Osmotic Pressure and Colloids

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Colligative Properties Osmosis Semipermeable membrane: permits passage of some components of a solution. Example: cell membranes and cellophane. Osmosis: the movement of a solvent from low solute concentration to high solute concentration. There is movement in both directions across a semipermeable membrane. As solvent moves across the membrane, the fluid levels in the arms becomes uneven.


Colligative Properties Osmosis Eventually the pressure difference between the arms stops osmosis.


Colligative Properties Osmosis Osmotic pressure, , is the pressure required to stop osmosis: Π = osmotic pressure M = Molarity (mol/L) R = Ideal Gas Constant T = Temperature (K)

Osmotic Pressure is a VERY sensitive measure of Molarity: 

Osmotic Pressure is a VERY sensitive measure of Molarity Seawater contains 3.4 g NaCl per liter M = 3.4 g/58.5 g/L = 0.0582 M Π =(0.0582 mol/L)(0.0821L atm/mol K )(298K) Π = 1.42 atm 1 atm supports column of water 10.34 m length (1.42 atm)(10.34 m/atm) = 14.68 m (14.68 m)(3.28 ft/m) ~ 48 feet

Pure solvent/solution: 

Pure solvent/solution Seawater 48 feet Sea water Pure Water Pure Water

Figure 17.8: Pure H20 solution: 

Figure 17.8: Pure H20 solution


Colligative Properties Osmosis Osmotic pressure, , is the pressure required to stop osmosis: Isotonic solutions: two solutions with the same  separated by a semipermeable membrane. Hypotonic solutions: a solution of lower  than a hypertonic solution. Osmosis is spontaneous. Red blood cells are surrounded by semipermeable membranes.


Colligative Properties Osmosis Crenation: red blood cells placed in hypertonic solution (relative to intracellular solution); there is a lower solute concentration in the cell than the surrounding tissue; osmosis occurs and water passes through the membrane out of the cell. The cell shrivels up.


Colligative Properties Osmosis Crenation and Hemolysis:

Crenation of Red Blood Cells, Electron Micrograph: 

Crenation of Red Blood Cells, Electron Micrograph


Colligative Properties Osmosis Hemolysis: red blood cells placed in a hypotonic solution; there is a higher solute concentration in the cell; osmosis occurs and water moves into the cell. The cell bursts. To prevent crenation or hemolysis, IV (intravenous) solutions must be isotonic. Examples of osmosis: Cucumber placed in NaCl solution loses water to shrivel up and become a pickle.


Colligative Properties Osmosis Limp carrot placed in water becomes firm because water enters via osmosis. Salty food causes retention of water and swelling of tissues (edema). Water moves into plants through osmosis. Salt added to meat or sugar to fruit prevents bacterial infection (a bacterium placed on the salt will lose water through osmosis and die). Active transport is the movement of nutrients and waste material through a biological system. Active transport is not spontaneous.

Drinking Seawater will Cause Dehydration of Body Tissues: 

Drinking Seawater will Cause Dehydration of Body Tissues

Reverse osmosis: 

Reverse osmosis

Reverse Osmosis Water Purification: 

Reverse Osmosis Water Purification

Desalination plant: 

Desalination plant


The facility is the largest seawater reverse osmosis desalination facility in the United States. The incoming seawater is pretreated in round horizontal media filters. There are two sets of filters — primary, consisting of sand, gravel, and anthracite, and secondary consisting of the same media as primary, plus garnet. Next, the cartridge filters act as a check to catch any material that gets through the primary and secondary stages. Then, pumps drive the water at 800 pounds per square inch (p.s.i.) through reverse osmosis membranes that separate the dissolved salt from the water. Approximately 45% of the pressurized seawater goes through membranes and becomes drinking water.

Desalination Schematic: 

Desalination Schematic

The Cost of Desalination: 

The Cost of Desalination

Cost of Water from Freshwater Sources: 

Cost of Water from Freshwater Sources

Home water purification by reverse osmosis: 

Home water purification by reverse osmosis


Colloids Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due to gravity. Particle size: 10 to 2000 Å. There are several types of colloid: aerosol (gas + liquid or solid, e.g. fog and smoke), foam (liquid + gas, e.g. whipped cream), emulsion (liquid + liquid, e.g. milk), sol (liquid + solid, e.g. paint), solid foam (solid + gas, e.g. marshmallow), solid emulsion (solid + liquid, e.g. butter), solid sol (solid + solid, e.g. ruby glass).


Colloids Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid.


Colloids Hydrophilic and Hydrophobic Colloids Focus on colloids in water. “Water loving” colloids: hydrophilic. “Water hating” colloids: hydrophobic. Molecules arrange themselves so that hydrophobic portions are oriented towards each other. If a large hydrophobic macromolecule (giant molecule) needs to exist in water (e.g. in a cell), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water.


Colloids Hydrophilic and Hydrophobic Colloids


Colloids Hydrophilic and Hydrophobic Colloids Typical hydrophilic groups are polar (containing C-O, O-H, N-H bonds) or charged. Hydrophobic colloids need to be stabilized in water. Adsorption: when something sticks to a surface we say that it is adsorbed. If ions are adsorbed onto the surface of a colloid, the colloids appears hydrophilic and is stabilized in water. Consider a small drop of oil in water. Add to the water sodium stearate.


Colloids Hydrophilic and Hydrophobic Colloids


Colloids Hydrophilic and Hydrophobic Colloids Sodium stearate has a long hydrophobic tail (CH3(CH2)16-) and a small hydrophobic head (-CO2-Na+). The hydrophobic tail can be absorbed into the oil drop, leaving the hydrophilic head on the surface. The hydrophilic heads then interact with the water and the oil drop is stabilized in water.


Colloids Hydrophilic and Hydrophobic Colloids


Colloids Hydrophilic and Hydrophobic Colloids Most dirt stains on people and clothing are oil-based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. Emulsifying agents help form an emulsion.


Colloids Removal of Colloidal Particles Colloid particles are too small to be separated by physical means (e.g. filtration). Colloid particles are coagulated (enlarged) until they can be removed by filtration. Methods of coagulation: heating (colloid particles move and are attracted to each other when they collide); adding an electrolyte (neutralize the surface charges on the colloid particles). Dialysis: using a semipermeable membranes separate ions from colloidal particles.

Coagulation of a Colloid: 

Coagulation of a Colloid

Kidney and Dialysis: 

Kidney and Dialysis

Artificial kidney: 

Artificial kidney

A Dialysis Unit: 

A Dialysis Unit

Principle of Dialysis: 

Principle of Dialysis

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