Impact of Molecular Structure on Properties of n-Hexadecane and Alkylbenzene Binary Mixtures.

Because of the complexity of petroleum-based fuels, researchers typically use simplified mixtures, known as surrogates, to study combustion behavior and to attempt to identify how physical properties are related to combustion. The process of determining the surrogate composition to yield a desired set of thermophysical properties can be a complicated and time-consuming task. As a result, the use of computer simulations to narrow the number of possible surrogate compositions is beginning to be explored. Herein, molecular dynamics (MD) simulations are used to model binary mixtures of n-hexadecane with either benzene, toluene, n-ethylbenzene, n-propylbenzene, or n-butylbenzene. Calculated densities are in quantitative agreement with experimental values. With the exception of the mixtures containing benzene, simulated excess molar volumes are also in very good agreement with measured values. Isentropic bulk moduli are in qualitative agreement with experiment, and reproduce interesting trends observed in the experimental data. Specifically, minima in the bulk moduli at intermediate compositions of several of the alkylbenzenes are correctly reproduced. In addition, the structures of the fluids are also examined. For mixtures of n-hexadecane with alkylbenzenes with longer chains, the orientation of the aromatic rings is not substantially impacted by composition. In contrast, increasing n-hexadecane content increases the ratio of parallel to perpendicular arrangements of benzene and toluene molecules. In those mixtures, this change in orientation of the aromatic rings could be responsible for the minima observed in the bulk moduli data. These results show that MD simulations can assist in development of fuel surrogates, both by predicting thermophysical properties and by providing insight into how molecular structure and composition affect those properties.

Luminescent tunable polydots: Charge effects in confined geometry.

Long-lived soft nanoparticles, formed by conjugated polymers, constitute a new class of far-from-equilibrium responsive structures for nano-medicine. Tethering ionizable groups to the polymers enables functionality. However concurrently, the ionic groups perturb the delicate balance of interactions that governs these particles. Using fully atomistic molecular dynamics simulations, this study probed the effects of charged groups tethered to poly para phenylene ethynylene substituted by alkyl groups on the polymer conformation and dynamics in confined geometry. We find that the ionizable groups affect the entire shape of the polydots and impact the conformation and dynamics of the polymer.

Structure of Rigid Polymers Confined to Nanoparticles: Molecular Dynamics Simulations Insight.

Nanoparticles (NPs) grafted with organic layers form hybrids able to retain their unique properties through integration into the mesoscopic scale. The organic layer structure and response often determine the functionality of the hybrids on the mesoscopic length scale. Using molecular dynamics (MD) simulations, we probe the conformation of luminescent rigid polymers, dialkyl poly(p-phenylene ethynylene)s (PPE), end-grafted onto a silica nanoparticle in different solvents as the molecular weights and polymer coverages are varied. We find that, in contrast to NP-grafted flexible polymers, the chains are fully extended independent of the solvent. In toluene and decane, which are good solvents, the grafted PPEs chains assume a similar conformation to that observed in dilute solutions. In water, which is a poor solvent for the PPEs, the polymer chains form one large cluster but remain extended. The radial distribution of the chains around the core of the nanoparticle is homogeneous in good solvents, whereas in poor solvents clusters are formed independent of molecular weights and coverages. The clustering is distinctively different from the response of grafted flexible and semiflexible polymers.

Internal Correlations and Stability of Polydots, Soft Conjugated Polymeric Nanoparticles.

Conjugated polymers collapsed into long-lived highly luminescent nanoparticles, or polydots, have opened a new paradigm of tunable organic particles with an immense potential enhancing intracellular imaging and drug delivery. Albeit the chains are not in their equilibrium conformation and are not confined by cross-links, they remain stable over astounding long times. Using fully atomistic molecular dynamics simulations with an innovative method to controllably collapse an inherently rigid polymer, we determined for the first time the internal structure and stability of polydots made of dialkyl-para-phenylene ethynylene, immersed in water, a biological relevant medium. In contrast to natural aggregates, the aromatic rings within the polydots are uncorrelated, with little to no water in its interior. This lack of correlation explains the differences of luminescence characteristics between spontaneously aggregated conjugated polymers and polydots. Resolving the conformation and stability of these particles will enable transforming an idea to a new effective tool.

Conformational study of a single molecule of poly para phenylene ethynylenes in dilute solutions.

The conformation of single molecules of dialkyl poly para phenylene ethynylenes (PPEs), electro-active polymers, is studied in solutions using molecular dynamics simulations. The conformation of conjugated polymers affects their electro-optical properties and therefore is critical to their current and potential uses, though only limited theoretical knowledge is available regarding the factors that control their configuration. The present study investigates the affects of molecular parameters including molecular weight of the polymer and chemical structure of the side chains of PPEs in different solvents on the conformation of the polymers. The PPEs are modeled atomistically where the solvents are modeled both implicitly and explicitly. The study finds that PPEs assume extended configuration which is affected by the length of the polymer backbone and the nature and length of substituting side chains. While the polymer remains extended, local dynamics is retained and no long range correlations are observed within the backbone. The results are compared with scattering experiments.