Thermodynamic studies on thin liquid films. III: Miscibility in adsorbed films at film interfaces

Thermodynamic treatment of surfactant mixture was developed for the adsorption at interfaces of thin liquid films and applied to the study of the foam film stabilized by decyl methyl sulfoxide (DeMS) in the presence of NaCl. The total surface density of NaCl and DeMS and the mole fraction of DeMS in the adsorbed film at the film surface were numerically evaluated by applying thermodynamic equations to the film tension as a function of the total molality of NaCl and DeMS and the mole fraction of DeMS in the mixture. Miscibility of NaCl and DeMS at the film surface was clarified by a phase diagram of adsorption and compared with that at the meniscus adjacent to the foam film. Judging from a phase diagram of phase transition, the transition in the DeMS foam film between common black and Newton black films, observed in part II, is a negative azeotropic transformation caused by the attractive interaction between the head group of DeMS molecule and Na⁺ or Cl^– in the adsorbed film.     

Thermodynamic studies on thin liquid films. II. Foam film stabilized by decyl methyl sulfoxide

Thermodynamic equations in Part I of this series were extended so as to be applicable to electrolyte mixtures and the resultant equations were applied to the experimental results of a NaCl-decyl methyl sulfoxide (DeMS) mixture. Film thickness and contact angle of the black foam film stabilized by DeMS were measured as a function of the total molality of NaCl and DeMS at constant mole fraction of DeMS in the mixture under constant disjoining pressure. Newton black film was observed only above a certain DeMS concentration and the phase transition between common black and Newton black films took place twice as NaCl concentration increased at constant DeMS concentration. The surface densities of NaCl and DeMS at the film surface and the differences in the surface densities between the adsorbed films at the film surface and bulk one coexisting at equilibrium were numerically evaluated by applying the thermodynamic equations to the film tension obtained from the contact angle. The film states and phase transitions were clarified in terms of the film thickness and surface densities.     

Charge number effect on the miscibility of inorganic salt and surfactant in adsorbed film and micelle: inorganic salt-octyl methyl sulfoxide mixtures

Adsorption and micelle formation of a surfactant in the presence of inorganic salts with different charge numbers of cations were investigated from the viewpoint of mixed adsorption and micelle formation of salt and surfactant. Surface tension of aqueous solutions of the mixtures of octyl methyl sulfoxide (OMS) with calcium chloride and lanthanum chloride was measured as a function of the total molality of the mixture and the mole fraction of OMS in the mixture at 298.15 K under atmospheric pressure. Composition of the adsorbed film and micelle was numerically evaluated from the dependence of the total molality at a given surface tension and the mixture CMC on the bulk composition to draw phase diagrams of adsorption and micelle formation. Judging from the phase diagrams together with the ones of the sodium chloride system, miscibility of inorganic salt and OMS in the adsorbed film and micelle increases with an increase in the charge number of inorganic cation, which is attributable to the attractive interaction between inorganic cation and the polar head group of OMS molecule in the adsorbed film and micelle.     

Effect of the Head Group of Surfactant on the Miscibility of Sodium Chloride and Surfactant in an Adsorbed Film and Micelle

The thermodynamic treatment of a surfactant mixture was applied to the mixture of sodium chloride, NaCl, with octyl methyl sulfoxide (OMS) and that with decyldimethylphosphine oxide (DePO). The surface tension of aqueous solutions of the mixtures was measured as a function of the total concentration and the composition of the mixtures at 298.15 K. The total surface densities of the mixtures and the composition of the adsorbed films and micelles were evaluated by applying thermodynamic equations to the expeimental results. It was found that the adsorbed film and micelle are almost composed of the surfactant and there is slight attractive interaction between the ions of NaCl and the head groups of OMS and DePO molecules in the adsorbed films and micelles. A difference in the miscibility of NaCl and surfactant was observed between the OMS and DePO systems and attributed to the difference in the hydration of the head group between OMS and DePO molecules. The comparison of these results with those of the mixtures of NaCl with tetraethylene glycol monooctyl ether (C(8)E(4)) and dodecylammonium chloride (DAC) indicated that the small difference in the miscibility in an adsorbed film and micelle among these nonionic surfactant systems arises from the difference in hydration and structure of the head groups and the large one between the nonionic surfactant and DAC systems results from electrostatic interactions between dodecylammonium and sodium ions. Copyright 2001 Academic Press.     

