Inhomogeneous binding of ionic surfactants to bulk gels and microgels consisting of poly(N-isopropylacrylamide)

The binding of sodium dodecylbenzene sulfonate (NaDBS) to bulk- and microgels of N-isopropylacrylamide (NIPA) was studied using wet and dry samples. The surfactant uptake by the dry gel sets in as soon as immersed in the surfactant solution, causing the scattering of surfactant molecules all over the gel phase. This makes it possible for the surfactant molecules to bind to the polymer network throughout the gel phase. In the wet gel, however, the surfactant binding begins at the surface of the gel because it already contains a saturated amount of water as the solvent. Even after a certain amount of surfactant molecules have penetrated into the gel, the binding will continue through its surface. Thus, with respect to the distribution of NaDBS concentration within the gel phase, we may predict a different surfactant-binding behavior between the dry and wet samples, regardless of whether we are dealing with a bulk gel or a microgel. In fact, both the volume phase transition temperature (T-v) and the volume at temperatures < T-v for the bulk gel to which surfactant binding took place were higher for the dry than for the wet sample. These characteristics were observed in the microgel system. In the case of the bulk gel system, the spatial distribution of bound NaDBS within the gel phase was directly monitored by microscopic spectroscopy. From these results, we concluded that the NaDBS ions inhomogeneously bind to the polymer networks not only within the bulk NIPA gel, but also within the microgel. (C) 1999 Elsevier Science B.V. All rights reserved.