Final answer:
Ion pairing is likely the reason not all ions transfer to the aqueous layer because cations and anions remain associated with each other, reducing the number of solvated ions.
Step-by-step explanation:
The question asks which factor might prevent ions from transferring completely to the aqueous layer during a process such as dissolution or extraction. Among the options given, ion pairing is the most likely cause. In ion pairing, a cation and an anion remain in close contact with each other in solution without being fully separated by solvent molecules. When ions form such a pair, they behave as a single particle until dissociation occurs. This phenomenon means that the ions are not as free or available as they would be if they were completely solvated, leading to a lower effective concentration of free ions in the solution. High-charged ions like Mg²⁺ and Al³⁺ are especially prone to this due to their greater electrostatic interactions. Furthermore, as per the Debye-Hückel theory, even in strong electrolytes, the ions don't behave as completely independent particles due to residual electrostatic attractions. This effective concentration is known as the ion's activity, which can be different from the actual concentration. As a result, not all the ions will transfer to the aqueous layer because some exist as ion pairs.