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A commonly stated requirement for the preparation of stable Langmuir monolayers of amphiphilic molecules at an air/water interface is that the surfactant must be insoluble in the subphase solution; however, a few prior studies have reported that some soluble surfactants can, under certain conditions, be compressed. The anomalous compression of soluble amphiphiles is extremely interesting and important, as it presents the possibility of greatly increasing the number of candidate compounds suitable for Langmuir monolayer studies and Langmuir-Blodgett deposition. The aim of this work was to obtain a better understanding of the factors that determine whether monolayers of a given water-soluble surfactant can be compressed. A series of amine oxide surfactants, including a novel gemini surfactant, were studied to explore the relationship between molecular structure and behavior at the air/water interface. Amine oxides are an especially interesting class of surfactants because their self-assembly in solution and at interfaces is pH-sensitive. Surface pressure-area isotherms show that the solubility of a surfactant in the subphase solution is not, in and of itself, a useful parameter in predicting whether the monolayer is compressible. Molecular modeling calculations suggest that the tendency of molecules to self-assemble plays a much more important role than solubility in this regard. The effect of pH was also investigated. We present a hypothesis that formation of dimers or small clusters of molecules at the interface inhibits the dissolution of these species into the subphase, and as a consequence the monolayer can be compressed.

The phase sequence of the N-dodecyl-N-octyl-N-methylamine oxide (C12C8MAO)/HCl/water system with increasing apparent degree of protonation, X, defined as [HCl]/[C12C8MAO], has been studied. For a 100 mM concentration of C12C8MAO the following sequence of phases has been observed: L1/L2, L1/Lalpha/L2, L1/Lalpha, Lalpha, Lalpha/L2. The single-phase Lalpha region begins at X = 0.007 and ends at X = 0.35. The upper phase boundary, X*, depends strongly on the acid that is used for the protonation of the surfactant. It is shifted for increasing hydrophilicity of the acid to higher X values. For formic acid X* = 0.95, and for HBr X* = 0.05. A weakly protonated 1% solution of the surfactant is an iridescent Lalpha phase. Both unilamellar vesicles and multilamellar vesicles are observed in cryo transmission electron microscopy and freeze fracture transmission electron microscopy images in the Lalpha phase. The phase sequence with protonation differs from that of single-chain amine oxide surfactants. The synergism between the protonated and the nonprotonated species is very weak in the range X < X*, while the transition from the Lalpha phase to the Lalpha/L2 two-phase region is considered to be due to synergism. Little or no synergism is observed regarding the surface tension, but synergism does appear in the interfacial tension between decane and the aqueous solution. The viscoelastic properties of the vesicle/Lalpha phase resemble those of densely packed hard spheres. The effects of electric charge on the elastic property of the vesicles could be understood in terms of the osmotic pressure of the solutions. The interlamellar spacing evaluated by small-angle X-ray scattering showed a minimum around X approximately 0.1, which is interpreted as a result of two opposing contributions. One contribution is the suppression of undulation of bilayer membranes by introduction of electric charges, and the other comes from the increasing total bilayer thickness due to the increasing hydrogen bond formation with increasing X.

Lyotropic phase behavior of the nonionic and the half-ionized oleyldimethylamine oxide (OlDMAO)/water systems was investigated using polarized light microscopy, small-angle X-ray diffraction, and differential scanning calorimetry. Nonionic OlDMAO formed isotropic micellar solution, nematic, hexagonal, cubic, and lamellar liquid crystalline phases as the surfactant concentration increased. In contrast, half-ionized OlDMAO (i.e., 1:1 mixture of the nonionic and the protonated species) had a greater tendency to form bilayer structures, and the phase diagram became quite similar to those of double-chained ionic surfactants rather than single-chained ones, despite the introduction of positive charges to the nonionic one. The preference of the bilayer structures in the half-ionized OlDMAO was interpreted in terms of the dimers stabilized by the hydrogen bond between the nonionic and protonated species. For alkyldimethylamine oxides with a saturated hydrocarbon chain (CnDMAO, chain length: n = 14, 16, and 18), the phase sequence of lyotropic liquid crystals was hardly affected by the protonation, but an elongation of the cylinders of the hexagonal phase was observed for the half-ionized C14DMAO. Consequently, it can be considered that the dominant bilayer formation of the half-ionized OlDMAO is attributed to the combined effect of the hydrogen-bonded dimer formation and the cis-double-bond configuration of the alkyl chain.

