On the nanoscale, particles flow in unexpected ways

October 24, 2011

Researchers studying how fluids travel through nanoscale channels were surprised to discover that the fluids don't flow equally well in all directions. Contrary to the behavior in the macroscale world, the researchers discovered that methyl alcohol, when it was placed in a network of nanoscale channels in a mineral known as a zeolite, diffused 1,000 times faster in one direction than another. This is the first known evidence of such highly unequal diffusion of molecules in a nanoporous material. This highly lopsided flow occurred despite the fact that the diameters of the respective channels are quite similar. In the mineral, two types of nanoscale channels are present: 8-ring and 10-ring channels. The numbers refers to the relative size of the pores in the material, though they are extremely close in size with only subtle differences in geometry. The methyl alcohol molecules were stored initially inside an optical cell.

At the beginning of the experiment, the pressure in the surrounding atmosphere is increased instantaneously and kept constant for the rest of the experiment. The methanol molecules then enter the zeolite voluntarily since they naturally prefer to be in the zeolite than in the gas phase. Once inside the mineral, the researchers measured the particle concentration at various points along the pores. From these profiles, they were able to calculate the particle flux (number of particles that cross a certain area in a certain time) and observed the highly biased flow.

Earlier research reported that the diffusivity of a guest molecule inside a pore network is extremely sensitive to the ratio of the pore window and molecule diameter, particularly if both quantities are close to each other, as was the case with the zeolite channels and the methyl alcohol atoms. The researchers in this study speculate that since the 8-ring window is slightly smaller than the 10-ring window, a smaller diffusivity (and therefore a smaller flux) might be expected. Another reason might be the different pore geometry (straight in the case of the 10-ring channels versus windows and cavities in the 8-ring channels).

Presented in the AIP's Journal of Chemical Physics, this apparently counterintuitive discovery has far-reaching implications for the understanding, development, and exploitation of novel microscopic materials, including nanotubes and "intelligent" cell membranes for purposeful drug delivery, the functionality of which is based on an extreme direction dependence of molecular mobilities.
Article: "Micro-imaging of transient guest profiles in nano-channels" is accepted for publication in the Journal of Chemical Physics.

Authors: F. Hibbe (1), V.R.R. Marthala (2), C. Chmelik (1), J. Weitkamp (2), and J. Kärger (1).

(1) Faculty of Physics and Earth Sciences, University of Leipzig, Germany

(2) Institute of Chemical Technology, University of Stuttgart, Germany

American Institute of Physics

Related Molecules Articles from Brightsurf:

Finally, a way to see molecules 'wobble'
Researchers at the University of Rochester and the Fresnel Institute in France have found a way to visualize those molecules in even greater detail, showing their position and orientation in 3D, and even how they wobble and oscillate.

Water molecules are gold for nanocatalysis
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals.

Water molecules dance in three
An international team of scientists has been able to shed new light on the properties of water at the molecular level.

How molecules self-assemble into superstructures
Most technical functional units are built bit by bit according to a well-designed construction plan.

Breaking down stubborn molecules
Seawater is more than just saltwater. The ocean is a veritable soup of chemicals.

Shaping the rings of molecules
Canadian chemists discover a natural process to control the shape of 'macrocycles,' molecules of large rings of atoms, for use in pharmaceuticals and electronics.

The mysterious movement of water molecules
Water is all around us and essential for life. Nevertheless, research into its behaviour at the atomic level -- above all how it interacts with surfaces -- is thin on the ground.

Spectroscopy: A fine sense for molecules
Scientists at the Laboratory for Attosecond Physics have developed a unique laser technology for the analysis of the molecular composition of biological samples.

Looking at the good vibes of molecules
Label-free dynamic detection of biomolecules is a major challenge in live-cell microscopy.

Colliding molecules and antiparticles
A study by Marcos Barp and Felipe Arretche from Brazil published in EPJ D shows a model of the interaction between positrons and simple molecules that is in good agreement with experimental results.

Read More: Molecules News and Molecules Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.