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

Molecular interactions are the key to understand the structure and properti es of liquids, solutions and solids. The relationship between the molecular structure and viscometric behaviour of fluids has been a subject of consid erable importance. Quantitative relationships between molecular structures and various bulk properties of lubricant base fluids, i.e. viscosity, visco sity-temperature and viscosity-pressure variations, pour point etc., are no t well defined. The understanding of these relationships is much more diffi cult in lubricating base fluids where only weak van der Waals interactions are predominant. The dynamic properties of molecular interactions such as c orrelation functions and relaxation times are of great interest to understa nd due structure-property relationships. In this paper, vibrational/rotational relaxation time (tau(IR)) 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 var ying physical properties. Relaxation time measurements have been carried ou t on a few model hydrocarbon compounds and a number of mineral base oils at varied temperatures (25-175 degrees C). Activation energies (E-a) for Newt onian viscous flow (macro process) and vibrational/rotational relaxations ( micro process) have been obtained from the temperature dependence of kinema tic viscosity and the relaxation data, respectively. The activation energie s for base oils containing higher isoparaffins (hydrocracking, HC class oil s) have shown lower activation energies compared to those having lower amou nts of isoparaffins (hydrofinishing, HF class oils). The activation energie s for the micro process involving relaxation of the 1360 cm(-1) band are lo wer than the 721 cm(-1) band. IR spectral half-bandwidths are correlated to the reciprocal of the viscosity of base fluids under study. The linear rel ationships are used to separate the temperature dependent reorientational e ffects from vibrational effects. (C) 2000 Elsevier Science Ltd. All rights reserved.