Adsorption of fatty acid potassium salts on diamond-like carbon: Effects of alkyl chain lengths and adsorptive conditions

Organic monolayers on solid substrates have received considerable attention due to their excellent versatilities in many fields. In this paper, molecular dynamics (MD) simulations were performed to study the adsorption of fatty acid potassium salts (FAPS) on diamond-like carbon (DLC) substrates. The results indicated that adsorbed molecules tend to aggregate on DLC surfaces due to the intense attractions between FAPS molecules. In order to obtain evenly distributed molecular films, the external energy input is needed to facilitate molecular adsorption and overcome the potential barriers. Also, the effects of alkyl chain lengths on molecular adsorption were investigated. A positive correlation was observed between the alkyl length and the adsorptive energy exerted by DLC substrates. Furthermore, the Coulomb interaction between adsorbed molecules is the most powerful interaction within the MD system. Affected by the Coulomb interactions, adsorbed molecules would undergo severe deformation. And a net-like structure composed of polar ends is formed in molecular films. The hollow area within the network is full of alkyl chains of molecules. It can be concluded that the powerful Coulomb interactions between adsorbed molecules play a dominant role in determining the adsorption behavior and structural properties of potassium salts on DLC substrates.