(The original is written in 1992)
(From: http://newnewhkcc1976.wordpress.com/2008/05/16/commentary-and-my-version...)
I am writing this as my own version of Euler’s ‘How salt dissolve in water’. Presumably, when its author is writing that article, he is describing it in the level of electrons since only the outlying electrons of atoms would repel each other. In my opinion, we can describe it in a more appropriate level: Atomic level. Moreover, that article consider only the thermal energy of the water molecules and salt, ignoring the electronic forces between the salt and water molecules. Therefore I attempted to offer a more comprehensive picture of the process by combing all forces.
Consider Salt itself is consisted of chloride ions and sodium ions packed together. As it is dropped into water, it is more likely that salt remove water molecules by displacement than electronic repulsion. As for the consideration of electronic forces, the repulsion and attraction of the same molecules of water by the ions is likely to resulted in molecules turning toward the salt, so the reaction on the ions is thus likely in the outward direction for the ions. Otherwise when the totality of electronic force acting on the molecules are repulsive rather than attractive, it also aid the dissolving process as water molecules would bounce back from the boundary (In case of a cup, it would bounce back from the glass; In case of ocean, it would bounce back from the shore.); thereby this time that water molecule is approaching the ions in the salt with increased kinetic energy. Thus, it speed up the next iteration of interactions between the ions and water molecules.
What are the effect of these interactions on the ions in the salt?
Undoubtedly, there are cases which the interaction resulted in the net repulsion between the water molecules and the ions. However, assume the water molecules are evenly distributed around the salt cube, the chance of ions repelled from the left side of the cube is identical with the chance of the ions repelled from the side of the cube. (If the forces are otherwise unbalanced, the net momentum gained by the cube would likely to erase this unbalance.) Since electrostatic force held the cube tightly, an effect on the left side is immediately felt on the right side of the opposite direction. Therefore the totality of the effect on the ions in the salt by the net repulsive interactions is very likely to neutralized. On the other hand, now, we consider the results of the net attractive interactions. Since there are more spaces outside the salt cube than inside, the pulling-outward effect on the left side of the salt cube is lesser likely to be ‘balanced’ by the pulling-outward effect of the right side of the salt cube. Thus the total effect of the net repulsive interactions between the water molecules and the ions, it is more likely to forcibly ‘expand’ the salt cubes. (which the thermal energy of the ions itself is most likely to aid the process as described by euler.) Combing the two net effect of interactions, the ions are thus stretched. As the distances between the ions increased, the electrostatic forces held them together become weaker. As the electrostatic force between the ions are weaker, the ions in the outermost layer of the salt cube thus has a higher chance of interacting with the water molecules, which resulted in more tearing effect on the salt cube(since the ions in the inner layer of the salt cube is insulated from the electrostatic interactions because the electrostatic force are decreased with distance squared, which it is furtherest from the water molecules.) Therefore, the outermost layer of the ions in the salt cube are gradually being tear away by the water molecules, exposing the next outermost layer of the ions in the salt cube to the electrostatic interaction between water molecules and ions. thereby the whole process happened again and again until the salt is completely dissolved in water.