Aqueous solutions amphiphilic compounds

Micelles the mostly spherical nanoscale aggregates formed by amphiphilic compounds above their critical micelle concentration in aqueous solution have a narrow size distribution and are dynamic, because there is a fast exchange of amphiphiles in solution and those incorporated in micelles. However, micelles are defined as self-assembled structures, since the structure is in thermodynamical equilibrium. 

Many micellar catalytic applications using low molecular weight amphiphiles have already been discussed in reviews and books and will not be the subject of this chapter [1]. We will rather focus on the use of different polymeric amphiphiles, that form micelles or micellar analogous structures and will summarize recent advances and new trends of using such systems for the catalytic synthesis of low molecular weight compounds and polymers, particularly in aqueous solution. The polymeric amphiphiles discussed herein are block copolymers, star-like polymers with a hyperbranched core, and polysoaps (Fig. 6.3). 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

The compound XXIII, consisting of two amphiphilic helical rods linked by an azobenzene moiety, was found to form micelles and ordered aggregates in aqueous solution in the dark, when the azo moiety is in the trans configuration. Photoisomerization of the azo linkage into the cis configuration, and the consequent bending in the structure of the molecules, induced disaggregation and disruption of the micelles.1118,1191... 

Poly(oxyethylene)octylphenyl ether (Triton X) is a nonionic amphiphilic compound. Triton X adsorbs at the aqueous]organic (W 0) solutions interface when it is added into W or O in a W-0 system. The oxygen atoms in the polyoxyethlene group of Triton X attract the positive charge of certain metal ions (M"+) in W, resulting in a rather hydrophobic M" -Triton X complex. On account of this complex formation, the facilitated transfer of M " " from W to O can be attained in the presence of Triton X at the W 0 interface when a potential difference is applied at this interface [12]. Elere, the appropriate potential difference is that more positive than about —0.4 V versus TPhBE if O is DCE. The facilitated ion transfer is accompanied by the desorption of Triton X (Fig. 11). 

An amphiphilic triblock copolymer consisting of poly(ethylene oxide) and PHB as the hydrophilic and the hydrophobic blocks, respectively, was synthesized by coupling two chains of methoxy-poly(ethylene oxide)-monocarboxylic acid with a PHB-diol in the presence of l,3-A,A -dicyclohexylcarbodiimide (Li et al. 2003, 2005a). This compound shows better thermal stability than its respective precursors, undergoes thermal degradation in two separate steps, and has a strong tendency to form micelles in aqueous solution (Li et al. 2003, 2005a). 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

---