J. Chem. SOC., Faraday Trans. I , 1984,80, 1539-1552 Adsorption of Ions at the Cellulose/Aqueous Electrolyte Interface Part 2.-Determination of the Surface Area of Cellulose Fibres BY THELMA M. HERRINGTON* AND BRIAN R. MIDMORE Department of Chemistry, University of Reading, Reading RG6 2AD Received 14th September, 1983 The surface areas of the fibres studied in Part 1, obtained by the method of negative adsorption, are reported. Corrections were applied for low surface potential using Gouy- Chapman theory and the charge/pH isotherms. The fibres were oxidised to increase the charge and surface areas obtained for oxidised and non-oxidised fibres were in good agreement. The surface area was also obtained for unbleached sulphite pulp which has a high charge. The results were compared with surface areas determined by B.E.T.nitrogen adsorption. All the pulps were initially in a ‘never-dried’ state and for the B.E.T. work they were specially prepared using solvent-exchange techniques such that all the water was replaced by dry pentane. The negative adsorption results for the unbleached sulphate pulp clearly showed the effect of overlapping double layers, and this was treated theoretically. The effect of beating the fibres, as in paper-making, on the surface area was also investigated by both methods as was the effect of drying the fibres once. In order to convert the surface charge obtained for cellulose fibres at different pH values in Part 1 from C 8-l to C m-2, the specific surface area is required. The accepted surface area for cellulose fibre obtained by B.E.T.nitrogen adsorption is < 10m2 g-l.l These authors obtained high values for the B.E.T. surface area of different types of pulp (100-300 m2 g-l) using a method of solvent exchange such that all the water in the pores of the fibre was replaced by dry pentane before drying the fibre for B.E.T. Schofield2 determined the surface area of jute fibres by the negative adsorption of chloride ions. He obtained values of between 130 and 200 m2 8-l for different sources of jute. It was decided to determine the surface area of different types of cellulose fibre by the two methods. One of the main advantages of the negative adsorption technique is that it eliminates all estimations of molecular sizes which are needed in other techniques like gas or dye adsorption. Since the double-layer theory assumes the ions to be point charges their size is immaterial.The degree of negative adsorption also becomes independent of the surface charge, once this has become sufficiently high. The fact that the measurements are carried out in aqueous electrolyte solution makes this technique very suitable for the evaluation of surface area for charge/pH isotherms as these are also determined in aqueous electrolyte solution. Hence ‘wet’ surface area is determined in contrast to a ‘dry’ surface area determined by gas adsorption. The main disadvantage of the technique lies in the generally small change in the concentration of co-ions. This was usually in the range of 1-2% for cellulose fibres. Also for these natural fibres, internal electrical double-layer overlap presents an added complication.Thus negative adsorption does not measure the smallest parts of the fibre and does not ‘see’ any roughness of the surface. Negative adsorption of course can only be used where there is a charged interface and an electrical double layer. 15391540 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE THEORY At the interface between a charged particle and the surrounding solution, the co-ions, bearing the same charge as the interface, are expelled from the surface, i.e. they are negatively adsorbed from the interface. Negative adsorption results in an increase in the concentration of that ion in the bulk solution. As this increase reflects the quantity of ions expelled from the interface, it can be related to the expelling surface area and can be used to determine this area.The interpretation of data in negative adsorption experiments depends upon a suitable application of the Gouy-Chapman theory for the diffuse component of the electrical double layer. This application was first made by S~hofield;~ he related the negative adsorption to