Bonding mechanisms and conformation of poly(ethylene oxide)-based surfactants in interlayer of smectite

A better understanding of the interactions between poly(ethylene oxide) (PEO)-based nonionic surfactants and smectite is important to fully comprehend the transport and the fate of nonionic surfactants in the environment and to design novel organo-clay composites. We studied the bonding between the surfactants and smectite and the molecular conformations of the surfactants in the interlayer of smectite. A reference polymer PEG and three nonionic surfactants—Brij 56, Brij 700, and PE-PEG—were intercalated into a smectite. The polymers and the composites were characterized with X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. The XRD and FT-IR results indicate that the bulk surfactants existed as crystalline materials at room temperature, and surfactant molecules had both helical/extended diblock and planar zigzag conformations. The surfactants intercalated smectite and expanded the d(001) spacing of smectite to nearly 1.8 nm. The shapes and positions of the IR bands of interlayer surfactants were similar to those of the melted (amorphous) bulk polymers: the wagging vibrations of the CH₂ merged to a single band at 1,350 cm⁻¹, the twisting bands of CH₂ had 9 cm⁻¹ or more blue shifts. These changes imply that the PEO segments of the surfactants existed with a distorted and extended conformation in the interlayer of smectite, and this extended conformation was an intermediate form of the helical and planar zigzag conformations. The molecular conformation of the interlayer surfactant was not affected by the seven types of exchangeable cations (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cu²⁺, Ni²⁺, and H⁺) tested. There were 20 cm⁻¹ or more red shifts from the C–O–C stretching bands when the surfactants were adsorbed. The red shifts suggest that surfactants were bonded to smectite mainly through (1) H-bonding between oxygen atoms of the PEO segments and water molecules in hydration shells of the exchangeable cations, and (2) direct coordination or ion–dipole interaction between the oxygen atoms of the PEO segments and the exchangeable cations. With the extended conformation, the oxygen atoms of the PEO segments have maximum exposure to the bonding water molecules and exchangeable cations.