Light scattering from mixtures of interacting, nonionic micelles with hydrophobic solutes.

Model equations for the Rayleigh ratio and the electric field autocorrelation function are derived using thermodynamic fluctuation theory applied to crowded solute-containing micellar solutions and microemulsions with negligible molecular species and polydispersity. This theory invokes non-equilibrium thermodynamics and enforces local equilibrium between molecular solute, surfactant, and the various micellar species, in order to elucidate the influence of self-assembly on light scattering correlation functions. We find that self-assembly driven variations in the average micelle radius and aggregation number along gradients in concentration, which were previously shown to drive strong multicomponent diffusion effects expressed via the ternary diffusivity matrix [D], do not affect the scattering functions in the limit of zero local polydispersity. Hence, theoretical predictions for the Rayleigh ratio and the field autocorrelation function for ternary mixtures of solute-containing, locally monodisperse micellar solutions are identical to those developed for binary mixtures of monodisperse, colloidal hard spheres. However, self-assembly driven multicomponent diffusion phenomena are predicted to influence the thermodynamic driving forces for diffusion in these mixtures. In support of our theoretical results, measurements for the Rayleigh ratio and the field autocorrelation function for ternary aqueous solutions of decaethylene glycol monododecyl ether (C12E10) with either decane or limonene solute were performed for several molar ratios and volume fractions up to ϕ ≈ 0.25, and for binary mixtures of C12E10/water up to ϕ ≈ 0.5. Excellent agreement between our light scattering theory and experimental data is achieved for low to moderate volume fractions (ϕ < 0.3), and at higher concentrations when our theoretical results are corrected to account for micelle dehydration.

Multicomponent diffusion of interacting, nonionic micelles with hydrophobic solutes.

Ternary diffusion coefficient matrices [D] were measured using the Taylor dispersion method, for crowded aqueous solutions of decaethylene glycol monododecyl ether (C12E10) with either decane or limonene solute. The matrix [D], for both systems, was found to be highly non-diagonal, and concentration dependent, over a broad domain of solute to surfactant molar ratios and micelle volume fractions. A recently developed theoretical model, based on Batchelor's theory for gradient diffusion in dilute, polydisperse mixtures of interacting spheres, was simplified by neglecting local polydispersity, and effectively used to predict [D] with no adjustable parameters. Even though the model originates from dilute theory, the theoretical results were in surprisingly good agreement with experimental data for concentrated mixtures, with volume fractions up to φ≈ 0.47. In addition, the theory predicts eigenvalues D- and D+ that correspond to long-time self and gradient diffusion coefficients, respectively, for monodisperse spheres, in reasonable agreement with experimental data.

Mechanism of Time-Dependent Adsorption for Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles: Effect of Temperature and Acyl Chain Length.

Dynamic surface tension measurements were used to track adsorption kinetics for dilauroylphosphatidylcholine (DLPC) or dimyristoylphosphatidylcholine (DMPC) from monodisperse vesicle dispersions to an air-water interface at elevated temperatures ≥30 °C. Effects of vesicle concentration, aqueous solubility of the lipids, and temperature T on the adsorption kinetics were determined, and the controlling transport pathway was identified. Adsorption dynamics were tracked for 0.1-10 mM DLPC at 30 and 38 °C and for 1-10 mM DMPC at 30, 50, and 58 °C. Experimental results were compared to theoretical predictions for a reaction-enhanced, molecular transport mechanism, which was previously shown to effectively predict DLPC adsorption kinetics at 22 °C. At higher temperatures, for DLPC concentrations ≥0.25 mM or DMPC concentrations ≥1 mM, a weak dependence of adsorption time on concentration was observed, again consistent with the reaction-enhanced molecular pathway. Molecular release rates from vesicles increased with increasing temperature or decreasing acyl chain length. At equivalent ratios T/Tm of the dispersion temperature to the lipid chain phase transition temperature Tm, measured adsorption times for DLPC were approximately 10-fold shorter than those for DMPC, suggesting that the fluidity of the acyl tails is not the only lipid property determining adsorption rates. Despite the significant difference in aqueous solubility and chain phase transition temperature between DLPC and DMPC, the results provide further evidence for an adsorption mechanism that is well described by diffusion of molecular lipid, with rates of molecular diffusion near the interface enhanced by release from nearby vesicles.

