Dynamics of supercooled liquids in the vicinity of soft and hard interfaces

Using solvation techniques we explore the dynamics of nanoconfined and interfacial supercooled liquids near their glass transition temperatures. Confinement is accomplished by the use of porous glasses with pore diameters between 4 and 7.5 nm, and by microemulsions with droplet sizes between 2.6 and 5 nm. Via the attachment of the probe molecules to the inner surface of porous glasses filled with 3-methylpentane, the interfacial layer is measured selectively for different surface curvatures and shows an increase of the relaxation time by more than three orders of magnitude over that of the bulk liquid. This frustration is most pronounced at the pore boundary and decreases gradually across the first few nanometers distance away from the interface. Nanoconfinement realized by glass-forming microemulsions with the confining material being more fluid than the intramicellar droplets reverse the situation from frustrated dynamics in hard confinement to accelerated relaxation behavior in the case of soft boundaries. Without changing the size of the geometrical restriction of propylene glycol, hard versus soft boundary conditions changes the glass transition shift Δ Tg from +6 K to -7 K. The findings can be rationalized on the basis of certain interfacial dynamics which differ from bulk behavior, with the penetration of this effect into the liquid being governed by the length scale of cooperativity. This picture explains both the more disperse relaxation times in confinement and the dependence of the average relaxation time on pore size.