Aqueous Foam Films Stabilized by a Non-Ionic Surface-Active Agent BY J. S. CLUNIE J. M. CORKILL J. F. GOODMAN AND B. T. INGRAM Procter & Gamble Limited Newcastle Technical Centre Basic Research Department Newcastle-upon-Tyne England Received 10th April 1970 The thicknesses and tensions of black films formed by aqueous solutions of a pure non-ionic surface-active agent n-decyl methyl sulphoxide (DMS) have been measured at 298 and 308 K as a function of sodium chloride concentration. In the DMS+NaCl+H20 system second black films are formed at low ionic strengths whereas first black films are formed at higher ionic strengths ( >lo0 mol m-3). Estimated values for the composite Hamaker constant for first and second black films have been obtained and compared with theoretical values. In most studies on aqueous foam films the surface-active agents used have been ionic in character.l By comparison studies using non-ionic surface-active agents are fewer in number and mostly confined to films formed by aqueous solutions of commercial alkyl phenol polyoxyethylene ethers (OP-7 to OP-20).2-4 Nevertheless some observations have been made on films formed by aqueous solutions of pure n-dodecyl hexaoxyethylene glycol rnon~ether,~ and a few results have also been reported for films stabilized by another pure non-ionic surface-active agent (a partially- fluorinated n-alkyl dimethylamine oxide 6).In the present investigation a study has been made on films formed by aqueous solutions of n-decyl methyl sulphoxide (n-CloH2 ,.SO.CH,.DMS). This lion-ionic surface-active agent has a compact hydrophilic group and unlike the n-alkyl dimethylamine oxides,' is protonated only under extremely acid conditions (PK < O).s We have determined the thicknesses and excess tensions of the foam films formed by solutions of this material in water and in the presence of added sodium chloride (to 5 kmol m-,).From these measurements estimated values for the composite Hamaker constant for first and second black films have been obtained and compared with calculated values. EXPERIMENTAL MATERIALS n-Decyl methyl sulphoxide (DMS) was prepared and purified as described previo~sly.~ The water used for preparing solutions was twice distilled in a silica vessel after initial distillation from aqueous alkaline potassium permanganate. At 298 K this water had a specific conductivity of less than A 1 krnol111-~ solution raised the surface tension of water by 1.6 mN rn-l at 298 K,1° indicating the absence of any surface-active impurities.30 R-l m-l and a surface tension of 72.0 mN m-l. A.R. sodium chloride was roasted in a platinum crucible at -900 K. J . s. CLUNIE J . M. COKKILL J . F . GOODMAN AND B . T . INGRAM 31 SURFACE TENSION MEASUREMENTS Solutions were contained in a double-walled thermostatted cell with a thermistor for temperature monitoring. Surface tensions were determined using a du Nouy tensiometer and the usual corrections were applied. l1 FILM THICKNESS MEASUREMENTS The apparatus consisted of a glass cell (3.5 x low4 m3 capacity) made vacuum tight by demountable seals liquid seals and greaseless vacuum taps (fig. 1). An auxiliary reservoir FIG.1 .-Cell used for optical reflectance measurements. (2.5 x m3 capacity) was connected to the cell by a siphon through which the surface- active solution was introduced into the evacuated cell. Both cell and reservoir were totally immersed in a constant temperature water bath controlled to fO.O1 K by a mercury-toluene regulator with two 100 W heaters and a cooling coil. The water bath was housed in a light- proof enclosure but was fitted with a window for viewing the film when necessary. Films were formed by totally withdrawing a rectangular glass frame (50 x 20 x 5.0 mm) from the surface-active SolLItioil. The frame was supported vertically from a Perspex vessel top through a liquid seal filled with the solution under investigation. Film thickness was determined by an optical reflection method similar to that described previously,12 with the addition of a beam splitter to allow continuous monitoring of the intensity of the incident monochromatic light beam.Throughout the course of any film’s lifetime the intensity of the