Impact of biogenic amine molecular weight and structure on surfactant adsorption at the air–water interface

Neutron reflection and the thermodynamics of the air-water interface

By means of isotopic substitution, measurements of the neutron reflectivity (NR) from a flat water surface generally give model independent measurements of the amount of a chosen solute at the surface irrespective of whether the layer is a mixture or whether there is any aggregation in the bulk solution. Previously, adsorption at air-water interfaces has been determined by applying the Gibbs equation to surface tension (ST) measurements, which requires assumptions about the composition of the surface and about the activity of the solute in the bulk, which, in turn, means that in practice the surface is assumed to consist of the pure solute or of a mixture of pure solutes, and that the activity of the solute in the bulk solution is known. The use of NR in combination with ST-Gibbs measurements makes it possible to (i) avoid these assumptions and hence understand several patterns of ST behaviour previously considered to be anomalous and (ii) to start to analyse quantitatively the behaviour of mixed surfactants both below and above the critical micelle concentration. These two developments in our understanding of the thermodynamics of the air-water interface are described with recent examples.

Multilayers formed by polyelectrolyte-surfactant and related mixtures at the air-water interface

The structure and occurrence of multilayered adsorption at the air-water interface of surfactants in combination with other oppositely charged species is reviewed. The main species that trigger multilayer formation are multiply charged metal, oligo- and polyions. The structures vary from the attachment of one or two more or less complete surfactant bilayers to the initial surfactant monolayer at the air-water interface to the attachment of a greater number of bilayers with a more defective structure. The majority of the wide range of observations of such structures have been made using neutron reflectometry. The possible mechanisms for the attraction of surfactant bilayers to an air-water interface are discussed and particular attention is given to the question of whether these structures are true equilibrium structures.

Effects of length and hydrophilicity/hydrophobicity of diamines on self-assembly of diamine/SDS gemini-like surfactants

This work studied gemini-like surfactants formed from anionic surfactant sodium dodecyl sulfate (SDS) and cationic charged bola-type diamines with hydrophilic or hydrophobic spacers of different lengths using surface tension, small angle neutron scattering, isothermal titration microcalorimetry and cryogenic transmission electron microscopy. The critical micelle concentrations (CMC) and the surface tension at CMC (γ_CMC) for all the diamine/SDS mixtures are markedly lower than that of SDS. The shorter diamines reduce γ_CMC to a greater extent regardless of the hydrophilicity/hydrophobicity of the diamines. Meanwhile, either the hydrophobic diamine with a longer spacer or the hydrophilic diamine with a shorter spacer is more beneficial to decrease CMC and leads to the transition from spherical micelles into rodlike or wormlike micelles. This is principally because of the formation of gemini-like surfactants by the electrostatic binding between SDS and the diamines, where the electrostatic repulsion between the adjacent headgroups of SDS becomes much weaker due to the electrostatic binding of oppositely charged diamine with SDS, and the longer hydrophobic spacer may also bend into the hydrophobic domain of micelles to promote micellar growth. However, the hydrophilic spacers are more compatible with the headgroup region, leading to micelles with a larger curvature. This work contributes to the understanding of the relationship between the properties of constructed gemini-like surfactants and the natures of connecting molecules, and provides guidance to efficiently improve the performance of surfactants.

Nanosheet Formation by an Anionic Surfactant-like Peptide and Modulation of Self-Assembly through Ionic Complexation

The surfactant-like peptide (Ala)₆-(Asp) (A₆D) is shown to self-assemble into ultrathin (3 nm thick) nanosheets in aqueous solution above a critical aggregation concentration. A combination of circular dichroism and FTIR spectroscopy and X-ray diffraction shows that the nanosheets comprise interdigitated bilayers of the peptide with β-sheet conformation. The self-assembly can be modulated by addition of hexamethylenediamine which is expected to interact with the anionic C terminus (and C-terminal D residue) of the peptide. Multiple ordered nanostructures can be accessed depending on the amount of added diamine. Nanosheet and bicontinuous network structures were observed using cryogenic-TEM and small-angle X-ray scattering. Addition of hexamethylenediamine at a sufficiently large molar ratio leads to disruption of the ordered nanostructure and the formation of a solution of A₆D-diamine molecular complexes with highly charged end groups. The multiple acid-functionalized nanostructures that are accessible in this system are expected to have many applications in the fabrication of new nanomaterials.