Multicomponent Diffusion in Aqueous Solutions of Nonionic Micelles and Decane.

Taylor dispersion and dynamic light scattering techniques were used to measure the ternary diffusivity matrix [D] and the micelle gradient diffusion coefficient, respectively, in crowded aqueous solutions of decaethylene glycol monododecyl ether (C12E10) and decane. The results indicate that C12E10 diffused down its own gradient with the micelle gradient diffusivity while decane diffused down a decane gradient at a much slower rate. Furthermore, strong diffusion coupling, comprising decane diffusion down a surfactant gradient and surfactant diffusion up a decane gradient, was also observed with cross diffusivities that were on the order of or larger than the main diffusivities. Measurements of the micelle aggregation number, hydration index, and the hydrodynamic radius, obtained using both static and dynamic light scattering methods, indicate that decane-containing micelles interacted as hard spheres and had radii and aggregation numbers that increased linearly with the molar ratio of solute to surfactant. A theoretical model, developed using Batchelor's theory for gradient diffusion in a polydisperse system of interacting hard spheres, was effectively used to predict [D] with no adjustable parameters. A comparison with the theory indicates that decane diffused down its own gradient by micelle self-diffusion while surfactant diffused down a surfactant gradient by micelle gradient diffusion. It is also shown that intermicellar interactions drove decane diffusion down a C12E10 gradient by a volume exclusion effect while an increase in the micelle aggregation number and hydrodynamic radius with decane was necessary to drive surfactant diffusion up a decane gradient.

Mechanism of Time-Dependent Adsorption for Dilauroyl Phosphatidylcholine onto a Clean Air-Water Interface from a Dispersion of Vesicles.

This study focused on mechanisms of adsorption for dilauroyl phosphatidylcholine (DLPC) from a dispersion of large, unilamellar vesicles (LUVs) onto a clean air-water interface. The adsorption kinetics were tracked using dynamic surface tension measurements for 0.01-10 mM concentrations of DLPC, contained within monodisperse LUVs with mean diameters between 100 and 300 nm. Any lipid in excess of the solubility limit, determined to be 1.1(±0.7) × 10-5 mM (1.1 × 10-8 M), was assumed to be in vesicle form. The adsorption rate was found to increase with increasing lipid concentration and decreasing vesicle diameter, indicating a clear mechanistic role for the vesicles. An induction regime was observed, during which lipid adsorption occurred without significantly changing the surface tension. Pressure-area isotherm data suggested that the surface concentration at the end of this induction period was ∼50% of the concentration at saturation, with the latter estimated as 4.2(±0.7) × 10-6 mol/m2. Convection was also introduced into these experiments to probe the importance of bulk transport mechanisms to the overall kinetics. Theoretical expressions for possible contributing mechanisms and pathways, via molecular and/or vesicle transport, were developed and used to predict associated transport time scales for different scenarios. These theoretical time scales were compared to experimentally measured characteristic times for a variety of DLPC concentrations, vesicle diameters, and convection rates. For DLPC concentrations ≥0.25 mM, our results were consistent with the monolayer formation arising from a molecular transport mechanism that is enhanced by vesicle-to-monomer exchange beneath the interface. At lower concentrations, experimental rates of adsorption increased with increasing convection, and a strong effect of lipid concentration was also observed. For DLPC ≤0.25 mM, transport controlled by direct interfacial vesicle adsorption reasonably captured the observed effect of lipid concentration; however, neither monomer nor vesicle pathway mechanisms captured the influence of convection. Understanding the adsorption kinetics for such nearly insoluble surfactant systems is important in several areas, including food emulsification, foam or microbubble formulation, spray drying techniques, and therapeutics.

