Micelles amphoterics

An L-B film is formed by the dispersion of amphoteric molecules at an air-water surface (Figure 8.20). These molecules have a polar group at one end, something like a carboxy substituent (in this respect they resemble the surfactant molecules which make micelles), and a long non-polar aliphatic chain. The polar group stays in the polar water phase, and the aliphatic chain stays in the non-polar air environment. The L-B film at the water surface is then made by the controlled compression of these molecules by means of a floating barrier. The molecules then line up to form a mono-molecular layer on the water surface. 

Surfactants are classified on the basis of the charge carried by the polar headgroup as anionic, cationic, nonionic, and amphoteric. Surfactant headgroups are dipoles, especially ionic ones that exist as ion pairs in hydrocarbon solvents. Electrostatic dipole-dipole attraction between headgroups in hydrocarbon solvents is the driving force for the formation of reverse micelles, or micellar aggregates, see Fig. 3.1 and Fig. 3.2. 

Other factors also affect the stability of eolloidal solutions, in particular the pH of the environment. The surface molecules of the nucleus of the micelles of different colloidal systems can have acid, basic, or amphoteric properties. Colloids characterized by this feature are called, respectively, acidosid, basoids, and ampholitoids (Marchenko, 1965). It has been established that the coagulation threshold of acidoids (sols of weak acids) increases and that of basoids decreases when the pH is increased. 

After cyclodextrins and co-solvents, other approaches can be applied including the use of surfactants and micelle forming agents. Surfactants can be classified as amphoteric (lecithin), non-ionic (Tween 80 or Cremophor EF) or ionic (sodium lauryl sulfate or sodium palmitate). Cremophor is a polyoxyethylenated castor oil derivative which is a common solubilizing excipient in a number of formulations including those for paclitaxel, propofol, teniposide and clanfenur... 

Amphoteric surfactants are sometimes referred to as 2witterionic molecules. They are soluble in water, but their solubility shows a minimum at the i.e.p. Amphoterics show excellent compatibility with other surfactants, forming muKd micelles they are also chemically stable both in acids and alkalis. The surface activity of amphoterics varies widely, and depends on the distance between the charged groups. Amphoterics display a maximum in surface activity at the i.e.p. 

The second example involves a mixture of two different types of commercial foam-forming surfactants anionic and amphoteric (7). Unlike the mixture of the previous example, an anionic—amphoteric surfactant mixture probably does not follow ideal mixed micelle behavior (138). The results of three core-floods, performed separately with each surfactant and with a mixture of the two surfactants, are summarized as follows. The anionic surfactant adsorbs negligibly when used either by itself or when mixed with the betaine (at least at the low salinity used in these particular core-floods). Betaine adsorption is lowered by about an order of magnitude by mixing it with the anionic surfactant, from 1.7 down to 0.2 mg/g. 

The size and shape of micelles are determined by a delicate balance between various factors, such as chemical constitution, electrical repulsion of head groups, amphiphile and solute concentration, and temperature. The addition of electrolytes will in general raise aggregation numbers of ionic micelles and may even induce sphere-rod transitions. Temperature has an enormous influence on aggregation numbers of nonionic micelles, but only a little effect on those of ionic and amphoteric micelles. There is a vast literature covering the subject (24,25,36). 

Surface active substances or surfactants are amphiphilic compounds having a lyophilic, in particular hydrophilic, part (polar group) and a lyophobic, in particular hydrophobic, part (often hydrocarbon chain). The amphiphilic structure of surfactants is responsible for their tendency to concentrate at interfaces and to aggregate in solutions into various supramolecular structures, such as micelles and bilayers. According to the nature of the polar group, surfactants can be classified into nonionics and ionic, which may be of anionic, cationic, and amphoteric or zwitterionic nature. 

To achieve the above-mentioned objective, the following processes have to be considered (a) formation of H+ at an oxidized anode area (decrease of pH) and OH at a reduced cathode area (increase of pH) (b) dissociation of soluble compounds within an entire length of cell (c) movement of cations and anions into respective electrodes (d) displacement of negatively charged and pH-dependent colloidal particles of clays toward the anode (e) formation of pH-dependent complexes of EDTA-metals and their transport toward the anode (f) electroos-motic transport of inert particles (phenanthrene compound) toward the cathode (g) amphoteric surfactant behavior (in the presence of a variable pH within the cell), formation of micelles, and desorption of phenanthrene (h) displacement of micelles and their transformation and (i) transport and removal of conditioning liquids. 

All hydrocarbon mixed surfactant systems with dissimilar head groups, such as ionic/nonionic, ionic/ amphoteric, and anionic/cationic, tend to have increased adsorption relative to the pure component adsorption at the same surfactant concentration. This synergisin is analogous to the effect of mixed surfactant systems in forming low CMC surfactant mixtures. It is easier to form a mixed admicelie rather than a pure component admicelie, just as it is easier to form a mixed micelle. 

For amphoteric surfactants, the critical micelle concentrations (CMCs) are often stated. However, most studies concerning the CMC have centred around betaines and the information extrapolated or applied to the other types of amphoteric surfactants. 

Jansson and co-workers (52-55) studied the interactions of amphoteric surfactants with other surfactants. They found that mixed micelles containing anionic surfactants are larger than those containing the corresponding cationic surfactants. 

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