Why does the solvent move upward in thin layer chromatography

The paper has capillaries in its fibers.

Here are two web sites. The first describes thin layer chromatography; the second describes paper chromatography. The medium for TLC is SiO2; the medium for paper chromatography is, of course, paper.

http://orgchem.colorado.edu/hndbksupport/TLC/TLC.html

http://en.wikipedia.org/wiki/Paper_chromatography

In thin layer chromatography (TLC), the solvent moves upward due to capillary action, which is the result of cohesive and adhesive forces.

Cohesive forces are the attractive forces between the particles of the same substance. In this case, the molecules of the solvent are attracted to each other, creating a cohesive force that helps hold the solvent molecules together.

Adhesive forces, on the other hand, are the attractive forces between particles of different substances. In TLC, the adsorbent material (usually a thin layer of silica or alumina gel) on which the sample is spotted attracts the solvent molecules through adhesive forces.

When a TLC plate is prepared by spotting a sample at the bottom, the adhesive forces between the adsorbent and the sample molecules cause them to stay in place. At the same time, the cohesive forces within the solvent itself encourage it to maintain its integrity.

As the solvent moves up the TLC plate, it is drawn up by capillary action. Capillary action is the phenomenon where a liquid, in this case, the solvent, is drawn into a narrow space or tube (like the pores of the adsorbent material) against the force of gravity. This occurs due to the balance between the cohesive forces among solvent molecules and the adhesive forces between the solvent and the adsorbent.

The solvent molecules at the leading edge of the liquid mixture are attracted to the adsorbent material through adhesive forces. As these molecules adhere to the surface, they create a meniscus, or a curved surface, which allows the solvent to rise in a capillary-like manner. This process continues until the solvent reaches the top of the chromatography plate.

Overall, the upward movement of the solvent in thin layer chromatography is driven by capillary action, which is a result of the cohesive and adhesive forces between the solvent molecules and the adsorbent material.