electrolyte concentration and surface charge. Lyklema and van den Hu14 gave a more lucid derivation in which potential rather than charge was the primary variable. They derived the following equation for a z : z electrolyte : Ani St = - & 2/ni B[ 1 - exp (- zi elys/2kT)]-l ni where B = (zi2e2/2ekT$. (All symbols are defined at the end of the paper.) The specific surface is given by S = St/o, where o is the mass of solid used.If the potential, ly6, at the start of the diffuse double layer is high, then the surface area is given by For and The two An. St = 2 F Bdn,. (2) ni work with natural fibres it is convenient to use the excluded volume then experimentally observed excluded volume, AGhs, in natural fibres is made up of A V = Ani F/(ni o) (3) AV = S/B2/ni. (4) components, one due to the negative adsorption and the other due to what is essentially a steric effect. This is caused by the inability of ions to penetrate into the smallest pores of the fibre, which can, however, be penetrated by water. Thus AV = A&,, - A & ( 5 ) where A is a constant independent of electrolyte concentration. After this correction a plot of A V against 1 / B d n i has slope S if lys is effectively infinite.In a natural fibre where part of the surface area measured is internal, the effect of overlap between nearby double layers must be considered. De Haan5 successfully solved the mathematical problems involved in an analysis similar in conception to that performed by Schofield3 for the single double layer. This analysis has been clarified by using potential as the primary variable.s The ratio between the negative adsorption for a single double layer and that for an overlapping double layer is given by Also (7) Values for u,, k and dare first calculated and then eqn (7) is solved for ud. These values for ud and u, enable An;/Ani to be calculated from eqn (6).T. M. HERRINGTON AND B. R. MIDMORE 1541 CORRECTION FOR LOW POTENTIAL The factor [l -exp ( -zf ey8/2kT)] only becomes unity, as assumed for eqn (2), when tys is infinite.This of course is never achieved in a real physical situation, so in practice the magnitude of y8 must be taken into consideration. The factor for a 1 : 1 electrolyte comes to within 2% of its limiting value at ca. 200 mV. However, the degree of negative adsorption is not effectively altered once y8 has become sufficiently high. The correction factor can be estimated in the following ways. (1) The charge/pH isotherm may be used. ys is the potential of the double layer at the outer Helmholtz plane, which is related to the diffuse double-layer charge os by the Gouy-Chapman equation. From the titration experiments of Part 1 we know the charge density per gram, So,. One approximation is that 08 = o,, which assumes the absence of cation absorption in the Stern layer.However, in order to determine o,, S must also be known. This problem was solved by two different methods. (a) According to the Gouy-Chapman equation o,, = as = A d n i sinh (zieys/2kT) (8) assuming no cation adsorption in the Stern layer. But from eqn (1) and (3) for negative adsorption of chloride ion (9) S = A VBdn, [ 1 - exp (eys/2kT)]-l and so eliminating ys where q = - So,/(ABni A V ) and the correction factor is [l - 1/(2q- l)]-l. Each individual negative adsorption point can be corrected in this way, the correction factor depending on AV, nt and a value of So, from the charge/pH isotherm. So AV?, the excluded volume if the surface potential were infinity, is given / 1 \-1 by and S = AVF B d n i from eqn (10).A plot of AV," against l/Bdn, then gives S. This correction method has the disadvantage of exaggerating any experimental errors, as values of AV which are initially too high owing to experimental error will lead to correction factors which are also too high, and vice versa for values of AV which are too low. Thus the standard deviation of a series of AVF will be relatively high. To overcome this effect another correction technique was employed. (b) A set of estimated surface areas, S,, from the set of negative adsorption points was used to calculate 0, and hence tys. For each S , the generated surface area S was then calculated in the following way. From the Gouy-Chapman