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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, , and Lewis acid–base parameters and , have been determined using V _N values. The values are decreased linearly with increase of temperature. The 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 and 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 …