An industrial chemist needs to extract an organic dye from an aqueous solution
CHEMISTRY · Multi-part · AP · Chemistry · Properties of Substances and Mixtures · Properties of Substances and Mixtures
Problem
An industrial chemist needs to extract an organic dye from an aqueous solution. The dye molecule is amphiphilic: it contains a long alkyl chain (C₁₂) terminated by a quaternary ammonium head group capable of strong ion–dipole interactions. Despite the presence of the charged head group, preliminary tests show that the dye partitions more strongly into chloroform than into water.
Partition coefficient Kchloroform/water=15 at 25 °C.
(a) Describe how the amphiphilic structure of the dye creates competing intermolecular interactions with each solvent.
(b) Explain why the hydrophobic effect drives preferential solubility of the dye in chloroform, despite its charged head group.
(c) Predict how addition of 1.0 mol·L⁻¹ NaCl to the aqueous phase would affect the partition coefficient and justify your answer.
(d) Propose one structural modification to the dye that would increase its affinity for water without dramatically reducing its amphiphilic character, and explain the molecular reasoning behind your proposal.