Aqueous Dielectric Effects

by Klaus Friedrich Rinne

Institution: Freie Universität Berlin
Department: FB Physik
Degree: PhD
Year: 2015
Record ID: 1115039
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000099215


The electrostatic interaction is one of the fundamental forces and is extremely important for the dynamics of highly polar water molecules and the network of hydrogen bonds. Experimental dielectric relaxation spectroscopy measurements established that the addition of substances such as salts to water changes the strength of the dielectric absorption and the position of the absorption maximum in the frequency domain. But an unambiguous assignment of spectral resonances to interactions between various components of the solution without the support from atomistic simulations is impossible. The dielectric spectrum of aqueous sodium-halide solutions is calculated from molecular dynamics simulations in this thesis and is in good agreement with the experimental measurements. By splitting the dielectric signal into cross-correlations between different contributions of water and ions, we show that the dominant contribution is due to the auto-correlation of the water polarization. The cross-term between water and ion polarization weakens the absorption. By a decomposition of the water signal into terms due to different solvation shells, we show that water in the first shell loses about half of its dielectric response and the signal is blueshifted. These effects in the first shell are the main cause for the observed reduction of the static dielectric constant and the faster dynamics seen in the overall spectrum. By a further division of the water contribution in self relaxation and collective relaxations, we show that only the dominant collective contributions are accelerated. In agreement with two-dimensional infrared vibrational echo spectroscopy experiments the self relaxation of single water molecules is slowed down by the presence of ions. Via projection into folded and unfolded states we calculate the dielectric spectra for native and non-native structures from equilibrium simulations of an eight monomer alanine chain solvated in water for native and non-native states. In addition to the pure water resonance at about 20 GHz, a peak occurs in the range of several hundred megahertz. It is mainly caused by the high polarization of the alanine peptide and is significantly stronger when the peptide is folded. However, we can also find a slow process in the collective interactions between different water molecules in the first hydration shell in this low frequency range via a decomposition. This collective water relaxation in the hydration shell is about two orders of magnitude slower than in pure bulk water. Interestingly, the self-rotation of water dipoles is only slightly retarded even in the first hydration shell. As a technological application of dielectric effects, we present a novel mechanism for pumping water on the nano scale. Our simulations show that water inside a carbon nanotube can be pumped using two electrodes which are positioned adjacent to the nanochannel. Prerequisite is that the electrode charges oscillate phase shifted to each other in the GHz-range. We derive an analytic polarization-dragging …