incident beam was constant to within f 2 x. The measured background light intensity was -20 % of the reflectance from the thinnest films studied. The uncertainties in reflection coefficient measurements lead on the basis of a single homogeneous layer model for the film,12 to a corresponding imprecision in film thickness of less than 2 %. Film thicknesses were calculated from the reflectance measurements using the sym- iiietrical three-layer model previously adopted.G The parameters of this model are the re- fractive indices of the surface monolayers (1.41) and the aqueous core (1.34) and also the 32 AQUEOUS FOAM FILMS thickness of the surface monolayers (1.4 nm).The uncertainty in the calculated thickness for possible variations in these parameters is unlikely to exceed 0.4 nm. FILM TENSION MEASUREMENTS The apparatus and technique were essentially those described previously 3;b~t to increase the sensitivity in the present series of measurements an 80 mm wide glass frame made of 1 mm thick glass was used to support the film. The electronic microbalance was calibrated with standard weights at 298 and 308 K. The estimated error in the measurement of excess tension was f2.5 pN m-l. RESULTS The (surface tension log concentration) curve for aqueous solutions of DMS showed no minimum at the critical micelle concentration (c.m.c.) indicating the ab- sence of any impurity of greater surface activity.The c.1n.c. for aqueous solutions was 2.0 mol m-3 at 298 K. The effect of added sodium chloride was to lower the c.m.c. (e.g. for DMS in 1 kmol m-3 sodium chloride solution the c.m.c. was 1.0 mol m-3 at 298 K). With DMS solutions stable films could only be formed at surface-active agent concentrations above the c.m.c. Variations in surface-active agent concentrations up to 1.5 x c.m.c. had no effect on measured film thicknesses. 10 100 1000 NaCl Conc. (mol m-') FIG. 2.-Equilibrium film thickness in the DMS+NaCl+H20 system as a function of NaCl concen- tration at 298 K. Results at 308 K are identical. I t - ~ 1 * [ I I ' I * I t 1 I ' ' 1 * 1 1 1 1 1 ' 3 FILM THICKNESS Measurements were made at 298 K and 308 K.All films showed mobile drainage behaviour and rapidly reached constant thicknesses which were maintained indefin- itely and which were reproducible to +0.2 nm. Fig. 2 shows the equilibrium thick- nesses of films formed from 2.2 mol M - ~ solutions of DMS containing varying amounts of sodium chloride. For any given film the measured reflection coefficients were the J . S . CLUNIE J . M. CORKILL J . F . GOODMAN AND B . T . INGRAM 33 same at both temperatures. Since any variation in the parameters of the optical model in this small temperature interval will be very small the calculated film thick- nesses are independent of temperature. Equilibrium film thicknesses were low (4.9 nm) and independent of electrolyte concentration up to a sodium chloride concentration of 70 mol m-3 where a sharp increase in film thickness to 7.3 nm was observed.With further increases in sodium chloride concentration the equilibrium film thickness decreased continuously returning to a value of 4.9 nm at the highest concentration studied (5 kmol m-3). The abrupt transition in film thickness from 4.9 to 7.3 nm occurred over a fairly narrow range of sodium chloride concentrations (70-100 mol m-3). In this concentra- tion range two co-existing black films could be formed in the vertical film holder viz. an upper film of thickness 4.9 nm in equilibrium with a lower film of thickness 7.3 nm. FILM TENSION Measurements were made at 5 K intervals in the temperature range 298-308 K. Fig. 3 shows the excess film tension Ao (Ao = of - 2y where of is the total film tension NaCl Conc.(mol m-3) FIG. 3.