Surface aggregates of surfactants

The fonnation of surface aggregates of surfactants and adsorbed micelles is a challenging area of experimental research. A relatively recent summary has been edited by Shanna [51]. The details of how surfactants pack when aggregated on surfaces, with respect to the atomic level and with respect to mesoscale stmcture (geometry, shape etc.), are less well understood than for micelles free in solution. Various models have been considered for surface surfactant aggregates, but most of these models have been adopted without finn experimental support. 

Chen M, Burgess I, Lipkowski J (2009) Potential controlled surface aggregation of surfactants at electrode surfaces - a molecular view. Surf Sci 603 1878-1891... 

The rates of multiphase reactions are often controlled by mass tran.sfer across the interface. An enlargement of the interfacial surface area can then speed up reactions and also affect selectivity. Formation of micelles (these are aggregates of surfactants, typically 400-800 nm in size, which can solubilize large quantities of hydrophobic substance) can lead to an enormous increase of the interfacial area, even at low concentrations. A qualitatively similar effect can be reached if microemulsions or hydrotropes are created. Microemulsions are colloidal dispersions that consist of monodisperse droplets of water-in-oil or oil-in-water, which are thermodynamically stable. Typically, droplets are 10 to 100 pm in diameter. Hydrotropes are substances like toluene/xylene/cumene sulphonic acids or their Na/K salts, glycol.s, urea, etc. These. substances are highly soluble in water and enormously increase the solubility of sparingly. soluble solutes. 

The same regression analysis methodology has been appHed for analyzing the model for aggregation of surfactants at the interface. The non-Hnear Eq. 28 has been numerically solved by the bisection method. The surface tension predicted by Eq. 27 has been fitted to the experimental data by min-... 

Fig. 4 Comparison between the experimental surface tension data for CnEs surfactant, data points [45], and the extended S-L adsorption model (line) by the aggregation of surfactant molecules at the sm-face described by Eq. 27-29. The best fit gives a>i = 557,691 m /mol, Tc = 1004.471 m /mol, and = 3.003...

Let s go back to much simpler systems, for example the self-aggregation of surfactant molecules. When surfactant molecules solubilize in water, often the process is slow at the very beginning, and gets faster with time the more surface bilayer is formed, the more the process speeds up, because there is more and more active surface where the next steps of aggregation can take place. The same... 

Pugnaloni, L.A., Ettelaie, R., Dickinson, E. (2003a). Growth and aggregation of surfactant islands during the displacement of an adsorbed protein monolayer a Brownian dynamics simulation study. Colloids and Surfaces B Biointerfaces, 31, 149-157. 

Based on the amount of surfactant adsorbed and on that present in solution, we concluded that it is unlikely that these nanoribbons consist of pure crystallised surfactant. On the basis of the approximate size of the SDS anions, we also ruled out the possibility that this image pattern resulted from aggregates of surfactant adsorbed onto the LDH s surface [11]. 

Other interesting examples of the organized molecular structures used to increase the quantum yield of charge photoseparation are micelles and vesicles. Micelles represent aggregates of surfactant molecules, one end of which is hydrophobic and the other hydrophilic. On reaching a certain critical concentration in a solution, these molecules group into spherical formations in which either the hydrophilic ends of the molecules are turned towards the micelle centre while their hydrophobic ends form its surface or vice versa. Micelles of the former type are usually formed in non-polar solvents and those of the latter type in polar solvents. The micelle is schematically represented in Fig. 1(d). 

The effect of a surfactant on the lowering of surface tension is shown in Fig. 1. The surface tension is lowered even at low concentrations of surfactant. As the surfactant concentration is increased, the surface layer becomes saturated with surfactant molecules, and micelles form within the bulk liquid as an alternative way of shielding the hydrophobic portions of the surfactants from the aqueous environment the surface tension tends to a constant value. Micelles are small aggregates of surfactant in which the surfactant molecules are arranged in such a way that the hydrophobic ends are shielded from the surrounding aqueous environment. The concentration at which micelles first appear in solution is termed the critical micelle concentration (CMC). 

Micelles are colloidal particles formed by the concentration-dependent aggregation of surfactant molecules (1). In an aqueous environment micelles form when the hydrophobic portions of the surfactant molecules begin to associate at a surfactant concentration that is referred to as the "critical micelle concentration", or CMC, as a result of hydrophobic effects In water, a micelle has a hydrophobic core and a charged surface that is the result of the orientation of ionizable or hydrophilic functional groups out into the bulk solution At concentrations prior to the CMC the surfactant molecules migrate to the solution-air interface which disturbs the structure of the water molecules and results in a decrease in the solution s surface tension (2), At concentrations greater than the CMC, increasing... 

Micelle - A particle formed by the aggregation of surfactant molecules (typically, 10 to 100 molecules) in solution. For aqueous solutions, the hydrophilic end of the molecule is on the surface of the micelle, while the hydrophobic end (often a hydrocarbon chain) points toward the center. At the critical micelle concentration (cmc) the previously dissolved molecules aggregate into a micelle. 

Single molecules or micelles associate spontaneously in a thermodynamic equihbrium at a definite critical micelle concentration within a biocoUoidal system [47]. Analogously to micelle formation in liquid systems, aggregation of surfactants at a surface depends on a critical hemi-micellar concentration [48, 49]. The removal of the hydrophobic molecular region from the hydrophihc interface... 

With surfactants in water, the decrease in surface tension at a lower concentration is accompanied by the adsorption of surfactants at an interface up to the point of CMC. Beyond this CMC, the surfactants form the micelles or the aggregates of surfactants. Whereas adsorption helps in making suspensions, dispersions, and emulsions, micellization helps in solubilization. 

Solloids. Surface colloids. Colloidal-sized aggregates of surfactant and/or polymer species adsorbed on a surface. Used as a more general term than admiceUes or hemimicelles. See reference [21],... 

Surfactants are not only applied to reduce the surface or interfacial tension. They can also be used to increase the solubility of substances. When dissolved in water above a certain minimum concentration, aggregates of surfactants are formed, also called micelles. Lipophilic substances can be brought into solution (solubilised) using these micellar solutions. Micelles can be prepared as follows. 

The interactions between surfactant solutions and solid surfaces play a key role in technologically important processes such as colloidal stabilization, ore flotation, and soil removal however, the interfacial aggregation of surfactant molecules is not yet well understood. 

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. 

---