Thinning and thickening transitions of foam film induced by 2D liquid-solid phase transitions in surfactant-alkane mixed adsorbed films.

Mixed adsorbed film of cationic surfactant and linear alkane at the air-water interface shows two-dimensional phase transition from surface liquid to surface frozen states upon cooling. This surface phase transition is accompanying with the compression of electrical double layer due to the enhancement of counterion adsorption onto the adsorbed surfactant cation and therefore induces the thinning of the foam film at fixed disjoining pressures. However, by increasing the disjoining pressure, surfactant ions desorb from the surface to reduce the electric repulsion between the adsorbed films on the both sides of the foam film. As a result, the foam film stabilized by the surfactant-alkane mixed adsorbed films showed unique thickening transition on the disjoining pressure isotherm due to the back reaction to the surface liquid films. In this review, we will summarize all these features based on the previously published papers and newly obtained results.

Study on the distribution of binary mixed counterions in surfactant adsorbed films by total reflection XAFS measurements.

The total reflection X-ray absorption fine structure (TR-XAFS) technique was applied to adsorbed films at the surface of aqueous solutions of surfactant mixtures composed of dodecyltrimethylammonium bromide (DTAB) and dodecyltrimethylammonium tetrafluoroborate (DTABF(4)). The obtained XAFS spectra were expressed as linear combinations of two specific spectra corresponding to fully hydrated bromide ions (free-Br) and partially dehydrated bromide ions adsorbed to the hydrophilic groups of surfactant ions (bound-Br) at the surface. The ratio of free- and bound-Br ions was determined as a function of surface tension and surface composition of the surfactants. Taking also the results in our previous studies on the DTAB - dodecyltrimethylammonium chloride (DTAC) and 1-hexyl-3-methylimidazolium bromide (HMIMBr) - 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixed systems into consideration, the relation between counterion distribution and miscibility of counterions at the solution surface was deduced for the surfactant mixtures having common surfactant ions but different counterions.

Specific counterion effect on the adsorbed film of cationic surfactant mixtures at the air/water interface.

To investigate the counterion effects, we employed dodecyltrimethylammonium bromide (DTABr)-dodecyltrimethylammonium tetrafluoroborate (DTABF(4)) mixed aqueous solutions and measured their surface tensions, then analyzed these data in a thermodynamic treatment. The tensiometry showed that DTABF(4) was more effective in lowering the surface tension of water. The phase diagram of adsorption demonstrated that the surface was enriched with BF(4)(-) ions, but the composition of Br(-) ions in the adsorbed film was slightly enhanced compared to the ideal mixing criteria. These were explained in terms of the size and polarizability of counterions. Moreover, the distribution of counterions of the DTABr-DTABF(4) mixtures in the adsorbed film is greatly different from that of the 1-hexyl-3-methylimidazolium bromide (HMIMBr)-1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF(4)) mixtures, where a stronger hydrogen-bonding exists between BF(4)(-) and HMIM(+) ions. These findings suggest that the adsorption of counterions in electric double layers is likely subject to two factors: the nature of counterion and their interactions with surfactant ions.

Surface adsorption and aggregate formation of cationic gemini surfactant and long-chain alcohol mixtures.

We measured the surface tension of aqueous solutions of octanol-butandiyl-1,4-bis(decyldimethylammonium bromide) using the drop-volume technique at 298.15 K under atmospheric pressure as a function of the total molality and bulk composition. The results of the surface tension measurements, which were analyzed by originally developed thermodynamic equations, suggested that octanol molecules filled the spaces among the hydrophobic chains of gemini surfactants and formed a densely packed monolayer with them in the adsorbed film. The turbidity of aqueous solutions was also measured to construct the concentration-composition diagram with the surface tension data. A transmission electron microscope was used to determine the aggregate morphology in the aqueous solutions. Disc-like micelle and microemulsion regions were found on the diagram prior to the spherical micelle formation; nevertheless, the butandiyl-1,4-bis(decyldimethylammonium bromide) itself formed only spherical (or small ellipsoid) micelles in the concentration range measured. We also studied the relationship between synergism and molecular packing in the aggregates.

