Surfactant hydrocarbon region

By small-angle neutron scattering experiments on water/AOT/hydrocarbon microemulsions containing various additives, the change of the radius of the miceUar core with the addition of small quantities of additives has been investigated. The results are consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water/surfactant interfacial region, decreasing the micellar radius, whereas toluene remains predominantly in the bulk hydrocarbon phase. The effect of n-alcohols on the stability of microemulsions has also been reported [119],... 

Most single-chain surfactants do not sufficiently lower the oil-water interfacial tension to form MEs, nor are they of the right molecular structure (i.e., HLB) to act as cosolvents. To overcome such a barrier, cosurfactant/cosolvent molecules are added to further lower the interfacial tension between oil and water, fluidize the hydrocarbon region of the interfacial film, and influence the curvature of the film. Typically small molecules (C3-C8) with a polar head (hydroxyl or amine) group that can diffuse between the bulk oil and water phase and the interfacial film are suitable candidates [11],... 

In the above equation ftjjj, and ftpg are the thickness of the region oeeupied by the surfactant hydrocarbon chain and... 

Becanse many types of polar groups exist and because the hydrocarbon portion of a surfactant may take different forms, many types of surfactants exist. Some simple polar groups are carboxylic acid (COOH), carboxylate ion (COO ), alcohol (OH), sulfate (S04 ), sulfonate (SO3-), and amine (NH3+). Hydrocarbon chains may be straight or branched, satnrated or unsaturated. Aromatic and naphthenic rings may also be present in the hydrocarbon region. Some surfactants have other nonpolar groups such as fluorocarbons. It is evident that, given suitable methods of synthesis, practically an infinite variety of surfactants could be made. 

A qualitative phase diagram or phase map study performed on these systems over the temperature interval (-20 +80) C, keeping constant both the surfactant/hydrocarbon and the alcohol/hydrocarbon ratio, showed that, as the water content is increased, four macroscopically different structure-regions may be identified I°- an optically transparent region, where the system is a stable and isotropic dispersion of water spherical droplets in a continuous oily phase (1-4-5) ... 

The classical model, as shown in Figure 1, assumes that the micelle adopts a spherical structure [2, 15-17], In aqueous solution the hydrocarbon chains or the hydrophobic part of the surfactants from the core of the micelle, while the ionic or polar groups face toward the exterior of the same, and together with a certain amount of counterions form what is known as the Stern layer. The remainder of the counterions, which are more or less associated with the micelle, make up the Gouy-Chapman layer. For the nonionic polyoxyethylene surfactants the structure is essentially the same except that the external region does not contain counterions but rather rings of hydrated polyoxyethylene chains. A micelle of... 

The sharp increase in adsorption in region II marks the onset of surfactant association at the surface through lateral interaction of the hydrocarbon chains. 

For spherical micelles, one of the more commonly accepted models for the micellar structure is that proposed by Gruen (Fig. 1). This model features a rather sharp interface between a dry hydrophobic hydrocarbon core and a region filled with surfactant headgroups, part of the counterions (for ionic surfactants), backfolding surfactant tails, and water, namely, the Stern region. In the remainder of this chapter, intramicelle volumes with specific features such as the Stern region and the hydro-phobic core will be referred to as zones. ... 

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