Question

Why do polar compounds move slower up the silica TLC plates than nonpolar compounds? Why does increasing the polarity of the mobile phase increase the rate at which all compounds move up the plate?

Answer 1

Due to electronegativity difference between Si and O, Si-O bond is polar.

so SiO2 is polar. Nonpolar compounds have no net dipole in molecule.

Due to dipole dipole interaction between polar compound and SiO2,  polar compounds move slower up the silica TLC plates than nonpolar compounds.

when increasing the polarity of the mobile phase, then competition between polar compound and SiO2 or mobile phase.

Due to higher interaction between mobile phase and polar compound, all compounds move up the plate.

That means we can not differentiate the compounds under study.