equation1542 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE where y = o,/(Az/n,).From the charge/pH isotherm, So, is known so y is replaced by p , where where Sn is calculated from eqn (4). Hence P = Soo/(ASnz/ni) (15) The intersect at the abscissa of the plot of ( S , / S ) - 1 against S , is an estimate for the best value of S . Another estimate is the minimum in the plot of the standard deviation of the generated surface areas against S,. AVF can then be calculated for each chloride concentration using this best value of S and eqn (16). (2) ws is increased to an effectively infinite value. In the case of the silver iodide and metal oxide sols this is easily accomplished by merely increasing the concentration of the potential-determining ions as there are a very large number of adsorption sites. In the case of cellulose fibres this can only be achieved by increasing the number of adsorption sites, either by chemical oxidation or by adsorption of charged species onto the surface.( 3 ) The assumption is made that tys is equal to the zeta potential. The zeta potential was determined using both microelectrophoresis and streaming potential techniques : this is discussed in Part 3. (4) ws may be estimated from an analysis of the So,/pH isotherm in terms of polyelectrolyte theory. This is also discussed in Part 3 . EXPERIMENTAL PREPARATION OF FIBRE The cotton linters, bleached sulphate pulp and unbleached sulphate pulp A were those used in Part 1. The unbleached sulphite and unbleached sulphate pulp B were obtained from Canada;' they were never-dried Black Spruce. All pulps were stored at 4-6 "C.The fibre was prepared in the same way as in Part 1 except that sodium chloride (0.1 mol dm-+) replaced hydrochloric acid as the electrolyte in which the fibres were soaked. The fibres were washed free of chloride as before. OXIDATION EXPERIMENTS Oxycellulose was prepared in two ways. (1) Oxidation at the C , position using chromate : 20 g of cellulose fibre were oxidised at room temperature in 0.2 mol dm-3 oxalic acid + 0.1 mol dmP3 potassium chromate solution (600 cm3) for 4 h. The fibre was washed thoroughly, soaked in 0.2 rnol dm-3 oxalic acid for 20 h and then reoxidised in 0.1 mol dm-3 sodium chlorate solution containing 3% acetic acid for a further 20 h. Finally the fibre was washed thoroughly. (2) Oxidative cleavage of the glucose ring using periodate: 20 g of cellulose fibre were soaked in 1 dm3 of 0.05 mol dm-3 potassium periodate solution in the dark for 48 h.The fibre was thoroughly washed and then soaked in 0.2 mol dmP3 sodium hypochlorite solution acidified to pH 3 with acetic acid, in the dark. Finally the fibre was well washed. DETERMINATION OF NEGATIVE ADSORPTION OF CHLORIDE ION The principle and method used to determine the negative adsorption were the same as that used in the single-point titration method for the determination of acid concentration. However, fibre in its sodium form was used, which, when dispersed in sodium chloride solution (2 x 10-4-0. 1 mol dm-3), gave a pH of ca. 6.5. The slurries were left overnight at 25 "C to ensure attainment of equilibrium, and the supernatant was analysed for chloride potentiometrically using a silver, silver chloride electrode and silver nitrate as the titrant.A glass electrode, keptT. M. HERRINGTON AND B. R. MIDMORE 1543 only for these titrations, was used as the reference electrode. The titration was carried out at pH 3, using dilute nitric acid, to prevent potential drift. All adsorption data were determined with reference to a blank, the moisture in the fibres being taken into account. A&,, was calculated from the increase in chloride concentration, Ani, using A &,s = Ani &/(ni 0). (17) The value of A%, the steric effect, or the negative adsorption not due to double-layer effects, was determined by carrying out the experiment at pH 1.3 (0.05 mol dmP3 HCl), where the surface potential is zero and there is no double layer.Then A 5 = A &,,(pH 1.3) (18) and A V is calculated from eqn (5). DETERMINATION OF SURFACE AREA BY NITROGEN ADSORPTION The water in the fibre was replaced by organic solvents so that on drying the fibre the pores did not collapse. Dry AnalaR methanol and pentane were