300 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 1 oo 25 •ODE-25 ODE- 16 OE-515 OE-510 C DE-8 0 0 0.5 1.0 Concentration of BP (w/v%) Figure 2. Effect of BP on cloud points of 3w/v% nonionic surfactants. The effect of BP on the cloud point of various nonionic surfactants is shown in Figure 2. The cloud points of all surfactants were depressed by the addition of BP. We believe that the cloud point depression is due to the hydrophobic interaction of the micelies with BP molecules. The results also indicate that the solubility of BP and its antimicrobial activity was influenced by the chemical structure of the surfactant used. The solubilization isotherms of BP in seven nonionic surfactants are also shown in Figure 3. The more hydrophobic the surfactant used, the more BP is solubilized. These solubilization isotherms show nonlinearity, which indicates Langmuir's solubilization isotherm. The interaction between solute and surfactant can be expressed by the reciprocal form of Langmuir's adsorption equation (8,9) as follows: 1 1 1 Cbx K 2 K• K 2 Cf Cbx is the solute solubilized (g/g micelie), Cf is the concentration of free solute (g/l), K• is the solubilization constant, and K 2 is the concentration, meaning the maximum number of solubilization loci (sites) in surfactant micelies respectively. Reciprocal Langmuir's plots of BP against seven nonionic surfactants (3w/v%) are shown in Figure 4. The straight line was obtained in a wide range of BP concentration for all of the nonionic surfactants used. According to Donbrow and Azaz (4), the combined parameter K•K2, which can be obtained from the slope (1/K• K2) of the straight line, is the value corresponding to the distribution coefficient of the solute between surfactant micelies and intermicellar aqueous phase. The values of K• K 2 obtained in seven nonionic surfactants are given in

ANTIMICROBIAL ACTIVITY OF BUTYLPARABEN 301 3.0 f ._o 2.5 E n,,_ n,,_ '• 1.,5 o 1.0 0.5 ODE-25 E-525 0 0.5 1.0 1.5 2.0 Concentration of Free BP (10'2w/v%) Figure 3. Solubilization isotherm of BP in 3w/v% aqueous nonionic surfactant solution. 6O 5O 4O "-Iu 30 2o 10 .•.1 ___1 _l - 1 Cb,,-- K:• K] K2C! SAA K•K2 OE-525 17.64 515 24.16 510 28.41 DE- 8 36.23 ODE-25 12.23 16 . .5 ., (•... . (•, . I 5 0 , I 15oo 1 Cf Figure 4. Reciprocal Langmuir's plots for interaction of BP with seven nonionic surfactants at 3w/v%.