| 1. |
- Elamin, Khalid, 1977, et al.
(författare)
-
Long-range diffusion in xylitol-water mixtures
- 2013
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 117:24, s. 7363-7369
-
Tidskriftsartikel (refereegranskat)abstract
- Dynamic light scattering (DLS) and small-angle neutron scattering (SANS) were employed to study mixtures of xylitol and water. The results were also related to a previous dielectric relaxation study on the same system. In the temperature range of the DLS measurements the viscosity related structural (α) relaxation is too fast to be observed on the experimental time scale, but a considerably slower exponential and hydrodynamic relaxation process is clearly observable in the polarized light scattering data. A similar ultraslow process has been observed in many other types of binary liquids and commonly assigned to long-range concentration or density fluctuations. In some studies this interpretation has been supported by observations of substantial structural inhomogeneities in static light scattering or SANS experiments. However, in this study we observe such an ultraslow process without any indication of structural inhomogeneities on length-scales above 2 nm. Hence, we suggest that our observed ultraslow process is due to long-range diffusion of single xylitol molecules or small clusters of a few xylitol molecules (and perhaps some associated water molecules) which are randomly dispersed and sufficiently small to not be structurally detected in our SANS study. In the q-range of the DLS measurements this ultraslow relaxation process is around room temperature several orders of magnitude slower than the structural α-relaxation. However, if its 1/q2-dependent relaxation time is extrapolated to q-values where relaxation times from dielectric spectroscopy and quasielastic neutron scattering are compatible (about 10 nm-1), a relaxation time similar to that of the dielectric α-relaxation is obtained. Thus, the large difference in time scale between the two relaxation processes in the q-range of a DLS study is due to the fact that the α-relaxation is cooperative in nature, rather than caused by long-range single particle diffusion, and thus q-independent at low q-values.
|
|
| 2. |
- Elamin, Khalid, 1977, et al.
(författare)
-
Different behavior of water in confined solutions of high and low solute concentrations
- 2013
-
Ingår i: Physical Chemistry Chemical Physics. - : Royal Society of Chemistry (RSC). - 1463-9084 .- 1463-9076. ; 15:42, s. 18437-18444
-
Tidskriftsartikel (refereegranskat)abstract
- Water-glycerol solutions confined in 21 angstrom pores of the silica matrix MCM-41 C10 have been studied using differential scanning calorimetry (DSC) and broadband dielectric spectroscopy (BDS). The results suggest a micro-phase separation caused by the confinement. Likely the water molecules coordinate to the hydroxyl surface groups of the pores, leaving most of the glycerol molecules in the centre of the pores. This makes the dynamics of glycerol almost concentration independent up to water concentrations of about 85 wt%. However, at higher water concentrations no substantial clustering of glycerol molecules should occur and the glass transition related dynamics exhibit an anomalous behaviour. Instead of a common plasticization effect of water, as for the corresponding bulk solutions (when no ice is formed), it is evident that water acts as an anti-plasticizer in the confinement at high water concentrations. We propose that the increased water concentration slows down the glass transition related dynamics in the deeply supercooled regime due to that a rigid hydrogen bonded network structure of water molecules is formed at low temperatures and low glycerol concentrations. This is in contrast to the situation in a homogenously mixed bulk solution of a high solute concentration where the water molecules will be less hydrogen bonded, and therefore are typically more mobile than the surrounding solute molecules. An almost complete hydrogen bonded network of water molecules may, even in confinements, be sufficiently rigid to slow down the relaxation of embedded solute molecules. It can also be expressed the other way around, i.e. small amounts of glycerol act as a plasticizer for water, due to its breaking up of the nearly tetrahedral network structure. From the here observed concentration dependent behaviour of the deeply supercooled bulk and confined solutions it seems, furthermore, evident that the T-g value of bulk water cannot be estimated from extrapolations of aqueous solutions.
|
|
| 3. |
- Swenson, Jan, 1966, et al.
(författare)
-
Why is there no clear glass transition of confined water?
- 2013
-
Ingår i: Chemical Physics. - : Elsevier BV. - 0301-0104. ; 424, s. 20-25
-
Tidskriftsartikel (refereegranskat)abstract
- To overcome the problem of crystallization of supercooled bulk water and water rich solutions we have studied water-glycerol mixtures confined in 21 angstrom pores of the silica matrix MCM-41 C10. The results from the differential scanning calorimetry (DSC) measurements shows an almost concentration independent glass transition temperature, T-g, at about 176 K for water concentrations up to 80 wt%, suggesting that the confined water has no influence on T-g in this concentration range. Rather, the findings indicate that the water molecules in the solutions have a stronger preference to coordinate to the hydroxyl surface groups than the glycerol molecules, which results in a micro-phase separation of the two liquids. The water phase does not give any sign of a T-g and therefore the observed T-g should be associated with the glass transition of the glycerol phase. Finally, we discuss why the confined water does not exhibit any clear calorimetric T-g. (C) 2012 Elsevier B. V. All rights reserved.
|
|
