Paul Scherrer Institut, 5232 Villigen PSI, Switzerland.
J Phys Condens Matter. 2010 Jun 16;22(23):235102. doi: 10.1088/0953-8984/22/23/235102. Epub 2010 May 21.
Ultrathin (<12 nm) films of tetrakis(trimethyl)siloxysilane (TTMSS) have been confined by atomically flat mica membranes in the presence and absence of applied normal forces. When applying normal forces, discrete film thickness transitions occur, each involving the expulsion of TTMSS molecules. Using optical interferometry we have measured the step size associated with a film thickness transition (7.5 Å for compressed, 8.4 Å for equilibrated films) to be smaller than the molecular diameter of 9.0 Å. Layering transitions with a discrete step size are commonly regarded as evidence for strong layering of the liquid's molecules in planes parallel to the confining surfaces and it is assumed that the layer spacing equals the measured periodicity of the oscillatory force profile. Using x-ray reflectivity (XRR), which directly yields the liquid's density profile along the confinement direction, we show that the layer spacing (10-11 Å) proves to be on average significantly larger than both the step size of a layering transition and the molecular diameter. We observe at least one boundary layer of different electron density and periodicity than the layers away from the surfaces.
在原子级平坦云母膜的限制下，研究了四（三甲硅基）硅氧烷（TTMSS）的超薄膜（<12nm），存在和不存在外加法向力的情况。当施加法向力时，会发生离散的膜厚转变，每次转变都涉及 TTMSS 分子的排出。使用光学干涉测量法，我们已经测量了与膜厚转变相关的台阶尺寸（压缩时为 7.5Å，平衡时为 8.4Å），其小于 9.0Å 的分子直径。具有离散台阶尺寸的分层转变通常被认为是液体分子在平行于约束表面的平面上强烈分层的证据，并且假设层间距等于测量的振荡力分布的周期性。使用 X 射线反射率（XRR），它直接沿约束方向给出液体的密度分布，我们表明层间距（10-11Å）平均明显大于分层转变的台阶尺寸和分子直径。我们观察到至少一个边界层的电子密度和周期性与远离表面的层不同。
