Flashcards in Lecture 4 Deck (47)
What is the relationship between the number of phases and the number of degrees of freedom?
the more phases we have the less degrees of freedom we have.
How many degrees of freedom in a system of 3 components and 1 phase?
How can we simplify a 4 dimensional graph?
by making two variables constant. e.g. temperature and pressure.
now the only thing we have to worry about is the 2 concentrations
which diagram is used to show this?
the triangular diagram which describes the concentration of the 3 different components of the system.
the apex/corners of the triangles are 100%
The area of the triangle represents all the possible combinations of the 3 components.
Everywere on the line you have a constant ratio of 2 of the components but the 3rd component is changing
in a ternary phase diagram with a pair of partially miscible liquids, how many regions are there?
1 region everything is completely miscible and the other region, everything is not miscible
How can we draw tie lines in this ternary phase triangluar diagram?
We can start at 100% water and add in bezene or vie versa.
why is the angle of the tie line not always horizontal in this case of a ternary phase diagram?
it depends on how miscible the substances are.
if the 3rd liquid has an equal effect on both immiscible liquids then the tieline might be parallel to the base
What is usually used as a constant and why?
pressure is fine to be a constant but temperature tends to vary a lot.
e.g. something at a certain phase at room temperature may be different inside the body at 37degrees
How can we study the effects of temperature on these systems?
we can construct these systems at different temperatures
what is the effect of temperature on the systems?
as temperature increases, the darker 2-phase region decreases because higher temperatures increase solubility so the one phase system increases
What does a solution require?
a solute and a solvent
what is a solute?
the substance that gets dissolved
what is a solvent
the substance that does the dissolving
Why cant you see clumps of a solutuion?
because it is a molecular dispersion
What does a molecular dispersion mean?
what has been dissolved has been broken apart to the level of indivdual molecules.
it is a 1 phase system where there are no boundaries or distinct regions
what types of solutes are there?
electrolytes and non-electrolytes
Why can electrolytes conduct electrcity?
Electrolytes ionise in water, they break apart into ions which are charged, so can conduct electricty
are most drugs electrolytes?
what is the difference between strong electrolytes and weak electrolytes?
strong electrolytes completely ionise in water,
weak ones only partially ionise in water
Why are there huge implications for electrolytes and solutions?
for something to be absorbed into the body, it has to be in liquid
What are the properties of a good drug considering the stomach is an aqueous environment?
a good drug should be ionised as ionised substances dissolve better in aqueous solutions so is more likely to dissolve in your gastric contents, ECF or your eye
What must the drug cross if we want it to be absorbed into the body?
a lipid bilayer membrane.
What form of the drug dissolves better through the lipid bilayer membrane?
the unionised form as it is not charged
What are the implications for non electrolytes like progesterone?
it will not dissolve in gastric contents and hence not available in absoprtion but can cross the skin.
This is why trasdermal progesterone formulations exist.
It also has relevance to colligative proerpties
What is bioavailability?
how much of the administered drug reaches your systemic circulation
often expressed as a %
What is electrolysis?
where a certain potential is applied which causes a current to generate a redox reaction.
However it cannot be used for anything that's unionised as they cannot conduct electrolysis
in a solution what does the dissolving?
the solute. this could be either the gas, liquid or solid
Is air a solution?
yes as we have many gases and they are molecularly dispersed into one another. There is no phase boundary
in pharmaceutics, which kind of solutions are we most interested in?
solutions where the solvent (dissolving agent) is liuid
What is an example of solid in liquid solution?
mixing a cup of tea with sugar
what is an example of a liquid liquid solution?
mixing alcohol and water
what is an example of a gas in liquid solution
a can of coke which has CO2 in the liquid, completely dissolved
Can a solute be filtered out from a solution?
you can filter out big grains of something but a solution is a molecular dispersion so you cant filter out individual molecules.
What other criteria are there of a solution?
the particles are always in motion
the volumes are not additive.
the 2 things you dissolve can have quite different properties
the dissolving agent appears in a greater extent, and the solute appears to a lesser extent
you may have more than one type solute or solvent
What is molarity (M)?
the number of moles in 1L of solution
what is molality (m)?
the number of moles per 1000g of solvent
What is the relationship between molality and molarity in the example of water?
molarity = molality
however in other solutions, 100g of solvent does NOT = 1L
what is % by weight?
the number of g per 100g of solution
what is % by volume?
the number of ml per 100ml of solution
What is 0.154M NaCl important to us?
it is isontonic saline, isotonic to our bodies
What is ideal?
what theoretical scientists want
- no properties to change other than dilution
- no heat
- no expansion or shrinkage
- we want the constitutive properties to be the weighted averages of the individual constituents
- to be formed by mixing substances of similar properties.
- complete uniformity of attractive forces
what happens when real solutions are mixted together?
there is a change of some sort, usually evolution of heat
what is Raoult's law?
if the solution is ideal, the partial vapour pressure of each volatile constituent is equal to the vapour pressure of the pure constituent multiplied by its mole fraction in the solution
What is abvoe the surface of liquid water?
a vapour pressure of molecules
What is the mole fraction?
the fraction of moles of a substance in that solution
What is the vapour pressure composition curve?
Y axis = vapour pressure on both sides
X axis = mole fraction
at 100% B, vapour pressure is all due to B
When you mix the two solutions in the middle, the total vapour pressure is a weighted pressure of the 2 components of the solution