Quantum mechanics investigations on noncovalent interactions controlling proprieties of materials used in membranes

Hydrogen-bonding and other noncovalent interactions appear of crucial importance for the interpretation of many membrane proprieties. For example, hydrogen-bonding are crucial in adsorption of polar molecules, such as water, on polymeric membranes. Quantum mechanics represents a powerful tool to investigate hydrogen-bonding and in general noncovalent interactions since it provides accurate and reliable values difficult or sometimes impossible to obtain experimentally. Thus, this presentation summarizes some case studies where quantum mechanics has provided useful and complementary information about the role of crucial noncovalent interactions. Firstly, hydrogen bonds, involving between an amphiphilic modifier (guest) and the polar functional groups of a block (ether/amide) polymer (host), have been analysed. The QM calculation of the interaction energies, in solvent and vacuum, showed a competition between the host-host and guest-host noncovalent interactions. This competition can be regarded as the responsible for an increase of the availability of polar functional group of polymer, therefore, of the hydrophilicity of the polymer membrane. The macroscopic result was the change in the sorption and transport of the water molecules through the polymer matrixes. The second example concerns the calculation of the activation energy that solutes should have to get into a Carbon Nanotube with specified diameter. It is worth noting that an efficient solutes rejection could be driven by an unfavourable trapping energy (activated process) rather than by a size exclusion mechanism. However, the QM calculations shown that for molecules such as tyrosol and CNTs with diameter of 1.66, this is not the case. The adsorption of antimicrobial particles (polyoxometalates, POM) on polymer surfaces represent the topic of the last cases study. The calculation of the binding energies related to the interactions between POM-3 ion and polymer functionalities (commercial surfactants) as well as chitosan are presented. The intermolecular energies were compared to bromine ion-surfactant interaction. The calculation predicts that the POM-3 anions can be exchanged with Br- ions on the polymer surface when a high concentration of surfactants on the surface is available. However, even under these conditions, there will be an equilibrium between the adsorbed Br- and POM anions . However, this means that when anions are present in water a release of these antibacterial nanoparticles from the functionalised polymer surface is possible.