Ultrafast dynamics in aqueous polyacrylamide solutions

Dynamics of Acrylamide Hydrogels, Polymers, and Monomers in Water Measured with Optical Heterodyne-Detected Optical Kerr Effect Spectroscopy

The ultrafast dynamics of acrylamide monomers (AAm), polyacrylamide (PAAm), and polyacrylamide hydrogels (PAAm-HG) in water were studied using optical heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Previous ultrafast infrared (IR) measurements of the water dynamics showed that at the same concentration of the acrylamide moiety, AAm, PAAm, and PAAm-HG exhibited identical water dynamics and that these dynamics slowed with increasing concentration. In contrast to the IR measurements, OHD-OKE experiments measure the dynamics of both the water and the acrylamide species, which occur on different time scales. In this study, the dynamics of all the acrylamide systems slowed with increasing concentration. We found that AAm exhibits tetraexponential decays, the longest component of which followed Debye-Stokes-Einstein behavior except for the highest concentration, 40% (w/v). Low concentrations of PAAm followed a single power law decay, while high concentrations of PAAm and all concentrations of PAAm-HG decayed with two power laws. The highest concentrations, 25% and 40%, of PAAm and PAAm-HG showed nearly identical dynamics. We interpreted this result as reflecting a similar extent of chain-chain interactions. At low concentrations, PAAm displays non-Markovian, single-chain dynamics (single power law), but PAAm displays entangled chain-chain interactions at high concentrations (two power laws). PAAm-HG has chain-chain interactions at all concentrations that arise from the cross-linking. At high concentrations, the dynamics of the entangled of PAAm become identical within error as those of the cross-linked PAAm-HG.

Intermolecular Dynamics of Positively and Negatively Charged Aromatics and Their Isoelectronic Neutral Analogs in Aqueous Solutions

In this study, we investigated the temperature dependence of intermolecular vibrations and orientational dynamics in the aqueous solutions of imidazole hydrochloride, imidazole, sodium triazolide, and triazole using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES) and steady-state Raman spectroscopy. The difference low-frequency Raman spectra under 250 cm-1 of the aqueous solutions relative to the neat water showed that the spectral shoulder in the high-frequency region at 60-100 cm-1, assigned to the libration of an aromatic ring, was higher in frequency for the imidazolium cation but lower for the triazolide anion than those of the respective neutral aromatics. The results of the ab initio quantum chemistry calculations of the clusters of the aromatics and water molecule(s) were consistent with the experimental spectra of the aqueous solutions. Further, the results of the temperature-dependent experiments showed that the signal intensity in the low-frequency region below 50 cm-1 increased for all solutions with an increase in temperature. In contrast, the spectral density in the high-frequency region above 80 cm-1 exhibited almost no shift for the 1.0 M solutions, while a significant red shift was observed for the 5.0 M solutions. In addition, the temperature-dependent densities, viscosities, and surface tensions were characterized for the aqueous aromatic solutions from 293 to 353 K.

Intermolecular Dynamics and Structure in Aqueous Lidocaine Hydrochloride Solutions

We investigated the intermolecular dynamics and static structure in the aqueous solutions of lidocaine hydrochloride (LDHCl) in the concentration range of [LDHCl] = 0-2.00 M using femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), small- and wide-angle X-ray scattering (SWAXS), and dynamic light scattering (DLS). For the fs-RIKES experiments, the concentration dependence of the difference low-frequency spectra of the aqueous LDHCl solutions relative to the neat water, which was mainly due to the intermolecular vibrations, was characterized using an exponential function with a characteristic concentration of ∼1 M. For the SWAXS experiments, we observed a manifestation of an excess scattering component centered within a range of 8-10 nm^-1 in the aqueous LDHCl solutions. The results of Fourier inversion and further deconvolution analyses unambiguously demonstrated that lidocaines assemble into a nanometer-sized micelle-like structure with the innermost core (∼0.3 nm) and outer shell (∼0.5 nm), respectively. The DLS experiments also found nanometer-sized aggregates and further indicated evidence of the clusters of the aggregates. The results of viscosities, densities, and surface tensions of the solutions and the quantum chemistry calculations supported the unique features of the microscopic intermolecular interaction and the micelle-like aggregation.

