Stephanie R. Dungan

Micellar properties of quillaja saponin. 2. Effect of solubilized cholesterol on solution properties

Abstract The interaction between cholesterol and the surfactant quillaja saponin has been investigated by measuring the effect of cholesterol on surface and micellar properties of quillaja saponin solutions. The aggregation properties of cholesterol in water were studied using fluorescent probe methods, with results indicating that cholesterol alone does not form micelles in aqueous solution. However, surface tension, dye solubilization, and light scattering measurements show that cholesterol and saponin mixtures do form micelles at well-defined critical micelle concentrations (cmc). The cmc for saponin solutions saturated with cholesterol was generally higher than that for saponin alone, with the extent of the increase dependent on the source — and most likely the composition — of the saponin. The addition of salt decreases the cmc values, while temperature dependence of these values is more complex. Surface adsorption studies show that cholesterol preferentially adsorbs at the air/water interface, forming a closely-packed monolayer, but that saponin can partially displace the cholesterol at high saponin concentrations. Finally, the size, intrinsic viscosity and the aggregation number of the cholesterol/saponin micelles are larger than those of saponin micelles alone, with the radius of the micelles between 20 and 40% larger at 298 K. These results indicate that cholesterol most likely solubilizes within quillaja saponin micelles, and in the process has a substantial impact on the micelle structure and the energetics of micelle formation.

Extraction of tungsten (VI), molybdenum (VI) and rhenium (VII) by diisododecylamine

Abstract The aggregation of amine-type extractants was studied in sulphuric acid media and applied to systems containing molybdenum, tungsten and rhenium. This article is mainly concerned with the micellization of diisododecylamine (DIDA) and compares the micellization of DIDA with that of two other extractants — trioctylamine (TOA) and dioctylamine (DOA). The degree of water solubility in the organic phase correlated with the degree of amine aggregation. The factors influencing the micellization of amine compounds-surfactant geometry, dipole moments, and amine concentration are described. The influence of diluent (kerosene and toluene) and long-chain alcohol on the micellization of amine compound are also discussed. The addition of octanol in the system TOA/toluene and DIDA/toluene (kerosene) results in increased micellar curvature. This increased micellar curvature causes a decrease in water solubility in the reversed micelles. Molecules of toluene penetrate deeper into the surfactant monolayers than does the nonpolar kerosene and decreases the polarity of the amine monolayers, providing stronger interactions between surfactant chains. Increasing acid concentration (0.2–2.0 M H 2 SO 4 ) leads to displacement of water molecules by acid molecules, thereby decreasing both water solubility and the distribution coefficient of metal between the organic and aqueous phases. Distribution coefficients of molybdenum, tungsten and rhenium was found to correlate with the water content in the organic phase.

Light scattering study of solubilization of emulsion droplets by non-ionic surfactant solutions

Abstract Light scattering (dynamic and static) and turbidity measurements were used to monitor the time dependence of the concentration and size of n -hexadecane emulsion droplets (initial diameter ≈ 0.43 mm) dispersed in non-ionic surfactant solutions (2 wt.% polyoxyethylene sorbitan monolaurate). The concentration of droplets decreased with time as n -hexadecane molecules were solubilized by surfactant micelles, but the droplet size distribution remained constant. The results are consistent with a mechanism in which oil molecules are exchanged between emulsion droplets and surfactant micelles at the oil/water interface. The measured solubilization kinetics appear to be dominated by interfacial transport processes, and can be predicted by an interfacial mass transfer coefficient of 9.7 × 10 −6 cm min −1 .

Oil exchange between oil-in-water emulsion droplets stabilised with a non-ionic surfactant

Abstract Differential scanning calorimetry has been used to monitor crystallization and melting of hydrocarbons in oil-in-water emulsions containing a mixture of pure n-hexadecane droplets (10 wt%) and pure octadecane droplets (10 wt%) stabilized by a non-ionic surfactant, (2 wt% polyoxyethylene sorbiton monolaurate). The crystallization and melting behavior of the emulsions changed with time in a manner consistent with exchange of oil between the n-hexadecane and octadecane droplets. Complete mixing of the oils in the droplets took over 7 days. The rate of exchange increased when additional surfactant was added to the aqueous phase of the emulsion. The droplet size of the emulsions did not change during the course of the experiment. The probable mechanism for oil exchange between the droplets involved the solubilisation of the oil in the aqueous phase by surfactant micelles.

