Micelle-water interface

Kalyanasundaram K, Thomas JK (1997) Solvent-dependent fluorescence of pyrene-3-car-boxaldehyde and its applications in the estimation of polarity at micelle-water interfaces. J Phys Chem 81 2176-2180... 

If, even if there were no penetration of the water in the interior of the core of the micelle, a part of the hydrocarbon chains were exposed to the micelle-water interface. 

Kinetic treatments are usually based on the assumption that reaction does not occur across the micelle-water interface. In other words a bimolecular reaction occurs between reactants in the Stern layer, or in the bulk aqueous medium. Thus the properties of the Stem layer are of key importance to the kineticist, and various probes have been devised for their study. Unfortunately, many of the probes are themselves kinetic, so it is hard to avoid circular arguments. However, the charge transfer and fluorescence spectra of micellar-bound indicators suggest that the micellar surface is less polar than water (Cordes and Gitler, 1973 Fernandez and Fromherz, 1977 Ramachan-dran et al., 1982). 

Similar considerations apply to situations in which substrate and micelle carry like charges. If the ionic substrate carries highly apolar groups, it should be bound at the micellar surface, but if it is hydrophilic so that it does not bind in the Stern layer, it may, nonetheless, be distributed in the diffuse Gouy-Chapman layer close to the micellar surface. In this case the distinction between sharply defined reaction regions would be lost, and there would be some probability of reactions across the micelle-water interface. 

The foregoing discussion of micellar charge effects has implicitly assumed that differences in water activity or substrate location in cationic and anionic micelles are not of major importance. If such differences were all important it would be difficult to explain the differences in k+/k for carbonyl addition and SN reactions, because increase of water content in an aqueous-organic solvent speeds all these reactions (Johnson, 1967 Ingold, 1969). As to substrate location, there is very extensive evidence that polar organic molecules bind close to the micelle-water interface in both anionic and cationic micelles, although the more hydrophobic the solute the more time it will spend in the less polar part of the micelle. Substrate hydrophobicity has a marked effect on the overall rate effects in both cationic and anionic micelles, but less so on values of k+/k. It seems impossible to explain all these charge effects in terms of differences in the location of substrates in cationic and anionic micelles. 

Kalyanasundaran K. and Thomas J. K. (1977a) Solvent-Dependent Fluorescence of Pyrene-3-Carboxaldehyde and its Applications in the Estimation of Polarity at Micelle-Water Interfaces,/. Phys. Chem. 81, 2176-2180. 

It has been established by many studies (2) that the charge at the micelle-water interface, or Stern layer, is high, and... 

The activation energies calculated for the two steps of the above reaction are + 160 kJ/mol for the ki step and -l- 78 kJ/mol for the k2 step [15]. The overall enthalpy of reaction is — 78 kJ/mol. It has been found that the half-life for the ki reaction is sensitive to the counterion concentration in case of SDS micelles. The effect of added counterion may be due to the charge neutralisation of the sulphate anion heads in the SDS micellar Stern layer, to facilitate approach and penetration of the CN- ions at the micelle-water interface. Hemin encapsulated in CTAB micelles reacts much faster with cyanide compared to that in SDS presumably because of the cationic Stern layer in CTAB. The... 

In our investigations, we have attempted to control the light-induced redox reactions through the use of the organized assemblies so that net useful chemical conversions can be obtained. We have also used some of these reactions as a probe to study the structure and binding properties of surfactant micelles, particularly the micelle-water interface region. 

P.in.r. spectroscopy Hexadecyltrimethylammonium Benzene Micelle-water interface Eriksson and Gillberg, 0... 

The hydrophobic part of the aggregate molecules forms the core of the micelle while the polar head groups are located at the micelle-water interface in contact with the water molecules. Such micelles usually have average radii of 2... 4 nm and contain 50... 100 monomers in water. Their geometric structure is usually roughly spherical or ellipsoidal. In non-aqueous nonpolar solvents, the micellar structures are generally the inverse of those formed in water. In these solvents, the polar head groups form the interior of the micelle while the hydrocarbon chains of the ions are in contact with the nonpolar solvent. 

