Project: Nanometer Confinement |
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The effects of spatial restrictions and interfaces on the properties of glass-forming liquids and on the glass transition are of considerable technological and fundamental interest. The dynamics of viscous liquids have been observed to change if subject to confinement on length scales of several nanometers. We use solvation dynamics experiments in order to probe the dynamics of a confined liquid as a function of pore size, temperature, and surface chemistry [59, 65, 86, 94]. We selectively tag the probe molecules to the silica surface in order to discriminate between interfacial and finite-size effects. Recently, we have found that a simple non-polar liquid, 3-methylpentane, experiences an increase in its viscosity by over three orders of magnitude in the immediate vicinity of a silica interface, equivalent to a surface induced glass transition [108, 126]. Consistent with length scales of cooperativity, these effects disappear for distances from the surface exceeding a few nanometers. |
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Relaxation time in confined 3-methylpentane for various distances x
from the silica
surface. The confinement is realized by a porous silica glass with 7.5 nm
(x = 0.9 nm) and 4 nm (x = 0.5 nm) average pore diameter.
The results are obtained from triplet state solvation dynamics measurements.
Note that the time scale of molecular motion changes by over 3 orders of
magnitude within several nanometers. [126, 187] |
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More recently, we have realized the case of soft confinement, again with characteristic length scales of only few nanometers, using water-in-oil type glass-forming microemulsions [116, 126]. Propylene glycol (PG, Tg = 170 K) is mixed with the nonpolar liquid decalin (DHN, Tg = 135 K) using AOT as surfactant. The dynamics of the intramicellar liquid PG is determined by solvation methods with quinoxaline as molecular probe. In contrast to the hard confinement of porous glasses, this soft confinement leads to accelerated dynamics relative to the bulk situation [187]. |
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Temperature dependent Stokes shift correlation times τ obtained from QX in PG for
various confinement situations: bulk liquid (solid circles), in 4.0 nm diameter porous
Vycor glass (triangles pointing up), and in 4.6 and 2.6 nm diameter droplets of PG/AOT/DHN
microemulsions (triangles pointing down). The solid line indicates the dielectric relaxation
trace for bulk PG, the dashed curve is for the dielectric result for bulk
decalin (DHN). The dotted horizontal line represents the τg = 100 s
criterion for identifying Tg. [116, 126] |
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Possible discrimination of finite size (top image) versus interfacial effects (lower
two images) of confinement induced changes in dynamics. Darker particles represent
those which relax more different than the bulk liquid case. Interfacial effects
generate a modification of relaxation times that depends strongly on the
distance to the pore surface. True finite size effects would be more uniform
within the pore volume. The lower two images indicate the difference between the
'gradient' and the 'two Tg' scenario's, where blue and red
respectively indicate slower and faster dynamics relative to the bulk. The arrow
indicates possible exchange between core and shell molecules. [187] |
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Reference numbers refer to the list of publications |
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Experimental techniques:
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Selected projects:
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