ABSTRACT
Overtones and combinational modes frequently play essential roles in ultrafast vibrational energy relaxation in liquid water. However, these modes are very weak and often overlap with fundamental modes, particularly in isotopologues mixtures. We measured VV and HV Raman spectra of H2O and D2O mixtures with femtosecond stimulated Raman scattering (FSRS) and compared the results with calculated spectra. Specifically, we observed the mode at around 1850 cm-1 and assigned it to H-O-D bend + rocking libration. Second, we found that the H-O-D bend overtone band and the OD stretch + rocking libration combination band contribute to the band located between 2850 and 3050 cm-1. Furthermore, we assigned the broad band located between 4000 and 4200 cm-1 to be composed of combinational modes of high-frequency OH stretching modes with predominantly twisting and rocking librations. These results should help in a proper interpretation of Raman spectra of aqueous systems as well as in the identification of vibrational relaxation pathways in isotopically diluted water.
ABSTRACT
HYPOTHESIS: The coil-to-globule transition is an essential phenomenon in protein and polymer solutions. Late stages of such transitions, >1 µs, have been thoroughly studied. Yet, the initial ones are a matter of speculations. Here, we present the first observation of a sub-nanosecond stage of the coil-to-globule transition of poly (vinyl methyl ether), PVME, in water. EXPERIMENTS: The detection of an early stage of the coil-to-globule transition has been possible thanks to a novel experimental approach - time-resolved elastic light scattering study, following an ultrafast temperature jump. We identified a molecular process active in the observed stage of the transition with use of broadband dielectric spectroscopy. FINDINGS: In the experiment's time window, from a few ps to around 600 ps, we observed an increase in the light scattering intensity 300-400 ps after the temperature jump that heated the sample above its lower critical solution temperature (LCST). The observed time coincides with the time of segmental relaxation of PVME, determined by broadband dielectric spectroscopy in the temperature range of the LCST of the PVME/water mixture. This coincidence strongly suggests that the observed herein stage of coil-to-globule transition is the rapid formation of local nuclei along the polymer chain. Those nuclei may grow and aggregate in later stages of the process, which are out of our experimental time window.
Subject(s)
Acrylic Resins , Water , Polymers , TemperatureABSTRACT
We have observed for the first time the surface-enhanced (SE) signal of water in an aqueous dispersion of silver nanoparticles in spontaneous (SERS) and femtosecond stimulated Raman (SE-FSRS) processes with different wavelengths of the Raman pump (515, 715, and 755 nm). By estimating the fraction of water molecules that interact with the metal surface, we have calculated enhancement factors (EF): 4.8 × 106 for SERS and (3.6-3.7) × 106 for SE-FSRS. Furthermore, we have tested the role of simultaneous plasmon resonance and Raman resonance conditions for the aν 1 + bν3 overtone mode of water (755 nm) in SE-FSRS signal amplification. When the wavelength of the Raman pump is within the plasmon resonance of the metal nanoparticles, the Raman resonance has a negligible effect on the EF. However, the Raman resonance with the aν 1 + bν3 mode strongly enhances the signal of the fundamental OH stretching mode of water.
ABSTRACT
We have proposed and constructed a setup for a novel method of ultrafast vibrational spectroscopy: femtosecond infrared pump-stimulated Raman probe spectroscopy. This is the first time-resolved spectroscopy providing simultaneously a sub-100 fs time resolution, a spectral resolution better than 10 cm-1 and a spectral window covering an extremely broad range of molecular vibrations (at least: 200-4000 cm-1) with a "single laser shot". The new method was applied to study vibrational relaxation pathways in the liquid HDO/D2O system. We determined the lifetimes of OH stretching vibrations to be in the range 310-500 fs depending on the isotopic dilution, which is in good agreement with the results from pump-probe femtosecond infrared spectroscopy. Moreover, we observed a strong coupling of OH stretch to OD stretch vibrations and possibly also to the librational modes of water.
ABSTRACT
Copolymers of 2-(2-methoxyethoxy)ethyl methacrylate (poly(MEO2MA)) are regarded as bioinert replacements of poly(N-isopropylacrylamide) in some biomedical applications. Networks of poly(MEO2MA) of various architecture form thermo-responsive hydrogels. Here, we present dielectric and mechanical spectroscopy studies on segmental motions and network relaxation processes in linear poly(MEO2MA) and its networks - bare network and the network grafted with short poly(MEO2MA) chains. We show that the α process assigned to the segmental motions of poly(MEO2MA) is independent on the polymer topology and the glass transition temperature, Tg, associated with this process equals 235-236 K for all investigated systems. The α' relaxation observed above Tg by dynamical mechanical analysis is assigned to the sub-Rouse process. It strongly depends on the polymer network architecture and slows down by four orders of magnitude upon network formation.
ABSTRACT
Water interacting with a polymer reveals a number of properties very different to bulk water. These interactions lead to the redistribution of hydrogen bonds in water. It results in modification of thermodynamic properties of water and the molecular dynamics of water. That kind of water is particularly well observable at temperatures below the freezing point of water, when the bulk water crystallizes. In this work, we determine the amount of water bound to the polymer and of the so-called pre-melting water in poly(vinyl methyl ether) hydrogels with the use of Raman spectroscopy, dielectric spectroscopy, and calorimetry. This analysis allows us to compare various physical properties of the bulk and the pre-melting water. We also postulate the molecular mechanism responsible for the pre-melting of part of water in poly(vinyl methyl ether) hydrogels. We suggest that above -60 °C, the first segmental motions of the polymer chain are activated, which trigger the process of the pre-melting.
ABSTRACT
We study the orientational dynamics of water molecules in solutions of formate salts using femtosecond mid-infrared spectroscopy. We observe that combining the formate ion with small cations like Na(+) or Li(+) leads to a cooperative effect on the water dynamics. This observation points at the formation of solvent-separated ion pairs.
ABSTRACT
Raman spectroscopy is a technique preferably used for studies of water structure because the proportions of intensities of main OH stretching modes are thought to reflect well a network of "intermonomer" hydrogen bonds as well as its disturbance by the presence of some solutes. The work presented herein demonstrates how the intensity ratio of two main components (around 3200 and 3400 cm (-1)) depends on the excitation wavelength in the visible range. Polarized Raman spectra indicate that the component at ca. 3200 cm (-1) is in resonance with light from the red range, which is in agreement with the presence of vibrational overtones in UV-vis absorption spectrum of water. These results are the first report on the occurrence of the Raman resonance effect in liquid water.
