107
CHAPTER 5
EFFECT OF SOLUBLE SALTS ON NUCLEATION
BEHAVIOUR OF Agl-AgCl-Cul
5.1 INTRODUCTION
As it has been already mentioned in Chapter I in cloud physics we are
concerned with three principal phase changes namely, vapour-liquid phase
transformation,
liquid-solid
phase
transformation
and
vapour-solid
phase
transformation.
It is to be noted that all these phase transformations require
nucleation. It is also well known that atmosphere contains a variety of particles in
abundance, of which some may favourably induce nucleation heterogeneously.
Heterogeneous nucleation of ice from supercooled water can take place by a
variety of mechanisms depending upon system variables and the exact nature of the
surface of the nucleant involved. A good amount of laboratory and field work have
been done in the last few decades to understand the phase change of water vapour
and liquid water into ice crystals in the atmosphere. In fact, experiments have shown
that an efficient ice nucleant should be relatively large in size and should be water
insoluble, with crystallographic properties similar to that of ice. It should also have
surface emanating bonds that are similar to the O-H-O bond in ice [135].
It is generally an accepted fact that the ice forming particles are a minor
fraction in the population of atmospheric particles. Still they play an important role
in assisting the onset of the ice crystals. But field observations have revealed that this
is not always true [136,137]. Upto about 4 orders of magnitude more of ice crystals
108
than the ice nuclei have been reported in some cumulus clouds depending upon how
near was the cloud top temperature. At present, it has been suggested that ice nuclei
act in three modes. The first is sublimation transformation, in which a direct transition
from the vapour to the solid (ice) occurs on a foreign particle (ice nucleus) in an
environment of supersaturated vapour without the formation of intermediate liquid
phase. The second is condensation of freezing nucleation, which initially requires
condensation of liquid water from the vapour on the surface of the nucleant and the
liquid freezes to form ice crystals. The third known mechanism is contact nucleation,
where the induced freezing of supercooled droplets by foreign particles. For a given
set of physical conditions, it is yet to be established which of these possibilities has
maximum efficiency.
The probability of finding a pure water droplet in the atmosphere is negligible.
Naturally condensed cloud nuclei (CCN) generally contain soluble salts, since cloud
droplets collect additional soluble aerosols throughout their life time. Salt content is
an inescapable factor which will have bearing upon the heterogenous freezing
nucleation of cloud droplets in the atmosphere [138]. Furthermore, the generator
effluent that contains appreciable amount of soluble salts largely affects the ice
nucleating properties of the seeding materials.
More relevant to natural or induced precipitation processes in cloud are
possibly the studies made with specific nucleants along with soluble salts. Hoffer
[139] examined the effect of a mixed solution of MgC^ and Na£S04 (to simulate
natural CCN) on the nucleation temperatures of Agl, illite, halloysite and kaolinite.
He found that nucleation temperatures were depressed, the depression being greatest
for saturated solutions and barely noticeable for solutions at 1/1,000th of saturation.
Heverly [140] studied the freezing of drops of water and dilute soluble salt. The
differences between the freezing temperatures of water and the salt solution have
been studied, taking the freezing temperature of water as the standard. Contradicting
the results of Heverly, Lafarque [141] has reported that the freezing temperatures of
the droplets of the salt solution were 6°C higher than the freezing temperature of pure
water. It was found that salt solutions of concentration 0.01 N froze around -9°C.
109
Kiryukhin and Pevsner [142] found that the droplets formed from saturated
solutions of sodium chloride froze below -50°C. Also, the droplets of HI and HF
solutions were found to freeze at higher temperatures whereas droplets of CsF, BaC^
and NaCl solutions froze at lower temperature than pure water [143]. Bigg [144]
investigated the effect of varying the concentration of the solute on the freezing
temperature of water droplets. It was noticed that the median freezing temperatures
become less with the dilutions of the solute concentration. Hosier and Hosier [145]
reported that the increase in nucleation temperature was large for solutions of NH4I
and W, while NaCl depressed the nucleation temperature.
After testing the freezing temperature of a number of solution at salt
concentrations of 10'^, 10"2 and 10'1 moles per litre, Pena et at [146] found that the
droplets of iodide and fluoride solution froze at much higher temperatures than pure
water drops. In the case of NH4I, nucleation temeprature could be elevated by 10°C.
