Propriedades estruturais da solução água-etanol: evidências pela RMN e cálculos computacionais

Abstract

Water-alcohol mixtures are not ideal solution within a large range ofconcentration, and display, therefore, anomalous thermodynamic properties such instance, density, viscosity and partial molar enthalpy.This work presents some results for experimental spin-lattice T1 relaxation time of water-ethanol solution in a ethanol molar fraction (xEtOH) range from 0.02 to 0.86. T1 shows a rather good dependence with the reciprocal of the respective solution viscosity. Based on this property it could retrieve the amount of dipole-dipole interaction contribution to the relaxation time for two different concentrations:xEtOH=0.18 and xEtOH=0.86. The largest dipole-dipole contribution to T1 occurs at ethanol mole fraction 0.18, T1(DD) = 84%, a concentration close to where the ethanol-water solutions show their maximum viscosity. The water-ethanol chemical shifts of 1H and 13C where also measured for eight different solutions on the range of xEtOH=0.02 to xEtOH=0.86 concentrations. The results were analyzed using internal and external reference. In the former case, the ethanol methyl group signals were taken as an fixed internal reference and the changes of the methylene group chemical shifts were then analyzed. The mostprominent relative changes were found for lower ethanol-water concentrations. Also, the changes were observed to be more pronounced for 13C than 1H nuclei. However, experiments using a external reference, a capillary containing DSS (sodium 2,2-dimethyl-2-silapentane-5-sulfonate) was introduced in the probe tube, showed a quite different pattern: for different solutions, both ethanol methylene and methyl groups 1H e 13C display uniform displacement on their chemical shifts. Also, with respect to this external reference, the methyl and methylene 1H chemical shifts increase with increasing of the ethanol concentration. This behavior is maintained for the methyl13C peaks, while the 13C methylene peaks show a decrease on their chemical shifts. The protons that make a chemical bound to the oxygen atoms in ethanol and water showed distinct signals for molar fraction greater than xEtOH=0.58; for xEtOH=0.86, it is seen a triplet coupled ethanol hydroxyl proton signal. This finding indicates that probably rather rigid ethanol-water clusters structures are formed with high interconversion energy for mole fractions above xETOH=0.58. It was possible to observe a hydrophobic ethanol solvation by water molecules, especially at low and intermediate ethanol concentrations. This solvation is one of the factors that affect the viscosity of the system. Another evidence for the development of clusters structures close to xETOH=0.86 for this system is the appearance of new signals seen at the 13C NMR spectra, indicating the presence of two different ethanol chemicalenvironments with proportion of approximately 2:1. At this concentration we can settle that there is not hydrophobic solvation since a vanishing experimental dipoledipole contribution to the spin-latice relation time was measured: %T1(DD) = 0%.