What is the difference of molar volume between dissolved 35Cl- and 37Cl-?
I don't have an exact answer for chloride specifically, but consider the theoretical equations in the following two articles:

Isotope Effects on Molar Volume and Surface Tension: Simple Theoretical Model
and Experimental Data for Hydrocarbons J. Chem. Phys. 44, 457 (1966)


  We assume the molecules are held together in a
  condensed phase by van der Waals "bonds" or covalent
  bonds represented by 6-12 or Morse potential-energy
  functions, V(x). For monatomic molecules x is taken
  to represent the distance between molecular centers.  


and

Lattice Constants of Separated Lithium Isotopes J. Chem. Phys. 27, 1030 (1957)

which finds atoms of Lithium-6 $4 \times 10^{-4}$ times more separated than those of Lithium-7 and also cited to a reference which considers lithium-6 versus lithium-7 fluoride-19.