prepared by refluxing over iodine + magnesium and sodium, respectively, followed by distillation. For the solvent exchange, 1 g of fibre was dispersed in distilled water and sucked dry on a Buchner funnel. A pad of fibre was formed in a vessel, from which damp air was excluded, and 500 cm3 of dry methanol passed through the pad over a period of 3 h followed by 500 cm3 of dry pentane. The solvent-exchanged fibre was then bottled in dry pentane.The B.E.T. surface area by nitrogen adsorption was determined by Dr A. McLeod of Brunel University using a Carlo Erba Strumentazione instrument. RESULTS Surface areas were determined for the cotton linters, bleached sulphate pulp and unbleached sulphate pulp, for which charge/pH isotherms are given in Part 1, both by negative and nitrogen adsorption. The surface areas of oxidised cotton linters and oxidised bleached sulphate pulp were also determined by the method of negative adsorption. The effect of drying on the surface area of the sample of unbleached sulphite pulp was determined by the negative-adsorption technique. The effect of drying on the surface area of the samples of unbleached sulphate pulp was investigated both by negative and nitrogen adsorption. The effect of beating on the sample of unbleached sulphate pulp B was investigated using negative adsorption.BLEACHED SULPHATE PULP The negative-adsorption results are shown in table 1. A value of 0.138 f 0.010 cm3 g-’ was determined for A V , by doing experiments in 0.05 mol dm-3 hydrochloric acid at sodium chloride concentrations of 0.05 mol dm-3 and zero. An error of f 0.01 cm3 g-l in A Gbs gives an error of f 0.014 cm3 g-l in A V. From fig. 1 the plot of A V against l/Bdni is not linear, indicating that t,us is too small. The low charge was corrected for in three ways, using the two procedures 1 ( a ) and 1 (b) and by oxidation with chromate and periodate as in method 2. Consider method 1 (a) : at a value of ni of 2.08 mol dm-3 and at pH 6.5 it is found from the charge/pH isotherm (fig.3 of Part 1) that So, is -2.80 C g-l. From table 1, AVis 0.347 cm3 g-l and using B = 5.19 x C mol-d m-1 then from eqn (1 1) q = 2.018. Hence from eqn (12), AVF, the excluded volume if the surface potential were infinite, is 0.517 cm3 g-l. Using this procedure for each point gave an average surface area of 128 24 m2 g-l (table 3). In method 1 (6) an approximate value of the surface area, S,, is calculated from A V using eqn (4). From this value and the So, value above, a value of S is calculated from eqn (1 5) and (16). A plot of ( S / S , ) - 1 against S , gave, from the intercept at the abscissa, a best value of S of 126 m2 g-l. From the minimum in the plot of the standard deviation of S against S,, the best mi mol-2 and A = 3.71 x1544 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE Table 1.Negative-adsorption results for bleached sulphate pulp A Kbs A V (l/Bdni) A Vr A Vg ni/mol dmP3 / cm3 g-' / cm3 g-1 / cm3 m-2 / cm3 g-l / 1 0-2 cm2 g-' 8.81 x 24.0+ 1.0 10.2f 1.4 20.8 18 22.3 6.56 x low2 29.6 15.8 24.1 35 31.9 4.47 x 36.5 22.7 28.8 48 41.4 2.08 x low2 48.5 34.7 42.8 52 52.8 8.75 x 78.0 64.2 66.0 86 84.6 6.94 x 83.4 69.6 74.1 89 89.8 2.18 x 63.4 49.6 41.3 6 . 5 7 ~ lop3 103.3 89.5 75.2 iodate oxidised pulp 2.23 x 83.0 69.2 40.8 9.11 x 139.8 126.0 63.9 chromate oxidised pulp (l/B,/ni)/104 cm3 m-2 Fig. 1. Negative-adsorption results for bleached sulphate pulp : A, experimentally observed excluded volume, AV; 0, AVcorrected by method (b) (AVg; S = 127 m2 g-l); m, chromate oxidised pulp (S = 120 m2 g-l); -.---, Acheor [eqn (16) using S = S, = 127 m2 g-11.value of S was 127 m2 g-l. A value of AVF was then calculated for this value of S and each chloride concentration from eqn (4). These are plotted as open squares in fig. 1. A value of A V that would be experimentally observed taking into account the low potential is given by putting S= S , = 127 m2 g-l in eqn (16); this is called and is plotted as a dotted line in fig. 1.T. M. HERRINGTON AND B. R. MIDMORE 1545 Table 2. Negative-adsorption results for cotton linters ~~ A &bs A V (1/Bz/ni) A VF A VT n,/mol dm-3 /10-2 cm3 g-1 /lop2 cm3 g-l cm3 rnP2 cm3 g-' cm3 g-l 2.02 x 10-2 14.2 _+ 1.0 9.9 _+ 1.4 43.4 33.9 