However, solution droplets of chlorides and bromides were found to lower the
freezing temperature. Evans [147] observed that the nucleation temperatures of
phloroglucinol dihydrate were raised in NaCl solutions of varying concentrations. He
even suggested that the increase in salt concentration induced two-dimensional
crystallization of the monolayer.
By drop freezing technique, Reischel and Vali [138] examined the influence
of about 20 soluble salts on the nucleating abilities of leaf derived nuclei, kaolinite,
cupric sulphide and silver iodide. They found that the salt contents of the cloud
droplets can be expected to alter the ice nucleation ability of the suspended particles
by as much as ± 4°C.
In order to have an overall estimate, the results of the
experiments which used, purified water are presented in Table 5.1 [138].
Gobinathan and Ramasamy [73] studied the ice nucleating behaviour of PW2
in the presence of some soluble salts. They found that among various iodides tested,
NH4I has remarkably increased the nucleation activity of PW2 by about 2°C. Recently
Palanisamy et al [148] examined the nucleation activity of Agl-AgBr-Cul in the
presence of soluble salts. They observed that the presence of soluble salts, in general,
Table 5.1
:
Summary of materials found active or inactive in promoting nucleation of "purified" water
110
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retards the nucleating activity of the material. Also, the activity is found to decrease
with increase in concentration of the salts.
From the survey of literature it has been found that the heterogeneous freezing
nucleation can be appreciably influenced by the soluble salts and the effects were
found to be specific to each nucleant tested. Also, controversy exists with respect to
the effect of a soluble salt on the freezing temperature of a droplet. Moreover in
Chapter 3 the author has reported that the 50:25:25 mole percent composition of
AgI,AgCl and Cul has the lowest misfit with ice and hence the highest nucleation
temperature (-0.9°C). In order to evaluate the solute effects on this specific Agl-AgClCul fusion nucleant, the present study has been undertaken. This will help to draw
a conclusion of the influence of soluble salts on the ice nucleability of Agl-AgCl-CuI.
5.2 SOLUBLE SALTS CHOSEN FOR STUDY
It is more relevant to examine the soluble materials which form the bulk of the
soluble compounds in natural aerosols.
Alkali halides such as sodium chloride,
magnesium chloride and sodium sulphate were chosen, since they include a number
of components of sea salt, which is one of the main sources of hygroscopic aerosols.
In addition, the halides of NH4 and Li, die sulphates of K, Na, NH4 and Mg were
considered. Byers et al [149] analysed the samples obtained during air craft flights
for the number of salt particles - chlorides and sulphates in the atmosphere. Twomey
[150] has found that in air, many salt particles associated with dust or clay and they
are of composition similar to ammonium sulphate.
5.3
5.3.1
EXPERIMENTAL DETAILS
Preparation of the samples
The fusion sample (nucleant) of 50:25:25 mole percent composition of Agl,
AgCl and Cul was first ground into fine powder using an agate mortar and pestle.
As the size of the particles is one of the important factors in the determination of the
112
threshold nucleation temperature [47], the sample powder size was maintained the
same (45//) throughout the experiment. For preparing the salt solutions, analytical
reagent grade salts were used. Salt solutions of 0.01, 0.1 and 0.5 M concentrations
were prepared first by weighing and dissolving appropriate amounts of salt in triple
distilled water. To remove, as many water insoluble particles as possible the solutions
were filtered through Grade 1 wattman filter paper. Just before the experiment, 50
mg of finely dispersed sample was suspended in 50 ml of the salt solution and stirred
well.
5.3.2 Ice nucleation studies
The freezing nucleation temperatures were measured. The detailed procedure
has been described in section 3.3 of chapter 3. The median of 20 observations with
a fluctuation of ± 0.2°C was taken to be the nucleation temperature in each salt
combination with the nucleant of each concentration. The experiment was repeated
and the mean of two readings was taken. Extreme care was taken to maintain the
time scale of the experiments, without which a proper comparison and interpretation
of the experimental data may not be possible. The time allowed in the tests for the
interaction of the nucleant with the solute ions was kept constant at about 10 minutes.
