The literature on the complexes of n-alkylammonium ions with layer silicates is characterized by marked divergences, not only in the deductions and conclusions of the different investigators but also in their experimental results. An attempt is made to reconcile and rationalize the differences, and to form a self-consistent picture of these systems.
Given a saturating solution of sufficient concentration, the complexes tend during their formation to adopt an arrangement in which silicate layers are separated by double layers of ions oriented so that the alkyl chains are extended away from the silicate surfaces at a high angle. Water molecules, salt and free alkylamine may also be present in the interlayer spaces. When the excess material is removed by washing and the complexes dried, ideally the alkyl chains from opposing surfaces move over one another and interpenetrate so as to form a single layer of ions. During the contraction, however, the chains interact with one another and tend to stick at some point short of complete interpenetration. Weak attractive forces acting to bring the silicate layers together, and consisting mainly of electrostatic forces originating in the mineral layers and van der Waals interactions arising from increasing association of opposing alkyl chains with increasing interpenetration, are insufficient to overcome this tendency fully. As a result, an alkyl chain of given length can give rise to a series of complexes differing in their XRD characteristics.
Previous workers have noted the difficulty of achieving reproducible results and some have recommended grinding the complexes, but it appears that this treatment significantly alters the nature of the end-product.