INVESTIGATIONS OF THE PHASE-CHANGES OF NONIONIC SURFACTANTS USING MICROSCOPY, DIFFERENTIAL SCANNING CALORIMETRY, AND RHEOLOGY .1. SYNPERONIC A7, A C-13 C-15 ALCOHOL WITH 7 MOL OF ETHYLENE-OXIDE/

The phase diagram of a nonionic surfactact, namely Synperonic A7 (C-13 /C-15 alcohol with 7 mol of ethylene oxide) was investigated using pol arizing microscopy, differential scanning calorimetry(DSC), and theolo gical measurements. Using polarizing microscopy, the various phases pr oduced were investigated from the texture observed at various surfacta nt concentrations. At 20 degrees C, the surfactant produced hexagonal (middle phase) structures when the concentration was increased above 3 0%. This middle phase structure changed to a lamellar (neat phase) str ucture at concentrations above similar to 55%. The neat phase remained up to a concentration of similar to 85%. Microscopic investigations w ere also carried out as a function of temperature (at constant surfact ant concentration). This established the temperature at which phase tr ansition from liquid crystalline structure to isotropic liquid phases occurred. Some results on these phase changes were also obtained using DSC measurements. Both steady state and oscillatory measurements were carried out as a function of surfactant concentration and temperature . At surfactant concentrations below 30%, i.e. in the L(1) region, the viscosity-temperature curves showed a maximum that shifted to lower t emperatures as the surfactant concentration increased. The initial inc rease in viscosity with increase of temperature was attributed to aggr egation and/or change in the shape of the micelle, whereas the reducti on was thought to be due to the breakdown of the micellar structure. V iscoelastic measurements of the concentrated surfactant solutions (30- 85%), i.e. in the hexagonal and lamellar regions, were carried out at 20 degrees C. The moduli-concentration curves showed two maxima at sim ilar to 45 and similar to 70% and this was explained in terms of the s tructural changes that occur in the system as the concentration was in creased. Measurements as a function of temperature showed a rapid redu ction in the moduli values at a critical temperature at which the liqu id crystalling structure undergoes ''melting'' to form a liquid-like s tructure. The theological results were used to establish the phase dia gram and the results were compared with those obtained using microscop y and DSC.