Extracellular fungal polyol lipids: A new class of potential high value lipids.

Extracellular fungal glycolipid biosurfactants have attracted attention because productivities can be high, cheap substrates can be used, the molecules are secreted into the medium and the downstream processing is relatively simple. Three classes of extracellular fungal glycolipid biosurfactants have provided most of the scientific advances in this area, namely sophorolipids, mannosylerythritol lipids and cellobioselipids. Polyol lipids, a fourth class of extracellular fungal glycolipid biosurfactants, comprise two groups of molecules: liamocins produced by the yeast-like fungus Aureobasidium pullulans, and polyol esters of fatty acids, produced by some Rhodotorula yeast species. Both are amphiphilic, surface active molecules with potential for commercial development as surfactants for industrial and household applications. The current knowledge of polyol lipids highlights an emerging group of extracellular fungal glycolipid biosurfactants and provides a perspective of what next steps are needed to harness the benefits and applications of this novel group of molecules.

Single-Step Partial Purification of Intracellular ß-Galactosidase from Kluyveromyces lactis Using Microemulsion Droplets.

Partial purification of ß-galactosidase from the crude extract of Kluyveromyces lactis was carried out using water-in-isooctane microemulsions formed by the anionic surfactant, sodium di-ethylhexyl sulfosuccinate (Aerosol OT). In order to obtain the crude extract, yeast cells of K. lactis were disrupted by a cell disrupter and separated. The purification of ß-galactosidase from the extract by a recently developed one-step reversed micellar (i.e., microemulsion-based) extraction method was then tested, by measuring total protein mass and enzyme activity in the product stream and by analyzing its composition using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration chromatography. Effects of salt concentration, protein concentration, and pH on the extraction were investigated. Using this approach, a 5.4-fold purification of ß-galactosidase was achieved with 96 % total activity recovery, using a feed containing crude extract and 50 mM K-phosphate buffer (pH 7.5) and 50 mM KCl. Gel filtration chromatography showed that the single extraction was successful at removing low molecular weight impurity proteins (molecular weight (MW) < 42 kDa) from the crude extract.

Effect of acidification on carrot (Daucus carota) juice cloud stability.

Effects of acidity on cloud stability in pasteurized carrot juice were examined over the pH range of 3.5-6.2. Cloud sedimentation, particle diameter, and ζ potential were measured at each pH condition to quantify juice cloud stability and clarification during 3 days of storage. Acidification below pH 4.9 resulted in a less negative ζ potential, an increased particle size, and an unstable cloud, leading to juice clarification. As the acidity increased, clarification occurred more rapidly and to a greater extent. Only a weak effect of ionic strength was observed when sodium salts were added to the juice, but the addition of calcium salts significantly reduced the cloud stability.

Multicomponent diffusion in solute-containing micelle and microemulsion solutions.

Holographic interferometry was used to obtain new results for the four coefficients that determine rates of multicomponent diffusion of hydrophobic solutes and surfactants in microemulsions. The three solutes pentanol, octanol, and heptane were examined in microemulsions formed from decaethylene glycol monododecyl ether (C12E10) and sodium dodecyl sulfate (SDS). These coefficients are compared with relevant binary and effective binary diffusion coefficients, and also with ternary diffusion coefficients reported in the literature. It is shown that a strong coupling exists between the diffusion of hydrophobic solutes and surfactant in solute-containing microemulsions. In particular, the presence of a gradient in the concentration of the solute can induce a surprisingly large flux of surfactant either up or down the solute gradient. Within the framework of irreversible thermodynamics, these results indicate that hydrophobic solute molecules significantly alter the chemical potential of the surfactant in microemulsions. These effects are present to a comparable degree for both the nonionic C12E10 and ionic SDS microemulsions.

Effect of pectin methylesterase on carrot (Daucus carota) juice cloud stability.

