Gelatin Adsorption onto Cellulose Nanocrystals Surfaces at Different pH: A QCM-D Study

By exploiting the pH-responsive behavior of gelatin, this study investigates the influence of pH on gelatin's properties both in solution and when adsorbed on cellulose nanocrystal (CNC) surfaces. To ensure a broad exploration of this system, the study was carried out below (pH 5), above (pH 11), and at the isoelectric point of gelatin (pH 8). In solution, gelatin exhibited strong pH-dependent behavior, with hydrodynamic diameters increasing from 15.7 nm at pH 5 to 27.9 nm at pH 8, and ζ-potential varying from 12.4 mV to -10.9 mV as pH shifted from 5 to 11. However, Nuclear Magnetic Resonance analysis revealed that gelatin does not undergo conformational changes in its secondary structure, suggesting that gelatin's pH responsiveness in solution is driven by self-aggregation or interactions with other polymers rather than conformational changes of the gelatin molecule itself. When adsorbed onto CNCs, gelatin showed a markedly different behavior. At pH 8, the frequency change observed in Quartz Crystal Microbalance with Dissipation (QCM-D) was 5-6 times higher than at pH 5 or 11, indicating greater adsorption, whereas dissipation changes were also 2-3 times higher at pH 8 than its counterparts. The reduction in surface charge and solubility of gelatin at its isoelectric point minimizes water release during adsorption, allowing more gelatin to bind to CNCs. At pH 5 and 11, when gelatin behaves as a polyelectrolyte, similar frequency and dissipation shifts suggest an adsorption mechanism primarily driven by entropic gain. pH also strongly affected the viscoelastic interfacial properties of CNC surfaces with adsorbed gelatin, with hydrodynamic thicknesses at pH 5 and 11 being smaller than the gelatin diameter in solution, indicating molecular reorientation of surface-bound gelatin molecules. Despite differing behaviors in solution and on the CNC surface, both scenarios suggest the presence of extended gelatin chains rather than globular structures under all pH conditions. These findings enhance understanding of pH-dependent gelatin behavior and offer insights for designing responsive nanostructured materials.