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The similarity between black foam films and neat mesomorphic phase
The Similarity between Black Foam Films and Neat Mesomorphic Phase R. R. BALMBRA, J. S. CLUNIE, J. F. GOODMAN, AND B. T. I N G R A M Procter &°Gamble Limited, Newcastle Technical Centre, Basic Research Department, Newcastle Upon Tyne, NE12, 9TS
Received March 6, 1972; accepted April 25, 1972 The equilibrium thicknesses of black foam films and the interlayer spacings of the structurally similar lamellar mesomophic phase (neat phase) formed by the decyltrimethylammonium decylsulphate + NaBr + H20 system have been measured at 298 K. A marked variation of film thickness and also neat-phase spacing occurs with changing system composition. This is attributed in both cases to the effect of inorganic ions on electrostatic interactions between surfactant layers, although the exact mechanism differs for each case. INTRODUCTION Analogies have been drawn (1,2) between the properties of foam films and those of the lamellar mesomorphic phase (neat phase) which commonly occurs in aqueous systems containing very high concentrations of surfaceactive agent. We have found that decyltrimethylammonium decylsulphate (C10+, C10-) not only gives exceptionally stable aqueous foam films but also forms neat phase as its only mesomorphic phase in concentrated aqueous solution, with or without added inorganic electrolyte. We have therefore compared the equilibrium thicknesses of films drawn from aqueous solutions of sodium bromide containing low concentrations of C10+, C~0- with the neat-phase interlayer spacing in the ternary system. EXPERIMENTAL Materials
Water was redistilled from a Pyrex vessel containing the distillate from an aqueous alkaline potassium permanganate solution. I t had a specific conductivity less than 10-4f~-lm-1 and a surface tension of 72 raN m -1 at 298 K. The sodium bromide was purified by recrystallization from aqueous solution and
roasting in a platinum crucible. The absence of surface active impurities was confirmed by measuring the surface tension (73.4 m N m -~) of a 1 kmole m -3 aqueous solution of the salt at 298 K. D e c y l t r i m e t h y l a m m o n i u m decylsulphate was prepared as described previously (3). The critical micelle concentration of its aqueous solution was 0.45 mole m -~ at 298 K. The p H of each solution was adjusted to 8.5 with sodium hydroxide to inhibit hydrolysis of the decylsulphate component to decanol, although it was subsequently shown that the deliberate addition of decanol to the film-forming solutions (up to one fifth of the C10+, C10- concentration) had no effect on the results. Methods (a) Bulk phase measurements. To obtain the ternary phase diagram, samples of known composition were first prepared and mixed in sealed glass ampoules as described previously (4) and then maintained at 298 K for several days to equilibrate. Samples were examined using low-angle X-ray diffraction (4) and polarization microscopy (5), although two phase systems were often more readily recognized by their bulk appearance than by their
Journal oJ Colloid and Inlet/ace Science, Vol. 42, No. I, January 1973
226
Copyright O 1973 by Academic Press, Inc; All rights of reproductior~ in any form reserved.