It is to be mentioned that this is the minimum period required for commencing the
experiment after a suspension is prepared. Throughout the investigation, this
procedure was uniformly adopted.
5.4 RESULTS
It has already been shown in section 3.7.1 of chapter 3 that the freezing
temperature
of the undoped nucleant, viz Agl-AgCl-Cul in triple distilled water
is -0.9°C. In the present study, the various salt solutions at different concentrations
with the nucleant have shown an appreciable influence on nucleation temperatures.
Among the soluble salts examined, the sulphates have better performance with
nucleant at 0.01M concentration. As a matter of fact, {NH/^SC^ at 0.01 M
113
concentration has significantly improved the nucleation activity of the nucleant and
the nucleation temperature is as high as -0.6°C.
All the chlorides in the concentration range of 0.1 M and 0.5M exhibited large
supercooling for freezing nucleation. But at the same time when the concentration
is 0.01 M, all chlorides show considerable improvement in the supercooling. They
have produced more or less the same effect and the supercooling is around -0.8°C.
The fluorides are bromides at 0.01 M concentration have exhibited basically
similar behaviour. They have exhibited larger supercooling at all concentrations, so
the nucleation activity of the Agl-AgCl-Cul nucleant has been largely reduced. The
nucleating performance of fluorides as additive with Agl-AgCl-Cul is better when
compared to that of bromides.
Sodium iodide has caused very small supercooling at 0.01M concentration
when compared to ammonium, potassium and lithium. For the same concentration
of 0.01M of the iodides of K, Li, NH4 and Na, KI has produced the largest
supercooling.
Further increase in concentration of the iodides has reduced the
nucleation activity of the nucleant considerably.
5.5 DISCUSSION
5.5.1 Comparitive effects of different sulphate salts
The presence of soluble salts with ice nucleant, in general increases the
supercooling. But at the same time exceptions are not rare and are of special interest.
It is clear from the present investigation that Agl-AgCl-Cul nucleant suspensions
containing sulphate salts have shown extreme variation in nucleation response when
experimented to different types of salts having different concentrations.
The author in the present study has observed that, the (NH^ SO4 has
improved the nucleation activity of Agl-AgCl-Cul nucleant when compared to other
114
sulphates of Na, Mg and K. The results are shown in Figure 5.1. The observed
feature can be explained qualitatively as discussed below. The metal cations and
sulphates are widely separated by the water molecules i.e. 2 Na+(10H2O) SO42'.
Thus a weak interaction exerted by the cations and anions on the water molecules
makes them easy to aggregate on nucleant. Since high nucleation temperatures were
observed with sulphate ions, the ionic charge should have some influence on the
freezing temperature. The SO^" has two negatively charged oxygen atoms and two
S=0 groups with lone pair of electrons. Water molecule has a lone pair of electrons
on its oxygen atom. Consequently, water molecules are bound strongly to SO^"
group.
The addition of sulphate to Agl-AgCl-Cul could, therefore, improve the
nucleation activity of Agl-AgCl-Cul. Moreover, all the cations have structure breaking
effect except NH4+ which has the least effect because of its charge being directed
tetrahedrally along the N-H bonds. NH4+ ion is bigger than K+ ion. On account of
this (NH4)2 SO4 would be expected to exhibit greater supercooling than K2SO4.
The other sulphates namely K, Na and Mg show an improved nucleation
activity on Agl-AgCl-Cul in the following order : K2SO4 > Na2S04 > MgS04- This
increased activity of K over Na can also be seen from the values of molar
polarisability, a factor which is responsible for the improvement of the nucleation
activity. The molar refraction of K2SO4 and Na2SC>4 are 19.1 and 14.8 as calculated
from the values of physical constant. Further, Mg++ ion has 7 water molecules
(7H2O) to form Mg2+ (7H2O) S042'. Since it has less hydration, the supercooling
produced by MgSC>4 would be small, but higher than that due to (NH^ SO4 and
K2SO4 salts. All these explanations are found to be true in this investigation.
5.5.2 Influence of the chloride salts on Agl-AgCl-Cul
NaCl, at a concentration of 0.01M has shown an increased nucleation activity
among the chlorides of Na, K, NH4, Li and Mg (Figure 5.2). The highest nucleation
temperature observed for NaCl with ice nucleant is -0.8°C. The earlier reports from
literature show [141,143,146] that NaCl needs more supercooling with the nucleant.
