PH2107 - Solutions Flashcards by Sam Robinson

What is a solution?

Homogenous distribution of individual molecules of a substance dispersed in a solvent

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What is a solution comprised of?

solvent or solvents

- solute

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What is a solvent?

that phase in which the distribution occurs

if liquid, referred to as the vehicle solid or semi solid: base

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What is a solute?

The component which is dissolved as discrete molecules or ions in the solvent

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What appearance may a disperse system have?

The appearance of a solution

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What are the two components in a disperse system?

one component is present as small particles or droplets (disperse phase)
the other forms the continuous phase or dispersant

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Give examples of solution preparations

IV injection
Infusion liquid solution molecules in a solvent (water or buffer)
Numerous OTC products (cough syrup)

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Give examples of disperse preparations

Lotion - calamine lotion
Suspension - Calpol
Mixture - kaolin and morphine
Inhaler - Evohaler
Total Parenteral Nutrition (TPN)

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What is dissolution?

The process whereby a molecule at the exterior particulate interface is no longer bound (or associated) to other molecules in the particle

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What is dissolution characterised by?

Dissolution is a kinetic process, characterised by a dissolution rate

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What factors is the dissolution rate determined by?

association constant (Ka)
temperature
free energy of solvation

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What increases the tendency to dissolve?

The more similar the solute and the solvent are in polarity, the greater the tendency to dissolve

LIKE DISSOLVES LIKE

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What are batches of synthetic drug produced as?

Large solid structures

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Describe a crystalline structure

Has a defined structure, arranged in a regular, periodic manner

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Describe an amorphous structure

Lacks any defined structure

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What does post synthesis milling reduce solid drug structures down to?

A finely powdered particles

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What is a particulate?

Particles in the range < 1um

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What are forces that bind two adjacent molecules together overcome by?

Attractive forces between molecules of solvent

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What happens when a solute molecule is freed from its crystal or other particle?

It becomes solvated within a solvation shell

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How do solvent molecules orientate themselves around a solute?

In a complementary manner

- solvent delta- towards delta+ of solute

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What happens to a solvation shell when more solvent is added to a solution?

A less strong, outer shell is formed

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Eventually, how will the solute and solvation shells exist?

In a ‘sea’ of free/bulk solvent molecules

What is caused when the association of solvent molecules is very strong?

Permanently bound water

How is permanently bound water removed?

Cannot be removed without breaking down molecule

What is a dissociation constant, Kd?

A specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt separates into its component ions

What is the equation for Kd in a pure crystal?

AA A + A

What is the equation for Kd in a non pure crystal (e.g. salt or formulation)?

AxBy xA + yB

How can dissolution be accelerated?

Heat input: Higher temperature increases Brownian motion causing greater collision between solvent molecules and molecules at crystal surface High shear: vortex mixing increases Interaction between solvent molecules and crystal Ultrasonication: ultrasonic waves pass through crystal, transmitted through water, physically causes them to shake apart and interact with solvent faster

How does heat input accelerate dissolution?

Higher temperatures increases Brownian motion causing greater collision between solvent molecules and molecules at crystal surface

How does high shear accelerate dissolution?

Vortex mixing increases interaction between solvent molecules and crystal

How does ultrasonification accelerate dissolution?

Ultrasonic waves pass through the crystal, transmitted through water, physically causes them to shake apart and interact with the solvent faster

What is solubility?

The amount of a substance (mass or moles) that will dissolve in a specific volume of solvent, Csol

How is solubility determined?

1. Excess solid drug added to solvent until no more dissolves, at a specific temperature 2. Separate excess (centrifugation) 3. Analyse supernatant (UV or HPLC) 4. Often, very large dilution of supernatant necessary (e.g. x 1000) 5. Replication > x3 6. Typical data form - mg/mL - mol/L 7. For precision purposes, always accompanied by SD or SEM

What does 'high solubility' or 'poor solubility' mean?

Not much! It depends on your frame of reference

Which factor is solubility highly dependent?

Temperature - higher temperature = higher solubility Solubility should have associated temperature

What is a saturated solution?

The point at which the solution has reached the solubility point - immediately prior to drug precipitation - at which no more drug can be solvated

Why is solubility important?

The first step in preparing a formulation is to dissolve the drug in an appropriate solvent - drugs in solid phase cannot penetrate biological membranes

What state must a drug be in before transport and absorption?

The drug substance must be in solution

The higher the concentration, the higher the . . .

- dose, hence pharmacological effect | - chemical potential (tendency to migrate from high to low concentration)

What factor affects the rate of migration?

The higher the difference in chemical potential, the greater the concentration gradient

What is solvent polarity determined by?

Dielectric constant

What features of a drug affect its solubility?

Molecular features (moieties) broadly described as polar or non-polar

What are the features of alkyl regions?

Don't ionise Non-polar Lipophilic

What is the relationship between alkyl regions and lipophilicity?

The greater the extent of alkyl regions, the greater the lipophilicity

What do functional groups introduce into the molecule?

Varying levels of polarity and LogP values

What effect does the ionic state of ionisable functional groups have?

The molecule's hydrophilicity or lipophilicity

How do charged drugs exist?

They cannot exist on their own - they must exist as salts

Which drugs need to be lipophilic to be effective?

Drugs for topical delivery to the skin

What is a salt?

An ion and a counterion

What effect can the counterion have?

- ion pair association constant (not too strong) - toxicity levels of counterion in formulation - counterion interaction with excipients / packaging (e.g. Cl- is corrosive) - lability of drug to acid used to make the salt

What is crystallisation?

The converse of solubilisation | - crystal purity desired

What is a polymorphic structure?

When more than 1 crystal lattice forms simultaneously

What effect does shear force in milling have?

Can alter crystal purity