Surface thermodynamics of Efavirenz and a blend of Efavirenz with cellulose acetate propionate by inverse gas chromatography

The purpose of this paper is to study the effect of an excipient on the surface energetics of Efavirenz (EFV) drug. The net retention volumes, VN, for n‐alkanes and polar solutes on the two columns, namely, EFV drug and a blend of EFV with cellulose acetate propionate have been measured by inverse gas chromatography. The dispersive surface free energy, γSd, Lewis acid parameter, Ka, and Lewis base parameter, Kb, have been determined using VN values. The γSd values are decreased linearly with increase of temperature for pure EFV as well as for the blend. Furthermore, the γSd values of EFV are higher than in the blend, for example, γSd values at T = 318.15 K for EFV, and for the blend are 28.09 ± 6.02 mJ/m2 and 28.30 ± 2.31 mJ/m2, respectively. The specific component of surface free energy −ΔHaSkJ/mol has been obtained by the Schultz method as well as by the Dong et al. method. The −ΔHaSkJ/mol values have been used to calculate Lewis acid‐base parameters for the EFV as well as the blend. The Ka values for the EFV and the blend are almost similar, where as the Kb values are higher in EFV than in the blend. Similar trend in Ka and Kb has been observed in both methods studied. Copyright © 2015 John Wiley & Sons, Ltd.