Page 151 - Chiral Separation Techniques

5.2 Chiral Membranes 129

b)

a)

Fig. 5-2. Three types of the liquid mem-
brane configuration: (a) emulsion liquid
membrane; (b) supported liquid membrane;
c) (c) classical bulk liquid membrane set-up.

5.2.1.1 Emulsion Liquid Membranes

The application of an emulsion liquid membrane system involves three consecutive
steps [7]. First, two immiscible phases are stirred with a surfactant to generate an
emulsion. Subsequently, the emulsion is mixed with another liquid containing the
material to be extracted. The phases are then separated, and the emulsifying agents
are recovered in a de-emulsification step. Two examples of the use of the liquid
emulsion membrane configuration are the selective extraction of phenylalanine
enantiomers, using copper(II)N-decyl-l-hydroxyproline (1) as the chiral selector [8],
and the permeation of dipeptides and derivatives, using Rokwin 60 (a commercial
nonionic surfactant, consisting of a mixture of esters of higher fatty acids with D-
sorbitol), as reported by Skrzypinski et al.[9].