Adsorption state of naphthoic acids on folded sheets mesoporous materials with different pore sizes

Adsorption properties of 1- and 2-naphthoic acids (1- and 2-NPAs), model compounds of medicines that have naphthalene moiety such as naproxen, naphazoline, rifampicin etc., in the presence of folded sheets mesoporous materials (FSM-16) were investigated by solid-state fluorescence emission spectroscopy. The molecular states of NPAs adsorbed onto FSM-16 mesopores changed depending on the NPA concentration on the FSM-16 surface. The redshift of the fluorescence emission spectra was attributed to intermolecular overlapping of the naphthalene moieties of the NPA molecules. Results of differential scanning calorimetry indicated that 2-NPA was selectively adsorbed onto FSM-16(Oc). The maximal amounts of 1-NPA and 2-NPA adsorbed onto FSM-16(Oc), having a pore diameter of 16 Ǻ, were estimated as 4.4% and 14.2%, respectively. In contrast, the maximal amounts of adsorbed NPAs onto FSM-16(Do), having a pore diameter of 21 A, were almost the same, i.e., about 40%. The amount of 2-NPA adsorbed onto FSM-16(Oc) was significantly higher than that of 1-NPA; this shows that it is possible to selectively separate 1-NPA and 2-NPA by using FSM-16(Oc) as a mesoporous molecular sieve.