120 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS the same pattern (Figure 10) so that the "bumps" in the curves should not be considered artifacts. These were distinguishable only in suspensions containing the smallest particle size benzocaine (Figure 8 and 9). The differences in F caused by daily shaking were probably due to rearrangement of particles in the suspensions into more compact formations. The dissolution of small particles may also have been promoted. Unfortunately, we have no data on the strength of particle-particle attraction in these systems. We could not produce smaller particles of butamben by jet milling because of the low melting point of this compound. Only one particle size material (23/am) was used in our suspensions. Suspension properties as a function of surfactant polyoxyethylene chain length are presented in Figure 11. The curves are parallel, again showing q 10 20 30 40 50 Figure 11. Sedimentation volume and apparent viscosity of butamben suspensions as a function of mean number of polyoxyethylene units per surfactant molecule. (©) sedimentation volume, F (o) apparent viscosity, r/. agreement between sedimentation volume and viscosity measurements. There are marked similarities to the data for suspensions of benzocaine of comparable particle size, and the same explanations for flocculation behavior apply. CONCLUSIONS Suspensions are complex systems in which a variety of physical-chemical processes interact. Flocculation of benzocaine and butamben suspensions, as assessed by measurements of sedimentation behavior, viscosity and refiltration, was a function of surfactant concentration, polyoxyethylene chain length and particle size. Surface tension and surfactant phase separation were involved in flocculation of some of the

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