Rheology as a tool for detecting mesophase transitions for a model nonyl phenol ethoxylate surfactant

The phase diagram of a non-ionic surfactant, namely NP-10(nonyl phenol with an average of 10 mol of ethylene oxide), was investigated using polarizing microscopy, rheological measurements and deuterium NMR spectroscopy. Using polarizing microscopy, the various phases were identified from the texture observed at various surfactant concentrations. Three liquid-crystalline phases and two micellar phases were observed, and they are normal micelles (L1), hexagonal liquid-crystalline phase (H1), isotropic cubic phase (V1), lamellar phase (La) and reversed micelles (L2) with increasing NP-10 concentrations, respectively. Frequency sweep rheological measurements were used to obtain characteristic relaxation times τs and plateau modulii G0. These data provide valuable information on the changes in microstructure induced by concentration at constant (20 °C). The changes in τs and G0 show that there is a co-existence regime of H1 and V1. These results powerfully supplement ones of polarizing microscopy and were proved by 2H NMR spectra.