Influence of Chain Length on the Sphere-to-Rod Transition in Alkyl Sulfate Micelles

Article abstract of DOI:10.1021/j100230a032

Using Quasielastic light scattering spectroscopy (QLS) we have deduced the mean hydrodynamic radius (R_h) of alkyl sulfate micelles as functions of chain length (number of carbons n_c = 8-16), temperature (0-85 deg C), detergent concentration (0.01-4g/dL), and NaCl concentration (0.1-2 M).In the region of low chain length (n_c h values (extrapolated to the cmc) increase approximately linearly with the chain length.These results combained with Huisman's aggregation numbers (n_w(cmc)) are consistent with a micellar shape that is close to spherical (axial ratio less than 2).Under conditions of high NaCl concentrations of high NaCl concentration the micelles exhibit a temperature-dependent growth from small spherical aggregates into long spherocylindrical micelles at concentrations above the cmc.With increasing chain length the temperature dependence of R_h becomes stronger and the NaCl and detergent concentrations needed for micellar growth become smaller.Light scattering intensity measurements confirm a rodlike growth for these micelles at all chain lengths.From these R_h measurements, values of the thermodynamic parameter K governing the sphere-to-rod transition are determined by using an extension of our previous thermodynamic model (Missel et al., J.Phys.Chem., 84, 1044 (1980)).A quantitative analysis of the dependence of K on chain length, temperature, and NaCl concentration provides new insights into energetic factors which govern the structure and growth of micelles.