The interaction of sodium dodecyl sulphate with colloidal microgel particles

The conformational transition of a 0.5% dispersion of poly(N-isopropylacrylamide) [poly-(NIPAM)] microgel, in aqueous solution, has been examined in the presence and absence of sodium dodecyl sulphate (SDS) using a variety of physico-chemical techniques. The conformational transition temperature (Tm) of the mixed microgel-SDS dispersion, as measured by turbidimetric analysis, was found to increase as a function of SDS concentration. These data were consistent with those obtained by high sensitivity differential scanning calorimetry (HSDSC). The same technique revealed that the specific heat (ΔHcal) of the mixed dispersion decreased as the concentration of SDS increased. However the ΔT1/2 (the width of the transition in °C at 1/2Cp,max) and the ΔCp,d (the difference in pre- and post-transition heat capacity) both increase with SDS concentration. Viscometric data shows an increase in the reduced viscosity under isothermal conditions at 25°C, while the viscosity-temperature profile of each mixed microgel-SDS dispersion revealed that the conformational transition behaviour mirrored that obtained from the turbidity and HSDSC measurements. Isothermal viscosity data, at 25°C, suggests that the structure of the microgel becomes more extended with increasing SDS concentration. This interpretation is confirmed by dynamic light scattering measurements, which clearly show that the hydrodynamic diameter of the microgel particles increases with SDS concentration, over the range 0–12 mM, by approx. 17%. Sorption isotherm 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 conductivity measurements, carried out in the presence and absence of SDS (below and above the Tm of the microgel dispersion), are also reported and show that the contraction of the microgel particle results in an expulsion of some surfactant from within the microgel structure.