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Intermediate Range Order and Structure in Colloidal Dispersions with Competing Interactions



P. Douglas Godfrin, Ramon Castaneda-Priego, Yun Liu, Norman J. Wagner


Recent interest in systems with a short range attraction and long range repulsion has brought attention to the formation of a low-q peak in the structure factor and the proposition that this peak directly indicates cluster formation. To understand the structure of the system and its relation with the structure factor obtained through scattering techniques, Metropolis Monte Carlo simulations are performed to calculate the partial structure factors after decomposing the system into cluster-cluster, monomer-monomer, and cross-correlations. We find that the low-q peak appears in fluids that have strong cluster-cluster correlations, but also appears in systems that are dominated by monomer-monomer correlations are those that are percolated. Thus, this low-q peak is more appropriately termed as the "intermediate range order" (IRO) peak. Even when there is cluster formation, it is not trivial to relate cluster formation and this low-q peak. Consequently, the presence of an IRO peak in the scattering does not necessarily signal the existence of a cluster state in solution. Rather, it reflects the presence of a preferred length scale that related to the two competing potential features defined in the interparticle potential. Further, the binodal calculated according to only the attractive portion of the potential is shown to provide a good estimate of the onset of a stable cluster fluid. Normalizing the phase behavior by the critical point of the estimated binodal transition demonstrates a general phase diagram of systems with short range attraction and long range repulsion. In addition to a fluid state of stable clusters, two distinct regimes of percolation are proposed and investigated. Above the estimated binodal, percolation arises from connectivity of monomers and small clusters. Within the estimated phase separation region, large stable clusters form which then merge and percolate. Deciphering the structure found in these complex systems is plausible by combining experiment with simulation.
Journal of Chemical Physics


Intermediate range order, simulation


Godfrin, P. , Castaneda-Priego, R. , Liu, Y. and Wagner, N. (2013), Intermediate Range Order and Structure in Colloidal Dispersions with Competing Interactions, Journal of Chemical Physics, [online], (Accessed December 6, 2023)
Created October 20, 2013, Updated October 12, 2021