Wetting and freezing of hexadecane on an aqueous surfactant solution: triple point in a 2-D film.

Wetting of water by hexadecane has been investigated by ellipsometry as a function of the concentration of the cationic surfactant dodecyltrimethylammonium bromide (DTAB) in the aqueous phase and temperature. Three phases are identified: a 2-D gas of hexadecane molecules and DTAB molecules, a 2-D liquid comprising a mixed monolayer of hexadecane and DTAB, and a 2-D 'solid' phase. Evidence is presented to support the hypothesis that the liquid-solid phase transition is actually a wetting transition in which a surface-frozen layer of pure hexadecane wets the liquid-like mixed monolayer of hexadecane and DTAB. The triple point, at which the three phases coexist, is located at a temperature of 17.3 degrees C and DTAB concentration of 0.75 mmol kg (-1). The slopes of the three phase boundaries are analyzed thermodynamically.

Adsorption and micelle formation of alkylammonium chloride-alkyltrimethylammonium chloride mixtures.

The surface tension of the aqueous solutions of binary cationic surfactant mixtures of (1) dodecylammnonium chloride (DAC)-tetradecyltrimethylammonium chloride (TTAC), (2) decylammonium chloride (DeAC)-dodecyltrimethylammonium chloride (DTAC), and (3) DAC-DTAC was measured as a function of the total molality and composition of surfactants at 298.15 K. The compositions of surfactants in the adsorbed film and micelle were evaluated and the phase diagram of adsorption and that of micelle formation were constructed. Furthermore the excess Gibbs energies of adsorption and micelle formation were calculated to estimate the deviation from the corresponding ideal mixing. It was found that the surface and micelle are enriched in trimethylammonium salts in (1) and (2), while in ammonium salt in (3) compared to the bulk solution. On the other hand, the micelle is enriched in trimethylammonium salts compared to the surface at the critical micelle concentration (CMC) in all the systems. The miscibility of the surfactants was clarified from the standpoints of the structure of the head group and of the matching between the size of polar head group of surfactants and the difference in hydrocarbon chain length.

Surface phase transition of C12E1 at the air/water interface: a study by dynamic surface tension, external RA FT-IR, and 2D IR correlation methods.

The surface conformational states of the Gibbs monolayer of ethylene glycol mono-n-dodecyl ether (C(12)E(1)) at the air/water interface was studied using dynamic surface tension, external reflection-absorption FT-IR spectroscopy (ERA FT-IR), and two-dimensional infrared (2D IR) correlation methods at constant temperature. The dynamic surface tensions were measured at different bulk concentrations of C(12)E(1), and it was observed that a constant surface tension region appears at approximately 38.5 mN m(-1) in a dynamic surface tension profile at concentrations higher than 11 micromol kg(-1). This constant surface tension region corresponds to the surface phase transition from liquid expanded (LE) to liquid condensed (LC). Two sets of ERA FT-IR spectra were collected, one at different bulk concentrations but after equilibrium time (equilibrium measurements) and another at constant bulk concentration (m = 16 micromol kg(-1)) but at different times (dynamic measurements). The first set of these measurements show that the peak area increases in the range of 11 < m < or = 16 micromol kg(-1), which means the increase in the number of surfactant molecules at the air/water interface. Also, the wavenumber of antisymmetric CH(2) stretching decreases gradually from approximately 2923 cm(-1) (for 10 and 11 micromol kg(-1)) to approximately 2918 cm(-1) (for m > or = 16 micromol kg(-1)) with increasing concentration. The wavenumbers of 2923 and 2918 cm(-1) were assigned to LE and LC phases, respectively, and the decrease of wavenumber in the concentration range of 11 < m < or = 16 micromol kg(-1) were correlated to the surface phase transition (LE --> LC), or in other words, in the mentioned concentration range, two phases coexist. The dynamic ERA FT-IR measurements at 16 micromol kg(-1) also confirm the surface phase transition from LE to LC. The 2D IR correlation method was applied to the both equilibrium and dynamic IR spectra of the C(12)E(1) monolayer. The synchronous correlation maps show two strong autopeaks at approximately 2922 and approximately 2851 cm(-1) and also show a strong correlation (cross-peaks) between antisymmetric CH(2) stretching (nu(a)) and symmetric CH(2) stretching (nu(s)). The asynchronous correlation maps show that both observed bands of nu(a) and nu(s) in one-dimensional IR split into two components with the characteristic of overlapped bands, which reveals the coexistence of two phases (LE and LC) at the interface at 11 < m < or = 16 micromol kg(-1). The synchronous and asynchronous maps that were obtained from dynamic IR spectra closely resembled the equilibrium map.

