Surfactants amphiphilic character

One question addressed in the literature is the relationship between the angle of orientation of the adsorbed species within the monolayer and their amphiphilic character. The case of surfactants like fatty acids or phospholipids is deferred until Section VI, since the technique of choice is SFG in order to perform a surface vibrational study. Phenol deri-... 

Drug molecules with amphiphilic character may form lyotropic mesophases, and amphiphilic excipients in drug formulations also form lyotropic liquid crystals. Especially surfactants, which are commonly used as emulsifiers in dermal formulations, associate to micelles after dissolution in a solvent. With increasing concentration of these micelles the probability of interaction between these micelles increases and thus the formation of liquid crystals. 

Figure 9.20 The amphiphilic character of vesicle-forming surfactant molecules the combination of oleic acid and oleate forms vesicles in the slightly alkaline pH range.

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

All surfactants have an amphiphilic character, meaning that the molecules have a hydrophilic and a hydrophobic part. For non-ionics the former is commonly a polyoj ethylene (Ej ) moiety, where m Is the number of -CHj - CH - O groups. Experience has shown that chciins of this nature dissolve well In water. It Is usually stated that the lone electrons on the ether oxygens are responsible for this solubility but this cannot be the complete story because polymethylene oxide, Is Insoluble in water. So Is polypropylene oxide. 

Figure 47.2. Pluronic block copolymers with various numbers of hydrophilic EO (n) and hydrophobic PO (m) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions at concentrations above critical micelle concentration (CMC) these copolymers self-assemble into micelles.

The controlled introduction of two lipophilic alkyl chains into mannuronate monomers represents a convenient way of producing double-tailed surfactants (Scheme 6). Indeed, the incorporation of longer hydrophobic alkyl chains than the two butyl groups brings a more amphiphilic character to the glycosides. 

As discussed by Binks and Lumsdon, amphiphilic Janus particles can exhibit an interfacial activity several times higher than simple homogeneous particles [54], Janus particles combine the amphiphilic character of surfactants and the physical properties of nanoparticles, which opens new opportunities in emerging areas of nanotechnology and emulsion stabilization. 

A special case of nanoparticle self-assembly is the Janus particle. It was shown that Janus particles are considerably more active than homogeneous particles of comparable size and chemical nature and that the interfacial activity can be increased by increasing the amphiphilic character of the particles. Thus, the Janus particles show a significant advantage in the stabilization of emulsions and foams over homogeneous particles as they unify the Pickering concept and the amphiphilicity of a simple surfactant. 

Alternatively, the hydrophilic and hydrophobic groups may be scattered all over the macromolecule. Here, the amphiphilic character of micellar polymers results from the presence of many independent, surfactant-like structural units which are covalently linked. This is realized in polymers which bear a limited number of ionic groups in their otherwise hydrophobic backbones - corresponding to a longitudinal linkage (Fig. le) - or in polymers with functional side-chains - corresponding to a lateral linkage of surfactant units (Fig. If). 

For non-ionic polysoaps the situation is quite different. Whereas in the case of oligoethyleneoxide head groups thermotropic mesophases seem to be absent [87, 121-124, 126, 231, 403, 409], polysoaps with liposaccharide [230,240, 300] surfactant fragments or with lipopeptide [244, 400-402] ones frequently show lamellar mesophases (e.g. of smectic A type) and even nematic ones (Fig. 39). It should be emphasized that the thermotropic mesophases here are not the result of mesogenic groups being present, but are the consequence of the amphiphilic character of the polysoaps and the resulting microphase separation. 

Amphiphilic Nature of Hydrolyzed TEOS. The TEOS molecules, readily solubilized in the external oil phase, interact with the water molecules present within the aggregates to produce hydrolyzed species. It is assumed that the products of hydrolysis, once formed, remain bound to the micellar aggregates (i.e., solubilized within the surfactant film or the aqueous core or both) because of their enhanced amphiphilic character (brought about by the formation of silanol groups). The exact locale for the solubilization of hydrolyzed TEOS molecules in a reverse micelle is expected to be dependent on the degree of hydrolysis for example, the most polar species [Si(OH)4] would be expected to reside within the aqueous core. Once the hydrolyzed TEOS becomes solubilized, all further reactions (further hydrolysis and condensation) are restricted to the locale of the surfactant aggregates. 

Dynamic properties of interfaces have attracted attention for many years because they help in understanding the behaviour of polymer, surfactant or mixed adsorption layers.6 In particular, interfacial rheology (dilational properties) is crucial for many technological processes (emulsions, flotation, foaming, etc).1 The present work deals with the adsorption of MeC at the air-water interface. Because of its amphiphilic character MeC is able to adsorb at the liquid interface thus lowering the surface tension. Our aim is to quantify how surface active this polymer is, and to determine the rheological properties of the layer. A qualitative and quantitative evaluation of the adsorption process and the dilata-tional surface properties have been realised by dynamic interface tension measurements using a drop tensiometer and an axisymmetric drop shape analysis. 

