Abstract
We present a fast optical screening method for the detection of sub-micro-structures nanostructuration in water-containing fluids. The optical method is based on Raman spectroscopy of the water stretching vibration. The shape of this Raman band reveals information about the development of hydrogen bonds, from which it can be extracted whether the water molecules are homogeneously distributed in the mixture, or they are heterogeneously distributed (sub-micro-structuration). The fast screening is enabled by analyzing the potentially structured ternary system CO2/acetonitrile/water between 10 and 22 MPa and at 308 K, 318 K and 328 K in continuously spilled microcapillary system (MCS). Compared to batch systems we specify the advantages of the MCS in which the operational conditions with respect to pressure, temperature and composition can be changed fast, reproducibly and across a wide range.
Highlights
In sub-micro-structured fluid systems, such as in microemulsions [1,2,3,4,5] or other non-surfactant containing systems [6,7,8,9,10,11], the characteristic size of structural heterogeneities is smaller than the wavelengths of visible light
The identification of sub-micro-structures in fluid systems in microcapillary system (MCS) is based on an approach that has already been applied in a similar manner in high-pressure variable volume view cells (HPVVVC) [6,18] It is based on the analysis of the shape of the symmetric stretching vibration Raman band of water
Considering the orange data points indicating the dilution of the initial equimolar water/acetonitrile mixture by addition of acetonitrile, one can see clearly that the vwater continuously increases with decreasing water concentration cwater. This tendency implies that water dilution is accompanied by a continuous – in the investigated range – weakening of the hydrogen bonds network
Summary
In sub-micro-structured fluid systems, such as in microemulsions [1,2,3,4,5] or other non-surfactant containing systems [6,7,8,9,10,11], the characteristic size of structural heterogeneities is smaller than the wavelengths of visible light. With the here proposed method the operational conditions are identified, at which sub-micro-tructuration develops, a much more targeted investigation of these promising operational points can follow with high-power X-ray or neutron scattering experiments during granted measurement times at LSFs, with the final goal to achieve a more detailed knowledge and understanding of structural and/or dynamic properties at submicron level. The identification of sub-micro-structures in fluid systems in MCS is based on an approach that has already been applied in a similar manner in high-pressure variable volume view cells (HPVVVC) [6,18] It is based on the analysis of the shape of the symmetric stretching vibration Raman band of water. The combination of a MCS with in situ Raman spectroscopy provides further potentials. [24] More specific advantages that mainly concern the precision and the accuracy of the evaluation method of the Raman spectra are mentioned in the sections that follow
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