Ellipsometric search for vapor layers at liquid-hydrophobic solid surfaces.

We use precision ellipsometry to evaluate the existence of nanometer thick vapor films at the surface between a liquid and a hydrophobic alkylsilane coated Si wafer. We find no evidence for such vapor films. All of our fluid-solid ellipsometry measurements can be explained using a double layer model consisting of an oxide plus silane layer between the fluid and bulk Si substrate. We have carefully checked our ellipsometer for residual phase shifts which might, under certain circumstances, cause a mis-interpretation of the experimental results. We find that the most reliable ellipsometric results for thin films (which are relatively immune to the presence of small residual phase shifts) are collected at the Brewster angle. The dielectric constant of the native oxide coating is also compared with similar measurements for two thick (approximately 100-300 nm) thermally grown oxide coatings on a Si wafer.

Lattice model for the wetting transition of alkanes on aqueous surfactant solutions.

Droplets of alkanes on aqueous solutions of the cationic surfactants C(n)TAB (CH3(CH2)(n-1)N+ (CH3)3Br-) exhibit a first-order wetting transition as the concentration of the surfactant is increased. A theoretical model is presented in which the surface free energy is broken down into a long-range dispersion interaction and a short-range interaction described by a 2D lattice gas, taking into account the interaction between oil and surfactant molecules. The model provides quantitative agreement with the observed wetting transitions and the variation in composition of the wetting film with bulk surfactant concentration. The behavior of oil drops on large reservoirs of dilute surfactant is discussed.

Thermodynamics of Formation of and Adsorption at Interfaces with a Floating Lens.

The thermodynamic equations of the formation of and adsorption at interfaces were derived for a three-phase system having a floating lens with curved interfaces. The surface excess quantities were defined both in the H convention based on the two dividing surfaces and in the P convention based on the four dividing surfaces. Their physical significance and utilities were discussed. The equations were simplified to be applicable to the three-component systems with the plane A/W interface and a relatively large size of lens, such as the air/water/long-chain alcohol systems. The formulation and geometrical consideration of the two conventions with respect to this simple cases revealed that the H convention introduces the distance between the two dividing surfaces and the P convention introduces three kinds of distances between the four dividing surfaces including a kind of thickness of the reference oil lens phase in the A/W interfacial region. Furthermore, the general equations were simplified to be applicable to four-component systems such as the air/oil solution of a surface-active substance/water or the air/oil/aqueous solution of a surfactant with a floating oil lens. Copyright 1999 Academic Press.

The Excess Thermodynamic Quantities of Adsorption of a Binary Nonionic Surfactant Mixture.

The thermodynamic relations were developed for examining the miscibility of surfactants in the adsorbed film from the viewpoints of the thermodynamic quantities of adsorption such as Gibbs free energy, entropy, volume, enthalpy, and energy. The deviation from the additivity relation, the straight line connecting the thermodynamic quantities of the respective pure surfactants as a function of the composition of the adsorbed film, was proved to yield the corresponding excess quantities of adsorption. The criterion of the ideal mixing was offered and summarized with respect to the composition of adsorbed film, the mean area occupied by surfactants, and the thermodynamic quantities of adsorption. The resulting equations were applied to the simple nonionic surfactants mixture of decyl methyl sulfoxide (DeMS) and octyl methyl sulfoxide (OMS). The surface tension of the aqueous solution was measured as a function of not only the total molality and mole fraction of surfactants but also temperature under atmospheric pressure. Copyright 1999 Academic Press.

