(667e) Parameterization of Coarse-Grained Models for Simulation of Self-Assembly: the Role of Molecule Rigidity

Micellization
of surfactant solutions is a ubiquitous phenomenon in natural systems and
technological processes, and its theoretical description is a cornerstone
problem in physical chemistry of colloidal systems. However, successful
attempts of quantitative modeling confirmed by experimental data remain
limited. We devised a novel approach to parameterization of coarse-grained
models: binary interaction parameters are fitted to the infinite dilution
activity coefficients of binary solutions formed by reference compounds that
represent coarse-grained fragments of surfactant molecules. Using this
protocol, we derived dissipative particle dynamics (DPD) models of several
surfactants of different chemical structures and obtained a quantitative
agreement with experimental critical micelle concentration (CMC) and
aggregation number (AN).

Another factor determining micellization in
surfactant solutions is the rigidity of the molecules. For the first time, we
present a systematic study of chain rigidity effect on CMC and AN using DPD
simulations. Molecule rigidity was controlled by second-neighbor (?1-3?)
harmonic bonds, or the harmonic angle potential between the nearest neighbor
bonds. Compared to flexible molecules with the nearest neighbor bonds being the
only type of bonded interactions, rigid molecules exhibited a lower critical
micelle concentration, larger and better-defined micelles. By varying the strength
of head-tail repulsion and chain stiffness, we constructed two-dimensional
diagrams presenting how the critical micelle concentration and aggregation
number depend on these parameters. We found that solutions of flexible and
rigid molecules that exhibited approximately the same critical micelle
concentration could differ substantially in the micelle size and shape
depending on the chain stiffness. With the increase of surfactant
concentration, primary micelles of more rigid molecules were found less keen to
agglomeration and formation of non-spherical aggregates characteristic to
flexible molecules.

Overall, our parameterization methodology
can be recommended for studies of self-assembly and dynamics in a wide range of
soft matter systems. However, rigidity has to be accurately accounted for in
coarse-grained modeling of self-assembly.