Amphiphilic surfactant molecules

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. 

The effect of electrolyte addition to oscillatory behaviour has also been considered in [235]. The disappearance of structural transition upon electrolyte addition was attributed to its electrostatic origin. The viscosity of the film did not differ much from that of the bulk solution in the case when micelles determined the structuring of the amphiphile surfactant molecules. It is worth to note that the length scale of the oscillations was large, about 10 nm and even reached about 50 nm. 

Amphiphilic surfactant molecules form spherical or nearly spherical aggregates called micelles, above a certain critical concentration, known as the critical micellar concentration (cmc) and above a critical temperature, called Kraft temperature [4,93]. The size of the micellar aggregates is usually 1-10 nm and the aggregation number, l.e., the number of surfactant molecules per micelle, ranges from 20 to 200. The structure of a typical cationic micelle is shown schematically in Fig. 

The basic idea employed by Lipshutz involves the use an amphiphilic or surfactant-type molecule as a platform to solubilize the components of an organic reaction in water. Essentially, the amphiphile forms micelles in an aqueous environment, and these can act as micellar nanoreactors, in which the metal-catalyzed organic transformation can proceed. This approach is attractive, as it employs commercially available catalysts without the need for time-consuming or costly catalyst modifications. The amphiphilic surfactant molecule initially employed for these studies was PTS (131), which contains an unsym-metrical, sebacic acid-derived diester functionalized at one end with a lipophilic a-tocopherol unit and a hydrophilic PEG unit at the other (Figure 5.28) [115]. 

Increases in the surfactant concentration lead to changes in the geometric conformation and packing of the micelles in solution. A direct relation between micellar shape and mesophase (surfactant phase in terms of structural properties) has been correlated, and it is known as the packing parameter, g, which describes the geometric parameters of the hydrophobic and hydrophilic sections of the amphiphilic surfactant molecule, and is described as follows ... 

CNTs lend themselves to a range of chemical modifications. Both covalent and non-covalent functionalizations are possible at intact CNT sidewalls, at defect sites on sidewalls or at the tip of the nanotubes. The most common modification is the formation of carboxyl residues [39, 40]. The non-covalent functionalization of CNTs can be carried out by coating CNTs with amphiphilic surfactant molecules or polymers (poly-ethylene-glycol). 

Amphiphilic Surfactant molecule having a polar group and a hydrocarbon chain Chemists have extended it to describe polar and nonpolar interactions... 

The second important attribute of amphiphiles is their affinity to both water and oil. This aspect is retained in the microscopic models, which will be discussed in Sec. Ill and IV. Oil, water, and surfactant molecules are represented by simplified pseudoparticles. 

In the structure with all the surfactant molecules located at monolayers, the volume fraction of surfactant should be proportional to the average surface area times the width of the monolayer divided by the volume, i.e., Ps (X Sa/V. The proportionality constant is called the surfactant parameter [34]. This is true for a single surface with no intersections. In our mesoscopic description the volume is measured in units of the volume occupied by the surfactant molecule, and the area is measured in units of the area occupied by the amphiphile. In other words, in our model the area of the monolayer is the dimensionless quantity equal to the number of amphiphiles residing on the monolayer. Hence, it should be identified with the area rescaled by the surfactant parameter of the corresponding structure. 

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. 

Surfactant molecules (also called amphiphiles or detergents) combine a polar or ionic head and a non-polar tail within the same molecule. The non-polar part, which is typically made up of one or more alkyl chains, causes these compounds to be sparingly soluble in water, whereas the polar or ionic part interacts strongly with water. Upon increasing the concentration of the amphiphilic compound in water, the solubility limit will be reached at a certain point and phase separation will set in. Due to the efficient interactions between the polar headgroups and the surrounding water molecules, a complete phase separation is usually unfavourable. Instead, the process halts in an intermediate stage... 

Performance of surfactants is closely related to surface activity and to micelle formation. Both these are due to amphiphilic nature of the surfactant molecule. The molecule contains a nonpolar hydrophobic part, usually, a hydrocarbon chain, and a polar hydrophilic group, which may be nonionic, zwitterionic, or ionic. When the hydrophobic group is a long straight chain of hydrocarbon, the micelle has a small liquid like hydrocarbon core (1,2). The primary driving... 

For instance, surfactants dissolve in water and give rise to low surface tension even at very low concentrations (a few grams per liter or 1-100 mmol/L) of the solution therefore, these substances are also called surface-active molecules (surface-active agents or substances). On the other hand, most inorganic salts increase the surface tension of water. All surfactant molecules are amphiphilic, which means that these molecules exhibit hydrophilic and hydrophobic properties. Ethanol reduces the surface tension of water, but one will need over a few moles per liter to obtain the same reduction as when using a few millimoles of surface-active agents. 

If one adds an inorganic salt, such as NaCl, instead of detergent, then no foam is formed. Foam formation indicates that the surface-active agent adsorbs at the surface, and forms a TLF (consisting of two layers of amphiphile molecules and some water). This has led to many theoretical analyses of surfactant concentration (in the bulk phase) and surface tension (consequent on the presence of surfactant molecules at the surface). The thermodynamics of surface adsorption has been extensively described by the Gibbs adsorption theory (Chattoraj and Birdi, 1984). 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

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