Investigation of the Interaction of Ionic Surfactants with Epoxy-Based Hydrogels by SANS

We investigated the effect of two ionic surfactants on the composition and structure of hydrogels obtained by swelling an epoxy amphiphilic polymer network (APN) using a combination of gravimetry and small-angle neutron scattering (SANS). Stoichiometric epoxy APN was synthesized by the reaction of diamino and diepoxy-terminated polypropylene (POP) and polyoxyethylene (POE). Sub- and supercritical solutions of surfactants with either cationic (myristyltrimethylammonium bromide (C₁₄TAB)) or anionic (sodium dodecyl sulfate (SDS)) headgroups in heavy water were used in the preparation of the hydrogels. At supercritical concentrations, SDS exhibited a much stronger effect on the composition and structure of the hydrogels than C₁₄TAB. The details of the hydrogel structure were deduced by general analysis and fitting of the experimental SANS profiles to two model scattering functions exploiting the Percus-Yevick hard-sphere (HS) model and rescaled mean spherical approximation (RMSA), respectively. In the former model, valid for the hydrogels prepared in low concentrated surfactant solutions, spherical domains of average radius of ca. 39-45 Å from a highly swollen network dispersed in a matrix of a poorly swollen network mixed with the bound alkyl surfactant tails were revealed. In the latter model, hydrogels prepared in surfactant solutions of sufficiently high concentrations consist of surfactant micelles dispersed in a matrix of a highly swollen network. The average radii of the C₁₄TAB and SDS micelles formed in the hydrogels were ca. 28 and 17 Å, respectively. The strong binding of surfactant tails with POP chains and dragging of large amounts of highly mobile counterions inside the hydrogels were responsible for the effects observed. This study provides important information about the surfactant organization inside polymer hydrogels, which is important for their applications.