On the contrary, in the bulk electrolyte, an abnormal ion association between Ca 2+ and Cl - in the CNTs with diameters from 1.085 nm to 1.220 nm is observed, where Ca 2+ and Cl - mainly exist in the form of contact ion pair (CIP) • An abnormal ion association between Ca 2+ and Cl - in 1.085 nm to 1.220 nm CNT. • Confinement enhances the hydration capacity of Ca 2+ in sub-nanoscale CNTs (d < 1.085 nm), while that of Cl - is greatly reduced. • The weakening of hydration capacity by confinement is the internal driving force for the ion association. Confined aqueous CaCl 2 solution plays an important role in biochemistry, geochemistry, electrochemistry, etc . In the present work, molecular dynamics simulation is employed to investigate the structures of aqueous CaCl 2 solution in carbon nanotubes (CNTs) with different diameters. An abnormal ion association between Ca 2+ and Cl - in the CNT with a diameter from 1.085 nm to 1.220 nm is observed, being completely different from the bulk electrolyte solution, where Ca 2+ and Cl - exist mainly in the form of contact ion pair. Confinement enhances the hydration capacity of Ca 2+ in subnanoscale CNTs (d < 1.085 nm), whereas the hydration capacity of Cl - is greatly reduced, although both Ca 2+ and Cl - have different degrees of dehydration. The hydration capacity of Cl - and Ca 2+ is the weakest in 1.085 nm and 1.220 nm CNTs, respectively, and the weakening of hydration capacity by confinement is the internal driving force for the ion association.