Surfactant aggregates Structures shapes

The aggregation numbers Nagg is determined as 27 for C1-(EO)53-C4-VB and 38 for Cr(EO)53-C7-VB micelles by analysis of fluorescence curves. A micelle formation mechanism is proposed for nonionic polymeric surfactants with weakly hydrophobic groups. At low concentrations of PEO macromonomers, large loosely aggregated structures involving the PEO chains are formed. At higher concentrations normal micelles form. These are star-shaped, with a hydrophobic core surrounded by a corona of PEO chains. 

I. V. Berezin, and K. Martinek, Catalysis by enzymes entrapped into hydrated surfactant aggregates having lamellar or cylindrical (hexagonal) or ball-shaped (cubic) structure in organic solvents,... 

Despite that the silicate-surfactant mesophase formation resembles the phase separation normally observed in surfactant-polyelecholyte systems, it is interesting to note that it is stiU possible to make qualitative predictions about the influence of inorganic-surfactant phase behavior based on models developed for dilute surfactant systems. The packing parameter concept - is based on a geomettic model that relates the geomehy of the individual surfactant molecule to the shape of the supramolecular aggregate structures most likely to form. N, is defined as... 

The quantity of insoluble substance which can be solubilised in micelles depends to a considerable extent on the chemical structure of the surfactant and is influenced by the presence of other components, which may influence either the micelle formation concentration (CMC) or the micelle geometry (aggregation number, shape). The transition from solubilisation to another important phenomenon, the formation of a micro-emulsions, is continuous. Microemulsions form spontaneously, whereas typical solubilisation systems attain their equilibrium state often only after extreme long periods of intensive mixing of both phases. 

The formation of surfaee aggregates of surfaetants and adsorbed micelles is a challenging area of experimental research. A relatively recent summary has been edited by Sharma [51], The details of how surfactants pack when aggregated on surfaces, with respect to the atomic level and with respect to mesoscale structure (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 firm experimental support. 

In aqueous solutions of surfactants at concentrations above the critical micelle concentration (CMC), the molecules self-assemble to form micelles, vesicles, or other colloidal aggregates. These may vary in size and shape depending on solution conditions. In addition to surfactant molecular structure, the effects of concentration, pH, other additives, cosolvents, temperature, and shear affect the nanostructure of the micelles. The presence of TLMs or cylindrical, rodlike, or wormlike micelles at concentrations > CMCii are generally believed to be necessary for surfactant solutions to be drag reducing [Zakin et al., 2007]. 

A typical spherical micelle in aqueous solution has an average number of surfactant molecules, or aggregation number, of 40-100. The diameter of a spherical or cylindrical micelle is around 4-6 nm for typical surfactants. The shape of the micelle depends on surfactant molecular structure, solution ionic strength, temperature, and presence of organic solutes in solution, among other factors. 

An alternative approach is to use a coarse-grained structural model for the surfactant and the other components so that the self-assembly of the surfactants can be monitored in the simulations [11,12]. The early studies of Karaborni, Smit, and coworkers [13-15] are such an example. These studies are mostly qualitative and primarily demonstrate that the basic features of surfactant self-assembly can be modeled by simple MD simulations. However, the more recent work of Esselink et al. [16] takes advantage of parallel processing techniques and faster computers to provide some insights into the forces of importance for surfactant aggregation and the effects of surfactant structure on the size and shape of the micelle. Moreover, Esselink et al. [16] also present a preliminary study of the mechanism of oil solubilization inside micelles. We restrict ourselves here to some of the details of the simulation and the results presented in their paper. 

In concentrated aqueous surfactant solutions, the sizes and shapes of the aggregates are also influenced by interaggregate forces. This leads to positionally ordered structures characterized by long-range orientational alignment and spatial periodicities that cannot be ascribed to spherical micelles [108]. Nevertheless, all three classical structural shapes—spheres, cylinders, and planes—are respectively revealed in hexagonal, discrete (globular) micellar. 

Here we mainly address one problem of microemulsion microstructure, namely that of connectivity, in particular the distinction between uni- and bicontinuous structures. We also, to some extent, consider the problem of size and shape of aggregates for discrete particle structures and that of different bicontinuous structures. NMR can also shed light on other aspects of microstructure, such as the distribution of surfactant molecules between surfactant films and the oil and water domains and the local packing and ordering of surfactant molecules. Here we merely note that there is overwhelming evidence (mainly from NMR) that, locally, surfactant aggregates in different phases (micellar solutions, microemulsions, different liquid crystalline phases) show only quite minor differences. 

Scheme 3 Example of relationships between the geometric shape of an aggregate (see Figure 3), surfactant structures, shapes of their tails relative to their headgroups of constant size, their packing parameters, and examples of surfactants that form these structures in some concentration range in their phase diagrams.

FIGURE 16.1 Surfactant molecules self-assemble into various aggregate shapes, depending on the surfactant molecular structure as described by the critical packing parameter (CPP), which is the ratio of the molecular volume (v) divided by its length (/) times the cross-sectional area of the head group (a) v/al. 

Lamellar liquid crystalline phases should be formed at the surface in order to attain good lubrication of a surfactant system. Surfactant molecular structure is one of the critical variables affecting the shape of the surfactant aggregates and, hence, the lubrication properties of surfactant systems. The aggregate shape is also determined by other factors, such as hydrophilic/hydrophobic additives, salt, solubilizates, and temperature. In addition, the aggregates present at the surface are also sensitive to surface properties as well as to co-ions. 

For surfactant aggregates of the interfacial complex type under discussion, Eq. (205) will be fulfilled for the same sequence of elementary shapes as the one we have already discussed for microemulsions, that is, spheres and cylinders with radii in correspondence with the spontaneous curvature, infinite periodical CMC structures for which tOsO H—Hq are equal to zero but where K is less than zero, yielding cubic surfactant phases and lamellar structures. However, it is probably worth stressing once more that noninteracting surfactant-laden interfaces are in focus here. With this in mind, we can discuss just interactionless aggregate geometries but not the whole issue about the possible formation of three-dimensional structures of these aggregates. 

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