Determination of thermodynamic interactions of poly(l-lactide) and biphasic calcium phosphate/poly(l-lactide) composite by inverse gas chromatography at infinite dilution

Inverse gas chromatography at infinite dilution was applied to determine the thermodynamic interactions of poly(l-lactide) (PLLA) and the composite of biphasic calcium phosphate and PLLA (BCP/PLLA). The specific retention volumes, \( V_{\text{g}}^{0} \), of 11 organic compounds of different chemical nature and polarity (non-polar, donor or acceptor) were determined in the temperature range of 308–378 K for PLLA and 308–398 K for BCP/PLLA. The weight fraction activity coefficients of test sorbates, \( \Omega_{1}^{\infty } \), and the Flory–Huggins interaction parameters, \( \chi_{12}^{\infty } \), were estimated and discussed in terms of interactions of the sorbates with PLLA and BCP/PLLA. Also, the partial molar free energy, \( \Delta G_{1}^{\infty } \), the partial molar heat of mixing, \( \Delta H_{1}^{\infty } \), the sorption molar free energy, \( \Delta G_{1}^{\text{S}} \), the sorption enthalpy, \( \Delta H_{1}^{\text{S}} \), and the sorption entropy, \( \Delta S_{1}^{\text{S}} \), were analyzed. A different chromatographic behavior of the two investigated samples, PLLA and BCP/PLLA, was observed. The values of \( \Omega_{1}^{\infty } \) indicated n-alkanes, diethyl ether, tetrahydrofurane (THF), cyclohexane, benzene, dioxane (except for 338 K), and ethyl acetate (EtAc) (except for 338 K) as non-solvents, and chloroform (CHCl₃) as good solvent (except for 378 K) for PLLA. For BCP/PLLA, CHCl₃, EtAc (for 378 K), dioxane (except for 378 K), and THF were indicated as good solvents.