Study of Intermolecular Interactions Involved in Capillary GLC with Liquid Crystal Compounds as Stationary Phases

Abstract

Abraham's equation was employed in order to investigate nonspecific intermolecular interactions involving liquid crystals. Several aromatic azo derivatives, proved to have liquid crystal behavior, were used as stationary phases in capillary gas chromatography. The polarizability, polarity, hydrophobicity and hydrogen-bond donor or acceptor characters were estimated using the Abraham solvation model. Calculations were based on the isothermal retention times for 25 compounds, in the temperature range between 70˚ and 170˚C, both in the heating and the cooling mode. The descriptors R, , , , and logL 16 (Abraham parameters) were converted into orthogonal descriptors following Randi 's procedure. The above-mentioned properties involve the r, s, a, b and l coefficients, respectively, which were determined by means of multilinear regression applied to the considered set. Following the orthogonalization procedure, the polarizability term (rR*) was found to be statistically insignificant in the case of the studied compounds. Plots of these coefficients vs temperature are discussed, in order to allow estimations regarding the retention mechanism and contributions of polar and Van der Waals interactions. The variation of properties (e.g. polarity, hydrogen-bond donor and hydrogen-bond acceptor character) with temperature was assigned to the changes in the molecular geometry of the compound used as stationary phase.

Liquid Crystals as Stationary Phases in Chromatography

“…Deliberating the solubility of the testing substances in LCSPs, it is necessary to consider that in some chromatographic systems, the different physical–chemical interactions can occur, namely, electrostatic, hydrophilic, hydrophobic, van der Waals, and the hydrogen bond (being a significant part of the acceptor–donor interactions) [ 66 ]. In the case of the elution of cresols and acetates, the specific interactions of hydroxyl and acetate groups with the LCs functional groups can decrease the selectivity of the isomers separations, which could be performed if the separation would only be connected with the differences in the chemical structures of the testing substances molecules.…”

“…The intermolecular interactions of the LC molecules with 25 testing substances of different properties ( n -alkanes, n -alcohols, and acetates) were studied in the mesophases, at the temperatures ranging from 343.2 to 443.2 K [ 66 ]. The Abraham solvation model was applied for the characterisation of these interactions for the systems: the testing substances—the LCSPs [ 66 , 67 ]: where k is the retention coefficient (in general the magnitude, which depends on the kind of the retention magnitude chosen), R is the descriptor of the polarizability, π is the descriptor of polarity, l represents the cavity formation and dispersion contributions taking part between the LCSP and the probes employed, is the air–hexadecane partition coefficient, determined at 298.2 K, a is the ability of the LCSP to interact as a base in the acceptor–donor interaction through hydrogen bonds with acid probes, is a descriptor of the hydrogen bond acceptor basicity, which is applied to self-associating compounds when they are used as solvents, b is the ability of the LCSP to interact as an acid in the acceptor–donor interaction through hydrogen bonds with basic probes, and is the descriptor of the hydrogen bond acceptor acidities, which are applied to self-associating compounds when they are acting as solvents.…”

“…It is necessarily to emphasise that the ‘effective’ or ‘summation’ solute hydrogen-bond acidity and basicity ( ) could be taken as the and scale [ 66 , 67 ] for the temperature ranges encompassing the phase transition temperatures for the LCs studied. Taking the Abraham and McGowan suggestions into account, Eq.…”