CATIONIC HYDROGELS FOR CONTROLLED DELIVERY 431 unambiguously, but it may be implied from circumstantial evidence, such as solution aging effects on viscosity and osmometry measurements (36). Water is a special solvent for PNIPAM in view of the large difference between the Hildebrand solubility parame- ters of water (δ23.4) (36). and of PNIPAM (δ 13–14 (38), 11 (39)). Normally a polymer is soluble in liquids, having solubility parameters only within the range of δ ± 3. From a thermodynamic point of view, the formation of hydrogen bonds between water molecules and the amide groups of the polymer contributes favorably to the free energy of mixing (ΔGM0), but unfavorably to the entropy of mixing (ΔSM). Hydrogen bonding Figure 9. AFM images of hydrogels at 40°C. Figure 10. Absorbance of hydrogel suspensions as a function of temperature at different pH values (λmax = 500 nm).

JOURNAL OF COSMETIC SCIENCE 432 between the water and the polymer triggers the formation of a layer of highly organized water molecules around the polymer, which decreases the entropy of the system. Associ- ation between polymeric chains via hydrophobic interactions between the alkyl substi- tutents can occur as well. These interactions become signifi cant as the temperature of the solution is increased and the bound water is released. Then the relative values of the ther- modynamic functions change: the entropic term becomes dominant, resulting in a positive free energy of mixing (ΔGM). This favors a two-phase system. Therefore, the absorbance of the hydrogel suspension increases signifi cantly after the LCST has been reached and the encapsulated water has been squeezed out. FLUORESCENCE The stimuli-responsive behavior of the hydrogel particles were also examined using pyrene fl uorescence. Pyrene has a much lower solubility in water (about 10−7 mol/l) than hydrocarbons (0.075 mol/l). It migrates from the water phase into hydrophobic regions once it is formed in aqueous solution, with remarkable photophysical changes (40,42). The hydrophobicity/hydrophilicity of the hydrogels at different pH values as a function of temperature can be assessed by examining the intensity ratio of the third/fi rst peak (I3/I1) of the pyrene fl uorescence emission spectrum. The temperature effects on the shape of hydrogels were also determined by plotting the intensity ratio of the pyrene excimer peak/pyrene monomer peak (Ie/Im) as a function of temperature at different pH values. The samples were prepared by adjusting the pH of the hydrogel suspen- sion to different values, and then pyrene was added to the pH-preadjusted hydrogel suspension. The variation of I3/I1 with temperature at different pH values is illustrated in Figure 11. An increase in I3/I1 with an increase in temperature was detected at all three pH values. Below the LCST, the hydrogels are very hydrophilic, as the hydrogels are fully hydrated with a large amount of water. Therefore, the encapsulated pyrene molecules experienced a Figure 11. I3/I1 ratios of pyrene fl uorescence of hydrogels as a function of temperature at different pH values.