ELAISSCH26 SurfaTech（ELAISSCH26 SurfaTech）.pdfVIP

Elaissari_4304-0_Ch26_R1_04-16-03 08:44:09 1 26 2 Preparation and Applications of 3 Silicone Emulsions Using Biopolymers 4 MUXIN LIU, AMRO RAGHEB, PAUL ZELISKO, and 5 MICHAEL A. BROOK McMaster University, Hamilton, Ontario, Canada 6 I. INTRODUCTION 7 A. Fundamentals 8 Dispersions of water in oil, or the inverse, are inherently unstable. Emulsifica- 9 tion is thus a nonequilibrium process such that the average droplet size in an 10 emulsion tends to increase over time. However, the characteristic time scales 11 for coarsening of emulsions can span a remarkably wide range, from seconds to 12 several years, that depends on the nature of the oil, the surfactants used to 13 stabilize the emulsion, and the processing history. It is fair to say that not all 14 the parameters affecting emulsion stability are completely understood. Current 15 practices in emulsion formulation thus combine art with science. With silicones, 16 being specialty materials and very unlike their organic counterparts, both theory 17 and art are less well explored than organic surfactants. 18 B. Silicone Properties 19 Silicones possess very unusual properties by organic standards. For example, the 20 low torsional force constant of the Si-O-Si-O linkage [1] results in exceptionally 21 flexible molecules: simple dimethylsilicone polymers [polydimethylsiloxane, 1 SC1 22 PDMS (Me SiO) , Scheme 1] have T values of approximately − 123°C irrespec- 2 n g 23 tive of molecular weight over a range of 1000–1,000,000 [2]. This backbone 24 flexibility, in combination with the high hydrophobicity of the gem-dimethyl