Fig. 19 Micelle and emulsion droplets in CaCl2 aqueous solutions The calcium-rich shell in the adsorbed state, at the a micelle-water interface b emulsion droplet surface. The surfactant molecules are represented by the zig-zag lines with Ca + heads...

The best fit for M = 64 corresponds to an interfacial energy at the micelle-water interface of y = 52 erg cm in agreement with the measured value of y, at the bulk oil-water interface. The interfacial energies of liquid hydrocarbon-water interfaces vary from about 50 to 54 erg cm (from section 4 with the spherical approximation, the best fit y was 37 erg cm- ). With a 20 A, the value of K2 corresponds to a hydrophobic energy of 13 300 cal mol Curvature corrections (see below) reduce this to about 12 000 cal mol close to the expected value deduced in section 4. 

Quantitative approaches to describing reactions in micelles differ markedly from treatments of reactions in homogeneous solution primarily because discrete statistical distributions of reactants among the micelles must be used in place of conventional concentrations [74], Further, the kinetic approach for bimolecular reactions will depend on how the reactants partition between micelles and bulk solution, and where they are located within the microphase region. Distinct microphase environments have been sensed by NMR spectrometry for hydrophobic molecules such as pyrene, cyclohexane and isopropylbenzene, which are thought to lie within a hydrophobic core , and less hydrophobic molecules such as nitrobenzene and N,N-dimethylaniline, which are preferentially located at the micelle-water interface [75]. Despite these complexities, relatively simple kinetic equations for electron-transfer reactions can be derived for cases where both donors and acceptors are uniformly distributed inside the micelle or on its surface. 

One particular property of micelles stands out above all others their ability to solubilize organic compounds in water. Benzene, for example, dissolves in SDS to the extent of 0.90 mol/mol surfactant, resulting in around 40 benzene molecules per micelle . NMR chemical shift data situate most of the benzene at the micelle-water interface, but the localization of small solubilizates in micelles is never uniform. 

Figure 6.39 Schematic representation of sites of solubilisation depending on the hydrophobicity of the solubilisate. Completely water-insoluble hydrophobic molecules are incorporated in the micelle core (case 1) water-soluble molecules may be solubilised in the polyoxyethylene shell of a nonionic micelle (case 4) solubilisates with intermediate hydrophobicities (cases 2 and 3) are incorporated in the micelle with the hydrophobic region (black) in the core and the hydrophilic region (white) at the micelle/water interface. Redrawn from V. Torchilin, J. Control. Release, 73, 1 37 [2001).

The fluorescent probe 4-aminophthalimide (63) was employed by several authors to study micelles. Samanta and coworkers187,188 used it to study micellization of common cationic (SDS), anionic (cetyltrimethylammonium bromide) and neutral (Triton-X 100) aqueous surfactants, and the same systems were also studied by Datta, Mandal and coworkers189 19°. The critical micelle concentration could be determined and it was found that the probe binds to the micelle water interface or to the cyclodextrin cavities that have also been studied187,188. Water-in-oil microemulsions of Triton-X 100 in a mixture of benzene and hexane showed190 the probe to reside in the water core of the reversed micelles, the polarity of which differs much from that of bulk water. 

Polarizable hydrocarbons, such as short-chain arenes (benzene, isopropylbenzene), have been shown to be solubilized in quaternary ammonium solutions initially by absorption at the micelle-water interface, replacing water molecules that may have penetrated into the outer core of the micelle close to the polar heads, but solubilization of additional material is either deep in the palisade layer or located in the inner core of the micelle (Eriksson, 1965). The polarizability of the ji-electron cloud of the aromatic nucleus and its consequent ability to interact with the positively charged quaternary ammonium groups at the micelle-water interface may account for the initial adsorption of these hydrocarbons in that location. In POE nonionics, benzene may be solubilized between the polyoxyethylene chains of the hydrophilic groups (Nakagawa, 1967). 

Small polar molecules in aqueous medium are generally solubilized close to the surface in the palisade layer or by adsorption at the micelle-water interface. The spectra of these materials after solubilization indicate that they are in a completely, or almost completely, polar environment. Short-chain phenols, when solubilized in POE nonionics, appear to be located between the POE chains (Nakagawa, 1967). 

---