-Excess film tension Aa in the DMS+NaCl+H,O system as a function of NaCl concentra- tion at 298 K (0) and 308 K (0). and y is the surface tension of the bulk solution) as a function of sodium chloride concentration at 298 and 308 K. The sharp minima in the curves at 100 mol m-3 corresponded to the abrupt transition in film thickness. Aa appeared to change linearly with temperature the extreme values of the temperature coefficient being -0.4 and + 1.1 pN m-1 K-l at sodium chloride concentrations of 50 mol M - ~ and 1 kmol m-3 respectively. DISCUSSION Mysels et aZ.149 l6 have defined " first " and " second " black films in terms of the behaviour of the equilibrium thickness with respect to the ionic strength of the bulk solution. First black films show a monotonic decrease of thickness with increasing SPI-B 34 AQUEOUS FOAM FILMS ionic strength whereas with second black films the thickness is independent of the ionic strength.It is generally considered l5 that the equilibrium thickness of the first black film is governed by the balance between the van der Waals attractive forces and the repulsion due to the overlap of the electrical double layers associated with a surface charge in the head-group planes. For the second black film the repulsion force is less well defined and is short range in nature. For films stabilized with non-ionic surface-active agents one might expect that since the head-groups are uncharged only second black films would be observed. In the DMS system second black films are indeed found at sodium chloride concentra- tions less than 70 rnol m-3 but after a restricted concentration region in which co- existing first and second black films are observed only first black films are formed.Due to salting out of the DMS the highest electrolyte concentration at which films could be formed was 5 kmol m-3 and at this composition the film thickness had returned to a value close to the electrolyte-free second black film thickness of 4.9 nm. In films formed from ionic surface-active agents,l6. l7 increasing electrolyte results in a transition from the first to the second black type ; in the present system the converse is the case. The transition from second to first black type in this system presumably results from ion adsorption in the head-group planes giving rise to a double-layer repulsion force as in first black films formed by ionic surface-active agents.This suggestion is supported by the effect on DMS films of other inorganic electrolytes having different adsorption potentials. For example with sodium hydroxide and with potassium thiocyanate the transition from second to first black film occurs at much lower electro- lyte concentrations and the first black films are correspondingly thicker (e.g. with sodium hydroxide the first black film has a thickness of 47 nm at a concentration of 0.1 mol m-3). Because of its thinness a black film has a surface tension lower than that of the adjoining bulk solution,18-20 and the tension difference ACT as defined earlier can be equated to the depth of the minimum in the potential energy-thickness curve 21 in which the film exists.For first black films the potential energy U is regarded as the sum of two terms a double layer repulsion U and a dispersion force attraction U, when the hydrostatic potential energy is negligible. For a film of total thickness h and surface layers of thickness d UE can be expressed in the form UE = (B/Ic) exp [ - K(h - 241 (1) where ic is the Debye-Huckel reciprocal length and B is a term whch includes the bulk solution concentration the Stern potential and temperature. UA is given (for non- retarded forces) by UA = -A"/12nh2 (2) where A* is the composite Hamaker constant for the film. is zero it follows from (1) and (2) that if A* is independent of h The potential energy (UE + UA) can consequently be expressed in the form Since at equilibrium the derivative of the potential energy with respect to thickness 1cU,+(2U,/h) = 0 (3) U = uA(1-2/~h) (4) or AO = (- A*/12xh2)( 1 -2/1ch).( 5 ) Since both h and AO can be directly measured eqn ( 5 ) affords a method for determining A*. J. S . CLUNIE J . M. CORKILL J . F. GOODMAN AND B. T . INGRAM 35 In fig. 4 the values of A* obtained from ACT and h for the first black films studied are shown as a function of h at 298 and 308 K. The large variation of A* with temperature 4 3 n b W 0 2 x2 t I 5 6 7 film thickness (nm) FIG. 4.