Surfactant orientation

Figure 1.22 This surfactant orientation is supported by the 4-spacing values that were obtained from transmission electron microscopy measurements. In (A) the first suggested orientation the surfactant length is 2.4 nm and in (B) another orientation the surfactant length is 1.85 nm. (Reproduced from ref. 49, with permission.)... 

To resolve the problem of negative /3 values obtained with the Frumkin theory, the improved Szyszkowski-Langmuir models which consider surfactant orientational states and aggregation at the interface have been considered [17]. For one-surfactant system with two orientational states at the interface, we have two balances, i.e., Ft = Fi + F2 and Ftco = Ficoi + F2C02, which can be used in conjunction with Eq. 24 to derive two important equations for determining the total surface excess and averaged molecular area required in the calculation of surface tension, i.e.,... 

A decrease in flotability is characterized by an increase in polarity of the adsorption layer of both surfactants due to the formation of a second adsorption layer with polar groups of the surfactant orientated toward the bulk phase, up to the CMC. The analysis of the flotation measurements was carried out on the basis of kinetic data for the shortest constant... 

The adsorption of nonionic surfactants on polar and nonpolar surfaces also exhibits various features, depending on the nature of the surfactant and the substrate. Three types of isotherms may be distinguished, as illustrated in Fig. 7. These isotherms can be accounted for by the different surfactant orientations and their association at the solid/liquid interface as illustrated in Fig. 8. Again, bilayers, hemimicelles, and micelles can be identified on various substrates. 

A nematic liquid crystal of negative dielectric anisotropy is aligned with the director aligned orthogonal to the cell walls by means of a surfactant orientation layer, see Figure 3.4. One or two linear, elliptical or circular polarisers are... 

Crystallization from the emulsified state may lead to different nucleation processes than observed for the same fat in bulk liquid form. It has been suggested that nucleation often occurs at the interface of the droplet where surface-active agents are located. The general similarity of the lipophilic components of surfactants oriented at the surface may provide some ordering and structure for the lipid molecules within the droplet and enhance nucleation, as found for example by Kaneko et al. (40) for a hydrocarbon emulsion. Walstra (11) also suggests that formation of compound crystals from emulsions of natural fats may be different than the same fat crystallized from bulk liquid. The initial polymorph formed may also be different, with more stable polymorphs more likely to form in the emulsion (38). 

Air consists of molecules that are mainly non-polar. Surface tension reduction by surfactants at the air-aqueous interface occurs due to adsorption of surfactants at the interface, with the hydrophilic end of the surfactant oriented toward the liquid. The presence of the surfactant molecules reduces the net inward pull toward the bulk liquid, and therefore reduces the surface tension. 

As an example, a fluorocarbon surface layer can be obtained by dissolving a small amount (less than 1%) of a polymerizable fluorosurfactant in a lacquer and cross-linking the surfactant monolayer formed at the surface. Figure 17.31 shows two fluorocarbon surfactants, one polymerizable (a) and the other non-reactive (b), used in such an experiment. The surfactants were added to a poly(methyl methacrylate) (PMMA) lacquer. PMMA is more polar than the hydrocarbon part of the surfactant so the surfactant orients at the film-air interface with... 

Micelles of surfactants in water are formed by molecular aggregates of surfactants oriented in such a way that the hydrophobic parts are directed internally, so that the hydrophilic parts are directed outwards. In this way the aggregates form spheres, cylinders, or laminar layers, dependent on its concentration. 

Rosen [43] also describes the surfactant adsorption isotherm for nonpolar, hydrophobic substrates as Langmuir type. At low concentrations, the surfactant orientation occurs with the hydrophobic tail group close or parallel to the surface and the hydrophilic head group towards the water. As more surfactant is adsorbed, the orientations of the surfactant molecules become more perpendicular to the surface until the CMC, where surface saturation is achieved. The chlferent orientations of the... 

The course of adsorption of a surfactant on the surface of a colloidal particle has been described by Void and Sivaramakrishnan (293) in discussing micellar adsorption. The polar surface is covered by the adsorbed surfactant oriented on the surface so that the exterior is hydrophobic. At a slightly higher concentration, additional adsorption occurs on this primary sorbed layer to form a condensed double layer over the surface, with the polar groups of the surfactant now oriented outward, thus making the surface hydrophilic. They presented experimental evidence to show that as more surfactant is added to the system, the concentration in solution decreases past a certain point because of the sudden aggregation of the adsorbed material to form the double layer of the surface micelles. 

The characterization of the adsorption behavior of a given surfactant onto a particular surface can be obtained through the determination of the adsorption isotherm. Its analysis is helpful in determining (1) surface excess concentration of the surfactant, (2) the packing of the surfactant at the solid surface and thus, in some cases, the surfactant orientation at... 

Flocculation and deflocculation of montmorillonite (Veegum) suspensions by a non-ionic surfactants has been studied by Ohno et al. [21]. Adsorption isotherms of the surfactant on to the clay showed points of inflexion due to bimolecular adsorption. Maximum flocculation occurred when the surface was covered by a layer of surfactant oriented with the alkyl chain pointing out into the aqueous phase. The deflocculation which occurs on further addition of surfactant is due to the hydrophilization of the surface by the second layer of surfactant molecules. The viscosity of the clay suspension reaches a maximum at the point of maximum monolayer coverage and then falls on further addition of surfactant. Fig. 9.9... 

Surfactants are surface-active compounds that consist of a hydrophihc head group attached to a hydrophobic tail (usually a long alkyl chain). They have a high affinity for water or oil depending on the dominant moiety (Walz, 1998 McClements, 2004). When present in sufficiently high concentrations, surfactants form a monolayer at the interface between the oil and water, with the hydrophobic tails of the surfactant orientated towards the oil phase and the hydrophilic head groups towards the aqueous phase. There are four categories of surfactants available in the food industry ionic, non-ionic, zwitterionic and cationic. 

---