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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5320_Библиотеки_им_академика_М_И_Перельмана
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166
FURTHER READINGS
Drug Stability: Principles and Practices
Applied Physical Pharmacy,
Physicochemical Principles of Pharmacy, 4th ed.,
Theory and Practice of
Contemporary Pharmaceutics
inhibition. J Pharm Sci 90
Advanced Pharmaceutics: Physical Principles
Biomaterials for Delivery and
Targeting of Proteins and Nucleic Acids
Developing
Solid Oral Dosage Forms
Int J Pharm 225
Pharmaceutical Dosage Forms and Drug Delivery
Developing Solid Oral Dosage Forms
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8
Interfacial Phenomena
LEARNING OBJECTIVES
On completion of this chapter, the students should be able to
1.
2. -
3.
4. Describe the importance of interfacial tension in pharmaceutical formulation.
6.
7.
8.1 Introduction
termed an interfacesurface.
interfacial phenomena. Interfacial phenomena result from the
8.2 Liquid– Liquid and Liquid– Gas Interfaces
DOI: 10.1201/9781003389378-10
167
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FIGURE 8.1
Pharmaceutical Dosage Forms and Drug Delivery
FIGURE 8.2
Figure 8.1).
cohesive forces,
and the resulting phenomenon is termed cohesion
adhesive forces, and the resulting phenomenon is termed adhesion.
-
face or interfacial tension.
8.2.1 Surface Tension
Surface tensionparallel
the surface, counterbalancing the net inward
Figure 8.2
the length of the bar.
γ
f
Lb2
γ
∆
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Interfacial Phenomena
169
=
(8.1)
fbL
surface being created.
8.2.2 Interfacial Tension
Interfacial tension is the force per unit length that must be applied in parallel
the interface, counterbalancing the net inward
γ
γ
γ
interfacial tensionsurface tension
8.2.3 Factors Affecting Surface Tension
tensiometers
• Nature of the liquid
• Temperature
the intermolecular attractive forces and, thus, the reduced inward pull of the molecules on the
surface.
8.2.4 Surface Free Energy
W) and surface tension (γ
(8.2)
γ
γ
∂∂G
A
γ
f
2L
γ
∆A
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Pharmaceutical Dosage Forms and Drug Delivery
W
A (cm2) for a liquid that has the surface tension γ
and pressure.
(8.3)
γ
Example 1: If the length of the bar (Figure 8.2
=
(8.4)
b
Since
=
∴ γ2
In addition,
∴ W2
8.3 Solid– Gas Interface
8.3.1 Adsorption
adsorption. Adsorption refers to the surface binding of a liquid
or gas molecule (adsorbate) onto a solid surface (adsorbent
molecule, such as a drug compound.
desorption
Adsorption is a surface
phenomenon, distinct from absorption
core of the solid.
8.3.2 Factors Affecting Adsorption
•
•
the surface area of the adsorbent increases. In modeling the adsorption phenomenon, the amount of
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Interfacial Phenomena
171
•
•
(adsorbate) partial pressure or concentration, greater the rate of adsorption.
8.3.3 Types of Adsorption
chemical adsorption.
8.3.3.1 Physical Adsorption
• Monolayer formation
• Multilayer formation
as solute concentration in the liquid or partial pressure of the gas) are supportive, multimolecular
• Condensation
TABLE 8.1
Properties Physical Adsorption Chemical Absorption
Adsorption forces
Reversible, that is, adsorbate can be
Rate of adsorption Rapid at all temperatures
an unchanged form
rise in temperature
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8.3.3.2 Chemical Adsorption (Chemisorption)
Pharmaceutical Dosage Forms and Drug Delivery
Chemical adsorption or chemisorption
8.3.4 Adsorption Isotherms
solid/ adsorbent as a function of the equilibrium partial pressure (P) of the gaseous solute or the concentration (c) of the solute in the liquid at a constant temperature (thus, the term isotherm).
8.3.4.1 Type of Isotherms
Figure 8.3):
•
•
sigmoid isotherms.
•
•
•
8.3.4.2 Modeling Isothermal Adsorption
Adsorption of a solute on a solid substrate at constant temperature (i.e., isothermal
FIGURE 8.3
x
m
n
x
m
n
p
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given drug overdose. In addition, modeling the adsorption data can be used to generate information about
8.3.4.2.1 Freundlich Adsorption Isotherm
equation (Figure 8.4a).
==
1/
kp
Y x)
adsorbed per unit mass (m) of adsorbent at the partial pressure of gas (pk and n are constants for a
k and n
FIGURE 8.4
glog log
=+
1
k
(8.6)
BB
k
k
y
KP
p
yKy
p
y
=+
maxmax
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Pharmaceutical Dosage Forms and Drug Delivery
A plot of log (x/ m) against log p/ n, and intercept log k
not reach saturation.
8.3.4.2.2 Langmuir Adsorption Isotherm
p) but also satur-
K-
tion) reaction rate constant (ka) to the reverse (desorption) reaction rate constant (kd
adsorption reaction,
a
→
A
+
←
k
d
a
→
K
=
←
k
d
=
max
+1
KP
y
y
y
is the total number of surface
yy
, even as the rates of
1
A plot of p/ y against p1/ y
y
(8.7)
as the slope and 1/ Ky
and K.
•
•
8.3.4.2.3 BET Adsorption Isotherm
P
YP
bYbP
11
()
−
mm
c
c
=+
maxmax
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Interfacial Phenomena
P
molecules (Y/ Ym
P
−
0
=+
Yb
(8.8)
P
0
p is the partial pressure of adsorbate; y is the mass of adsorbate per unit mass of adsorbent; P0 is
Ymb is the constant proportional to the
8.4 Solid– Liquid Interface
-
pension or the adsorption of the drug on activated charcoal administered in the case of oral drug overdose.
8.4.1 Modeling Solute Adsorption
1
yKy
y
(8.9)
c is the equilibrium concentration of the solute in the solution and replaces p, the partial pressure
of the gas. A plot of c/ y against cy
and K can be obtained from the slope and
intercept of this plot.
8.4.2 Factors Affecting Adsorption from Solution
1. Solubility of adsorbate/ solute
higher its rate of adsorption onto the solid adsorbent.
2. Solute concentration: An increase in the solute concentration increases the rate of adsorption that
occurs at equilibrium until a limiting value is reached.
3. Temperature
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