Micelle electric double layer

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Washing and Cleaning Action. The properties of alkyl ether sulfates, due to the good solubility and the special hydrophilic/hydrophobic properties of the molecule, are of particular practical interest. From the investigations described in sections 2 and 3, it can be concluded that, in addition to the decrease in the Krafft Point, favorable properties for practical applications can be expected as a result of the inclusion of the oxyethylene groups into the hydrophobic part of the molecule. As is true for other anionic surfactants, the electrical double layer will be compressed by the addition of multivalent cations. By this means, the adsorption at the interface is increased, the surface activity is raised, and, furthermore, the critical micelle concentration decreased. In the case of the alkyl ether sulfates, however these effects can be obtained without encountering undesirable salting out effects. 

An illustration of the effect of micelle/nanoparticle volume fraction on contact line motion is found in [57]. They used 0.1 M NaCl solution to reduce the electrical double layer thickness surrounding the NaDS micelle. At a given number concentration of micelles, decreasing the size of each micelle decreases the volume fraction greatly, since the volume of each spherical micelle varies as the third power of the radius. Thus, the addition of electrolyte effectively reduced the micellar volume fraction in the aqueous medium. The authors found that the oil droplet that would otherwise become completely detached from the solid surface, came back to reattach itself to the solid when electrolyte was present. They rationalized this finding as being caused by the inability of the weakened structural disjoining forces to counteract the attraction of the oil drop to the solid surface. 

It was recently ascertained that the behavior of the adsorbed film of two surfactants in equilibrium with their micelle can be explained by assuming both the surface region and the micelle particle to be mixtures of the surfactants (1 - ) - Further, the application of the regular solution theory to the mixtures was shown to be useful to describe the nonideal behavior of ionic surfactants ( - ) However, the above treatments are incomplete from the thermodynamic viewpoint, because they do not consider the dissociation of surfactants and ignore the presence of solvent (T). In addition, it is impossible to suppose that the regular solution theory is applicable to both the adsorbed film and the micelle of ionic surfactants accompanied by the electrical double layer ( ). 

To this point, only models based on the pseudo—phase separation model have been discussed. Mixed micelle models utilizing the mass action model may be necessary for micelles with small aggregation numbers, as demonstrated by Kamrath and Franses ( ). However, even for large micelles, the fundamental basis for the pseudophase separation model needs to be examined. In micelles, how much solvent or how many counterions (bound or in the electrical double layer) should be included in the micellar pseudo-phase is unclear. The difficulty is normally surmounted by assuming that the pseudo—phase consists of only the surfactant components i.e., solvent or counterions are ignored. The validity of treating the micelle on a surfactant—oniy basis has not been verified. Funasaki and Hada (22) have questioned the thermodynamic consistency of such an approach. 

CFSE Crystal Field Stabilization Energy CMC Critical micelle concentration DDAC1 N-Dodecylammonium Chloride DEDTC Diethyl Dithiocarbamate EDL Electric Double Layer GChSG Gouy/Chapman/Stern/Grahame IP Isoelectric Point... 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

The stability of inverse micelles has been treated by Eicke (8,9) and by Muller (10) for nonaqueous systems, while Adamson (1) and later Levine (11) calculated the electric field gradient in an inverse micelle for a solution in equilibrium with an aqueous solution. Ruckenstein (5) later gave a more complete treatment of the stability of such systems taking both enthalpic (Van der Waals (VdW) interparticle potential, the first component of the interfacial free energy and the interparticle contribution of the repulsion energy from the compression of the diffuse part of the electric double layer) and entropic contributions into consideration. His calculations also were performed for the equilibrium between two liquid solutions—one aqueous, the other hydrocarbon. 

The discussion of the relative stability of solutions with inverse micelles and of liquid crystals containing electrolytes may be limited to the enthalpic contributions to the total free energy. The experimentally determined entropy differences between an inverse micellar phase and a lamellar liquid crystalline phase are small (12). The interparticle interaction from the Van der Waals forces is small (5) it is obvious that changes in them owing to added electrolyte may be neglected. The contribution from the compression of the diffuse electric double layer is also small in a nonaqueous medium (II) and their modification owing to added electrolyte may be considered less important. It appears justified to limit the discussion to modifications of the intramicellar forces. 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

For our purposes, adsorption from solution is of more direct relevance than gas adsorption. Most, if not all, topics in the five volumes of FICS Involve one or more elements of it. In the present chapter, the basic elements will be introduced, restricting ourselves to low molecular weight, uncharged adsorbates and solid surfaces. Adsorption of charged species leads to the formation of electrical double layers, which will be treated in chapter 3. Adsorption at fluld/fluid Interfaces follows in Volume III. Adsorption of macromolecules will be Introduced in chapter 5. Between monomers, short oligomers, longer oligomers and polymers there is no sharp transition in the present chapter we shall go as far as non-ionic surfactants, but omit most of the association and micelle formation features, which will be addressed in a later Volume. There will be some emphasis on aqueous systems. 

Electric double layers at phase boundaries pervade the entire realm of Interface and colloid science. Especially in aqueous systems, double layers tend to form spontaneously. Hence, special precautions have to be taken to ensure the absence of charges on the surfaces of particles. Insight into the properties of double layers is mandatory, in describing for Instance electrosorption, ion exchange, electrokinetics (chapter 4), charged monolayers (Volume III), colloid stability, polyelectrolytes and proteins, and micelle formation of ionic surfactants, topics that are intended to be treated in later Volumes. The present chapter is meant to Introduce the basic features. 

