In Situ Nanostructural Analysis of Concentrated Wormlike Micellar Fluids Comprising Sodium Laureth Sulfate and Cocamidopropyl Betaine Using Small-Angle Neutron Scattering

Salt-Induced Linker Dehydration Modulates Micellar Structure in Ether-Linked Sulfate Surfactants

Ether-linked surfactants are widely used in formulations such as liquid soaps, but despite their ubiquity, it is unclear how n-ethylene glycol linkers in surfactants, such as sodium lauryl n-(ethylene glycol) sulfate (SLEnS), influence micellar packing in the presence of NaCl. In the present work, we probe the structure and hydration of ether linkers in micelles comprising monodisperse SLEnS surfactants using contrast-variation small-angle neutron scattering (CV-SANS) and small-angle X-ray scattering (SAXS). Using SAXS, changes in micellar structure were observed for SLEnS (n = 1, 2, or 3) arising from the extent of ethoxylation. Scattering profiles indicated a clear transition from elongated cylindrical micelles to shorter ellipsoidal micelles with increasing ethoxylation. With CV-SANS, micellar structure and linker geometries of SLE3S were able to be resolved, indicating that a change in micellar architecture is modulated by dehydration of the tri(ethylene glycol) linker, offering new insights into the role of water and ions in the self-assembly of this key class of surfactants.

Influence of Surfactant Structure on Polydisperse Formulations of Alkyl Ether Sulfates and Alkyl Amidopropyl Betaines

Surfactants provide detergency, foaming, and texture in personal care formulations, yet the micellization of typical industrial primary and cosurfactants is not well understood, particularly in light of the polydisperse nature of commercial surfactants. Synergistic interactions are hypothesized to drive the formation of elongated wormlike self-assemblies in these mixed surfactant systems. Small-angle neutron scattering, rheology, and pendant drop tensiometry are used to examine surface adsorption, viscoelasticity, and self-assembly structure for wormlike micellar formulations comprising cocoamidopropyl betaine, and its two major components laurylamidopropyl betaine and oleylamidopropyl betaine, with sodium alkyl ethoxy sulfates. The tail length of sodium alkyl ethoxy sulfates was related to their ability to form wormlike micelles in electrolyte solutions, indicating that a tail length greater than 10 carbons is required to form wormlike micelles in NaCl solutions, with the decyl homologue unable to form elongated micelles and maintaining a low viscosity even at 20 wt % surfactant loading with 4 wt % NaCl present. For these systems, the incorporation of a disperse ethoxylate linker does not enable shorter chain surfactants to elongate into wormlike micelles for single-component systems; however, it could increase the interactions between surfactants in mixed surfactant systems. For synergy in surfactant mixing, the nonideal regular solution theory is used to study the sulfate/betaine mixtures. Tail mismatch appears to drive lower critical micelle concentrations, although tail matching improves synergy with larger relative reductions in critical micelle concentrations and greater micelle elongation, as seen by both tensiometric and scattering measurements.

Influence of tail group length, amide functionality and added salt ion identity on the behaviour of betaine surfactants

Hypothesis The behaviour of surfactants in solution and at interfaces is governed by a combination of steric and electrostatic effects experienced by surfactant molecules as they interact with solvent, other species in solution, and each other. It would therefore be anticipated that highly interacting groups would significantly influence surfactant behaviour. The widely used amide functionality has polar H-bond donor/acceptor properties, and therefore its inclusion into a surfactant structure should have a profound effect on surface activity and self-assembly of that surfactant when compared to the equivalent molecule without an amide linker. Further, chaotropic or kosmotropic salt ions that affect water structuring and hydrogen bonding may provide opportunities for further tuning surfactant interactions in such cases. Experiments A library of betaine surfactant with tail lengths n=14-22 both with and without an amidopropyl linker were synthesised to study the effect of the amide functionality on surfactant properties. Characterisation of the molecules interfacial properties were performed using pendant drop tensiometry and their solution state formulation properties were probed using small-angle neutron scattering (SANS) and rheological measurements. Findings Presence of an amidopropyl linker had little effect on aggregation propensity (as evidenced by critical micelle concentration) and aggregate morphology of betaine surfactants, but did increase the Krafft temperature of these surfactants. SANS analysis indicated that aggregate morphology of alkyl betaine surfactants could be influenced by the addition of sodium salts with chaotropic counterions (I- and SCN-), but they were insensitive to more kosmotropic anions (SO42-, F- and Cl-), providing unique and novel solution control methods for this (supposedly salt-insensitive) class of surfactants.

