Interactions of ions and odorant molecules with graphene-based nanostructures in synthetic urine: a molecular dynamics exploration

Odor control is a burning question in many practical applications, especially in hygiene products. At present, there is a lack of understanding of how odorant molecules interact or adsorb on solid surfaces in complex solutions like urine. In this work, we have used Molecular Dynamics (MD) simulations to investigate the interaction of the odorant molecule, p-cresol at concentrations relevant to human urine (500-100 pm) on carbon surfaces. The carbon surface was modelled simply as a graphene sheet. Moreover, the graphene sheet was edge functionalized with carboxylic groups, hydroxylic and epoxy groups on its basal plane to mimic the surface chemistry of oxidized active carbon. Charged graphene surfaces were created by deprotonating either 50% or 100% of functional groups, corresponding to experimentally determined charge at various pH values. The MD simulation results showed that PO₄ ^3-, SO₄ ^2-, Ca²⁺, and Na⁺ ions form strong clusters in synthetic urine in the presence of neutral as well as charged graphene surfaces. However, ions present in the synthetic urine showed specific affinities for charged functional groups. For example, NH₄ ⁺, K⁺, and Ca²⁺ showed specific affinities for hydroxyl groups, whereas Na⁺ and Mg²⁺ were complexed with both carboxylic and hydroxyl groups. Charge density and hydration have significant affect on the affinity of divalent ions. At 50% charged surface Ca²⁺ interact strongly than Mg²⁺, however at 100% charged surface Mg²⁺ showed stronger affinity than Ca²⁺ ions. The MD results also revealed that for effective adsorption of p-cresol, the hydrophobic surface such as pristine graphene is the best candidate because charging of the surface weakened the p-cresol interaction due to counterions accumulation near the charged groups, which pushed away the neutral p-cresol molecules. A general conclusion which can be drawn from the results of MD simulations is that p-cresol interaction with graphene surfaces becomes weaker in synthetic urine compared to pure water. To the best of our knowledge, this is the first study on such a system and shall trigger further work in this direction.