Solute osmotic pressure

For example, the measurements of solution osmotic pressure made with membranes by Traube and Pfeffer were used by van t Hoff in 1887 to develop his limit law, which explains the behavior of ideal dilute solutions. This work led direcdy to the van t Hoff equation. At about the same time, the concept of a perfectly selective semipermeable membrane was used by MaxweU and others in developing the kinetic theory of gases. 

Reverse Osmosis. Osmosis is the flow of solvent through a semipermeable membrane, from a dilute solution to a concentrated solution. This flow results from the driving force created by the difference in pressure between the two solutions. Osmotic pressure is the pressure that must be added to the concentrated solution side to stop the solvent flow through the membrane. Reverse osmosis is the process of reversing the flow, forcing water through a membrane from a concentrated solution to a dilute solution to produce pure water. Figure 2 illustrates the processes of osmosis and reverse osmosis. 

Peptides are now included in some parenteral feeds for patients because they have less effect than amino acids on the osmotic pressure of the solution. Osmotic pressure can be a problem since the feed enters the blood directly. 

The effects of the most important operating parameters on membrane water flux and salt rejection are shown schematically in Figure 5.2 [14]. The effect of feed pressure on membrane performance is shown in Figure 5.2(a). As predicted by Equation (5.1), at a pressure equal to the osmotic pressure of the feed (350 psi), the water flux is zero thereafter, it increases linearly as the pressure is increased. The salt rejection also extrapolates to zero at a feed pressure of 350 psi as predicted by Equation (5.6), but increases very rapidly with increased pressure to reach salt rejections of more than 99% at an applied pressure of 700 psi (twice the feed solution osmotic pressure). 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

At this point the system is finally ready to stick a knife in the invader. One of the pieces of C5 sticks to C6 and C7. This structure has the remarkable property of being able to insert itself into a cell membrane. C5b,6,7 then binds to a molecule of C8 and a variable number (from one to eighteen) of molecules of C9 adds to it. The proteins, however, do not form an undifferentiated glob. Rather, they organize themselves into a tubular form that punches a hole in the membrane of the invading bacterial cell. Because the insides of cells are very concentrated solutions, osmotic pressure causes water to rush in. The in-rushing water swells the bacterial cell till it bursts. 

Staudinger and Husemann determined the osmotic pressure of solutions of a potato starch acetate which had been fractionated into four parts by precipitation of its chloroform solution with ether. The molecular weights of the fractions ranged from 45,000 to 275,000. All of the fractions were soluble in chloroform, but fractions of low molecular weight were also soluble in acetone. For various concentrations of solute in either chloroform or acetone, the osmotic pressure did not increase in direct proportion to the solute concentration, but the deviation from van t Hoff s law was the smallest in the case of the acetone solutions. Osmotic pressure measurements on amylose and amylopectin tri-acetates dissolved in tetrachloroethane have been made by Meyer and co-workers, who have deduced molecular weights for these substances of approximately 78,000 and 300,000, respectively (see above discussion of the purity of these fractions). 

Water, salt, and blood pressure are related. The blood volume is closely related to the blood pressure. A loss in blood volume can occur with water deficiency or because of extensive bleeding. The lack of enough blood to fill up the vessels of the circulatory system leads to a drop in blood pressure. A severe drop in blood pressure results in the inability of the heart to pump vital nutrients to the brain and other tissues. A loss in blood volume can also result from sodium deficiency. The concentrations of sodium and its counterion chloride must be maintained to maintain the osmotic strength of the blood plasma. Osmotic strength is expressed by the term osmolality. Osmolality is equal to the sum of the molarities of the separate particles (ions or molecules) in a liquid. For example, a solution of 1 mole of NaCl in 1 liter has an osmolality of 2.0 osmol/liter. Na and Cl ions dissociate completely in solution. Osmotic pressure develops when two solutions of differing osmolalities are placed in contact with each other but separated by a semiperme-able membrane. The walls of capillaries are semipermeable membranes. The renal... 

Osmometry is a technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution. Osmotic pressure governs the movement of solvent (water in biological systems) across membranes that separate two solutions. Different membranes vary in pore size and thus in their ability to select molecules of different size and shape. Examples of biologically important selective membranes are those enclosing the glomerular and capillary vessels that are permeable to water and to essentially aU small molecules and ions, but not to large protein molecules. Differences in the concentrations of osmoticaUy active molecules that carmot cross a membrane cause those molecules that can cross the membrane to move to establish an osmotic equilibrium. This movement of solute and permeable ions exerts what is known as osmotic pressure. 