227
BLACK FOAM FILMS
q-isotropic solution, and neat phase+crystals. It was not possible to determine the neatphase boundary very precisely at low C~0+, C10- concentrations and in this region the boundary is represented by a broken line on the phase diagram. The structure of neat phase consists of bilayers of surfactant interleaved with layers of aqueous solution of varying thickness (8,9). The interlamellar periodicity (d001) for neat-phase compositions was determined by low-angle X-ray diffraction, and Fig. 2 shows the interlamellar spacing at constant C~0+, C10- compositions and varying sodium bromide solution concentration, the latter being the amount of sodium bromide per unit volume of water in the whole ternary system taking the molar volume of water to RESULTS be the same as in the binary (NaBr -t- H20) Bulk Phase solution. The X-ray camera resolution limit was 11 nm and spacings greater than this Figure 1 shows the ternary-phase diagram for the Cx0+, Cx0-+ NaBr + H20 system at could not be measured because the theoretically 298 K. For reasons of scale the isotropic phase accessible second-order reflections were too at low C~0+, C~0- concentrations (<0.02%) weak relative to background to be distinhas not been included in the diagram. No guished on the X-ray photographs. attempt was made to identify any of the cryIn the absence of sodium bromide, the neatstalline phases present. Only one mesomorphic phase spacings increased linearly from 3.25 phase, neat (lamellar) phase, was observed, to 4.00 nm as the proportion of C10+, C10- was either as an homogeneous phase or in hetero- decreased from 80 to 60% by weight; further geneous two phase regions, viz., neat phase addition of water then produced a hetero-
microscopic textures. In those cases where one of the coexisting phases was isotropic solution, the two phases were separated by centrifuging at 1500 rpm. (Centrifuging at speeds greater than 15,000 rpm caused complete separation of surfactant and electrolyte solution.) The separated phases were examined by chemical analysis, optical microscopy, and X-ray diffraction (4). (b) Film measurements. Film thicknesses were determined by the optical reflection method (2,6). The thickness was calculated assuming an isotropic film structure and a correction of -0.7 nm was then applied to allow for the heterogeneous structure of the real film (7).
'lO0 ~ NaBr
1007, H20
1007. ¢~**C~o-
FIG. 1. Partial phase diagram for decyltrimethylammonium decylsulphate -}- sodium bromide + water at 298 K. (A) crystalline phases present, (B) neat phase only, (C) neat phase + isotropic solution. Journal of Colloid and Interlace Science,
Vol. 42, No. 1, January 1973
228
BALMBRA ET AL.
"~"i
10
~'!
"" !
i
1
10
'!
10 2 NaBr
Concentration
'
i
i
10 3
10 4
(tool m " 3 )
Fro. 2. Interlayer spacing in C~0+, C10- ~- NaBr -}- H20 neat phase at 298 K. %C10+, C10-: 20%, A; 30%, [] ; 40%, ©; 50%, A; 60%, i ; 70%, e. Broken lines show limiting volume hypothesis values for each of the compositions. geneous mixture of isotropic solution coexisting with neat phase. The addition of sodium bromide up to a concentration of about 100 mole m -~ had no significant effect on the heterogeneity of the system or on the X-ray spacing (Fig. 2), but above this concentration a homogeneous neat phase was formed and a marked increase in interlayer distance occurred for the lower surfactant concentrations. Further increase in the salt concentration produced a decrease in the neat-phase spacing but the phase remained homogeneous. The interlayer spacing was found to be inversely proportional to the concentration of suffactant. From X-ray spacing and phase density measurements the interracial area per surfactant ion in the bilayer was calculated (9) to be 0.30 4- 0.02 nm 2 over the entire homogeneous neat-phase region studied. This value agrees well with that found for the limiting area per long-chain ion in the close-packed monolayer at the air/solution interface (3). Films
Figure 3 shows the equilibrium film thicknesses at 298 K as a function of sodium bromide Journal of Colloid and Interface Science,
