PROPERTIES AND STRUCTURE OF SILICAS 503 10.0 1.0 WATER PORE VOLUME STRUCTURE INDEX 6.5 ml/g VHS 5.0 ml/g HS 3.0 ml/g MS 1.5 ml/g LS 1. 40 50 60 70 80 90 Percent Wet Cake Moisture Figure 5. Plot of water pore volume versus WET cake moisture cles. Therefore, at a fixed set of reaction conditions, the water associated with the precipitate or the filter cake is called the structural water. The amount of water associated with one part of the dry, recoverable product is defined as the water pore volume. The amount of water associated with 100 parts of the dry recoverable product is defined as Structure Index (SI) (15 a, b). Thus the water pore volume (PV) and SI are related to the % WCM by the following equation. % WCM ) (1) PV= 100 - % WCM SI ( %WCM ) = x 100 (2) 100 - % WCM A plot of water pore volume versus the % WCM is shown in Figure 5. Thus a product with water pore volume above 6.5 or SI values above 650 is arbitrarily defined as a VHS (very high structure silica). A low structure silica (LS) is one which exhibits a water pore volume above 1.5 or SI values above 150. These definitions are clearly explained in Figure 5. It must be pointed out that the log of the water pore volume or the log SI is linearly related to the % WCM. The SI values of products between the % WCM values of 80 to 85 are given in Table II (15). C. STRUCTURE CONTROL One of the key variables which can be used for controlling the properties and the struc- ture of silica is the fraction of the theoretical silicate which is added to the reaction me-

504 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II WET Cake Moisture (WCM) Versus Structure Index (SI) % WCM 8O 81 82 83 84 85 Sl 400.0 426.3 455.5 488.2 525.0 566.7 dium prior to the acidulation. Figure 6 shows how the silica structure increases as the fraction of the theoretical silicate in the reaction medium increases. D. ELECTRON PHOTOMICROGRAPHS OF CONTROLLED STRUCTUKE SILICAS The electron photomicrographs of VHS (very high structure) and HS (high structure) silicas are shown in Figure 7. The comparative photomicrographs of MS (medium structure) and LS (low structure) silicas are shown in Figure 8. In Table III we have compared the particle size of the VHS to LS silicas. Examination of Table III clearly points to the fact that the primary particle size increases as the silica structure level decreases. Thus the control of particle size via controlling the silica structure provides an important tool for producing silicas for diverse specialty and cosmetic applications. 84 • 80 _• 75 • 70 •. 65 60 I I I I I I I ' ' 0 10 20 30 40 50 60 70 80 90 Percent Theoretical Silicate Figure 6. Method of controlling the silica structure