Micelle in nonpolar solvents

It appears from a survey of the literature that the essential properties of micelles in nonpolar solvents are understood, namely their stability and variations of size, the dissociation behavior, and their solubilizing capacities. Reverse micelles can dissolve relatively large amounts of water (1-10% w/v depending on emulsion formula) as well as polar solutes and, of course, water-soluble compounds. Consequently, they can be used as media for a number of reactions, including enzyme-catalyzed reactions. Very few attempts to investigate such reverse micelles at subzero temperatures are known, in spite of the fact that hydrocarbon solutions present very low freezing points. 

It may be expected that other, highly structured solvents with a tri-dimensional network of strong hydrogen bonds, would also permit micelle formation by surfactants, but little evidence of such occurrences has been reported. On the other hand, surfactants in non-polar solvents, aliphatic or aromatic hydrocarbons and halocarbons tend to form so-called inverted micelles, but these aggregate in a stepwise manner rather than all at once to a definite average size. In these inverted micelles, formed, e.g., by long-chain alkylammonium salts or dinonyl-naphthalene sulfonates, the hydrophilic heads are oriented towards the interior, the alkyl chains, tails, towards the exterior of the micelles (Shinoda 1978). Water and hydrophilic solutes may be solubilized in these inverted micelles in nonpolar solvents, such as hydrocarbons. 

In this connection the surprising thermal stability of AOT micelles in nonpolar solvents has been demonstrated again with the help of photon correlation spectroscopic experiments227 in the AOT/isooctane system between —85 and +95 °C. This was explained by the formation of a hydrogen bond network where it was assumed... 

The reactions of organic compounds can be catalyzed markedly in micellar solution. Catalysis by both normal micelles in aqueous medium and by reversed micelles in nonpolar solvents is possible (Fendler and Fendler, 1975 Kitahara, 1980). In normal micelles in aqueous medium, enhanced reaction of the solubilized substrate generally, but not always, occurs at the micelle-aqueous solution interface in reversed micelles in nonaqueous medium, this reaction occurs deep in the inner core of the micelle. 

The Winsor R parameter and the Mitchell-Ninham VH /lca0 parameter are related to each other in that both specify that when the value of the parameter exceeds 1, normal micelles in aqueous media in the presence of excess nonpolar solvent will be converted into reverse micelles in nonpolar solvent in the presence of excess aqueous phase. The former concept bases this on molecular interactions, the latter on molecular geometry. 

Water-soluble soil (sodium chloride, sugar) appears to be removed by solubilization (Chapter 4, Section II) into free water in the interior of surfactant micelles in the solvent. Surfactant micelles in nonpolar solvents are formed with the polar heads oriented into the interior of the micelle. Water is added to the dry-cleaning solvent and is solubilized into the interior of these micelles. Some of this water in the interior is bound strongly to the polar heads of the surfactants in the interior of the micelle and some is essentially free water. Studies (Aebi, 1959) have shown that it is the free water that dissolves water-soluble soil rather than the bound water. In the absence of any free water in the solvent, water-soluble soil is not removed to any significant extent. The water-soluble soil appears to be removed from fibrous surfaces by a process involving hydration of the soil followed by solubilization (Monch, 1960 Rieker, 1973). 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Eicke, F. H. Surfactants in Nonpolar Solvents. Aggregation and Micellization, in Topics in Current Chemistry (ed. Boschke, F. L.), p. 85, Berlin—Heidelberg—New York, Springer 1980... 

Eicke, H.-p. Surfactants in Nonpolar Solvents. Aggregation and Micellization. 87, 85—145... 

Surfactants can aggregate in nonpolar solvents in the presence of small amounts of water with the tails oriented towards the bulk nonpolar solution and head groups interacting with water in the center (Fig. 2). The water pool formed in reverse micelles has been used as a medium to study chemical and biological reactions [22]. 

