Investigations of the solubilization kinetics of binary mixtures of non-polar oils by non-ionic surfactants

Solubilization rates of oils in surfactant solutions and their relationship to mass transport in emulsions

Information on solubilization rates of oils in aqueous micellar solutions is reviewed. For ionic surfactants electrostatic repulsion prevents close approach of micelles to the oil-water interface, so that solubilization results from oil molecules dissolving individually in the solution and being taken up by micelles during and/or after transport across a diffusion boundary layer to the bulk solution. Experiments with SDS solutions and single oil drops having low (but not negligible) solubility indicate that mass transfer is often not rate-controlling. Instead phenomena near the oil-water interface including, but not limited to, the rates of micellar uptake of oil from the aqueous solution seem to control the solubilization rate. In contrast, Ostwald ripening experiments involving multiple oil drops in SDS solutions are often interpreted in terms of molecular dissolution and diffusion alone since ripening rates are typically only slightly different from those observed in the absence of surfactant micelles, where this mechanism is considered to hold. For many nonionic surfactant systems and oils of low or negligible solubility the principal mechanism of solubilization is incorporation of surfactant at the oil-water interface from micelles, which coalesce or "adsorb" at the interface or else dissociate nearby, permitting individual surfactant molecules to be adsorbed. Subsequently the excess surfactant is emitted as oil-containing micelles. Most experiments have indicated that this process, which appears in the analyses as an interfacial resistance, is rate controlling. New results are presented here supporting this model and showing that resistance to mass transfer is often quite low because natural convection can arise near an oil drop owing to the density change produced by solubilized oil in micelles near the drop surface. Provided that polydispersity of drop sizes is properly accounted for, experiments on solubilization and compositional ripening in emulsions stabilized with nonionic surfactants can be interpreted using the interfacial resistance model with values of resistance obtained from single-drop experiments. However, it is unclear whether mass transfer, interfacial resistance or perhaps some combined mechanism controls the rate of Ostwald ripening. One uncertainty limiting predictions of the interfacial resistance model is the lack of information on the oil-to-surfactant ratio in micelles when the concentration of individually dissolved oil molecules slightly exceeds the equilibrium value for a plane oil-water interface, the situation during Ostwald ripening.

Degradation of kinetically-stable o/w emulsions

This article summarizes the studies on the degradation of the thermodynamically unstable o/w (nano)emulsion--a dispersion of one liquid in another, where each liquid is immiscible, or poorly miscible in the other. Emulsions are unstable exhibiting flocculation, coalescence, creaming and degradation. The physical degradation of emulsions is due to the spontaneous trend toward a minimal interfacial area between the dispersed phase and the dispersion medium. Minimizing the interfacial area is mainly achieved by two mechanisms: first coagulation possibly followed by coalescence and second by Ostwald ripening. Coalescence is often considered as the most important destabilization mechanism leading to coursing of dispersions and can be prevented by a careful choice of stabilizers. The molecular diffusion of solubilizate (Ostwald ripening), however, will continuously occur as soon as curved interfaces are present. Mass transfers in emulsion may be driven not only by differences in droplet curvatures, but also by differences in their compositions. This is observed when two or more chemically different oils are emulsified separately and the resulting emulsions are mixed. Compositional ripening involves the exchange of oil molecules between emulsion droplets with different compositions. The stability of the electrostatically- and sterically-stabilized dispersions can be controlled by the charge of the electrical double layer and the thickness of the droplet surface layer formed by non-ionic emulsifier. In spite of the similarities between electrostatically- and sterically-stabilized emulsions, there are large differences in the partitioning of molecules of ionic and non-ionic emulsifiers between the oil and water phases and the thickness of the interfacial layers at the droplet surface. The thin interfacial layer (the electrical double layer) at the surface of electrostatically stabilized droplets does not create any steric barrier for mass transfer. This may not be true for the thick interfacial layer formed by non-ionic emulsifier. The interactive sterically-stabilized oil droplets, however, can favor the transfer of materials within the intermediate agglomerates. The stability of electrosterically-stabilized emulsion is controlled by the ratio of the thickness of the non-ionic emulsifier adsorption layer (delta) to the thickness of the electrical double layer (kappa(-1)) around the oil droplets (delta/(kappa(-1))) = (deltakappa). The monomer droplet degradation can be somewhat depressed by transformation of coarse emulsions to nano-emulsion (miniemulsion) by intensive homogenization and by the addition of a surface active agent (coemulsifier) or/and a water-insoluble compound (hydrophobe). The addition of hydrophobe (hexadecane) to the dispersed phase significantly retards the rate of ripening. A long chain alcohol (coemulsifier) resulted in a marked improvement in stability, as well, which was attributed to a specific interaction between alcohol and emulsifier and to the alcohols tendency to concentrate at the o/w interface to form stronger interfacial film. The rate of ripening, according to the Lifshitz-Slyozov-Wagner (LSW) model, is directly proportional to the solubility of the dispersed phase in the dispersion medium. The increased polarity of the dispersed phase (oil) decreases the stability of the emulsion. The molar volume of solubilizate is a further parameter, which influences the stability of emulsion or the transfer of materials through the aqueous phase. The interparticle interaction is expected to favor the transfer of solubilizate located at the interfacial layer. The kinetics of solubilization of non-polar oils by ionic micelles is strongly related to the aqueous solubility of the oil phase (the diffusion approach), whilst their solubilization into non-ionic micelles can be contributed by interparticle collisions.

A novel cloud-point extraction process for preconcentrating selected polycyclic aromatic hydrocarbons in aqueous solution

A novel but simple cloud-point extraction (CPE) process is developed to preconcentrate the trace of selected polycyclic aromatic hydrocarbons (PAHs) with the use of the readily biodegradable nonionic surfactant Tergitol 15-S-7 as extractant. The concentrations of PAHs, mixtures of naphthalene and phenanthrene as well as pyrene, in the spiked samples were determined with the new CPE process at ambient temperature (23 degrees C) followed by high performance liquid chromatography(HPLC) with fluorescence detection. More than 80% of phenanthrene and pyrene, respectively, and 96% of naphthalene initially present in the aqueous solutions with concentrations near or below their aqueous solubilities were recovered using this new CPE process. Importantly Tergitol 15-S-7 does not give any fluorometric signal to interfere with fluorescence detection of PAHs in the UV range. No special washing step is, thus, required to remove surfactant before HPLC analyses. Different experimental conditions were studied. The optimum conditions for the preconcentration and determination of these selected PAHs at ambient temperature have been established as the following: (1) 3 wt% surfactant; (2) addition of 0.5 M Na2SO4; (3) 10 min for equilibration time; and (4) 3000 rpm for centrifugal speed with duration of 10 min.

Solubilization kinetics of a triglyceride/n-alkane mixture in a non-ionic surfactant solution

The kinetics of solubilization in a non-ionic surfactant solution of pure n-hexadecane, pure triolein and a mixture of the two oils have been investigated. The rates were determined from observations of the dissolution of a single, microscopic droplet, using a drop-on-fibre technique. Conditions were such that negligible saturation of the micellar surfactant with solubilizate occurred. The rate of solubilization of each of the pure oils was found to be independent of time, hexadecane solubilization on a volume basis being about 8 times as rapid as that for triolein. The rate of solubilization of a mixture of the two oils, having a volume fraction 0.69 for triolein, was found to vary in the medium term, the rates apparently being steady both initially and (at a lower value) in the long term. These observations can be related quantitatively to the rates found for the pure oils.