25
FREEZE—DRYING
Ï
from the vapeur at subwater saturation.
Nucleation by crystals of symmetry other than
is most marked for organic compounds 5.
Many
of these substances (for
example triclinic chloresterol) nucleated crystals from the vapour at a temperature as
high as— 1°C, the nucleation temperature not being related to the difference in lattice
parameters,— as was the case with the inorganic compounds. This leads to the idea that the
surface arrangement of OH groups may be a more important consideration. The mode
of action may be related to the suggested ice—like conguration of water in contact With
protein molecules which is responsible for bonding in cell membranes at temperatures
well above O°C 6.
Many of the substances nucleated in crystals preferentially at steps whose height was
greater than about 0.1 micron.
It was also found that some substances (for example
kaolinite) nucleated at temperatures about 8 °C_lower when they were completely embedded
'Î
in liquid, compared with nucleation from the vapour.
To summarise, the nucleating ability of a particular nucleus, measured as the maximum
:
'
supercooling which can be achieved in its vicinity, or the lowest temperature at any super—
saturation which just produces ice crystals, depends on :
'
-
a. the size of the particle.
b. the molecular characteristics of the surface.
c. the surface topography.
_
The extent to which the size of a nucleus or step exceeds that calculated from equation [l]
for the critical size of an ice nucleus will be a measure of the difference between the surface
energy between the ice—water interface and the substrate-water interface.
SUPERCOOLING OF SOLUTIONS
In order to investigate the effect of dissolved substances on the supercooling, it is rst
necessary to remove the suspended particulate matter. Ion exchange ltration is impracticable for ionic compounds, and the best which can be achieved is ltration by 10 milli—
micron millipore lters 2’7.
The supercooling is now dened as the depression below
the equilibrium melting point of the solution.
It was found that the supercooling of
alkali halide solutions equalled or actually exceeded that of the water from which the solu—
tions were made.
The excess was never greater than about 2°Ç for
molar solution,
the maximum effect occurring for solutes With larger ions (for eXample Cs++) which could
give appreciable structure-breaking e‘ects. A somewhatdi‘erent effect has been observed
for
solutions8 where a
ltration technique failed to inuence the
ability of the solution to nucleate around— 10°C. This result may be interpreted either
as nucleation by particles with“ adsorbed solute of size less than 10 milli—microns or as
nucleation by the Solute' molecules themselves.
,
.
‘
,
_
Nucleation of solutions may take place in two Ways, depending on concentration of. the
solute and on
If a saturated solution
cooljed,the‘n it may—become not '
only supercooled with respect to the ice phase,; but
'solute (gure 7). In
absence of aîppr0priate freezingçnucle‘i,
by (say) 30°C. Similay, in the absence of…appropriate
mayalso supersaturaté3; 'Hence
Willibe possible * for—the
FREEZE—DRYING
Ï
from the vapeur at subwater saturation.
Nucleation by crystals of symmetry other than
is most marked for organic compounds 5.
Many
of these substances (for
example triclinic chloresterol) nucleated crystals from the vapour at a temperature as
high as— 1°C, the nucleation temperature not being related to the difference in lattice
parameters,— as was the case with the inorganic compounds. This leads to the idea that the
surface arrangement of OH groups may be a more important consideration. The mode
of action may be related to the suggested ice—like conguration of water in contact With
protein molecules which is responsible for bonding in cell membranes at temperatures
well above O°C 6.
Many of the substances nucleated in crystals preferentially at steps whose height was
greater than about 0.1 micron.
It was also found that some substances (for example
kaolinite) nucleated at temperatures about 8 °C_lower when they were completely embedded
'Î
in liquid, compared with nucleation from the vapour.
To summarise, the nucleating ability of a particular nucleus, measured as the maximum
:
'
supercooling which can be achieved in its vicinity, or the lowest temperature at any super—
saturation which just produces ice crystals, depends on :
'
-
a. the size of the particle.
b. the molecular characteristics of the surface.
c. the surface topography.
_
The extent to which the size of a nucleus or step exceeds that calculated from equation [l]
for the critical size of an ice nucleus will be a measure of the difference between the surface
energy between the ice—water interface and the substrate-water interface.
SUPERCOOLING OF SOLUTIONS
In order to investigate the effect of dissolved substances on the supercooling, it is rst
necessary to remove the suspended particulate matter. Ion exchange ltration is impracticable for ionic compounds, and the best which can be achieved is ltration by 10 milli—
micron millipore lters 2’7.
The supercooling is now dened as the depression below
the equilibrium melting point of the solution.
It was found that the supercooling of
alkali halide solutions equalled or actually exceeded that of the water from which the solu—
tions were made.
The excess was never greater than about 2°Ç for
molar solution,
the maximum effect occurring for solutes With larger ions (for eXample Cs++) which could
give appreciable structure-breaking e‘ects. A somewhatdi‘erent effect has been observed
for
solutions8 where a
ltration technique failed to inuence the
ability of the solution to nucleate around— 10°C. This result may be interpreted either
as nucleation by particles with“ adsorbed solute of size less than 10 milli—microns or as
nucleation by the Solute' molecules themselves.
,
.
‘
,
_
Nucleation of solutions may take place in two Ways, depending on concentration of. the
solute and on
If a saturated solution
cooljed,the‘n it may—become not '
only supercooled with respect to the ice phase,; but
'solute (gure 7). In
absence of aîppr0priate freezingçnucle‘i,
by (say) 30°C. Similay, in the absence of…appropriate
mayalso supersaturaté3; 'Hence
Willibe possible * for—the