To determine the effect of residual enzyme activity on carrot juice cloud, 0 to 1 U/g pectin methylesterase (PME) was added to pasteurized carrot juice. Cloud stability and particle diameters were measured to quantify juice cloud stability and clarification for 56 days of storage. All levels of PME addition resulted in clarification; higher amounts had a modest effect in causing more rapid clarification, due to a faster increase in particle size. The cloud initially exhibited a trimodal distribution of particle sizes. For enzyme-containing samples, particles in the smallest-sized mode initially aggregated to merge with the second peak over 5-10 days. This larger population then continued to aggregate more slowly over longer times. This observation of a more rapid destabilization process initially, followed by slower subsequent changes in the cloud, was also manifested in measurements of sedimentation extent and in turbidity tests. Optical microscopy showed that aggregation created elongated, fractal particle structures over time.

One-step separation of ß-galactosidase from ß-lactoglobulin using water-in-oil microemulsions.

Solubilisation of ß-galactosidase from Kluyveromyces lactis in Aerosol-OT water-in-isooctane microemulsions was measured as a function of buffer type and concentration, pH, and protein concentration. For buffer concentrations above ∼40mM, the enzyme was largely excluded from the droplets. Based on these results, a one-step separation was developed. A protein-containing aqueous feed was injected into an AOT/isooctane solution, with the feed volume slightly in excess of the predicted water solubility. Impurity proteins were entrapped inside the microemulsion droplets that then formed in the organic phase, while the high MW target protein was excluded and entered a newly formed, excess aqueous phase. The separation of ß-galactosidase from the test protein ß-lactoglobulin was most complete at 100mM KCl salt concentration, where the droplets were large enough to carry ß-lactoglobulin but too small for ß-galactosidase. At 100mM [KCl], 92% of the total enzyme activity was recovered in a concentrated and virtually pure form.

Measuring local equilibrium flavor distributions in SDS solution using headspace solid-phase microextraction.

Solid-phase microextraction (SPME) sampling of the headspace above an aqueous micellar solution of sodium dodecyl sulfate (SDS) was shown to be effective for quantifying the equilibrium partitioning of limonene solute between water and SDS micellar aggregates. Concentrations in the headspace were determined from the amount absorbed by the SPME fiber during 1 min extractions, with the quantity on the fiber determined using gas chromatography/mass spectrometry (GC/MS). Headspace concentrations as a function of surfactant concentration were fit to a mass balance to yield the partition coefficient and critical micelle concentration. When the total limonene in the system was low enough that it could be completely dissolved by water in the absence of micelles, a constant value for the partition coefficient of 1700 M(-1) was obtained, independent of the limonene concentration. However, at higher total limonene concentrations, the partition coefficient became a function of the amount of limonene in the micelles, as confirmed by separate experiments in which either limonene or SDS concentration was varied. The observed increase in partition coefficient with increasing limonene likely signals a shift from micelles to swollen micelles and ultimately to microemulsion droplets. The effect of SDS concentration on the aqueous solubility limit of limonene could also be observed in HS-SPME experiments where either SDS or limonene was varied.

Measuring gas-liquid partition coefficients of aroma compounds by solid phase microextraction, sampling either headspace or liquid.

Hydrophobic compounds are important odorants and nutrients in foods and beverages, as well as environmental contaminants and pharmaceuticals. Factors influencing their partitioning within multi-component systems and/or from the bulk liquid phase to the air are critical for understanding aroma quality and nutrient bioavailability. The equilibrium partitioning of hydrophobic analytes between air and water was analyzed using solid phase microextraction (SPME) in the headspace (HS-SPME) and via direct immersion in the liquid (DI-SPME). The compounds studied serve as models for hydrophobic aroma compounds covering a range of air-water partition coefficients that extends over four orders of magnitude. By varying the total amount of analyte as well as the ratio of vapor to liquid in the closed, static system, the partition coefficient, K(vl), can be determined without the need for an external calibration, eliminating many potential systematic errors. K(vl) determination using DI-SPME in this manner has not been demonstrated before. There was good agreement between results determined by DI-SPME and by HS-SPME over the wide range of partitioning behavior studied. This shows that these two methods are capable of providing accurate, complementary measurements. Precision in K(vl) determination depends strongly on K(vl) magnitude and the ratio of the air and liquid phases.

