Analysis of the Self-Association of Aliphatic Alcohols Using Fourier Transform Infrared (FT-IR) Spectroscopy

A number of industrially important systems contain molecules, such as alcohols, that form hydrogen bonds. To correlate the thermodynamic properties of such systems, assumptions must be made in modeling the chemical equilibrium. It has been found that the data can be fit to a high degree of accuracy if it is assumed that the system contains monomers, dimers, and trimers or tetramers. Alcohols are often modeled by an infinite equilibrium model that takes into account associated species of all sizes. In this model and in others as well, an additional assumption must be made concerning the values of the various equilibrium constants; it usually is assumed that all of the equilibrium constants for a given species are equal. In this work, Fourier transform infrared (FT-IR) spectroscopy has been used to study the chemical association of aliphatic alcohols. Analysis of FT-IR spectroscopic data was used to determine the species present in a mixture and the corresponding equilibrium constants and also to evaluate the validity of the assumptions commonly made. The effects of molecular size on the enthalpic and entropic contributions to the equilibrium constant were determined by analyzing spectroscopic data for alcohols with carbon numbers of 5, 7, and 9 at different temperatures. A qualitative analysis of the spectra revealed that there are at least three types of hydrogen bonds in the mixtures. Equilibrium constants, enthalpies and entropies of association, and absorptivities of the unassociated OH stretch were determined. The equilibrium constants obtained were found to be a function of temperature and chain length.