Montmorillonite modified with unconventional surfactants from the series of octylammonium-based cations: Structural characterization and hydration properties

The properties of organoclays, highly attractive materials utilizing in a variety of applications, depend largely on the type and arrangement of organic cations present in clay mineral interlayer. The systematic study of organoclays containing surfactants with shorter alkyl chains, however, is rare. In this paper, the effect of the chain number, head-group structure and amount of octylammonium cations on their conformation and hydration properties of organo-montmorillonites (OMt) is studied using mainly near-IR spectroscopy. With increasing number of the octylammonium chains and amount of loaded surfactant a gradual transition from disordered gauche to prevailing ordered all-trans conformation has been observed. Creation of H-bonds between -NHX+ head-groups and basal oxygens of the montmorillonite has restricted the mobility of the alkyl tails resulting in a higher portion of gauche conformers comparing to surfactant with -N(CH2)(7)CH3 head-group. The presence of H2O also affects the ordering of the surfactants. In dried OMt the head-groups are anchored close to the hexagonal cavities, but in hydrated samples water molecules can partly remove the head-groups from the vicinity of the hexagonal cavity, making alkyl chains easier to stretch to the all-trans conformation. This study demonstrates, for the first time, different conformation and mobility of the octylammonium-based cations intercalated in OMt.