An Investigation of the Micellar Phase of Sodium Sulfopropyl Octadecyl Maleate in Aqueous Sodium Chloride Solutions by Light Scattering Techniques. Evidence of Nearly Rodlike Micelles

The micellar phase of sodium sulfopropyl octadecyl maleate (SSPOM) in aqueous solution in the presence of NaCl has been investigated by static and dynamic light scattering. Below 80 mmol/L NaCl, small micelles with a mean aggregation number of 180 and an effective ionization degree of 0.13 are detected. Their size is salt independent and they show a small shape asymmetry. At higher salt concentrations a spheroid-to-rod transition occurs and elongated micelles are detected. The light scattering measurements are interpreted by using a structural model of nearly rodlike particles. At 250 mmol/L NaCl the contour and persistence lengths are about 500 and 120 nm, respectively. For higher surfactant concentrations than 3 × 10 -3 g/mL the scattering behavior features that of a network of entangled micelles. The measured data are described by simple power laws with exponents differing from those predicted for semidilute solutions of flexible chains which is suggested to result from the high stiffness of structures. The micelles are directly visualized by cryo-TEM.