-Hamaker constant A* for first (0) and salt-free second (0) black films as a function of film thickness at 298 K (closed symbols) and 308 K (open symbols). Dashed line represents the caicu- lated Hamaker constant for the non-retarded van der Waals forces. is most unexpected since the densities and polarizabilities of the film components should show only a small change for a lOK temperature difference.Taking the value of A* = 2.5 x J into eqn (3) leads to a value for U and hence the Stern potential can be calculated. At the transition concentration a potential of 20 mV is obtained which is similar to that obtained for first black films stabilized by the non-ionic surface-active agent OP-20. Approximate values of A* may be calculated by the method of Duyvis 2 2 from the Hamaker constants and thicknesses of the film surface layers and aqueous core. The Hamaker constants A for water and decane can be calculated from a form of the London equation 27 n2-1 64 ( n 2 + 2 ) A = -hv - where Iz is Planck's constant n the refractive index and v the characteristic frequency. Using the values of v given by Gregory,24 we obtain A values for water and decane of 3.9 x and 5.8 x J respectively.Inserting these into the Duyvis equation leads to A* values that are in moderate agreement with those calculated from the experimental data using eqn (5). An alternative method of estimating A' is to assume that for the second black films in the absence of electrolyte the short-range repulsion is represented by a cut-off potential and hence that U = UA at the equilibrium thickness. The value of A* for the electrolyte free films is comparable to that calculated from eqn (5) for the first black films (fig. 4). However for these films although h is independent of electrolyte concentration Aa shows large changes with electrolyte concentration passing through a maximum at - 1.0 mol M - ~ .It seems improbable that composi- tional changes could lead to variations in A'% large enough to account for these effects, 36 AQUEOUS FOAM FILMS and hence some other forces besides the van der Waals attraction and a steric (cut-off) repulsion must operate in these films. The nature of these additional short-range forces remains obscure but assuming U to be unaffected by the presence of ions then the results indicate that the only restriction on the net contribution Us of these forces to the film potential energy is that I Us I < I U I and dU,/dh>dU,/dh at all thicknesses. It is reasonable to suppose that the variations of Ao with electrolyte concentration reflect changes in solvent orientation ion adsorption and electrostatic screening. A. Scheludko Adu.Colloid Interface Sci. 1967 1 391. B. V. Deryaguin A. S. Titievskaya and V. K. Vybornova Colloid J. U.S.S.R. 1960,22,407. E. M. Duyvis and J. Th. G. Overbeek Proc. Kon. Ned. Akad. Wetens. B 1962 65,26. A. Scheludko Proc. Kon. Ned. Akad. Wetens. B 1962 65,97. J. M. Corkill J. F. Goodman D. R. Haisman and S. P. Harrold Trans. Faraday Soc. 1961 57 821. J. M. Corkill J. F. Goodman and C. P. Ogden Trans. Faraduy Soc. 1965,61 583. F. Tokiwa and K. Ohki J. Phys. Chem. 1966,70,3437. D. Landini G. Modena G. Scorrano and F. Taddei J. Amer. Chem. Soc. 1969 91,6703. J . M. Corkill J. F. Goodman and J. R. Tate Trans. Faraday Soc. 1969 65 1743. lo G. W. C. Kaye and T. H. Laby Tables of Physical and Chemical Constants 13th ed. (Longmans London 1966). W. D. Harkins and H. F. Jordan J. Amer. Chem.SOC. 1930 52 1751. l 2 J. M. Corkill J. F. Goodman C. I?. Ogden and J. R. Tate Proc. Roy. SOC. A 1963 273 84. l3 J. H. Clint J. S. Clunie J. F. Goodman and J. R. Tate Nature 1969 223 291. l4 K. J. Mysels K. Shinoda and S. Frankel Soup Films Studies of their Thinning and a Bibliography (Pergamon Press London 1959). l 5 J. Lyklema and K. J. Mysels J. Amer. Chem. SOC. 1965 87 2539. l6 M. N. Jones K. J. Mysels and P. C. Scholten Trans. Faraduy SOC. 1966 62 1336. l7 G. Ibbotson and M. N. Jones Trans. Furaday Soc. 1969 65 1146. l9 K. J. Mysels H. F. Huisman and R. Razouk J. Phys. Chem. 1966,70 1339. 2o A. Scheludko B. Radoev and T. Kolarov Trans. Furadzy Soc. 1968 64 2213. 21 F. Huisman and K. J. Mysels J. Phys. Chem. 1969,73,489. 22 E. M. Duyvis The Epdibrium Thickness of Free Liquid Films (Thesis Utrecht 1962). 23 D. Tabor and R. H. S. Winterton Proc. Roy. SOC. A 1969 312,435. 24 J. Gregory Adv. Colloid Interface Sci. 1970 2 396. R. V. Deryaguin G. A. Martinov and Yu. V. Gutop Colloid J. U.S.S.R. 1965 27,298.