapid a t first but de- creases more and more until equilibrium is established. Oxygen on Tungsten. While most of the results described relate to electropositive materials, similar results appear probable for electronegative substances such as -6 -8 AMOUNT OF OXYGEN FIG.5. oxygen. varies with the concentration of adsorbed oxygen. Fig. 5 shows how the logarithm of the emission from tungsten The absolute valueADSORPTION OF SATURATED VAPOURS of the amount of oxygen is not known. One might venture to gtless that the maximum amount in this figure corresponds to half a layer. From the fact that the emission decreases it follows that oxygen forms negative adions. Since the magnitude of the slope decreases asfincreases, the tendency to form adions decreases as the oxygen concentration increases. that the rate of evaporation of adsorbed oxygen increases very rapidly with the surface concentration. Hence the heat of adsorption must decrease as the concentration increases. Other experiments show that oxygen migrates very rapidly a t 1400' K. Experiment shows Summary. Thermionic emission can be very useful in the study of adsorption phenomena. The primary reason is that very minute amounts of electro- positive elements, such as caesium, barium, or thorium, or electronegative gases, such as oxygen, change the thermionic emission from surfaces of tungsten, platinum, molybdenum, etc., by very large factors and in a characteristic manner. They do this by changing the work function of the surface. This effect, as well as other surface effects, can be best ex- plained by the adion grid theory : The adsorbed particles can exist on the surface either as adions (adsorbed ions) or as adatoms; the adions act like a positively charged, open meshed grid placed very close to the surface. (I) That the ratio of adions to adatoms decreases as the surface concentration increases (Table I.) ; (2) that the work required to remove an adion from the surface increases while the work to remove an adatom decreases as the surface concentration increases ; (3) the mean life of an adsorbed particle depends on the surface concentration as well as on the temperature {Table 11.) ; (4) the rate of diffusion from the surface into the interior depends upon the temperature and on the amount by which the surface concentra- tion exceeds its equilibrium value. Thermionic experiments show the existence of surface migration and can be used to make a quantitative study of this phenomenon. The techniques involved in these various ex- periments are described and references given to previous publications. From this theory and the experimental facts it follows :

