Formation from amphiphilic molecules

Impressive materials properties can also be achieved by the solvent-specific aggregation properties of appropriately designed amphiphilic materials. A brief account on the organogel or hydrogel formation by amphiphilic molecules is now presented, citing a few examples from the current literature. Both... 

As it is well known, the contacts between drops (in emulsions), solid particles (in suspensions) and gas bubbles (in foams) are accomplished by films of different thickness. These films, as already discussed, can thin, reaching very small thickness. Observed under a microscope these films reflect very little light and appear black when their thickness is below 20 nm. Therefore, they can be called nano foam films. IUPAC nomenclature (1994) distinguishes two equilibrium states of black films common black films (CBF) and Newton black films (NBF). It will be shown that there is a pronounced transition between them, i.e. CBFs can transform into NBFs (or the reverse). The latter are bilayer formations without a free aqueous core between the two layers of surfactant molecules. Thus, the contact between droplets, particles and bubbles in disperse systems can be achieved by bilayers from amphiphile molecules. 

Let me begin by reminding the reader of the physico-chemical basis for the formation of membranes in water from amphiphilic molecules. The physical characteristics of water lead to the solubility of salts and of molecules able to participate in extensive hydrogen bonds, such as sugars (see Figure 19.4). Hydrocarbons and other non-polar molecules are by contrast insoluble. To be dissolved, they would require... 

Artificial hydrothermal vents might be constructed and supplied with plausible concentrations of simple reactants such as CO, H2, NH3, and H2S. Appropriate levels of amino adds induding a small chiral excess, along with the sorts of amphiphilic molecules described above, can be rationalized by the findings from the Murchison meteorite. Organic molecules such as found in irradiated interstellar ice models, including HMT, can also be induded. The system should indude weathered feldspars, which can be modified to indude the reduced transition-metal minerals that they are known to contain. [134] Such minerals as Fe,Ni sulfides are likely to have been both present and stable in the environment of early Earth and are known [153, 155] to catalyze formation of organic molecules from simpler precursors. 

The aggregation of amphiphilic molecules into micelles is, from a physico-chemical point of view, an example of the formation of a molecular complex. The total thermodynamic description of the aggregation would involve a series of stability constants, including their variation with salt concentration. In most applications, it is neither feasible to obtain such detailed information nor necessary from a practical point of view. The characteristic cooperative nature of the micellization makes it often possible to describe the aggregation process using only a few parameters. It has,... 

A necessary prerequisite for the formation of microscopic foam films is the adsorption of surfactants at the solution/air interface. Different ways have been sought in order to obtain adsorption layers from insoluble surfactants at such interface. The easiest way to form a foam film is to blow a freely floating gas bubble at a liquid surface covered with a monolayer of insoluble amphiphile molecules. This approach has been used by various authors [138-142]. 

Microscopic foam films are most successfully employed in the study of surface forces. Since such films are small it is possible to follow their formation at very low concentrations of the amphiphile molecules in the bulk solution. On the other hand, the small size permits studying the fluctuation phenomena in thin liquid films which play an important role in the binding energy of amphiphile molecules in the bilayer. In a bilayer film connected with the bulk phase, there appear fluctuation holes formed from vacancies (missing molecules) which depend on the difference in the chemical potential of the molecules in the film and the bulk phase. The bilayer black foam film subjected to different temperatures can be either in liquid-crystalline or gel state, each one being characterised by a respective binding energy. 

It is also interesting to study the role of surface forces in formation of multilayer structures from amphiphiles in the foam film as well as the comparison with the lamellar mesomorphic phases, produced in aqueous dispersions of amphiphile molecules. 

Thermodynamics can also be used for determining of the work W, for fluctuation formation of an / -sized hole, i = 1,2,3,... being the number of amphiphile molecules that would fill the hole. Since the hole appears as a result of the passage of / molecules from the bilayer into the solution, the work associated with this process is - /A//. On the other hand, work equal to the hole total peripheral free energy /, > 0 is done in creating the hole periphery. Hence, W, is given by [399,402,403]... 

The determination of the binding energy of DMPC molecule in the foam bilayer was carried out using the experimental results for the temperature dependence of the critical concentration for formation of foam bilayer (Fig. 3.95) and the theory of Kashchiev-Exerowa (see Section 3.4.4.2). The concentrations Cc and Ce (Eq. (3.129)) are specific constants of each system which determine the ability of a foam bilayer to exist in a metastable state within the concentration range Cc< C < Ce. When C >Ce the foam bilayer is thermodynamically stable (there is no driving force for the whole nucleation process in the foam bilayer). It follows from the theory that the critical concentration of amphiphile molecules in the solution equals the equilibrium one (Cc = Ce) in the case of a missing metastable region when only thermodynamically stable foam bilayers are formed. As mentioned above, the DMPC foam... 

The above mentioned threshold dilution d, and critical concentration for formation of a bilayer are used as measures for bilayer stability [19] being determined by the first neighbour lateral and normal interactions in the foam bilayer. This is the difference of the parameter d, from the change in the free surface energy which is usually used as a measure of the surface activity. Thus, the parameters d, and C, are proposed as new characteristics of the surface activity of an amphiphile molecule, evaluated with high accuracy from the sharp W(d) and W(C) dependences, respectively. 

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