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Rheology
Maryam kazemiPh.D student of pharmaceutics
Shiraz university of medical sciences
Measurement of thixotropy The most apparent characteristic of a thixotropic
system area of hysteresisPlastic( bingham) bodies :
Thixotropic coefficient:
Thixotropic coefficient
B=thixotropic coefficientU=plastic viscosityF=Shear stress G=Shear ratef=Bingham yield value
Determine the structural break down • M=thixotropic coefficient• V=maximum shearing rate
Bulgs & Spures
Agueous bentonit gel (10-15% by weight) hysteresis loop with bulge in the up curve.
Procain penicillin gel IM
Bulgs & spures cont …A sharp point of structural break down
spurevalue or yield value at the low shear rate.
The spur value is obtained by using an instrument in which the rate of shear can be slowly and uniformly increased preferably automatically
Penicillin gels having definite I values were very thixotropic, forming intramuscular depots upon injection that afforded prolonged blood levels of the drug.
Negative thixotropy or Anti thixotropy
Represents an increase rather than a decrease in consistency on the downcurve.
This increase in thickness or resistance to flow with increased time of shear.
It was detected at shear rates of greater than 30 1/sec Below 30 1/sec the magma showed normal thixotropy
It was observed that when magnesia magma was alternately sheared at increasing and then decreasing rates of shear, the magma continuously thickened (an increase in shearing stress per unit shear rate) but at a decreasing rate, and it finally reached an equilibrium state in which further cycles of increasing-decreasing shear rates no longer increased the consistency of the material.
The equilibrium system :
1) readily pourable
2)the material returned to the Sol like properties.
Anti thixotropy compare with dilatancy or rheopexy
Anti thixotropy:1)Floculated2)Low solid content(1-10%)3)Equilibrium form is sol
Rheopexy or Dilatancy1)Defloculated2)Greater than 50% by volume of solid dispersed phase .3)Equilibrium form is gel
Increased collision frequency of dispersed particles or polymer molecules in suspension
Anti thixotropy results from
Inter particle bonding with time
Eventual equlibrium
Small number of relatively large floccules (at rest )
Large floccules break up and gratually return to the original state of small floccules and indivitual particles
Thixotropy in Formulation Desirable properly in liquid pharmaceutical sys
Ideally should have a high consistency in the container
Spread easily
Degree of thixotropy & rate of sedimentation Concentrated parenteral suspensions containing 40-70% w/v
procain pen g in water high inherent thixotropy .
Break down of the structure occurred (pass through the needle)
Consistency was then recovered as rheologic structure reformed.
Formulation of a depot of drug at the site of IM injection drug was slowly removed & made available to the body
Degree of thixotropy was related to the specific surface of the Penicillin used.
Determination of the rheological behavior and viscosity of various materials and compounds
Choise of viscometer:1) Single point viscometer : Newtonian systemSingle shear stress & shear rate .
2) Multipoint viscometer/rheometer: non-Newtonian, Newtonian systemMultiple shear stress & shear rate
Instrumentation
Newtonian Fluids Viscometers1- Capillary Viscometer
Ostwald U-tube viscometer
• Example:Acetone (45 s, 0.786 g cm-3) Water (100 s, 0.997 g cm-3, 0.8904 cp) viscosity acetone?
Poiseuilles law• Application:Calculate the pressure difference in the arteries and capillaries.Poiseulles law :r : the radius of the inside of the capillaryt : the time of flowp : the pressure head in dyne/cm2 under which the liquid flows.I : the length of the capillaryV : the volume of liquid flowing
pressure head is depended on density ,height, gravity .
• Poiseuille law according to the r:
• Falling sphere viscometer
2- Falling Sphere ViscometerHoeppler falling ball viscometer 1.59 mm diameter ball (glass or steel cylinder
(320 mm height x 25 mm width)
• Bob and cup viscometer
Multipoint Viscometers1- Bob and Cup Viscometer (concentric cylinder viscometer)
Couette (Cup rotates, and Bob stationary)Searle (Cup stationary and Bob rotates) Rotovisco , Stormer
rotovisco:
• F = S x KF Shear stress (dyne cm-2) = Stress reported by viscometer x Shear
stress factor (dyne cm-2)
G = KG x n
Shear rate (s-1) = Shear rate factor x Speed of rotation (rpm)
omega : the angular velocity in radians/sec produced byT: the torque in dynes cm. H: The depth to which the bob is immersed in the liquids, Rb and Rc :the radii of the bob and cup.
• Newtonian liquid
• Plastic viscosityU : the plastic viscosity in poises wf :the yield value intercept in grams
• yield value of a plastic system
• Kf is equel to:
• The Brookfield viscometer is a rotational viscometer of the Searle type that is popular in the quality-control laboratories of pharmaceutical manufacturers
• Pulg flowFlow of paste Bob and cup