EFFECT OF POLYOXYETHYLENE ON FLOCCULATION 111 REFILTRATION The refiltration studies were performed with the apparatus set up as shown in Figure 1. A fixed quantity of suspension was introduced into the filtration cylinder, D, after gentle shaking. The diameter of the cylinder was 1.90 cm. A wide tip measuring piper was used in order to avoid disruption of fioc structure. A 0.8-/.tm film membrane (Millipore Co., Bedford, Mass.) properly trimmed and secured to the glass frit, F, served as the filter unit. Pressurized air, which was supplied from outlet A, precisely adjusted to a desired pressure by a bleeder, B, and monitored by a mercury manometer, C, exerted a positive driving force in the filtration process. A pressure of 140 mmHg was applied in the first step to obtain the slightly compacted sediment bed which was formed by the solid content of a suspension. At the moment the supematant surface just met the sediment bed, the air pressure was discontinued. At once, a 15-ml portion of the suspension flitrate was re-introduced carefully onto a glass column with a measuring pipet. With benzocaine suspensions, a refiltration pressure of 140 mmHg read from the manometer was applied and the time to collect flitrate in the graduated cylinder from mark 2 ml to 12 ml was taken. For butamben suspensions, a 20-ml portion was used and the time to collect 3 to 18 ml mark at a pressure of 60 mmHg was measured. The reproducibility of time t was within +_ 10 sec. The height of sediment bed was measured by a caliper. Reproducibility was within 0.1 cm. The results were analyzed by Darcy's Law (7) 1 dV K ß Ap -- - (3) .4 dt •/ ß L where V is the volume of the fluid of viscosity r/flowing through the sediment bed of crossectional area A and of thickness L in the period t Ap is the total pressure and K the permeability constant. K, which is strongly related to the porosity of the sediment bed, is a measure of interparticulate structure. By applying a constant air pressure to the vertically loaded column in which a sediment bed has been formed, the total pressure, AP = Po --C'V (4) and Po = Pa h- ,o. V'. g/A (5) where Pa represents the supplying air pressure and/•- V' are the density and original volume of loaded supernatant. C' is a proportionality constant. By substituting Eq. 4 into Eq. 3 and integrating over time from zero to t, the final equation may be obtained: C' V) A ß C' ß K In 1 - ß t (6) Po r/ ß L K may be calculated since all other parameters are experimentally accessible.

112 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS RESULTS & DISCUSSION Surfactant HLB values and surface tension of 50 x 10--4M solutions are presented in Table I. Surface tension values ranged from 29.3 for the surfactant with the shortest Table I Properties of Polyoxyethylene Nonylphenols (50 x 10-4 M) n (Average Number of Polyoxyethylene Surface Tension Units/Molecule) (dynes/cm) HLB • 5 29.3 10.0 6 -- 10.8 7.5 -- 12.2 9 31.1 13.0 10.5 31.8 13.6 12 32.7 14.2 15 35.6 15.0 30 43.0 17.2 50 46.2 18.2 •Technical Bulletin 2303-036 GAF Corp., N.Y. (1979). polyoxyethylene chain (n=5), to 46.2 dynes/cm for the surfactant with the longest chain length (n=50). Figure 2 shows plots of cosine of the advancing contact angle on the local anesthetics as a function of surface tension of the surfactant solutions used in the study. The critical surface tension of benzocaine and butamben (from Figure 2) were 31.1 and 28.5 dynes/cm, respectively. These values represent the maximum surface tension required by liquids for spontaneous wetting of the solid surfaces. The values are indicative of hydrophobic surfaces, and indeed, the powders could not be wet by water. Various characteristics of the powders are shown in Table II. 1.0 0.9 0.8 o 0.7 0.6 0.5 - ] I I i I 30 35 40 45 surf=ce tension (dyne/cm ) Figure 2. Critical surface tension of powders.(A) benzocaine (•x) butamben.