Rheology: Compounding PDF

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Summary

The details of how medicines are put together and equations to illustrate the pharmaceutics of the drug ...

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Khadijah Iqbal

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Dr.Fei’s Work – Rheology What is rheology?  

The study of flow properties or the viscosity of liquids, powders and semisolids (creams). How do we relate this to Pharmacy?

Basic principles    

Materials flow depending on the applied force There are intermolecular forces between the fluid layer and the static surface the stationary layer will retard (slow down/resistance to flow) the motion of the fluid layer due to intermolecular attraction between the fluid molecules The level of retardation is dependent on the viscosity of the fluid

The boundary layer   

This is the region in which differences in velocity is measured It depends on the viscosity of the liquid and the rate of flow E.g. A high viscosity and a low flow rate = thick boundary layer o A low viscosity and a high flow rate = thinner boundary layer

Fluid Flow- investigated by Reynolds  

Each subsequent layer will be retarded by the layer immediately below it As the distance from the surface increases, the extent of retardation decreases (therefore fluid velocity increases as the liquid is less viscous and can flow more easily).

Laminar flow  

Where the velocity in a liquid remains constant with time (generally at a low flow velocity). This can be seen in a river where there is very little movement and the water molecules move parallel to one another.

Turbulent flow 

When the speed of the fluid increases and a critical velocity is reached. This breaks the surface tension and causes chaotic flow r = fluid density (kg.m-3) u = mean fluid velocity (m.s-1) d = pipe diameter (m) µ = fluid viscosity (Pa.s or kg.m-1.s-1)

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Heat transfer This can be maintained by ensuring the thickness of the boundary layer is kept low Mass transfer By decreasing the thickness of the boundary layer it allows for greater mass transfer by molecular diffusion (Thus increasing the dissolution rate of the drug). Shear stress and shear rate Shear stress (τ): the force that needs to be applied to induce flow Shear rate: the rate of change of velocity at which one layer of fluid passed over an adjacent layer Viscosity Rate of flow is related to shear stress

This is viscosity (Nm-2 s) 

Dissolving polymers in a fluid affects its viscosity

Viscosity ratio (µr) 

This Is the ratio of the viscosity of a solution (µ), to the viscosity of the solvent (µ0).



Specific viscosity (µsp) is simply (relative velocity) -1



The concentration of the polymer (C) in solution is related to the specific viscosity Intrinsic

viscosity

(k ) 1 characterise the polymer.

can

be

used

to

Huggins constant (k ) shows how the polymer 2 interacts with the solvent, decreasing with more interaction.

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Newtonian Fluids 

Shear rate is related to shear stress (rate of flow)

Shear rate (γ) = shear stress (τ) A constant viscosity (η) is dependent on the shear rate τ = ηγ

Viscosity η

Shear stress τ

 

Shear rate γ

Shear rate γ Simple fluids and their viscosity   

Pure liquids consisting of small molecules These flow by newton’s law The viscosity of simple fluids depends only on o Composition: viscosity increases as concentration increases o Temperature: viscosity decreases when temp increases o Pressure: viscosity increases when pressure increases

Plastic Flow o o

This shows Newtonian behaviour once a yield stress is reached Tooth paste, jellies

Non Newtonian fluids   

They do not obey Newton’s law of flow Their viscosity changes with shear rate These can either be Shear Thinning or Shear Thickening, Time independent or Time dependant

Pseudoplastic liquids o

These are shear thinning

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As its agitated more violently it becomes less viscous. Blood, paint, ketchup

Examples of polymers include: o o o

Tragacanth Methylcellulose Carmellose

Dilatent liquids o o o

These are shear thickening As the liquid is agitated more violently its apparent viscosity increases may contain a high concentration of deflocculating particles (corn starch in water).

Time independent non-Newtonian fluids o o

Although viscosity of a fluid varies with shear rate, it is independent of the length of time that the shear rate is applied Ideally, the same shear rate would always produce the same viscosity.

Time dependant non-Newtonian fluids  

Thixotropic pseudoplastic fluids: These are shear thinning When these are agitated they will take time to return to their initial state (gels)

 

Rheopectic fluids (Thixotropic dilatant fluids) : These are shear thickening it may not return to its original structure instantly.

When the shear stress is removed the fluids do not adapt immediately to the new conditions A hysteresis loop forms which shows the fluid structure has been broken down Returning to original structure     

The molecules return to their original state due to Brownian movement (erratic movement of microscopic particles). Recovery time can vary from minutes to days The length of time the shear stress is applied will affect the degree of breakdown The degree of breakdown can be measured by looking at the area of the hysteresis loop. In some cases, the damage is not repaired and there is no loop in this case

The effects of temperature on viscosity     

For non-Newtonian fluids: An increase in temperature reduces the viscosity. However, in chemical change (heating sugar and water = syrup thus increasing viscosity) Crystallisation of dissolved solid form a solution Phase change: increase temperature a solid can melt Boiling off one or more components (changing the chemical composition..

Viscoelasticity: demonstrate solid and fluid behaviour when stressed for a period of time

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How to measure viscosity 



Ostwald Viscometer (U-tube) o For Newtonian liquids o Water used as a reference liquid then compared with test liquid against time Falling sphere Viscometer o Spherical ball is dropped in viscous liquid o Time measured between 2 points to get terminal velocity

Flocculated particles  

Newtonian vehicles o Slows the rate of sedimentation, easy to re-disperse but the suspension is gritty Non-Newtonian vehicle o Slows the rate of sedimentation, easily re-dispersed in the suspension and the suspension looks elegant

Deflocculated particles 



Newtonian Particles o Not ideal to have compact sediment (cake) as it is hard to re-disperse. However, the Newtonian vehicle slows sedimentation rate o If it does settle though it would be very hard to re-disperse Non-Newtonian Particles o Particles do not deep sediment (Cake) o Shaking will break the structural network formed when stored, thus allowing the particles to be re-dispersed o Suspending agents such as Tragacanth can be used

Non-newtonian vehicles (Flocculated particles)    

Does not settle quickly (increase viscosity) Readily re-dispersed Looks elegant With Newtonian the suspension would look gritty

Deflocculated particles     

Newtonian vehicles = cake formation Hard to re-disperse However, they have a high viscosity which means they have a low sedimentation rate Non-newtonian vehicles = structural network formed upon storage which prevents deep sediment being formed Pseudplastic fluids = shaking breaks down the network and causes re-dispersion of particles

Hydrocolloids are usually used as suspending agents

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Emulsions       

These are usually pseudoplastic Usually water in oil (w/o) emulsions is more viscous than oil in water. By increasing the viscosity of the discrete phase= more viscous emulsion However this can lead to Phase Inversion Decreasing droplet size = increase viscosity = Creaming Viscosity depends on the shear rate and haematocrit + flexibility of the RBC. Haematocrit: the % of RBC in the body

Eye drops  

Stay on the eye but not too thick as it will cause visual impairment Too thin it will not stay on the eye

Synovial liquid: 

lubricant between joints

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