Hydrogen Bonding. 42. Characterization of Reversed-Phase High-Performance Liquid Chromatographic C18 Stationary Phases

The linear free energy equation logk′ = c + rR2 + sπ2H + a∑α2H + b∑β2 + vVx was applied to the capacity factors for various sets of solutes on C18 stationary phases with aqueous methanol and acetonitrile eluents. Here, k′ are the capacity factors for a series of solutes with a given C18 phase and a given eluent, and R2, π2H, ∑α2H, ∑β2, Vx are parameters or descriptors of the solutes as follows: R2 is an excess molar refraction, π2H is the solute polarizability/dipolarity, ∑α2H and ∑β2 are the solute hydrogen–bond acidity and basicity and Vx is the solute volume. It is shown that although the regression coefficients r, s, a, b and v vary widely within the C18 column and mobile phase used, the ratios r/v, s/v, a/v and b/v are remarkably constant. Thus, for the retention of 25 series of solutes on six different C18 columns with 30-90% aqueous methanol as the eluent, all the 25 LFER equations can be combined into one general equation: logk′ = c + v(0.13 R2 − 0.32 π2H − 0.22 ∑α2H − 0.90 ∑β2O + 1.00 Vx) where only c and v vary from system to system. For 11 other phases for which data are available, the ratios v/A and (v + c)/A are constant, where A is the quantity of stationary phase per unit surface area. Similar results were found with C18 phases and aqueous acetonitrile as eluents. Although a first examination of equations based on the first equation above suggests that various C18 phases behave differently, for example the v coefficient, that is related to the observed hydrophobicity of a stationary phase relative to the mobile phase, varies considerably from phase to phase with the same eluent, a detailed analysis led to the conclusion that all the C18 phases examined have roughly the same hydrophobicity, when the v coefficients are corrected for the quantity of stationary phase per unit surface area. It is suggested that these corrected v coefficients, v/A and (v + c)/A, can be regarded as the ‘intrinsic’ phase hydrophobicity.