The concentrations of aqueous calcium chloride solutions are increased and it is seen the surface tension increase although insignificantly.
The ions desorb from the surface due to the action of image force at low concentration and hydration forces at high:
https://www.researchgate.net/publication/236918649_Surface_tension_and_surface_Dch-potential_of_concentrated_ZZ-_electrolyte_solutions/figures
There is CaCl2 data in the 3rd supplement of this article.
Article Surface tension and surface Δχ-potential of concentrated Z+:...
https://en.wikipedia.org/wiki/Method_of_image_charges
A charge near the interface between a polar/conductive and a non-polar/non-conductive phase is pulled toward by the polar phase. For example, an ion at water|gas is attracted to the water bulk by the water dipoles oriented by the charge itself, i.e., the image force is the ion-dipole interaction. The hydration force is the short-ranged part of the same ion-dipole interaction (affected by the structure of the first 1-2 hydration layers of the ion).
The integral of the sum of these two forces from z = infinity (gas phase) to z = -infinity (water) yields the Born energy of the ion. Or the Born + the Debye-Hueckel energy, in case it is an electrolyte solution. Onsager's theory for the screened image force is the generalization of the Born-Debye-Hueckel model for an ion near an interface.
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