Structure Effect on Nonideal Mixing of Alkyl Methyl Sulfoxide and Alkyldimethylphosphine Oxide in Adsorbed Film and Micelle.

The surface tension of aqueous solutions of the octylmethylsulfoxide (OMS)-decyldimethylphosphine oxide (DePO) and decylmethylsulfoxide (DeMS)-octyldimethylphosphine oxide (OPO) mixtures was measured as a function of the total molality of the surfactants and the composition in the surfactants. Compositions in the adsorbed film and micelle were thermodynamically evaluated from the total molality vs composition and the critical micelle concentration (CMC) vs compositon curves, respectively. The phase diagrams of adsorption and micelle formation were drawn and compared with those of the DeMS-DePO mixture previously studied. A large gap in composition was observed between the adsorbed film and the micelle coexisting at the CMC and ascribed to the differences in geometric structure between them and the constituent molecules. Negative deviation from ideal mixings in adsorbed film and micelle was observed for the OMS-DePO mixture, whereas a positive one was observed for the DeMS-OPO. The structure effect on the nonideal mixings was clarified by using the excess thermodynamic functions of adsorption and micelle formation calculated from the activity coefficients in the adsorbed film and micelle. Copyright 1999 Academic Press.

Miscibility of Butanol and Cationic Surfactant in the Adsorbed Film and Micelle.

Mixed adsorbed films and micelles were investigated by measuring surface tension of the aqueous solutions for the following four mixtures: the n-butanol (NBA)-dodecylammonium chloride (DAC), t-butanol (TBA)-DAC, NBA-dodecyltrimethylammonium chloride (DTAC), and TBA-DTAC. Experimental results were analyzed thermodynamically. Phase diagrams of adsorption and micelle formation and that exhibiting the composition relation of micelle and adsorbed film at CMC were compared among the four systems. Excess Gibbs energy in the adsorbed film gH,E and activity coefficient in the micelle fM2+/- were calculated from the phase diagrams to estimate the miscibility of the butanol and cationic surfactant quantitatively. By comparing the results of the pairs of NBA-DAC and TBA-DAC systems with those of the NBA-DTAC and TBA-DTAC systems, it was shown that the substitution of the polar head group from ammonium to trimethylammonium changes the gH,E value from positive to negative and increases fM2+/-. Furthermore, by comparing the results of the pairs of NBA-DAC and NBA-DTAC systems with those of the TBA-DAC and TBA-DTAC systems, it was shown that the bulky hydrophobic group weakens the molecular interaction between the butanol and the cationic surfactant in the adsorbed film. Copyright 1998 Academic Press.

Structure Effect on Miscibility of Butanol and Dodecyltrimethylammonium Chloride in Adsorbed Film.

Miscibility of components in the adsorbed film for the n-butanol (NBA)-dodecyltrimethylammonium chloride (DTAC) and tert-butyl alcohol (TBA)-DTAC mixtures were investigated by measuring surface tension of the aqueous solutions. The experimental results were analyzed thermodynamically. Diagrams of the excess Gibbs energy in the adsorbed film gH,E vs the composition in the adsorbed film XH2 were obtained from the phase diagrams of adsorption. The gH,E values of the NBA-DTAC system are more negative than those of the TBA-DTAC system. It is concluded that the molecular interaction between NBA and DTAC is stronger than that between TBA and DTAC. With respect to the mean molar area in the adsorbed film A vs XH2 curve at a fixed surface tension gamma, the criterion of ideal mixing is demonstrated to be a straight line connecting the pure values. The A vs XH2 curves are convex downward in both systems and the deviation from the straight line is attributed to the dependence of gH,E on gamma. Copyright 1998 Academic Press.

Nonideal Mixing of Decyl Methyl Sulfoxide and Decyldimethylphosphine Oxide in Adsorbed Films and Micelles.