Diffusion of sodium dodecyl sulfate micelles in agarose gels.

The gradient diffusion of ionic sodium dodecyl sulfate micelles in agarose gel was investigated at moderate concentrations above the CMC. Of particular interest were the effects of micelle, gel, and sodium chloride concentration on the micelle diffusivity. Holographic interferometry was used to measure the gradient diffusion coefficient at three sodium chloride concentrations (0, 0.03, 0.10 M), three gel concentrations (0, 1, 2 wt%), and several surfactant concentrations. Time-resolved fluorescence quenching was used to measure aggregation numbers both in solution and gel. The micelle diffusivity increased linearly with surfactant concentration at the two larger sodium chloride concentrations and all gel concentrations. In general, the strength of this effect increased with decreasing sodium chloride concentration and increased with gel concentration. This behavior is evidence of decreasing micelle-micelle electrostatic interactions with increasing sodium chloride concentrations, and increasing excluded volume effects and hydrodynamic screening with increasing gel concentration, respectively. The only exception was at 0.1M sodium chloride and 2 wt% agarose, which showed a slight reduction in the slope compared to 1 wt% agarose. It was found that the concentration effect is quite strong for charged solutes: at a NaCl concentration of 0.03 M in a 2% agarose gel, in a solution with 3% SDS micelles by volume, the micelle diffusion coefficient is doubled relative to its value in the same gel at infinite dilution. The extrapolated, infinite-dilution diffusion coefficients and the rate at which the micelle diffusivity increased with surfactant concentration were compared with predictions of previously published theories in which the micelles are treated as charged, colloidal spheres and the gel as a Brinkman medium. The experimental data and theoretical predictions were in good agreement.

Effect of emulsion drop-size distribution upon coalescence in simple shear flow: a population balance study.

A population balance is used to examine the effect of the shape of the initial drop-size distribution of an emulsion upon its short and long-time evolution in simple shear flow. Initial distributions that are monodisperse, multidisperse, lognormal, bimodal, multimodal, and step functions are considered. At short times, it is shown that the rate of coalescence decreases by up to 25% for step distributions and up to 75% for lognormal distributions as the width of the distribution increases. Bimodal, multidisperse and multimodal distributions show intermediate decreases in the rate of coalescence, between these two values, with increases in the distribution width. Furthermore, it is found that the initial rate of coalescence is strongly dependent upon the presence of large drops. As the number fraction of large droplets within the distribution increases, the rate of coalescence also increases. At long times, all distributions move toward an asymptotic distribution shape in which the frequency of drops decreases algebraically with drop diameter at small drop diameters, and decreases exponentially with drop diameter at large drop diameters. Though portions of each distribution showed the expected asymptotic scaling behavior at long times, each asymptotic distribution nevertheless retains 'fingerprints' of the respective initial distribution. Overall, the rate of coalescence for a system is bounded by the initial rate, which is a function of the initial distribution shape, and the asymptotic rate, which is dependent upon the long-time scaling behavior. Finally, it is shown that the resolution with which the drop-size distribution of an emulsion is experimentally measured can have a significant effect upon predicted rates of coalescence.

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.

alpha-lactalbumin-AOT charge interactions tune phase structures in isooctane/brine mixtures.

