Effect of added surfactant on temperature-induced gelation of emulsions

This paper involves an investigation of the effect of added ionic surfactant on the temperature-induced gelation of oil-in-water (O/W) emulsions stabilized by a responsive copolymer. The oil phase used in this study is 1-bromohexadecane. The copolymer is poly(NIPAM-co-PEGMa) (NIPAM and PEGMa are N-isopropylacrylamide and poly(ethylene glycol) methacrylate, respectively). The lower critical solution temperature for the copolymer was 39.5 degrees C. The ionic surfactant used in this work was sodium dodecylbenzenesulfonate (NaDBS). The critical association concentration for NaDBS and poly(NIPAM-co-PEGMa) was measured at 0.30 mM using fluorescence measurements (pyrene was the probe molecule). Gelation temperatures were measured for the O/W emulsions to establish the effect of added NaDBS and copolymer concentration (Cp) on the gelation temperature (Tgel). The strength of the gels was measured using dynamic oscillatory measurements. These measurements allowed the shear modulus of the gel at Tgel to be estimated as 100 Pa. A theoretical model based on transient network theory was developed that predicts the dependence of Tgel on Cp. The study revealed that NaDBS has two effects on the overall cross-link density of the emulsion gels: it contributes a source of cross-linking via micellar cross-links and also decreases the proportion of transient cross-links due to electrostatic repulsion.