SELF-ASSOCIATION OF MEDIUM-CHAIN ALCOHOLS IN N-DECANE SOLUTIONS

Self-association of medium-chain alcohols in n-decane solutions has be en studied by infrared absorption of the fundamental OH stretching vib ration. The alcohols investigated were 1-propanol, 2-methyl-1-propanol , 2-methyl-2-propanol, 1-butanol, 1-pentanol, and 1-hexanol. Infrared spectra were acquired for varying alcohol molalities, the highest conc entration being 0.2 mol/kg. The spectra for each alcohol were collecte d in a data matrix, The bilinear multicomponent data were successfully resolved into spectra and concentration profiles by a multivariate me thod. The result indicates that monomers dominate the spectral varianc e in the low-molality region, while cyclic oligomers dominate in the u pper concentration range. It further indicates that minor amounts of o pen-chain aggregates may be present. The monomer and cyclic tetramer a ppear to be the dominant species, while the amount of open-chain aggre gates was negligible even in the low-molality region. The equilibrium constants for the monomer-tetramer association reactions (K-1-4) were calculated by a least-squares method. The calculated values for the eq uilibrium constants, based on the molality, range from 138 to 106 for the linear alcohol molecules. The result shoes that 1-butanol, 1-penta nol, and 1-hexanol have similar constants, while 1-propanol displays a markedly higher value. The equilibrium constants obtained for 2-methy l-1-propanol and 2-methyl-2-propanol were 77 and 39, respectively, The considerably lower values for the branched alcohol molecules indicate that steric interaction between the chain prevents self-association i nto larger aggregates.