Session Y1: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments V

Weighted Random Mixing and Exact Finite Lattice Descriptions of Molecular Aggregation Equilibria

Entropic and energetic contributions to a broad class of molecular aggregation and self-assembly processes are described by performing a mean field Boltzmann average over aggregate size distributions pertaining to an idealized random mixture. Predictions obtained using the resulting weighted random mixing (WRM) model are compared with exact finite lattice and fluid molecular dynamics simulation results for systems in which each aggregate resembles a central molecule with multiple ligand binding sites. Good agreement between the exact and WRM results is found for systems with interaction energies of various magnitudes (and signs), both in the large and small cohesive interaction energy regimes (or at low and high temperature, respectively). The latter two regimes are separated by a critical point on either side of which qualitatively different aggregation behavior is predicted and observed. More specifically, both the WRM model and exact finite lattice aggregation results reveal that when half the ligand binding sites are filled, the corresponding aggregate size distributions are bimodal below and unimodal above the corresponding critical temperature, whose value depends on the ligand-ligand interaction energy, but is independent of the binding energy of each ligand to the central molecule.   

Rotational-Rate Heterogeneity in Polymers on the Picosecond and Second Timescales through 2D Kinetics

In a polymer, the anisotropy decay of a small solute is nonexponential, i.e. it has rate dispersion. This dispersion could reflect heterogeneity in the local structure and dynamics of the polymer. On the other hand, homogeneous mechanisms are also possible: anisotropic local structure, multiple, independent rotational processes, or a relaxation hierarchy. Two-dimensional (2D) kinetics separate heterogeneous and homogeneous relaxation by monitoring dynamics over two time periods before ensemble averaging. Far from the glass transition, subnanosecond rotation in PDMS has been measured by MUPPETS (multiple population-period transient spectroscopy), a nonequilibrium technique. These experiments are the first to use polarization in MUPPETS to make 2D anisotropy measurements. Near the glass transition, rotation on the seconds timescales in poly(cyclohexyl acrylate) has been analyzed with 2D correlation functions calculated from single-molecule trajectories [\textit{J. Phys. Chem. B}, \textbf{113} 2253 (2009)]. This method is an equilibrium approach to 2D kinetics. By averaging over the entire ensemble, the known problems of subensemble averaging are avoided. Both measurements indicate that rotational-rate dispersion in polymers is primarily due to heterogeneous local environments.   

Computer simulation study of structure and dynamics of supercooled water in silica nanopores

In narrow hydrophilic pores, interactions with pore walls and confinement dimensions allow water to remain liquid well below the normal freezing point. We investigate the properties of nanoconfined supercooled water by means of molecular simulation. The focus of our study is confinement in approximately cylindrical silica pores, with diameters in the 20-40 {\AA} range, a model for MCM-41 materials. We use Gibbs-ensemble Monte Carlo method to determine water density in the pores in equilibrium with the bulk and molecular dynamics simulation to study the properties of confined water [1]. We study the translational and rotational mobilities of molecules in different interfacial layers and the effects on water dynamics of interfacial hydrogen bonding. We make contact with quasi-elastic neutron scattering experiments on supercooled water in MCM-14 silica pores by calculating and analyzing self-intermediate scattering functions of water hydrogens. [1] A. A. Milischuk and B. M. Ladanyi, J. Chem. Phys. \textbf{135}, 174709 (2011).   

Electrolyte Structure near Electrode Interfaces in Lithium-Ion Batteries

The performance of lithium-ion secondary batteries (LIBs) is strongly tied to electrochemistry and ionic transport near the electrode-electrolyte interface. Changes in ion solvation near the interface affect ion conductivity and also are associated with the formation and evolution of solid-electrolyte interphase (SEI) layers, which impede transport but also passivate the interface. Thus, understanding these effects is critical to optimizing battery performance. Here we present molecular dynamics (MD) simulations of typical organic liquid LIB electrolytes in contact with graphite electrodes to understand differences in molecular structure and solvation near the interface compared to the bulk electrolyte. Results for different graphite terminations are presented. We compare the results of density-functional based MD to the empirical reactive forcefield ReaxFF and the non-reactive, non-polarizable COMPASS forcefield. Notable differences in the predictive power of each of these techniques are discussed.   

