Thermodynamics and Viscoelastic Property of Interface Unravel Combined Functions of Cationic Surfactant and Aromatic Alcohol against Gram-Negative Bacteria

Lipopolysaccharides(LPSs), the major constituents ofthe outermembranes of Gram-negative bacteria, play a key role in protectingbacteria against antibiotics and antibacterial agents. In this study,we investigated how a mixture of cationic surfactants and aromaticalcohols, the base materials of widely used sanitizers, synergisticallyact on LPSs purified from Escherichia coli using isothermal titration calorimetry (ITC), surface tension measurements,and quartz crystal microbalance with dissipation (QCM-D). ITC datameasured in the absence of Ca2+ ions showed the coexistenceof exothermic and endothermic processes. The exotherm can be interpretedas the electrostatic binding of the cationic surfactant to the negativelycharged LPS membrane surface, whereas the endotherm indicates thehydrophobic interaction between the hydrocarbon chains of the surfactantsand LPSs. In the presence of Ca2+ ions, only an exothermicreaction was observed by ITC, and no entropically driven endothermcould be detected. Surface tension experiments further revealed thatthe co-adsorption of surfactants and LPS was synergistic, while thatof surfactants and alcohol was negatively synergistic. Moreover, theQCM-D data indicated that the LPS membrane remained intact when thealcohol alone was added to the system. Intriguingly, the LPS membranebecame highly susceptible to the combination of cationic surfactantsand aromatic alcohols in the absence of Ca2+ ions. Theobtained data provide thermodynamic and mechanical insights into thesynergistic function of surfactants and alcohols in sanitation, whichwill enable the identification of the optimal combination of smallmolecules for a high hygiene level for the post-pandemic society.

Automated summary

In this study, the effects of a mixture of cationic surfactants and aromatic alcohols on LPSs purified from Escherichia coli were investigated using ITC, surface tension measurements, and QCM-D. The results showed that the interaction between cationic surfactants and LPSs involved electrostatic binding and hydrophobic interaction, while the interaction between aromatic alcohols and LPSs exhibited negatively synergistic effects. Interestingly, the combination of cationic surfactants and aromatic alcohols made the LPS membrane highly susceptible in the absence of Ca2+ ions.