Structural origin of apparent surface basicities of p-aminosulanamide polymorphs

Perichromism of two polymorphic forms of p‐aminosulfonamide (SNM) affecting spectroscopic properties of adsorbed thymol blue (TB) was studied. The consistent protocol for assessment of indicator interactions with solids was proposed and validated. This relies on the deconvolution of spectra into individual contributions both in solution and on the surface. The obtained relationships between pH and degree of protonation of indicator allows for surface basicity quantification. It has been found that γ‐SNM is significantly more basic than β‐form with difference exciding 3 pH units. Besides, the signal profiles measured in solution and adsorbed on either of SNM form were used for examination of TB perichromism. Although, only small red‐shift has been notices, this effect is not identical for both polymorphs. The differences in apparent surface pH of polymorph are explained in terms of structural properties. The morphology analysis allows for identification of habits responsible for structural origin in observed differences of polymorphs basicities. The noticed coincidental distributions of active centres of TB conformers and amine basic/acidic sites on γ‐polymorph might be considered as the source of apparent pH differences. Thus, basicities of studied system are to be related to morphology differences of polymorphs and recognition by iorn‐pairs formation rather than changes in characters of proton donating or accepting sites. Perichromism of two polymorphic forms of p‐aminosulfonamide (SNM) affecting spectroscopic properties of adsorbed thymol blue (TB) was studied. The noticed coincidental distributions of active centres of TB conformers and amine basic/acidic sites on γ‐polymorph might be considered as the source of apparent pH differences. Thus, basicities of studied system are to be related to morphology differences of polymorphs and recognition by ion‐pairs formation rather than changes in characters of proton donating or accepting sites.