Influence of surfactant structure on the contribution of micelles to Ostwald ripening in oil-in-water emulsions

The rate of Ostwald ripening was measured, using light scattering, in 2 wt.% and 10 wt.% decane-in-water and dodecane-in-water emulsions. Sodium dodecyl sulfate and several nonionic ethylene oxide dodecyl ethers--surfactants with tails containing 12 carbons, but with various headgroups--were used to form the emulsions. Emulsions were formed with sufficient quantities of the surfactant to saturate the droplet interfaces. The influence of surfactant micelles in the continuous phase was then explored by adding 1-5 wt.% surfactant to the water. The increase in the average droplet radius in the absence of micelles was found to agree qualitatively with Lifshitz-Slyozov-Wagner theory for the different surfactant types. The addition of micelles increased the rate of Ostwald ripening, by factors between 2 and 50, depending on the type and concentration of surfactant. However, there was no systematic correspondence between the increased rate and the equilibrium solubilization capacity of the micelles, nor was the rate decreased with increased strength of repulsive interactions between micelle and the droplet interface. It is proposed that the complex influence of surfactant on Ostwald ripening kinetics may depend on the ability of micelles to become supersaturated with oil--i.e., to incorporate solute temporarily above their equilibrium solubilization capacity.

Transport mechanisms in the micellar solubilization of alkanes in oil-in-water emulsions

Abstract The theory governing mass transfer rates associated with moving a solute from an oil droplet into an aqueous micellar solution is discussed. This theory is used to determine critical dimensionless groups that characterize transfer mechanisms, and to obtain rate equations for several limiting cases. These theoretical results are compared with experimental measurements of emulsified alkane solubilization into nonionic and anionic micellar solutions, with varying levels of convection and values of the aqueous phase viscosity. We conclude that two mechanisms are consistent with our observations: a molecular diffusive mechanism coupled with fast, irreversible reaction as proposed by (Kabalnov and Weers, Langmuir 12 (1996) 3442), or an interfacially controlled mechanism. The ability to understand and control these kinetic mechanisms is important in such processes as flavor and drug release, surfactant-based environmental remediation, detergency, multiphase chemical reactions, emulsion stability and in vivo digestion.

Contribution of Molecular Pathways in the Micellar Solubilization of Monodisperse Emulsion Droplets

It is often proposed that oil solubilization in anionic and nonionic micelles proceeds by different mechanisms, with diffusion of the oil molecule thought to control the former, and the latter interfacially controlled. In order to investigate this hypothesis, the effect of aqueous phase viscosity, salt, and surfactant concentration during the solubilization process was studied. The progressive decrease in average droplet size of nearly monodisperse emulsions during solubilization in SDS or Tween 20 micellar solutions was monitored by light scattering, and the change in turbidity was measured by UV-vis spectrophotometer. The solubilization rates were analyzed using a population balance approach to calculate the mass transfer coefficients. Increasing the aqueous viscosity by adding sucrose reduced the mass transfer coefficients of n-tetradecane and n-dodecane but had a smaller effect on n-hexadecane. The strong dependence of the solubilization rate for the shorter chain length alkanes on aqueous viscosity supported a mechanism in which the oil undergoes molecular diffusion before being taken up by micelles. The dependence of the solubilization kinetics on surfactant concentration appeared consistent with this mechanism but yielded a slower micellar uptake rate than previously predicted theoretically. As the solute chain length increased in nonionic surfactant solutions, an interfacial mechanism mediated by micelles appeared to contribute substantially to the overall rate. Addition of salt only slightly increased the solubilization rate of n-hexadecane in SDS solutions and, thus, indicated a weak role of electrostatic interactions for ionic surfactants on the overall mechanism.