Charge number effect on the miscibility of inorganic salt and surfactant in adsorbed film and micelle: Inorganic salt-dodecylammonium chloride mixtures

The effect of inorganic salts with different charge numbers of cations on the adsorption and micelle formation of dodecylammonium chloride (DAC) was clarified by applying the thermodynamic treatment of surfactant mixtures to mixtures of DAC with calcium and lanthanum chlorides and comparing the results with those of the sodium chloride-DAC mixture in the previous study. Surface tension of aqueous solutions of the salt-DAC mixture was measured as a function of the total molality of the mixture and the mole fraction of DAC in the mixture at 298.15 K under atmospheric pressure. Judging from the phase diagrams of adsorption and micelle formation obtained from the surface tension measurement, dodecylammonium cations expel inorganic cations from the adsorbed film and micelle and the repulsive interaction between dodecylammonium and inorganic cations increases with increasing charge number of the inorganic cation.     

Miscibilities of methanol and ethanol with dodecylammonium chloride in the adsorbed film and micelle

The surface tension of the aqueous solutions of methanol– dodecylammonium chloride (DAC) and ethanol–DAC mixtures has been measured as a function of the total molality of the mixture m^ and the mole fraction of DAC X^₂ in the mixture at 298.15?K under atmospheric pressure. The compositions of the adsorbed film at 50, 40, and 30?mN?m^-1, and that of the micelle at the critical micelle concentration (CMC) have been evaluated by applying the thermodynamic equations derived previously and shown in the form of phase diagrams of adsorption and micelle formation. It has been found that (i) methanol is hardly miscible in M28.8nthe adsorbed film and micelle, and (ii) ethanol molecules are very slightly incorporated into adsorbed film of DAC at high-surface tension and into the micelle, although hardly miscible in the adsorbed film at low-surface tension. By comparing the corresponding phase diagrams of ethanol-2- (octylsulfinyl)ethanol (OSE) mixture, furthermore, it has been shown that there exists a difference in the miscibility of ethanol between DAC and OSE.     

Miscibility of sodium chloride and ionic surfactant in adsorbed film and aggregate

The adsorption, micelle formation, and salting-out of dodecylammonium chloride in the presence of NaCl were studied from the viewpoint of mixed adsorption and aggregate formation. The surface tension of aqueous solutions of a NaCl–dodecylammonium chloride mixture was measured as a function of the total molality and composition of the mixture. Judging from phase diagrams of mixed adsorption and aggregate formation, NaCl and dodecylammonium chloride are miscible in the adsorbed film and coagel particle at high NaCl concentrations due to specific (nonelectrostatic) interaction between dodecylamonium ion and the counterion, while they are immiscible in the micelle. The difference in miscibility among the oriented states was ascribed to the difference in geometry among the states and to the interaction between bilayers in a coagel particle. Miscibility and specific interaction are compared between the mixtures of NaCl with dodecylammonium chloride and sodium dodecylsulfate.     

