10 z i, LL I,I U Z .5 I I I I 1 2 :3 4 ESTER oeHA•N LENGTH Figure 7. Relation between paraben ester chain length and partition coefficient (fillerless membrane: aqueous solution). • 2.0 o •J o 1.5 1.!3 • I I I 1 2 :3 4 ESTER IZHA]•N LENGTH Figure 8. Observed and theoretical diffusivities as a function of ester chain length. Key: •, observed value •, theoretical value based on the square root of molecular weight &, theoretical value based on the cube root of the molecular weight.

440 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table III Permeation of Methylparaben Through Polydimethylsiloxane Membranes a From Hydroalcoholic Solutions Solvent Vehicle Flux Diffusion Solubility (mole/cm2/hr Partition Coefficient (mg/ml) x 106) Coefficient (cm2/sec x 106) Non-Interactive Ethanol 20% w/w 23.6 Ethanol 40% w/w 197. Ethanol 60% w/w 361. Ethanol 80% w/w 426. Ethanol 90% w/w 408. Ethanol 100% 382. 0.596 + 0.028 1.52 1.10 + 0.01 0.0337 1.94 1.47 + 0.16 0.00525 1.99 1.83 - 0.04 0.00355 2.00 2.67 --- 0.09 0.00368 2.39 4.15 + 0.14 0.00600 2.37 11.3 -+ 0.8 0.0155 2.68 Data for commercial membranes of 0.0254-cm thickness. ethanol values (mole fractions of 0.8 and above), there was a much steeper rise in flux (Figure 9). The flux from neat ethanol was close to twenty times greater than flux from non-interactive systems. This enhancement in flux was not directly related to the par- aben solubility. Solvent uptake measurements using ethanol-water mixtures revealed that imbibition of solvent into the membrane occurs (Figure 10). Imbibition became more pronounced as the ethanol activity within the solvent system was increased. Membrane solubility was found to rise in a similar fashion as flux from the ethanol- water mixtures (Figure 11). The polymer solubility of methylparaben was increased [ ! • I , I I I 0.0 .2 .4 .6 .8 1.0 ETHANOL MOLE FRACTION Figure 9. Effect of ethanol content on steady-state methylparaben flux from saturated ethanol-water systems.