Modeling second harmonic generation at alcohol/air interfaces. A molecular multi-layer approach

Abstract

The liquid/air interface asymmetry leads to specific molecular interactions, compared to those observed in the bulk, and its characterization has significant implications for describing and controlling chemical and physical processes inherent to catalysis and environmental sciences. The second harmonic generation (SHG) is an interfacial selective spectroscopy often used to characterize interfaces. In this study, a computational multiscale methodology that combines molecular dynamics and quantum chemistry calculations has been worked out to scrutinize the SHG responses of 1-alcohols/air interfaces, and to unravel the role of the aliphatic chain length by exploring alcohols ranging from ethanol to pentanol. First, their molecular responses (β, the first hyperpolarizability) and then their macroscopic responses (χ 2, the second-order NLO susceptibility) have been computed and analyzed by decomposing the global responses into the contributions of successive molecular layers, providing relationships between the structure of the interfacial molecules and their dipolar SHG responses. The results show (i) an enhancement of the β ZZZ component (the component perpendicular to the interface) attributed to collective responses arising from the structural alignment of the interfacial molecules. Furthermore, (ii) longer aliphatic chains tend to create a bilayer structure, which was clearly observed at the pentanol/air interface where the polar heads of the molecules in the first and second molecular layers point toward each other. Such structural organization leads to a partial centrosymmetry at the interface, which is detrimental to the dipolar SHG response. On the other hand, (iii) owing to their smaller number density, the χ ZZZ 2 response of longer alcohols is smaller than for the smaller ones. Furthermore, (iv) a correlation between χ ZXX 2 and the aliphatic chain length is observed for the third molecular layer.</p