Like molecules of an ideal gas, solute particles are widely separated in very dilute solutions and do not interact significantly with one another. For very dilute solutions, osmotic pressure, tt, is found to follow the equation... 

Molar masses and molar mass distributions are usually measured in dilute solutions. Osmotic pressure measurements determine and light scattering measurements determine M. The entire molar mass distribution can be measured using properly calibrated SEC. 

For instance, the establishment of when an ultraflltratlon process Is osmotic pressure limited or gel limited needs to be more clearly defined. In macromolecular ultraflltratlon solution osmotic pressure is often a strong function of moderate-to-high solute... 

Figure 5. Solution osmotic pressure vs. solute concentration 0./5M NaCl BSA...

Osmosis is the movement of water across a semipermeable membrane from a dilute solution to a more concentrated solution. Osmotic pressure is the pressure exerted by water on a semipermeable membrane due to a difference in the concentration of solutes on either side of the membrane. 

Under conditions of partly screened interactions in dilute solutions (high added salt concentration cs and low polymer concentration c), the solution osmotic pressure can be expressed via a virial expansion (Eq. 24). Then light scattering becomes a useful tool to obtain values of second virial coefficients characterizing interactions in solution. The second virial coefficient can be calculated from the slope of the dependence given by Eq. 25. The relation between the true and the apparent second virial coefficient is similar to the relation between the true and the apparent molecular weight (see the previous section for more details and the meaning of the symbols) ... 

Fluids move through the body continuously. The heart pumps the blood, pressure is exerted on the vessels from outside the body, and muscles relax and contract to help the heart move the fluid through the vascular system. Fluid moves into and out of the cells and the extracellular spaces by osmotic pressure. This is the pressure exerted by the flow of water through a semipermeable membrane separating two solutions with different concentrations of solute. Osmotic pressure is determined by the concentration of the electrolytes and other solutes in water and is expressed as osmolarity or osmolality. However, the terms are used interchangeably. 

If the cell and the surroundings have the same osmotic pressure then turgor pressure is zero and the system is in thermodynamic equilibrium. Osmotic pressure of the surroundings lower than that of the cell causes transfer of water into the cell. The cell swells, but the rigid cell wall limits the extent of swelling. A cell placed in a hypertonic solution (osmotic pressure higher than that of the cell) will lose water. The dehydration of a protoplast causes decrease of its volume and, in consequence, detachment of plasma lemma from the cell wall. This process is called plasmolysis (Figure 32.3). As the cell wall is permeable the volume between the cell wall and plasma lemma fills with the hypertonic solution. 

The RO and NF membrane processes are discussed in detail in Chapter 1. RO membranes are weU-suited to rejecting dissolved ions and most organics (some organics such as ethanol and acetone have very low rejections of 45-55%). The rate of water transport through a membrane depends on membrane properties (polymeric, chemical, morphological), water temperature, and the difierence in applied pressure across the membrane, less the difference in osmotic pressure between the concentrated and dilute solutions. Osmotic pressure is proportional to the solution concentration and temperature, and depends on the type of ionic species present. For solutions of predominandy sodium chloride at 25°C, a mle of thumb is that the osmotic pressure is 0.7 bar per 1000 mg/1 concentration (see Table 6.11 for osmotic pressures of various solutions). 

In the present sections on the behavior of mixtures, the solutions will occupy the foremost place of interest, because in discussing high polymers, investigations of the different properties of their solutions—osmotic pressure, viscosity, diffusion etc.—are of principal interest. 

We wish to emphasise in this connection that we are not here dealing with a fanciful analogy, but with one which is fundamental for the mechanism which, according to our present conceptions, produces gaseous pressure, and in solutions osmotic pressure, is essentially the same. In the first case it is due to the impacts of the gas molecules on the containing walls, in the second to the impacts of the dissolved molecules on the semipermeable membrane. The molecules of the solvent present on both sides of the membrane, since they pass freely through it, need not be taken into consideration. ... 

This is a simplified picture of osmosis. No one has ever seen the hypothetical passages that allow water molecules and other small molecules or ions to pass through them. Alternative explanations have been proposed, but our discussion has been confined to water solutions. Osmotic pressure is a general colligative properly, however, and is known to occur in nonaqueous systems. 

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