Self-assembly of the anionic surfactant AOT with the protein alpha-lactalbumin in isooctane/brine mixtures results in phase structures whose type, size, and shape differ considerably from those formed by the surfactant alone. Small-angle X-ray scattering was used to determine the size and shape of these structures for 5.4 < pH < 11.2 and 0.25, 0.33, and 0.4 wt % NaCl. All pH values were above the reported isoelectric point for the protein. The composition of the system (except for salt) was fixed, with 2.5 wt % surfactant in equivolume mixtures of oil and water and either 0 or 0.4 wt % protein. Under these conditions, AOT in the absence of protein always formed spherical, water-in-oil (w/o) microemulsion droplets in the organic phase with no self-assembly in the aqueous phase. In the presence of alpha-lactalbumin, self-assembled structures were formed in both aqueous and organic phases, and the size and shape of these was tuned by both pH and [NaCl]. Protein-surfactant interaction was weakest at the most alkaline pH, with protein-free, spherical droplets forming in the organic phase and surfactant-decorated soluble protein clusters forming in the aqueous phase. As pH was decreased, protein increasingly partitioned to the organic phase and droplets became ellipsoidal and much larger in volume, with these effects enhanced at lower salt concentration. Aqueous structures were also strongly affected by pH, shifting from prolate protein/surfactant aggregates at alkaline pH to oil-in-water, oblate microemulsion droplets at neutral pH. At acidic pH and higher salt concentration, self-assembly shifted toward a third, anisotropic aqueous phase, which contained discoid bilayer structures. It is proposed that hydrophobic attraction causes association of the protein with the surfactant monolayer, and pH and [salt] tune the system via the protein by modifying electrostatic repulsion and monolayer curvature.

Effect of alpha-lactalbumin on aerosol-OT phase structures in oil/water mixtures.

The ability of water-soluble, globular proteins to tune surfactant/oil/water self-assemblies has potential for the formation of biocompatible microemulsions and also plays a role in protein function at biological interfaces. In this work, we examined the effect of the protein alpha-lactalbumin on Aerosol-OT (AOT) phase structures in equivolume mixtures of oil and 0.1 M brine. In this pseudo-ternary system, surfactants are free to move to either oil or water phase to adopt phase structures close to the spontaneous curvature of the surfactants. Using small-angle X-ray scattering, we observed that addition of this protein changed the spontaneous curvature of the surfactant monolayer substantially. In the absence of protein, AOT adopted a negative spontaneous curvature to form spherical w/o microemulsion droplets. When less than 1 wt % of alpha-lactalbumin was added into the system, the w/o droplets became nonspherical and larger in volume, corresponding to an increase in water uptake into the droplets. As the protein-to-surfactant ratio increased, protein, surfactant, and oil increasingly partitioned toward the aqueous phase. There the protein triggered the formation of o/w microemulsions with a positive spontaneous curvature. These protein-containing structures exhibited significant interparticle attraction. We also compared the influence of two oil types, isooctane and cyclohexane, on the protein/surfactant interactions. We propose that the more negative natural curvature of the AOT/cyclohexane monolayer in the absence of protein prevented protein incorporation within organic phase structures and consequently pushed the system self-assembly toward aqueous aggregate formation.

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.

Solubilization in monodisperse emulsions.

The kinetics of oil solubilization into micelles from nearly monodisperse alkane-in-water emulsion droplets was investigated. Emulsions containing either hexadecane or tetradecane oils were fractionated to be narrowly distributed, using a method developed by Bibette [J. Bibette, J. Colloid Interface Sci. 147 (1991) 474]. These monodisperse emulsions were mixed with SDS or Tween 20 aqueous micellar solutions of various concentrations. Time-dependent solubilization was monitored using light scattering and a decrease in average droplet size over time was observed, in contrast to what has been observed previously with polydisperse emulsions. The rate at which the droplet size decreased was found to be independent of the initial droplet size. Turbidity measurements were also used to track the solubilization kinetics, and a population balance analysis used on both types of measurements to extract effective mass transfer coefficients. The dependence of these transfer coefficients on droplet size, alkane type, surfactant type and concentration provide insights into plausible mechanisms of emulsion droplet solubilization within micellar solutions.