Abstract
While there are many theories, there is still no clear view why osmosis occurs? Three of such explanations have been reviewed in this paper [1] diffusion due to a presumed water-concentration gradient [2] bound water explanation and [3] Van't Hoff's particle explanation. None of the mechanisms seems as such to be contributory in making the osmosis happen.
General
Osmosis is a selective movement of solvent from a solution through a semipermeable membrane separating two solutions of different concentrations. Solvent moves from a region of higher concentration to a region of lower concentration. It can also be restated that osmosis involves movement of water across a semipermeable membrane separating two solutions of different concentrations from a region of lower solute concentration to higher solute concentration.
Osmotic pressure is the external pressure to be applied to stop flow of water across the membrane.
Semipermeable membrane does not allow solute to pass through membrane.
The obvious question that arises is how the solvent separated by a membrane which is permeable to solvent moves across the membrane from a region of lower osmotic pressure to higher osmotic pressure overcoming the hydrostatic pressure. A movement of solvent from higher concentration to lower concentration implies that water moves from lower osmotic pressure to higher osmotic pressure.
For example, in the diagram below, the two arms of the U tube is separated by a semipermeable membrane with right side arm containing solution having lower concentration of solute than on the left side arm. Initially, the level is the same in both arms of the U tube. As osmosis starts, it pushes the solvent from right arm to left arm and pushes the solution up even though the membrane is permeable to solvent until the osmotic pressure in the right arm is equal to the hydrostatic pressure exerted by the left arm.
Questions:
[1] What drives solvent into the left arm when there is a solvent permeable membrane between two arms of the U tube?
[2] Are osmotic pressure and hydrostatic pressure additive? Or, it is simply an effect of density in the right arm of the U tube. As osmosis begins, the concentration of solute on the right arm concentrates, its density increases and simultaneously the pressure in the right arm increases which pushes the solvent into the left arm.
While there are many theories, there is still no clear view why osmosis occurs?
Following are some of the theories.
Concentration of water
Very simple explanation for osmosis is the concentration of water explanation - water in pure water is simply more concentrated than water in solutions because the solute has to take up some room in the solution. The dilution of water by solute resulting in lower concentration of water on the higher solute concentration side of the membrane and therefore a diffusion of water along the high to low concentration gradient occurs. . If this is true, then the concentration of water should be able to predict the direction of osmosis when different solutes are used. For example, 0.2 molal sucrose solution has a water concentration of 937 g/L and a 0.2 molal NaCl solution has a much higher water concentration—989 g/L. The sucrose solution should gain water from the NaCl solution if the two were separated by a semipermeable membrane. The number of molecules of NaCl and sucrose shouldn’t matter—only the water concentration should matter.
But the fact is a typical molal concentrations of sucrose with a lower water concentrations than the same concentrations of NaCl, because sucrose is a much larger molecule than NaCl displaces more water. Therefore, water concentration gradient does not seem to be important.
So this explanation is untenable.
Bound water explanation
This says that any hydrophilic solute (like sucrose or NaCl) will bind up hydrating water and prevent it from moving freely. Therefore, the side of a semipermeable membrane with pure water has a higher “free” water concentration than the side with the solute molecules.
If the bound water explanation were true, we would expect that a greater mass of hydrophilic solute would bind more water. Also, when predicting osmosis, we would have to carefully consider how hydrophilic the solute is (that is, how many water molecules it binds per molecule).
The actual position is, the number of solute molecules present has a dominant effect on osmosis, and not the hydrophilicity of solute.
So, this explanation is ruled out.
Number of particles explanation
This explanation is based on Van't Hoff's Law. According to this law for a dilute solution at constant temperature, the osmotic potential is proportional to the concentration of solute particles. The size or nature of the solute particles does not matter. So, for example, a small sodium ion would have the same osmotic effect as a large sucrose molecule, and both of these would be equivalent to a very large starch molecule. This also means that ionizing substances like NaCl should have a greater osmotic effect than non-ionizing substances like sucrose because when they ionize, they generate more particles. Van't Hoff's Law to an extent explains osmosis better than the two earlier explanations, but the fact is that Van't Hoff's Law is an empirical relationship, not a physical description of why osmosis occurs.
The best known expression for Osmotic pressure is below Π = iMRT where Π is the osmotic pressure in atm i = Van 't Hoff factor of the solute M = molar concentration in mol/L R = universal gas constant = 0.08206 L·atm/mol·K T = absolute temperature in K Osmotic pressure depends on [1] Van't Hoff factor [2]molar concentration of solute. Van’t Hoff’s factor, expresses the extent of association or dissociation of solutes in solution.It is the number of particles a solute dissociates in water. Example: for sucrose it is 1 and for NaCl it is 2