The effect of adding a small amount of dodecyl dimethylamine oxide (DDAO) on adsorption on silica from an aqueous solution of dodecyl maltoside (C12G2) has been investigated. The C12G2 itself does not adsorb significantly on silica at any concentration. DDAO on the other hand readily adsorbs in a bilayer-like structure at concentrations approaching the critical micelle concentration (cmc), but the adsorbed amount at the concentrations it has been applied in these mixtures is small. In contrast, by combination of the two surfactants, significant adsorption is observed at concentrations where the adsorption of the pure DDAO, as well as pure C12G2, is very low. We thus see a strong enhancement of the adsorption from the mixed system. The adsorption is suggested to be a two-step process, where individual DDAO molecules first adsorb to the silica surface through electrostatic interactions, and then C12G2 adsorbs at the hydrophobic sites the DDAO tails constitute through hydrophobic bonding. A minimum concentration of DDAO is required to induce adsorption from a solution with constant C12G2 concentration. This concentration is lower for C12G2 solutions below and equal to cmcC12G2 than above cmcC12G2. In addition, the total adsorbed amount shows a maximum around cmcC12G2 of the mixture for a solution with low DDAO contents. Both these effects are explained by incorporation of DDAO in the mixed micelles above cmc(mix), which leads to a desorption of DDAO from the surface.

We have used streaming potential measurements to determine the ζ-potential of amorphous, hydrophilic, mesoporous silica gel in aqueous nonionic−cationic solutions of the amphoteric alkyldimethylamine oxide surfactants (CnDAO with n = 8, 10, and 12) and alkyldimethylphosphine oxide surfactants (CnDPO with n = 10 and 12). The effect of different mixtures of the nonionic and cationic (protonated) forms of the amine oxide surfactant on the ζ-potential was studied at natural pH and at pH = 4. The nonionic phosphine oxide homologues were chosen as references. The surfactant contents ranged from dilute solutions to well above the cmc. The ζ-potential isotherms of the alkyldimethylamine oxides featured break points at the critical surface aggregation concentration (CSAC), recognized as the onset of counterion association with formed surface aggregates. The surface aggregates of all alkyldimethylamine oxide surfactants studied are charged at the cmc. At natural pH evidence of ζ-potential reversal was found as a re...

The method of Rathman and Christian for determining the micellar activities of dimethyldodecylamine oxide by pH titration in the presence of swamping electrolyte has been extended to accommodate nonswamping electrolyte conditions. Dimethyltetradecylamine oxide (AO±) was used as a model surfactant in this study. The effect of the added chloride on the critical micelle concentration (cmc) values of HAO+ and AO± was experimentally determined and incorporated into the titration method. Under the experimental conditions used, the change in the cmc value of AO+ was negligible while the cmc of HAO± changed logarithmically with increasing chloride concentration (slope = −0.58). Results show a dependency of the interaction parameter, β, between AO± and HAO+ on the ratio of the initial chloride to surfactant concentrations. The pKa and cmc values are also reported. To overcome the need for experimentally determining the change in the cmc value of HAO+ with respect to the added chloride concentrations, an approximat...

