Organic surfactant molecule

Figure 1.2 September 2007 cover of a leading international chemistry journal dedicated to sol-gel materials shows mesostructured silicas derived from the cooperative assembly of soluble sol-gel precursors and organic surfactant molecules. (Reproduced from acs.org, with permission.)...

The organic surfactant molecules can be removed by calcination or extraction of the as-synthesized material. The walls of the obtained mesoporous solid consist of partially condensed silica. They carry silanol groups which can be used to modify the pore walls, e.g. by trimethylsilylation with hexamethyldisilazane (hydrophobization) or phenylsilylation with phenyltrimethoxysilane (functionalization). 

In contrast to these post-synthetic modifications, it is also possible to functionalize the pore walls directly during the synthesis, as was first shown by Mann and co-workers [7,8] and Stucky and coworkers [9], who used trialkoxysilanes R-Si(OR )3. In our approach, such R Si(OR )3 molecules substitute for part of the TEOS. After hydrolysis, they serve as additional framework components during the hydrothermally induced condensation. An essential condition for this approach is that the trialkoxysilane does not destroy the micellar arrangement of the surfactant, which gives rise to the mesostructure. In mesostructures produced in this way, the R residues should be covalently linked to the silica walls. After the synthesis, the organic surfactant molecules can be removed by extraction so that a modified mesoporous material should remain. For example, when using phenyltrimethoxysilane (PTMOS), phenyl groups may become attached to the walls of the mesopores these can be utilized for further modifications, e.g. the immobilization of metal complexes. 

These materials, unlike the other nanophase materials described in this chapter, are nano-sized in only one dimension and thereby act as nanoplatelets that sandwich polymer chains in composites. Mont-morillonite (MMT) is a well-characterized layered silicate that can be made hydrophobic through either ionic exchange or modification with organic surfactant molecules to aid in dispersion [5,23]. Polymer-layered silicates may be synthesized by exfoliation adsorption, in situ intercalative polymerization, and melt intercalation to yield three general types of polymer/clay nanocomposites. Intercalated structures are characterized as alternating polymer and siHcate layers in an ordered pattern with a periodic space between layers of a few nanometers [13], ExfoHated or delaminated structure occurs when silicate layers are uniformly distributed throughout the polymer matrix. In some cases, the polymer does not intercalate... 

The introduction of low quantities of surfactants (50 to 125 ppm) helps solve these two problems. The surfactant molecule has a lipophilic organic tail and a polar head that is adsorbed selectively on the metal walls of the admission system. These products have a double action ... 

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

Since the compartmentalization occurs as a result of microphase separation of an amphiphilic polyelectrolyte in aqueous solution, an aqueous system is the only possible object of study. This limitation is a disadvantage from a practical point of view. Our recent studies, however, have shown that this disadvantage can be overcome with a molecular composite of an amphiphilic polyelectrolyte with a surfactant molecule [129], This composite was dissolvable in organic solvents and dopable in polymer film, and the microphase structure was found to remain unchaged in the composite. This finding is important, because it has made it possible to extend the study on photo-systems involving the chromophore compartmentalization to organic solutions and polymer solid systems. 

Modem commercial detergents are mixtures. Their most important component is a surfactant, or surface-active agent, which takes the place of the soap. Surfactant molecules are organic compounds with a structure and action similar to those of soap. A difference is that they typically contain sulfur atoms in their polar groups (4). 

Enzymes are suspended in hydrated microemulsion surrounded by a monolayer of surfactant molecules dispersed in an apolar solvent [53-60,135] [Fig. 1(b)]. Micelles ( 2 nm sphere) are formed when lyophilized or aqueous preparation of enzymes are introduced with stirring or shaking into a solution of synthetic or natural surfactant in an organic solvent. 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

Compared to inorganic materials, organic materials such as polymers, surfactant molecules and micelles also act as a capping material or soft template. Figure 5.15 shows TEM images of gold nanorods and nanoparticles synthesized by sonochemical reduction of Au(I) in the presence of cetyltrimethylammonium bromide,... 

Based on the above results and discussion, the mechanism for the rhythmic oscillations at the oil/water interface with surfactant and alcohol molecules may be explained in the following way [3,47,48] with reference to Table 1. As the first step, surfactant and alcohol molecules diffuse from the bulk aqueous phase to the interface. The surfactant and alcohol molecules near the interface tend to form a monolayer. When the concentration of the surfactant together with the alcohol reaches an upper critical value, the surfactant molecules are abruptly transferred to the organic phase with the formation of inverted micelles or inverted microemulsions. This step should be associated with the transfer of alcohol from the interface to the organic phase. Thus, when the concentration of the surfactant at the interface decreases below the lower critical value, accumulation of the surfactant begins and the cycle is repeated. Rhythmic changes in the electrical potential and the interface tension are thus generated. 

IPEC or hydrogen-bonded complexes may form not only between mutually interacting polymer blocks but also between a polymer block and low-MW molecules. Complexes between surfactants and block copolymers have been investigated for the formation of micelles. As illustrated by the work of Ikkala and coworkers [313], one of the major interests of these systems is that they combine two different-length scales of supramolecular organizations, i.e., the nanometer-scale organization of the (liquid) crystalline surfactant molecules and the ten-nanometer scale relative to block copolymers. This gives rise to the so-called hierarchical systems. The field of (block)... 

Over the narrow concentration range centering about the CMC, individual surfactant molecules form aggregates, usually containing 20-100 individual molecules. These aggregates are usually spherical, organized spherically so that the hydrophobic chains are hidden within the structure and thus isolated from the aqueous environment. The hydrophilic head groups are exposed on the micellar surface (Fig. 6.9). 

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