Orientation, surfactant molecules

In water the surfactant molecules orient themselves with their hydrophobes at the centre of the cluster. The CMC is typically quite low, perhaps 0.5-0.2 g/l. At concentrations lower than this the molecules orient themselves only at the interfaces of the solution, and it is this effect which brings about the lowering of surface tension. Once the CMC is reached the... 

It is important to consider the different stages when producing microemulsions from macroemulsions. It was mentioned earlier that surfactant molecules orient with the hydrophobic group inside the oil phase, while the polar group orients toward the water phase. The orientation of surfactants at the interfaces cannot be measured by any direct method, although much useful information can be obtained from mono-layer studies of the air-water or oil-water interfaces. 

In non-polar solvents, hydrophilic head groups interact due to dipole-dipole attractions and produce aggregates called reverse micelles. With this structure, head groups of surfactant molecules orientate towards the interior and the hydrophobic tails orientate towards the nonpolar solvents. In the absence of additives such as water, the aggregation numbers of reverse micelles are small (mostly less than 10). On the other hand, in polar solvents such as glycol,... 

Surfactants accumulate at interfaces, a process described as adsorption. The simplest interfaces are the air/water (A/W) and oil/water (O/W). The surfactant molecule orients itself at the interface, with the hydrophobic portion orienting towards the hydrophobic phase (air or oil) and the hydrophilic portion orienting at the hydrophihc phase (water) this is shown schematically in Figure 10.14. As a result of adsorption, the surface tension of water is reduced from its value of 72 mN m before adsorption to 30-40mN m, while the interfacial tension for the O/W system decreases from a value of 50 mN m (for an alkane oil) before adsorption to a value of 1-lOmN m , depending on the nature of the surfactant. 

Surface Tension Measurement. The most commonly used cmc determination method is the surface tension measurement [22]. Figure 2.2 shows that the surfactant molecules orientate at the solution air-water interface. This surfactant adsorption decreases the surface tension. The magnitude of the tension decrease depends on the free monomer surfactant concentration. As shown by Figure 2.5, the free surfactant monomer concentration reaches a plateau for surfactant concentrations above the cmc. As the surfactant concentration is increased in the aqueous solution, the surface tension... 

D. The polymer is an uncharged random or multiblock copolymer. The surfactant molecules orient themselves at domain boundaries separating the polymer segments of different polarities. 

A micelle comprises 50-100 molecules of surfactant and is approximately 40A across. The micelles form because the surfactant molecules orientate to remove the hydrophobic part as far as possible from the water and are stabilised at the water micelle interface by the electrical charge on the polar (hydrophilic) portion of the molecule. 

Rasing Th, Shen Y R, Kim M W, Valint P Jr and Bock J 1985 Orientation of surfactant molecules at a liquid-air interface measured by optical second-harmonic generation Phys. Rev. A 31 537-9... 

Effects of Surfactants on Solutions. A surfactant changes the properties of a solvent ia which it is dissolved to a much greater extent than is expected from its concentration effects. This marked effect is the result of adsorption at the solution s iaterfaces, orientation of the adsorbed surfactant ions or molecules, micelle formation ia the bulk of the solution, and orientation of the surfactant ions or molecules ia the micelles, which are caused by the amphipathic stmcture of a surfactant molecule. The magnitude of these effects depends to a large extent on the solubiUty balance of the molecule. An efficient surfactant is usually relatively iasoluble as iadividual ions or molecules ia the bulk of a solution, eg, 10 to mol/L. 

The above results show that the structure of the system with the molecules self-assembled into the internal films is determined by their correlation functions. In contrast to simple fluids, the four-point correlation functions are as important as the two-point correlation functions for the description of the structure in this case. The oil or water domain size is related to the period of oscillations A of the two-point functions. The connectivity of the oil and water domains, related to the sign of K, is determined by the way four moleeules at distanees eomparable to their sizes are eorrelated. For > 0 surfactant molecules are correlated in such a way that preferred orientations... 

In ternary mixtures of oil, water, and surfactant the ordering properties of the system follow from the vectorial character of the interactions of the surfactant molecules with both the oil and the water molecules. The typical size of the ordered domains, much larger than the molecular size, justifies the application of the mesoscopic Landau-Ginzburg approach to the ordering. In the simplest approach of Gompper and Schick [3,12], which we call here the basic Landau-Ginzburg model, the orientational degrees of free-... 

NOTE The orientation of surfactant molecules at an interface (water-solvent, water-gas, water-metal) confers performance characteristics on the molecule that permit it to be employed as an emulsifier, demulsifier, wetting agent, antifoam, lubricant, or other agent. 

In highly diluted solutions the surfactants are monodispersed and are enriched by hydrophil-hydrophobe-oriented adsorption at the surface. If a certain concentration which is characteristic for each surfactant is exceeded, the surfactant molecules congregate to micelles. The inside of a micelle consists of hydrophobic groups whereas its surface consists of hydrophilic groups. Micelles are dynamic entities that are in equilibrium with their surrounded concentration. If the solution is diluted and remains under the characteristic concentration, micelles dissociate to single molecules. The concentration at which micelle formation starts is called critical micelle concentration (cmc). Its value is characteristic for each surfactant and depends on several parameters [189-191] ... 

The interfacial activity is determined by the sterical properties of the molecule. At the interface the spatial demand A0 of the hydrophobic part of the molecule is higher because of the second chain of the internal sulfonate compared with the terminal sulfonate. Thus, the surface concentration of the surfactant molecules is lower. That means that the hydrocarbon chains are laterally oriented and therefore cover the interface between the solution surface and air more completely. Because the ratio of the spatial demand of the head group to the volume of the alkyl chain governs the radius of the micellar surface, it... 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

Of special interest in liquid dispersions are the surface-active agents that tend to accumulate at air/ liquid, liquid/liquid, and/or solid/liquid interfaces. Surfactants can arrange themselves to form a coherent film surrounding the dispersed droplets (in emulsions) or suspended particles (in suspensions). This process is an oriented physical adsorption. Adsorption at the interface tends to increase with increasing thermodynamic activity of the surfactant in solution until a complete monolayer is formed at the interface or until the active sites are saturated with surfactant molecules. Also, a multilayer of adsorbed surfactant molecules may occur, resulting in more complex adsorption isotherms. 

Another means of measuring the properties of insoluble films at the air-water interface is through the use of surface potentials. Surface potential (AF) measures the charge separation created by the vector component of the surfactant s molecular dipole that is perpendicular to the air-water interface. Thus, the surface potential yields information about the orientation of the surfactant molecules. Surface potential values are often expressed alternatively as surface dipole moments /i according to (2), where n is the... 

---