Hydrogen Bonding of Carboxylic Acids in Aqueous

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these wavelengths, the carboxylic acids exhibit an absorption peak, attributed ... pure acetic acid at 20°C is comprised mainly of cyclic dimers, with some linear.

Journal of Solution Chemistry, Vol. 27, No. 10, 1998

Hydrogen Bonding of Carboxylic Acids in Aqueous Solutions—UV Spectroscopy, Viscosity, and Molecular Simulation of Acetic Acid Graciela Ruderman,* Ernesto R. Caffarena, Ines G. Mogilner, and Eduardo J. Tolosa Received January 2, 1998; accepted July 6, 1997 The UV spectra of aqueous acetic acid solutions up to 2M were investigated. At these wavelengths, the carboxylic acids exhibit an absorption peak, attributed to the C=O group, which shifts when hydrogen bonds are formed.. The measured spectra were best fitted to several bands, either of Gaussian or Lorentzian shape, which can be explained as several types of structural units formed by hydrogen bonds established between acetic acid and water molecules and between acetic acid molecules themselves. Molecular dynamics simulation of these mixtures was also performed, confirming the occurrence of several types of hydrogen bonds and showing the presence of dimers at higher concentrations. The viscosity and density of these solutions were also measured at different concentrations and temperatures. These results give a more complete picture of the hydrogen bond network of the system. KEY WORDS: Hydrogen bonding; carboxylic acids; acetic acid; UV spectroscopy; viscosity; molecular simulation.

1. INTRODUCTION Acetic acid, an excellent solvent for many organic compounds and several other substances, is a weakly ionized acid in aqueous solutions. As an example of the carboxylic acids, it shows a marked tendency for association, not only in the pure liquid, but also in the vapor phase and in aqueous solutions.(1)

Instituto de Fisica de Lfquidos y Sistemas Biologicos, Universidad Nacional de La Plata, cc 565, 1900 La Plata, Argentina.

935 0095-9782/98/1000-0935$15.00/0 © 1998 Plenum Publishing Corporation

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X-ray diffraction studies(2) of neat acetic acid indicate the presence of cyclic dinners, while H nmr(3) and volumetric and dielectric studies(4) suggest the presence of cyclic trimers. Some Raman spectra studies(5,6) indicate that pure acetic acid at 20°C is comprised mainly of cyclic dimers, with some linear dimers, chain polymers, and monomers also present. Similar experiments'71 suggest that the major equilibrium in glacial acetic acid at 25°C is between cyclic and linear dimers, with possibly a small contribution from chain polymers. On dilution with water, the polymers and cyclic dimers break down into linear dimers, monomers, and their hydrated forms; the dominant species are dependent on the extent of dilution. (5-7) The dimerization constant at concentrations between 0.01 and 3M acetic acid in water was determined by emf measurements, (8-10) conductometry, (11-13) and ultrasonic absorption.(14) In all these studies, an equilibrium between monomers and cyclic dimers was considered. From several physical properties some author (15) infer that a dihydrated monomeric structure predominates from approximately 5M to infinite acetic acid dilution in water. Density measurements(16,17) of acetic acid-water mixtures indicate the presence of monohydrated monomers over the complete concentration range. From dielectric properties(18) in the high concentration range, the presence of monohydrated linear dimers and, below 15M, dihydrated dimers were suggested. Calorimetric measurements(19,20) indicate rather the prevalence of bond-breaking processes, than the creation of new bonds, with formation of hydrates on dilution with water at concentrations lower than 15M. In addition, an important hydration of the acid radical was inferred for diluted concentrations and an attempt to summarize in a diagram the composition complexity of the aqueous mixtures was made.(20) Among the spectroscopic studies of acetic acid-water mixtures, the more extensive and successful are those of IR and Raman. (5-7,21-23) These experiments suggest that at the lowest concentration studied (1.7M acetic acid, at 20°C), the dihydrated monomer is the predominant species, with some dihydrated linear dimers and cyclic dimers also present.(5) From similar experiments, (7) it was inferred that the major equilibrium in a 3.9M solution is between dihydrated linear dimers and monomers, with possibly a small contribution of cyclic dimers and monohydrated linear dimers. In the present work, the absorption spectra, between 180 and 600 nm, of aqueous solutions of acetic acid at concentrations between 2 x 10~3 and 2M at various temperatures, have been measured. Acetic acid presents an absorption maximum for X = 195 nm, which can be attributed to a transition from a nonbonding orbital on the oxygen atom to an antibonding orbital in the C=O bond.(24) At the lowest concentration measured, we found a maximum near 198 nm. However, as the concentration was increased, the shape of the absorption curves became more complex and the maximum of the