Miscibility of sodium chloride and sodium dodecyl sulfate in the adsorbed film and aggregate

The adsorption, micelle formation, and salting out of sodium dodecyl sulfate in the presence of sodium chloride were studied from the viewpoint of their mixed adsorption and aggregate formation. The surface tension of aqueous solutions of a sodium chloride–sodium dodecyl sulfate mixture was measured as a function of the total molality and composition of the mixture. Phase diagrams of adsorption and aggregate formation were obtained by applying thermodynamic equations to the surface tension. Judging from the phase diagrams, sodium chloride and sodium dodecyl sulfate are miscible in the adsorbed film at very large composition of sodium chloride and in the salted-out crystalline particle, while they are immiscible in the micelle. The miscibilities in the adsorbed film, micelle, and crystalline particle increase in the following order: particle > adsorbed film > micelle. The difference in miscibility among the oriented states was ascribed to the difference in geometry between the adsorbed film and micelle and to the interaction between bilayer surfaces in the particle.     

Nonideal mixing of dodecyltrimethylammonium halides and nonionic surfactant in adsorbed films and micelles

The ion–dipole interaction between dodecyltrimethylammonium cations and nonionic surfactant molecules in adsorbed films and micelles was investigated by concentrating on the difference in the degree of counterion binding by employing dodecyltrimethylammonium chloride (DTAC)–octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide (DTAB)–OMS mixtures. The phase diagrams of adsorption and micelle formation were constructed and then the nonideal mixing of different species of surfactants was demonstrated in terms of the excess Gibbs free energies of adsorption and micelle formation, and the surface excess areas. Furthermore the dependence of them on the counterion was clearly shown. All these results were found to support our previous view that the direct interaction between surfactant cation and the dipole of the hydrophilic part of a nonionic surfactant is essential in cationic-nonionic surfactant mixtures, i.e., the DTAC system with a lower counterion biding has more negative excess thermodynamic quantities than the DTAB system with a higher one.     

Miscibility of sodium dodecyl sulfate and sodium decyl sulfate in the adsorbed film and micelle

The interfacial tension of the aqueous solution of sodium dodecyl sulfate (SDS) and sodium decyl sulfate (SDeS) mixture against hexane was measured as a function of the total molality and composition of the surfactant mixture at 298.15 K under atmospheric pressure. The compositions of adsorbed film and micelle were evaluated numerically by applying the thermodynamic relations to the experimental results. These results were shown in the form of the phase diagrams of adsorption and micelle formation and compared with those of the aqueous solution of sodium perfluorooctanoate (SPFO) and SDeS mixture. It was found that the diagrams of SDS and SDeS system have swollen cigar shapes and are quite different from those of SPFO and SDeS system which show non-ideal mixing both in the adsorbed film and micelle. This finding was attributed to the fact that the interaction between fluorocarbon and hydrocarbon chains is weaker than that between hydrocarbon chains.     

Selective determination of surface density of bromide ion through XAFS and its application to verification of a criterion of an ideal mixing of surfactant mixture

The mole fraction of chloride ion of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium chloride (DTAC) mixture in the adsorbed film XHC was estimated not only by the thermodynamic analysis of the surface tension data but also by analyzing the Br K-edge jump of the XAFS spectrum under the total reflection condition (TRXAFS method). The phase diagrams of adsorption (PDA) at several surface tensions from the two methods were in good agreement. On the basis of the PDA obtained, it was clearly shown that the criterion of an ideal mixing for the DTAB-DTAC system is not given by the linear relation between the total molality of surfactant mixture m and XHC, m = m0B + (m0C - m0B)XHC, but by the one between m2 and XHC, m2 = (m0B)2 + [(m0C)2 - (m0B)2]XHC. Furthermore, it was demonstrated that the theoretical approach that provides the latter relation draws a distinction between the criteria for an ionic surfactant mixture without a common ion and that for an ionic surfactant mixture with a common ion.     

