I have found several different sources of data which show that a solution with a higher concentration of salt has a higher coefficient of volumetric thermal expansion than a lower concentration salt solution or distilled water. However there does not appear to be any explanation for this. How does a salt solution expand more than distilled water when both are heated over the same temperature range? Could it be related to specific heat capacity or convection?
I think a little chemistry can help. Here is what I think. Water molecules are polar in nature and are associated (not bonded) to each other by hydrogen bonds. When salt is added, it dissociates into ions. For example, Common salt dissociates into Na and Cl. These ions occupy space between the water molecules and reduce there Hydrogen Bonding. The water molecules are now less tightly bonded to each other and the same amount of heat supplied results in greater change in volume then before. In other words, coefficient of volumetric expansion increases.