Introduction

High performance liquid chromatography (HPLC) is often the method of choice for the separation of nonvolatile solutes. LC commonly exhibits longer analysis times, larger pressure drops and lower efficiency than either supercritical fluid chromatography (SFC) or gas chromatography (GC). The greater viscosities and lower diffusivities of liquids compared to those of supercritical fluids or gases are the primary cause of these differences in the three types of chrom-atography. The ideal mobile phase for a chromato-graphic separation of nonvolatile solutes would combine the positive attributes of liquids and supercritical fluids. That is, a solvent with low viscosity, high diffusivity and high solvent strength is desirable. Enhanced-fluidity liquid mixtures are solvents that have these attributes. These solvents are prepared by dissolving large proportions of liquefied gases, such as CO2, in commonly associated liquid solvents such as methanol.

To date, enhanced-fluidity mixtures such as meth-anol-CO2, methanol-CO2-H2O, tetrahydrofuran-CO2, hexane-CO2, methanol-fluoroform and meth-anol-fluoroform-H2O have been used to improve the performance in a range of different types of liquid chromatographies.

fore, Figure 1 shows that the addition of 50% v/v CO2 causes the diffusion coefficient of benzene to increase by approximately a factor of 2.

If liquefied gas addition and temperature elevation are combined, improvements in mass transport are substantial. For example, at 58 °C with a 0.49 : 0.21 : 0.30 mol fraction methanol-H2O/CO2 mixture, the diffusion coefficient of benzene increases ninefold relative to that in the same methanol-H2O mixture at 26°C. Without temperature elevation, the diffusion coefficient of benzene increases by a factor of 2 by adding 0.30 mol fraction CO2 to the 0.70 : 0.30 mol ratio methanol-H2O mixture (Figure 2).

These improvements in the transport properties of the liquid mixtures are only advantageous if the solvent strength of the mixtures remains high when the liquefied gas is added. Figures 3 and 4 show the variation of the hydrogen bond acidity, hydrogen-bond basicity and dipolarity for methanol-CO2 mixtures as measured by Kamlet-Taft a, ft, and n* solvent strength parameters. Clearly the hydrogen bond acidity and basicity of the methanol-CO2 mixtures is similar to that of methanol, even with 70% added CO2. The dipolarity of the mixture decreases faster than the hydrogen bond acidity or basicity with added CO2. However, the dipolarity remains relatively high, even with 50% added CO2. Data collected on a number of enhanced-fluidity mixtures have shown that often as much as 50% liquefied gas can be added to the organic solvent without significantly affecting the solvent strength.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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