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Speciale, 3. modul, 2003, id:193
Vejleder:Dorthe Posselt
Findes på RUb:Ja
English abstract
In this thesis results of studies carried out using differential scanning calorimetry and Raman spectroscopy are presented. Using differential scanning calorimetry the effects of KF, KCl, KBr, KI and KSCN on the phase behavior of aqueous solutions of the triblock copolymer Pluronics P85 and the homopolymer poly(propylene oxide) (PPO) are examined. The change in hydrogen bonding network in liquid water with increasing temperature, is examined by Raman spectroscopy. These results are used to examine the structure of hydrogen bonding network of water in aqueous solutions of P85, PPO and salts. These studies shows that KF, KCl and KBr lower the temperature at which the PPO solutions phase separate. In the solution of P85 these salts lower the temperature at which micelle formation starts, the temperature for the transition at which the micelles change from spherical to a rod-like geometry and the temperature at which the solution phase separates. KSCN has the opposite effect as this salt increase these transitions temperatures. KI has only a small effect, and the directions of the effect vary among the different transition temperatures. The order (KF, KCl, KBr, KI and KSCN) in which the salts affect the phase behaviour is identical with the order of the anions in the Hofmeister series. Temperature dependencies of both the OH-stretching band and a signal at low frequencies which is assigned to hydrogen-bond-stretching-vibration from water molecules interacting by hydrogen bonding with four other water molecules are studied in pure liquid water by Raman spectroscopy. The OH-stretching band in the isotropic spectrum is assigned to OH-oscillators from water molecules involved in different degrees of hydrogen bonding. This assignment give cause to dissolve the band into tree populations of OH-oscillators. From this assignment it was found that the amount of fully hydrogen-bonded water molecules decrease with increasing temperature while the amount of partly hydrogen-bonded water molecules increases with increasing temperature. By combining these results with a simple thermodynamic model, the strength of a hydrogen bond in water was calculated. This value of 12 kJ/mol is close to similar values found by other techniques. Studies of the signal at low frequencies show that the intensity decrease with increasing temperature, which is also a consequence of the breaking down of the hydrogen-bonding network. An observed downshift in frequency with increasing temperature is suggested to be due to the thermal expansion of the liquid. A comparison of the results found by differential scanning calorimetry and Raman spectroscopy indicates that the hydration of P85 and PPO only has a minor effect on the water structure. However, Raman spectroscopic measurements show that salts have a great influence on the water structure. KI, KBr, KCl and KSCN is found to have a structure breaking effect on the water, which decrease in strength in the order mentioned, while KF is shown only to have a very little effect on the water structure. These results show that while the order in which the salts affect the phase behaviour of P85 and PPO follows the Hofmeister series, the order is different when it comes to salts effect on the water structure. It is therefore concluded that a more complicated mechanism is needed to explain the effect of salts on the phase behaviour of the polymer solutions.

Database opslag den: 22-09-2017