The linear free energy equation logkЈ =c+rR 2 +s H 2 +a⌺a H 2 +b⌺ 2 + vV x was applied to the capacity factors for various series of solutes on C 18 stationary phases with aqueous methanol and acetonitrile eluents. Here, kЈ are the capacity factors for a series of solutes with a given C 18 phase and a given eluent, and R 2 , H 2 , ⌺␣ H 2 , ⌺ 2 and V x are parameters or descriptors of the solutes as follows: R 2 is an excess molar refraction, H 2 is the solute polarizability/dipolarity, ⌺␣ H 2 and ⌺ 2 are the solute hydrogen-bond acidity and basicity and V x is the solute volume. It is shown that although the regression coefficients r, s, a, b and v vary widely with the C 18 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 C 18 columns with 30-90% aqueous methanol as the eluent, all the 25 LFER equations can be combined into one general equation:
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 C 18 phases and aqueous acetonitrile as eluents. Although a first examination of equations based on the first equation above suggests that various C 18 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 C 18 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 or (v+c)/A, can be regarded as the 'intrinsic' phase hydrophobicity.