Surface aggregation concentration

Some theoretical models based on the thermodynamics of surfactant self-assembly have also been recently used to predict the critical surface aggregation concentration (the bulk concentration at which surfactants start to self-assemble at the solid-liquid interface), and the self-assembled surfactant structure at the solid-liquid interface (11). These models, although providing useful insight into the surfactant self-assembly process, require an estimate of the interaction energies, which are difficult to determine experimentally. Variations in the estimated interaction energies can lead to different self-assembled surfactant structures, depending on the values used for the calculations. 

Micelles are spherical aggregates of surfactant molecules that can be represented by fig. 1.2a. The concentration at which micelles form in solution is known as the critical micelle concentration (CMC). The concentration at which surfactants aggregate at surfaces to form monolayer-level surface coverage (see fig. 1.2b) is referred to as the surface aggregation concentration (SAC). The SAC is usually very similar to the CMC, although the SAC is usually lower due to interactions with immobile lattice atoms. Other aggregate structures such as bilayers and cylindrical micelles can also form above the CMC or SAC. 

FIGURE 3.3 Surface tension y plotted against the concentration c of lysine derivative 3.20. Critical aggregate concentration (O) cM=2430mg/L=2.43mM, yc =58mN/m. (Reprinted from Nalum Naess, S. et al., Chem. Phys. Lipids, 148, 63, 2007. With permission.)M... 

What characterizes surfactants is their ability to adsorb onto surfaces and to modify the surface properties. At the gas/liquid interface this leads to a reduction in surface tension. Fig. 4.1 shows the dependence of surface tension on the concentration for different surfactant types [39]. It is obvious from this figure that the nonionic surfactants have a lower surface tension for the same alkyl chain length and concentration than the ionic surfactants. The second effect which can be seen from Fig. 4.1 is the discontinuity of the surface tension-concentration curves with a constant value for the surface tension above this point. The breakpoint of the curves can be correlated to the critical micelle concentration (cmc) above which the formation of micellar aggregates can be observed in the bulk phase. These micelles are characteristic for the ability of surfactants to solubilize hydrophobic substances in aqueous solution. So the concentration of surfactant in the washing liquor has at least to be right above the cmc. 

Equations 27 and 28 present the extension of the Szyszkowski-Langmuir model to the adsorption of one-surfactant systems with aggregation at the interface. For the formation of dimmers on the surface, n = 2 and Eqs. 27 and 28 can be expanded to obtain the Frumkin equation of adsorption state. In general, the surface aggregation model described by Eqs. 27 and 28 contains four free parameters, including coi, n, b and Fc, which can be obtained by regression analysis of the data for surface tension versus surfactant concentration in the solution. 

Surface aggregates formed by ionic surfactant adsorption on oppositely charge surfaces have been shown to be bi layered structures (1.) and are called admicelles<2) in this paper, though they are sometimes referred to as hemimicelles. The concentration at which admicelles first form on the most energetic surface patch is called the Critical Admicellar Concentration (CAC) in analogy to the Critical Micelle Concentration (CMC), where micelles are first formed. Again, in much of the literature, the CAC is referred to as the Hemimicellar Concentration (HMC). 

Figure 5.3 Self-assembly of a vesicle. Water-soluble molecules can be entrapped inside, ionic molecules on the polar head groups of the surface, amphiphatic molecules in the hydrophobic bilayer, (cac critical aggregate concentration).

Long-chain fatty acids are insoluble in water, and their titration curves are concentration-dependent because of the formation of organized aggregates (acid soaps, soap micelles, fatty acid precipitates) which concentrate protons at the surface. At concentrations above the critical micellar concentration, solutions of long-chain fatty acid soaps manifest a diprotic curve when they are titrated from pH 10 to 4 (23). The first... 

Y. Li, S. Zhang, Q. Wang, and J. Yang, Study on surface activity and critical aggregation concentration of sucrose esters containing different isomers of mono-, di- and polyesters, Ten-side Surfactants Deterg., 41 (2004) 26-30. 

G. Garofalakis, B. S. Murray, and D. B. Sarney, Surface activity and critical aggregation concentration of pure sugar esters with different sugar headgroups, J. Colloid Interface Sci., 229 (2000) 391-398. 

The properties of surfactant at low concentration in water are similar to those of simple electrolytes except that the surface tension decreases sharply with increase in concentration. At a certain concentration, surfactant monomers assemble to form a closed aggregate (micelle) in which the hydrophobic tails are shielded from water while the hydrophilic heads face water. The critical aggregation concentration is called the critical micelle concentration (CMC) when micelles form in an aqueous medium. The CMC is a property of the surfactant. It indicates the point at which monolayer adsorption is complete and the surface active properties are at an optimum. Above the CMC, the concentrations of monomers are nearly constant. Hence, there are no significant changes in the surfactant properties of the solution since the monomers are the cause of the surface activity. Micelles have no surface activity and any increase in the surfactant concentration does not affect the number of monomers in the solution but affects the structure of micelles. 

Silicone surfactants in aqueous solutions show the same general behavior as conventional hydrocarbon surfactants - the surface tension decreases with increasing concentration until a densely packed film is formed at the surface. Above this concentration, the surface tension becomes constant. The concentration at the transition is called the critical micelle concentration (CMC) or critical aggregation concentration (CAC). The surface and interfacial activity of silicone surfactants was reviewed by Hoffmann and Ulbricht [27]. Useful discussions of the dependence of the surface activity of polymeric silicone surfactants on molecular weight and structure are given by Vick [28] and for the trisiloxane surfactants by Gentle and Snow [29]. 

A second set of possibilities arises if the electroactive species is adsorbed onto the surface of the micelle under these circumstances the main effects will be a marked decrease (normally) in apparent diffusion coefficient and again the possibility of inhibited electron transfer due to surface aggregate formation. Finally, the micelle itself may be formed from electroactive surfactant species, and their electrochemistry under controlled hydrodynamic conditions used to explore changes in structure with increasing surfactant or background electrolyte concentrations. 

A dispersion Is a system made of discrete objects separated by a homogeneous medium In colloidal dispersions the objects are very small In at least one dimension. Colloidal sizes range from 1 to 100 nm however these limits are somewhat arbitrary, and It Is more useful to define colloids as dispersions where surface forces are large compared to bulk forces. Here we are concerned with systems where the dispersion medium Is a liquid examples are droplets In emulsions or mlcroemulslons (oll/water or water/oll), aggregates of amphiphilic molecules (surfactant micelles), foams, and all the dispersions of solid particles which are used as Intermediates In the manufacture of ceramics. At this stage we are not too concerned with the nature of the constituents, but rather with the structures which they form this Is a geometrical problem, where the system Is characterized by Its surface area A, by the shapes of Its Interfaces (curvatures - b ), and by the distances between opposing surfaces (d — concentration parameter). 

The variation of the surface tension with the surfactant concentration was determined for several sur tant molecules, and the critical aggregation concentration (CAC) was calculated fix>m the... 

Equations (2)-(4) show that the total potential energy of interaction between two colloidal spherical particles depends on the surface potential of the particles, the effective Hamaker constant, and the ionic strength of the suspending medium. It is known that the addition of an indifferent electrolyte can cause a colloid to undergo aggregation. Furthermore, for a particular salt, a fairly sharply defined concentration, called critical aggregation concentration (CAC), is needed to induce aggregation. 

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