Electrolyte effect on mixed micelle and interfacial properties of binary mixtures of cationic and nonionic surfactants

In the present work, the adsorption behavior at the liquid-air interface and micellization characteristics of mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl) polyoxyethylene (TritonX-100) in aqueous media containing different concentrations of NaBr were investigated by surface tension and potentiometry measurements. From plots of surface tension (gamma) as a function of solution composition and total surfactant concentration, we determined the critical micelle concentration (CMC), minimum surface tension at the CMC (gamma(CMC)), surface excess (Gamma(max)), and mean molecular surface area (A(min)). On the basis of regular solution theory, the compositions of the adsorbed film (Z) and micelles (X(M)) were estimated, and then the interaction parameters in the micelles (beta(M)) and in the adsorbed film phase (beta(sigma)) were calculated. For all mole fraction ratios, the results showed synergistically enhanced ability to form mixed micelles as well as surface tension reduction. Furthermore beta was calculated by considering nonrandom mixing and head group size effects. It was observed that, for both the planar air/aqueous interface and micellar systems, the nonideality decreased as the amount of electrolyte in the aqueous medium was increased. This was attributed to a decrease of the surface charge density caused by increasing the concentration of bromide ions.     

Blending effects on adsorption and micellization of different membrane protein solubilizers: a thermodynamic study on three mixed systems of CHAPS with MEGA-8, -9 and -10 in pH 7.2 phosphate buffer solution

By means of surface tension measurement (Wilhelmy method), micellization and adsorbed film formation were investigated for three combinations of mixed surfactant systems, all of which are used for solubilizing membrane proteins: a typical zwitterionic surfactant, CHAPS (a derivative of cholic acid) with n-alkyl (octyl, nonyl and decyl)-N-glucamides, MEGA-n (n=8, 9, 10). The data based on plotting of surface tension (gamma) versus logarithmic total molarity (or molality) (Ct or Mt) as a function of mole fraction of surfactant 2 (2 corresponds to MEGA-n's) enabled us to determine critical micellization concentration (CMC), minimum surface tension at CMC (gammaCMC), surface excess (Gamma(t)), mean molecular surface area (Am), the minimum Gibbs energy (Gmin(S)) of adsorbed film of both single and mixed surfactant systems and partial molecular area (PMA) in addition to parameters such as pC20 and CMC/C20 being related to synergism accompanied by blending (mixing) in regard to surface activity as well as micelle forming ability. On the basis of the regular solution theory, the relations of compositions of singly dispersed phase (X2), micellar phase (Y2) and adsorbed film (Z2) were estimated, and then the interaction parameters in micelles (omegaR) and in the adsorbed film phase (omegaA) were also calculated. From both the CMC-X2 and CMC-Y2 curves, it was found for all the combinations to show synergistically enhanced ability of mixed micelle formation as well as surface tension reduction. The resultant synergism coming from blending CHAPS with MEGA-n's was discussed in comparison with different combinations of various types of surfactants including membrane proteins solubilizers.     

Influence of NaCl on the behavior of PEO-PPO-PEO triblock copolymers in solution, at interfaces, and in asymmetric liquid films

The solution behavior of the polymeric surfactant Pluronic F127 (PEO(99)PPO(65)PEO(99)) and its adsorption behavior on aqueous-silica and aqueous-air interfaces, as well as the disjoining pressure isotherms of asymmetric films (silica/aqueous film/air) containing F127, are studied. The interfacial properties of adsorbed F127 layers (the adsorbed amount Gamma and the thickness h) as well as the aqueous wetting film properties [film thickness (h) and refractive indexes] were studied via ellipsometry. The solution properties of F127 were investigated using surface tensiometry and light scattering. The interactions between the air-water and silica-water interfaces were measured with a thin film pressure balance technique (TFB) and interpreted in terms of disjoining pressure as a function of the film thickness. The relations between the behaviors of the asymmetric films, adsorption at aqueous air, and aqueous silica interfaces and the solution behavior of the polymeric surfactant are discussed. Special attention is paid to the influence of the concentrations of F127 and NaCl. Addition of electrolyte lowers the critical micelle concentration, diminishes adsorption on silica, and increases the thickness of the asymmetric film.     

