CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 433 hydrophilic environment. With an increase in temperature above the LCST of the poly- mer, the particles are dehydrated, resulting in an increase in the hydrophobic environment in the core. Hence the I3/I1, which is the measure of the polarity of the environment, in- creases signifi cantly once the pyrene molecules sense the hydrophobic environment of the dehydrated hydrogel core. Similar behavior was observed at all the pH levels. Also, in this fi gure, the magnitude of I3/I1 difference between the lower and upper values should be considered as an indicator for the thermosensitivity of hydrogel dispersion. The lower I3/I1 values observed at the low temperature range correspond to the completely hydrated hy- drogel dispersion, whereas higher values at high temperature give the I3/I1 values of shrunken particles. Furthermore, a consistent decrease in I3/I1 with an increase in pH was detected over the entire temperature range used in this study, suggesting pH dependence of the hydrophobic/hydrophilic behavior of the hydrogels. When a solution of PNIPAM/Py in water is heated from 20°C to 60°C, several events seem to be occurring, as illustrated in Figure 12. The ratio Ie/Im increases slightly to reach a maximum value. Then it decreases sharply to reach a limiting value. These changes are reversible: upon slow cooling of the solution, the ratio Ie/Im increases and returns to its initial value. EFFECTS OF SALT ON THE SWELLING BEHAVIOR OF HYDROGELS The volume phase transition behavior of hydrogels in the presence and the absence of salt was characterized using an atomic force microscope and is illustrated in Figure 13. It is evident from the fi gure that in the presence of 0.1M NaCl, at pH 7.0, the hydrogels deswell signifi cantly. For example, the cubic ratio of the particle sizes at 0 and 0.1M NaCl (d0/d0.1)3 was 15.08. The observed phenomenon can be clearly explained using simple physical evidence. At low ionic strength or in the absence of any salt, the concentration of bound charges (and accompanying co-ions) within the gels exceeds the concentration of salt in the external Figure 12. The ratio of excimer to monomer intensities (Ie/Im) of pyrene in hydrogel suspensions as a func- tion of temperature at different pH values.

JOURNAL OF COSMETIC SCIENCE 434 solution therefore, a large ion-swelling pressure causes the hydrogels to expand, thereby lowering the concentration of co-ions within the gels. As the external salt concentration rises, the difference between the internal and external ion concentrations decreases and the gel deswells. The gel continues to deswell with rising external salt concentration until the mobile ion concentrations within and surrounding the gel are approximately equal. Fur- thermore, the swelling behavior of the hydrogels in Figure 13 can also be explained on the basis of repulsions between fi xed charge groups on the hydrogel particles. At low ionic strength, (large Debye lengths), repulsions are long-range and the gel expands to minimize the repulsive free energy as ionic strength rises (smaller Debye lengths), repul- sions are shielded and the gel deswells. Therefore, in the presence of 0.1 M NaCl, the particle size decreases from 4.1 μm to 1.6 μm this is clearly demonstrated using atomic force microscopy. Furthermore, it is clearly shown using both AFM and SEM (Figure 14) that if the hydrogel samples are allowed to dry in the presence of an excess amount of NaCl, the particles seem to lose their spherical shape, due to non- uniform dehydration. EFFECT OF UREA ON THE TEMPERATURE-INDUCED VOLUME TRANSITION Urea has been frequently employed in the biochemical fi eld as a means to identify hy- drogen bonds, since it is generally believed that urea can break up intra- or intermolec- ular hydrogen bonding of proteins/polymers in aqueous systems. Therefore, we studied Figure 13. AFM images of hydrogels in the (a) absence and (b) presence of 0.1 M NaCl.