Linse [59] made a similar simulation for water near a charged surface with mobile counterions constituting an electric double layer such as in the interior of a reverse micelle formed with ionic surfactants in oil. He reported that water in the aqueous core of reverse micelles has a reduced rate of translational and rotational motions by a factor of 2-4. 

The outer surface of the Stern layer is the shear surface of the micelle. The core and the Stern layer together constitute what is termed the kinetic micelle. Surrounding the Stern layer is a diffuse layer called the Gouy-Chapman electrical double layer, which contains the aN counterions required to neutralise the charge on the kinetic micelle. The thickness of the double layer is dependent on the ionic strength of the solution and is greatly compressed in the presence of electrolyte. 

In aqueous media, the surfactant molecules are oriented, in all these structures, with their polar heads predominantly toward the aqueous phase and their hydro-phobic groups away from it. In vesicles, there will also be an aqueous phase in the interior of the structure. In ionic micelles, the aqueous solution-micelle interfacial region contains the ionic head groups, the Stern layer of the electrical double layer with the bound counterions, and water. The remaining counterions are contained in the Gouy-Chapman portion of the double layer that extends further into the aqueous phase. For POE nonionics the structure is essentially the same, except that the outer region contains no counterions, but includes coils of hydrated POE chains. 

The value of o varies not only with the structure of the hydrophilic head group, but also with changes in the electrolyte content, temperature, pH, and the presence of additives in the solution. Additives, such as medium-chain alcohols that are solubilized in the vicinity of the head groups (Chapter 4, Section IIIA), increase the value of oq. With ionic surfactants, o decreases with increase in the electrolyte content of the solution, due to compression of the electrical double layer, and also with increase in the concentration of the ionic surfactant, since that increases the concentration of counterions in the solution. This decrease in the value of ao promotes change in the shape of the micelle from spherical to cylindrical. For POE nonionic surfactants, an increase in temperature may cause a change in shape if temperature increase results in increased dehydration of the POE chain. 

The addition of neutral electrolyte to solutions of ionic surfactants in aqueous solution causes an increase in the aggregation number, presumably because of compression of the electrical double layer surrounding the ionic heads. The resulting reduction of their mutual repulsion in the micelle permits closer packing of the head groups (a0 is reduced), with a consequent increase in n. 

The same phenomenon was observed for ionic surfactant solutions (16, 55), such as a-olefin sulfonates (1), but here the height of the steps was close to the effective micellar diameter, which includes the electric double layer around the ionic micelles. Furthermore, foam films formed from concentrated monodisperse suspensions of polystyrene latexes (16) or silica particles (56) stratify in a similar way. 

By increasing the electrolyte concentration, the electric double layer is compressed and the compensation ions are situated tightly above the charging layer. In the case of a low electrolyte concentration, or in the absence of ions of a low valency, the thickness of the compensation layer increases, the compensation ions dissociate and they are separate from the charging layer by diffusion. This results in the formation of a micelle charge and increase of the value of the electrokinetic potential. The electrokinetic po-... 

These reactions are regarded as electron tunnelling processes, and the results are interpreted in terms of the estimated distributions of occupied and unoccupied electronic levels in the redox systems aq-e, D-D+, and A-A- involved. If the overlap of the occupied levels of the donor with the unoccupied levels of the acceptor is not good, transfer is slow, irrespective of how thermodynamically favourable it may be. These levels are shifted, relative to one another, by the charge on the micellar head-group, which determines the direction of the electrical double layer at the interface. Reaction rates are therefore influenced both by the charge on the micelle and the concentration of electrolyte in the aqueous phase.47 48 The lifetime of normally unstable intermediates may be enormously enhanced by working in a micellar system.49... 

A popular representation of spherical micelles was devised by Hartley (26). As indicated in Fig. 1, the Hartley model of, e.g., an anionic micelle exhibits a spherical electric double layer composed of bulky, hydrated anionic heads of surfactant molecules and their counterions in the aqueous phase, while the hydrophobic tails, visualized as sticks, form a hydrocarbon-like micellar interior. Because of the high surface charge density of the micelle, there is only little electrolytic dissociation of counterions. The Hartley model explains the low conductivity of micellar solutions and the way surfactants work as detergents by solubilizing (i.e. incorporating) hydroi obic substrates. The model fails to explain certain NMR and fluorescence data that demonstrate some contact of... 

The remarkable capability of micelles to solubilize hydrophobic substances in water and locally to concentrate ions has a decisive influence on the course of elementary photochemical processes. In this respect micelles are sometimes referred to as supercages and as microscopic reactors which favor bimolecular processes (17,18). Also the term micellar catalysis is used to describe the higher rate at which bimolecular photochemical reactions proceed. Moreover, ionic micelles, being surrounded by an electric double layer, may promote or inhibit unimolecular processes involving ionization as well as electron transfer. 

In order to explain this effect it was assumed that micelles can be formed by fusion of two smaller aggregates and can disappear by fission into two small particles [116]. The aggregates formed by ionic surfactants are charged particles. At low concentrations they are stable in relation to coagulation because of repulsive electrostatic forces. When the concentration of counterions increases, the electric double layer around the aggregates shrinks, the repulsive electrostatic forces between the aggregates decrease, and the reversible fusion and fission processes can proceed... 

The electrostatic charges of surfactants seriously affect the localization of host molecules in the water pool. Monte Carlo simulation in which ionic reversed micelles are treated as spherical entities showed the presence of the electrical double layer in the interface of the water pool, and the distribution of counterions followed the Poisson-Boltzmann approximation [51]. Mancini and Schiavo [52] assumed recently, by the yield of halogenation, that the specific interactions between bromide or chloride ions and an ammonium head-group in cationic reversed micelles keep the ions in a defined position on the interface. 

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