The effect of counterions on the surface adsorption and micelle formation in mixed surfactant systems of dodecyl sulfates

 The surface tension of the aqueous solutions of sodium dodecyl sulfate (SDS) and tetramethylammonium dodecyl sulfate (TMADS) was measured as a function of total molality of the surfactants at fixed composition of TMADS at 298.15 K under atmospheric pressure. The phase diagrams of adsorption and of micelle formation, the activity coefficients, and the excess Gibbs energy were calculated to estimate the deviation from the ideal mixing quantitatively. The preferential adsorption and the micelle formation for TMA⁺ to Na⁺ is attributable to some extent to the hydrophobicity of the methyl groups of TMA⁺. The composition of TMA⁺ in the micelle is larger than that in the adsorbed film at equilibrium. That is, a larger hydrated counterion is more likely to exist in the micelle than in the adsorbed film owing to geometrical benefit. The negative values of the excess Gibbs energy of the adsorbed film and of the micelle arise from the positive ones of the excess entropy greater than that of excess enthalpy. The counterions of very similar size are mixed ideally in the micelle and the size effect appears sensitively in the adsorbed film. Received: 23 May 2001 Accepted: 16 July 2001     

Novel phase behavior in adsorbed film of fluoroalkanol and cationic surfactant mixture

The surface tension of the aqueous solution of the binary mixture of 1H,1H-heptafluoro-1-butanol (FC4OH) and dodecyltrimethylammonium bromide (DTAB) was measured as a function of the total molality of the mixture and the composition (mole fraction in the surfactant mixture) of DTAB at 298.15 K under atmospheric pressure to examine the phase behavior in the adsorbed film. The results of the surface tension measurement were analyzed by the thermodynamic procedure proposed by us and the composition of the mixed adsorbed film in equilibrium with their bulk solution was calculated. Three different phases of the adsorbed film appeared by a subtle balance between the attractive interaction of the polar head groups and weak dispersion interaction of the hydrophobic chains. In the low-concentration regime, FC4OH molecules and DTAB molecules form a gaseous film and mix attractively in the whole composition by the long-range ion–dipole attraction between hydrophilic groups. The effect of the attractive dispersion interaction between CH and CF chains became more influential in the expanded film within a restricted composition region, where it should be noted that the interaction between CH and CF is weaker than that between CH chains or between CF chains alone. Furthermore, the adsorbed films at two specific compositions are stabilized by the stoichiometric arrangements of the molecules, which help ion–dipole attraction, in them.     

Effect of preferential adsorption on the synergism of a homologous cationic surfactant mixture

To evaluate the effect of preferential surface adsorption of bromide ions on the synergism of homologous cationic surfactant mixtures reported previously, the surface tension of the aqueous solutions of the hexadecyltrimethylammonium chloride (HTAC)-dodecyltrimethylammonium bromide (DTAB) system was measured as a function of the total molality of surfactants and the relative proportion of DTAB at 298.15 +/- 0.05 K under atmospheric pressure. The excess Gibbs energies calculated from them were -2.6 kJ mol(-)(1) in the mixed adsorbed film and -2.0 kJ mol(-)(1) in the mixed micelle, respectively. A useful analytical procedure to evaluate the composition of individual ions (hexadecyltrimethylammonium, dodecyltrimethylammonium, chloride, and bromide ions) in the adsorbed film and micelle was developed and applied.     

Surface adsorption and micelle formation of aqueous solutions of polyethyleneglycol and sugar surfactants

The surface tension of aqueous solutions of tetraethyleneglycol octyl ether (C8E4) and octyl-β-d-maltopyranoside (OM) mixture was measured as a function of the total molality of surfactants and the composition of OM under atmospheric pressure at 298.15 K by drop volume technique. The results of surface tension measurements were analyzed by originally developed thermodynamic equations, then phase diagrams of adsorption and micelle formation were constructed. From the analysis of the surface tension data, it was found that the C8E4 and OM molecules interact attractively in the adsorbed film and the excess Gibbs energy of adsorption can be compared with those observed in typical cationic–nonionic surfactant systems; nevertheless, they are mixed almost ideally in the mixed micelle. Judging from a negative excess surface area calculated by differentiating the excess Gibbs energy by the surface tension, we concluded that the attraction between C8E4 and OM molecules is a short-range one originated in the hydrogen bonding between them which favors the planar configuration.     

