Surfactants clusters

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

CMC), the concentration at which the monomeric form, in which the surfactant exists in very dilute solution, aggregates to form a surfactant cluster known as a micelle (Chapter 3). Above this concentration the surface tension of the solution remains essentially constant since only the monomeric form contributes to the reduction of the surface or interfacial tension. For concentrations below but near the CMC the slope of the curve is essentially constant, indicating that the surface concentration has reached a constant maximum value. In this range the interface is considered to be saturated with surfactant (van Voorst Vader, 1960a) and the continued reduction in the surface tension is due mainly to the increased activity of the surfactant in the bulk phase rather than at the interface (equation 2.17). For ionic surfactants in the presence of a constant concentration of counterion, this region of saturated adsorption may extend down to one-third of the CMC. 

These mesoporous molecular sieves are prepared using a liquid crystal templating mechanism in which micelles, which are assemblies of cationic alkyl trimethylammonium surfactants [CH3(CH2) N+(CH3)3] X , act as a template for the formation of the silicaceous material (Figure 6.2). In the silicate-rich aqueous solution, the hydrophobic tails of the surfactant cluster together, leaving the positively charged heads to form the outside of the rod-like liquid crystal micelles. The silicate anions are attracted to, and surround the micelles, aggregating into an open-framework amorphous solid, which precipitates. The solid is filtered off, and heated in air at up to 700 °C (calcination), which removes the surfactant and leaves the... 

Lorenz [22] considered the case that a two-dimensional association occurs in the adsorption layer. Here the surfactant monomers are in equilibrium with surfactant clusters of constant size. He derived the corresponding frequency dependence of cot = < C par/ par- For lower frequencies, a characteristic decrease in cot a should appear compared to cot (5a given by Eqs. (IL5.46) and (II.5.47). Such behaviour was in fact detected for higher concentrations of caproic as well as caprylic acid. 

Figure 23.1. Network-like superstructure of a surfactant film of cetyltrimethylammonium bromide (c = 6 x 10 mol/1). The cross-linking process between the molecules was induced by adding multivalent counterions such as cerium sulfate c = 10 mol/1). The Brewster-angle image was obtained during the adsorption process of the amphiphilic molecules and represents the situation near the sol-gel transition, where the first infinite large clusters were formed. Black zones represent the pure water surface, while clear areas correspond to the presence of surfactant clusters...

Binding sites are localized on charged groups in the case of macroions, but delocalized in uncharged polymers so that surfactant clusters move along the macromolecular chains. In any case, bound aggregates are viewed as hydrophobic domains dispersed in aqueous media by host macromolecules. Cooperativity is a common feature in surfactant binding to my class of macromolecules such as nucleic acids. 

Next, the number of bound surfactant jons is given by d In o /d In a. The average bound surfactant cluster, m, can be calculated from28... 

The main feature of the proposed mechanism is that it is expressed in terms of complex subimit (subaggregate) formation in excess form. Equations 5 and 8 permit one to determine the standard free energy change of the process and the ag egation number of the surfactant clusters in polymer complex, as well as the total number of the active centers in the polymer molecules. 

The process of surfactant clustering or micellization is primarily an entropy-driven process. When surfactants are dissolved in water, the hydrophobic group disrupts the structure of water and therefore increases the free energy of the system. Surfactant molecules therefore concentrate at interfaces, so that their hydrophobic groups are directed away from the water and the free energy of the solution is minimized. The distortion of... 

At low concentrations surfactant molecules adsorbed at the surface are in equilibrium with other molecules in solution. Above a threshold concentration, called the critical micelle concentration (cmc, for short), another equilibrium must be considered. This additional equilibrium is that between individual molecules in solution and clusters of emulsifier molecules known as micelles. 

When micelles are formed just above the cmc, they are spherical aggregates in which surfactant molecules are clustered, tails together, to form a spherical particle. At higher concentrations the amount of excess surfactant is such that the micelles acquire a rod shape or, eventually, even a layer structure. 

Surfactants have a unique long-chain molecular structure composed of a hydrophilic head and hydrophobic tail. Based on the nature of the hydrophilic part surfactants are generally categorized as anionic, non-ionic, cationic, and zwitter-ionic. They all have a natural tendency to adsorb at surfaces and interfaces when added in low concentration in water. Surfactant absorption/desorption at the vapor-liquid interface alters the surface tension, which decreases continually with increasing concentrations until the critical micelle concentration (CMC), at which micelles (colloid-sized clusters or aggregates of monomers) start to form is reached (Manglik et al. 2001 Hetsroni et al. 2003c). 

It has been found that at surfactant concentrations higher than 0.1 M, water-containing reversed micelles of AOT are not randomly dispersed in an isolated state in n-heptane but form clusters through intermicellar flocculation [241,242]. 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

The catalytic applications of Moiseev s giant cationic palladium clusters have extensively been reviewed by Finke et al. [167], In a recent review chapter we have outlined the potential of surfactant-stabilized nanocolloids in the different fields of catalysis [53]. Our three-step precursor concept for the manufacture of heterogeneous egg-shell - nanocatalysts catalysts based on surfactant-stabilized organosols or hydrosols was developed in the 1990s [173-177] and has been fully elaborated in recent time as a standard procedure for the manufacture of egg-shell - nanometal catalysts, namely for the preparation of high-performance fuel cell catalysts. For details consult the following Refs. [53,181,387]. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

Another example of chemical-potential-driven percolation is in the recent report on the use of simple poly(oxyethylene)alkyl ethers, C, ), as cosurfactants in reverse water, alkane, and AOT microemulsions [27]. While studying temperature-driven percolation, Nazario et al. also examined the effects of added C, ) as cosurfactants, and found that these cosurfactants decreased the temperature threshold for percolation. Based on these collective observations one can conclude that linear alcohols as cosurfactants tend to stiffen the surfactant interface, and that amides and poly(oxyethylene) alkyl ethers as cosurfactants tend to make this interface more flexible and enhance clustering, leading to more facile percolation. 

---