G ! ,_....­ / ! I I . I \! h 2006 ANNUAL SCIENTIFIC MEETING ! \ \ h G a. electrostatic b. steric c. electrosteric Figure 1 Energy distance curves for electrostatic, steric and electrosteric stabilisation 191 So that is the theory. In practice, particulate powders are supplied in an aggregated state. They must be in order to be handleable. However they must be milled down to their individual units in order to provide their designed function. This process must allow transport of the dispersant to the particle surface and adsorption there. Finally the dispersion must remain stable to dilution or addition of further formulation components. Discussion There are two main consequences of instability in particulate dispersions- flocculation or aggregation and sedimentation. Both depend ultimately on the interparticle interactions and the viscosity of the medium and will determine the product quality. The required functional attributes of a particulate used in colour cosmetics often depends critically upon its state of dispersion. A titanium dioxide pigment for example, designed to provide opacity in a formulation will not realise its maximum hiding power unless it is dispersed and remains dispersed in its constituent particles of 200-300nm. A UV attenuating grade of TiO2 on the other hand must be dispersed down to its primary particle size of 50- 100nm in order to be optimally functional as a sunscreen agent. Both powders as supplied (in order to be handleable) however have similar agglomerate sizes of several microns. Particle aggregation or flocculation can also affect the skin feel of a formulation since the presence of large aggregates can be detrimental to sensory properties Stability is very dependent on particle size. Larger particles or aggregates particularly those consisting of dense pigment will be inclined to settle out in a low viscosity formulation. Remembering the density differences of some of these pigments even in their dispersed state this may require some tricks. Rheology modification can help to minimise these effects and indeed some pigment particles cannot be stabilised without help from rheological aids. This settling is often preceded by flocculation or aggregation if particles are inadequately stabilised. Thus the particle stabilisation again plays an important role. Although In most cases aggregation is detrimental to performance. Some interaction between the particles may be desirable in order to enhance the long-term physical stability of the dispersion. Weak aggregation or flocculation resulting in a reversible gel can provide enhanced dispersion stability effectively eliminating sedimentation. This dependence of rheology on interparticle interaction is a keen area of interest and will be demonstrated by some examples. In many cases however rheology modification is achieved by addition of a rheology modifier such as xanthan gum in water or a bentonite clay in non-aqueous media. The rheology characteristics can be described in terms of creep or oscillation measurements. A thorough understanding of the nature of gels and the selection of appropriate conditions to give both the state of dispersion and rheology for optimum performance is again the role of the dispersion scientist. References Fleer GJ, Cohen-Stuart MA, Scheutjens JMHM, Cosgrove T, Vincent B: Polymers at Interfaces. Chapman and Hall, London, 1993. Napper FH: Polymeric stabilization of Colloidal Dispersions. Academic Press, London, 1989. Kessell LM, Naden BJ, Tadros ThF: Attractive and Repulsive Gels From Inorganic Sunscreen Actives, Proceedings of the IFSCC 23rd Congress, October 2004.

192 JOURNAL OF COSMETIC SCIENCE THE EFFECT OF VARYING THE ALKYL GROUP ON THE PERFORMANCE OF ALKYL TRIALKOXYSILANES USED TO SURFACE MODIFY COSMETIC PIGMENTS AND FILLERS Jane Hollenberg, Barry Arkles, Ph.D. and Yu-Lin Pan, Ph.D. Ge/est, Inc. Pigments and fillers treated with alkyl trialkoxysilanes are widely used in color cosmetics and sunscreens. Although hydrophobicity is the most obvious property imparted by the silanes, the effect on wetting in a variety of vehicles is most frequently the reason for the use of the surface modified pigments. Although the octyl trialkoxysilanes are the silanes best known to cosmetic formulators, alkoxysilanes having alkyl groups ranging from C1 to C24 are available. Measurement of physical properties of the treated pigments and incorporation into typical color cosmetic vehicles were performed to compare the performance of the longer and shorter chain alkoxysilanes.