Michael Fayer, Doctoral Dissertation Advisor (AC)
Dynamics in a Water Interfacial Boundary Layer Investigated with IR Polarization-Selective Pump-Probe Experiments
JOURNAL OF PHYSICAL CHEMISTRY B
2017; 121 (17): 4530-4537
The dynamics of water molecules near the surfactant interface in large Aerosol-OT reverse micelles (RMs) (w0 = 16-25) was investigated with IR polarization-selective pump-probe experiments using the SeCN(-) anion as a vibrational probe. Linear absorption spectra of RMs (w0 = 25-2) can be decomposed into the weighted sum of the SeCN(-) spectra in bulk water and the spectrum of the SeCN(-) anion interacting with the interfacial sulfonate head groups (w0 = 1). The spectra of the large RMs, w0 ≥ 16, are overwhelmingly dominated by the bulk water component. Anisotropy decays (orientational relaxation) of the anion for w0 ≥ 16 displayed bulk water relaxation (1.4 and 4.5 ps) plus an additional slow decay with a time constant of ∼13 ps. The amplitude of the slow decay was too large to be associated with SeCN(-) in contact with the interface on the basis of the linear spectrum decomposition. The results indicate that the observed slow components arise from SeCN(-) in a water boundary layer, in which water molecules are perturbed by the interface but are not directly associated with it. This layer is the transition between water in direct contact with the interface and bulk water in the large RM cores. In the boundary layer, the water dynamics is slow compared to that in bulk water.
View details for DOI 10.1021/acs.jpcb.7b01028
View details for Web of Science ID 000400881300022
View details for PubMedID 28379003
Water of Hydration Dynamics in Minerals Gypsum and Bassanite: Ultrafast 2D IR Spectroscopy of Rocks.
Journal of the American Chemical Society
2016; 138 (30): 9694-9703
Water of hydration plays an important role in minerals, determining their crystal structures and physical properties. Here ultrafast nonlinear infrared (IR) techniques, two-dimensional infrared (2D IR) and polarization selective pump-probe (PSPP) spectroscopies, were used to measure the dynamics and disorder of water of hydration in two minerals, gypsum (CaSO4·2H2O) and bassanite (CaSO4·0.5H2O). 2D IR spectra revealed that water arrangement in freshly precipitated gypsum contained a small amount of inhomogeneity. Following annealing at 348 K, water molecules became highly ordered; the 2D IR spectrum became homogeneously broadened (motional narrowed). PSPP measurements observed only inertial orientational relaxation. In contrast, water in bassanite's tubular channels is dynamically disordered. 2D IR spectra showed a significant amount of inhomogeneous broadening caused by a range of water configurations. At 298 K, water dynamics cause spectral diffusion that sampled a portion of the inhomogeneous line width on the time scale of ∼30 ps, while the rest of inhomogeneity is static on the time scale of the measurements. At higher temperature, the dynamics become faster. Spectral diffusion accelerates, and a portion of the lower temperature spectral diffusion became motionally narrowed. At sufficiently high temperature, all of the dynamics that produced spectral diffusion at lower temperatures became motionally narrowed, and only homogeneous broadening and static inhomogeneity were observed. Water angular motions in bassanite exhibit temperature-dependent diffusive orientational relaxation in a restricted cone of angles. The experiments were made possible by eliminating the vast amount of scattered light produced by the granulated powder samples using phase cycling methods.
View details for DOI 10.1021/jacs.6b05589
View details for PubMedID 27385320
Unraveling the dynamics and structure of functionalized self-assembled monolayers on gold using 2D IR spectroscopy and MD simulations
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
2016; 113 (18): 4929-4934
Functionalized self-assembled monolayers (SAMs) are the focus of ongoing investigations because they can be chemically tuned to control their structure and dynamics for a wide variety of applications, including electrochemistry, catalysis, and as models of biological interfaces. Here we combine reflection 2D infrared vibrational echo spectroscopy (R-2D IR) and molecular dynamics simulations to determine the relationship between the structures of functionalized alkanethiol SAMs on gold surfaces and their underlying molecular motions on timescales of tens to hundreds of picoseconds. We find that at higher head group density, the monolayers have more disorder in the alkyl chain packing and faster dynamics. The dynamics of alkanethiol SAMs on gold are much slower than the dynamics of alkylsiloxane SAMs on silica. Using the simulations, we assess how the different molecular motions of the alkyl chain monolayers give rise to the dynamics observed in the experiments.
View details for DOI 10.1073/pnas.1603080113
View details for Web of Science ID 000375395700026
View details for PubMedID 27044113
View details for PubMedCentralID PMC4983838
- Molecular Anion Hydrogen Bonding Dynamics in Aqueous Solution. journal of physical chemistry. B 2015; 119 (42): 13407-13415
- Structural Influences on the Fast Dynamics of Alkylsiloxane Monolayers on SiO2 Surfaces Measured with 2D IR Spectroscopy JOURNAL OF PHYSICAL CHEMISTRY C 2015; 119 (29): 16811-16823