Weak amphiphiles

Nutrients. Amphipathic lipids are used by cells to build membranes (see p. 214). Typical membrane lipids include phospholipids, glycolipids, and cholesterol. Fats are only weakly amphiphilic and are therefore not suitable as membrane components. 

The adsorption isotherm for pentanol is typical for lyophobic substances, i.e., substances which do not like to stay in solution, and for weakly amphiphilic substances. They become enriched in the interface and decrease the surface tension. If water is the solvent, most organic substances show such a behaviour. The LiCl adsorption isotherm is characteristic for lyophilic substances. Most ions in water show such behaviour. 

Cosurfactants are molecules with weak amphiphilic properties that are mixed with the surfactant(s) to enhance their ability to reduce the interfacial tension of a system and promote the formation of a ME [3]. Cosolvents have also been described as weak amphiphilic molecules that tend to distribute between the aqueous phase, the... 

Microemulsions are fluid, transparent, thermodynamically stable oil and water systems, stabilized by a surfactant usually in conjunction with a cosurfactant that may be a short-chain alcohol, amine, or other weakly amphiphilic molecule. An interesting characteristic of microemulsions is that the diameter of the droplets is in the range of 100-1000 A, whereas the diameter of droplets in a kinetically stable macroemulsion is 5000 A. The small droplet size allows the microemulsion to act as carriers for drugs that are poorly soluble in water. The suggested method of preparation of microemulsions is as follows the surfactant, oil, and water are mixed to form a milky emulsion and titrated with a fourth component, the cosurfactant,... 

Another class of lipids, called steroids, is a large group of molecules that includes cholesterol and is only weakly amphiphilic due to few polar groups (Figure 10,9). Cholesterol is a prominent component of lipid bilayers, but its bulky shape tends to disrupt the regularity of the membrane. 

The variation in phase behavior as the strength of the amphiphile is increased can also be followed in the three-component model [44]. One finds that for weak amphiphiles, characterized by a small negative value of L in Eq. (3) of Sec. II.A.2, there is a continuous transition to oil-water coexistence from a disordered phase that is not structured. As... 

As to the first question, lattice models do exhibit oil/water inlerfacial tensions that are reduced to various degrees from the value in the absence of amphiphile. For example, in the three-component model solved within mean-field theory, a reduction on the order of 30 was found in the oil/water interfacial tension at three-phase coexistence with the microemulsion [101]. When simulated so that fluctuations were included [102], the reduction increased to about a factor of 100, which is characteristic of a weak amphiphile. Other lattice models [103] have obtained reductions as large as a factor of 800, larger than that provided by even the strong amphiphile C6E3 [104]. 

It has long been known that one of the principal distinctions between the middle phase brought about by a weak amphiphile and the microemulsion brought about by a strong one is that the latter will not wet the interface between the coexisting oil and water phases whereas the former will [107]. One of the successes of the Ginzburg-Landau approach is in explaining this phenomenon in terms of the difference between the bulk structure of the microemulsion and that of an ordinary fluid [42]. This difference is manifest in the correlation function of Eq. (32), Sec. III.Cl,... 

A further complication arises when the surfactant is not confined to the surfactant films but has significant solubility in one of the solvents. This retards solvent diffusion due to solvation. The effect occurs in general for weakly amphiphilic, mainly short alkyl chain, surfactants. Another example is that of nonionic ethylene oxide (EO) surfactants, which have a relatively high monomer solubility in oil, especially at higher temperatures. 

Properties of the interfacial film are profoundly modified by the presence of certain constituents. Other amphiphilic molecules or weak amphiphiles of alcohol, amine or hydrocarbon type, capable of inserting themselves into the inter facial film, may cause new structures to appear. 

We can suppose that weak amphiphilic properties of amine molecules become stronger due to hydrogen bonding with polyacids. As a result microheterogeneous system appears which contains regions where the interaction of alkylaromatic residues takes place and regions where polar groups are localized. In this case polyacid performs like "aqueous" interlayer. 

In the microemulsion systems the primary alcohols are frequently considered as cosurfactants, which are usually weakly amphiphilic molecules that help the amphiphilic surfactants to reduce the surface tension of the interface between the immiscible components of the system. In this way they usually enhance and emphasize the internal structure of the system at the colloidal level. Remarkably, the short-chain alcohols, which are sufficiently soluble in water, themselves show surfactant-like behavior in plain binary water mixtures. As was shown by Kahlweit et al. [87], this specific behavior can be observed from the break in the curves of surface tension versus molar fraction of alcohol in water. Similar breaks were observed by Zana et al. [7] in the curves of fluorescence intensity versus molar fraction of alcohol, where changes in the environment polarity are sensed by the pyrene fluorescence probe. Interestingly, with increasing the length of the... 

A good example of the biological relevance of solubilization in micelles is provided by the mechanism by which fats are digested by animals. This is achieved using bile salts, which act as surfactants in the stomach and intestine. Examples of cholesterol derivatives that are bile salts are illustrated in Fig. 4.21. Cholesterol itself is weakly amphiphilic due to the presence of a terminal -OH group (in fact, it is an example of a lipid... 

We have seen that thermotropic liquid crystal molecules that tend to form ID and 2D structures in the form of smectic and columnar liquid crystals have weakly amphiphilic anisometric (rod-, disc-, banana-, and bowl-shaped) molecules, where the rigid cores tend to avoid the flexible aliphatic chains. These... 

In some cases (with hydrophilic substrate and only weakly amphiphilic materials) the monolayers are deposited only in the upstroke with the same orientation, leading to a Z-type film. In case of hydrophobic substrates the monolayers are only deposited on the downstroke, and the result is the so-called X-type film. The combination of different deposition types is also possible. We note that similar methods were used by traditional candle makers. 

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