Synthesis of pH-Degradable Nonionic Surfactants and Their Applications in Microemulsions

An oil-soluble pH-degradable nonionic surfactant with poly(ethylene glycol) monomethyl ether as the hydrophile and a cyclic ketal as the hydrophobe was synthesized for use in microemulsion-based protein extraction. The surfactant solubilized water in isooctane. Dynamic light-scattering measurements showed formation of fairly monodisperse water-in-oil microemulsions of radii 4−6 nm, with very strong intermicellar attractive interactions. The ternary phase diagram for the system surfactant/water/isooctane at 23 °C consists of one-, two-, and three-phase regions as well as gel-like phases. The well-known “fish” pattern occurred for the phase diagram of temperature vs surfactant concentration at a fixed ratio of water-to-oil (1/1 g/g). The surfactant remained stable at neutral pH for several days but degraded rapidly when a mildly acidic phosphate buffer (pH = 5) was encapsulated in the water-in-oil microemulsion solution. Degradation occurred more rapidly when the microemulsion solution was brought in contact with an equal volume of pH 5 buffer solution in the presence of agitation. The encapsulation of protein (lysozyme) and its subsequent release upon contact with pH 5 buffer were observed, with 70% recovery of lysozyme mass in 0.5 h and 90% recovery in 1.0 h. The specific activity of the recovered lysozyme was within 90.4 ± 4.0% of the value for untreated lysozyme.