concentration. The equilibrium thicknesses were independent of C10+, C10- concentration over the range 0.8-2.0 times the critical micelle concentration. (Stable films could not be obtained at lower surfactant concentrations.) At low concentrations of sodium bromide ( < 100 mole m -~) the films drained slowly and irregularly for several hours, although at concentrations below 5 mole m -3, the decrease in thickness from about 20 nm to the equilibrium value of 4.4 nm occurred very rapidly. At sodium bromide concentrations >100 mole m -~ there was regular (mobile) drainage and rapid attainment of equilibrium thickness. The heating of small portions of the films at salt concentrations of 5 mole m -8 and above, using the method of Jones et aL (10), failed to produce thinner stable regions. DISCUSSION
There are obvious qualitative similarities in the variations of black foam film thickness and neat-phase interlamellar spacing with changing electrolyte concentrations (Figs. 2 and 3). At low salt concentrations both the film thickness an d neat-phase spacing remain virtually con-
Vol.42,No.I,January1973
229
BLACK FOAM FILMS
range effects possibly such as that resulting from the charge mosaic nature of the surfaces (11). But at higher salt concentrations, ionic adsorption could produce a long-range repulsion which, above a certain electrolyte concentration, becomes sufficient to balance the long-range dispersion force and arrest the film's drainage at a greater thickness. A similar explanation has been proposed to account for the abrupt transition in thickness observed in films stabilized by the nonionic surfactant, decylmethyl sulphoxide (6). Moreover, there is evidence from studies on insoluble monolayers that specific ion adsorption effects do occur with alkyl sulphate and alkyl trimethylammonium salts (12), and conductivity and radiotracer studies of C10+, C10films have also given indications of inorganic ion adsorption (13). The DLVO theory of colloid stability 04) may be used to calculate the equilibrium thickness of the first black foam film (2), but values for the effective Hamaker constant and surface potential of the film are required. By using a value of 3 X 10-2°J for the effective Hamaker constant which was obtained from thickness and tension measure-
stant with ionic strength at a value slightly greater than twice the length of the extended surfactant molecule. In a foam film such behavior is typical of a "second black" or "Perrin" film (2,10). At higher salt concentrations there is a large increase in both film thickness and neat-phase spacing followed by a decrease as the ionic strength is increased even further. The behavior of the foam film in the highest concentration region is similar to that of a "first black" film (2,10). The discontinuous transition from a second black film at low ionic strengths to a first black one at higher concentrations could be the result of preferential adsorption of one type of inorganic ion giving rise to a long-range repulsive force within the film. At low concentrations of inorganic salt, the net surface charge in the 1:1 anionic-cationic surfactant film system is likely to be very small (3), and thus the repulsive forces between the film surfaces are probably fairly short range ones such as those between the oriented solvent dipoles associated with each surfactant monolayer. These balance the attractive forces arising from the longrange London dispersion effect and short-
\
22
10mV °
20
20mV
18 16
14
: I0-
o
.u_
8-
.c:
I"
6"
4'
O
o---
~ ~
O
1
1()
NaBr Concentration
10 z
1; 3
1()'
(tool m - a )
Fie,. 3, Equilibrium thickness of Ci0+, Ct0- + NaBr + H20 black-foam films at 298 K. Broken line shows equilibrium thicknesses calculated for a surface potential of 80 mV. Journal o/Colloid and Inter/ace Science, Vol. 42, No. 1, January 1973
230
BALMBRA E T AL.
ments made on films stabilized by decylmethyl sulphoxide (6) and decyltrimethylammonium decylsulphate (15) we have calculated the surface potentials corresponding to the observed equilibrium thicknesses. These are shown in Fig. 3 with a complete thickness curve (broken line) for an 80 mV potential. The Gouy-Chapman simplifying assumptions used in the calculations are unlikely to be completely valid at ionic strengths greater than about 100 mole m -~. However these calculated potentials are consistent with increasing ion adsorption as the salt concentration increases. The neat-phase expansion probably results from the suppression of short-range electrostatic attraction (ll) between the surfactant bilayers caused by the increasing concentration of inorganic ions in the aqueous layers which disrupts the crystal-like charge symmetry. Since the van der Waals attraction between the bilayers is very small---at least an order of magnitude smaller than the van der Waals attraction in the foam film--there is then no force in the neat phase suff_cient to prevent the bilayers separating to incorporate the liquid from the isotropic solution and forming a homogeneous phase. By analogy with the foam films, preferential ion adsorption should also occur in the neat phase at low electrolyte concentrations but it obviously does not do so to an extent sufficient to suppress the short-range attraction. The apparent paradox of the foam-film expansion can be resolved by noting that, unlike the neat phase, the film approaches its equilibrium state by thinning from very large thicknesses and must therefore first come under the influence of the longrange forces. Hence, although a very strong attraction may exist at small thicknesses it is the long-range repulsion of the overlapping electrostatic double-layer kind which determines the equilibrium thickness of the film. The transition from a nonexpanding to an expanding neat phase also occurs in systems of ionic surfactants (16,17) under conditions where a change in charge distribution seems likely. The neat-phase spacing in the homogeneous phase must, if the expansion is one
dimensional, depend solely on the amount of electrolyte solution available to fill the interlamellar region. Figure 2 shows the expected neat-phase spacings for different compositions based on this limiting volume hypothesis. It has been assumed that the bilayer thickness and area per surfactant ion remain constant at 3.0 nm and 0.30 nm 2, respectively, and that the molar volumes of interlamellar constituents, taken to be only water and inorganic ions, are the same as in normal bulk solution. It will be seen that the marked dependence of bilayer spacing o n e l 0 +, C10- concentration is fairly well predicted. The difference between the sodium bromide concentrations at which the foam-film and neat-phase transitions occur follows from the different causes of the expansions. In the film, the transition concentration is the concentration required to provide suffcient ion adsorption-induced repulsion to balance the longrange van der Waals attraction, whereas in the neat phase it is the concentration at which the short-range charge mosaic attraction is suppressed. It is important to note, however, that the salt concentrations shown in Fig. 3 are those in bulk solution and not those in the film. If ionic adsorption occurs the concentration in the film will be greater. Radiotracer measurements carried out with C10+, C10films containing sodium sulphate (12) have shown that at a bulk concentration of 5 mole m -3 the concentration of inorganic salt in the film is about 16 mole m -s. Although bromide ions may be more strongly adsorbed than sulphate ions, a sevenfold greater adsorption seems unlikely, so that this effect alone would not account for the difference in transition concentrations. In summary we conclude that, although inorganic salt has a similar effect on the separation of the surfactant layers in the black foam film and in the lamellar mesomorphic phase, the mechanisms by which their respective expansions are brought about are different as are also the forces responsible for maintaining that separation in the expanded neat phase and first-black foam film.
Journal of Colloid and Interface Science, Vol. 42, No. 1, January 1973
BLACK FOAM FILMS REFERENCES 1. CORKILL,J. M., GOODMAN,J. F., HAISMAN,D. R., AND HARROLD, S. P.~ Trans. Faraday Soc. 57, 821 (1961). 2. CLUNIE, J. S., GOODMAN,J. F., AND INGRAM,B. T., in "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 3, Wiley, New York, 1971. 3. CORKILL, J. ~/[., GOODMAN,J. F., OGDEN, C. P., ANDTATE, J. R., Proc. Roy. Soc. A273, 84 (1963). 4. CLUNIE, J. S., CORKILL, J. M., AND GOODMAN, J. F., Proc. Roy. Soc., A285, 520 (1965). 5. ROSEVEAR, F. B., J. Amer. Oil. Chem. Soc. 31, 628 (1954). 6. CLUNIE, J. S., CORKILL, J. ~V~., GOODMAN,J. F., AND INGRAM,B. T., Special Disc. Faraday Soc. 1, 30 (1970). 7. CORKILL,J. M., GOODMAN,J. F., AND OGDEN, C. P. Trans. Faraday Soc. 61, 583 (1965).
