Determination of surface properties and Gutmann's Lewis acidity-basicity parameters of thiourea and melamine...

Melamine and thiourea-derived graphitic carbon nitrides (M-GCN/T-GCN) were synthesized and characterized. The surface properties were investigated using inverse gas chromatography (IGC) and compared. Net retention volumes of M-GCN and T-GCN were measured with n-alkanes (C₅-C₁₀) and polar probes. The London dispersive surface free energies (γ_s^d), calculated using Schultz and Dorris-Gray methods, decreased linearly with increasing temperature. The specific components of the enthalpy and entropy of adsorption for the polar probes were obtained using the Schultz, Dong, and Sawyer-Brookman methods. The Gutmann's Lewis acid-base parameters, K_a and K_b, were determined using the surface free energy (ΔG_a^S) via these methods. The surface character 'S' values (K_b/K_a) of M-GCN and T-GCN using the respective methods are 4.04, 3.78, and 5.08 and 4.11, 5.27, and 2.86. Hence, the surfaces contain more basic than acidic sites and could interact strongly with acidic media. Thus, IGC elucidates the surface charges of the GCN matrix resulting from surface chemical modification.