Semisolid dosage forms and rheology part II Flashcards
rheology
The study of flow and deformation of matter
-Addresses the viscosity characteristics of a material
The study of flow and deformation of matter
-Addresses the viscosity characteristics of a materia
An expression of the resistance to flow; the higher the viscosity, the
greater the resistance
The unit of viscosity is
the poise (p)
Viscosity is involved in
the mixing and flow of materials, their packaging into containers
and their removal prior to use, and can affect patient acceptability, physical
stability and even biological availability
Rheology
• Materials divided into two general categories:
- Newtonian flow
2. Non-Newtonian flow
- Newtonian flow
•Characterized by constant viscosity, independent of the shear rates applied
• Material can be defined by a single viscosity at a specific temperature
• Direct proportionality between shearing stress and rate of shear
➢ Examples: Water and mineral oil
- Non-Newtonian flow
Characterized by a change in viscosity with increasing shear rate
• Materials cannot be defined by a single viscosity at a specific temperature
• May be time dependent
• The majority of fluid pharmaceutical products are non-Newtonian systems
➢ Examples: Liquid and solid heterogeneous dispersions such as colloidal solutions,
emulsions, liquid suspensions and ointments
Rate of shear* (G) is the
difference of velocity (dv) between two planes of liquid separated by an infinitesimal distance (dr)
➢ G = dv/dr
Shearing stress (F) is the
force (F’) per unit area (A) required to bring about flow
➢ F = F’/A
*Shear
To cause to move along the plane of contact
Higher the viscosity of a Newtonian liquid, the
greater is the shearing stress required to produce a certain rate of shear
F′/A = n(dv/dr), where n =
viscosity coefficient or viscosity
➢ More simply, n = F/G
Poise (p)
The shearing force required to produce a velocity of 1 cm/s between two parallel planes of liquid each 1 cm2
in area and separated by a distance of 1 cm
➢ g/cms
Capillary viscometer
-Time of flow of liquid under test is compared with the time
required for a liquid of known viscosity to pass between two
marks
- Can be used to determine the viscosity of a Newtonian
material, with the following relationship: n = ktp
Kinematic viscosity =
n/p, where n is the absolute viscosity and p is the density of a liquid at a specific temperature, and its units are the stoke (s)
‒ Used in the USP and NF
Viscosity of a liquid
increases or decreases when temp is raised?
decreases as temperature is raised
What equation shows dependence of viscosity of liquids on temperatre
Modified Arrhenius equation
n = AeEv/
RT, where A is a constant depending on the molecular weight and molar
volume of the material, Ev
is an activation energy required to initiate flow, R is the
gas constant and T is temperature
Non-Newtonian materials
Plastic
Floc
Pseudoplastic
DIlatant
Plastic
Flow does not begin until a shearing stress corresponding to a certain
yield value is exceeded
-The flow curve intersects the shearing stress axis and does not pass
through the origin
• Material is elastic below the yield value
• Associated with the presence of flocculated* particles in concentrated
suspensions
• A yield value exists because of the contacts between adjacent particles
(brought about by van der Waals forces), which must be broken before flow
can occur
In plastic, Once the yield stress is exceeded,
, it flows like a Newtonian material
Materials that exhibit this type of behavior : Plastic non-newtonian materials
are called Bingham bodies
*Floc
A loose aggregation of particles held together by weak bonds
Loose structure of particlaes permits them to
break up easily and distribute readily with a
small amount of agitation
*Floc prevents
the rigid cohesion of small particles of a suspension, which leads to formation of a solid cake
Pseudoplastic
There is no yield value as there is in a plastic system
– “Shear-thinning”
– Viscosity decreases with increasing shear rat
-
Increased shear rate deforms or rearranges particles, resulting in
lower flow resistance
Example of pseudoplastic
Polymeric solutions exhibit this type of flow
For pseudoplastic, As shearing stress in increased
normally disarranged molecules begin
to align their long axis in the direction of the flow
Dilatant
Type of flow is the inverse of that possessed by pseudoplastic systems
– “Shear-thickening”
– Viscosity increases with increasing shear rate
dilatant is characterized
by having a high percentage of deflocculated particles in
the formulation
Deflocculated system of titanium dioxide in water exhibits
dilatant flow
In Newtonian systems, when the rate of shear is reduced
the down curve is
super imposable on the up curve
In non-Newtonian systems, the down curve can be
displaced relative to the
up curve
Thixotropy
An isothermal and comparatively slow recovery, on standing of a material, of a consistency lost through shearing
-can be applied only to shear-thinning systems
Thixotropy are desirable in
pharmaceutical systems – Pour and spread easily, but have a high consistency in its container
Thixotropy consistency?
Material has a lower consistency at any one rate of shear on the down curve than it had on the up curve
Antithixotropy
Increase rather than a decrease in consistency on the down
curve
Antithixotropy -Example in a pharmaceutical system
Milk of magnesia at shear rates greater
than 30 sec-1 ; below a shear rate of 30 sec-1 , the magma shows normal thixotropy
Which of the following ointment bases provides the greatest degree of occlusion? A. Oleaginous B. Absorption C. Water-removable D. Water-soluble
A. Oleaginous
Which of the following ointment bases contains no oleaginous components? A. Oleaginous B. Absorption C. Water-removable D. Water-soluble
D. Water-soluble
A water-in-oil emulsion that permits incorporation of additional
quantities of aqueous solutions is considered what type of base?
A. Oleaginous
B. Absorption
C. Water-removable
D. Water-soluble
✓B. Absorption
Which of the following is a factor in the selection of an appropriate
ointment base?
A. Desired release rate of the drug substance from the ointment base
B. Stability of the drug in the ointment base
C. Characteristics of the surface to which it is applied
D. All of the above
D. All of the above
Which of the following statements about the preparation of ointments
via the incorporation method is NOT true?
C. The components are melted together
Which of the following statements is NOT true of Newtonian systems?
A. They are characterized by a constant viscosity, independent of the
shear rates applied
B. There is direct proportionality between shearing stress and rate of
shear
C. The majority of fluid pharmaceutical products are this type of
material
D. They can be defined by a single viscosity at a specific temperature
C. The majority of fluid pharmaceutical products are this type of
material
As the temperature is raised, the viscosity of a Newtonian liquid does what?A. Increases B. Decreases C. Remains the same D. It is dependent on its yield value
✓B. Decreases
In which of the following systems does viscosity increase with increasing shear? A. Plastic B. Pseudoplastic C. Dilatant D. None of the above
C. Dilatant
Which of the following statements is true of thixotropy?
A. It refers to the recovery of viscosity in time when flow is discontinued
B. It is applied only to shear-thinning systems
C. It is desirable in pharmaceutical systems
D. All of the above
✓D. All of the above
Plastic flow is associated with the presence of flocculated particles in
concentrated suspensions. What is a floc?
A. A solid cake of particles
B. A loose aggregation of particles
C. Component of a two-phase gel system
D. Both B and C
D. Both B and C
