Roles of Polymer Concentration and Ionic Strength in the Deposition of Chitosan of Fungal Origin onto Negatively Charged Surfaces

This study examines the potential of fungal chitosan derived from Aspergillus niger as a sustainable alternative to traditional petrochemical-based ingredients in cosmetic products. Specifically, the research examines the solubility of fungal chitosan in aqueous solutions of varying ionic strength and its adsorption onto negatively charged surfaces that mimic human hair keratin. The adsorption behavior, water content, and frictional properties of chitosan films were evaluated using a quartz crystal microbalance with dissipation monitoring and a surface force apparatus (SFA). The findings indicated that fungal chitosan exhibits good solubility at a pH of 4.5. Conversely, the adsorption of chitosan is subject to the influence of both polymer concentration and ionic strength. At the lowest ionic strengths, a screening-enhanced adsorption process occurs as a consequence of the reduction in chitosan solubility in the presence of salt. This results in the depletion of polymer chains from the solution and their subsequent deposition. An increase in ionic strength above 15–20 mM results in a worsening of the chitosan–surface interaction, due to the simultaneous screening of both the chitosan and the surface charges. This results in a hindrance to the adsorption process. The deposited films are highly hydrated, and this hydration increases with both polymer concentration and ionic strength. Furthermore, the films exhibit a predominantly elastic behavior, and the response of the films under shear deformations shows a strong dependence on the polymer concentration. These findings contribute to the development of environmentally friendly cosmetic formulations that meet consumer demands for sustainability.