Atomic-scale Insight into Molecular Adsorption of Sodium Dodecyl Sulfate (SDS) on Ideal Bcc-Fe Surface: A First-Principles Investigation

Adsorption is the most significant first step for boundary lubrication film formation, catalysis and chemical synthesis processes. Nevertheless, at present, the adsorption process and its mechanism of dodecyl sulfate (DS) molecules on solid surfaces are largely conjectured. Here, adsorption properties of DS on clean ideal bcc-Fe surfaces have been computational investigated via first principles method at atomic scale. At ground state, adsorption energies Eads are ranged from -11.75 eV to -3.09 eV. Adsorption Fe-O bond is a mixture of ionic bond and covalent one. Bond strength is affected by interfacial electron transfer and its stability is related to electron gain/loss of sub-interfacial atoms. Boltzmann distributions of adsorption conformations are calculated by partition functions that only depend on relative energies, and they are used as weight coefficients to estimate mixed-Eads. The temperature-induced Fermi-softness SF is calculated to reveal relationship between Eads and SF. Considered about solvent and temperature effects, the competitive adsorption and thermal activation can reduce the difference of adsorption energy barriers between conformations. Their adsorption probabilities become closer with the distribution proportion increase of the conformations with low Eads. This can provide theoretical foundations for further understanding of boundary lubrication film formation mechanism.