FREEZING TEMPERATURE
( #C
)
115
FIGURE. 5.1
FREEZING TEMPERATURE AS A FUNCTION OF
SALT CONCENTRATION OF THE SULPHATES
WITH ' Agl-AgCt Cul' NUCLEANT.
FREEZING
TEM PERATURE (°C )
116
SALT CONCENTRATION
FIGURE.5.2
(mol. I"1 )
FREEZING TEMPERATURE AS A FUNCTION OF
SALT CONCENTRATION OF THE CHLORIDES WITH
' Agl-AgCl-Cul ' NUCLEANT.
117
In order to confirm our results with the present nucleant, the experiment with NaCl
was repeated many times and the readings were found to be consistent.
The increased nudeation activity of chlorides can be explained as follows. It
is known that when electrolytes are dissolved in water, they are partly or fully
dissociated into ions. Due to the interaction between the electric field of the ions and
the electric dipole of the water molecules, water molecules behave in a manner similar
to that of the molecules in the ice structure. In this way, bonds between the oriented
molecules and the neighbouring molecules are broken [70]. Most of the ions do not
sit perfectly into the water structure, since breaking of water structure also takes place.
As such, the water lattice is either expanded or contracted by means of the electric
forces of the ions. Thus, the disruption of the water lattice and the breaking of the
bonds cause an increased number of vacant lattice sites and interstitial molecules.
Moreover, molecular shape and size are important for the activity of the ice nuclei
and in the formation of surfaces where the density of hydrogen bonding groups is
high. Crystal lattice and flexibility of hydrogen bonding groups may further help to
reduce the strain energy for embryo formation [35]. In order to the formation an ice
nucleus of critical size in supercooled water, ice-like clusters have to be formed among
the water molecules. When the water lattice is more disrupted and more bonds are
broken, the formation of such a cluster becomes very difficult.
It is believed that larger the size of the ion, greater is the polarizability. This,
in turn, causes greater structure breaking effect. The ionic radii of Na+, K+, NH4+,
Li+ and Mg++ are 0.097, 0.133, 0.143, 0.068 and 0.066 nm, respectively. Since the
Na+ ion is smaller than K+ and NH4+ ions, the structure breaking effect of Na+ ion
is very small. Similarly, the structure breaking effect of K+ ion is smaller than to
NH4+ ion.
Greater structure breaking effect will ultimately lower the nudeation
activity of the material. The experimental results fully agree with the above statement.
It is found that at 0.01M concentration, NaCI has improved the nudeation activity of
the system. The structure breaking effect may, perhaps, be the least in this case.
118
Eventhough Li+ and Mg+ + ions are smaller than Na+ ion, they cause greater
supercooling at all concentrations. This is due to the process of hydration. It is well
known that smaller the ion, greater is the hydration. During this process, greater
energy of enthalpy will be released. So, in the case of such substances, the process
of hydration which sets in after the crystallite comes into contact with water, continues
until saturation is reached. This characteristic of hydration is attributable to the
diffusion of water molecules into the interior of the crystallite, which is invariably timedependent and reaches saturation finally [151,152]. As a result, the aggregation of
water molecules which facilitates ice formation, will be fast in the beginning, slows
down thereafter and ceases finally. The decrease noticed in the nudeation activity in
tire case of LiCl and MgCi2 is a dear experimental evidence. Also Mg++ ion has two
units of positive charge as well as smaller radius than Li+ ion and hence MgC^ has
produced more supercooling than LiCl.
5.5.3 Influence of the fluoride salts
The fluorides have exhibited smaller supercooling at 0.01M concentration
(Figure 5.3). When the concentration is increased further, the supercooling of the
system is also found to increase. This is expected because any dissolved substance will
lower the freezing nudeation temperature more at higher concentrations. Comparing
the results produced by the chlorides and fluorides on the activity of the nudeant,
one can conclude that when (Figure 5.2 and 5.3) cations are the same but the anions
are different, the smaller F' ion (0.133 nm radius) has reduced the nudeation activity
at all concentrations.
