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- Elamin, Khalid, 1977, et al.
(author)
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Brownian motion of single glycerol molecules in an aqueous solution as studied by dynamic light scattering
- 2015
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In: Physical Review E. - 2470-0045 .- 2470-0053. ; 91:3
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Journal article (peer-reviewed)abstract
- Aqueous solutions of glycerol are investigated by dynamic light scattering (DLS) over the whole concentration range (10-98 wt.% water) and in the temperature range 283-303 K. The measurements reveal one slow relaxation process in the geometry of polarized light scattering. This process is present in the whole concentration range, although it is very weak at the highest and lowest water concentrations and is considerably slower than the structural alpha relaxation, which is too fast to be observed on the experimental time scale in the measured temperature range. The relaxation time of the observed process exhibits a 1/q(2) dependence, proving that it is due to long-range translational diffusion. The Stokes-Einstein relation is used to estimate the hydrodynamic radius of the diffusing particles and from these calculations it is evident that the observed relaxation process is due to the Brownian motion of single or a few glycerol molecules. The fact that it is possible to study the self-diffusion of such small molecules may stimulate a broadening of the research field used to be covered by the DLS technique.
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| 2. |
- Elamin, Khalid, 1977, et al.
(author)
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Dynamics of aqueous binary glass-formers confined in MCM-41
- 2015
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In: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 17:19, s. 12978-12987
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Journal article (peer-reviewed)abstract
- Dielectric permittivity measurements were performed on water solutions of propylene glycol (PG) and propylene glycol monomethyl ether (PGME) confined in 21 angstrom pores of the silica matrix MCM-41 C10 in wide frequency (10(-2)-10(6) Hz) and temperature (130-250 K) ranges. The aim was to elucidate how the formation of large hydrogen bonded structural entities, found in bulk solutions of PGME, was affected by the confined geometry, and to make comparisons with the dynamic behavior of the PG-water system. For all solutions the measurements revealed four almost concentration independent relaxation processes. The intensity of the fastest process is low compared to the other relaxation processes and might be caused by both hydroxyl groups of the pore surfaces and by local motions of water and solute molecules. The second fastest process contains contributions from both the main water relaxation as well as the intrinsic beta-relaxation of the solute molecules. The third fastest process is the viscosity related alpha-relaxation. Its concentration independency is very different compared to the findings for the corresponding bulk systems, particularly for the PGME-water system. The experimental data suggests that the surface interactions induce a micro-phase separation of the two liquids, resulting in a full molecular layer of water molecules coordinating to the hydrophilic hydroxyl groups on the surfaces of the silica pores. This, in turn, increases the geometrical confinement effect for the remaining solution even more and prevents the building up of the same type of larger structural entities in the PGME-water system as in the corresponding bulk solutions. The slowest process is mainly hidden in the high conductivity contribution at low frequencies, but its temperature dependence can be extracted for the PGME-water system. However, its origin is not fully clear, as will be discussed.
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