231
8. McBAIN, J. W., AND MARSDEN, S. S., Acta Crystallogr. 1,270 (1948). 9. LUZZATI, V., MUSTACCHI, H., AND SKOULIOS, A., Disc. Faraday Soc. 25, 43 (1958). 1O. JONES, M. 1'%;.,MYSELS, K. J., AND SCHOLTEN, P. C., Trans. Faraday Soc. 62, 1336 (1966). 11. GOODMAN,J. F., Disc. Faraday Soc. 42, 64 (1966). 12. GODDARD, E. D., KAO, O. AND KUNG, K. C., I. Colloid Interface Sci. 27, 616 (1968). 13. CLUNIE, J. S., GOODMAN,J. F., AND TATE, J. R., Trans. Faraday Soe. 64, 1965 (1968). 14. VERWEY, E. J. W., AND 0VERBEEK, J. T. G., "Theory of the Stability of Lyophobic Colloids," Elsevier, Amsterdam, 1948. 15. INGRAM, B. T., Trans. Faraday Soc. 68, 2230 (1972). 16. EKWALL, 1)., MANDELL, L., AND FONTELL, K., Acta Chem. Stand. 22, 1543 (1968). 17. GULIK-KRzYWICKI,T., TARDIEU, A., AND LUZZATI, V., Mol. Cryst. 8,285 (1969).
Journal o/ Colloid and Inter/ace Science, Vol. 42, No. 1, January 1973
Received March 6, 1972; accepted April 25, 1972 The equilibrium thicknesses of black foam films and the interlayer spacings of the structurally similar lamellar mesomophic phase (neat phase) formed by the decyltrimethylammonium decylsulphate + NaBr + H20 system have been measured at 298 K. A marked variation of film thickness and also neat-phase spacing occurs with changing system composition. This is attributed in both cases to the effect of inorganic ions on electrostatic interactions between surfactant layers, although the exact mechanism differs for each case. INTRODUCTION Analogies have been drawn (1,2) between the properties of foam films and those of the lamellar mesomorphic phase (neat phase) which commonly occurs in aqueous systems containing very high concentrations of surfaceactive agent. We have found that decyltrimethylammonium decylsulphate (C10+, C10-) not only gives exceptionally stable aqueous foam films but also forms neat phase as its only mesomorphic phase in concentrated aqueous solution, with or without added inorganic electrolyte. We have therefore compared the equilibrium thicknesses of films drawn from aqueous solutions of sodium bromide containing low concentrations of C10+, C~0- with the neat-phase interlayer spacing in the ternary system. EXPERIMENTAL Materials
Water was redistilled from a Pyrex vessel containing the distillate from an aqueous alkaline potassium permanganate solution. I t had a specific conductivity less than 10-4f~-lm-1 and a surface tension of 72 raN m -1 at 298 K. The sodium bromide was purified by recrystallization from aqueous solution and
roasting in a platinum crucible. The absence of surface active impurities was confirmed by measuring the surface tension (73.4 m N m -~) of a 1 kmole m -3 aqueous solution of the salt at 298 K. D e c y l t r i m e t h y l a m m o n i u m decylsulphate was prepared as described previously (3). The critical micelle concentration of its aqueous solution was 0.45 mole m -~ at 298 K. The p H of each solution was adjusted to 8.5 with sodium hydroxide to inhibit hydrolysis of the decylsulphate component to decanol, although it was subsequently shown that the deliberate addition of decanol to the film-forming solutions (up to one fifth of the C10+, C10- concentration) had no effect on the results. Methods (a) Bulk phase measurements. To obtain the ternary phase diagram, samples of known composition were first prepared and mixed in sealed glass ampoules as described previously (4) and then maintained at 298 K for several days to equilibrate. Samples were examined using low-angle X-ray diffraction (4) and polarization microscopy (5), although two phase systems were often more readily recognized by their bulk appearance than by their
Journal oJ Colloid and Inlet/ace Science, Vol. 42, No. I, January 1973
226
Copyright O 1973 by Academic Press, Inc; All rights of reproductior~ in any form reserved.