The interaction of an anionic gemini surfactant with conventional anionic surfactants

The interaction in two mixtures of an anionic gemini surfactant having N ,N -dialkylamide and carboxylate groups in a molecule, (CH2)2[N(COC11H23)CH(CO2H)CH2(CO2H)]2. 2NaOH (GA), and conventional anionic surfactants have been investigated in 0.1 M NaCl at pH 5.0. The two mixtures are GA/sodium dodecylsulfate (SDS) and GA/sodium N -dodecanoylglutamate (AGS) at a molar fraction of GA, alphaGA = 0.25 . Mixtures of both GA/SDS and GA/AGS exhibit synergism in surface tension reduction effectiveness. The GA/SDS mixture also exhibits synergism in surface tension reduction efficiency and mixed micelle formation, whereas the GA/AGS mixture does not. The interaction in mixed adsorption film formation is stronger than that in mixed micelle formation for the two mixtures. The interaction in the formation of the mixed adsorption film and the mixed micelle for the GA/SDS mixture is stronger in both formations than that for the GA/AGS mixture. The stronger interaction for the GA/SDS mixture may be caused by the combination of the smaller minimum area per molecule at the air/water interface (Amin) of the head groups in the GA molecule and the larger Amin in the SDS molecule.     

Variegated micelle surfaces: correlating the microstructure of mixed surfactant micelles with bulk solution properties

Electron paramagnetic resonance, viscosity, and small-angle neutron scattering (SANS) measurements have been used to study the interaction of mixed anionic/nonionic surfactant micelles with the polyampholytic protein gelatin. Sodium dodecyl sulfate (SDS) and the nonionic surfactant dodecylmalono-bis-N-methylglucamide (C12BNMG) were chosen as "interacting" and "noninteracting" surfactants, respectively; SDS micelles bind strongly to gelatin but C12BNMG micelles do not. Further, the two surfactants interact synergistically in the absence of the gelatin. The effects of total surfactant concentration and surfactant mole fraction have been investigated. Previous work (Griffiths et al. Langmuir 2000, 16 (26), 9983-9990) has shown that above a critical solution mole fraction, mixed micelles bind to gelatin. This critical mole fraction corresponds to a micelle surface that has no displaceable water (Griffiths et al. J. Phys. Chem. B 2001, 105 (31), 7465). On binding of the mixed micelle, the bulk solution viscosity increases, with the viscosity-surfactant concentration behavior being strongly dependent on the solution surfactant mole fraction. The viscosity at a stoichiometry of approximately one micelle per gelatin molecule observed in SDS-rich mixtures scales with the surface area of the micelle occupied by the interacting surfactant, SDS. Below the critical solution mole fraction, there is no significant increase in viscosity with increasing surfactant concentration. Further, the SANS behavior of the gelatin/mixed surfactant systems below the critical micelle mole fraction can be described as a simple summation of those arising from the separate gelatin and binary mixed surfactant micelles. By contrast, for systems above the critical micelle mole fraction, the SANS data cannot be described by such a simple approach. No signature from any unperturbed gelatin could be detected in the gelatin/mixed surfactant system. The gelatin scattering is very similar in form to the surfactant scattering, confirming the widely accepted picture that the polymer "wraps" around the micelle surface. The gelatin scattering in the presence of deuterated surfactants is insensitive to the micelle composition provided the composition is above the critical value, suggesting that the viscosity enhancement observed arises from the number and strength of the micelle-polymer contact points rather than the gelatin conformation per se.