Hydrogen Bonding of Carboxylic Acids

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band contour shifts to higher wavelengths, up to A. = 236 nm for 2M acetic acid. We assume this complexity is related to the several species, which appear to form in this diluted concentration range. Unfortunately, the extent of overlap is severe enough to prevent an unambiguous assignment of the bands. Instead, a qualitative attempt is made, with the aid of other complementary measurements. In the present work, we have also performed a molecular dynamics simulation of acetic acid aqueous solutions. This simulation for 0.5M acetic acid in water reveals that some hydrogen bonds between the solute molecules exist, the corresponding lifetimes being calculated. For 2M the results show that the number of these hydrogen bonds is considerably increased and the corresponding calculated lifetimes are fairly high. This simulation was not possible for concentrations lower than 0.5M, since the acetic acid behaves like at infinite dilution. In order to obtain a better scope in studying the hydrogen bond net of the system, the viscosity and the density of the solutions were measured, thus obtaining the specific viscosity for concentrations between 5X10 -2 and 2M. 2. EXPERIMENTAL Glacial acetic acid of analytical grade was purchased from Merck and used without further purification. Water was double distilled of d.c. conductivity lower than 1 mS-cm - 1 in all cases. The ultraviolet spectra were recorded using a spectrophotometer Metrolab 2500, with quartz cells of 1 cm path-length. The reference cell was filled with water. The data were digitized by an analog digital convenor Singular SC-series, coupled to a personal computer system. After each spectrum was recorded, it was fitted to the number of bands, either Gaussians or Lorentzians, which best represents the experimental data. This process was performed using a nonlinear least-squares algorithm with a personal computer. Accuracy of the spectrophotometric measurements was checked as follows. For values of absorbances near 1, the measurements agree with those obtained with a spectrophotometer Beckman DU 640 and also with a diode array spectrophotometer Hewlett Packard 8452A, within ±2X 10 -3 . The stray light level, as evaluated by current methods,(25) was lower than 2% at all wavelengths. For the highest registered absorbance, i.e., A (X max ) corresponding to 2M acetic acid, with over ten measurements performed, all values fall within the interval 4.90±0.16. Linearity of absorbance measurements, for values higher than 2, was calibrated using glasses of known absorbance. Viscosity was measured with a digital viscometer School Gerate AVS 300, equipped with a Ubbelohde microcell. Density was measured using an Anton Paar DMA46 digital densimeter.

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Samples were thermostatized at several temperatures between 5° and 25°C. 2.1. Molecular Dynamics Simulation The interaction between acetic acid and water molecules and between acetic acid molecules themselves was simulated by molecular dynamics using the Gromos Package (Biomos n.v. Gronningen),X 4 ,X 5 = 238

0.90 17.7 11.54 19.95 22.84 28.67 12.08 16.09 29.42 29.33 34.11 40.80 6.30 19.00 30.42

0.10 0.57

0.04 0.06 0.08 0.10 0.20 0.30 0.40 0.50 0.60 0.80 1.00 1.50 2.00

0.4 0.71 0.87 1.06 0.70 1.16 1.68 1.88 2.18 2.68 0.56 2.10 3.62

the acid anion, which at high dilution possibly becomes an important contribution to the viscosity, cannot be inferred from the simulation. Unlike viscosity, the measured spectra show no significant effect of temperature between 5 and 25°C. A change in the amplitudes related to a variation in the concentrations of the species with increasing temperature should be expected. Nevertheless, to detect these variations, a more extensive interval of temperature should be investigated, as shown from Raman studies.(7) The temperature behavior of the increment of viscosity represented in Fig. 5 for 2M acid seems to be related to the hydration (see Fig. 7) and also to the lifetime of the hydrogen bonds between the acetic acid molecules (see Fig. 6). As a conclusion, UV spectroscopy, which requires only current equipment, is presented as a method for detection of hydrogen bonds in this type of mixture. In the present work, this method was found to be useful to investigate the acetic acid aqueous solutions in the low concentration range, where no other spectroscopic method has been used. The results obtained by molecular simulation and the measured viscosity contributed to the development of a fairly complete picture of the hydrogen bond network of these particular systems.

