Surfactants aggregated, structure formation

The spontaneous formation of surfactant aggregate structures or micelles at relatively low concentrations opens the door to a veritable zoo of larger, more structurally complex, and certainly more theoretically complex self-assembled structures that inhabit our natural world, and make life as we know it possible. While the concepts presented in Chapter 4 to explain micelle formation in aqueous and nonaqu-eous solutions are relatively straightforward, chemically speaking, it should be obvious that there is a great deal of room for complications to set in as a system becomes more complex. 

It is generally accepted that surfactant-polymer interactions may occur between individual surfactant molecules and the polymer chain (i.e., simple adsorption), or in the form of polymer-surfactant aggregate complexes. In the latter case, there may be a complex formation between the polymer chain and micelles, premiceUar or submiceUar aggregates, liquid crystals, and bicontinuous phases—that is, with any and all of the various surfactant aggregate structures described in Chapters 4 and 5. Other association mechanisms may result in the direct formation of what are sometimes called hemimicelles along the polymer chain. The term hemimicelle may be defined, for present purposes, as a surfactant aggregate formed... 

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

Some surfactants aggregate at the solid-liquid interface to form micelle-like structures, which are popularly known as hemimicelles or in general solloids (surface colloids) [23-26]. There is evidence in favor of the formation of these two-dimensional surfactant aggregates of ionic surfactants at the alumina-water surface and that of nonionic surfactants at the silica-water interface [23-26]. 

Chapter 8 has been revised to include a discussion of the critical packing parameter of surfactants and its relation to the structure of resulting surfactant aggregates. This simple geometric basis for the formation of micelles, bilayers, and other structures is intuitively easier to understand for a beginning student. 

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. 

Anionic surfactants complexation, charge neutralization, aggregation, precipitation, structure formation... 

This process leads to a stable hydrogen bond network and consequently to the micelles. This micelle formation is accompanied by a partial compensation of the individual dipole moments of the soap molecules. Also in the case of DAP a dipole compensation interaction of the ammonium compared to the sodium ion and the correspondingly inferior bond formation the tendency towards structure formation of these aggregates is expected to be less pronounced. Moreover, structural variation, should be considered comparing these two surfactants. 

Considerations of the packing parameter concept of Israelachvili et al. [1] suggest that double-chain surfactants, which form the basis of measurements described in this article, cannot readily form spherical micelles. With double-chain surfactants, a more likely aggregate structure is the formation of bilayer vesicles, which can be also thought of as a dispersed lamellar phase (La) as such the vesicular dispersed form is likely to be preferentially formed at low concentrations ( 1 mmol dm-3) of surfactant. Furthermore, it is necessary to consider the possibility, unlike in the case of micelles, that such vesicles, formed by self-assembly of surfactant monomers, will not be thermodynamically stable. The instability is then likely to be in the direction of growth to a thermodynamically-stable lamellar phase from the vesicles. This process will be driven, at least initially, by fusion of two vesicles. 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

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... 

Another new concept formulated in conjunction with the formation of the M41S structures was supramolecular templating, i.e. one involving assemblies of molecules (surfactant aggregates, micelles, liquid crystals) [31]. In contrast, microporous materials are templated by isolated molecules acting as structure directing agents. 

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. 

The success of NMR for studies of microemulsions has basically been that it can address one of the key questions, that of microheterogeneity. This is, for various reasons, very difficult to study by other physicochemical approaches and often informative regarding other types of complex formation, including certain aspects of surfactant self-assembly. To understand this we need to look at microemulsions in a broader context, and we start with micelles. The spherical micelle was the first firmly established surfactant self-assembly structure, and the spherical aggregate structure has penetrated thinking in this field of study to such an extent that almost every new phase or phenomenon in surfactant solutions has at some point been considered as based on spherical micellar units ... 

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