Discerning the Structure Factor of Charged Micelles in Water and Supercooled Solvent by Contrast Variation X-ray Scattering

Abstract

Sodium dodecyl sulfate (SDS) is a well-known anionic surfactant that forms micelles in various solvents including supercooled sugar−urea melt. Here, we explore the application of contrast variation smallangle X-ray scattering (SAXS) in discerning the structure and interactions of SDS micelles in aqueous solution and in a room-temperature supercooled solvent. The SAXS patterns can be analyzed in terms of a core−shell ellipsoid model. For aqueous SDS micelles, at low volume fractions, the features due to intermicellar interaction, S(q), in the SAXS pattern are poorly resolved because of the prominent contribution from shell scattering. Increasing the electron density of the solvent by the addition of the urea or fructose−urea mixture (at a weight ratio of 6:4) permits the systematic variation of shell scattering without influencing the structure drastically. For a 10% solution of SDS in water, the contribution from the shell can be completely masked by the addition of 40% urea or fructose−urea mixture. The fructose−urea mixture is a preferred additive as it can vary the scattering length density over a wide range and serves as a matrix to form supercooled micelles. The structural parameters of micelles in supercooled fructose−urea melt are obtained from contrast variation SAXS, small-angle neutron scattering, and high-resolution transmission electron microscopy.

Foam film stratification studies probe intermicellar interactions

“…The P(q) fits reveal that SDS molecules self-assemble to form core-shell ellipsoids with the minor and the major axis radii R p ≈ 1.3 nm and R e ≈ 2 nm, respectively (an aspect ratio of ≈0.65), and a shell thickness ≈0.75 nm (see Fig. 3B), in good agreement with previous scattering studies on SDS micelles (46,65,66,72,73). The computed values of total charge (Q < 30 e − ) show that only a fraction of molecules is ionized.…”

“…For SDS micelles, the hydrophobic core has a lower electron density than water, while the shell comprising the hydrophilic group and counter ions possesses a higher electron density than water. The combined influence of P(q), S(q), and the scattering length density differences (between solvent and shell and between shell and core) create distinct peaks and valleys in the X-ray scattering profiles of SDS micelles in contrast with the neutron scattering profiles (66). An analysis of the features in I(q) and their corresponding length scale (∼1/q) reveal that the I(q) peaks in the range 1 nm −1 < q < 3 nm −1 emerge owing to the contribution of P(q), whereas the peaks in the range 0.4 nm −1 < q < 1 nm −1 are the contribution of S(q).…”

Salt Weakens Intermicellar Interactions and Structuring in Bulk Solutions and Foam Films

“…The I ( q ) profile depends on two key contributions: the structure factor S ( q ) is set by intermicellar interference effects, whereas the form factor P ( q ) captures the intramicellar interference − is expressed as P ( q ) ∼ (1/ V ) F 2 ( q , R e ,δ,σ, T ), with …”

Altering the X-ray Scattering Contrast of Triton X-100 Micelles and Its Trapping in a Supercooled Solvent

“…Moreover, at appropriate compositions such molten mixtures can be supercooled to form amorphous matrix that are amenable for trapping self-assembled structures. Detailed structural studies of SDS micelles in such supercooled matrix are obtained from contrast variation small-angle X-ray scattering (SAXS) . Here, we examine the structure of Triton X-100 (TX-100) micelles in glucose–urea (GU) melt using complementary techniques like SAXS, small angle neutron scattering (SANS), and cryo-TEM.…”

“…Detailed structural studies of SDS micelles in such supercooled matrix are obtained from contrast variation small-angle X-ray scattering (SAXS). 16 Here, we examine the structure of Triton X-100 (TX-100) micelles in glucose−urea (GU) melt using complementary techniques like SAXS, small angle neutron scattering (SANS), and cryo-TEM. Scattering techniques in combination with microscopy permit deep understanding of the structure of micelles in various solvents.…”