Femtosecond optical force microscopy

Investigating the nonlinear optical properties of individual nanoscale objects, including single molecules, requires exquisitely sensitive tools. Although several optical microscopy approaches have demonstrated single molecule sensitivity, the acquisition of the nonlinear optical response from individual objects with femtosecond resolution has remained a challenge. In this presentation, we will discuss a new type of microscopy, femtosecond optical force microscopy, which is designed to sensitively probe ultrafast excitation dynamics at the nanoscale. Optical force microscopy detects the molecular response after optical manipulation through minute changes in the force between an atomically sharp tip and the molecule. This approach achieves spectroscopy with femtosecond time resolution and 10 nm spatial resolution. We will highlight the principles of this technique and outline several applications in molecular spectroscopy, including measurements sensitive to excited state dynamics (pump-probe) and experiments that probe ground state vibrational dynamics (stimulated Raman).   

Spatio-spectral infrared vibrational nano-imaging of intermolecular coupling

Molecular self-assembly, the function of biomembranes, and the performance of organic solar cells rely on molecular interactions on the nanoscale. The understanding and design of such heterogeneous functional soft matter has long been impeded by a lack of spectroscopic tools with sufficient nanometer spatial resolution, attomolar sensitivity, and intermolecular spectroscopic specificity. We implement vibrational scattering-scanning near-field optical microscopy ($s$-SNOM) in a multi-spectral modality to investigate the structure-function relationship in PS-$b$-PMMA block copolymers. Using a vibrational resonance as a sensitive reporter of local structure, coupling, and dynamics, we resolve spectral Stark shifts and line broadening correlated with molecular-scale morphologies. By creating images of solvatochromic vibrational shifts we discriminate local variations in electric fields between nanoscale bulk and interface regions, with quantitative agreement to dielectric continuum models. This new nano-chemometric ability to directly resolve nanoscale morphology and associated intermolecular interactions can form a basis for the systematic control of functionality in multicomponent soft matter systems.   

Synchrotron Infrared Nano-Spectroscopy

Heterogeneity underlies many fundamental physical processes and biological functions, and characterizing or ultimately controlling these requires spectroscopic imaging with simultaneous nanometer spatial resolution and sensitivity to chemical structure and composition. In ultrahigh resolution spectromicroscopies, however, spectroscopic sensitivity and spatial resolution often oppose one another. We overcome this limitation with scattering scanning near-field optical microscopy using synchrotron infrared radiation. In this method, the tip of an atomic force microscope acts as an optical antenna, localizing broadband synchrotron infrared radiation with high irradiance and low noise, enabling tip-limited imaging at $\le$40 nm resolution. Optical heterodyne amplified, Fourier-transform detection enables rapid spectral acquisition, spanning 700-5000cm$^{-1}$, with zeptomole (10$^{-21}$) sensitivity. Synchrotron infrared nano-spectroscopy (SINS) is broadly applicable, which we demonstrate through investigations of surface phonon polaritons, biominerals and proteins. Finally, we show preliminary results incorporating advanced optical-antenna designs, with the goal of single molecule infrared spectroscopy.   

Crystallization of supercooled liquids

We investigate the crystallization process on the basis of the free energy landscape (FEL) approach to non-equilibrium systems. In this approach, the crystallization time is given by the first passage time of the representative point arriving at the crystalline basin in the FEL. We devise an efficient method to obtain the first passage time exploiting a specific boundary condition. Applying this formalism to a model system, we show that the first passage time is determined by two competing effects; one is the difference in the free energy of the initial and the final basins, and the other is the slow relaxation. As the temperature is reduced, the former accelerates the crystallization and the latter retards it. We show that these competing effects give rise to the typical nose-shape form of the time-temperature transformation curve and that the retardation of the crystallization is related to the mean waiting time of the jump motion.   

Single Molecule Approaches to Studying Heterogeneity in Molecular Supercooled Liquids

Supercooled liquids display behaviors consistent with the presence of heterogeneous dynamics. We investigate the length scales over which such heterogeneities exist and the time scales over which they persist using single molecule (SM) fluorescence microscopy. In previous work, multiple perylene diimide (PDI) probes were employed to investigate whether probe properties affected breadth of heterogeneity reported in the fragile supercooled liquid ortho-terphenyl (OTP) as well as in less fragile supercooled glycerol. In both cases, the fastest rotating probes reported the greatest breadth of heterogeneity in the host, regardless of physical probe size, suggesting slow probes were averaging over dynamic changes in the environment in time. Here, we introduce a new set of BODIPY-core based probes that are both smaller and more quickly rotating in OTP than the PDI probes. These probes show qualitatively different behavior than the PDI probes, reporting more spatial and temporal heterogeneity than previously studied probes. The newly employed probes open the door to studying the full range of consequences of dynamic heterogeneity in supercooled liquids on the molecular length scale.