POLYMERIC EMULSIFIERS BASED ON REVERSIBLE FORMATION OF HYDROPHOBIC UNITS

Emulsions consist of mixtures of immiscible liquids where one liquid i s finely dispersed within the continuous phase of another, They are ge nerally not thermodynamically stable: the dispersion tends to separate over time. Aqueous emulsions, widely used in food, pharmaceutical, an d many other industries, are often stabilized by block copolymers cont aining alternating hydrophilic and hydrophobic segments (typically bas ed on ethylene oxide/ propylene oxide diblock and triblock systems) th at penetrate into the oil and aqueous phase, respectively(1,2). Here w e describe a conceptually new type of emulsifier whose hydrophobic blo cks are formed spontaneously and reversibly by the complexation of hyd rophilic segments, thereby allowing the stabilizing properties of the system to be switched on and off. We illustrate this approach using a comb-type graft copolymer containing a poly(methacrylic acid) backbone and short grafts of poly(ethylene glycol), The uncomplexed polymer is hydrophilic, but acidic conditions induce the formation of hydrogen-b onded hydrophobic complexes between parts of the backbone and the graf ts. As a result, the grafted copolymer forms alternating blocks of hyd rophilic (uncomplexed) and hydrophobic (complexed) segments that stabi lize acidic emulsions, An increase in pH suppresses complex formation and thus leads to the breakup of the emulsion. Emulsion tests show tha t although the performance of the grafted copolymers is not Vet compet itive with existing emulsifiers, this approach provides an efficient s trategy for the design of fully reversible emulsifiers.