Infrared spectroscopy of methanol-hexane liquid mixtures. I. Free OH present in minute quantities

Abstract

Methanol and hexane mixtures covering the whole solubility range are studied by Fourier transform infrared attenuated total reflectance spectroscopy in order to evaluate OH groups that are H-bond-free. The mixtures from 0 to 0.25 and from 0.75 to 1.00 mole fractions form homogeneous solutions, whereas those from 0.25 to 0.75 mole fractions are inhomogeneous, forming two phases. Factor analysis (FA) was used to find out if free OH groups were present. These were found in minute quantities at the lowest mole fraction by evaluating the OH stretch absorption. The bulk of the absorption is due to the greater than 99.9% of hydrogen-bonded methanol molecules, with a band maximum situated at 3340 cm-1. The stretch band of the free OH groups absorbs at 3654 cm -1, with a full width at half maximum of 35 cm-1. The concentration is very weak but constant at less than 5 mM in the mole fraction between 0.252 and 0.067. Below this range, OH concentrations are even smaller. This represents less than 1% of the amount of methanol at the mole fraction of 0.067 (0.543M). Above 0.25 mole fraction, free methanol OH groups are not observed. Since the free OH band is very weak, almost at the noise level, we verified its presence with mixtures of hexanol in hexane. There, we found a similar free OH band with almost the same band characteristics, but with almost three times the concentrations found with methanol, which we attribute to the difference in the hydrocarbon chain length. The present study indicates clearly that solutions of methanol in hexane contain free OH groups but in minute quantities and only in the low methanol concentrations. This situation is much different from that observed in solutions of methanol in CCl4, where free OH groups are clearly observed at all concentrations except at the concentration limits. Whereas in CCl4, methanol is believed to form H-bonded chains, the situation is different in n -hexane: methanol in the low concentration region would form reverse micelles with the OH groups in the core and the CH3 groups mixed with n -hexane molecules. © 2008 American Institute of Physics.