Fig. 7. Radial distribution functions of water around the center of mass of an acetic molecule for 0.5 and 2M concentrations.

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ACKNOWLEDGMENTS This work was partially supported by the Consejo Nacional de Investigaciones Cientificas y Tecnicas of Argentina (CONICET), the Comision Cientffica de la Prov. de Buenos Aires (CIC), and the Universidad Nacional de La Plata. G. Ruderman is a member of the Carrera del Investigador of CONICET. I.G. Mogilner is granted by the CONICET and E. J. Tolosa by the CIC. We wish to thank Prof. J. R. Grigera for helpful advise and correcting the manuscript. REFERENCES 1. K. H. Gustavson, The Chemistry and Reactivity of Collagen (Academic Press, New York, 1956) p. 185. 2. N. I. Gulivets, A. E. Lutskii, and I. V. Radchenko, J. Struct. Chem. 6, 20 (1965). 3. J. H. Clark and J. Emsley, J. Chem. Sac., Dalton, p. 2154 (1973). 4. H. E. Affsprung, G. H. Findenegg, and F. Kohler, J. Chem. Soc. A, p. 1364 (1968). 5. J. B. Ng and H. F. Shurvell, Can. J. Spectrosc. 30, 149 (1985). 6. J. B. Ng and H. F. Shurvell, J. Phys. Chem. 91, 496 (1987). 7. J. Semmler and D. E. Irish, J. Solution Chem. 17, 805 (1988). 8. H, N. Farrer and F. J. C. Rossotti, Acta Chem. Scand. 17, 1824 (1963). 9. G. R. Nash and C. B. Monk, J. Chem. Soc., p. 4274 (1957). 10. L. Barcza and K. Mihalyi, Z. Phys. Chem.. Newe Folge 104, 213 (1977). 11. D. R. Cartwright and C. B. Monk, J. Chem. Soc., p. 2500 (1955). 12. A. Katchalsky, H. Eisenberg, and S. Lifson, J. Am. Chem. Soc. 73, 5889 (1951). 13. K. Suzuki, Y. Taniguchi, and T. Watanabe, J. Phys. Chem. 77, 1918 (1973). 14. E. Freedman, J. Chem. Phys. 21, 1784 (1953). 15. R. W. Sims, M. R. Willcott, III, and R. R. Inners, J. Chem. Phvs. 70, 4562 (1979). 16. J. J. Kipling, J. Chem. Soc. 8, 2858 (1952). 17. A. N. Campbell, E. M. Kartzmark, and J. M. T. M. Gieskes, Can.J. Chem. 41, 407 (1963). 18. A. N. Campbell and J. M. T. M. Gieskes, Can. J. Chem. 42, 1379 (1964). 19. A. N. Campbell and J. M. T. M. Gieskes, Can. J. Chem. 43, 1004 (1965). 20. R. Vilcu and E. Lucinescu, Rev. Roitm. Chim. 19, 791 (1974). 21. P. Krtshnamurti. Nature 128, 639 (1931). 22. Ph. Traynard. Bull. Soc. Chim. Fr.. p. 316 (1947), 23. S. Feneant, Compt. Rend. 235, 1292 (1952). 24. G. F. Lothian, Absorption Spectmphotometry (Hilger and Watts, London, 1958), p. 90. 25. R. E. Poulson, Appl. Opt. 3, 99 (1964). 26. W. F. van Gusnteren and H. J. Berendsen, Groningen Molecular Simulation (GROMOS) (Biomos, Groningen, 1987). 27. H. J. C. Berendsen, J. R. Grigera, and T. P. Straatsma, J. Phys. Chem. 91, 6269 (1987). 28. Handbook of Chemistry and Physics, 72nd edn., (CRC Press, Boca Raton, 1982), p. D-200. 29. E. R. Caffarena and J. R. Grigera, J. Chem. Faraday Trans. 92, 2285 (1996). 30. G. E. Maciel and D. D. Traficante, J. Am. Chem. Soc. 88, 220 (1966). 31. C. R. Cantor and P. R. Schimmel, Biophysical Chemistry, (Freeman, San Francisco, 1980), part II, p. 648. 32. A. Einstein, Ann. Phys. 19, 298 (1906); 34, 591 (1911).