Physicochemical and antibacterial properties of surfactant mixtures with quaternized chitosan microgels

Antibacterial surfactant mixtures attract widespread interest in the design of consumer product formulations, but often use toxic biocidal agents such as cationic surfactants, triclosan or bleach. To address this, we explored replacing these toxic ingredients with quaternized chitosan microgels, which combine high antibacterial activity with cytocompatibility with mammalian cells. Specifically, three essential properties of microgel mixtures with model anionic (sodium dodecyl sulfate, SDS) and nonionic (Triton X-100, TX-100) surfactants (and with SDS/TX-100 mixtures) were investigated: (1) colloidal stability, (2) antibacterial activity, and (3) hydrophobe solubilization. Additionally, the effect of surfactant on dispersion turbidity, which can be important in the formulation of aesthetically-appealing products, was explored. The microgels formed more-stable dispersions when mixed with nonionic TX-100, but quickly precipitated when mixed with the electrostatically-binding SDS and SDS/TX-100 surfactant systems at fairly-low (millimolar) surfactant concentrations. At higher SDS concentrations the microgels were redispersed and ultimately dissolved when mixed with only SDS, but remained precipitated in SDS/TX-100 mixtures. Furthermore, the electrostatic binding of SDS to quaternized chitosan diminished its antibacterial activity and (because the SDS-bearing mixtures with strong biocidal activity were limited to low surfactant concentrations) also resulted in limited hydrophobe solubilization. Conversely, microgels mixed with TX-100 maintained their biocidal activity even when the surfactant was in excess, and exhibited good solubilization properties. This suggests that surfactant-based products that use quaternized chitosan microgels as antibacterial agents should optimally be formulated using nonionic surfactants.