Conformational Behavior of Dodecyldiamine inside the Confined Space of Montmorillonites

Abstract

We have investigated the change of the intercalated amount and conformational behavior of dodecyldiamine (C12H28N2, di-C12amine) inside the confined space of the montmorillonite (MMT) intergallery at different pHs of the intercalation solution (H2O/ethanol/Na+-MMT/di-C12amine), and these results were compared with those of dodecylmonoamine (C12H27N, mono-C12amine). The mono-C12amine with one end-functional amine (−NH2) has a constant intercalated amount independent of the pH of the intercalation solution, confirming the tail conformation in the MMT intergallery. On the other hand, the intercalated amount of di-C12amines remains quite low at low pH due to the long-range Coulombic repulsion among protonated amines. The di-C12amines dominantly take the bridge conformation inside the MMT intergallery up to pH 9.5 and adopt a fraction of the tail conformation at pH 11.5, which is verified by Fourier transform infrared spectra with an abrupt frequency shift of the CH2 stretching band and the emergence of a new NH2 scissoring band at 1598 cm-1 related to unanchored free amines. The d-spacing and the square of the half-width at half-maximum of the MMT based on X-ray diffraction measurements are also constant up to pH 9.5, while these two variables are suddenly increased at pH 11.5. The bridge conformation of di-C12amines in the confined space of the MMT intergallery prevents poly(ethylene oxide) chains from intercalation into the MMT intergallery, called the gluing effect.