Molecular dynamic studies of lubricant related systems—variable temperature IR spectroscopic studies

Abstract

Molecular interactions are the key to understand the structure and properties of liquids, solutions and solids. The relationship between the molecular structure and viscometric behaviour of fluids has been a subject of considerable importance. Quantitative relationships between molecular structures and various bulk properties of lubricant base fluids, i.e. viscosity, viscosity–temperature and viscosity–pressure variations, pour point etc., are not well defined. The understanding of these relationships is much more difficult in lubricating base fluids where only weak van der Waals interactions are predominant. The dynamic properties of molecular interactions such as correlation functions and relaxation times are of great interest to understand the structure–property relationships.In this paper, vibrational/rotational relaxation time (τIR) data along with half-bandwidth data obtained from infrared (IR) spectra have been used to study the several aspects of the molecular dynamics of base oils of varying physical properties. Relaxation time measurements have been carried out on a few model hydrocarbon compounds and a number of mineral base oils at varied temperatures (25–175°C). Activation energies (Ea) for Newtonian viscous flow (macro process) and vibrational/rotational relaxations (micro process) have been obtained from the temperature dependence of kinematic viscosity and the relaxation data, respectively. The activation energies for base oils containing higher isoparaffins (hydrocracking, HC class oils) have shown lower activation energies compared to those having lower amounts of isoparaffins (hydrofinishing, HF class oils). The activation energies for the micro process involving relaxation of the 1360cm−1 band are lower than the 721cm−1 band. IR spectral half-bandwidths are correlated to the reciprocal of the viscosity of base fluids under study. The linear relationships are used to separate the temperature dependent reorientational effects from vibrational effects.