227
BLACK FOAM FILMS
q-isotropic solution, and neat phase+crystals. It was not possible to determine the neatphase boundary very precisely at low C~0+, C10- concentrations and in this region the boundary is represented by a broken line on the phase diagram. The structure of neat phase consists of bilayers of surfactant interleaved with layers of aqueous solution of varying thickness (8,9). The interlamellar periodicity (d001) for neat-phase compositions was determined by low-angle X-ray diffraction, and Fig. 2 shows the interlamellar spacing at constant C~0+, C10- compositions and varying sodium bromide solution concentration, the latter being the amount of sodium bromide per unit volume of water in the whole ternary system taking the molar volume of water to RESULTS be the same as in the binary (NaBr -t- H20) Bulk Phase solution. The X-ray camera resolution limit was 11 nm and spacings greater than this Figure 1 shows the ternary-phase diagram for the Cx0+, Cx0-+ NaBr + H20 system at could not be measured because the theoretically 298 K. For reasons of scale the isotropic phase accessible second-order reflections were too at low C~0+, C~0- concentrations (<0.02%) weak relative to background to be distinhas not been included in the diagram. No guished on the X-ray photographs. attempt was made to identify any of the cryIn the absence of sodium bromide, the neatstalline phases present. Only one mesomorphic phase spacings increased linearly from 3.25 phase, neat (lamellar) phase, was observed, to 4.00 nm as the proportion of C10+, C10- was either as an homogeneous phase or in hetero- decreased from 80 to 60% by weight; further geneous two phase regions, viz., neat phase addition of water then produced a hetero-
microscopic textures. In those cases where one of the coexisting phases was isotropic solution, the two phases were separated by centrifuging at 1500 rpm. (Centrifuging at speeds greater than 15,000 rpm caused complete separation of surfactant and electrolyte solution.) The separated phases were examined by chemical analysis, optical microscopy, and X-ray diffraction (4). (b) Film measurements. Film thicknesses were determined by the optical reflection method (2,6). The thickness was calculated assuming an isotropic film structure and a correction of -0.7 nm was then applied to allow for the heterogeneous structure of the real film (7).
'lO0 ~ NaBr
1007, H20
1007. ¢~**C~o-
FIG. 1. Partial phase diagram for decyltrimethylammonium decylsulphate -}- sodium bromide + water at 298 K. (A) crystalline phases present, (B) neat phase only, (C) neat phase + isotropic solution. Journal of Colloid and Interlace Science,
Vol. 42, No. 1, January 1973
228
BALMBRA ET AL.
"~"i
10
~'!
"" !
i
1
10
'!
10 2 NaBr
Concentration
'
i
i
10 3
10 4
(tool m " 3 )
Fro. 2. Interlayer spacing in C~0+, C10- ~- NaBr -}- H20 neat phase at 298 K. %C10+, C10-: 20%, A; 30%, [] ; 40%, ©; 50%, A; 60%, i ; 70%, e. Broken lines show limiting volume hypothesis values for each of the compositions. geneous mixture of isotropic solution coexisting with neat phase. The addition of sodium bromide up to a concentration of about 100 mole m -~ had no significant effect on the heterogeneity of the system or on the X-ray spacing (Fig. 2), but above this concentration a homogeneous neat phase was formed and a marked increase in interlayer distance occurred for the lower surfactant concentrations. Further increase in the salt concentration produced a decrease in the neat-phase spacing but the phase remained homogeneous. The interlayer spacing was found to be inversely proportional to the concentration of suffactant. From X-ray spacing and phase density measurements the interracial area per surfactant ion in the bilayer was calculated (9) to be 0.30 4- 0.02 nm 2 over the entire homogeneous neat-phase region studied. This value agrees well with that found for the limiting area per long-chain ion in the close-packed monolayer at the air/solution interface (3). Films