ISSN:0014-7672    

DOI:10.1039/TF9322800148

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. ADSORPTION OF SATURATED VAPOURS ADSORPTION OF SATURATED VAPOURS BY POROUS SUBSTANCES.EXPERIMENTAL METHODS. BY F. G. TRYHORN AND W. F. WYATT. Received 8th December, I 93 I The comparatively few adsorption measurements in which values have been obtained for the amounts of two components adsorbed from a solution or a mixture led the authors to develop simple methods for studying the adsorption by porous substances of saturated vapours under conditions in which the composition as well as the amount of the adsorbate could be determined. Most of the methods described have been previously recorded in these Transactions, and are therefore pre- sented in summary form for the purposes of the present discussion,F. G. TRYHORN AND W. F. WYATT 159 1. Adsorption of a Saturated Vapour. (a) In early experiments a rather tedious procedure was adopted in which a weighed quantity of activated coconut charcoal or silica gel was suspended in a small pan by a fine platinum wire from the left arm of a balance beam so that it swung freely over the surface of a pure liquid of which the vapour was to be adsorbed. The liquid was contained in a jacltetted vessel maintained a t constant temperature by circulation of water from a thermostat : this vessel stood on a bridge placed across the balance pan, and its top was closed by a wooden cover carrying a glass tube drawn out to a fine aperture through which passed the sus- pension wire.The process of adsorption was followed by counterpoising the suspended adsorbent by weights on the right balance pan a t fre- quent intervals until saturation occurred. With liquids of low vapour pressure saturation required several days.Measurements were made by this method of the adsorption of the vapours of pure liquids by charcoal and gave concordant evidence of the two stages in the adsorption, zuk. adsorption of vapour qua vapour, followed by the sudden formation of a liquid surface on the charcoal through condensation of the adsorbed vapour. (b) The labour involved in the above method was great and to lessen it an automatic recording balance was devised. In this device the sus- pended pan of adsorbent was hydrostatically counterpoised by a short metal rod, hung from the right arm of the balance beam and partly immersed in a liquid of high density and low vapour pressure. The liquid was kept a t constant temperature in a jacketted glass vessel through which water a t 25' was pumped.By this arrangement, as the weight of the adsorbent increased the balance beam moved through a series of equilibrium positions, raising the metal rod progressively from the immer- sion liquid : the movement of the balance beam caused a corresponding movement of a small concave mirror of stainless steel, mounted on a rocking lever, one end of which was attached to the right end of the beam by a fine hooked platinum wire. Movements of this mirror were recorded by the reflexion of a spot of light on to a piece of bromide paper carried on a rotating drum which was driven electromagnetically. The pro- portions of the lever and the distance of the drum from the mirror were so calculated that the vertical displacement of the end of the balance beam over its maximum range of travel (0.6 cm.) corresponded with a movement of 14 cm.of the spot of light on the bromide paper. The liquid used was a paraffin oil fraction, b.p. 250-260°, density 0.8352 at 20° : in conjunction with a rod of 3.18 mm. radius this gave a range for the balance of 0.1592 gm. : by using p-bromo-naphthalene (d.oa 1.605) as immersion liquid and a rod 1.0 cm. radius the range could be increased to 3.026 gm. Separate light sources, flashing a t variable periods, were focussed on the drum to give time signals to calibrate the weight record in measuring adsorption velocity. The electromagnetic driye to the drum was controlled by an electric seconds pendulum which drove a clock mechanism in which variable contacts on the minute wheel completed the drum circuit a t 2, 15, 30 or 60 sec.intervals, causing the drum to rotate in 1.25, 9.375, 18-75 or 37.5 hours respectively. The balance was calibrated before and after an experiment by the addition of weights to the balance pans: when the balance had come into equilibrium after the addition of a weight the lamp focussed on to the mirror was flashed momentarily to record aI 60 ADSORPTION OF SATURATED VAPOURS spot on the drum which corresponded with the weight in question. After adsorption was complete the second calibration was made in a similar manner by adding weights to the right balance pan to bring back to beam to its initial position. After the record had been developed the calibration and time marks enabled it to be scaled off in both time and weight L1nits.l A balance of this type gives very satisfactory service and by building up the rotating drum on a broken gramophone motor, and using a simply constructed electric clock the cost of conversion of a normal balance to an automatic one was kept down to l 2 10s.11. Adsorption from Binary Saturated Vapours. (a) If from a mixture containing A, and B, grams of two components X and Y respectively, the amounts a and b are adsorbed, the final con- centration of X in the mixture will be 100 (A, - .)