Lecture 11/10/14

• The energy comes from the surroundings in the form of heat.
• The driving force is entropy.
• There is sometimes a misconception that the energy comes from entropy.

No osmotic pressure

Higher osmotic pressure.

Lower osmotic pressure.

• Mammalian kidney - an effective osmotic device that removes metabolic waste products and other impurities from the bloodstream.
• Freshwater fish - draw in water by osmosis through gills -> excrete with urine
• Water absorption by plants

• Apply a greater pressure than equilibrium osmotic pressure, then water can be driven against its osmotic gradient into the compartment with purer water (water purification).
• Additional note: In the normal osmotic pressure pressure setup, the water can be driven against its osmotic gradient into the compartment with pure water. For sea water, you need about 60 atm for producing pure water from sea water.

• It is a substance that generates charge carriers when dissolved in solution.
• It could be an acid, base, or salt.

• Dissolving salt crystals requires breaking the attractions in the lattice, which is possible in water because of the dielectric constant of water and its capability of forming ion-dipole interactions.
• Additional view: Requires the breaking of the electrostatic forces in the crystal lattice, the forces being the Coulomb force between two charged species.

Because it is a polar molecule and therefore can stabilize the ions through ion-dipole interaction that results in hydration.

his number is directly proportional to the charge and inversely proportional to the size of the ion.

Smaller ions and ions of higher charges will have agreater hydration radius: the hydration shell is stronger and bigger. Small ions or multi-chargesions (Li+, Mg2+) are structure-making ions: the viscosity is increases due to strong polarizationof water beyond first hydration layer. Big ions with one diffusive charge have smaller cells due totheir weak ion-dipole interaction.

Important for ion channel selectivity

• When salt concentrations are bigger than 0.01 M, you can no longer assume an ideal solution.
• This is due to strong Coulomb attractions.

Potassium channel selects the hydrated radius; sodium channel selects the ionic radius by arrangement of carboxyl groups in the channel.

• In most cases dissolving a salt into water is an exothermic process and the water heats up during this process.
• The breaking of the bonds in the crystal lattice of most salts requires less energy than the energy released during the formation of ion-dipole interactions when water solvates (hydrates) the dissolved ions.
• 

Ice Pack: Generally speaking, dissolving processes are exothermic; there are exceptions such as NH4NO3,the hydration energy is lower due to a greater ionic radius. In this case, the process is entropy driven.