Amphiphilic solute

The Amphiphilic Solute Facilitator family of transporters are simple in the sense that no specific source of energy is used for operation (such as hydrolysis of ATP or gradients of inorganic solutes). 

Ionic, polar, and amphiphilic solutes display local concentrations very different from the overall. 

The present work reports preliminary studies with a variety of amphiphilic solutes. The results, while somewhat fragmentary at this stage, suggest that GPC may be a rapid and versatile way to examine the size and stability of reversed micelles. 

The intermicellar equilibrium which, according to the R theory (I, 2, 3) is responsible for the succession of micellar amphiphilic solution phases, both amorphous and liquid crystalline, is shown in Figure 1. 

Stupp, S.I., Hartgerink, J.D., and Beniash, E. Peptide Amphiphile Solutions and Self Assembled Peptide Nanofiber Networks, 2003-US10051 2003084980 (2003a). 

Figure 3. Simplified cross section of an aqueous normal micelle showing possible solubilization sites. A charged solute (A) would be electrostatically repelled from the micelle surface if it were of the same charge-type as the ionic micelle while an oppositely charged solute (B) would be electrostatically attracted to the micellar surface. Nonpolar solutes (C) would partition to the outer part of the more hydrophobic core region. Amphiphilic solutes (D) would attempt to align themselves so as to maximize the electrostatic and hydrophobic interactions possible between itself and the surfactant molecules. "Reproduced with permission from Ref. 49. Copyright 1984, Elsevier. "...

Figure 6. Phase diagram for a typical nonionic surfactant. region refers to an isotropic amphiphile solution whereas and indicates the two co-existing isotropic phases.

One of the most important theoretical predictions is the existence of truly (i.e. infinitely) stable bilayers for C > Ce provided Ce < CMC. By fitting theoretical to experimental rfC) dependences it is possible to determine the equilibrium amphiphile concentration Ce and thus to judge whether in a given C range a bilayer, and in some cases, the corresponding disperse system, can be infinitely stable. BLMs, for example, are known to live for months and years. Thermodynamically, there is no difference between foam bilayers and BLMs so that the long lifetime of BLMs is apparently due to their existence in contact with amphiphile solutions of concentrations C either slightly bellow or above Cr. 

The existence of micelles in solutions of large ions with hydrocarbon chains is responsible for the observation that certain substances, normally insoluble or only slightly soluble in a given solvent, dissolve very well on addition of a surfactant (detergent or tenside). This phenomenon is called solubilization and implies the formation of a thermodynamically stable isotropic solution of a normally slightly soluble substrate (the solubilizate) on the addition of a surfactant (the solubilizer) [128, 133], Non-ionic, nonpolar solubilizates such as hydrocarbons can be trapped in the hydrocarbon core of the micelle. Other amphiphilic solutes are incorporated alongside the principal amphiphile and oriented radially, and small ionic species can be adsorbed on the surface of the micelle. Two modes of solubilizate incorporation are illustrated in Fig. 2-13. 

To a stirred 0.3 mM dispersion of PS-fi e rfr-(NH2) in a 0.01 M KCl solution, a 0.3 mM amphiphile solution in toluene was added dropwise. By measuring the conductivity of the system as a function of the ratio toluene-water, it could be estimated whether toluene or water was the continuous phase. At the point where the conductivity dropped to zero, the phase inversion point was reached and toluene became dispersing phase. The effect of dendrimer generation on the position of this inversion point was investigated with PS-c enrfr-(NH2) with n — 2-16. VS-dendr-(NH2)32 could not be measured in the same manner, because this product proved to be insoluble in toluene. The conductivity measurements show a distinct difference between PS-denc r-(NH2)i6 and the lower generations. For PS-cfendr-(NH2) with n — 2-8 there is a strong tendency to stabilize toluene as a continuous phase. PS-dendr-(NE.2)2 even showed a remarkable phase inversion at 2 vol% of toluene. This can be explained by the fact that polystyrene is the dominant part in the amphiphilic... 

