Methanol interaction with dissolved

Just prior to analysis the contents of the tube were evaporated to dryness. The residue was then re-dissolved in 100 pi of methanol and then diluted with 100 pi of 0.5 % acetic acid in water. 100 pi of the resulting mixture solution was then placed cm the column and the chromatogram obtained is shown in figure 9. It is seen that an excellent separation was obtained and again the retention and selectivity was dominated by dispersive interactions with the reversed phase. 

Since iodine is a solid and sulphur dioxide is a gas, a polar solvent must be used for the reaction and as a dilution agent. Methanol is generally used as the solvent however, methylmonoether glycol or diethylene glycol can also be used. In these conditions, sulphur dioxide is not simply dissolved in the solvent but actually interacts with it. For example, with methanol, S02 is transformed into methylhydrogen sulphite that reacts with I2 in the presence of water. This leads to the following reaction ... 

Methanol, which has the shortest carbon chain, is more polar and soluble than other alcohols. SDS monomers are more easily solvated in an aqueous-methanol medium. This inhibits them from interacting and forming micelles. A similar behavior is expected for acetonitrile. Ethanol and propanol, which are also miscible with water, remain outside the micelles, dissolved in the bulk liquid, but interact with the micelle surface. Repulsion among the... 

Polarity may be qualitatively defined as the ability of a solute to dissolve in a polar solvent, which results from interaction with surrounding molecules by dipolar, non-dispersive forces. By this definition, hydrocarbons are nonpolar because they possesses no permanent dipole moments, and the entire molecular surface must solely interact with its environment via dispersion forces. Thus methanol is more polar than octanol because the surface area of methanol that interacts only via dispersion forces (hydrophobic surface area) is much less than that of octanol. For liquids, increasing solute polarity generally causes an increase in water solubility. This is not necessarily true for solids because polarity... 

While there are other solvents that may also be used effectively with reversed-phase sorbents, these solvents are capable of dissolving the range of compounds that are generally isolated by SPE. If more hydrophobic solvents are used in bonded-phase SPE, then the sorbent must be carefully dried by vacuum to remove all traces of water in the silica matrix in order that the eluting solvent can interact with all areas of the sorbent and not be stopped by residual water trapped in the pores. If this is not done, then the hydrophobic solvent (let us say, methylene chloride) will not effectively wet the surfaces of the C-18 bonded phase and poor recoveries will result. If methanol, acetonitrile, or ethyl acetate is used, then complete drying of the sorbent is not required because the solvent will either be miscible with water (methanol and acetonitrile) or displace the water from the silica (ethyl acetate). 

Since iodine is a solid and sulfur dioxide is a gas, an auxiliary polar solvent is used which serves as both a diluent and as a reaction medium. Generally, methanol is used, though in rare cases the methylmonoether glycol, or diethylene glycol, are also used. Under these conditions sulfur dioxide is not simply dissolved in the solvent but interacts with it, leading with methanol to methylhydrogenosulfite. This latter becomes the species that reacts with the iodine in the presence of water. 

---