The interaction of sodium dodecyl sulphate with colloidal microgel particles

The conformational transition of a 0.5% dispersion of poly(N-isopropylacryl amide) [poly-(NIPAM)] microgel, in aqueous solution, has been examined in t he presence and absence of sodium dodecyl sulphate (SDS) using a variety of physico-chemical techniques. The conformational transition temperature (T- m) of the mixed microgel-SDS dispersion, as measured by turbidimetric analy sis, was found to increase as a function of SDS concentration. These data w ere consistent with those obtained by high sensitivity differential scannin g calorimetry (HSDSC). The same technique revealed that the specific heat ( Delta H-cal) of the mixed dispersion decreased as the concentration of SDS increased. However the Delta T-1/2 (the width of the transition in degrees C at 1/2C(p,max)) and the Delta C-p,C-d (the difference in pre- and post-tr ansition heat capacity) both increase with SDS concentration. Viscometric d ata shows an increase in the reduced viscosity under isothermal conditions at 25 degrees C, while the viscosity-temperature profile of each mixed micr ogel-SDS dispersion revealed that the conformational transition behaviour m irrored that obtained from the turbidity and HSDSC measurements. Isothermal viscosity data, at 25 degrees C, suggests that the structure of the microg el becomes more extended with increasing SDS concentration. This interpreta tion is confirmed by dynamic light scattering measurements, which clearly s how that the hydrodynamic diameter of the microgel particles increases with SDS concentration, over the range 0-12 mM, by approx. 17%. Sorption isothe rm data shows that, below a SDS concentration of 4 mM, sorption is low then increases steadily until a concentration of 12 mM. The results of conducti vity measurements, carried out in the presence and absence of SDS (below an d above the T-m of the microgel dispersion). are also reported and show tha t the contraction of the microgel particle results in an expulsion of some surfactant from within the microgel structure. (C) 2000 Elsevier Science Lt d. All rights reserved.