The attenuated total reflection-infrared (ATR-IR) spectra in the 4800-700 cm -1 range of nine carboxylic acids and their sodium salts in aqueous solutions are obtained and analyzed. Overall, 22 species are studied. Six IR titrations are made with five different acids: acetic acid, malic acid, betaine, glycine, and N,N-((butyloxy)propyl) amino diacetic acid (BOPA). From the spectra of these titrations, the spectra of four types of water (acidic, basic, saline, and pure water) are subtracted, giving spectra with flat baselines without any artificial adjustment. Factor analysis (FA) made on the water-subtracted spectra yield the spectra of the principal species, and their abundances. Titration curves obtained from these precisely fit the theoretical curves and the pK a values in the literature. The remaining water bands that are not subtracted are assigned to water solute close-bound situations. The hydration number varied from 5 to 1, with an average of almost 2 per carboxyl carbonyl group. The IR CO band positions ((16 cm -1 ) are assigned to the different species: 1723 and 1257 cm -1 for the un-ionized acid double and single bonds; 1579 cm -1 for CO 2asymmetric stretch; 1406 cm -1 for CO 2symmetric stretch; and 1094 cm -1 for noncarboxylic ethoxy groups. The OH absorption covers the full region, from 3700 to 1700 cm -1 , in four bands that are ∼220 cm -1 wide. The near-3400 cm -1 band is assigned to solvated water, alcoholic OH, and NH groups, because these are hydrogen-bonded groups. The 3000 and 2600 cm -1 bands are assigned to the carboxyl OH groups that are hydrogen-bonded to other carboxyl groups in the pure acrylic species or to water in the aqueous solutions cases. The 2100 cm -1 band is assigned to a combination band that involves the far-IR absorption. The absorption from 3700 to 1700 cm -1 , which is sometimes called the "continuous absorption", cannot be attributed to the hydronium ion (H 3 O + ), because the acids are not ionized; rather, it results from the strong hydrogen bonds between water and the carboxylic acids.

The infrared spectra (IR) of pure liquid light (H(2)O) and heavy (D(2)O) water were obtained with attenuated total reflection (ATR) and transmission measurements in the mid-IR and far-IR. With these and with other values obtained from the literature, the real (n) and imaginary parts (k) of the refractive index were meticulously derived in the complete IR region from 6000 to 0 cm(-1). The reliability of the results resides in the critical comparison of our experimental data with that obtained from other laboratories and between calculated and experimental spectra, obtained by ATR and transmission techniques. The new optical properties (n and k) can now be used as standards for liquid H(2)O and D(2)O. To these we have added the water (H and D) absorption coefficients (K) that are derived from the k values. These can be used as references for spectra obtained by transmission with an absorbance intensity scale because they are almost the same.

The light and heavy liquid water (H2O–D2O) mixtures in the 0–1 molar fraction were studied in the mid-infrared by Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopy. Five principal factors were retrieved by factor analysis (FA). When D2O is mixed with H2O, the HDO formed because of the hopping nature of the proton (H or D) results in three types of molecules in equilibrium. Because of the nearest-neighbor interactions, the three molecules give rise to nine species. Some of the species evolve concomitantly with other species giving the five principal factors observed. We present the spectra of these factors with their abundances. The calculated probability of the species present at different molar fractions which when the concomitant species are combined gives the observed abundances. To appreciate clearly the difference between the principal spectra, a Gaussian simulation of the bands was made. Because of the numerous components that make up the stretch bands, they are not very sensitive to changes in composition of the solutions; nevertheless, they do indicate the presence of new entities other than the pure species. The deformation bands, more sensitive to such changes than the stretch bands, clearly indicate the presence of the three types of molecules as well as of intermediate species. These bands are sensitive to the two hydrogen bonds on the oxygen atom that a reference molecule makes with its nearest-neighbors, but not to the hydrogen bonds that the nearest-neighbors make with the next nearest neighbors.

Extrapolation techniques were used to obtain pure salt-solvated water spectra from the attenuated total reflection infrared spectra (ATR-IR) of aqueous solutions of the nine alkali halide salts LiCl, NaCl, KCl, CsCl, NaBr, KBr, NaI, KI, and CsI and the alkaline–earth chloride salt MgCl2. These salts ionize completely in water. The ions by themselves do not absorb in the IR, but their interactions with water can be observed and analyzed. A pure salt-solvated water spectrum is easier to analyze than that of a combined solution of pure water and salt-solvated water. Although the salt-solvated water spectra examined have distinctive signatures, they can be classified in three categories: those similar to NaCl; those not similar to NaCl; and MgCl2, in a class by itself. Each of the pure salt-solvated water spectra differs from that of liquid water, though the number of bands is the same. From the Gaussian band fitting, we found that the positions of the bands were fairly constant, whereas their intensities differed. The salt hydration numbers were determined: for NaCl, KCl, NaBr, KBr, and CsI solutions it is 5; for KI and MgCL2 it is 4; for NaI it is 3.5; for CsCl it is 3; and for LiCl it is 2. From these results we found that each pair of ions (monoatomic ions) of the ten salt solutions studied are close bound and form a complex in a cluster organization with a fixed number of water molecules.

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