| Popis: |
Water-alcohol solutions have been extensively studied to correlate molecular structure to thermodynamic properties, which are known to deviate strongly from ideality. This project offers an analysis of cluster formation in ethanol-water binary solutions and an approach to identify distinctive structures within water and alcohol (W-A) solutions. Thermodynamic properties (such as enthalpy) and hydrogen bond strength can provide physical evidence of clustering within the solution. The heat of mixing was observed as a function of concentration. The concentration dependence of hydrogen bonding strength was analyzed through Infrared spectroscopy. Molecular aggregation in solution was explored experimentally with light scattering. Some of the results were consistent with published data and some results did not support data in the current literature. The calorimetry data were consistent with the published data showing a minimum at ~20 mol%. In the water rich region, we assume the hydrogen-bonded water clusters isolate individual ethanol molecules. The frequency shift observed with IR spectroscopy shows that the C-O bond strength increases with increasing ethanol concentration. Similar to the observations of D’Angelo, the frequency shift of the C-O and C-H stretching is concentration-dependent and is attributed to the transfer of electron density from the carbon of the methyl group. The observed minimum (17 mol%) in the H-OH bending absorption band frequency indicates an increase in hydrogen bond strength between water molecules in the water-rich region. The light scattering analysis was complex and required multiple systems to draw conclusions from the results. Two systems, ethanol in water and butoxy-ethanol in water, were studied with light scattering to observe the Rayleigh ratio (RR). The RR is proportional to the scattered light intensity. The ethanol-in-water system did not show a peak in the plot of RR as a function of concentration. However, for the butoxy-ethanol system a maximum in the RR vs. concentration occurs at ~2.5 mol%, close to the 3 mol% reported by Ito. The average scattering intensity, however, was four times less than reported by Ito and Schmitz. The major finding of the research is that the cluster nature, which is intimately related to solution properties, changes with concentration. The calorimetry and IR data suggest enhanced hydrogen bond strength of water in the water rich region that transitions to increased self association of ethanol in the alcohol rich region. |
