Amphiphiles in Water

Solubility is governed by the energy difference between the solid or liquid and dissolved states of materials, and especially by the stability of the solid state. The more stable a material, the smaller (and the more difficult to determine) is its solubility. However, the coexistence of a solute phase is thermodynamically significant it reduces the degrees of freedom of the system by one. In any case, the differences between the physicochemical properties of a solution and those of the pure solvent come about by the dissolution of solutes into the solvent. This chapter discusses the process of dissolution from a thermodynamic standpoint. 

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In a somewhat wider sense, one can define amphiphiles as molecules in which chemically very different units are linked together. For example, the structures formed by A B block copolymers in demixed A and/or B homopolymer melts and their phase behavior are very similar to those of classical amphiphiles in water and/or oil [13,14]. Copolymers are used not only to disperse immiscible homopolymer phases in one another, but also to create new, mesoscopically structured materials with unusual and interesting properties [15]. 

A force-distance curve between layers of the ammonium amphiphiles in water is shown in Figure 8. The interaction is repulsive and is attributed to the electric double-layer... 

Vesicles [10, 11] these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase. 

FIGURE 7.44. Transmission electron micrographs of the micellar assemblies formed by the aggregation of a lipase-polystyrene giant amphiphile in water. Expansion reveals a single micellar fiber with a diameter of 20-30 nm. Schematic representation of the micellar rod which possesses a polystyrene core. 

Micelles are loose aggregates of amphiphiles in water or organic solvents which form above a certain temperature (Krafft point) and concentration (critical micellar concentration, cmc). Below the Krafft temperature, clear micellar solutions become turbid and the amphiphile forms three-dimensional hydrated crystals. Below the cmc, micelles dissociate into monomers and small aggregates. Above the cmc, the micelles of an aggregation number n are formed n then remains stable over a wide concentration range . Table 1 gives some typical cmcs and three Krafft point values. 

Fig-i Schematic representations of a individual aggregate structures commonly formed by amphiphiles in water and b common LLC phases formed by amphiphiles in water. A small element in Fig. lb is partially reproduced with permission from [176], 1997 by the American Chemical Society... 

Discuss the effect that interactions between micelles could have on their growth as a function of the volume fraction of amphiphile. For fixed number of amphiphiles in water would the addition of salt (which screens polar-head repulsions) result in longer or shorter cylinders in equilibrium ... 

Fig. 12.1 Schematic representations of common UjC-phases formed by amphiphiles in water (Taken from [1])...

Tubular shaped superstructures formed by certain amphiphiles in water aggregate through intermediate helical ribbon structures.Since these amphiphiles feature a polar head in combination with a chiral hydrophobic group, the sol-gel... 

The simplest experiment consists in measuring the solubility of an amphiphile in water (e.g., from the remaining solid residue). We observe a wide variation. At room temperature and pressure, there is a variation with molecular structure. For example, if the lipophilic sequence contains more than 14 or 16 carbon atoms, the molecule is practically insoluble. But solubility is also strongly dependent on temperature for a given chemically pure molecule. We observe a sudden increase in solubility at a particular temperature Tk, the Krafft temperature (see Fig. 4.2). 

Before considering BCPs in detail, it is worth reflecting on the phase behavior of surfactants (see Self-Assembly of Facial Amphiphiles in Water and Soft Matter Science—a Historical Overview with a Supramolecular Perspective, Soft Matter). Classically, a simple geomettic... 

As organic compounds with an amphiphilic structure, they show solubility in both oils or organic solvents and water and, in fact, with respect to water, because these molecules have one hydrophilic and one hydrophobic part, they are pushed toward the areas of contact, concentrating at the interfaces between air (or oil) and water through the adsorption process. Further insight on the self-assembly and self-organization including surfactants in liquids can be found in Introduction to Surfactant Self-Assembly, Concepts, Soft Matter Science—a Historical Overview with a Supramolecular Perspective and Self-Assembly of Facial Amphiphiles in Water, Soft Matter, respectively. 

Another potential of nonfnnctionalized LLCs phases is related to the catalysis of polymerization reactions. For instance, hexagonal LLCs from poly(oxyethylene)-alkyl amphiphiles in water have been used to solubilize and to polymerize 3,4-ethyldioxythiophene (EDOT) within their 3 nm hydrophobic cores (Fignre 28). ... 

Surfactants play an integral role in detergents, lubricants, and stabiUzation agents for coUoids (see Self-Assembly of Facial Amphiphiles in Water and Self-Assembly of Surfactants at Solid Surfaces, Soft Matter). A 1% solution of SDS, which is an anionic surfactant, successfully dispersed CNTs in aqueous suspensions (Figure 10a). 

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