A dynamic light scattering study of hydrogels with the addition of surfactant: a discussion of mesh size and correlation length

Abstract

We report a study of the interaction of sodium dodecyl sulfate (SDS) with hydrogels made of hydroxyethylcellulose (HEC) and hydrophobically modified hydroxyethylcellulose (HMHEC) by means of dynamic light scattering. The results reveal that the interaction between the hydrogels and SDS only affects the cooperative diffusion, while the slow relaxation of the hydrogels was not influenced by the interaction. Gel swelling due to the presence of SDS is also examined by measuring the weight change of the gel. The main observation is that when the gel swells due to increasing SDS concentration, the cooperative diffusion becomes faster. As such, we raise the question of whether the correlation length obtained from DLS is still proportional to the mesh size inferred from gel swelling. In the discussion, we attribute the cause to the quantity of bound surfactants. The mutual diffusion of SDS micelles could be detected at high SDS concentrations, and the diffusion coefficient of SDS in the gel matrix was determined. HEC (or HMHEC) gel swells due to increasing SDS concentration, while the cooperative diffusion becomes faster. This indicates that the HEC (or HMHEC) gel can swell until the osmotic pressure equals the modulus when the charge effect dominates over the effect of configurational entropy for uncharged gels interacting with charged surfactants.