A novel surfactant of N–dodecanoyl–N–(2-hydroxyethyl)–β–alanine (coded as C12–EtOH–βAla) was synthesized by modifying the methyl group of N–dodecanoyl–N–methyl–β–alanine (coded as C12–Me–βAla). Amino-acid-type surfactants (C12–EtOH–βAla and C12–Me–βAla) are more healthy and environmentally friendly compared to sodium dodecyl sulfate (SDS). To investigate the microstructures of these new surfactants, we employed a method of time-of-flight small-angle neutron scattering (TOF SANS) at a pulsed neutron source, Tokai Japan (J–PARC). The advances in TOF SANS enable simultaneous multiscale observations without changing the detector positions, which is usually necessary for SANS at the reactor or small-angle X-ray scattering. We performed in situ and real-time observations of microstructures of collapsing shampoo foam covering over a wide range of length scales from 100 to 0.1 nm. After starting an air pump, we obtained time-resolved SANS from smaller wave number, small-angle scattering attributed to (1) a single bimolecular layer with a disk shape, (2) micelles in a bimolecular layer, and (3) incoherent scattering due to the hydrogen atoms of surfactants. The micelle in the foam film was the same size as the micelle found in the solution before foaming. The film thickness (~27 nm) was stable for a long time (<3600 s), and we simultaneously found a Newton black film of 6 nm thickness at a long time limit (~1000 s). The incoherent scattering obtained with different contrasts using protonated and deuterated water was crucial to determining the water content in the foam film, which was about 10~5 wt%.

The interfacial adsorption properties of the novel hydroxy-group-containing amino-acid-type surfactant, N-alkanoyl-N-(2-hydroxyethyl)-β-alanine (Cn-EtOH-βAla, where n is the length of the alkyl chain; n = 8, 10, 12, 14, and 16; EtOH is the hydroxyethyl group; βAla is β-alanine), and the hydroxy-group-free conventional amino-acid-type surfactant, N-dodecanoyl-N-methyl-β-alanine (C12-Me-βAla), were investigated for comparison purposes in an alkaline solution. Cn-EtOH-βAla (where n = 10–14) possessed a significant minimum critical micelle concentration (CMC) in surface tension vs. concentration plots, contrary to the conventional surfactant C12-Me-βAla. The area occupied per molecule for C12-EtOH-βAla (0.316 nm2) was considerably decreased compared with C12-Me-βAla (0.683 nm2); this was attributed to hydrogen bonding between the hydroxy group and water/carboxylate ions in Cn-EtOH-βAla, which facilitated the dense adsorption and efficient orientation of the surfactant at the air/water interface. The relationship between the logarithm of the CMC and the n value of Cn-EtOH-βAla indicated that the surfactant exhibited nonionic-like behavior because the hydrogen bonding between the hydroxy groups and carboxylate ions suppressed the electrostatic repulsion between hydrophilic groups.

Small-angle neutron scattering, which has not been extensively utilized for foam characterization, can provide important insights into the microstructure of surfactant-stabilized foam. Small-angle neutron scattering in combination with several other techniques was herein employed to determine the microstructure of foams stabilized by hydroxy group-containing (C12-EtOH-βAla) and hydroxy group-free (C12-Me-βAla) surfactants of the amino acid type. Hydroxy group introduction at the amide nitrogen had no effect on the foam film thickness (~26 nm in both cases) but increased the foam stability and suppressed draining, as hydrogen bonding between hydroxy groups and carboxylate ions increased the foam film strength. Moreover, the obtained foam films were shown to contain micelles identical to those in the bulk solution.