23.1 4.33 x 10-3 28.1 23.8 98.4 33.2 37.5 3.22 x 39.2 34.9 108.8 53.9 55.2 9.80 x 76.9 72.6 197.2 91.9 92.1 8.52 x 83.7 79.4 21 1.5 99.4 99.6 8.80 x 62.0 57.7 65.0 2.29 x 43.8 39.5 40.3 oxidised by IO;/ClO- The values of AV for the chromate-oxidised pulp (table 1) are plotted in fig.1 as filled squares; they seem to fit eqn (4) and give a very similar surface area (see table 3 below) to the corrected values for the non-oxidised pulp, implying that the surface potential is high. In support of this, the surface charge was determined by titration in 0.01 mol dm-3 NaCl to be 13.0 C g-l at neutralisation, which provides a high surface potential of 150 mV. The periodate oxidation caused a degree of degradation to the fibre, which almost certainly accounts for the high surface area (table 3). The charge density was determined to be 17.0 C g-l at neutralisation.The surface area of the unoxidised bleached sulphate pulp was determined by nitrogen adsorption to be 190 m2 g-l (see table 5 later). COTTON LINTERS The negative-adsorption results for cotton linters are given in table 2. (The value of A& was determined to be 0.043 +O.OlO cm3 g-l.) From fig. 2 it can be seen that they again imply a low surface potential. The surface area was calculated using methods 1 (a) and 1 (b) to give values of 51 and 48 m2 g-l, respectively, (table 3). Oxidation with IOy/ClO- gave a much higher value of 94 5 m2 g-l, possibly because of degradation of the fibre. However, nitrogen adsorption gave a yet higher value of 130 m2 g-l (table 5). UNBLEACHED SULPHITE PULP The effect of drying the sample of never-dried unbleached sulphite pulp was investigated by the negative-adsorption technique.The wet pulp was dried in an air oven at 104 "C for 24 h and then redispersed in water. A& was determined to be 0.15 & 0.01 cm3 g-l, which is the same as unbleached sulphate pulp. The plots of A V against l / B z / n , are shown in fig. 3 for the never-dried and the once-dried pulps. The surface areas were corrected for 'low charge' by method 1 (a), but the corrections were small. The charge density at neutralization was 14.9 C g-l. The never-dried pulp gave a value of 210 m2 g-l, which reduced to 122 m2 g-l for the once-dried pulp (table 3). UNBLEACHED SULPHATE PULP From the negative-adsorption experiments, AK = O.lS+O.Ol cm3 g-l. From the plot of A V against 1 /Bz/n, (fig. 4) it can be seen that this pulp clearly shows the effect of overlapping double layers for chloride concentrations < 1 x lop2 mol dm-3.The pore volume, 5, was estimated by extrapolating the plot of A V against l/Bdni to l/Bz/n, = 0 0 . ~ The value of S was calculated from the negative adsorption at the 51 FAR 11546 1.0- 0.8 - 0.6- I M m E . 2 Q 0.4 - 0.2- THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE / I / /’ 80 160 240 0 (1 /B&.)/ 1 o+ cm3 Fig. 2. Negative-adsorption results for cotton linters : A, experimentally observed excluded volume, A V ; 0, A V corrected by method (b) (A V c = 48 m2 g-l). (l/B,/ni)/lO+ cm3 m-2 Fig. 3. Negative-adsorption results for unbleached sulphite pulp : 0, never-dried pulp; 0, once-dried pulp.T. M. HERRINGTON AND B. R. MIDMORE 1547 Table 3.Comparison of the surface areas of different fibres obtained by negative adsorption method 1 method 2 Pulp (4 (6) CrOt- IO,/ClO- bleached sulphate 128 & 24 126+ 10 120+ 1 184+ 14 unbleached sulphite cotton linters 51 f 14 48+5 - 94i- 5 never dried 210 f 10 once dried 122+5 never dried 210+9 once dried 110+4 unbleached sulphate; sample A 16C 12c - I 04 m E PI 0 80 6 - 2 40 1 0 80 160 (l/B,/ni)/104 cm3 m-2 Fig. 4. Negative-adsorption results for unbleached sulphate pulp : 0, Never-dried pulp; a, Once-dried pulp; ----, Acheor (table 4) for box-shaped pores. highest chloride concentration, where the overlap should be least, using eqn (4). From these two values the average separation between the two double layers, 2d, was estimated assuming the pores to be box-shaped by d = VJS. (19) Values for V, of 1.48 cm3 g-l and for S of 210 m2 g-l [obtained by correction method l(a)] give d = 70.1 A.From this value of d the theoretical negative adsorption could 51-21548 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE Table 4. Effect of double-layer