Sune Backlund

Solubilization of alcohols and alkanes in aqueous solution of sodium dodecyl sulfate

Abstract The distribution coefficient of some alcohols and alkanes between water and micelles at 298.15 K have been calculated from solubility measurements in aqueous solutions of sodium dodecyl sulfate (NaDDS) with and without added salt. The total solubilities of alcohols and alkanes in aqueous solutions of NaDDS increase with the solubility in the aqueous medium. Simultaneously the distribution coefficients decrease. Addition of salt leads to a decrease in the total solubilities and a corresponding increase in the distribution coefficients. The results suggest that the solubility of the alcohol or alkane in the aqueous phase as well as the micellar aggregation number play a significant part in the solubilization process. The localization of the solubilizate in the micelles have been briefly discussed, and it is suggested that the alcohols are solubilized both at the micellar surface and in the micellar interior.

Aggregation of tetradecyltrimethylammonium bromide in water, 1,2-ethanediol, and their mixtures

Abstract The formation of surfactant aggregates of tetradecyltrimethylammonium bromide in water, 1,2-ethanediol, and their mixtures was monitored by conductivity and density measurements. In addition, phase diagrams were determined for the systems. In water, the surfactant molality at the critical micellar concentration, CMC, is 4 mm, while, e.g., in a solvent mixture with a mass fraction of 0.40 of water and 0.60 of 1,2-ethanediol, the molality has risen to 20 mm. The phase diagrams reveal the formation of mesophases even in pure 1,2-ethanediol, showing that the surfactant aggregates in this solvent; however, no evidence for micelle formation in the solution phase was found. The decreasing sizes of the existence regions for the mesophases with increasing mass fraction of 1,2-ethanediol were found to correlate with the increasing molality at the CMC. The phenomenon is discussed in terms of the lowered solvophobic interaction; this has been directly monitored through measuring the interfacial tension between water-1,2-ethanediol mixtures and dodecane.

Shape transitions in the aqueous phase of the system hexadecyltrimethylammonium bromide—hexanol—water

Abstract Viscosity, conductivity, density, and ultrasonic velocity measurements have been used to characterize aqueous solutions of hexadecyltrimethylammonium bromide and hexanol, i.e., the L 1 phase of the system. At surfactant molalities below 0.055 m , conductivity and ultrasonic velocity measurements show that a change in the solubilization pattern takes place as hexanol is added. It seems that above a certain hexanol content, the additive is solubilized not only in the palisade layer but also in the interior of the micelles, thus producing swollen micelles. The viscosity remains practically constant as hexanol is added. At surfactant molalities above 0.055 m the relative viscosity increases drastically above a certain hexanol content and the conductivities show a maximum at this point. This suggests a shape transition from spherical to larger rod- or disklike micelles. At surfactant molalities between 0.055 and 0.09 m hexadecyltrimethylammonium bromide, the viscosities start to decrease at even higher hexanol contents. The conductivity and ultrasonic velocity measurements also show that a change in solubilization pattern takes place at this hexanol content. It seems likely that hexanol becomes solubilized in the micellar interior and that a breakdown from large aggregates to spherical swollen micelles takes place. It seems that the L 1 phase of the system hexadecyltrimethylammonium bromide—hexanol—water can be divided into four structurally different regions; one of monomeric species, one of ordinary spherical micelles, one of swollen spherical micelles, and one of rod- or disklike micelles.

The effect of additives on the micellar systems as studied by membrane osmometry

Abstract The effect of the addition of n-propanol to various micellar systems (e.g., sodium dodecyl sulfate, cetyltrimethylammonium bromide, sodium laurate) in aqueous media was studied. The critical micelle concentration (CMC) as well as the micellar molecular weight (number average as determined by membrane osmometry) were determined as a function of the concentration of n-propanol. The analysis of the data indicated that the additive, i.e., n-propanol, does not take part in the micelle formation. The second virial coefficient changed sign (i.e., from positive to negative) when the propanol concentration was greater than ca. 0.8 mole kg−1, in all the systems studied. The data also showed that aggregation number (number average) decreased with increasing concentration of propanol, until at a concentration of ca. 20% (weight percent) no micelles are formed.

Solubilization of heptanols and α,ω-alkanediols in aqueous solutions of sodium dodecyl sulfate

Abstract The distribution coefficients of 1- to 4-heptanol and 1,8-octanediol, 1,9-nonanediol, and 1,10-decanediol between micelles and water have been determined from solubility measurements at 298.15 K. The surfactant was sodium dodecyl sulfate. The data show a negative correlation between the distribution coefficient and the solubility of the alcohol or diol in water: the lower the solubility in pure water, the higher the distribution coefficient. The total solubilities of the alcohols and diols increase with the sodium dodecyl sulfate (NaDDS) content, but for 1-heptanol a maximum is reached at a NaDDS content of about 0.04 m , suggesting a change in the solubilization mechanism. The localization of the solubilizate in the micelles has also been discussed. The data suggest that 3- and 4-heptanol and the diols are solubilized at the micellar surface only, while 2-heptanol and especially 1-heptanol are solubilized both at the surface and in the interior of the micelles.

