ABSTRACT
Introduction ........................................................................................................ 639
Materials and Methods ..................................................................................... 640
Water-Uptake Measurements........................................................................... 641
Isothermal Microcalorimetry ........................................................................... 641
Results and Discussion ..................................................................................... 642
Conclusions......................................................................................................... 645
Acknowledgments ............................................................................................. 645
References ........................................................................................................... 645
Water adsorbed on the surface of pharmaceutical or food powders, even at
low levels, has a significant impact on the behavior of such powders during
processing, and ultimately on the quality and performance of the final
product. Bakri (1993) reported the application of heat conduction micro-
calorimetry to study the adsorption of water vapor onto solid pharmaceu-
ticals. The high sensitivity of the calorimeter permits detection, even at low
humidities, of heat flow as a result of interaction of solvent vapor with the
solid. The heat flow ðP
Þ can be described by:
P
¼
dQ
dt
¼ DH
dn
dt
ð51:1Þ
where dQ
=dt and DH
are the rate and enthalpy of adsorption,
respectively, and n denotes the number of moles adsorbed. This equation
is related to two aspects of the process having place during interactions, a
thermodynamic aspect (what interactions will occur, extent of adsorption)
and kinetic aspect (how fast, rate of adsorption). For a given polymorphic
drug quantity, Q
, is given by Bakri (1993)
Q
¼ DH
n ð51:2Þ
where DH
is the adsorption enthalpy of the organic probe vapor on the
polymorphic drug, and n is the quantity of sorbed water. DH
can be
obtained directly from the calorimetric data. Physical adsorption is
considered a reversible process. The physical adsorption and magnitude
of adhesion of drug particles on excipient particles in a powder blend are
likely to be affected by the interactive forces. Physical adsorption and
adhesion phenomena are in turn likely to influence physical and mechanical
properties of powders including blend stability, flowability, dispersibility,
compressibility, and dissolution characteristics. The physical properties of
powders are also affected by the crystalline form of an active ingredient or
excipient entity. Staniforth et al. (1981) investigated the use of excipients,
including lactose, prepared with different crystal habits. His experiments
resulted in various nonsegregated and de-agglomerated system behavior.
Powder behavior in terms of mixing and the physical stability of powder
blends is very much dependent on the energetics of the interaction between
powder particles. Since the components in a powder blend interact with
each other at the level of their particles’ surfaces, the surface properties of
powders are a determining factor in their mixing behavior.