To study the electric properties of adsorption monolayers of an amphoteric surfactant, in our case lauryl dimethyl amine oxide (LDAO), we measured the ζ-potential of latex particles covered with this surfactant. Its adsorption on the latex particles is estimated by dynamic surface tension measurements coupled with light scattering experiments. The influence of pH, ionic strength, and surfactant concentration on the electric properties of the adsorption monolayer has been examined. We propose a theoretical model which describes the dependence of ζ-potential on both pH and the ionic strength. The model accounts for the adsorption of H+, Na+, and Cl- ions on the amphoteric surfactant headgroups. Very good agreement between theory and experiment is achieved. The ζ-potential measurements, coupled with the theoretical model, can be considered as a method for quantitative characterization of the ionization properties of amphoteric surfactants in adsorption layers. The latter properties are, in general, different...

The synthesis of N,N-dimethyldodecylamine N-oxide in aqueous solutions by micellar autocatalysis was investigated. Micellar autocatalysis is a novel variation of conventional micellar catalysis in which surfactant micelles catalyze the reaction by which the surfactant itself is synthesized. The lipophilic reactant, dimethyldodecylamine, was initially solubilized in micellar solutions of the amine oxide surfactant, resulting in substantially higher reaction rates. Amine conversions of 90-100% were obtained within 2 h at 70 °C. The effects of reactant concentrations, temperature, and initial surfactant concentration were studied. For systems with no surfactant at time zero, the system was initially an emulsion and reaction rates were low. A sharp increase in the rate was observed when enough surfactant had been produced to form micelles. Activation energy calculations indicate that enhancements of the rate were due primarily to the localized concentration of reactants in the micelle. A simple pseudophase model was used to model reactions under pseudo-first-order reaction conditions.

The apparent molar volumes and heat capacities of n-octyldimethylamine oxide hydrochloride (OAOHCl) were measured in water as a function of concentration between 2 and 55 °C and the apparent molar relative enthalpies of the same system at 25°C. The apparent molar expansibilities were calculated from the temperature dependence of the volumes. The volumes and heat capacities were also measured at 25°C for trimethylamine oxide hydrochloride in water in order to check the group additivity. The data for OAOHCl were fitted with a mass-action model (J. Phys. Chem. 1983, 87, 1397) and compared with those of n-octyldimethylamine oxide (OAO) and n-octylammonium bromide (OABr) in order to investigate the effect of the charge on the thermodynamic micellar properties. A comparison between the properties of OAOHCl and OABr shows that OAOHCl behaves as a typical cationic surfactant. The differences in the thermodynamic properties of micellization between OAO and OAOHCl are as expected from a stronger hydration in the premicellar region and counterion binding beyond the critical micellar concentration (cmc) of OAOHCl. The temperature dependence of most micellar properties is comparable for the two surfactants. As it is generally observed with ionic surfactants, the enthalpy of micellization of OAOHCl is endothermic at low temperature and exothermic at high temperature and the cmc goes through a minimum at about 35°C. However, the cmc of both surfactants at 25 °C are of the same magnitude. From the mass-action model, the thermodynamic properties of the surfactant can be predicted at any temperature and concentration in the range measured.

The influence of the nonionic water-soluble polymers poly(vinyl methyl ether) (PVME), poly(propylene oxide) (PPO), and poly(ethylene oxide) (PEO) on the aggregation behavior of n-dodecyldimethylamine oxide (DDAO), at various stages of protonation, has been studied. Critical micelle concentration (cmc) values were determined by the pH method and revealed an increase in stabilization of the micelles by association with PVME and PPO, upon increasing the average charge of the surfactant. The micelles formed from nonionic DDAO are not stabilized by association with PVME or PPO, but association was apparent from the reduction in aggregation number. This reduction in aggregation number is even more pronounced at higher surfactant charge. The results are interpreted in terms of a reduction in electrostatic inter-head-group interaction upon formation of the smaller polymer-bound micelles in the case of the charged surfactant molecules. PEO does not exert any influence on either the cmc or the aggregation number of DDAO micelles at any degree of protonation indicating the absence of polymer-micelle interaction. The effect of neutral and protonated DDAO on the clouding behavior of PVME and PPO has also been studied. 36 refs., 4 figs., 4 tabs.