Chloroacetic Acids in Water: Structure of the Hydrated Ions from Mass Spectra of Electrolyte Solutions

The mass spectra of negative ions from the aqueous solutions of monochloroacetic (MCA), dichloroacetic (DCA), and trichloroacetic (TCA) acids, obtained earlier using a mass spectrographic method involving electrospraying of electrolyte solutions in a vacuum, were reanalyzed, enabling the acquisition of new information about the hydration of the anions of acid residues (RCOO^–). The structures of the hydration shells at both the Cl and COO^– sites of the residue were determined due to the presence of the lines of both hydrated ions of acid residues (RCOO)⁻ and chlorine ions Cl^– in the mass spectra. Eight water molecules in the hydration shell at the Cl site were observed in the spectrum of MCA in a 0.001 M solution. It was found that the hydration of the COO^– site depends on the number of chlorine atoms in the acid: the most probable number of water molecules in the first hydration layer around COO^– is 1 for MCA, 1–2 for DCA, and 1–3 for TCA. The importance of the hydration state of the chloroacetic acids for their transformation in the liquid phase is discussed.