Sodium cholate is insoluble in chloroform and in nonpolar solvents in general, but it is very soluble in alcohol and in water. Lecithin, on the contrary, is soluble in chloroform and only swells in water without dissolving in it. These differences in solubility are evidently related to the molecular structure and to the position of the hydrophilic groups in each of these molecules. The lecithin molecule has two important paraffinic chains and a group of hydrophilic functions (choline phosphate) localized at one end. In the presence of water, the lecithin molecules are oriented with their hydrophilic groups toward the water, and they hide their paraffinic chains inside a structure formed of two superposed layers of molecules. Conversely, in a nonpolar solvent the paraffinic chains are turned toward the solvent, while the polar groups are hidden inside the micelle. 

A nonpolar solubilizate such as hexane penetrates deeply into such a micelle, and is held in the nonpolar interior hydrocarbon environment, while a solubilizate such as an alcohol, which has both polar and nonpolar ends, usually penetrates less, with its polar end at or near the polar surface of the micelle. The vapor pressure of hexane in aqueous solution is diminished by the presence of sodium oleate m a manner analogous to that cited above for systems in nonpolar solvents. A 5% aqueous solution of potassium oleate dissolves more than twice the volume of propylene at a given pressure than does pure water. Dnnethylaminoazobenzene, a water-insoluble dye, is solubilized to the extent of 125 mg per liter by a 0.05 M aqueous solution of potassium myristate. Bile salts solubilize fatty acids, and this fact is considered important physiologically. Cetyl pyridinium chloride, a cationic salt, is also a solubilizing agent, and 100 ml of its A/10 solution solubilizes about 1 g of methyl ethyl-butyl either m aqueous solution. 

Surfactants with very small head group areas such as cholesterol tend to form inverted micelles (Fig. 12.7). Their head groups point into the center of the micelle while the hydrophobic tails form the continuous, hydrophobic outer region. Inverted structures such as inverted liposomes, are also formed in nonpolar solvents such as toluene, benzene, cyclohexane instead of water [534]. 

H. F. Eicke. Surfactants in nonpolar solvents. Aggregation and micellization. Topics Curr. Chem., 87 85-145, 1980. 

Table 7. Proposed reaction scheme for topological transformation during micelle formation in nonpolar solvents. AOT in C6H12 [Ber. Bunsenges. Physikal. Chem. 79, 667 (1975)]... 

It should be noted that the micelle formation of AOT in nonpolar solvents is primarily determined by hydration interactions between sodium ions and water molecules and the concomitant hydrogen bond formation linking the surfactant molecules. 

In this context, the chiral hyperbranched polyglycerols (-)-PG [Mn = 3000, with bis(2,3-dihydroxypropyl)undecenylamine as the initiator] and (+)-PG [Mn = 5500, with trimethylolpropane (TMP) as the initiator] were used. Esterification of the hydroxyl groups of these hyperbranched polyglycerols with hydrophobic alkyl chains as palmitoyl chloride, yielded amphiphilic molecular nanocapsules with reverse micelle-type architecture, in which approximately 50% of the hydroxyl groups were functionalized with palmitoyl chains [96-98]. These materials exhibit low polydispersity (Mw/Mn < 2), and the amphiphilic molecular nanocapsules are soluble in nonpolar solvents and irreversibly encapsulate various polar, water-soluble dye molecules in their hydrophilic interior by liquid-liquid extraction [96,98]. 

Surfactants are organic molecules that possess a nonpolar hydrocarbon tail and a polar head. The polar head can be anionic, cationic, or nonionic. Because of the existence of the two moieties in one molecule, surfactants have limited solubility in polar and nonpolar solvents. Their solubility is dependent on the hydrophile-lipophile balance of their molecular structure. At a critical concentration, they form aggregates in either type of solvent. This colloidal aggregation is referred to as micellization, and the concentration at which it occurs is known as the critical micelle concentration. The term micelle was coined by McBain (7) to designate the aggregated solute. In water or other polar solvents, the micellar structure is such that the hydrophobic tails of the surfactant molecules are clustered together and form the interior of a sphere. The surface of the sphere consists of the hydrophilic heads. In nonpolar solvents, the orientation of the molecules is reversed. 

Theoretically, triacylglycerols and phosphohpids have similar molecular weights, which make them difficult to separate by membrane technology. However, the structural differences are exemplified in nonpolar solvents such as hexane. Phospholipids are surfactant in nature, and in hexane miscella, they form micelles with a molecular weight of 20,000 Da or more. 

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