Low-Frequency Spectra of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Poly(ethylene glycol) by Femtosecond Raman-Induced Kerr Effect Spectroscopy

This is the first report on low-frequency spectra of ionic liquid (IL)/polymer mixtures using femtosecond Raman-induced Kerr effect spectroscopy. We studied mixtures of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF₄]) and poly(ethylene glycol) (PEG) with M_n = 400 (PEG400) at various concentrations. To elucidate the unique features of the IL/polymer mixture system, mixtures of PEG400 with a molecular liquid, 1-octhylimidazole (OIm), which is a neutral analog of the cation, were also studied. In addition, mixtures of [MOIm][BF₄] with ethylene glycol (EG) and poly(ethylene glycol) with M_n = 4000 (PEG4000) were also investigated. The first moments of broad low-frequency spectra, mainly due to intermolecular vibrations for the [MOIm][BF₄]/PEG400 and OIm/PEG400, increased slightly with increasing concentration of PEG400, indicating that microscopic intermolecular interactions, in general, are slightly enhanced. We also compared the [MOIm][BF₄] mixtures with EG, PEG400, and PEG4000 at concentrations of 5 and 10 wt % PEG or EG. The low-frequency spectra of samples with the same concentrations were quite similar, but a comparison of the normalized spectra showed that the spectral intensity in the low-frequency region below ∼50 cm^-1 of the [MOIm][BF₄] mixtures with PEG400 and PEG4000 is somewhat lower than that of the [MOIm][BF₄] mixtures with EG. Although the effect of the polymer is small compared to other polymer solution systems, this feature is attributed to a suppression of translational motion in the mixtures of [MOIm][BF₄] with PEG compared to the mixtures of [MOIm][BF₄] with EG due to the greater mass of PEG than EG. Density, surface tension, viscosity, and electrical conductivity were also estimated. From Walden plots, it was found that the [MOIm][BF₄]/PEG4000 system showed more ideal electrical conductive behavior than the [MOIm][BF₄]/PEG400 and [MOIm][BF₄]/EG systems.

Low-Frequency Vibrational Motions of Polystyrene in Carbon Tetrachloride: Comparison with Model Monomer and Dependence on Concentration and Molecular Weight

In this study, the low-frequency vibrational dynamics of polystyrene (PS) in CCl₄ was investigated by femtosecond Raman-induced Kerr effect spectroscopy. Ethylbenzene (EBz) was also investigated as a model monomer of the polymer to elucidate the unique dynamical features of PS in solution. The broadened low-frequency spectrum of the PS/CCl₄ in the frequency region below 150 cm^-1 is significantly different from that of the EBz/CCl₄. Difference spectra between the PS or EBz solutions and neat CCl₄, normalized to an internal vibrational mode of CCl₄, clearly show a much lower spectral intensity for the PS/CCl₄ than the EBz/CCl₄ in the low-frequency region below ca. 20 cm^-1. This indicates that translational motions are suppressed in the PS/CCl₄ compared to the EBz/CCl₄. Moreover, the high-frequency motion at ca. 70 cm^-1, mainly due to phenyl ring librations, occurs at higher frequency in PS (78 cm^-1) than EBz (65 cm^-1). In addition, the results of concentration-dependent experiments show that the first moment (M₁) of the low-frequency difference spectra of both PS/CCl₄ and EBz/CCl₄ is almost independent of the concentration. The molecular weight dependence of the low-frequency spectrum in the PS/CCl₄ shows that the M₁ value of the low-frequency spectral band of PS shifts to higher frequencies when the molecular weight of PS increases up to M_w = ∼1000, which corresponds approximately to the decamer, and then remains constant upon further increasing the molecular weight.

Effect of Salt on the Ordinary-Extraordinary Transition in Solutions of Charged Macromolecules

Using dynamic light scattering technique, we address the role of added salt at higher concentrations on the "ordinary-extraordinary" transition in solutions of charged macromolecules. The "ordinary" behavior has previously been associated with a "fast" diffusion coefficient which is independent of salt concentration C_s and polymer concentration C_p if the ratio C_p/ C_s is above a threshold value. The "extraordinary" transition is associated with formation of aggregates, with a "slow" diffusion coefficient, formed from similarly charged macromolecules. By investigating aqueous solutions of sodium poly(styrenesulfonate) and sodium chloride with variations in C_p, C_s, and polymer molecular weight, M_w, we report the emergence of a new diffusive "fast" relaxation mode at higher values of C_p, C_s, and M_w, in addition to the previously known "fast" and "slow" relaxation modes. Furthermore, we find that M_w plays a crucial role on the collective dynamics of polyelectrolyte solutions with salt, instead of just the C_p/ C_s ratio as previously postulated. As M_w is progressively decreased, the salty solution exhibits dynamical transitions from three modes to two modes and then to one mode of relaxation. The emergence of the new fast mode and the dynamical transitions are in marked departure from the general premise of the ordinary-extraordinary transition developed over several decades. In an effort to rationalize our experimental findings we present a theory for the collective dynamics of polyelectrolyte solutions with salt by addressing the coupling between the relaxations of polyelectrolyte chains, counterions from the polymer and added salt, and co-ions from the salt. The predictions are in qualitative agreement with experimental findings. The present combined work of experiments and theory forms the basis for accurately characterizing dynamics of charged macromolecules in salty solutions, which are ubiquitous in biological systems and polyelectrolyte-based technologies.

