Solution Structures of Anionic–Amphoteric Surfactant Mixtures near the Two-Phase Region at Fixed pH

Abstract

We examine the solution structures in a mixed surfactant system of sodium dodecyl sulfate (SDS) and N , N -dimethyldodecylamine N -oxide (DDAO) in water, on both sides of the two-phase boundary, employing dynamic light scattering, small-angle neutron scattering, and Fourier transform infrared spectroscopy. The precipitate phase boundary was accessed by lowering pH to 8, from its floating pH 9.5 value, and was experimentally approached from the monomeric and micellar regions in three ways: at fixed DDAO or SDS concentrations and at a fixed (70:30) SDS:DDAO molar ratio. We characterize the size, shape, and interactions of micelles, which elongate approaching the boundary, leading to the formation of disk-like aggregates within the biphasic region, coexisting with micelles and monomers. Our data, from both monomeric and micellar solutions, indicate that the two phase structures formed are largely pathway-independent, with dimensions influenced by both pH and mixed surfactant composition. Precipitation occurs at intermediate stoichiometries with a similar SDS:DDAO ratio, whereas asymmetric stoichiometries form a re-entrant transition, returning to the mixed micelle phase. Overall, our findings demonstrate the effect of stoichiometry and solution pH on the synergistic interaction of mixed surfactants and their impact on phase equilibrium and associated micellar and two-phase structures.

Fourier transform infrared spectroscopy in surfactant science: A personal view

“…In the case of SDS mixed micelles with dodecyldimethyl amine oxide (C12AO), a study of composition‐dependent changes in various parts of the phase diagram by a combination of FT‐IR, dynamic light scattering and neutron scattering techniques provided additional details on mixed micelle shape and size. Confirmation of an increase in the protonation of the amine oxide in the presence of SDS was also obtained (Tyagi et al, 2022).…”

Concentration Dependent Asymmetric Synergy in SDS–DDAO Mixed Surfactant Micelles

“…Our SANS data expand and complement the findings of Kakitani et al 11 where a higher concentration of 80 mM SDS− DDAO micellar system was found to exhibit near symmetric synergy with DDAO composition, suggestive of twin-tail (gemini) surfactant behavior between SDS and DDAO. Further, work by Prevost et al 12 and Tyagi et al 13 revealed that the precipitation boundary for higher concentrations of this system is symmetric and centered at the equimolar solution. Measurements of the kinematic viscosity of SDS− DDAO solutions by Safonova et al 16 corroborate a transition of the maximum viscosity from 80% to 50% DDAO with increasing total surfactant concentration in the 1−15% w/w range, and here we show that this transition also applies for a range of micellar structural and interaction properties.…”

“…This gave the most self-consistent results and minimized scatter in other fitting parameters at these low concentrations. We also note that our ellipsoidal form factor model is in contrast to previous studies that used a core−shell ellipsoid form factor for this system, 11,13,24 albeit at higher concentrations. However, due to a greater scatter in other fitting parameters obtained from trialling the core−shell model and uncertainty on the SLD of the "shells" (headgroups), as will become evident when regular solution theory results are discussed, we therefore report the SANS data fitted with an ellipsoid form factor and an SLD of −0.691 × 10 −6 Å −2 , corresponding to the surfactant tails.…”

“…Changes in their relative contribution have been associated with changes in the symmetry environment around the sulfate headgroup whereby a strong interaction between SDS and DDAO headgroups leads to a further lowering in the symmetry of the sulfate group. 6,13 Data for the 50 mM solution indicate the largest variations near 80% DDAO, corresponding to greater sulfate−amine oxide interaction. Our proposed cycle of Na + dissociation and DDAO protonation (from regular solution theory) agrees with these FTIR findings as we expect a high proportion of both SDS and DDAO headgroups being charged.…”

Phase Behavior and Aggregate Transition Based on Co-assembly of Negatively Charged Carbon Dots and a pH-Responsive Tertiary Amine Cationic Surfactant

“…Meanwhile, some small spherical particles could be observed, which had similar morphologies to those of the Se-CQDs. The coexistence of monomers (or building blocks) and aggregates is commonly observed in self-assembling systems. − However, in the co-assembly of CQDs and the surfactant, the single free CQD theoretically can rarely be seen in the solutions because of electrostatic interaction. It is possible that each CQD is binding to a few molecules of surfactants, whereas these combined surfactant molecules have no visible contribution to the result of DLS and microscopic images.…”