Figure 3 shows the equilibrium film thicknesses at 298 K as a function of sodium bromide Journal of Colloid and Interface Science,
concentration. The equilibrium thicknesses were independent of C10+, C10- concentration over the range 0.8-2.0 times the critical micelle concentration. (Stable films could not be obtained at lower surfactant concentrations.) At low concentrations of sodium bromide ( < 100 mole m -~) the films drained slowly and irregularly for several hours, although at concentrations below 5 mole m -3, the decrease in thickness from about 20 nm to the equilibrium value of 4.4 nm occurred very rapidly. At sodium bromide concentrations >100 mole m -~ there was regular (mobile) drainage and rapid attainment of equilibrium thickness. The heating of small portions of the films at salt concentrations of 5 mole m -8 and above, using the method of Jones et aL (10), failed to produce thinner stable regions. DISCUSSION
There are obvious qualitative similarities in the variations of black foam film thickness and neat-phase interlamellar spacing with changing electrolyte concentrations (Figs. 2 and 3). At low salt concentrations both the film thickness an d neat-phase spacing remain virtually con-
Vol.42,No.I,January1973
229
BLACK FOAM FILMS
range effects possibly such as that resulting from the charge mosaic nature of the surfaces (11). But at higher salt concentrations, ionic adsorption could produce a long-range repulsion which, above a certain electrolyte concentration, becomes sufficient to balance the long-range dispersion force and arrest the film's drainage at a greater thickness. A similar explanation has been proposed to account for the abrupt transition in thickness observed in films stabilized by the nonionic surfactant, decylmethyl sulphoxide (6). Moreover, there is evidence from studies on insoluble monolayers that specific ion adsorption effects do occur with alkyl sulphate and alkyl trimethylammonium salts (12), and conductivity and radiotracer studies of C10+, C10films have also given indications of inorganic ion adsorption (13). The DLVO theory of colloid stability 04) may be used to calculate the equilibrium thickness of the first black foam film (2), but values for the effective Hamaker constant and surface potential of the film are required. By using a value of 3 X 10-2°J for the effective Hamaker constant which was obtained from thickness and tension measure-
stant with ionic strength at a value slightly greater than twice the length of the extended surfactant molecule. In a foam film such behavior is typical of a "second black" or "Perrin" film (2,10). At higher salt concentrations there is a large increase in both film thickness and neat-phase spacing followed by a decrease as the ionic strength is increased even further. The behavior of the foam film in the highest concentration region is similar to that of a "first black" film (2,10). The discontinuous transition from a second black film at low ionic strengths to a first black one at higher concentrations could be the result of preferential adsorption of one type of inorganic ion giving rise to a long-range repulsive force within the film. At low concentrations of inorganic salt, the net surface charge in the 1:1 anionic-cationic surfactant film system is likely to be very small (3), and thus the repulsive forces between the film surfaces are probably fairly short range ones such as those between the oriented solvent dipoles associated with each surfactant monolayer. These balance the attractive forces arising from the longrange London dispersion effect and short-
\
22
10mV °
20
20mV
18 16
14
: I0-
o
.u_
8-
.c:
I"
6"
4'
O
o---
~ ~
O
1
1()
NaBr Concentration
10 z
1; 3
1()'
(tool m - a )
Fie,. 3, Equilibrium thickness of Ci0+, Ct0- + NaBr + H20 black-foam films at 298 K. Broken line shows equilibrium thicknesses calculated for a surface potential of 80 mV. Journal o/Colloid and Inter/ace Science, Vol. 42, No. 1, January 1973
230
BALMBRA E T AL.