overlap on the negative adsorption of unbleached sulphate pulp 2.06 x 71.2 42.5 21.2 8.80 x 10-3 102.1 65.0 32.4 6.94 x 10-3 112.0 73.2 36.5 2.82 x 10-3 132.8 114.8 57.3 9.50 x 10-4 141.3 197.8 98.7 0.207 0.1350 0.1204 0.0774 0.045 1 'd 2.06 x 0.840 98.6 100 70.5 8.80 x 10-3 0.473 84.8 86.5 100.1 6.94 x 10-3 0.385 79.1 82.8 107.0 2.82 x 10-3 0.158 54.7 59.7 121.7 9.50 x 10-4 0.0527 32.5 35.6 129.1 be calculated from eqn (6) in the following way. For ni = 6.94 x mol dm-3 (table 4) and pH 4.5, So, is given by fig.4 of Part 1 as 8.20 C g-l. A value for d u o is calculated from the Gouy-Chapman eqn (14). Rearranging eqn (7) gives rcd+22/uddu, = 22/UdF(Ud,Z/2). (20) This eqn was solved for ud. For this value of ud, the elliptic integrals in eqn (6) were found from tables and hence (An;/Ani)theor was calculated (table 4). These values are compared in table 4 with values of (Ani/Ani)obs obtained by assuming no double-layer overlap at the highest chloride concentration. Using this value of S, eqn (4), Ani was calculated at the other chloride concentrations; An: is the experimentally measured negative adsorption. As can be seen from table 4, the agreement between the two sets of values for An;/Ani is close, lending credance to the theoretical model.In further support of the pore model, A&heor was calculated from (An;/Ani)theor; this is plotted as a dotted line in fig. 4 and shows the same trend as the experimental AV. The effect of drying upon the surface area of this 'never-dried' pulp (sample A) was investigated as for the unbleached sulphite. The surface areas were corrected for low charge by method 1 (a). As can be seen from fig. 3 and 4, their behaviour is similar, dropping from ca. 210 m2 g-l to ca. 110 m2 g-l after drying (table 3). The effect of beating and drying on the second sample of unbleached sulphate pulp (sample B) was investigated both by the negative-adsorption technique and by B.E.T. nitrogen adsorption. Sample B had a slightly lower charge at neutralisation than sample A (6.1 C g-l compared with 8.2 C g-l).It was beaten in a standard PFI mill to three degrees of beating. The extent of beating was defined by a Schopper number (determined by the Schopper-Reigler method). These samples were then dried as before. The surface area obtained by negative adsorption was determined at two chloride concentrations (1 x and 2.5 mol dm-3) and corrected for low charge by method 1 (a); the average value was taken. The surface areas by the two methods are compared in table 5. The two values of the nitrogen surface area for the never-beaten, never-dried pulp B are for the same sample but two different runs of solvent exchange.T. M. HERRINGTON AND B. R. MIDMORE 1549 Table 5. Comparison of the surface areas of different pulps obtained by negative and nitrogen adsorption by by - scr, negative nitrogen /c g-' adsorption adsorption ~ bleached sulphate 2.80 cotton linters 0.46 unbleached sulphate; sample B (1) zero beating, 15 O SR 6.12 (2) beaten to 38.5 O SR (3) beaten to 78 O SR- (1) zero beating, 15 O SR (2) beaten to 38.5 O SR (3) beaten to 78 O SR - - unbleached sulphate; sample B; once-dried 5.02 126+ 10 190 48f5 130 207 & 9 265a, 240a 216f 10 - 226& 10 - 112+6 120 122f7 - 137+7 - a Same sample but for two different runs.The pore size distribution from B.E.T. nitrogen desorption gave a broad band of porosity 22-60 A in radius, which is comparable with our d value of 70 A for the wet pulp. This increase is probably explained by swelling of the pulp in electrolyte solution. DISCUSSION The linear form of the negative-adsorption plots seems to be good quantitative evidence for the applicability of electrical double-layer theory to the cellulose/water interface.It suggests that a double layer does exist which is reasonably well described by the Gouy-Chapman equation. The linearity of the plots increases with increasing charge density, which is as expected since lya is closer to being effectively infinite over a greater range of chloride concentration. The oxidised bleached sulphate (fig. 1) and the unbleached sulphite (fig. 3) pulps show this linearity well. In the case of the bleached sulphate pulps and cotton linters, because of their relatively low