/(Ao + B, - a - b) - - AF. Thus if we can measure the total weight adsorbed, x, and also the final concentration of X, we can calculate the values of a and b from the equations a + b = X , and (100 - Ap)Aq - AF(B, - x ) = Iooa. The most rapid and accurate method of analysis of binary mixtures of pure liquids is by refractometric examination, the accuracy increasing with the difference in the refractive index of the two liquids.The H, line, and 20' were taken as reference standards throughout the work, and refractive index curves for the various pairs of liquids were constructed from measurements of mixtures carefully made up by weight. In making an adsorption measurement by this method a gram of activated adsorbent was placed in a flat-bottomed 6" x I" glass tube, and a small test-tube (about X I&"), with a short leg about Q" long sealed on to the bottom, was dropped into the larger tube : the latter was then drawn out in the blowpipe flame to a narrow tube about 2" from the top. After cooling and weighing, 2 or 3 C.C.of a given binary liquid mixture was run into the small test tube by means of a capillary pipette : the tube was then sealed off a t the constriction and both parts weighed together. The tube was placed in a thermostat, and after a definite time interval was dried, opened! and the refractive index of the residual liquid determined, and the increase in weight of the char coal measured. For a given pair of liquids about 20 such tubes were pre- pared, and were opened a t times ranging from I hour to five days. From these measurements it was possible to construct curves showing the composition of the adsorbate as functions of total adsorbate or of time, by means of the above equations. (b) Vapour Composition Curves.-Since the experiments measured primarily the adsorption from the vapour phase it was essential to know the composition of the vapour in equilibrium with the liquid mixtures of known composition.As no values were recorded in the literature for the systems examined the necessary data were obtained by a separate series of experiments, by a method based on the equations given above. A weighed amount of a mixture of known composition was placed in a long-necked bulb of about 10 C.C. capacity which was connected through an expansion vessel to a manometer, pump, and constant pressure de- vice. The bulb and expansion vessel were submerged in a thermostat 1 Tryhorn and Wyatt, Trans. Faraday Soc., 21, 399, 1925 ; 22, 139, 1926 : 2 Electvzc Clocks, Messrs. Cassell & Co., Ltd. 23, 238, 1927 ; 24, 1, 1928.F.G. TRYHORN AND W. F. WYATT I 61 a t 20'. The liquid in the bulb was shaken in the earlier experiments, but in later ones stirred by an electromagnetic stirrer, while the pressure in the apparatus was slowly lowered until slow boiling commenced : this mas continued until about 0.1 to 0.15 gm. of the mixture had evaporated : the loss of weight and the composition of the residue were then accurately determined. By substituting these values in the above equations the weights of each component in the vapour phase were found, and from this the weight percentage composition of the vapour in equilibrium with the liquid in the bulb. 111. Determination of the Pore Volume of the Adsorbents. Several methods were used in measuring the pore volumes of the charcoal and silica gel utilised in this work.These methods gave results which were in agreement, but were all based on the assumption that the molecular volume of the adsorbed liquid is the same as that of the bulk liquid : on this ground the values found are open to criticism, but the consistency of results obtained with liquids of very different compressi- bility renders it probable that, except perhaps for those molecules actually in contact with the surface of the adsorbent, the bulk molecular volume does apply to the adsorbate. (a) The first derivatives of the curves obtained when the weight of vapour of a pure liquid adsorbed per unit weight of adsorbent was plotted against time showed sudden breaks which were identified with the moments a t which the pores were filled and a liquid film formed on the surface of the adsorbent.From the amount of vapour adsorbed a t these points, assuming that the molecular volume is the same as in bulk, pore volumes of reasonable consistency (0.16 to 0.18 C.C. per gram of adsorbent) were obtained. (b) A check on the above value was obtained by soaking charcoal in a liquid until saturation was ensured, and then determining the loss in weight as a function of time : the volume of liquid present when the liquid film just disappeared from the surface was taken as the pore volume. .These two methods gave results which fell close together, those from the second method being slightly lower than those from the former. (c) The relative accuracy of the above methods was checked in the case of silica gel by determining the differential heats of wetting of the gel when various amounts of liquid were present in it. Samples of the gel were prepared containing known percentages of a liquid such as acetone : the heats of wetting of these specimens by acetone were then measured, and the heats of wetting per gram of gel plotted against the acetone contents. Extrapolation of such curves to zero heats of wetting was made, the intercept on the acetone-content axis being taken as the pore volume of the gel. The values so obtained were found to be the mean of those obtained by the above methods. 12

