How Do the Valency and Radii of Cations Affect the Rheological Properties of Aqueous Solutions of Zwitterionic and Anionic Surfactant Mixtures?

Abstract

Despite extensive research on the use of salts to enhance micellar growth, numerous questions remain regarding the impact of ionic exchange and molecular structure on charge neutralization. This study looks into how certain cations (Na⁺, Ca²⁺, and Mg²⁺) affect the structure of a cocamidopropyl betaine CAPB and sodium dodecylbenzenesulfonate SDBS surfactant mixture, aiming toward applications in targeted delivery systems. The mixture consists of a zwitterionic surfactant, cocamidopropyl betaine (CAPB), and an anionic surfactant, sodium dodecylbenzenesulfonate (SDBS), combined in varying molar ratios at a total concentration of 200 mM. We characterized the macroscale properties through rheological measurements and obtained detailed structural insights using small-angle X-ray scattering (SAXS) and cryogenic electron microscopy (cryo-EM). The findings reveal that increasing the concentration of cations in the CAPB/SDBS mixture induces the formation of peaks in the zero-shear viscosity as a function of salt concentration (salt curve). Analysis through cryo-EM and SAXS showed that these viscosity peaks are related to the change of micellar assemblies from entangled worm-like micelles to branched worm-like micelles and then to bilayer structures (vesicles). The specific cation concentration at which the zero-shear viscosity peak occurs, as well as the maximum viscosity, is strongly influenced by the type of cation present in the CAPB/SDBS solutions, a phenomenon explained by the Hofmeister series. Notably, the differing affinities of cations for the carboxylate COO^- and sulfite SO^3- groups and the partial dehydration of micelles contribute to the lower concentration of magnesium cations required to reach the viscosity peak compared to calcium cations.

Figure 1

Structure of (a) cocamidopropyl betaine (CAPB) and (b) sodium dodecylbenzenesulfonate (SDBS) (drawings created by ChemSketch).

Figure 2

Dependence of zero-shear viscosity on the molar ratio of CAPB/SDBS in aqueous solutions with the addition of NaCl, MgCl₂, and CaCl₂ (0.1 M). The images on the right represent samples with Mg²⁺, but the same tendency was observed for other types. A line has been marked indicating the threshold at which the samples become turbid, with representative images displayed on the right.

Figure 3

Influence of metal ions on the zero-shear viscosity of different aqueous CAPB/SDBS solutions. (a) Fixed molar ratio of 2.0 for Na⁺, Ca²⁺, and Mg²⁺. (b) Molar ratios based on the zero-shear viscosity peaks observed in Figure 2 (2.0, 3.1, and 3.5 for Na⁺, Ca²⁺, and Mg²⁺, respectively). The dashed lines on the graph denote the concentrations at which turbidity begins.

Figure 4

Storage modulus G′ and loss modulus G″ vs angular frequency for CAPB/SDBS solutions with molar ratio 3.5 and Mg²⁺ concentration 0.12 M and 0.15 M. Solid lines correspond to the Maxwell model fitted to the experimental data.

Figure 5

(a) Midfilament diameter as a function of time for CAPB/SDBS solutions containing different amounts of Mg²⁺ ions. The values of zero-shear viscosity were, respectively: 10.12 Pa·s for the sample with 0.12 M Mg²⁺, 9.95 Pa·s (0.15 M), 2.81 Pa·s (0.18 M), 1.16 Pa·s (0.20 M). (b) Images of the filament thinning, i.e., evolution of D_mid(t) during the extensional rheometry experiment.

Figure 6

Cryo-EM images of CAPB/SDBS = 3.5 mixtures at increasing concentrations of Mg²⁺ ions. The first row presents a schematic representation of the microstructure based on the images with ion concentration values.

Figure 7

Small-angle X-ray scattering data (black circles) and model fits (solid colored lines). (a) Pure CAPB (fit with core–shell sphere model) and pure SDBS (fit with core–shell ellipsoid model). (b) Mixtures of CAPB/SDBS (77:23) with increasing Mg²⁺ concentration (0.12 and 0.15 M: fit with core–shell cylinder; 0.20 M: fit with core–multishell/vesicle model). (c) Scattering data from Figure 6a,b are compared and shifted by a factor of 2.5 for better illustration.