DIMETHYL SULFOXIDE 121 Figure 1. Unassembled diffusion chambers Assembled diffusion chambers mounted in thermostated water bath Figure 2. skin was cut away leaving a circular membrane area of approximately 3 cm •. The entire assembly was then mounted on a Burrell wrist-action shaker which positioned the diffusion assembly in a thermostated water bath (Figs. 1 and 2). In a typical diffusion experiment, picric acid was dissolved in an 80% (v/v) DMSO/pH 7.0 buffer solution (monopotassium phosphate-so- dium hydroxide 0.025 M). Fifteen milliliters of this solution was then placed in the epidermal chamber and 15 ml. of pH 7.0 buffer was placed in the derreal chamber. Glass beads were added to ensure proper mix- ing and a genfie shaking motion was then initiated. All diffusion experi- ments reported in this paper were performed at pH 7.0 and 30 ø q- 0.05 øC.

122 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The passage of pierate ion from the epidermal chamber to the dermal chamber was followed spectrophotometrically using a Beckman DU Spectrophotometer equipped with a Gilford Model 220 absorbance In- dicator. Samples were withdrawn at various times from the dermal chamber. Prior to withdrawing the first sample, a small piece of glass wool was submerged in the dermal chamber. Thus, by placing the tip of the sampling piper onto the wad of glass wool, any small particles re- sulting from the use of intact skin could be filtered. The optical density reading at 362 m/• was immediately obtained and the sample was placed back in the dermal chamber. In practice, this operation can be carried out in several minutes. Concentrations of pierate were calculated by using a value of 14700 M -• cm -• for the molar extinction coefficient at 362 mu. Initial studies indicated that, after a lag period, the pierate ion con- centration in the dermal chamber increased in a linear manner with re- spect to time. The data have been treated in accordance with Fiek's First Diffusion Law in the following manner: dc d• = kM (Ce - C•) (•) where: C• = initial concentration of pierate in epidermal chamber C, = concentration of pierate at time t in epidermal chamber C,• = concentration of pierate at time t in derreal chamber A = area of membrane k•, = absolute rate constant If C• C•, equation 1 reduces to dc - k•A Ce (2) dt At early times during the diffusion process, Ce is approximately equal to Ci and equation 2 becomes dc dt- k•.4 C• (3) de/dr can be obtained from the linear portion of the progression curve of concentration versus time. A and Ci are experimentally measurable. Thus, the absolute rate constant for a given experiment can be calcu- lated. In experiments in which dimethyl-C TM sulfoxide was utilized, the diffusion rate of DMSO was determined by sampling 0.1 ml aliquots of