GATE 2024 BT Q.11: solution and answer

Official answer: DSource: Official answer key, GATE 2024 BT (IISc Bengaluru)

1. Type of retention. Adsorption chromatography on silica gel (or alumina) works by physical adsorption (physisorption). A solute molecule sticks to the polar surface for a short time, comes off again, and re-adsorbs further down the column. The solute can only move and separate if this binding is weak and reversible, so that a change of mobile phase can displace it.

2. What the silica surface offers. The silica surface carries silanol groups ($\text{Si-OH}$). A solute with no net charge has no charge for an ion–ion attraction to act on. It is held only by weak, short-range physical forces: dispersion (London) forces and dipole–dipole or dipole–induced-dipole attractions, which together make up the van der Waals forces. Polar solutes are also helped by hydrogen bonds to the silanols. All of these are weak (a few ${\text{kJ mol}}^{-1}$ up to roughly $40{\text{kJ mol}}^{-1}$) and reversible, as physisorption has to be.

3. Rule out (A), covalent bonds. A covalent bond between solute and surface is chemisorption. Breaking it takes a chemical reaction, not just a change of eluent. That is the principle of the specialised technique called covalent chromatography, not of ordinary adsorption of uncharged solutes on silica.

4. Rule out (B) and (C), electrostatic interactions and ionic bonds. In chromatography these options mean attraction between net charges on the solute and on the stationary phase. That is the retention mechanism of ion-exchange chromatography. The question says the solutes are uncharged, so there is no net charge to attract.

5. Conclusion. Uncharged solutes are adsorbed on silica by weak, reversible van der Waals forces. Answer (D).