There have not been such detailed formulation studies for O/W microemulsions based on hydrophobic monomers. The main reason is that the monomer most investigated so far is styrene trapped in droplets stabilized by aliphatic surfactants. According to the criterion defined above, there is a chemical mismatch between styrene (aromatic) and the hydrophobic tail of the surfactant. In addition, styrene has no amphiphilic character and cannot act as a cosurfactant. As a result, the domain of existence of microemulsions is very limited. 

Because of their amphiphilic character, alkali resinates have been exploited both as polymer latex stabilizers and as surfactants in emulsion polymerization from the early development of these techniques, as in the pre-Second World War industrial example of the polymerization of 2-chloro-l,3-butadiene, to produce neoprene [68]. In the following decades, other emulsion polymerizations systems, like the synthesis of styrene-butadiene copolymers [68, 69], also called upon these surfactants, which are still being envisaged today, for example, for the polymerization of styrene [70] and chloroprene [71]. However, the reactivity of the conjugated double bond towards free radicals has made it more profitable to use hydrogenated or dehydrogenated rosins rather than their natural forms [68, 72]. 

The amphiphilic character of surfactant molecules is due to the association of two parts with very differing polarities inside the same molecule [2]. One part is highly nonpolar, hydrophobic or lipophilic, usually an alkyl chain. Another part of the surfactant molecule is polar or hydrophilic. It can be a nonionic chain with polar groups, such as ether, alcohol or amine groups, or an ionic (anionic or cationic) group. Figure 2.1 shows the schematic representation of a surfectant molecule. Some surfactants have two nonpolar tails or two polar heads, as illustrated in the figure. The nature of the surfactant polar head is used to classify the molecules. 

The amphiphilic character of surfactant molecules explains their trend to adsorb at any interface. Two phases of different polarity are separated by an interface. This polarity difference attracts the surfectant molecules because this can minimize the entropy change by putting their polar part in the more polar phase and their nonpolar part in the less polar phase. Figure 2.2 shows the surfactant molecule arrangement at the liquid-liquid interface and at the liquid-air interface. 

Solutions of molecules with pronounced amphiphilic character exhibit unusual concentration dependent properties dilute solutions behave like normal electrolytes (if ionic surfactants are considered), at higher, rather well-defined concentrations, quasisudden changes of several physical properties are observed (see Fig. 1). This phenomenon has been successfully ascribed to the formation of organised aggregates, i.e. micelles. The concentration above which micelles exist in equilibrium with monomers (and eventually small subunits) is the so-called critical micelle concentration (cmc). 

Evans and coworkers have published an interesting study of the electrochemical behavior of a ferrocene-carboxylic acid derivative in the presence of BCD [18] and a summary of general guidelines on the electrochemical methodology that can be used to assess the complexation of redox-active molecules by cyclodextrin hosts [19]. However, the amphiphilic character of the surfactant viologens used in this work prevented us from applying most of these methods because the voltammetric... 

Amphiphilic molecules contain a polar and an apolar part. As a result, such molecules have an ambiguous (amphi) affinity (philos) for water. The apolar parts behave hydrophobically the water molecules tend to escape from contact with these parts. The polar parts are hydrophilic. They interact favorably with water. The consequence of the amphiphilic character is that the molecules are preferably located at interfaces with water, where the polar parts are exposed to the aqueous phase and the apolar parts to the nonaqueous phase. Low-molecular-weight, amphiphilic compounds are often called surfactants. Well-known examples of surfactants are the classical soaps (single-chain fatty acids), phospholipids, cholesterol, bile acids, lung surfactant, and so on. In Figure 7.1, the chemical structures showing the polar and apolar parts of some of these surfactants are given. Monolayers may also be formed by polymers, polyelectrolytes, and proteins that contain polar and apolar parts. 

At very low values of n, say < 0.5 mN m", the monolayer exhibits gaseous (G) behavior. At these conditions, the molecules in the monolayer are so far apart from each other that they do not interact. Because of their amphiphilic character, the surfactant molecules have different interactions with the two phases at either side of the interface so that they adopt a preferential orientation. In the G state, the isotherm obeys the relation... 

Condensation of hydrolyzed proteins with fatty acid groups determines reduction of the skin and eye compatibility, resulting from the increase of their amphiphilic character the skin/eye tolerability of acylated proteins is reported to increase with the average length of the peptide moiety (127). Quaternized derivatives are generally less compatible than fatty acid condensates (128). The irritation potential of protein-surfactant complexes is intermediate between those of the parent protein and the pure tenside. 

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