Interaction of hydrophobically modified cationic dextran hydrogels with biological surfactants

Cross-linked cationic dextran hydrogels with 20-25 mol % N-alkyl-N,N-dimethylammonium chloride pendant groups were prepared, and their interaction with sodium salts of some bile acids was investigated, The binding parameters, initial ionization constant K-0, stability constant K-0 and cooperativity parameter II, were calculated according to the nearest-neighbor interaction model. The influence of the length of the alkyl substituent of the hydrogel, the ionic strength, and the bile acid hydrophobicity on the binding parameters was studied. The increase in the length of the alkyl substituent of the hydrogel strongly increases the binding constants K-0 and K, but decreases the cooperativity parameter It. This was explained by the formation of mixed micelles between pendant groups of the gel and surfactant molecules. The increase in the ionic strength leads to a significant decrease of the binding constants K-0 and K and an increase in the cooperativity parameter for the binding of the bile salts by hydrogels with alkyl substituent length less than or equal toC(4). However, ionic strength has a small influence on the parameters for bile acid binding by hydrogels with alkyl substituent as long as C-8 or C-12. We have also found that the binding constants are higher for more hydrophobic dihydroxylic bile acids than those for trihydroxylic ones, but u values are less influenced by the hydrophobicity of the surfactant. Finally, we have shown that the water uptake at equilibrium by the hydrogels decreases rather monotonically with the increase in the binding degree, and the decrease is more significant for the more hydrophobic hydrogels, both in pure water and in the presence of 10 mM NaCl.