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University of Sulaimani School of Pharmacy Dept. of Pharmaceutics Pharmaceutical Compounding
Pharmaceutical compounding I Colloidal and Surface-Chemical Aspects of Dosage
Forms
Dr. rer. nat. Rebaz H. Ali
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Outlines
• Rheology
• Introduction
• Mechanical properties of solids
• Flow of liquids
• Newtonian liquids
• Non-Newtonian liquids
• Colloidal dispersions
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Introduction
• Rheology is the branch of physics that deals with deformation and flow of matter.
• Deformation describes the change of matter in terms of shape or volume, or both.
• Increasing interest in rheological methods in the medical and biological sciences:
A. A change of the rheological behavior of certain body fluids such as mucus, saliva,
blood, or synovial fluid.
B. In pharmaceutics more viscous materials require larger amounts of energy during
mixing.
• The viscosity might be reduced by the application of heat, which could
reduce the mixing time and improve product homogeneity.
C. Patient compliance.
• Application of stiff creams
• Flow through a hypodermic needle,
• Pouring from bottle
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• Rheology systems and stresses:
• Solids: have a constant volume and permanent shape, and are capable of
supporting loads.
• Liquids have constant volumes at constant temperature, variable shape, and
support no loads.
• Gases have neither constant volume nor permanent shape.
• Stress and Strain
• Deformation is the result of a force acting on or within a body, its extent depends on
the magnitude of force per unit area (F/A), i.e., the stress (Pa, Nm-2).
• As a consequence of the stress applied, the body will change its shape, and as a result,
there will be a change in length of the body, i.e., strain.
Introduction
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Outlines
• Rheology
• Introduction
• Mechanical properties of solids
• Flow of liquids
• Newtonian liquids
• Non-Newtonian liquids
• Colloidal dispersions
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Mechanical properties of solids
• Elastic solids are deform under stress, but once the stress is removed, they regain their
original shape, and the strain returns to zero.
• Point Y is the yield point or elastic limit, and the corresponding stress is the yield stress.
• If the stretching process of polyethylene or steel is
stopped before point Y, they will snap back to their
original length.
• Beyond Y, the solids undergo permanent deformation
from which they do not recover upon the removal of
stress, this called plasticity.
Stress, dyne/cm2
Pe
rce
nt é
lon
ga
tion
R
P G
S
Yield point
Breaking point
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Mechanical properties of solids cont.
• Steel has a high modulus (young’s modulus E; it’s a characteristic property of solids,
representing their stiffness or hardness).
• 𝐸 = 𝑠𝑡𝑟𝑒𝑠𝑠
𝑠𝑡𝑟𝑎𝑖𝑛 =
𝐹/𝐴
∆𝑙
• The horizontal portion YAH, the material is
ductile; it flows under practically constant stress
like a viscous liquid.
• If the stress is released at A, the sample
retracts along AC. The nonrecoverable
deformation OC is called permanent set.
• R is the elongation at the break, and the stress
corresponding to B is the ultimate strength or
tensile strength.
B
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Mechanical properties of solids cont
B
• The area OLYAHBRCO under the stress-strain
curve is the energy or work required to
break or rupture the material. It measures
the material’s toughness.
• Glass is hard and brittle.
• Steel is tough.
• Plastics are medium-hard or soft.
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Outlines
• Rheology
• Introduction
• Mechanical properties of solids
• Flow of liquids
• Newtonian liquids
• Non-Newtonian liquids
• Colloidal dispersions
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Flow of liquids
• Liquids do not retain their shape; the smallest
stresses, if applied for long enough time,
produce infinite deformation.
• If water and castor oil are poured from
bottles, water flows a thousand times faster,
i.e., its rate of shear is a thousand times
greater.
• γ =𝑉
𝐻=
𝑐𝑚
𝑠𝑒𝑐.𝑐𝑚= 𝑠𝑒𝑐−
• When liquid flows through a cylindrical tube
of small diameter the velocity is zero at the
wall of the tube, and maximum in the center.