ments made on films stabilized by decylmethyl sulphoxide (6) and decyltrimethylammonium decylsulphate (15) we have calculated the surface potentials corresponding to the observed equilibrium thicknesses. These are shown in Fig. 3 with a complete thickness curve (broken line) for an 80 mV potential. The Gouy-Chapman simplifying assumptions used in the calculations are unlikely to be completely valid at ionic strengths greater than about 100 mole m -~. However these calculated potentials are consistent with increasing ion adsorption as the salt concentration increases. The neat-phase expansion probably results from the suppression of short-range electrostatic attraction (ll) between the surfactant bilayers caused by the increasing concentration of inorganic ions in the aqueous layers which disrupts the crystal-like charge symmetry. Since the van der Waals attraction between the bilayers is very small---at least an order of magnitude smaller than the van der Waals attraction in the foam film--there is then no force in the neat phase suff_cient to prevent the bilayers separating to incorporate the liquid from the isotropic solution and forming a homogeneous phase. By analogy with the foam films, preferential ion adsorption should also occur in the neat phase at low electrolyte concentrations but it obviously does not do so to an extent sufficient to suppress the short-range attraction. The apparent paradox of the foam-film expansion can be resolved by noting that, unlike the neat phase, the film approaches its equilibrium state by thinning from very large thicknesses and must therefore first come under the influence of the longrange forces. Hence, although a very strong attraction may exist at small thicknesses it is the long-range repulsion of the overlapping electrostatic double-layer kind which determines the equilibrium thickness of the film. The transition from a nonexpanding to an expanding neat phase also occurs in systems of ionic surfactants (16,17) under conditions where a change in charge distribution seems likely. The neat-phase spacing in the homogeneous phase must, if the expansion is one
dimensional, depend solely on the amount of electrolyte solution available to fill the interlamellar region. Figure 2 shows the expected neat-phase spacings for different compositions based on this limiting volume hypothesis. It has been assumed that the bilayer thickness and area per surfactant ion remain constant at 3.0 nm and 0.30 nm 2, respectively, and that the molar volumes of interlamellar constituents, taken to be only water and inorganic ions, are the same as in normal bulk solution. It will be seen that the marked dependence of bilayer spacing o n e l 0 +, C10- concentration is fairly well predicted. The difference between the sodium bromide concentrations at which the foam-film and neat-phase transitions occur follows from the different causes of the expansions. In the film, the transition concentration is the concentration required to provide suffcient ion adsorption-induced repulsion to balance the longrange van der Waals attraction, whereas in the neat phase it is the concentration at which the short-range charge mosaic attraction is suppressed. It is important to note, however, that the salt concentrations shown in Fig. 3 are those in bulk solution and not those in the film. If ionic adsorption occurs the concentration in the film will be greater. Radiotracer measurements carried out with C10+, C10films containing sodium sulphate (12) have shown that at a bulk concentration of 5 mole m -3 the concentration of inorganic salt in the film is about 16 mole m -s. Although bromide ions may be more strongly adsorbed than sulphate ions, a sevenfold greater adsorption seems unlikely, so that this effect alone would not account for the difference in transition concentrations. In summary we conclude that, although inorganic salt has a similar effect on the separation of the surfactant layers in the black foam film and in the lamellar mesomorphic phase, the mechanisms by which their respective expansions are brought about are different as are also the forces responsible for maintaining that separation in the expanded neat phase and first-black foam film.
Journal of Colloid and Interface Science, Vol. 42, No. 1, January 1973
BLACK FOAM FILMS REFERENCES 1. CORKILL,J. M., GOODMAN,J. F., HAISMAN,D. R., AND HARROLD, S. P.~ Trans. Faraday Soc. 57, 821 (1961). 2. CLUNIE, J. S., GOODMAN,J. F., AND INGRAM,B. T., in "Surface and Colloid Science" (E. Matijevic, Ed.), Vol. 3, Wiley, New York, 1971. 3. CORKILL, J. ~/[., GOODMAN,J. F., OGDEN, C. P., ANDTATE, J. R., Proc. Roy. Soc. A273, 84 (1963). 4. CLUNIE, J. S., CORKILL, J. M., AND GOODMAN, J. F., Proc. Roy. Soc., A285, 520 (1965). 5. ROSEVEAR, F. B., J. Amer. Oil. Chem. Soc. 31, 628 (1954). 6. CLUNIE, J. S., CORKILL, J. ~V~., GOODMAN,J. F., AND INGRAM,B. T., Special Disc. Faraday Soc. 1, 30 (1970). 7. CORKILL,J. M., GOODMAN,J. F., AND OGDEN, C. P. Trans. Faraday Soc. 61, 583 (1965).
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Journal o/ Colloid and Inter/ace Science, Vol. 42, No. 1, January 1973