Rheological investigation of the influence of dextran on the self-assembly of lysine derivatives in water/dimethylsulfoxide mixtures

Hybrid molecular/macromolecular gels were formulated by mixing amino-acid based gelators with dextran, a nonionic bacterial polysaccharide, in mixtures of water and dimethylsulfoxide. Gelators were hydrophobic derivatives of L-lysine in which the N alpha was acylated with dodecanoic acid and the N epsilon was acylated by benzoyloxycarbonyl or o-chloro-benzoyloxycarbonyl group. Increasing dextran concentration led to a decrease of the stationary values of storage and loss modulus of the gels. In addition, gel formation was slowed down upon increasing dextran concentration. Nevertheless, the characteristics of aggregates as well as their kinetics of growth, as deduced from rheological data, remained identical to those observed with gelator alone (mainly linear fractal aggregates and unidirectional growing mechanism) whatever dextran concentration. Raman microscopy confirmed conclusions drawn from rheological data. It was concluded that dextran macromolecules were mainly at the origin of steric hindrance with supramolecular aggregates.