Effect of Salt on Swelling Behaviors of Thermosensitive Hydrogels: Applicability of the Nonrandom Contact Model

Abstract

Thermosensitive nanosized particle gels are prepared by precipitation polymerization of the N-vinylcaprolactam (NVCL) monomer. The photon correlation spectroscopy (PCS) technique detects a significant effect of the added sodium chloride (NaCl) on thermosensitive swelling behaviors. For theoretical treatment of the salting-out effect on the swelling behaviors, this study proposes a nonrandom contact (NRC) model that considers the occupation of salt molecules on the nearest lattice sites of gel molecules, inspired from Guggenheim's approach. To be specific, salt-dependent interchange energy ((epsilon) over tilde (salt)) is introduced to the mixing contribution term to overcome a major limitation of classical Donnan-type approaches, which is that such approaches have poor applicability of conventional ionic models to noncharged gel systems and thermosensitivity. This study then combined three closed-packed lattice models (modified double lattice (MDL) model, modified Flory-Huggins (MFH) model, and Xin model) with the NRC model to evaluate its general applicability. Remarkably improved results are observed with one additional model parameter from the NRC model when applied to the three lattice models. The calculated results show good agreement with the experimental data for thermosensitive and salt-sensitive swelling behaviors of various types of hydrogels.