Surfactants hydrophobic regions

For aqueous solutions, the chemical constituents most commonly responsible for foaming arc surfactants, i.e.. surface-active agents. Such molecules find wide use in other settings, and are distinguished by having both hydrophilic and hydrophobic regions. 

Enhancement of the aqueous solubility by surfactants occurs as a result of the dual nature of the surfactant molecule. The term surfactant is derived from the concept of a surface-active agent. Surfactants typically contain discrete hydrophobic and hydrophilic regions, which allow them to orient at polar-nonpolar interfaces, such as water/air interfaces. Once the interface is saturated, th surfactants self-associate to form micelles and other aggregates, whereby their hydrophobic region are minimized and shielded from aqueous contact by their hydrophilic regions. This creates a discrete hydrophobic environment suitable forsolubilization of many hydrophobic compounds (Attwood and Florence, 1983 Li et al., 1999 Zhao et al., 1999). 

Micelles are formed in order to protect the hydrophobic regions of the amphiphilic surfactant from the aqueous solution. The surface area S occupied by the surfactant molecule can be determined by ... 

Structure Formation in Surfactant Solutions. Surfactants, also referred to as soaps, detergents, tensides, or surface active agents, are amphiphilic molecules possessing both hydrophilic and hydrophobic regions. They can be classified as anionic, cationic, zwitterionic, or nonionic (neutral) depending upon the nature of the polar... 

The first source of confusion was the fact that minimal surfaces represent local minima in surface area under Plateau (or fixed boundary ) boundary conditions. The importance of this property with respect to cubic phases must be considered to be limited, however, because the surface area of the interfacial dividing surface — drawn between the hydrophilic and the hydrophobic regions of the microstucture - is given simply by the product of the number of surfactant molecules, times the average area per siufactant which is strongly fixed by the steric, van der Waals, and electrostatic interactions between surfactant molecules. Therefore this interfacial area does not in general seek a minimum but rather an optimum value, which does not tend to zero because of the electrostatic repulsion between surfactant head groups. Furthermore,... 

The rate of transmembrane diffusion of ions and molecules across a membrane is usually described in terms of a permeability constant (P), defined so that the unitary flux of molecules per unit time [J) across the membrane is 7 = P(co - f,), where co and Ci are the concentrations of the permeant species on opposite sides of membrane correspondingly, P has units of cm s. Two theoretical models have been proposed to account for solute permeation of bilayer membranes. The most generally accepted description for polar nonelectrolytes is the solubility-diffusion model [24]. This model treats the membrane as a thin slab of hydrophobic matter embedded in an aqueous environment. To cross the membrane, the permeating particle dissolves in the hydrophobic region of the membrane, diffuses to the opposite interface, and leaves the membrane by redissolving in the second aqueous phase. If the membrane thickness and the diffusion and partition coefficients of the permeating species are known, the permeability coefficient can be calculated. In some cases, the permeabilities of small molecules (water, urea) and ions (proton, potassium ion) calculated from the solubility-diffusion model are much smaller than experimentally observed values. This has led to an alternative model wherein permeation occurs through transient hydrophilic defects, or pores , formed by thermal fluctuations of surfactant monomers in the membrane [25]. 

Some of the amino acid side chains in proteins are hydrophobic, generally buried in the interior of the folded protein molecule but exposed if the protein is unfolded. Sometimes these hydrophobic regions are partially exposed even in the native folded protein, and they are often referred to as hydrophobic patches on the protein surface. The lipophilic parts of surfactants interact with these hydrophobic regions,... 

Surface-active agents (surfactants) are substances which, at low concentrations, adsorb onto the surfaces or interfaces of a system and alter the surface or interfacial free energy and the surface or interfacial tension. Surface-active agents have a characteristic structure, possessing both polar (hydrophilic) and non-polar (hydrophobic) regions in the same molecule. Thus surfactants are said to be amphipathic in nature. The wide range of uses for surfactants in pharmaceutical products and systems is the subject of this article. 

For surfactants, the hydrophobic free energy of transfer of the lipophilic hydrocarbon tail from water to oil provides the driving force for aggregation. But the hydrophilic head-groups prefer an aqueous environment and an interface between the polar region and the lipophilic domains results. With hydrocarbon tails, like alkanes, there are a large number of accessible tail conformations, so that the hydrophobic region is usually fluid-like aroimd room temperature. The interface can be a dynamic one of a well-defined... 

Figure 4.2 View of the curvature of aggregates formed by surfactant molecules of various siufactant parameters, v/al. (Left ) If the surfactant parameter is less than one, the interface between the polar and hydrophobic regions curves towards the chain region, and the average molecular shape is tapered towards the hydrophobic end of the molecule. (Middle ) If the surfactant parameter is exactly equal to one, the interface exhibits no preferential curr atiue, and the molecules are on average cylindrical. (Right) If the surfactant parameter exceeds one, the interface curves towards the polar regions, and the molecule tapers towards the head-group.

Setting v(l) equal to the chain volume, v, I to the chain length and (0) to the head-group area, a, leads to a simple general expression for the surfactant parameter in terms of the curvatures of the interface between hydrophilic and hydrophobic regions, scaled by the characteristic distance, 1 ... 

Consider, for example a spherical micelle. By our convention, if the interface encloses hydrophobic regions, the mean curvature is negative. Consequently, the surfactant parameter for a spherical micelle of radius R is given by ... 

The details of the calculations for both reversed and normal bilayers (for which the tunnels are filled with water and surfactant respectively) are given elsewhere [7-9]. We characterise the concentration of the surfactant by the volume fraction of the hydrophobic region, chains- The relation between composition and molecular shape for hyperbolic bilayers is ... 

The most water-dilute botmdaries of the microemulsion phase regions within the ternary phase triangle of our systems are invariably lines of constant surfactant to water fraction (see Fig. 4.19). It is easy to show that this implies the micellar radii at the upper water limit of the microemulsion region are constant, irrespective of the oil content. The implication is that the curvatures of the surfactant interface, which separates the hydrophilic from hydrophobic regions, are fixed within this region. 

Surfactant molecules have hydrophilic and hydrophobic regions and adsorb at interfaces in such a way that their hydrophobic regions are removed from the aqueous environment. The forces of attraction between surfactant and water molecules in the interface are less than those between two water molecules and hence the surface tension is reduced as a result of adsorption. 

In the context of utilizing surfactant assemblies for building nanostructured polymeric materials, one other approach that deserves mention is the polymerization of standard monomers partitioned into the hydrophobic regions of surfactant aggregates one in particular that has received a lot of attention is vesicle templating [84]. The basic idea in this approach is to generate vesicles using appropriate surfactants and then to solubilize standard monomers, such as styrene, within the... 

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