An NMR investigation into the range of the surface effect on the rotation of water molecules

Abstract It is well known that the properties of water in the vicinity of a surface or interface are different from those of pure bulk water. An important question is the thickness of the water layer with properties different from bulk water, i.e., the range of the effect. Different physical properties might have different ranges. The NMR relaxation time T 1 reflects the molecular motions in the megahertz frequency range. The deuteron T 1 of D 2 O depends only on the molecular rotational properties, whereas the proton T 1 of H 2 O depends on both the translational and rotational properties. In order to determine the range of the surface effect on the rotational properties of water, the room temperature deuteron T 1 was measured as a function of water layer thickness for a carefully prepared series of hectorite-D 2 O samples. This clay was chosen because it has very low concentration of paramagnetic impurities which can complicate the experiment. The results are in complete agreement with the simple picture that at room temperature the hectorite surface influences and slows down the rotation of only the first layer or two of water molecules.