Hydrophobically Modified Pullulans: Characterization and Physicochemical Properties

Abstract

Pullulan was hydrophobically modified by reaction with dodecanoic acid, modulating the polymer-to-acid ratio. The resulting products, termed PULAUN, have been characterized, and their physicochemical properties in aqueous solution, at 25 °C, are reported. The hydrophobic substitution was determined by Fourier transform infrared, FTIR, and NMR analyses. NMR also quantifies the substitution degree, N. Viscosity, surface tension, quasi-elastic light scattering, QELS, and gel permeation chromatography, GPC, methods were used to characterize the solution properties of PULAUN. Depending on the polymer concentration and N, PULAUN form micelle-like aggregates. The critical micelle concentration, cmc, decreases with increasing N. Addition of sodium dodecyl sulfate, SDS, results in the formation of mixed micelles. In the investigated concentration range, no polymer−surfactant-based gels are formed by mixing PULAUN and SDS. According to NMR self-diffusion and QELS findings, the number-average aggregation numbers of PULAUN micelles, 〈Z〉, depend on the concentration and substitution degree. Large 〈Z〉 values are observed in the more substituted PULAU5.0. A very slow kinetics of adsorption at the air−solution interface (some days long) is observed for PULAU5.0. The effect, observed at concentrations below the cmc, is modulated by the concentration of hydrophobically modified polymer. The kinetics of adsorption is related to the combination of different effects. A thermodynamic model was developed to account for the above behavior. The process was rationalized by taking into account terms due to the surface spreading, self-diffusion, and elastic properties of the PULAU5.0 chains.