charge densities, lya cannot be described as effectively infinite. The negative adsorption therefore declines considerably as the increasing electrolyte concentration reduces lya as given by eqn (8).This reduction seems to agree well with the theoretical reduction predicted from eqn (9), as shown by the dotted line of A Kheor in fig. 1 ; the slope decreases as the concentration of chloride ion increases. This again is not only good evidence for the applicability of the negative adsorption aspects of double-layer theory but also more generally for the Gouy-Chapman equation, as this is involved in the estimation of lya. Both of the correction procedures applied to account for a low surface potential seem to work quite well. The estimated surface area for the oxidised pulp of 126 m2 g-l agrees within experimental error with that obtained for the oxidised pulp (120 m2 g-l) where the surface potential was raised effectively to infinity by chemical means.The effect of double-layer overlap is shown clearly by the negative-adsorption plot for the unbleached sulphate pulp (sample A) in fig. 4. The theoretical negative adsorption AKheor of table 4 is plotted as a dotted line in the figure and agrees1550 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE reasonably well with the experimental values. Exact agreement was unlikely, as the calculations are based on a single double-layer separation value of d which is a simple numerical average obtained by assuming box-shaped pores. This situation is most unlikely within the complex porous structure of the cellulose fibre. However, the results do show that both the calculated surface area S and the pore volume are at least of the right order of magnitude, and hence give a reasonable estimate for d.Indeed for an unbleached sulphate pulp has been independently determined by Stone and Scallan.8 They determined the negative adsorption of dextran molecules with sufficiently high molecular weight that they were excluded from the interior of the fibre. A value of 1.4 cm3 g-l was obtained, which compares very well with the value of 1.48 cm3 g-l obtained in this work. They found a higher pore volume for sulphite pulp (ca. 1.7 cm3 g-l); from fig. 3 it can be seen that the pore volume for unbleached sulphite pulp must be > 1.8 cm3 g-l. It is clear that in every case drying the fibre at 104 "C for 24 h and redispersion in water considerably reduced the surface area.In general there is a reduction by about a half (tables 3 and 5). This reduction must be caused by permanent closure of some of the pores in the fibres. They are permanently closed because they are not reopened on redispersal in water. There is evidence that these pores are small pores. The surface area of the unbleached sulphate pulp (B) obtained by nitrogen adsorption is 30% larger than that obtained by negative adsorption (265 m2 g-l compared with 207 m2 g-l; table 5). This is presumably because the negative-adsorption technique does not de- tect the smallest pores. However, after one drying the difference reduces to only 7% (120 m2 g-l compared with 112 m2 g-l; table 5). This may be explicable by the closing up of the smallest pores which are now no longer accessible to either technique.From the results in table 5 on the unbleached sulphate pulp (B), beating increases the surface area of the fibres, but not dramatically so. This is considered to be achieved by the increased external fibrillation of the fibre^.^ The fact that the reduction of surface area on drying for all three degrees of beating is the same indicates that this loss in surface area is due to reduction of internal and not external fibrillation. The negative-adsorption surface-area values compare reasonably well with those obtained by nitrogen adsorption. They are all, however, lower than the latter, which is as expected for the internal surface of a porous material. This effect was found by Tadros and LyklemalO for the surface area of silica gel, where the surface area obtained by negative adsorption was 35 m2 g-l and that by nitrogen adsorption was 50 m2 g-l.This represents a reduction of 30%. So 30% of the surface (that due to the smallest pores) was not available for negative