Infrared dichroism measurements on the alkyl chain packing of an ionic detergent intercalated between silicate layers

Microcrystals of the metal silicate hydrate ilerite orient macroscopically on the surface of a ATR-crystals and thus, are accessible for infrared line ar dichroism measurements. We present first results which indicate that the alkyl chain packing and the orientation of the polar group of dodecyl-trim ethylammoniumbromide (DTAB) intercalated between silicate layers can be det ermined in terms of infrared order parameters. The properties of DTAB can b e modulated by the relative humidity of the surrounding atmosphere and by t emperature. Upon heating DTAB undergoes a phase transition from a paraffin- like solid to a fluid phase. The former is characterized by the orthorhombi c perpendicular packing of the frozen alkyl chains with tilted long axes. T he interactions between the ionic groups of the surfactant and that of the host matrix stabilize the lamellar arrangement of DTAB in the crystalline a nd in the fluid phases.