Amphoteric surfactants, interactions with

Low charge density, hydrophobically modified polybetaines were shown to interact and comicellize with nonionic, anionic, cationic, and amphoteric surfactants [181-183] and many ionic organic dyes [264,265]. The association mechanism of hydrophobically modified polymers and surfactants in dependence on the concentration of interacting components can be modeled by two pathways (Scheme 21) [183]. 

The following rationale accounts for these experimental findings. Sodium lauryl sulfate interacts with polymer primarily through an electrostatic interaction, and the net result is to neutralize the charge of the polymer and thereby reduce its affinity for keratin. Amphoterics do not neutralize the charge of the cationic polymer as effectively as do anionics. Therefore, cationic polymers demonstrate a greater affinity for keratins in an amphoteric surfactant system than in an anionic surfactant system. 

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. 

The cationic charge on a polymer affects its behavior considerably when surfactants are included in formulations. Cationic polymers (a member of the broader class of polymers termed polyelectrolytes) usually interact strongly with anionic surfactants, weakly with cationic surfactants, and unpredictably with nonionic and amphoteric surfactants (14,167-169). Anionic surfactant binds to cationic polymers at concentrations well below the critical micelle concentration (cmc). The low surfactant concentration where polymer and surfactant begin to interact is known as the critical aggregation concentration (cag). 

The behavior of anionic polysaccharides with nonionic and amphoteric surfactants (betaines) is more complex. For example, nonionic surfactants can form charge transfer complexes with highly charged polysaccharides. Amphoteric surfactants can have various charges and various degrees of interaction with anionic polysaccharides depending on the... 

These four classes behave differently in ion-exchange systems, and the best method of isolation depends on which amphoteric and which other classes of surfactant are present. The behaviour of WW and WS amphoterics in ion-exchange systems is well characterised, at least for those with only one acid and one basic group, but that of SW amphoterics is not. They are believed to behave as described here, but there is little direct experimental evidence. SS amphoterics (sulphobetaines) do not interact with ion-exchange resins. 

Amphoteric surfactants by definition are chimeric, exhibiting anionic character in alkaline solution, nonionic character near their isoelectric point, and cationic character in acidic solution [73]. As a result of their complex charge characteristics, their interactions with interfaces must be examined iudividually and as a function of pH. For example, the adsorption of alkyl betaines firom solution onto wool keratin is much greater at acidic than alkalide pH values. Although amphoteric surfactants are used extensively to improve the cosmetic attributes of many consmner products [74], their interactions with skin have received little attention. 

As in the case of surfactants, four general types of polymer can be defined with respect to the electronic nature of the species anionics, cationics, nonionics, and amphoterics. Not surprisingly, each polymer type will exhibit characteristic interactions with each surfactant class, with variations occurring within each group. It is little wonder, then, that surfactant-polymer interactions can produce some very interesting effects and become the subject of some lively discussions. 

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