A Computational Chemistry Approach to the Molecular Design of SiO₂ Nanoparticles Coated with Stearic Acid and Sodium Stearate in Ethanol Solvent.

Preparation of hydrophobic SiO₂ nanoparticles (NPs) coated with different surfactants is important due to their potential application in different fields of chemistry. In this work, a combined experimental and Molecular Dynamics (MD) simulation approach, is advanced to characterize the adsorption process, in ethanol as a solvent, of stearic acid and sodium stearate on SiO₂ spherical NPs with different ionization degrees (0%, 10%, and 23.3%). The main objective is to gain molecular insight into the factors involved in the preparation of hydrophobic coated NPs, which involves the intervention of ion-dipole, electrostatic, and hydrogen bond-type interactions depending on the surfactants and the nature SiO₂ NPs. Our results demonstrate that the SiO₂ NPs have a good affinity for ethanol solvent medium., as confirmed through the analysis of the Radial Distribution Functions (RDFs)), which indicates that hydrogen bonds are formed at a distance of ∼0.192 nm between ethanol and SiO₂ NPs. The presence of Na⁺ ions reduce the electrostatic repulsion between the -COO^- and -SiO^- groups in NPs with degrees of ionization of 10% and 23.3%, because it acts as a bridge and thus favors the adsorption between the silanol and carboxylic groups. The investigation of the Potentials of Mean Force (PMFs) suggests that the adsorption on these NPs, is a spontaneous process compared with the case with 0% ionization degree. The experimental coating of the NPs was studied using Atomic Force Microscopy (AFM), a technique that allows the indirect measure of the Work of Adhesion (W_adh), a key quantity to estimate the energy needed to separate the interfaces AFM tip-sample. The experimental values of W_adh for the pure SiO₂ NPs and two modified SiO₂ NPs correspond to 2.01 J/m², 1.72 J/m² and 1.43 J/m², respectively. The main conclusion is that the interaction energies between surfactants and SiO₂ NPs, estimated from MD simulations, and the W_adh obtained from AFM measurements are correlated, in the sense that the reduction in the W_adh, in a solvent-free environment, corresponds to an increment of the interaction energy in the presence of the solvent. This reduction in W_adh is also associated with the fact that the nature of the coating of the SiO₂ NPs surfaces increases the NPs hydrophobicity. Our analysis provides a path for the computational design and the prediction of hydrophobicity of coated NPs, which is the main focus of our work.