Effect of surfactants on the swelling behaviors of thermosensitive hydrogels: applicability of the generalized Langmuir isotherm

Abstract

Herein, the surfactant effects on the thermosensitive swelling behaviors of nanometer-sized particle gels are investigated. N-Diethylacrylamide (NDEA) particle gels are prepared by precipitation polymerization with a neutral initiator (tert-butyl hydroperoxide (TBHPO)) to prevent neutralization of the anionic groups in the gel network by the ionic initiators with oppositely charged surfactants. A photon correlation spectroscopy (PCS) technique is utilized to detect the effect of ionic surfactants (sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB)) on the thermosensitive swelling behaviors of the hydrogel. A thermodynamic framework for describing the surfactant effect on the swelling behavior of the hydrogel is established by applying the generalized Langmuir isotherm (GLI), which considers both the binding isotherm of the surfactant and the effect of the electrical double layer of the bound surfactant on the conventional thermodynamic model. For comparison, two lattice models, the modified double lattice temperature (MDL T) and modified Flory–Huggins (MFH) models, are employed as mixing contributions and the Flory–Rehner (FR) model is used as an elastic contribution. Experimental results show an obvious increase in both the volume phase transition temperature (VPTT) and the swelling ratio of hydrogels after adding surfactants. The calculation results show that the ability to describe the VPTT and swelling ratio of various hydrogels in surfactant solutions is significantly improved when the GLI is combined with the conventional thermodynamic model.