ISSN:0014-7672    

DOI:10.1039/TF9322800158

出版商:RSC    

年代:1932

数据来源: RSC

摘要:

118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. PRESSURE MEASUREMENTS FOR INVESTIGAT- ING THE MUTUAL BEHAVIOUR OF ADSORBED HYDROGEN ATOMS. BY M.C. JOHNSON, M.A., DSc. Physics Department, Birmingham University. Received 23rd December, 1931. I . Introduction.-The present note is concerned with the cohesion of an adsorbed layer, or mutual attraction and repulsion between its constituent particles, rather than with their adhesion to the adsorbent. There are many important cases in which adhesion to individual atoms of the solid is small enough not to be the main factor in controlling the lateral extension or aggregation of the adsorbed layer. There is well- known experimental evidence from electron scattering and from con- densation of molecular rays that such cohesion can range from lattice-like to fluid-like behaviour of an adsorbate. The state of aggregation of adsorbed atomic hydrogen is of particular interest, since in this case the intermolecular forces cannot be deduced from knowledge of cohesion in any pure phase; we are dealing with a substance which can only maintain stable existence in the adsorbed condition, the atoms being recombined into molecules in the gaseous, liquid and solid phases.We proceed to assess some of the conditions under which it becomes possible to study HI atoms remaining in proximity to each other without forming h. The first need is for quantitative data as to closeness of this proximity in a layer adsorbed a t the gas-solid interface. 2. Density of Packing of Atoms in Layer.-If hydrogen in a perfectly sealed enclosure a t constant temperature is known to be undergoing partial dissociation by some agency confined to the gaseous phase, measurements of its fall of pressure may yield information as to the structure of an adsorbed layer, on the very probable general hypothesis that atoms can remain on surfaces they strike much longer than can molecules.For quantitative accuracy the following conditions must be fulfilled :- (a) The initial stage when no atoms exist on the surfaces exposed, and the final stage when surface density of atoms has reached its maximum for given conditions of the gaseous phase (stage of “ saturation,” but not necessarily close packing), must both be precisely determinable. For this it is necessary t o ensure a complete tracing of the various “ fatigue ” phenomena affecting rate of pressure fall. (b) It is necessary to choose an adsorbent which is not itself likely to dissociate molecules a t its surface, as for instance may occur with Tungsten and Oxygen. Vitreous surfaces are preferable to metals in this particular; for a t many metallic surfaces dissociation of the gas may have to be regarded as a consequence of, not a cause of, adsorption, in which case an atomic layer might have begun to form immediately on exposure to any stray H,, thus making impossible the fulfilment of (a).(c) No other solids beside the given adsorbing surface must make any contribution to loss or gain from the gaseous phase. I 62M. C. JOHNSON 163 ( b ) and (c) can be satisfied only if metals are completely eliminated from the adsorption vessel. This was not possible in the pioneer measure- ments of Langmuir on atomic hydrogen, since the hot filaments used as dissociating agent themselves contribute to gas losses, by processes such as those investigated later by Dillon., For this reason I investigated pressure losses in adsorbable hydrogen, devising continuous-reading methods of elucidating the factors in “ fatigue ” to comply with (a), and avoiding all metal surfaces for the sake of (b) and (c) by using as dissociating agents (i) electrodeless in- duced discharges, and (ii) the impact of mercury vapour atoms after their excitation by k2537.2 The further assumptions then needed for determination of the packing of atoms in the adsorbed layer are : - (d) Loss of N molecules from a given volume of the gaseous phase implies gain of zN atoms to the adsorbed phase over a given area.This is based on the assumptions, valid for many practical conditions, that adsorption of normal H, is negligible compared with that of H,, that excited H,’ (which is not necessarily as slow to adsorb as normal H,) is not the principal product of the disturbances (i) or (ii) proceeding in the gas, and that the temperature is not low enough for condensation of H, which possibly occurs under liquid air conditions. (e) Magnitude of the given area of adsorbing solid is equal to the geometrically measurable area. This assumption is the weakest and is certainly invalid, in the strict sense, for all surfaces except those just solidified; but if fresh glass or silica walls are thoroughly baked, but not acid treated or bombarded, this unavoidable error is probably less than for microcrystalline metals whose surface has suffered oxidation, etc.The error may reach, under optimum conditions, the order of 50 per cent. ; and this accessible surface may itself be not so much vitreous as covered with the gases which diffuse continuously out of the best baked vitreous materials. Subject to the error (e), the methods described in the papers enable initial and final stages in saturation to yield a sequence of pressure measurements fulfilling ( d ) ; these gave, under varied conditions, the following order of variation in measured maximum packing of H, atoms in the adsorbed layer, incidentally confirming monomolecular thickness and atomic state of the adsorbate :- (i) 5.9 x 1olS, 5.3 x 1015, 3-2 X 1015 per cm.,, using electrodeless discharge.