The <sup>1</sup>H-nuclear magnetic resonance (NMR) chemical shifts of ethanol and water hydroxyl groups show a pattern change at a critical ethanol concentration. Below the critical value (20 mol% at 400 Hz), only one hydroxyl peak appears due to fast proton exchange, whereas above the critical concentration, the ethanol hydroxyl peak splits from the water peak emerging as an individual chemical shift. The structural basis of the NMR pattern change was interpreted by a multivariate curve resolution–alternating least squares (MCR-ALS) analysis of the mid-infrared (mid-IR) spectra obtained for ethanol–water solutions. Results suggest that the NMR pattern change is due to the formation of ethanol–ethanol clusters. Below the critical concentration, no ethanol–ethanol clusters exist. Therefore, the NMR does not detect the ethanol environment. Above the critical ethanol concentration, ethanol–ethanol clusters first appear such that a distinct ethanol hydroxyl peak emerges. The basis for the dependence of the critical concentration on working frequency is also interpreted. High frequency NMR measurements are more sensitive to ethanol content, resulting in a lower critical ethanol concentration.

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