CONDUCTIVITY OF SKIN IN DETERGENT SOLUTIONS 65 I ToeMP. CONTROL UNIT [ C DM 8:3 {WATER BATH, 37'C) ½ONDUCTOMF'TER CELL SELECTOR IlL & I'lklJ•l [.LICl I• [IZT IlOO! Pt ATINU# •K N SKIN SKIN I CELL I -- CELL 2 __ CELL Figure 1. Laboratory setup for electroconductometric skin permeation studies. cm apart and at a depth of 1 cm. The solutions were stirred by a matched set of magnets at a constant rate of 600 rpm by an external syncronized driving unit. Both donor and receptor compartments were thermostated at a constant temperature of 37øC. The con- ductivity across the cream-treated skin could be monitored continuously by connection with a strip-chart recorder or manually recorded at predetermined intervals. Each value in the plots represents the mean of triplicate determinations with a standard deviation of less than 7%. RESULTS AND DISCUSSION EXPERIMENTAL SET-UP Passage of current through an electrolyte solution and skin involves the migration of positively charged species toward the cathode and of negatively charge species toward the anode. When a known voltage (V) is applied across the two electrodes in a conduc- tivity cell, the resultant current (I) will be a measure of the conductivity (K) of the conductive medium, with the relationship presented as follows (12): K = G(d/A) = (1/R)m = (I/V)m (1) where R is the resistance of the medium G is the conductance and m, a cell constant, is defined by the ratio of d, the distance between the electrodes, over A, the projected surface area of the electrodes, here equivalent to the surface area of the skin sample exposed to the electrolyte solution.

66 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS SKI N P,a.t. inum Electrode _.. W--2.75crn 4-' 2.75 cm .--• Diagram I. Schematic of conductivity cell used in experiments. Platinum Elect. rode To calibrate the V-C skin permeation cells used, the cell constant for each cell was measured and the conductivities were correlated to one another using the following relationship: K 1 = (ml/m2)K 2 (2) The cell constants for each of the V-C skin permeation cells can be directly obtained from the conductometer when calibrated with 0.05% NaCI solution (Table II). The data indicate that the cell constant is dependent upon the distance between electrodes and is rather reproducible in this system. Hence, the conductivity data can be compared if the cell constant in each system is known or predetermined. Alternating current (AC) was used in these conductivity studies instead of direct cur- rent. Since the polarity of the electrodes changes very rapidly with AC voltage, the Table II Cell Constants for the V-C Skin Permeation Cells Used Distance between Cell Constant a Cell # electrodes (cm) (per cm) b 1 5.5 2.75 2 5.5 2.75 3 5.5 2.75 8.21 + 0.36 4.20 --+ 0.36 8.20 + 0.52 4.20 + 0.52 9.63 + 0.55 4.82 + 0.55 A 0.05% NaC1 solution at 37øC was used for calibration. Mean --- standard deviation of triplicate determinations.