Probing the persistence length of large, rigid chromonic aggregates from Computer Simulation

Chromonic molecules are non-conventional amphiphiles. In dilute solution, rather than forming micelles, chromonic mesogens self-assemble into elongated aggregates or stacks, which can then orient into nematic and other ordered... [ view full abstract ]

Chromonic molecules are non-conventional amphiphiles. In dilute solution, rather than forming micelles, chromonic mesogens self-assemble into elongated aggregates or stacks, which can then orient into nematic and other ordered phases. The self-assembly process is unlike that observed for many conventional amphiphiles; occurring without a critical micelle concentration[1].

The formation of orientationally ordered phases, such as the nematic phase, are driven by the size of the aggregate. For isodesmic self-assembly, at a set temperature, the distribution of aggregate sizes should follow an exponential distribution. The average stack size, along with the cross-sectional area of the aggregate can be used in an Onsager theory to predict the onset of the nematic phase. However, current experimental studies show binding energies (and hence average stack sizes) to be much lower than expected within the nematic phase[2]. The persistence length of the aggregates is often used to explain this discrepancy, but to simulate such a system at an atomistic level of detail would require prohibitively large aggregates studied for prohibitively long time scales.

We apply constant force atomistic simulations to explicitly calculate the persistence lengths of chromonic aggregates from smaller(accessible) aggregate sizes. These calculations are performed by applying a constant force, normal to the aggregate orientation, and measuring the aggregate's deviation from its mean position. Comparing the persistence length and the potential of mean force (PMF)[3,4], that describes the binding energy of a molecule within an aggregate,provides a good comparison with experimental systems and highlights the features that dictate the persistence length in chromonics.

References

[1] J.Lydon, J.Mater.Chem.,20,10071-10099 (2010)

[2] H.-S.Park, S.-W. Kang, L.Tortora, Y.Nastishin, D.Finotello, S.Kumar and O.D.Lavrentovich J.Phys. Chem. B,112,16307-16319 (2008)

[3] F.Chami and M.R.Wilson, J.Am. Chem. Soc.,132,7794-7802 (2010)

[4] A.Akinshina, M.Walker, M.R.Wilson, G.J.T.Tiddy, A.Masters and P.Carbone, Soft Matter,11,680-691 (2015)