Application of a polytetrafluoroethylene (PTFE) flat membrane for the treatment of pre-treated ASP flooding produced water in a Daqing oilfield

Pre-treated ASP flooding oilfield water produced in Daqing, China was treated by a PTFE microfiltration membrane and the removal efficiency of the main pollutants in the oilfield-produced water was studied.

Femtosecond optical Kerr effect setup with signal "live view" for measurements in the solid, liquid, and gas phases

We present the experimental setup constructed in our laboratory for measurement of the femtosecond optical Kerr effect. The setup allows measurements with high temporal resolution and acquisition speed. The high signal to noise ratio is obtained with use of a homemade balanced detector. Due to the high acquisition speed and good signal to noise ratio, it is possible to have a "live view" of the signal and to easily tune the sample position and orientation before the measurement. We show the example results obtained in the solid, liquid, and the gas phases and we use them in order to check on the precision of our setup. As the samples we have used a YAG crystal, liquid acetone, and atmospheric air. In the latter two cases, a good agreement with the literature data has been found. The measurements in the gas phase confirm that our setup, although utilizing low energy pulses from the sapphire oscillator, is able to acquire high quality rotational signal in a low density sample.

Ultrafast charge transfer in nickel phthalocyanine probed by femtosecond Raman-induced Kerr effect spectroscopy

The recently developed technique of femtosecond stimulated Raman spectroscopy, and its variant, femtosecond Raman-induced Kerr effect spectroscopy (FRIKES), offer access to ultrafast excited-state dynamics via structurally specific vibrational spectra. We have used FRIKES to study the photoexcitation dynamics of nickel(II) phthalocyanine with eight butoxy substituents, NiPc(OBu)8. NiPc(OBu)8 is reported to have a relatively long-lived ligand-to-metal charge-transfer (LMCT) state, an essential characteristic for efficient electron transfer in photocatalysis. Following photoexcitation, vibrational transitions in the FRIKES spectra, assignable to phthalocyanine ring modes, evolve on the femtosecond to picosecond time scales. Correlation of ring core size with the frequency of the ν10 (asymmetric C-N stretching) mode confirms the identity of the LMCT state, which has a ∼500 ps lifetime, as well as that of a precursor d-d excited state. An even earlier (∼0.2 ps) transient is observed and tentatively assigned to a higher-lying Jahn-Teller-active LMCT state. This study illustrates the power of FRIKES spectroscopy in elucidating ultrafast molecular dynamics.

MOLECULAR DYNAMIC SIMULATION OF CARBOXYETHYL CHITOSAN: EFFECT OF TEMPERATURE, SALT CONCENTRATION AND pH

The effect of temperatures and NaCl concentrations on the structures of polyampholyte(carboxyethyl chitosan) under different pH conditions were investigated using molecular dynamics simulations. The radius of gyration (ROG) of the backbone Cα of carboxyethyl chitosan was analyzed and the accessible surface areas (ASA) of entire polyampholytes were estimated. The variation of the ROG and the ASA of polyampholyte with temperatures and different NaCl concentrations under different pH conditions are discussed. The present simulation makes it possible to understand the structures of polyampholyte in aqueous solutions at the molecular level.

Inter- and intramolecular hydrogen bonding in phenol derivatives: a model system for poly-L-tyrosine

The ultrafast dynamics of solutions of phenol and two phenol derivatives--hydroquinone (1,4-benzenediol) and pyrocatechol (1,2-benzenediol)--have been studied with Optically Heterodyne-Detected Optical Kerr-Effect (OHD-OKE) spectroscopy. The solvents, methanol and acetonitrile, were selected to provide strong and weak solvent-solute hydrogen-bonding interactions, respectively, while pyrocatechol features an intramolecular hydrogen bond. Together these provide a series of model systems for polypeptides such as polytyrosine, which facilitate the direct study of inter- and intramolecular hydrogen bonding. A broad contribution to the Raman spectral density of the methanol solutions at frequencies between 150 and 300 cm(-1) has been observed that is absent in acetonitrile. This contribution has been assigned to solvent-solute hydrogen-bond stretching vibrations. The OHD-OKE response of poly-L-tyrosine has been measured and was found to contain a similar contribution. Density functional theory geometry optimizations and normal mode calculations have been performed using the B3LYP hybrid functional and 6-311++G** basis set. These have yielded a complete assignment of the low-frequency Raman and far-infrared spectra of pyrocatechol for the first time, which has provided information on the nature of the intramolecular hydrogen bond of pyrocatechol.

