Dimerization of decanoic acid in mixtures of cyclohexane and dioxane: FTIR and density studies.

Abstract

The effect of different solvents on the dimerization of decanoic acid was investigated using Fourier-transform infrared (FTIR) spectroscopy and high-precision density measurements. The solvents studied were pure cyclohexane, pure 1,4-dioxane, and mixtures of the two. Association of decanoic acid via hydrogen bonding was followed as a function of the acid concentration (0-50 mM). The equilibrium dimer dissociation constant was extracted. The obtained Kd values follow the expected solvent effect: increasing the dioxane content reduces the degree of dimerization by stabilizing the monomers through solvent-solute interactions (polar, quadrupolar, and hydrogen-bond-accepting character of dioxane). Simultaneously, the nature of dimers changes from the double-bonded 'closed' structure in nonpolar solvents to 'open' single-bonded dimers at higher dioxane content. The results demonstrate good agreement between the two methods. The suitability of the density method was evaluated; we show it provides a useful thermodynamic route for quantifying association equilibria in cases where FTIR is inapplicable - like CO2 media under high pressure, where solute dimerization is important in the context of carbon capture and storage processes. However, the density approach requires many measurements, it works for a limited range of values of Kd, and it cannot distinguish between open and closed dimers. The hydrogen bonding produces a contraction by 8-15 mL mol-1 per dimer.

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