Selective enzymatic reactions using microemulsion-based gels

Abstract Aliphatic acids were allowed to react at 298.2 K with racemic 2-octanol catalysed by the commercial lipase of Chromobacterium viscosum immobilized in microemulsion-based gelatin gels. The lipase was solubilized in microemulsions having different microstructures: a water-in-oil microemulsion based on waterhexanesodium 1,4-bis(2-ethylhexyl)sulfosuccinate and a microemulsion having bicontinuous structure based on water/ethanol-hexadecane-soybean lecithin. The syntheses were carried out in various hydrophobic solvents using different initial reactant concentrations. The immobilized lipase exhibits stereoselective action which, however, decreases with use. Enantiomeric excess values in this work are the same or somewhat higher compared with syntheses performed in organic solvents.

Solubilization of 1-hexanol in hexadecyltrimethylammonium bromide, sodium dodecyl sulfate, and sodium decanoate

The solubility of hexanol has been measured in aqueous solutions of hexadecyltrimethylammonium bromide (HTAB), sodium dodecyl sulfate (NaDDS), and sodium decanoate (NaC10). In the HTAB and NaDDS solutions the hexanol solubility decreased abruptly above a surfactant content of about 0.05 mole kg−1, suggesting a structural change from spherical to anisodiametric micelles. The total solubility of hexanol shows that it must be solubilized both in the palisade layer and in the micellar interior of these spherical micelles. After the structural change has taken place less hexanol is solubilized so that all can be accomodated in the palisade layer. No decrease in solubility has been observed for the NaC10 system, and the total hexanol solubility is so small that all can be accommodated in the palisade at all NaC10 concentrations. ΔHo and ΔSo of solubilization for spherical micelles have been calculated from the temperature dependence of the distribution coefficient.

Aggregation behavior of ionic surfactant micelles in aqueous alcoholic solutions at different temperatures

In some recent studies Vikingstad has pointed out that when water-soluble alcohols are dissolved in aqueous micellar solutions, most of the alcohol molecules are solubilized in the micelles, thereby decreasing the aggregation number. A literature study has been made on the effect of different n-alcohols on either the average micellar molar mass, M, or the average aggregation number, N. The conclusion is that addition of small amounts of alcohol may increase the aggregation number. Larger amounts of water-soluble alcohols (methanol, propanol) seem to decrease N, while sparingly soluble alcohols (octanol, dodecanol) seem to increase N. With very large amounts of water-soluble alcohols there is no evidence of micelle formation.

Lipase-catalyzed enantioselective esterifications using different microemulsion-based gels

Chiral esters with high optical purity have been synthesized at 298.2 K from racemic 2-octanol and alkanoic acids using the commerical lipases fromChromobacterium viscosum (CV) orCandida sp. (SP 525) immobilized in microemulsion-based gelatin gels. The microemulsions consisted of water and alkanes stabilized by the anionic surfactant sodium 1,4-bis(2-ethylhexyl) sulfosuccinate (AOT) and the naturally occurring zwitterionic surfactant soybean lecithin, respectively. The enzymes were solubilized both in water-in-oil (W/O) microemulsions and in microemulsions with a bicontinuous structure. Different microstructures of the gels were chosen since the enzyme may undergo conformational changes in different environments resulting in different catalytic efficiencies toward competing substrates. Therefore, it is of great fundamental interest to know the phase behaviour and the microstructures of the used microemulsion systems. Phase diagrams were determined at 298.2 K for the systems water-hexane-AOT and ethanol/water (1∶1)-hexadecane-soybean lecithin. The former system exhibited a large one-phase W/O microemulsion region, while in the latter a small one-phase region with bicontinuous structure was present. The kinetic enantiomeric ratios (E-values), as determined from enantiomeric excess (e.e.) values at a conversion below 0.5, were higher both in the W/O microemulsion as well as in the bicontinuous microemulsion using the SP 525 lipase, than using the CV lipase. On the other hand, the conversions were higher using gels based on W/O microemulsions (AOT stabilized) than using gels based on microemulsions with a bicontinuous structure (lecithin stabilized).

Aggregation studies in alkanoic acidalkylaminewater systems

Abstract Alkylamines show amphiphilic properties such as self-association in polar solvents and accumulation at interfaces. However, their complexation with alkanoic acids changes drastically their colloidal properties. Phase equilibria at different temperatures in the systems hexanoic acid hexylamine water, heptanoic acid heptylamine water and octanoic acid octylamine water were determined in order to elucidate the structure in the isotropic solution phases. Densities, surface tensions, electrical conductivities and viscosities were measured at 298.2 K. Self-diffusion coefficients were also determined both in the acid- and in the amine-rich part of the phase diagram in order to confirm if a component is confined to closed aggregates. The aim of this study was to contribute to a better understanding of the complexation in relation to the properties of catanionic surfactants.