Spehr, Tinka Luise
Water Dynamics In Soft Confinement - Neutron Scattering Investigations On Reverse Micelles.
Technische Universität, Darmstadt
[Ph.D. Thesis], (2010)
Dissertation Dipl.-Phys. Tinka Spehr -
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|Item Type:||Ph.D. Thesis|
|Title:||Water Dynamics In Soft Confinement - Neutron Scattering Investigations On Reverse Micelles|
In the present experimental study we investigate the dynamical behaviour of water in soft conﬁnement. As a model system we choose the AOT (sodium bis[ethylhexyl] sulfosuccinate) based water-in-oil droplet microemulsion which we precharacterize in detail. Using small angle neutron scattering we determine the microemulsion structure as a function of composition. Spherical water droplets (reverse micelles) coated by a monomolecular layer of the surfactant are dispersed in the continuous oil matrix. By variation of the molar ratio of water to surfactant one may control the droplet size between a few Angström and several nanometers. We extend the phase diagram to temperatures far below the freezing point of bulk water to determine the range of structural stability of the droplets as a function of water loading. The supercooling of the conﬁned water is investigated by means of elastic ﬁxed window scans on neutron backscattering. The freezing point of the conﬁned water as well as the lower stability temperature decrease with decreasing droplet size. Moreover we use neutron- spin-echo spectroscopy to measure droplet diﬀusion and form ﬂuctuations, whereof we deduce the temperature dependency of the bending modulus of the surfactant shell. The inﬂuence of conﬁnement on the water mobility is then studied using two systems with diﬀerent droplet sizes. We access the water dynamics over three orders of magnitude from pico- to nanoseconds by the combination of neutron backscattering and time-of-ﬂight spectroscopy. We analyze the data taking into account rotation and jump diﬀusion. We ﬁnd the water diﬀusion inside the droplets on average to be considerably slowed down with respect to bulk water. The translational mobility further decreases with decreasing droplet size. We extract values for rotational and translational diﬀusion coeﬃcients. Dependent on water loading, two dynamically separated water fractions inside the droplets are resolved, one probably corresponding to surfactant bound water and the other one corresponding to less hindered water in the middle of the droplet core. We determine the absolute number of tightly bound water molecules per AOT molecule.
|Place of Publication:||Darmstadt|
|Classification DDC:||500 Naturwissenschaften und Mathematik > 530 Physik|
|Divisions:||05 Department of Physics > Institute for condensed matter physics|
|Date Deposited:||15 Feb 2010 09:31|
|Last Modified:||07 Dec 2012 11:56|
|Referees:||Stühn, Prof. Dr. Bernd and Vogel, Prof. Dr. Michael|
|Refereed:||25 January 2010|