Thermodynamics and Viscoelastic Property of Interface Unravel Combined Functions of Cationic Surfactant and Aromatic Alcohol against Gram-Negative Bacteria.Lipopolysaccharides (LPSs), the major constituents of the outer membranes of Gram-negative bacteria, play a key role in protecting bacteria against antibiotics and antibacterial agents.  In this study, we investigated how a mixture of cationic surfactants and aromatic alcohols, the base materials of widely used sanitizers, synergistically act on LPSs purified from Escherichia coli using isothermal titration calorimetry (ITC), surface tension measurements, and quartz crystal microbalance with dissipation (QCM-D).  ITC data measured in the absence of Ca2+ ions showed the coexistence of exothermic and endothermic processes.  The exotherm can be interpreted as the electrostatic binding of the cationic surfactant to the negatively charged LPS membrane surface, whereas the endotherm indicates the hydrophobic interaction between the hydrocarbon chains of the surfactants and LPSs.  In the presence of Ca2+ ions, only an exothermic reaction was observed by ITC, and no entropically driven endotherm could be detected.  The obtained data provide thermodynamic and mechanical insights into the synergistic function of surfactants and alcohols in sanitation, which will enable the identification of the optimal combination of small molecules for a high hygiene level for the post-pandemic society.
THERM