Sistema ETD

banca dati delle tesi e dissertazioni accademiche elettroniche


Tesi etd-07022011-103037

Tipo di tesi
Tesi di dottorato di ricerca
Indirizzo email
Dynamics of supercooled aqueous systems at low temperature and high pressure.
Settore scientifico disciplinare
Corso di studi
tutor Dott. Capaccioli, Simone
Parole chiave
  • α- and ν- relaxation processes
  • saccharides
  • pressure
  • glycol oligomers
  • glass transition
  • fragility
  • crossover
  • aqueous mixtures
  • activation energy
Data inizio appello
Riassunto analitico
Aqueous mixtures are excellent systems to deepen the understanding of the relaxation dynamics of supercooled water in the temperature region usually unaccessible without crystallization. The first part of this thesis provides an investigation, by means of dielectric spectroscopy, of the molecular dynamics of water mixed with propylene glycol oligomers (n-PG, n = 1, 3, 7) at ambient and extremely high pressure (up to 1.8 GPa). By changing in a systematic way the relative concentration of water (starting from small amount of water, 2.0 wt.%) added to polypropylene glycol, PPG400, three relaxation processes show up: a) the slowest process, whose dynamic properties indicate that it is the structural α-relaxation of the aqueous mixture leading to glass transition at T equal to the calorimetric Tg; b) a faster process (ν-relaxation process), which appears close to β- (secondary) process of PPG400 in dielectric spectra even for the lowest water concentration mixture (similarly as in the case of many other water mixtures) and has the characteristics of an intermolecular Johari-Goldstein secondary relaxation; c) the fastest process (γ-relaxation process), originating from hydroxyl group rotation, which is prominent for anhydrous PPG400, but this process is weak and becomes hidden by the much stronger ν-relaxation for high water concentration mixture.
In addition, a change of the dynamics of the water related (ν-) process occurs in all isobaric measurements: the temperature dependence of ν- relaxation time, τn, that has an Arrhenius behaviour in the glassy state, becomes stronger on increasing temperature in the liquid state. Such a crossover is in a way significantly different from similar phenomenon observed in literature for confined water. Moreover, our studies under pressure revealed that the secondary relaxation process for aqueous mixtures of PPG400 with low (4 wt.%) and high (26 wt.%) amount of water behaves in a different way. Our results show that the secondary relaxation in the mixtures with low content of water is mainly influenced by the dynamics of the solute molecules and has the characteristics of the Johari-Goldstein β-relaxation occurring in van der Waals and polymeric glassformers. In the case of high water content, where each molecule of water is mainly surrounded by other water molecules, the ν- (secondary) relaxation has very specific characteristics, quite different from those of conventional glassformers in its ability to rotate and translate after breaking two hydrogen bonds.
Finally, mixing water with n-PG oligomers (increasing the number of repeating units, n) with similar weight concentration of water allowed us to study how the dynamics of water mixtures at ambient and elevated pressure is influenced by the number of OH-groups and by the connectivity of the solute. Similar studies have been carried out for aqueous mixtures with ethylene glycol oligomers.
Among the numbers of new experimental findings coming from this work, three results can be mentioned, of particular interest for the current debate in literature on dynamics of aqueous systems. The first is the increasing of timescale separation between α- and ν- process in dielectric spectra at given (the same) α- relaxation time, τα, on increasing temperature and pressure. This behaviour is usual for the timescale separation between α- and β- processes of hydrogen bonded systems (like sorbitol) and not shown in the case of van der Waals liquids where the α-β timescale separation is the same on increasing temperature and pressure but keeping constant τα. This effect is related to the fact that the H-bond network in such systems is weakened by elevating temperature and pressure and the structure of the system is changing with the thermodynamic conditions.
The second result of particular interest is that, when it has been possible to clearly reveal the ν-relaxation from the glassy state up to well above Tg, a strong deviation from the glassy state trend has been found for both the temperature and pressure dependence, with an apparent kink in the relaxation map always occurring near the glass transition of the aqueous mixtures. The only explanation for such result could be that such crossover in temperature or pressure dependence of ν-process is related to glass transition. This occurrence helps to rule out some of the hypotheses on dynamic crossover recently published in literature.
The third important result is that the dielectric strength of the water-specific relaxation also exhibits a crossover from a weaker to a stronger dependence with increasing T, at the temperature where the slow process attains a very long timescale (> 1 ks) and becomes structurally arrested.

All these facts support the idea that water-specific ν-relaxation can be identified as the Johari-Goldstein β-relaxation of water. This interpretation of ν-relaxation is in full agreement with the temperature, pressure, concentration dependence of its dynamic features and dielectric strength.
Taking into account the experience gained from the work on aqueous mixtures of glycol oligomers in the first part, the second part of the thesis is devoted to the study of aqueous mixtures with saccharides or carbohydrates (mono-, di- and polysaccharides). The main idea was to investigate, by means of dielectric spectroscopy, the dynamics and glass transition phenomena of water-sugar mixtures under pressure for possible application to cryoprotection, food preservation, biological functioning of hydrated biomolecules. In order to widen the possible applications of this study, sugars of strong interest in bio-science were chosen, such as fructose, glucose, sucrose, trehalose, dioxyribose, glycogen. For water-fructose solutions a systematic study was done in a wide pressure range by varying concentration of water. A preliminary study of the dynamics of proteins solved in sugar-water mixture was also begun.

Summarizing, our results show that the relaxation time of the ν-process, τn, represents qualitatively very similar features for all studied aqueous systems, almost universal, irrespective of the chemical and structural differences, especially for high water concentration mixtures. The ν-relaxation has an Arrhenius T-dependence at low temperature with an activation enthalpy of about 50 kJ/mol universally found in the glassy state for aqueous mixtures at ambient pressure. Such activation enthalpy is comparable with that enough for breaking two hydrogen bonds whereupon the water molecules can rotate and translate. This is because the secondary relaxation originating from water in mixtures with high water content is effectively not so different from that in bulk or confined water. Our study confirms that the aqueous mixtures can be a useful "tool" for understanding the water dynamics in the temperature and pressure regions usually unaccessible without crystallization.