The surface tension of aqueous solution of decyl methyl sulfoxide (DeMS) and decyldimethylphosphine oxide (DePO) mixture was measured as a function of the total concentration of the surfactants and the composition in the surfactants. The compositions of the adsobed film and micelle were evaluated by applying thermodynamic equations to the surface tension and the phase diagrams of adsorption and micelle formation were drawn. The DeMS and DePO mixture showed a negative deviation from the ideal mixing in adsorbed film and a positive deviation in micelles. The activity coefficients of surfactant in the adsorbed film and micelle were calculated from the phase diagrams and the nonideality of mixing in the adsorbed film and micelle was clarified in terms of the excess thermodynamic quantities of adsorption and micelle formation. Judging from the negative values of the excess Gibbs free energy and area of adsorption and a positive one of the excess Gibbs free energy of micelle formation, it was concluded that the difference in head group between DeMS and DePO molecules causes a favorable packing in the adsorbed film and an unfavorable one in the micelle. Copyright 1998 Academic Press.

Nonideal Mixing in Adsorbed Film and Micelle of Ionic-Nonionic Surfactant Mixtures

The surface tension of aqueous solutions of dodecylammonium chloride-octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide-OMS mixtures was measured as a function of the total molality of surfactants and the composition of OMS. The results were analyzed according to our thermodynamic procedure; the phase diagrams of adsorption and micelle formation and the diagram illustrating the composition relation between the micelle and the adsorbed film at the critical micelle concentration were drawn. The activity coefficient and excess Gibbs energy in the adsorbed film and the micelle were defined and then evaluated from the phase diagrams. They were found to give useful information on the deviation from ideal mixing and intermolecular interaction in the adsorbed film and micelle. Copyright 1997 Academic Press. Copyright 1997Academic Press

Extremely Strong Interaction of Sodium Decyl Sulfate and Decyltrimethylammonium Bromide in Molecular Aggregates

The thermodynamic behavior of a mixture of sodium decyl sulfate (SDeS) and decyltrimethylammonium bromide (DeTAB) in aqueous solution and in molecular aggregates such as surface adsorbed films and micelles was investigated by measuring the electric conductivity and surface tension of the aqueous solutions. The thermodynamic quantities in solution and those in the molecular aggregates were evaluated from the experimental conductivity and surface tension data. The results for molar conductivity showed that dimerization or ion-pair formation of the SDeS and DeTAB molecules does not occur in aqueous solution and the mixture behaves as uni-univalent strong electrolyte below the critical micelle concentration (CMC). Contrary to the results in the aqueous solution, we found significant nonideal behavior in the phase diagrams of surface adsorption; that is, equimolar mixture of SDeS and DeTAB exists in the adsorbed film at almost all compositions irrespective of the bulk composition in the solution. The same result was also observed in the phase diagram of micelle formation. There was no difference in phase diagrams between surface adsorption and micelle formation at the CMC. The great nonideal mixing of SDeS and DeTAB in the molecular aggregates is undoubtedly attributable to the extreme attractive interaction between oppositely charged polar head groups of surfactants as well as to cohesion between hydrophobic groups. Further, in a low concentration region, it turned out that equimolar composition is preserved in the film during phase transition of the mixed adsorbed film of SDeS and DeTAB from a gaseous state to an expanded state.

Thermodynamic Study on the Surface Formation of the Mixture of Water and Ethanol

The formation of the water + ethanol mixture/air surface was studied by measuring the surface tension as a function of temperature and the mole fraction of ethanol in the mixture. The surface composition, the entropy Deltas , and energy Deltau of surface formation were evaluated by adopting the thermodynamic equations. It was shown that the surface was enriched in ethanol molecules compared with the bulk solution. The values of Deltas and Deltau were examined closely by estimating the contribution from the nonideal mixing in the bulk solution and that at the surface to Deltas and Deltau . It was concluded that the surface formation is driven by entropy at lower x 2 and by energy at higher x 2 , respectively, and that the surface formation becomes more energetically favorable with rising temperature.