Concerning the nature of coal solutions and suspensions

Coal extract and suspensions of coal particles are capable of forming viscoelastic gels. The nature of the connections or interactions in these systems is not clear. Simple experiments reported in this article indicate that NMP/CS₂ extracts from Upper Freeport coal are predominantly solutions of molecular species that nevertheless form gels at certain concentrations. The physical gels formed in these mixtures must therefore involve some specific association that, in turn, can lead to aggregation, such as π-cation interactions, or a phase separation. This latter mechanism must involve bicontinuous domains where one of the phases is glassy, to provide the cohesive properties necessary for elastic properties. In contrast, gels formed from insoluble coal particles most probably associate through forces common to many colloidal suspensions. In such systems, it is not the local pairwise interactions between molecules alone that is important, but the sum of all interactions between the molecules in each particle. This results in a large interaction energy that is proportional to particle size and the association of these particles to form a space filling network and a gel with viscolelastic properties.