Surface tensometry studies on formulations of surfactants with preservatives as a tool for antimicrobial drug protection characterization

The aim of the work was to quantify possible interactions between surfactants and preservatives, comparing surface properties, in model pharmaceutical formulations. Surface parameters of 2-component surfactant-preservative aquous mixtures were determined with a Wilhelmy plate technique, for the so-called principal surfactants (polysorbate 80, egg lecithin, phosphatidylcholine) and preservatives, which were methylparaben and benzalkonium chloride (BAC). A generalized surface tension vs. surfactant concentration plot signatures, in the presence of preservative at a fixed amount, allowed: the critical micellar concentration (cmc) shift, additive molecules partition from the surface to the bulk, mixed micelles formation concentration, and additive surface removal concentration to be determined in reference to surface activity of the added substance. Methylparaben is a compound of lower (in comparison to BAC) surface activity, lower partitioning coefficient possessing lower energy and concentration of its removal from the surface, that makes it play effectively an antimicrobial protection role in the bulk of pharmaceutical products, as already shown by chemical tests.