The bigger Cl' ion (0.181nm radius) has caused lesser
supercooling. This can be accounted by the fact that the smaller F' ion has greater
hydration. In such case more energy of enthalphy is released. This release of energy
may ultimately hinder the ice formation. Hence, the fluorides have produced more
supercooling than chlorides.
FREEZING TEMPERATURE
( °C
)
119
SALT CONCENTRATION ( mol. I"1)
FIGURE . 5.3
FREEZING TEMPERATURE AS A
FUNCTION OF SALT CONCENTRATION OF
THE FLUORIDES WITH ' Agl - AgCl-Cut'
120
5.5.4 Influence of the bromide salts
The influence of the different bromide salts on the nucleation activity of the
Agl-AgCl-Cul system and the results of the measurements are represented in Figure
5.4. It is observed that the nucleation activity of the system has been largely reduced
at all concentrations. As the Br ion is comparatively bigger (0.196 nm radius) than
Cl' and F' ions and the cations being the same, the disruption produced in the water
lattice is more. This would necessarily be responsible for greater supercooling. This
is in accordance with the experimental observations.
5.5.5 Influence of the Iodide salts
It is observed (Figure 5.5) that the freezing temperature of Agl-AgCl-Cul system
has been differently affected by the presence of different iodides. Ice formation on
the ’solute adsorbed’ nucleating substrate depends on the aggregation of water
molecules. The extent to which aggregation of water molecules takes place around
the adsorbed ion depends upon the polarizability of that ion, its electronic
configuration and the polarizing power of the oppositely charged ion in the pair.
Weyl [152] observed that a liquid surface exists in the most polarizable form since
polarizability permits an adjustment of the force field of the surface ions. As a result,
the lowering of the surface free energy is a function of the polarizability of the ions.
The presence of more polarizable ions are molecules on the substrate will reduce the
amount of supercooling, a favourable condition to form ice crystals. Therefore, it is
expected that the nucleating capability of the present nucleant will be affected
differently by the different iodides, depending upon the polarizing powers of the
cations. This is found to be true at 0.1 and 0.5M concentrations of the iodies except
KI, which exhibits very high supercooling. Also, at 0.01M concentration, KI behaves
in the same fashion whereas Lil has shown abruptly a reverse trend. Such behaviour
is not quite uncommon and it is the characteristics of the iodides as has been
observed with other nucleant such as Agl [138,153,154]. In cold cloud modification
programmes, Agl is widely used, the products of generator systems using KI or Nal
have been found to consist of complex salts of Agl and KI or Agl and Nal [155,156].
FREEZING TEMPERATURE
{ *C )
121
FIGURE.5.4 FREEZING TEMPERATURE AS A FUNCTION OF
SALT CONCENTRATION OF THE BROMIDES
WITH 'Agl - AgCt ~ Cul* NUCLEANT
FREEZING TEMPERATURE
(
*C
)
122
SALT
CONCENTRATION ( mol l-')
FIGURE.5.5 FREEZING TEMPERATURE AS A FUNCTION OF
SALT CONCENTRATION OF THE IODIDES
WITH 'Agl-AgCl-Cul' NUCLEANT
123
The soluble parts of the generator products were believed to decrease the nucleation
activities of the Agl particles. Similar processes are also expected to happen with the
present nucleant. Also, the high ionic character of KI may be the reason for the large
supercooling.
5.6 CONCLUSION
In the present study, the observations have revealed that the presence of
soluble salts, in general, retard the nucleating activity of the material. Also, the
activity is found to decrease with the increase in concentration of the salts, due to the
production of water disruption and breaking of bonds by the salts. It is noticed that
the nucleant in combination with (NH^ S04, at 0.01M concentration shows highest
nucleating ability. As the data for the solute concentration in cloud droplets are not
readily known, the importance of this experimental finding is not fully understood.
However, as a first approximation, taking the salt concentration present in the natural
aerosols to be a maximum of 0.01M, the combination of the nucleant Agl-AgCl-Cul
and (NH4)2S04 can be hopefully expected to be a better substitute for Agl in weather
modification experiments. This assumption can also be justified because the cloud
volumes which are saturated with respect to water, present the best opportunity for
droplets to freeze as the salt solution will be the most dilute in these volumes. Hence,
these criteria are to be taken into consideration before entering to any effective cloud
seeding operations.