Surfactants ambidextrous

Recently, the concept of an ambidextrous surfactant, which is active at an organic-C02 and organic-water interface, was demonstrated as shown in Figure 8.10. The surfactants included a PDMS block, a poly(methacrylic acid) (PMA) or poly(acrylic acid) block, and, in some cases, a third PMMA... 

Figure 8.10. Trifunctional ambidextrous surfactant for steric stabilization of organic-C02 latexes and electrostatic stabilization of organic-water latexes (Yates et al., 1999).

Yates, M. Z. Li, G. Shim, J. J. Maniar, S. Johnston, K. P. Lim, K. T. Webber, S. Ambidextrous Surfactants for Water-Dispersible Polymer Powders from Dispersion Polymerization in Supercritical C02. Macromolecules 1999, 32, 1018-1026. 

Formed by synthesis or phase separation Single ambidextrous surfactants or multiple surfactants... 

A variety of organic colloids including emulsions and polymer latexes have been dispersed in carbon dioxide in the presence of surfactants (3,13). In most cases, owing to the lower interfacial tension of the former as explained shortly it is easier to form organic-in-C02 emulsions than water-in-C02, emulsions. Sterically stabilized colloids are stable above the critical flocculation density (CFD) and precipitate below this density. In some cases the CFD occurs at the upper critical solution density of the steric stabilizer, that is, the density at which the stabilizer phase separates from CO2, as has been shown by theory (14,15) and experiment (16). So-called ambidextrous surfactants have been designed to allow polymer latexes produced in CO2 to be transferred to an aqueous solution to form a dispersion (17,18). 

Figure 7 Ambidextrous surfactant for stabilization of latexes in CO2 or water tails C02-philic stabilizer segments solid circles, anchor group open circles, an ionizable group. (From Ref. 18.)...

Yates MZ, Li G, Shim JJ, Maniar S, Johnston KP. Ambidextrous surfactants for water-dispersible polymer powders from dispersion polymerization in supercritical CO2. Macromolecules 1999 32 1018-1026. 

Li G, Yates MZ, Johnston KP, Lim KT, Webber SE. Trifunctional ambidextrous surfactants for latexes in supercritical carbon dioxide and water. Macromolecules 2000 33 1606-1612. 

For applications in CO2, silicones are generally considered less effective than their fluorinated counterparts. Poly(dimethyl siloxane) (PDMS) solubility in CO2 was first reported in 1996 at a level of 4 %wt, PDMS n 13k) is soluble in CO2 at 35 °C and 277 bar (55). Block copolymer surfactants consisting of C02-philic PDMS and C02-phobic ionizable poly(methaciylic acid) (PMA) or poly(acrylic acid) (PAA) were used to form w/c and c/w emulsions (17). These PDMS-based block copolymers exhibited remarkable ambidextrous behavior to stabilize PPMA particles both in CO2 on the one hand, and in water on the other hand (56, 57). Steric stabilization is imparted in CO2 by PDMS, which is significantly more soluble in a CO2/MMA mixture than in pure CO2 (58). 

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