Structure and Dynamics of Nonionic Surfactant Aggregates in Layered Materials

The aggregation of surfactants on solid surfaces as they are adsorbed from solution is the basis of numerous technological applications such as colloidal stabilization, ore flotation, and floor cleaning. The understanding of both the structure and the dynamics of surfactant aggregates applies to the development of alternative ways of preparing hybrid layered materials. For this purpose, we study the adsorption of the triethylene glycol mono n-decyl ether (C₁₀E₃) nonionic surfactant onto a synthetic montmorillonite (Mt), an aluminosilicate clay mineral for organoclay preparation with important applications in materials sciences, catalysis, wastewater treatment, or as drug delivery. The aggregation mechanisms follow those observed in an analogous natural Mt, with the condensation of C₁₀E₃ in a bilayer arrangement once the surfactant self-assembles in a lamellar phase beyond the critical micelle concentration, underlining the importance of the surfactant state in solution. Solid-state ¹H nuclear magnetic resonance (NMR) at fast magic-angle spinning (MAS) and high magnetic field combined with¹H-¹³C correlation experiments and different types of ¹³C NMR experiments selectively probes mobile or rigid moieties of C₁₀E₃ in three different aggregate organizations: (i) a lateral monolayer, (ii) a lateral bilayer, and (iii) a normal bilayer. High-resolution ¹H{²⁷Al} CP-¹H-¹H spin diffusion experiments shed light on the proximities and dynamics of the different fragments and fractions of the intercalated surfactant molecules with respect to the Mt surface. ²³Na and ¹H NMR measurements combined with complementary NMR data, at both molecular and nanometer scales, precisely pointed out the location of the C₁₀E₃ ethylene oxide hydrophilic group in close contact with the Mt surface interacting through ion-dipole or van der Waals interactions.