CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 429 ZETA POTENTIAL Figure 6 shows the variation of the zeta potential as a function of pH. It was found that the zeta potential decreased when the pH increased above 7.0, which can be attributed to the partial neutralization of surface charges, in agreement with the previously reported results (34). ABSORBANCE Figure 7 represents the absorbance (light scattering) as a function of temperature. The amount of light scattered showed a dramatic increase when the temperature was raised above the LCST of poly(NIPAM). Therefore, it would appear that when the temperature was raised past the LCST of the polymer, the particles shrank, which resulted in an increase in the refractive index and in light scattering. It should be noted that the experiments in Figure 6. The variation of the zeta potential with pH. Figure 7. Change in optical density and hydrodynamic diameter of cationically modifi ed hydrogels as a function of temperature.

JOURNAL OF COSMETIC SCIENCE 430 Figure 8. Reversible swelling and deswelling behavior of hydrogel particles when the temperature was cycled between 20°C and 40°C. Figure 7 were conducted in the absence of any added electrolyte and that the particles remained colloidally stable over the temperature range shown here. Indeed, the absorbance curves were reversible. To further verify the reversibility of the swelling and deswelling behavior of hydrogel particles, the hydrogel suspension as obtained was thermally cycled between 25°C and 40°C. Figure 8 shows the reversible swelling and deswelling of hydro- gel particles when thermally cycled between 25°C and 40°C. It can be noticed that hydrogel particles exhibit similar values of effective diameter at each temperature even after several thermal cycles, suggesting that the hydrogel particles are capable of maintaining structural integrity during the repeated swelling-deswelling cycles. Fur- thermore, it was found that the kinetics of swelling and deswelling of hydrogels in re- sponse to thermal stimuli were very fast. The effect of temperature on the hydration/dehydration behavior of hydrogel particles was further verifi ed using AFM and is illustrated in Figure 9. It is clearly evident from the fi gure that if the particles are deposited on the glass surface at a temperature above the LCST, the size of the hydrogel particles decreases markedly—for example, from 1.6 μ to around 680 nm, which is in agreement with the light-scattering and absorbance results. The degree of absorbance change was found to be relatively less affected with an increase in pH (Figure 10). AAPTAC, a quaternized ammonium salt, strongly dissociates in aqueous solution, probably rendering the degree of hydrogel swelling relatively insensitive to pH. The continuous changes in water content in the hydrogels as a function of temperature can be explained in the light of different proposed models. According to Heskins and Guillet (35), PNIPAM particles behave as fl exible coils that undergo extensive intermo- lecular association near the LCST. This model was based on an analysis of sedimentation and viscosity measurements at ambient temperature of an unfractionated PNIPAM sample and on the temperature dependence of these properties. However, whether these intermo- lecular associations are also prevalent at ambient temperature has not been established