Effects of the spacer length on the aggregate formation and the gelation of alkylamide amine oxides

Abstract

Three kinds of alkylamide amine oxides having one, two, and three amides were synthesized, and their solution properties such as the surface tension and the viscosity were examined. The effects of the spacer group length n between amide and dimethyamine oxide group as well as the number of amide groups were examined with respect to the aggregation behavior. For short spacer lengths, amide groups join the hydrophilic head group, while for long spacer lengths, amide groups and a part of the spacer groups are suggested to reside in the micelle palisade layer. In a series of amphiphile having three amide groups, gels formed at a high concentration of 50 mM were tentatively grouped into two types: A and B, depending on the spacer length n. For Gel-A, the gelling temperature T gel increased linearly with n but it was scarcely affected by the protonation of amine oxide (AO) group. For Gel-A, fiber-like aggregates were observed in cryo-TEM images. For Gel-B, values of T gel were higher by 18±2 K than those of Gel-A for a given n. Cryo-TEM images of Gel-B showed wide ribbon-like aggregates. On ionization of AO group, Gel-B transformed into Gel-A.

Amidoamine Oxide Surfactants as Low-Molecular-Weight Hydrogelators: Effect of Methylene Chain Length on Aggregate Structure and Rheological Behavior

“…At room temperature, the aqueous AAO solution is highly viscous and does not flow, and the obtained cryo-SEM images ( Figure 3 a,b) clearly show the presence of aggregates. An aqueous solution of 9-2-2-6 contains thin and straight rod-like structures ( Figure 3 a), whereas the solution of 11-2-2-6 contains twisted ribbon-like structures ( Figure 3 b) that are wider than those in 9-2-2-6 [ 30 , 31 ]. Figure 3 c shows a quick-frozen aqueous solution of 9-2-2-6 heated to about 60 °C (higher than T gel ), and Figure 3 d shows a quick-frozen aqueous solution of 11-2-2-6 heated to about 80 °C (higher than T gel ).…”

“…We previously reported the relationship between T gel and the length of the methylene chain between the amide and amine oxide groups ( n ) in aqueous AAO solutions [ 30 , 31 ]. Briefly, T gel increased with n for all methylene chain lengths ( k = 9, 11, and 13 in the hydrophobic part), and cryo-TEM observations showed the formation of thin and linear rod-like aggregates in the region where T gel linearly increased with n [ 30 , 31 ].…”

“…We previously reported the relationship between T gel and the length of the methylene chain between the amide and amine oxide groups ( n ) in aqueous AAO solutions [ 30 , 31 ]. Briefly, T gel increased with n for all methylene chain lengths ( k = 9, 11, and 13 in the hydrophobic part), and cryo-TEM observations showed the formation of thin and linear rod-like aggregates in the region where T gel linearly increased with n [ 30 , 31 ]. The slope of the plot of T gel against n (d T gel /d n ) was 30, 20, and 15 for k = 9, 11, and 13, respectively, indicating that the effect of n on T gel diminishes with increasing k , i.e., with the lengthening of the hydrophobic chain.…”

“…Although polymer materials are commonly used as thickeners/gelators, they have significant disadvantages: their molecular weight is difficult to control, and once the polymer is dissolved, the viscosity of the gel does not decrease even at high temperatures, resulting in poor operability. In contrast to conventional polymer gels, supramolecular gels have attracted attention in recent years [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ]. Supramolecular gels are formed by the self-assembly of low-molecular-weight gelators (LMWGs), which form fibrous aggregates and 3D network structures with them.…”

“…We have previously reported that amidoamine oxide surfactants (AAOs), which are long-chain alkylamine oxides with multiple amide groups, gelate water and aqueous salt solutions [ 29 , 30 , 31 ]. Long-chain alkylamine oxides are general-purpose surfactants used in kitchen detergents.…”