61st Annual Report on Research 2016

Gasoline-ethanol blends are common motor fuels in several countries. The blend with 10% ethanol E10 is standard in the US. Addition of bio-ethanol increases the octane number and decreases the carbon footprint. Among disadvantages are increased fuel consumption and the drawbacks coming from the presence of water: a shorter shelf life, possible phase separation, and corrosive aggressiveness. Gasoline is a complex mixture of thousands of different hydrocarbons. It is virtually impossible to identify and quantify them all. At the same time, there are several molecules which have proton NMR peaks in “empty” regions of the spectrum. They can be easily identified, and their spectral intensities (concentrations) can be measured with high accuracy. NMR study using such “probe” molecules, existing in the blend or artificially added, can give valuable information about molecular organization, physical and chemical properties of the blends. In terms of solubility, “triple” gasoline-ethanol-water solutions are interesting physical systems: water is infinitely soluble in ethanol but insoluble in gasoline, while ethanol is infinitely soluble in gasoline. Only 0.5 mass % of water in E10 blend causes fast phase separation.

Our first goal was to study formation of ethanol clusters in gasoline-ethanol fuel blends. We applied pulsed-field-gradient NMR technique to measure coefficients of diffusion of ethanol and water molecules in the fuel blends. In addition, we developed a theoretical model of clusterization, which has been successfully applied to interpret the experimental data and extract physical parameters. The major, already published, findings of this part of the work are the following:

1. Ethanol molecules in gasoline-ethanol fuel blends form small short-lived clusters.

2. The size of the clusters increases at decreased temperature and increased ethanol concentration.

3. Water molecules, which are always present in the blends, are attached to ethanol clusters and have the same coefficient of diffusion.

4. Average cluster size in E10 fuel at 25C is 2.9, and its lifetime is about one microsecond.

5. Additional proof of the formation of hydrogen-bonded ethanol clusters has been obtained by measuring NMR chemical shifts at varying temperature and ethanol concentration.

Another ongoing project is the study of proton exchange between water and ethanol molecules in the fuel blends. We have found that the proton exchange is indirect and it is catalyzed by acids and bases present in the fuels. The effect on the proton exchange rates of added acids/bases (at micro-molar concentrations) is fully reversible after neutralization by bases/acids. We have also found that the width of the water NMR peak, which is determined by the exchange rate and varies significantly between different gasoline brands, can be an excellent indicator of the fuel acidity (gasoline is not an aqueous system, where acidity can be conventionally measured by simple means). This new acidity indicator is easy to use for monitoring quality of gasoline-ethanol fuel blends. Acidity decreases long-term chemical stability of the fuels and increases corrosive aggressiveness in engines.

In the course of this work, we noticed some peculiarities in NMR spectra of symmetric molecules, unusual spectral features which were not yet well understood. We did some experimental and theoretical work to clarify these issues. The result of this side project, already published, is a convenient method of measuring J-couplings between chemically equivalent protons.