The Ultrafast Dynamics of Hydrogen-Bonded Liquids: Molecular Structure-Dependent Occurrence of Normal Arrhenius or Fractional Stokes−Einstein−Debye Rotational Diffusive Relaxation

The ultrafast rotational-diffusive dynamics of the peptide linkage model compounds N-methylacetamide (NMA), acetamide (Ac), and N,N-dimethylacetamide (DMA) have been studied as a function of temperature using optically heterodyne-detected optical Kerr effect (OHD-OKE) spectroscopy. Both NMA and Ac exhibit a non-Arrhenius temperature dependence of the rotational diffusive relaxation time. By contrast, the non-hydrogen-bonding DMA exhibits normal hydrodynamic behavior. The unusual dynamics of NMA and Ac are attributed to the decoupling of single-molecule rotational diffusive relaxation from the shear viscosity via a transition between stick and slip boundary conditions, which arises from local heterogeneity in the liquid due to the formation of hydrogen-bonded chains or clusters. This provides new insight into the structure and dynamics of an important peptide model compound and the first instance of such a phenomenon in a room-temperature liquid. The OHD-OKE responses of carboxylic acids acetic acid (AcOH) and dichloroacetic acid (DCA) are also reported. These, along with the terahertz Raman spectra, show no evidence of the effects observed in amide systems, but display trends consistent with the presence of an equilibrium between the linear and cyclic dimer structures at all temperatures and moderate-to-high mole fractions in aqueous solution. This equilibrium manifests itself as hydrodynamic behavior in the liquid phase.

Single particle and collective hydration dynamics for hydrophobic and hydrophilic peptides

We have conducted extensive molecular dynamics simulations to study the single particle and collective dynamics of water in solutions of N-acetyl-glycine-methylamide, a model hydrophilic protein backbone, and N-acetyl-leucine-methylamide, a model (amphiphilic) hydrophobic peptide, as a function of peptide concentration. Various analytical models commonly used in the analysis of incoherent quasielastic neutron scattering (QENS), are tested against the translational and rotational intermediate scattering function, the mean square displacement of the water molecule center of mass, and fits to the second-order rotational correlation function of water evaluated directly from the simulation data. We find that while the agreement between the model-free analysis and analytical QENS models is quantitatively poor, the qualitative feature of dynamical heterogeneity due to caging is captured well by all approaches. The center of mass collective and single particle intermediate scattering functions of water calculated for these peptide solutions show that the crossover from collective to single particle-dominated motions occurs at a higher value of Q for high concentration solutions relative to low concentration because of the greater restriction in movement of water molecules due to confinement. Finally, we have shown that at the same level of confinement of the two peptides, the aqueous amphiphilic amino acid solution shows the strongest deviation between single particle and collective dynamics relative to the hydrophilic amino acid, indicating that chemical heterogeneity induces even greater spatial heterogeneity in the water dynamics.

Intermolecular Interactions and Dynamics of Room Temperature Ionic Liquids That Have Silyl- and Siloxy-Substituted Imidazolium Cations

The intermolecular interactions and dynamics of novel ionic liquids with alkylsilyl and alkylsiloxy substitutions on the cations are studied by measuring the intermolecular vibrational spectra and reorientational dynamics using femtosecond Kerr effect methods. The new ionic liquids include 1-dimethylphenylsilylmethyl-3-methylimidazolium (PhSi-mim+), and 1-methyl-3-pentamethyldisiloxymethylimidazolium (SiOSi-mim+) cations paired with the bis(trifluoromethylsulfonyl)imide (NTf(2)-) anion. Measured ionic liquid viscosities are surprisingly low for such bulky cation substituents. DFT electronic structure calculations on the isolated ions provide additional information about the electrostatic interactions.

The dynamics of water-protein interaction studied by ultrafast optical Kerr-effect spectroscopy

Changes in the ultrafast dynamics and terahertz Raman spectrum accompanying a helix-to-coil transition of a homo-polypeptide have been observed for the first time. Formation of the alpha-helix is associated with a shift to lower frequency of a broad Raman band attributable to solvent-peptide intermolecular hydrogen bonding. This band facilitates direct spectroscopic observation of so-called hydration water near a peptide and yields the first quantitative estimate of the time scale of the ultrafast dynamics in the solvation shell, which range from 0.18 to 0.33 ps (185-100 cm(-1)) depending on the secondary structure of the peptide. Such fast motions of solvent molecules have been referred to as the "lubricant of life" and are thought to play key roles in determining structure and activity of proteins.