Effect of carboxymethyl chitosan on the structural and rheological properties of NaC/lecithin or NaDC/lecithin aggregates

Carboxymethyl chitosan (CMCS) is a water-soluble derivative of chitosan of great interest due to its biocompatible and biodegradable properties [1]. Its incorporation into self-assembled systems, such as those formed by lecithin and bile salts, can significantly alter their structural and rheological characteristics [2]. Lecithin is a natural zwitterionic surfactant characterized by two hydrophobic tails [3]. Bile salts are surfactants composed of a polar and a nonpolar face that are responsible for the solubilization of fats [2]. Above the critical micelle concentration (CMC), lecithin and bile salts self-assemble into various morphologies and forming hydrogels with potential for controlled drug release [3].

This study evaluated the effect of carboxymethyl chitosan (CMQ) on the structural and rheological properties of self-assembled colloidal systems composed of lecithin and the bile salts sodium cholate (NaC) or sodium deoxycholate (NaDC). Solutions of lecithin (0.1 mol·L⁻¹) and bile salts (0.05 mol·L⁻¹) were prepared in methanol, mixed and dried by solvent evaporation. The resulting aggregates were redispersed in aqueous solutions of CMQ (0.5%, 1% or 2% w/w). The systems were characterized by Dynamic Light Scattering (DLS), Zeta Potential (ZP) and rheological analysis.

The NaC-based systems showed significant changes in particle size distribution with increasing CMQ concentration, indicating structural reorganization. Zeta potential values ​​increased from 1% CMQ, increasing colloidal stability. These systems showed non-Newtonian behavior, with an increase in viscosity over time and a predominance of the elastic modulus (G') over the viscous modulus (G''), suggesting the formation of more organized viscoelastic giant micelles. In contrast, the NaDC-based systems showed a different behavior. The initial distributions of bimodal particles became more uniform at low CMQ concentrations. However, ZP values ​​decreased, indicating a reduction in electrostatic repulsion and stronger intermolecular interactions. Although viscosity initially increased with CMQ, a progressive decrease in viscosity and viscoelasticity occurred over time, suggesting destabilization of the micelles. After storage, all systems showed similar rheological profiles, pointing to a common structural reorganization. These results highlight the fundamental role of the bile salt type in modulating the stability and mechanical behavior of these colloidal systems.

References:

[1] SHI H. et al. Journal of Polymer Research, v. 30, n. 9, p. 342, 2023.

[2] AZUM, N. et al. International Journal of Electrochemical Science, v. 15, n. 12, p. 12380-12394, 2020.

[3] CAUTELA, J. et al. Colloids and Surfaces A, v. 532, p. 411-419, 2017.