Another important aspect of water sorption is that it can
vary from one batch of drug or excipient to another, and in cases
where the moisture level is critical, the water sorption profile
becomes an important characterization technique for raw ma-
terials. There can be energetic and micromeritic reasons for the
water sorption level variation. Typically, raw materials are also
tested for particle size and specific surface area, and the results
can be used to try to normalize the moisture sorption profile by
these properties, for example, water content per unit of surface
area. It is also possible to analyze a moisture sorption isotherm
in terms of the Brunauer–Emmett–Teller (BET) equation to
extract a
C
B
value related to energy of adsorption of the first
layer or the Guggenheim-Anderson-de Boer (GAB) equation to
extract
C
G
and
K
values related to adsorption energies of the
first and an intermediate layer.
25,26
Both models also provide
the
W
m
parameter formally related to monolayer capacity, and
although it does not provide specific surface areas in accord
with values from N
2
gas adsorption, it can have interpretive
value.
27
The BET or GAB sorption energy parameters fail to
give the complete characterization of a vapor’s interaction with
2772
Sacchetti, JOURNAL OF PHARMACEUTICAL SCIENCES 103:2772–2783, 2014
RESEARCH ARTICLE –
Pharmaceutics, Drug Delivery and Pharmaceutical Technology
2773
a solid, in particular water’s energetic interaction with a drug
or excipient. Specifically, the BET or GAB models do not in-
corporate interactions between adsorbed vapor molecules, in
particular water–water interactions, which presumably are an
importantfactorinthesorptionprocessastheRHincreases, es-
pecially for the water molecule that exhibits strong H-bonding
with itself. Indeed, IR spectroscopic experiments of moisture
sorption on NaCl have shown H-bonding of water with itself
at submonolayer coverage.
28
It is in fact well established from
sorption calorimetry experiments that heats of sorption vary
continuously as the vapor pressure (or RH in the case of water)
of the probe increases, indicating that a single energy param-
eter such as
C
B
can miss important detail in characterizing
water–solid interactions.
2
Another important aspect of water sorption is that it can
vary from one batch of drug or excipient to another, and in cases
where the moisture level is critical, the water sorption profile
becomes an important characterization technique for raw ma-
terials. There can be energetic and micromeritic reasons for the
water sorption level variation. Typically, raw materials are also
tested for particle size and specific surface area, and the results
can be used to try to normalize the moisture sorption profile by
these properties, for example, water content per unit of surface
area. It is also possible to analyze a moisture sorption isotherm
in terms of the Brunauer–Emmett–Teller (BET) equation to
extract a
C
B
value related to energy of adsorption of the first
layer or the Guggenheim-Anderson-de Boer (GAB) equation to
extract
C
G
and
K
values related to adsorption energies of the
first and an intermediate layer.
25,26
Both models also provide
the
W
m
parameter formally related to monolayer capacity, and
although it does not provide specific surface areas in accord
with values from N
2
gas adsorption, it can have interpretive
value.
27
The BET or GAB sorption energy parameters fail to
give the complete characterization of a vapor’s interaction with
2772
Sacchetti, JOURNAL OF PHARMACEUTICAL SCIENCES 103:2772–2783, 2014
RESEARCH ARTICLE –
Pharmaceutics, Drug Delivery and Pharmaceutical Technology
2773
a solid, in particular water’s energetic interaction with a drug
or excipient. Specifically, the BET or GAB models do not in-
corporate interactions between adsorbed vapor molecules, in
particular water–water interactions, which presumably are an
importantfactorinthesorptionprocessastheRHincreases, es-
pecially for the water molecule that exhibits strong H-bonding
with itself. Indeed, IR spectroscopic experiments of moisture
sorption on NaCl have shown H-bonding of water with itself
at submonolayer coverage.
28
It is in fact well established from
sorption calorimetry experiments that heats of sorption vary
continuously as the vapor pressure (or RH in the case of water)
of the probe increases, indicating that a single energy param-
eter such as
C
B
can miss important detail in characterizing
water–solid interactions.
2
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