Antimicrobial activity of organoclays based on quaternary alkylammonium and alkylphosphonium surfactants and montmorillonite

Abstract Organoclays are mostly materials composed of clay minerals modified with organic surfactants, predominantly of an alkylammonium type. Such hybrid materials often exhibit properties which are superior to those of their components. The main objective of this study was to characterize the antimicrobial activity and physico-chemical properties of tetrabutylammonium (TBA), dodecyltrimethylammonium (DDA), and tetrabutylphosphonium (TBP) cations, and the effectiveness of the organoclays composed of montmorillonite SWy-2 (Mt) and those cations. Their activity was tested on the Gram-positive bacteria Staphylococcus aureus and the Gram-negative bacteria Escherichia coli. Prior to the determination of their antimicrobial activity, organoclays were characterized using X-ray diffraction and infrared spectroscopy. The results confirmed the irreversible adsorption of organic cations onto the surface of Mt and the stability of the complexes in the medium used for biological experiments. The cytotoxicity of the organic cations on a HeLa cell line determined by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) reduction assay proved to be dependent mainly on the type of surfactant. While DDA with long alkyl chains fully inhibited surviving HeLa cells at concentrations of 1 and 0.5 mmol L−1, lower concentrations (0.1 and 0.05 mmol L−1) were not toxic. On the other hand, TBA and TBP also exhibited a very low toxicity at the higher concentrations. DDA alone exhibited the highest antimicrobial properties. It inhibited surviving S. aureus and E. coli at a concentration of 1 mmol L−1, estimated by the determination of colony forming units (CFU). Both TBA and TPB were only effective at a concentration of 10 mmol L−1. All organoclays were prepared with a ratio of organic cation/Mt of 10−3 mol g−1 at a concentration ≤ 1 mmol L−1. The antimicrobial effect of Mt alone was negligible and did not interfere with the effect of the organic cations. At this concentration, the most effective organoclay was based on DDA, which reduced the survival of both S. aureus and E. coli by over 93%. The organoclays based on TBA and TBP exhibited much lower antimicrobial effectiveness. The effectiveness of organic cations bound to the surface of Mt particles at a concentration of 0.1 mmol L−1 was similar with S. aureus and lower with E. coli compared to that of the respective surfactant solutions. The results of this work provided new knowledge on the biological activities of the selected quaternary ammonium and phosphonium cations, which are often used in organoclays. The antimicrobial activity of the organoclays should be considered when they are applied as carriers of bioactive substances or in other applications dealing with biological systems.