H
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Flow of liquids cont.
• Vasodilator drugs like nitroglycerin increase the radius of blood vessels by relaxing the
vascular smooth muscles.
• The viscosity of a fluid may be described simply as its resistance to flow or
movement. Thus water, which is easier to stir than syrup, is said to have the lower
viscosity.
• Viscosity η is defined as the ratio of shear stress τ to rate of shear γ.
• η = τγ
= 𝐹/𝐴
1/𝑠𝑒𝑐 =
𝑑𝑦𝑛𝑒/𝑐𝑚2
1/𝑠𝑒𝑐= Pa.s
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Outlines
• Rheology
• Introduction
• Mechanical properties of solids
• Flow of liquids
• Newtonian liquids
• Non-Newtonian liquids
• Colloidal dispersions
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Newtonian liquid
• The viscosity of simple liquids (i.e., pure liquids consisting of small molecules and
solutions where solute and solvent are small molecules) depends only on
composition, temperature, and pressure.
• It increases moderately with increasing pressure and markedly with decreasing
temperature.
• For solutions of solid solutes, the viscosity usually increases with concentration.
• When the viscosity is independent of the shear stress or the rate of shear, the
liquid called Newtonian liquid .
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Newtonian liquid
• Plots of shear stress (on the y axis) as a function of the rate of shear (on the x axis) are
referred to as flow curves or rheograms.
• A has a higher viscosity than B because α > β
• The slope, f, is known as fluidity and is the reciprocal of viscosity, η:
Rheograms or flow curves of two Newtonian liquids.
Shea
r ra
te, S
-1
Shear stress, N/m2
B
A
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Outlines
• Rheology
• Introduction
• Mechanical properties of solids
• Flow of liquids
• Newtonian liquids
• Non-Newtonian liquids
• Colloidal dispersions
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Non-Newtonian liquid
A. Plasticity: the flocculated particles in concentrated suspensions do not flow at low
shear stresses (exhibiting reversible deformation like elastic solids) but flow like liquids
above their yield value (i.e., yield stress)
• This termed plastics or Bingham bodies.
• The more flocculated the suspension, the higher will be the yield value
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Non-Newtonian liquid cont.
B. Shear-thinning Fluids. Many colloidal systems, especially polymer solutions and
flocculated solid/liquid dispersions, become more fluid the faster they are stirred.
• Shear-thinning behavior is often referred to as pseudoplasticity.
• Shear-thinning behavior is an example of non-Newtonian flow because the viscosity, at
constant temperature and composition, decreases with increasing shear.
• Examples are solutions of polymers, such as
MC or NaCMC and gums such as tragacanth
or acacia.
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Non-Newtonian liquid cont.
• The macromolecules tend to assume roughly
spherical shapes, which surrounded by a sheath
of water of hydration.
• The viscosity of the solution is reduced in these
ways:
• The polymer chain become elongated and
thus offer less resistance to flow.
• The amount of water trapped inside the
coils decreases.
• Brownian motion will lead to a rebuilding of
the inner structure in many cases
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Non-Newtonian liquid cont.
C. Dilatancy or shear-thickening is an increase in viscosity with increasing shear.
• It is shown by concentrated (> 50%) deflocculated dispersions as the amount of
liquid present is not much larger than that needed to fill the voids between the
particles
• As shear stress is increased the particles are rearranged which leads to a significant
increase in interparticle void volume.
• The amount of vehicle remains constant, accordingly, resistance to flow increases
because particles are no longer completely wetted, or lubricated, by the vehicle.
• Example is suspensions of starch in water.
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Non-Newtonian liquid cont.
• Thixotropy is the gradual decrease in viscosity with increased shear followed by a
gradual recovery of the original structure.
• Their apparent viscosity depends on temperature, composition, shear stress, and the
previous shear history and time under shear.
• It usually related to shear thinning materials.
• For shear thickening samples, called negative thixotropy.
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Thank you for your attention!