McBain micelles

Isoelectric casein Nag [(O ") Ca,] ->-Nas (C ") Na Cg,. Whether the complex salt [NusC" casein] must be regarded as a complex salt or an ionic micelle of the type investigated by McBain in his investigations on the soap solutions is not yet clearly established, whilst information on the acid and basic dissociation constants of the higher stages of dissociation of the complex protein is still lacking. 

Aqueous soap solutions can be obtained in three distinct forms, the sol form containing the ionic micelle, a clear gel, and a white opaque solid the curd. The sol and gel forms of various soaps have been examined by McBain and his co-workers and shown to differ only in elasticity and rigidity, whilst the electrical conductivity, refractive index, concentration of metallic ion and lowering of the vapour pressure are all identical, results to be anticipated on the fibrillar theory. The gel as we have seen is fibrillar in nature and the conversion of a gel into a curd is brought about by the removal of soap fi om solutions in the form of relatively coarse fibres, a process similar to crystallisation. The experiments of Laing and McBain... 

Although McBain suggested over 80 years ago that soap molecules form micellar structures of lamellar and spherical shape (McBain 1913), most of the subsequent work focused on spherical micelles. The earliest concrete model for spherical micelles is attributed to Hartley (1936), whose picture of a liquidlike hydrocarbon core surrounded by a hydrophilic surface layer formed by the head groups, has been essentially verified by modern techniques, and the Hartley model still dominates our thinking. We present an overview of the structure of the micelle first and then go on to examine the details a little bit more closely. 

McBain pointed out that this seemingly anomalous behaviour could be explained in terms of organised aggregates, or micelles, of the surfactant ions in which the lipophilic hydrocarbon chains are orientated towards the interior of the micelle, leaving the hydrophilic groups in contact with the aqueous medium. The concentration above which micelle formation becomes appreciable is termed the critical micelle concentration (c.m.c.). 

Two other shapes of micelles may be considered, namely, the rod-shaped micelle suggested by Debye and Anacker and the lamellar micelle suggested by McBain. The rod-shaped micelle was suggested to account for the light-scattering results of cetyl trimethyl ammonium bromide in KBr solutions, whereas the lamellar micelle was considered to account for the X-ray results in soap solutions. A schematic picture of the three type of micelles is shown in Fig. 2. 

FIGURE 2 Various shapes of micelles following McBain (II). [Adapted from Hartley (1936) and Debye and Anaker (1951).]... 

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. 

In aqueous solution, dilute concentrations of surfactant act much as normal electrolytes, but at higher concentrations very different behavior results. This behavior (illustrated in Figures 8 and 9) was explained by McBain in terms of organized aggregates called micelles in which the lipophilic parts of the surfactants associate in the interior of the aggregate and leave hydrophilic parts to face the aqueous medium. (Details are given in refs. 16 and 31.) The concentration at which micelle formation becomes... 

Although, the spherical micelle model accounts for many of the physical properties of solutions of surfactants, a number of phenomena remain unexplained, without considering other shapes. For example, McBain [9] suggested the presence of two types of micelle - spherical and lamellar - in order to account for the drop in molar conductance of surfactant solutions. The lamellar micelles are neutral and hence they account for the reduction in conductance. Later, Harkins et al. 

The process of enhanced solubility in micellar solutions is normally referred to as solubilization, or in the words of McBain solubilization is the term given to a particular mode of bringing into solution substances that are otherwise insoluble in a given medium. Similar definitions were proposed later, with the only significant change that solubilization includes increased solubility caused by the presence of micelles. Over the years a considerable amount of empirical information relating to solubihzation has been pubhshed. The early studies have been reviewed by McBain, Klevens, McBain and Hutchinson, and by Elworthy et ah Later developments have been described in several reviews, the most comprehensive is the book edited by Christian and Scamehom. ... 

McBain J W1913 Mobility of highly charged micelles Trans. Faraday Soc. 9 99-101... 

McBain and Harkins, who studied concentrated soap solutions (20-30 per cent) with the aid of x-ray diffraction, suggest that lamellar micelles are formed by soap molecules in such a way that the hydrocarbon chains of these molecules line up parallel to each other and form double layers, as shown in Fig. 15-5a. The hydrophilic ends of the soap molecules face outward, and each double layer is separated from the next one by a zone of water. If a hydrocarbon is now added to a strong soap solution, it is solubilized by locating in the hydrocarbonic part of the micelles. Fig. 15-5b. It is assumed that a similar solubilization occurs when a hydrocarbon monomer is added to a dilute soap solution (0.5-2 per cent), as used in emulsion polymerizations. In this case though, the aqueous Zone between the layers is assumed to be much wider than in concentrated soap solutions. 

Fig. 1. The fatty acid soap-water phase diagram of McBain (58) modified (1) to show the molecular arrangement in relation to aqueous concentration (abscissa) and temperature (ordinate). Ideal solution, i.e., true molecular solution, is to the left of the vertical dashed line, indicating the critical micellar concentration (CMC), which varies little with temperature. At concentrations above the CMC, provided that the temperature is above the critical micellar temperature (CMT), a micellar phase is present. At high concentrations, the soap exists in a liquid crystalline arrangement, provided that the solution is above the transition temperature of the system, i.e., the temperature at which a crystalline phase becomes liquid crystalline. The Krafft point is best defined (D. M. Small, personal communication) as the triple point, i.e., the concentration and temperature at which the three phases (true solution, micelles, and solid crystals) coexist, but in the past the Krafft point has been equated with the CMT. The diagram emphasizes the requirement for micelle formation (a) a concentration above the CMC, (b) temperature above the CMT, and (c) a concentration below that at which the transition from micelles to liquid crystals occurs. Modified from Hofmann and Small (1).

The hypothesis of an aggregation of surfactants into micelles at concentrations in excess of the critical micelli-zation concentration of the surfactant is old (McBain, Laring and Titlay, 1919 Jones and Bury, 1920 and Ekwall, 1927). The phenomenon can be empirically understood in the following manner. 

We take the example of an ionic surfactant. When placed in water, a group consisting of a number p of surfactant molecules can associate to form a spherical structure illustrated in Figure 8.2, where all the polar heads face water and all the aliphatic tails are protected from it. Such an aggregate, or micelle, was conceived of by McBain in 1913. 

The term solubilization was coined by McBain [152] to denote the increased solubility of a given compound, associated with the presence of surfactant micelles or inverted micelles in the solution. The most popular solubilization process is the transfer of oil molecules into the core of surfactant micelles. Thus, oil that has no solubility (or limited solubility) in the aqueous phase becomes water soluble in the form of solubilizate inside the micelles. This process has a central importance for washing of oily deposits from solid surfaces and porous media, and for removal of oily contaminants dispersed in water. The great practical importance of solubilization is related to its application in the everyday life in the personal care and household detergency, as well as in various industrial processes [153]. 

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