A method was developed allowing in situ adjustment of water-in-oil-in-water double emulsion (W/O/W) morphologies by tailoring the osmotic pressure of the water phases. The control of internal droplet size is achieved by altering the chemical potential of the external and internal water phases by dissolving neutral linear polysaccharides of suitable molecular weights. As a consequence of the different chemical potentials in the two aqueous phases, transport of water takes place modifying the initial morphology of the double emulsion. Self-diffusion 1H nuclear magnetic resonance (1H NMR) was used to assess transport mechanisms of water in oil, while a numerical model was developed to predict the swelling/shrinking behavior of W/O/W double emulsions. The model was based on a two-step procedure in which the equilibrium size of a single internal water droplet was first predicted and then the results of the single droplet were extended to the entire double emulsion. The prediction of the equilibrium size of an internal droplet was derived by the equalization of the Laplace pressure with the osmotic pressure difference of the two aqueous phases, as modeled by mean-field theory. The double emulsion equilibrium morphologies were then predicted by upscaling the results of a single drop to the droplet size distribution of the internal W/O emulsion. Good agreement was found between the theoretical predictions and the measurement of double emulsion droplet size distribution. Therefore, the present model constitutes a valuable tool for in situ control of double emulsion morphology and enables new possible applications of these colloidal systems.
The foaming behavior of the anionic surfactant sodium dodecyl sulfate (SDS) has been studied in the
presence and in the absence of the nonionic polymer poly(vinylpyrrolidone) (PVP). A current model of
surfactant−polymer aggregations in bulk solution and at the air/water interface is related to the foam and
thin-film stabilities. Tensiometry, foaming tests, and a thin-film balance are used to obtain this relationship.
It is found that, at very low surfactant concentrations, where the surfactants are present as unimers in
the bulk solution, there is association between surfactants and polymer at the liquid/air surface, giving
increased foam and thin-film stabilities as compared to cases for the same surfactant concentrations but
without polymer. As the surfactants and polymers associate in the bulk solution, there is desorption of
surfactants and polymers from the surface, rendering decreases in foam and thin-film stabilities. At higher
surfactant concentrations, the bulk viscosity is significantly increased owing to the presence of both micelles
and saturated micelle−polymer complexes. Also, the surfactant surface coverage at the liquid/air surface
has reached its maximum value and is similar to that of SDS solution above the cmc when no polymer
is present. Both the increased surface viscosity and the increased bulk viscosity contribute to the observed
foam and film stabilities. In the thin-film studies, several stratification steps are observed, probably owing
to micelles that are being pushed out of the film.
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