For semipermeable membranes with different concentrations of aqueous solutions on both sides, the minimum additional pressure applied on the high concentration side to prevent water from permeating from the low concentration side to the high concentration side is called osmotic pressure.Osmotic pressure and solution cannot pass throughSemipermeable membraneOfparticulateThe number is related to the ambient temperature.[1]
Schematic Diagram of Osmotic Pressure Generation and Measurement
The extra pressure exerted above the liquid level just enough to prevent permeation is called osmotic pressure (osmotic pressure for short)[1-2]。
brief introduction
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For semipermeable membranes with different concentrations of aqueous solutions on both sides, the minimum additional pressure applied on the high concentration side to prevent water from permeating from the low concentration side to the high concentration side is called osmotic pressure.Osmotic pressure and solution cannot pass throughSemipermeable membraneOfparticulateThe number is related to the ambient temperature.[1]
influence factor
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In 1877, German botanist Pfeffer found two laws based on his experimental data:
(1) When the temperature is constant, the osmotic pressure of dilute solution is proportional to the concentration of solution
(2) When the concentration is constant, the osmotic pressure of dilute solution is proportional to the thermodynamic temperature[1]
In 1886, van't Hoff, a Dutch theoretical chemist, theoretically deduced that non-volatileNon electrolyteThe relationship between osmotic pressure of dilute solution, solution concentration and thermodynamic temperature is as follows:
The above formula is called Vantov formula, also called osmotic pressure formula.
C is the molar concentration, unit: mol/L, which can also be calculated as C=n/V (amount of substance (mol)/volume (L)).
R is the ideal gas constant. When π is in kPa and V is in liter (L), R is 8.314J · K-1 · mol-1.
T is the thermodynamic temperature, in K (Kelvin). The conversion relationship with Celsius is T (K)=273+T (C), for example, 25 ℃=298 Kelvin.
The van Toff formula indicates that at a certain temperature, the osmotic pressure of the solution is proportional to the number of particles (molecules or ions) of the solute contained in the solution per unit volume that cannot pass through the semipermeable membrane, but is independent of the nature of the solute.
Solution osmotic pressure
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explain
Measurement of osmotic pressure[3]
The so-called solution osmotic pressure, in brief, refers to the attraction of solute particles in the solution to water.The osmotic pressure of solution depends on the number of solute particles in unit volume of solution: the more solute particles, that is, the higher the concentration of solution, the greater the attraction to water, and the higher the osmotic pressure of solution;Conversely, the smaller the number of solute particles, that is, the lower the concentration of solution, the weaker the attraction to water and the lower the osmotic pressure of solution.It is related to the content of inorganic salt and protein.Among the various inorganic salt ions that make up extracellular fluid, Na has obvious advantages in content+And Cl-，Extracellular osmotic pressureMore than 90% of them are from Na+And Cl-。At 37 ℃, humanPlasma osmolalityAbout 770kPa, equivalent toIntracellular fluidOsmotic pressure of.
Colligativity
Since the equilibrium osmotic pressure follows the ideal gas law (the interaction of solute molecules is ignored in dilute solution), this mathematical derivation process is omitted here, and finally the van der Hoff relationship can be obtained: π=cRT (or π=kTN/V; N/V is the molecular number density). From the formula, it can be known that the osmotic pressure of solution is only determined by the molecular number of solute, so the osmotic pressure is also a colligative property of solution.This relationship does not give the true pressure, but the pressure that may be required to prevent seepage flow, that is, the pressure difference required for the system to reach equilibrium.
Cell related
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plant cellThe power of water absorption is osmotic pressure.So how much does osmotic pressure mean to the cell world?We must also take specific data to illustrate that if the cell radius is R, the osmotic expansion will become R+dR, so the increase of the area is dA=8 π RdR, and its energy consumption is ∑× dA.To balance the cell expansion, let the free energy pdV equal to the surface tension, which can be obtained through calculationLaplace formula：Σ=Rp/2。If red blood cells are in pure water, the pressure preventing water from entering the cells is 300 pa, assuming that the cell diameter is 10 μ m, how big is it for the cells?Substitute the estimated p to get ∑=10-5m × 300pa/2=1.5 × 10-3Nm - 1.This force is enough to tear open eukaryotic cells and destroy them.So you can't dilute red blood cells with pure water, so they will burst, that is, lysate.
Hydrophobic action
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Emptying effect is the ideal situation of hydrophobic interaction (hydrophobic force is essentially the mixed effect of entropy and free energy), and osmotic pressure is the reason for macromolecules to produce this emptying force.Osmotic pressure can be regarded as the change of free energy per unit volume.The evacuation effect is that small particles can push large particles together to maximize the entropy of small particles themselves. If the two surfaces match accurately, the free energy on the corresponding unit contact area will be reduced to Δ F/A=ckBT × 2R, and R is the radius of small particles (where c is not the concentration but the molecular density).
Small particles can effectively help macromolecules find mutually specific recognition sites. In biological experimentsserum albumin(BSA) andpolyethylene glycol(PEG) acts as small particles, which are called blocking agents.For example, they can help deoxyhemoglobin stick together with other large proteins, reducing the solubility by 10 times;Glucan or PEG can stabilize the complex from thermal decomposition, which can increase the solubility of DNA;PEG and BSA can also increase the self-assembly rate of motorized protein filaments or the activity of different enzymes by several orders of magnitude;The E. coli DNA replication system cannot work without adding blocking reagent.It is not important to choose the blocking agent, but the key is the size and number density of its relatively assembled molecules.This is an ordered assembly driven simultaneously in the process of disorder, and this order is at the cost of smaller particles and larger disorder.
Related figures
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Van terhoffchemical kineticsHe won the first Nobel Prize in Chemistry.
In 1886, Vantehoff drew a rule based on experimental data: for dilute solutions, osmotic pressure is proportional to the concentration and temperature of the solution, and its proportional constant is the constant R in the gas equation of state.This law is called Vantehoff's law.
The principle is to place the solution and water in the U-shaped tube, and place a semi permeable membrane in the middle of the U-shaped tube to separate the water and solution. It can be seen that the water runs to one end of the solution through the semi permeable membrane. If the pressure is applied to the solution end, and this pressure can just prevent the penetration of water, then this pressure is called osmotic pressureMolar mass concentration, solution temperature and solute dissociation degree are related, so sometimes if the osmotic pressure and other conditions are obtained, the molecular weight of macromolecules can be inversely deduced.Van terhoff won the first nobel prize in chemistry for his research on osmotic pressure and chemical kinetics.
