Amine oxides interaction with anionic surfactants

Another important class of nonionics are amine oxides, such as DMDAO (dimethyldodecyl amine oxide) and CAPAO (cocoamidopropyldimethyl amine oxide). This type of surfactant is nonionic at pH values above its pKa and cationic below that point. When functioning as a nonionic, amine oxides have many useful properties. They interact strongly with anionics which can result in performance benefits [17]. Amine oxides help to mitigate anionic surfactant irritation, act as foam stabilizers, and can also function to improve grease removal. 

The second factor, namely the head group interaction, can also influence the surface properties of mixed surfactant markedly. In particular, anionic/catlonic surfactant mixtures exhibit the largest effect (17,18). In nonionic/anionic surfactant mixtures, synergistic effects can still take place to a significant extent, as revealed in Figure 3 (pH 10.9, nonionic amine oxide with anionic long chain sulfate), since insertion of nonionic surfactant molecules into an ionic surfactant molecular assembly minimises electrostatic repulsion (19). 

In acid solutions, the amino group is protonated and acts as a cationic surfactant, whereas in neutral or alkaline solution the amine oxides are essentially nonionic in character. Alkyl dimethyl amine oxides are water-soluble up to Cj, alkyl chain. Above pH 9, amine oxides are compatible with most anionics, but at pH 6.5 and below some anionics tend to interact and form precipitates. In combination with anionics, amine oxides can be used as foam boosters (e.g., in shampoos). 

The skin irritancy of anionics can be diminished by the addition of positively charged materials such as protein hydrolysates (Taves, 1986) or long-chain amine oxides that interact with the anionic and decrease its tendency to adsorb onto the skin, or by polymers that interact with them (Chapter 5, Section 1B5) to reduce the CMC and, consequently, the concentration of monomeric anionic surfactant (Goddard, 1994), since it is the latter that produces the skin irritation. 

Amine oxides (shampoos and light-duty liquid detergents) are able to interact by electrostatic forces with the anionic surfactant. The coupling of surfactants shows greater surface activity and leads to a more stable and closely packed film [79,80]. 

Sayari et al [202,203] extended the LCT technique to the synthesis of mesostructured zirconium oxide. The use of long chain quaternary ammonium salts or primary amines as templates led to the formation of hexagonal and lamellar Zr02 phases, respectively. Zr(S04)2 was used as zirconium source, which provided a highly acidic medium, pH < 1.5. Consistent with the synthesis conditions and EDX analysis data a S X I mechanism where the surfactant-inorganic interaction is mediated by sulfate anions was proposed. Unfortunately, both structures collapsed upon removal of the surfactant either by high temperature calcination or by solvent extraction [203]. However, the hexagonal form was successfully... 

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