Flocculation and coalescence of oil-in-water poly(dimethylsiloxane) emulsions

The stability of poly(dimethylsiloxane) (PDMS) oil-in-water emulsions has b een investigated in the presence of added NaCl as well as in the presence o f added surfactant. The emulsions were prepared using a combination of noni onic (CxEy, x and y represent the number of methylene (C) and ethylene oxid e (E) groups, respectively) and cationic (quarternary alkylammonium) surfac tants. The droplets were observed to exhibit weak flocculation in the prese nce of high NaCl concentration (1 M). Phase separation and optical microsco pic observations revealed that the principal mechanism for emulsion destabi lization at high salt concentration was coalescence? which was accelerated at elevated temperature (50 C). The effective coalescence rate for diluted emulsions was investigated using photon correlation spectroscopy. The small effective Hamaker constant for PDMS is the primary reason for the slow rat e of coalescence observed for the emulsions at neutral pH in the presence o f NaCl. The stability of PDMS emulsions to flocculation is qualitatively si milar to that reported for low Hamaker constant dispersions (e.g, microgel particles). Addition of cationic surfactants (cetyltrimethylammonium chlori de and dodecyl dimethylbenzylammonium chloride) to the negatively charged d roplets after preparation was shown to decrease the emulsion stability once the surfactant concentration exceeded the CMC. Electrophoretic mobility me asurements showed that added cationic surfactant changed the sign of the dr oplet charge from negative to positive at concentrations well below the CMC . Charged micelles of the same sign as the droplets are electrostatically e xcluded from close approach to the droplet surface within a distance (epsil on) which results in depletion flocculation. (C) 2000 Academic Press.