(ii) 1-2 x 1015, 1.0 x 1ols per cm.2, using photosensitised dissociation. The experimental conditions of (i) must have included molecules and atoms in various states of excitation and ionisation in the gaseous phase, and it is only the degree of packing which makes it unlikely that the layer contains much else beside neutral H,. The experimental conditions of (ii) can include no ions and no excited H,’ but only normal neutral atoms, and possibly the bye-product HgH, which itself may possibly be adsorbable. In comparing the packings, it must be remembered that forces of attraction and repulsion between excited H,’ are probably different from those between normal H1,3 and hence layers formed under T. J. Dillon, Proc. Physical SOC., 41, 546, 1929. *M. C.Johnson, Proc. Roy. SOC., 123, 603, 1929; 128, 447, 1930; Proc. Kemble and Zener, Physic. Rev., 33, 512, 1929; Eisenschitz and London, Physical SOL, 4, 490, 1930. 2. Physik, 60, 523. 1930.PRESSURE MEASUREMENTS conditions (i) and (ii) would exhibit (if fluid-like), different two-dimen- sional pressures, and (if lattice-like), different spacings. 3. Conditions under which Adsorbed H, can Recombine and De- sorb as H,.-A necessary (and until lately considered sufficient) condition that an H, atom encountering another H, shall form the desorbable H,, is that a third body shall be present to receive the surplus energy in the formation of the stable molecule. This third body may be a gas molecule or a “catalytic” surface. Atoms packed to the above measured surface densities must be continually within “ collision )’ distance unless they constitute a very rigid lattice.The adsorbed layer of H, would accordingly be expected t o be in a constant state of desorp- tion by pairing of its constituents, and only capable of being maintained by a constant supply of freshly adsorbing atoms from the gaseous phase. It is possible that such a non-static maintenance of layer plays consider- able part in electrode phenomena a t the liquid-solid interface, but at the gas-solid interface under the above conditions this is not the case : for the pressure curves of all the above experiments show no rise a t room temperature, following a removal of either of the types of dissociating agent used, partial desorption only setting in a t 2 0 0 O - 3 0 0 ~ C.This con- tinued stable existence of a sheet of non-combining atoms might be explained by saying that their valency is saturated by the adsorption itself, were i t not that they recombine readily enough with fresh atoms striking them from the gaseous phase. This has been investigated by many workers beside the present author, by means of the heating of the surface during attack by fresh atoms of however small kinetic energy. Accordingly the experiments enforce a conception of HI atoms at mean distances of mutual separation only slightly exceeding the diameter of their normal Bohr orbits, and yet in mutual repulsion from each other, while still capable of attraction by and combination with similar HI impinging from the gaseous phase. 4. Cohesion Between Atoms in the Layer.-The above facts might be taken as pointing to a perfect gas state of the adsorbed atomic hydrogen, if i t were not that the further investigation yielded some indirect qual- itative evidence of cohesion between neighbouring adsorbed atoms ; this is distinct both from their common adhesion to the solid surface, and from the mutual repulsion which gives them such rigid preference for gas atoms instead of adsorbed neighbours in the far stronger attraction needed for recombination.By analysing the curves of pressure fall into terms representing condensation, spontaneous desorption, and desorption due to recombination with atoms from the gas, the last of these three processes was found to be variable, and to depend on the packing density in the layer ; * this was only explainable if, when a gas atom strikes an adsorbed atom, the latter’s neighbours can exhibit a tendency to restrain the recombination from taking place.The work done in releasing an adsorbed atom cannot, therefore, be solely against the attraction of the solid, but is also against a slight cohesion with other adsorbed atoms. Adding such data to the previous, i t becomes necessary to regard the adsorbed atoms as exhibiting both (a) a mutual repulsion preventing molecule formation among themselves, and, superposed on this, (b) a slight mutual attraction giving an incipient tendency to a state of aggregation whose packing would be loose (due to the repulsion), com- pared with the closeness of atoms within a single molecule. 4 M . C. Johnson, Proc. Roy. Soc., 132, 67, 1931.M. C. JOHNSON 165 That is, the adsorbed layer possesses both of the characteristics of a (very) imperfect gas below its critical state and tending to condense into a lattice of wide spacing. In classical theory no such combination of properties could rationally be ascribed to HI, though of course H, molecules exhibit mutual attrac- tion in very slight degree. The quantum mechanical treatment of intermolecular forces alone explains the existence of H, atoms mutually repelling at distances such as found in the above experiments: the most recent development of this to include van der Waals’ forces accounts also for the slight super- posed attraction. It is possible that such treatment of combined at- traction and repulsion nowadays considered possible to non-combining H atoms in certain states will evaluate a theoretical structure of this adsorbed layer, to which the above experiments may possibly provide, in certain features, a rough demonstration.

ISSN:0014-7672    

DOI:10.1039/TF9322800162

出版商:RSC    

年代:1932

数据来源: RSC