Water Evaporation Analysis of L-Phenylalanine from Initial Aqueous Solutions to Powder State by Vibrational Spectroscopy

Abstract

The water evaporation from L-phenylalanine (L-phe) aqueous solutions at different initial pH (0–13) was studied by vibrational spectroscopy. Next, the attenuated total reflection–Fourier transform infrared (ATR-FT-IR) spectra of aqueous solutions were compared to those recorded after drying for 72 h at 21 °C at appropriate initial pH values. Micro-Raman results collected after the water evaporation process are also presented and interpreted. Between pH = 2.5 and 8.76 a white non-transparent gel was observed, possibly due to the presence of the NaCl salt. The significant differences of the band intensities of L-phe functional groups noticed at pH near p Ka values indicate the structural changes of L-phe molecules due to dimer formation (hydrogen bonds between the –COOH and –CO2− groups, and the –NH3+ and –NH2 groups). The presence of the hydrophobic interactions leads to the aggregation of L-phe molecules, most probably via phe–phe stacking as well as complexes of phe with Na+ ions, HCl, or H2O molecules.