Surface characterization of cellulose acetate propionate by inverse gas chromatography

The purpose of this paper is to study the surface energetics of the polymer excipient cellulose acetate propionate (CAP) in the solid form. The net retention volumes, V _N, for n-alkanes and polar solutes have been measured in the temperature range 353.15–403.15 K by inverse gas chromatography. The dispersive surface free energy, \( \gamma_{\text{S}}^{\text{d}} \), and Lewis acid–base parameters \( K_{\text{a}} \) and \( K_{\text{b}} \), have been determined using V _N values. The \( \gamma_{\text{S}}^{\text{d}} \) values are decreased linearly with increase of temperature. The \( \gamma_{\text{S}}^{\text{d}} \) value at 353.15 K is 24.50 ± 1.54 mJ/m², and the temperature gradient was found to be −0.287 mJ/m²/K¹. The \( K_{\text{a}} \) and \( K_{\text{b}} \) values are 0.410 ± 0.021 and 1.708 ± 0.388, respectively, which suggest that the CAP solid surface contain relatively more basic sites. The K _a and K _b values of CAP are compared with the similar values obtained on the cellulose acetate butyrate solid surface.