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Structural studies of mixed solvents

The rate of a catalytic reaction in the liquid phase can be affected by the choice of solvent. This applies equally to pure solvents and to mixtures of solvents as shown by the results for mixtures of water and 2-propanol (isopropanol or IPA).

It is known from Small Angle X-ray Scattering (SAXS) and neutron diffraction that mixtures of water and alcohol are not completely miscible.

At the microscopic level there are separate hydrophobic and hydrophilic regions.

 

 

Neutron diffraction experiments on water-isopropanol mixtures were performed at the ISIS SANDALS facility (Small Angle Neutron Diffractometer for Liquids and Amorphous Solids) at the Rutherford Appleton Laboratory

 

Empirical Potential Structure Refinement Analysis was used to analyse the neutron diffraction results and a model of the structure of the mixed solvent was obtained.

Simulation of a box containing 300 IPA and 700 H2O molecules

 

There is good agreement between the experimental and the simulated results (below) indicating that the model (above) could be a reliable model of the structure of a 30% IPA - 70% water mixture.

Probability distributions give pictorial displays that represent the arrangement of different molecules in a mixed solvent. An example for water-IPA at two different concentrations is shown below:

                        Probability distributions of (a) IPA around IPA and (b) water around IPA for IPA-water mixtures (with 70 mol% water).

                                                (c) IPA around IPA and (d) water around IPA for IPA-water mixtures (with 90 mol% water)