Cryo-TEM and rheological study on shear-thickening wormlike micelles of zwitterionic/anionic (AHSB/SDS) surfactants

HYPOTHESIS

Shear-thickening micelles were mostly made of cationic surfactants, but shear-thickening was rarely reported in the zwitterionic/anionic surfactants. Since wormlike micelles were essential in shear-thickening systems, it should be common for the hybrid wormlike micelles formed by zwitterionic/anionic surfactants, and their fundamental features need to be clarified.

EXPERIMENTS

The micellization of zwitterionic surfactant homologies alkyl dimethyl amidopropyl hydroxyl sulfobetaine (AHSB) and sodium dodecyl sulfate (SDS) in brine was studied, and various environmental factors were considered systematically. Light scattering, rheology, zeta potential, ¹H NMR and cryo-TEM techniques were employed to characterize the AHSB/SDS wormlike micelles.

FINDINGS

AHSB/SDS hybrid wormlike micelles were formed in a wide x_SDS region to endow them with apparent viscosities, in which the electrostatic and hydrophobic interactions between AHSB and SDS molecules were critical. AHSB with the longer tail, the higher c_AHSB and c_NaCl were advantageous to enhance the viscosity because of the longitudinal growth of wormlike micelles. The shear-thickening AHSB/SDS samples were commonly composed of unbranched wormlike micelles with various length, and the shear-induced alignment of wormlike micelles was the major cause as verified by cryo-TEM. Moreover, the quantitative relationships on the critical shear rate ɣ̇_c were established, and the activation energies were obtained from the temperature-dependent ɣ̇_c.

Shear-induced nanostructural changes in micelles formed by sugar-based surfactants with varied anomeric configuration

HYPOTHESIS

The self-assembly of long tail sugar-based surfactants into worm-like micelles has recently been demonstrated, and the rheological properties of such systems have been shown to be tuneable through subtle modifications of the molecular characteristics of the surfactant monomer. In particular, the anomeric configuration of the hexadecylmaltoside headgroup was shown to induce profound changes in the nanostructure and rheology of the system. The origin of such changes is hypothesised to arise from differences in the structure and relaxation of the micellar networks in the semi-dilute regime.

EXPERIMENTS

Here we explore the molecular background to the flow properties of the two anomers of hexadecylmaltoside (α- and β-C₁₆G₂) by directly connecting their rheological behaviour to the micelle morphology. For this purpose, 1-3 plane rheo-small-angle neutron scattering measurements, using a Couette cell geometry, probed the structural changes in the micellar phase under shear. The effect of surfactant anomeric configuration, surfactant concentration, temperature and mixing ratio of the two anomers were investigated. The static micelle structure in the semi-dilute regime was determined using the polymer reference interaction site model.

FINDINGS

The segmental alignment of the micellar phase was studied under several flow conditions, showing that the shear-thinning behaviour relates to the re-arrangement of β-C₁₆G₂ worm-like micelles, whilst shorter α-C₁₆G₂ micelles are considerably less affected by the flow. The results are rationalised in terms of micelle alignment and disruption of the entangled network, providing a detailed mechanism by which sugar-based surfactants control the rheology of the fluid. To further enable future studies, we provide the complete code for modelling micelle structure in the semi-dilute regime using the polymer reference interaction site model.