For low molecular mass amphiphiles, hydrophobic interactions and surface effects determine the critical concentration at which micellar aggregates are favored over the molecularly dispersed amphiphilic solutes. For polysurfactants, however, the amphiphiles are linked together and the dynamic exchange of associated and non-associated amphiphilic monomer units is prevented. Consequently the micelle formation does not only depends on the hydrophilic/ hydrophobic balance of the monomer... 

Microemulsions have the ability to partition polar species into the aqueous core or nonpolar solutes into the continuous phase (See Fig. 1). They can therefore greatly increase the solvation of polar species in essentially a nonpolar medium. The surfactant interfacial region provides a dramatic transition from the highly polar aqueous core to the nonpolar continuous-phase solvent. This region represents a third type of solvent environment where amphiphilic solutes can reside. Such amphiphilic species will be strongly oriented in the interfacial film so that their polar ends are in the core of the microemulsion droplet and the nonpolar end is pointed towards or dissolved in the continuous phase solvent. When the continuous phase is a near-critical liquid (7)j = r/7 > 0.75) or supercritical fluid, additional parameters such as transport properties, and pressure (or density) manipulation become important aids in applying this technology to chemical processes. 

The paper is organized as follows In the next section, the literature results regarding some features of the nanostructure of pure water or methanol and the water/methanol binary clusters will be summarized. Then, the quantum mechanical ah initio method that was employed will be described. This will be followed by the presentation of the results that were obtained for the dilute clusters of methanol and water. Furthermore, the results will be compared to the available information regarding the liquid methanol/water mixtures that were obtained experimentally and by simulations. Finally, they will be used to shed some light on the structure and other features of water molecules in the vicinity of an amphiphilic solute. 

Intermembrane exchange Trans bilayer movement of amphiphilic solutes Lipid flip-flop... 

The importance of the hydrophobic effect in the self-association of amphiphilic solutes means that any factor which disrupts three-dimensional solvent structure also disrupts normal micelles. For example, addition of relatively large amounts of such organic solvents as ethanol or acetone to water makes the micelles smaller, and eventually they disappear and the surfactant behaves as a simple solute. On the other hand apolar solutes can enter the micelle and stabilize it and so lower the cmc, and added electrolytes also lower the cmc [15]. 

Figure 16.1. Molecular structure of some amphiphilic solutes, (a) Dimethyl sulphoxide (DMSO), (b) methanol (MeOH), (c) ethanol (EtOH), (d) tert-butyl alcohol (TEA), (e) acetone. For all these solutes the partial charges are indicated on the corresponding atoms according to the GROMOS-96 force field, (f) Molecular structure of 1,4-dioxane. For 1,4-dioxane the partial charges are indicated on the respective atoms according to / Am. Chem. Soc., 127 (2005), 11019-11028.

Liquid-liquid structural transformation in aqueous binary mixtures a generic phenomenon for amphiphilic solutes... 

As noted earlier, copolymer solutions are similar in many respects to short-chain amphiphilic solutions, and additional insights into the usefulness of lattice MC simulations for studying surfactant mixtures can be gathered from studies of copolymers. Mattice... 

It is well known that the air/solution interface of an amphiphile solution is well populated (Clint, 1992) by the adsorbed molecules. Accordingly, it has been shown that the concentration of the surfactant is always greater at the surface due to adsorption over and above the concentration of surfactant in the bulk. For calculation of Gibbs free energy changes, required different surface properties (e.g., the surface excess concentration, Tmax, minimum area per surfactant molecule at the air/water interface, Amin etc.). The surface excess concentration is an effective measure of the Gibbs adsorption at liquid/air interface, which was calculated by applying equation (Chattoraj Birdi, 1984)... 

Although this adsorption concerns only a tiny proportion of the molecules (see Fig. 4.5), it has a significant effect on the properties of amphiphilic solutions. By reducing the surface tension 7, it favours wetting (cf. Chap. 1) and foaming properties. This explains the term surface active agent or surfactant. 

Fig. 5.2. Schematic representation showing the structure of two mesomorphic structures occurring in concentrated amphiphilic solutions, (a) Hexagonal phase. Amphiphilic molecules assemble into long cylinders organised periodically in space, (b) Lamellar phase. Amphiphilic molecules make up infinite bilayers, stacked periodically...

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