I want to understand the forces involved in osmosis. If I have a molecule of water and one of a salt in the left side of a semi-permeable membrane, and a water molecule at the right side, what forces make the right side molecule travels left and remains there what forces make water molecules be more time at the left side of the membrane?
I read about a chemical potential difference between the initial and final states, but I can't understand why there is less energy in solution with more solvent. Why the impurities cause the chemical potential to reduce at the left side?
I understand that osmosis happen in other scenarios, like gases, therefore I discard think in the ionic solution or polarity of water molecules, etc.
On this site, I found:
The decrease in chemical potential occurs because there is a lower
concentration of water in the solution than in the pure liquid.
Statistically, fewer water molecules escape a solution into the vapor
phase or freeze out onto the solid phase. That's why salt lowers the
chemical potential of water in the solution.
But is still unclear to me why the salt has this effect on water molecules if their kinetic energy remains the same with or without the salt.
Best Answer
Usually the driving force for diffusion is a concentration gradient, however that is not strictly true. It is actually a gradient in chemical potential which is the driving force for diffusional processes. For some simplifying assumptions this reduces again to concentration gradient as driving force.
For osmosis, the chemical potential is the driving force as the presence of any impurities cause the chemical potential to reduce compared to the pure side of the membrane. This is modeled by an equation of the form: $$\mu_i = \mu_{i,0} + RT\ln x_i$$ where $\mu_i$ is the chemical potential of species i, $\mu_{i,0}$ some reference state of chemical potential, $RT$ is the specific molar energy and most importantly $x_i$ is the mole fraction of solute $i$ in the solvent.
Now as $0\le x_i\le 1$, it follows that $\ln x_i<0$ and $\Delta\mu_i=\mu_i-\mu_{i,0}<0$, i.e. there is decrease in chemical potential in the solvent leading to diffusion of across the membrane.