Characterization of the solvation properties of sodium n-dodecyl sulfate micelles in buffered and unbuffered aqueous phases by solvatochromic indicators

The solvation properties of a micellar electrokinetic chromatography (MEKC) system composed of an aqueous phosphate buffer at pH 7.0 and sodium n-dodecyl sulfate (SDS) have been determined with solvatochromic indicators and compared with those of pure unbuffered water-SDS micellar system. The solvatochromic measurements allow calculation of the critical micellar concentrations (CMC), partition coefficients for the test compounds, and solvation properties of the two micellar systems studied. The results obtained are in good agreement with data obtained here and in previous works from MEKC measurements for the phosphate-SDS system. The solvent properties and partition coefficients in phosphate-SDS buffer are very similar to those obtained in water-SDS system, but the CMC values are lower in the phosphate buffer. The obtained results indicate that SDS micelles are very polar(pi* = 1.02 +/- 0.01), quite hydrogen-bond donor (HBD) acidic (alpha = 0.80 +/- 0.02), and moderately hydrogen-bond acceptor (HBA) basic (beta = 0.62 +/- 0.01).