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 pK_a 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.