Structure of surfactant aggregates

Surfactants not only aggregate to spherical micelles but also form cylinders, bilayers, inverted micelles, etc. [524], The type of aggregate structure formed depends on different factors. An important factor is the so-called surfactant parameter, also referred to as the packing ratio [533]  

Vc is the volume of the hydrophobic part of the surfactant, Lc is the length of the hydrocarbon chains, and a a is the effective area per head group. 

From the density of pure hydrocarbons a simple equation for the volume of a saturated hydrocarbon chain (no double bonds) can be obtained (see exercises)  

The first number, 0.127 nm, is the carbon-carbon bond length (0.154 nm) projected onto the direction of the alkyl chain in all-trans configurations. The second number, 0.15 nm, is the van der Waals radius of the terminal methyl group, minus 0.127 nm. 

The most problematic quantity in the definition of the surfactant parameter is the head group area. For ionic surfactants a a depends on both the electrolyte and the surfactant concentration. In this case the surfactant parameter is only of limited usefulness for a quantitative 

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These examples allow us to describe tiie structure of surfactant aggregates in terms of the value of the surfactant parameter. Indeed, this is the case for simple closed surfaces, where the interior contains the hydrophobic fraction (v/al[Pg.145]

Although many techniques have been employed to determine both the concentration and the properties of adsorbed surfactants, most of these techniques do not directly probe that lateral organization of surfactants. In 1994, Manne et al. [27] showed that the AFM could be used to image directly the structure of surfactant aggregates at the solid-liquid interface above the surfactant cmc. This method is fast, does not require special sample preparation, is not restricted to particular surfactant molecules, and determination of the shape is not model dependent. 

For obvious reasons, we need to introduce surface contributions in the thermodynamic framework. Typically, in interface thermodynamics, the area in the system, e.g. the area of an air-water interface, is a state variable that can be adjusted by the observer while keeping the intensive variables (such as the temperature, pressure and chemical potentials) fixed. The unique feature in selfassembling systems is that the observer cannot adjust the area of a membrane in the same way, unless the membrane is put in a frame. Systems that have self-assembly characteristics are conveniently handled in a setting of thermodynamics of small systems, developed by Hill [12], and applied to surfactant self-assembly by Hall and Pethica [13]. In this approach, it is not necessary to make assumptions about the structure of the aggregates in order to define exactly the equilibrium conditions. However, for the present purpose, it is convenient to take the bilayer as an example. 

Another important area of progress is the enlargement of the scope of the structure of aqueous aggregates. The bilayer membrane formed from dialkyl amphiphiles belongs to a new class of the aqueous aggregate, totally different from the conventional surfactant micelle. A trialkylammonium compound gives yet another type of aggregation. 

Because of the hmited solubility in different solvents surfactants form different types of surfactant assemblies in solutions and on solids. These organized assemblies are formed when different proportions of surfactants, oils, cosurfactants and water are mixed together. The types of surfactant aggregate formed depends on its chemical structure and the nature of the medium. 

A detailed study of the structure of the aggregates of the ionic surfactants in polyelectrolyte networks was presented in Refs. [66,68]. The dynamics of the changes in the microenvironment of the fluorescent probe, pyrene, in slightly crosslinked networks of poly(diallyldimethylammonium bromide) (PDADMAB) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by means of fluorescence spectroscopy. In Ref. [66], an analogous investigation was reported for complexes formal by the sodium salt of PMAA with cetyltrimethylammonium bromide (CTAB). 

Polymer-surfactant aggregates formed "In-sltu" In solution can dissociate or change their structure depending upon the solution conditions. Such changes can be avoided by either polymerizing appropriately chosen structures of surfactants or by using preformed entitles which combine the two features In the same molecule. We refer here to polymeric surfactants. The concept of polymeric surfactants Is not new. To some extent proteins themselves embody this principle. Strauss and co-workers studied "polysoaps" derived from... 

In addition to giving information about the shape and internal structure of colloidal aggregates, SANS studies can also be used profitably to determine the thickness and conformation of polymer layers adsorbed onto the surface of colloidal particles such as latex nanoparticles, and in some special cases, the surface of emulsion droplets. ° In such studies, the particles on which the polymer is adsorbed must generally be very accurately contrast matched to the solvent so as to allow information to be obtained only about the adsorbed layer. SANS studies have also been recently used in combination with differential scanning calorimetry and visual inspection of the solutions, to draw up a (simplified) partial phase diagram of the aggregation behavior of a polymeric surfactant in water.t ... 

The molecular shape is not the sole determinant of the structure of the aggregate. If the suifactant-water mixture is to form a single phase, the smface and volume requirements set by the composition of Ae mixture must be satisfied. Introducing the global constraint set by the composition leads to an estimate of the relation between the local geometry (expressed by the surfactant parameter) and the composition at which the surfactant mixture is expected to form a bilayer - or reversed bilayer - wrapped onto an IPMS (illustrated in Fig. 4.7). 

Fig. 2 Examples of aggregate structures of surfactants in solution (a) micelle (b) inverted micelles (c) bilayer vesicle (d) bilayer. (From Ref. 7.)...

The nonpolar portion of surfactant ions has an important role in promoting the adsorption process because it increases the affinity of these organic ions to the interfacial region. The effect derives from mutual attraction between the hydrophobic tails as well as their tendency to escape from an aqueous environment. That mechanism is precisely the same one which causes the spontaneous formation of micelles in aqueous solution and is known as the hydrophobic effect [78]. In the case of surfactant adsorption, it is responsible for the formation of surface aggregates. However, it is not easy to accurately predict the shape and the size of such molecular associations in the same way that the structure of bulk aggregates can be determined from the geometry of the molecule. This is because the surface imposes different restrictions on the organization of the adsorbed layer. 

There has been much interest in studying surfactant aggregation in polar solvents other than water over the last few decades. In a large number of studies various surfactant systems have been mapped and evidence for self-assembly of surfactants in some nonaqueous polar solvents has been published. During the last few years more detailed information on the structure of the aggregates and on the characteristics of the aggregation processes have been provided. 

Surface active substances or surfactants are amphiphilic compounds having a lyophilic, in particular hydrophilic, part (polar group) and a lyophobic, in particular hydrophobic, part (often hydrocarbon chain). The amphiphilic structure of surfactants is responsible for their tendency to concentrate at interfaces and to aggregate in solutions into various supramolecular structures, such as micelles and bilayers. According to the nature of the polar group, surfactants can be classified into nonionics and ionic, which may be of anionic, cationic, and amphoteric or zwitterionic nature. 

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