The uptake and reaction of methanol at the air-liquid interface of 0-96.5 wt% sulfuric acid (SA) solutions has been observed directly using vibrational sum frequency generation spectroscopy (VSFG) and Raman spectroscopy. Evidence for the formation of methyl hydrogen sulfate (MHS) was obtained by the presence of a new peak in the 800 cm -1 region, not present in either the neat methanol or concentrated sulfuric acid spectra. This peak is attributed to the singly bonded OSO symmetric stretch of MHS. The maximum yield of MHS with a large SA excess is shown to be (95 ± 5)% at –(15 ± 2) °C. No evidence was found to suggest formation of dimethyl sulfate.
As the concentration of SA increases from 0 – 96.5 wt%, the SFG spectra shift from that of methanol to that of methyl hydrogen sulfate. The surface is saturated with a mixture of the three methyl compounds after 15 minutes, although the relative amounts of MeOH, MeOH2+, and MHS vary with SA concentration. Uptake occurred on a much longer timescale, suggesting that uptake of methanol by sulfuric acid solutions is diffusion-limited.
The diffusion coefficients for methanol into 0 – 96.5 wt% sulfuric acid solutions were measured by passing MeOH vapor in N2 over the SA solutions and monitoring the uptake using Raman spectroscopy. The value obtained for methanol into water, D = (0.7 ± 0.2) x 10-5 cm2/s, is in agreement with values found in the literature. The values of D in 39.2-96.5 wt% SA range from (1 ± 2.7) x 10 -6 cm2/s with the maximum value occurring for the 59.5 wt% SA solution. This may be due to the speciation of MeOH in the SA solutions or to speciation of the SA solutions.
The organization of 1-butanol and 1-hexanol, at air-liquid interfaces was investigated using VSFG. There is evidence for centrosymmetric structures at the surface of pure butanol and hexanol. At most solution surfaces, butanol molecules organize in all-trans conformations. In contrast, the spectrum of 0.052 M butanol in 59.5 wt% sulfuric acid solution possesses a significant number of gauche defects. Relative to surface butanol, surface hexanol chains are significantly more disordered at the surface of the solutions.
|School:||The Ohio State University|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Air-liquid interface, Broad bandwidth sum frequency generation, Organosulfate formation, Raman spectroscopy, Surface structure and organization, Vibrational sum frequency generation spectroscopy|
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