Amphiphiles ionic surfactant molecules

Surfactant molecules (also called amphiphiles or detergents) combine a polar or ionic head and a non-polar tail within the same molecule. The non-polar part, which is typically made up of one or more alkyl chains, causes these compounds to be sparingly soluble in water, whereas the polar or ionic part interacts strongly with water. Upon increasing the concentration of the amphiphilic compound in water, the solubility limit will be reached at a certain point and phase separation will set in. Due to the efficient interactions between the polar headgroups and the surrounding water molecules, a complete phase separation is usually unfavourable. Instead, the process halts in an intermediate stage... 

Physical properties of the protein structure should be considered in designing strategies to achieve stable formulations because they can often yield clues about which solution environment would be appropriate for stabilization. For example, the insulin molecule is known to self-associate via a nonspecific hydrophobic mechanism66 Stabilizers tested include phenol derivatives, nonionic and ionic surfactants, polypropylene glycol, glycerol, and carbohydrates. The choice of using stabilizers that are amphiphilic in nature to minimize interactions where protein hydrophobic surfaces instigate the instability is founded upon the hydro-phobic effect.19 It has already been mentioned that hydrophobic surfaces prefer... 

Performance of surfactants is closely related to surface activity and to micelle formation. Both these are due to amphiphilic nature of the surfactant molecule. The molecule contains a nonpolar hydrophobic part, usually, a hydrocarbon chain, and a polar hydrophilic group, which may be nonionic, zwitterionic, or ionic. When the hydrophobic group is a long straight chain of hydrocarbon, the micelle has a small liquid like hydrocarbon core (1,2). The primary driving... 

Neutral. A bis(ethylenediamine) structure has been incorporated into the surfactant molecule -Ci6H33C(H)[CON(H)(CH2)2NH2]2 in older to incorporate metal ions in an LB film structure via coordination instead of ionic complexation as occurs for anionic/cationic amphiphiles (14). Also, films of n-octadecylacetoacetate containing Cu2+ have been prepared, and exposure to H2S has resulted in the formation of a copper sulfide (39). Ditetradecyl-A-[4- [6-(A, N, W -trimethyl-ethylenediamino)-hexyl]oxy]benzoyl]-L-glutamate (DTG), which also contains the ethylenediamine unit, was used to make self-assembled films containing Cd2+ (40). 

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. "...

The polar character of the liposomal core enables the encapsulation of polar drug molecules. Amphiphilic and lipophilic molecules are solubilized within the phospholipid bilayer depending on their affinity for the phospholipids. Participation of non-ionic surfactants instead of phospholipids in the bilayer formation results in Niosomes . The term sphingosomes is suggested for vesicles from sphingolipids. However, nomenclature is not consistent the term liposomes is used as a general term, although vesicles would be a better term. 

Clearly, there are some similarities between the structural features of ionic liquids and those displayed by some amphiphilic molecules, namely ionic surfactants. 

The formulation has been related with the type and properties of emulsions since Bancroft s rule of thumb (1913) and Langmuirs wedge theory (1917). The hydrophilic-lipophilic balance (HLB) was introduced by Griffin 60 years ago, probably as a selling argument for the (by the time) new non-ionic surfactants. It accounts for the relative importance of the hydrophilic and lipophilic parts of an amphiphilic molecule on a weight basis [19]. For decades there was no other numerical yardstick. The simplicity of the HLB concept was its main advantage in spite of very serious limitations, such as an inaccuracy sometimes over two units, and the fact that it does not take into account several variables which are known to alter the phase behaviour, independently of the surfactant. 

The self-assembly of amphiphilic molecules in non-aqueous polar solvents is usually attenuated compared to that in water. The CMCs increase significantly upon the substitution of water by polar solvents [2, 57, 58]. For example, the CMC of ionic surfactants in ethylene glycol are two orders of magnitude larger than that in water [57], while the monomeric solubility of sodium dodecyl sulphate in formamide is so high that micelles do not form at all [58]. Attenuation of self-assembly in non-aqueous polar solvents is the result of the reduced free energy of repulsion between polar solvents and the solvent-phobic parts of amphiphiles compared to that in water. 

Surfactants in Aqueous Solution A very important component that is usually present in the lyophobic colloids is the surfactant. These molecules are amphiphilic, that is, a part of the molecule is much more polar than the other part. On the basis of the nature of the polar groups in the surfactant molecule, they are classified as ionic (anionic or cationic) and nonionic. When ionic-type surfactants are adsorbed onto polymer particles, they provide stabilization by electrostatic repulsion between them and when the nonionic type are adsorbed instead the mode of stabilization is by steric repulsion. Electrosteric stabilization is provided by polyelectrolyte chains that give place to both modes of repulsion electrostatic and steric. 

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

The flexibility of the possible structure depends, in part, on the lateral interactions between neighboring amphiphile molecules. If the interaction is highly directional as with many ionic surfactants, an almost solid layer is formed that leads to liquid crystal or vesicle structures. To produce a liquidlike microemulsion, some disorder should be introduced. There are several ways to do so. 

With respect to the properties of polar groups, surfactants can be subdivided into ionic (cation- and anion-active, ampholytic, and zwitterionic) and nonionic surfactants. If the effect produced by the polar group of the surfactant molecule is more significant than that of the lipophilic group, this substance is soluble in water. It is less surface active as compared to any substance characterized by an optimum balance between the activities of hydrophilic and lipophilic groups. Similar conclusions can be drawn also with respect to the solubility in oil here, the role of the lipophilic group is determining. Clearly, the efficiency of a surfactant is not determined solely by the amphiphilicity, but depends on the hydrophilic/lipophilic balance (HLB) characteristic for this compound. Therefore, this balance is an important characteristic of both the surfactant and the interface. 

Most often, commercially available and purely organic amphiphilic, self-assembling molecules are applied in the synthesis of mesostructured materials such as ionic surfactants or block copolymers, i.e. Pluronic surfactants (PEO-f>-PPO-f>-PEO with PPO = poly(propylene oxide)) or poly(ethylene oxide) alkyl ether surfactants (Brij ). However, due to the restricted availability of amphiphilic block copolymers, not only are the accessible pore sizes and phases limited, but commercial products are sometimes inhomogeneous and have high molecular weight distributions [2]. 

Whatever the degree of dissociation of the counterions and their incorporation in the system (e.g. in the water pools), it is easily understood that these counterions act as impurities and may affect the properties of the final product. In some procedures of particle synthesis, ionic surfactants like NaAOT have been avoided for this reason [56] and non-ionic surfactants like Span 80 selected [57]. It will be seen on the other hand that NaAOT molecules have been ion-exchanged by various authors to M CAOT) and used in microemulsions for particle synthesis where the exchanged cation is a part of the final product, e.g. NaAOT Cd(AOT)2 for synthesis of CdS (see Section 5.2). These ion-exchanged amphiphiles are called functionalized surfactants . 

Expectedly, the outlook is quite different in case of non-ionic surfactants, like those belonging to poly(oxyethylene)alkyl or alkylphenyl ethers [61]. The hydrophilic part of these molecules can be in the form of chains longer than the corresponding hydrophobic part. An example is Triton X-100. As a result of the above, the structure of the polar interior of a reverse micelle of such amphiphiles does not resemble that of a reverse micelle of an ionic surfactant. The resemblance, indeed, is more with the interior of a normal micelle (of ionic surfactants). In reverse micelles of surfactants like Triton X-100, the polar interior can be invaded to an extent by a non-polar solvent like cyclohexane. 

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. 

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