host matrices for confinement. If the size d of the confinement is finite the cooperatively rearranging regions cannot grow beyond any bound, becoming saturated at ξ = d < ∞ . This should lead to a confinement induced acceleration of the dynamics resulting in a downshift of the glass transition, and even impeding it [16] as ξ > d . 

Since the pioneer work of Jackson and McKenna [17] in 1991, uncovering a reduction of the glass transition temperature Δ T$_{g}$ ∝ 1 /d in confinement of size d , a variety of confinement geometries and experimental methods have been used. Both weak and strong glass forming liquids, showing strong and weak interaction with the 2D or 3D confinement media were investigated. Single, double, even multiple transitions have been observed. Extensive topical overviews are found in Refs. 18, 19 and 20. 

The abundance and diversity of experimental findings shows that an accurate discussion of side effects in discussing results of glass forming liquids in confinement is essential. Negative pressure due to mismatching thermal expansion coefficients of liquid and confining matrix is such a side effect. It was discussed by various authors [21, 22] and sometimes even made responsible for the whole downshift of T$_{g}$ in confinement [21]. Being true, it would disprove the idea of a growing length scale of cooperativity. In a former paper [23] the authors have determined negative pressure effects for salol in natural un-coated pores of size 7.5 to 2.6 nm from high resolution thermal expansion measurements. An upper bound for the contribution of negative pressure to the total downshift of T$_{g}$ of ≈ 30% was found. 

A second, and much larger effect on the glass transition of liquids in confinement arises from the interaction of the molecules with the huge inner surface of confining host matrices which can take values up to 600 m 2 /g (see Tab. I). Confined liquids tend to form H-bonds with the hydrophilic pore surface, which leads to an immobile surface layer of molecules and a retarded relaxation behavior at the glass transition. 

In recent dynamic elastic measurements [23, 24] of salol filled into matrices of Vycor and Gelsil with natural untreated pores we studied this competition between surface induced slowing down and confinement induced acceleration of the dynamics. Here we present new results of salol confined in mesoporous Vycor and Gelsil with silanated pore surfaces. The 

