Surface and interfacial tension study of interactions between water-soluble cationic and hydrophobically modified chitosans and nonylphenol ethoxylate

The interactions between two water-soluble chitosan derivatives and a non-ionic surfactant were investigated using surface and interfacial tension measurements, in the absence and in the presence of salt. Specifically, aqueous mixtures of trimethylated and hydrophobically modified cationic chitosans with a polyoxyethylene nonylphenol ether were studied. The results showed that the surface tension profile of the non-ionic surfactant was almost unaffected by chitosan and its trimethylated derivative, while a pronounced effect was observed in the case of the hydrophobically modified chitosan (HMTC), due to the presence of the hydrophobes in the polymer structure. Molar ratios estimated by tensiometry were of 45,000 molecules of surfactant to one molecule of HMTC. Inter and intramolecular associations between HMTC molecules were enhanced at high polymer concentrations, and upon addition of salt, which hindered polymer-surfactant interactions. These results suggest that hydrophobic interactions between the hydrocarbon chains of the modified polymer and the nonpolar tails of the surfactant are the dominant forces involved in aggregate formation. Low interfacial tension values at the heptane/water interface were obtained for both modified polysaccharide:surfactant systems, indicating that there is a synergy towards the reduction of interfacial tension at low concentrations and short equilibrium times. This behavior is attributed to a co-adsorption phenomenon, where the presence of strong hydrophobic interactions between the molecules enhances adsorption at the oil/water interface, compared to individual behavior.