Influence of the tridecane and heptadecane molecule chain length on the structural characteristics of their epitropic liquid-crystal layers

Abstract

Introduction. The structural characteristics of epitropic liquid-crystal (ELC) layers [1] of saturated hydrocarbons incorporated into the composition of lubricating oils determine the wear-resisting properties of the latter [2]. The ELC layers of n-alkanes (and of aromatic compounds) formed in the field of the fluctuation forces of dielectric substrates have a parameter of the order of S 0.3, comparable with thermotropic liquid crystals, but small values of birefringence Δn 10 −4 and of the ELC thickness of a layer equal to d 10 −7 m, which increases with the molecule chain length [1]. The thickness of the ELC layer formed on a metallic substrate (in particular, for mineral oils [3] and n-hexadecane [4] in model friction triads) is an order of magnitude higher than the thickness of the layer formed on a dielectric substrate, but the influence of the molecule chain length on it has not been elucidated. Statement of the Problem and Research Subjects. In the present work, using polarimetry and viscosimetry methods [3–5], we investigated the birefringence and viscosity of interlayers (between metallic substrates) of two n-alkanes, * tridecane (C 13 H 28 ), and heptadecane (C 17 H 36 ), with a molecule chain length of 15 and 20 A ° , respectively [6]. The ELC layer parameters in such interlayers were determined within the framework of its simplified structural model as a homogeneous layer separated from the isotropic liquid by a phase boundary [3, 4]. Results of Optical Measurements of Birefringence. The magnitude of birefringence (Δn = n ex − n od ) that characterizes the structural ordering of ELC layers in the alkane interlayers was determined with the aid of a micropolarimetric device, which is a slit light guide of variable thickness [3]. The device measured the "extinction azimuth," viz., the Senarmont compensator rotation angle ϕ = f(D) corresponding to the minimum of the intensity of the light that passed through interlayers of different thicknesses D. The Δn-related phase shift δ between the components of the elliptically polarized light was calculated according to [7] from the formula