Factors which affect oil exchange between oil-in-water emulsion droplets stabilized by whey protein isolate: Protein concentration, droplet size and ethanol

Abstract The ability of whey protein isolate (WPI) to enhance the exchange of oil molecules between droplets in an oil-in-water emulsion was investigated. Emulsions stabilized by WPI were prepared which initially contained a mixture of 10 wt.% n-hexadecane droplets and 10 wt.% octadecane droplets. The composition of the droplets changed with time because of interdroplet oil exchange. The extent of exchange was monitored by measuring the change in the crystallization behavior of the droplets, which is related to their composition, using differential scanning calorimetry. The rate of oil exchange increased when additional protein was added to the aqueous phase of the emulsions (0–4.4 wt.%) and was proportional to the specific surface area of the droplets (0.3–2.3 m2 g−1 of emulsion). Emulsions containing no additional WPI in the aqueous phase exhibited an appreciable rate of oil exchange. We propose that oil exchange occurs because of a dynamic equilibrium between oil in the emulsion droplets and that solubilized in the aqueous phase. The non-zero rate of oil exchange in the emulsions containing low WPI concentrations may occur because of the finite solubility of hydrocarbons in water, whereas the increase in interdroplet oil exchange with increasing protein concentration is probably because the proteins bind and transfer oil molecules. The addition of ethanol (0–30 wt.%) to the aqueous phase of the emulsions dramatically increased the rate of oil exchange, most likely due to the ability of ethanol to enhance the solubility of the hydrocarbons in water.

Interdroplet heterogeneous nucleation of supercooled liquid droplets by solid droplets in oil-in-water emulsions

The crystallization of oil droplets inn-hexadecane oil-inwater emulsions was monitored by pulsed nuclear magnetic resonance (NMR). The emulsions initially contained an equal mixture of solid droplets and supercooled liquid droplets at either 6 or 8°C. The degree of crystallization in the droplets was determined by measuring the NMR signal 30 μs after a 90o radio frequency pulse was applied to the sample. The signal from the solid droplets decayed rapidly after the radio frequency pulse, allowing the measured signal to be related to the fraction of liquid droplets. No crystallization was observed in a sample that contained only supercooled liquid droplets, but crystallization was observed when solid droplets of the same material were present. This indicated that crystallization was induced in the liquid droplets by the solid droplets and was most likely caused by interactions between solid and liquid droplets-that is, by interdroplet heterogeneous nucleation. The rate of induced crystallization decreased as the viscosity of the continuous phase was increased and the size of the droplets was increased, but was independent of droplet concentration (20–40%).

Effect of added α-lactalbumin protein on the phase behavior of AOT–brine–isooctane systems

We have found that the presence of <1 wt% of the globular protein α-lactalbumin has a significant impact on the equilibrium phase behavior of dilute sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane systems. Nuclear magnetic resonance (NMR), Karl Fischer titration, and ultraviolet spectroscopy were used to determine the surfactant, oil, water, and protein content of the organic and aqueous phases as a function of the total surfactant and protein present. As a small amount of α-lactalbumin is added to the mixture, there is a substantial increase (up to 80%) in the maximum water solubility in the water-in-oil microemulsion phase. Dynamic light scattering measurements indicate that this increase is due to a decrease in the magnitude of the (negative) spontaneous curvature of the surfactant monolayer, as droplets swell in size. As the molar ratio of α-lactalbumin to AOT surpasses approximately 1:300, the partitioning of water, protein, and surfactant shifts to the excess aqueous phase, where soluble assemblies with positive curvature are detected by dynamic light scattering. Significant amounts of isooctane are solubilized in these aggregates, consistent with the formation of oil-in-water microemulsion droplets. Circular dichroism studies showed that the tertiary structure of the protein in the microemulsion is disrupted while the secondary structure is increased. In light of these findings, the protein most likely expands to a molten-globule type conformation in the AOT interfacial environment, but does not substantially unfold to become an extended chain.