adsorption. This compares well with the results obtained for cellulose, where for the unbleached sulphate pulp 22% of the surface is undetected by negative adsorption and 34% in the case of bleached sulphate pulp. The 'wet' value for the cotton linters (48 m2 g-l) is, however, considerably lower than the nitrogen-adsorption value (130 m2 g-l). This is accounted for by the very low charge density of the cotton linters. At these low charge densities (4 x C m-2, calculated from the charge/pH isotherm with S = 130 m2 g-l) the double-layer theory, which assumes an indiscreet and smeared surface charge density, can no longer apply. The charges have a separation of 63 A at this char e density, compared with pulp (B) (2.3 x C m-2).The degree of effective shielding the discreet charges will experience will be greater than that of a ' smeared ' charge, and so the effective surface potential will be considerably lower. Thus our method of correction using Gouy- Chapman theory cannot apply, as it relies on an estimation of the surface potential that is too high; from this method a surface area which is too low is obtained. The only other negative-adsorption study of cellulose fibres is that of Schofield2 on jute. 32 A for bleached sulphate pulp (1.6 x C m-2) and 26 x for unbleached sulphateT.M. HERRINGTON AND B. R. MIDMORE 1551 He obtained a surface area for jute from different sources of 150-200 m2 g-l, which is the same range as the surface areas for cellulose fibres in this work. The nitrogen- adsorption surface areas obtained by Stone and Scallans also compare well with this work; four of the pulps they studied had surface areas in the range 182-230 m2 g-l. They also found a loss of surface area on bleaching; the surface area of unbleached sulphite pulp dropped from 182 to 93 m2 g-l after bleaching. This is paralleled by the lower surface area of bleached sulphate (1 26 m2 g-l negative adsorption, 190 m2 g-l nitrogen adsorption) compared with unbleached sulphate (2 10 m2 g-l negative adsorp- tion) in this work. Probably the digestion of finely divided areas in the fibre occurs in the bleaching process.It is considered that the negative-adsorption technique offers the best approximation for the surface area in the context of the adsorption of ions from solution on to the cellulose surface as in determining chargelpH isotherms. A B d E e F ni Ani P 4 S Sn st K A Vobs AKi Ud UO AV A VF A V g Y zi u) & K *b 0 0 Wb W O ryd GLOSSARY OF SYMBOLS = ( 8 ~ k T ) ; = (z2e2/2ckT); half the distance apart of two overlapping double layers elliptic integral of the first kind charge of electron elliptic integral of the second kind concentration of co-ion i in bulk solution increase in concentration of co-ion i in bulk solution = - Soo/(ABni A V ) surface area per gram estimated surface area total surface area = exp ( -zi etyd/kT) = exp ( - zi ey/,/kT) total volume of solution excluded volume, A V = Ani VJ(ni co) experimentally observed excluded volume excluded volume accessible to water but not ions excluded volume corrected for low potential by method 1 (a) excluded volume corrected for low potential by method 1 (b) mass of solid = *o/(Adni) valency of ion i, sign included electrical permittivity of the medium Debye reciprocal length, K = ( 2 4 zf e2/&kr): charge per unit area at Stern plane charge per unit area at surface potential at Stern plane potential at surface potential at midpoint of two overlapping double layers = S~oI(ASndni)1552 THE CELLULOSE/AQUEOUS ELECTROLYTE INTERFACE M. B. Donnan, T. W. Healy and P. F. Nelson, Colloids Surf., 1981, 2, 133. R. K. Schofield and 0. Talibuddin, Discuss. Faraday SOC., 1948,3, 51. R. K. Schofield, Nature (London), 1947, 160,408. H. J. van den Hul and J. Lyklema, J. Colloid Interface. Sci., 1967, 23, 500. F. A. M. de Haan, J. Phys. Chem., 1964, 68, 2970. B. R. Midmore, Thesis (Reading University, 1983) communication. J. E. Stone and A. M. Scallan, Pulp Paper Mag. Can., 1965, 66 (8), 407. ' A. M. Scallan (Pulp and Paper Research Institute of Canada, Point Claire, P.Q., Canada), personal * J. E. Stone and A. M. Scallan, Tappi, 1967,50 (lo), 496. lo Th. F. Tadros and J. Lyklema, J. Electroanal. Chem., 1968, 17, 267. (PAPER 3/1621)