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Unformatted text preview: Nucleation of Water by Hydrophobic Silicas ITILIER SALAZAR' AND LUIS SEPULVEDA‘ Memento tie Quintin. Fucked J: Garcia: Bosh»: y Femaeérrrim. Universidcd d: Chit:
to: Palm 3425. Guide 653. Somme. Gritt- Received July 3!). I982; accepted December 2. 1982 INTRODUCTION In the process of nucleation the structure
of water may play a fundamental role. Ac-
cording to the model proposed by Frank and
Evans (l) and by Nemethy and Scheraga [2)
liquid water has a considerable number of
water molecules with an ioelike structure
which increases as the temperature is de-
creased. 'l'hesc same authors have also sug-
gested the existence ofsolutee which destroy
the iee~lilre stntcture and others which pro-
mote it. Hydrocarbons are among these latter
solutes. On the other hand, Zettlemoyer at
of. (3) in their study on the adsorption of
water onto silver iodide have also mod
that a nucleatins agent must possess “the
common attribute of having hydrophilic ad-
sorption sites each located in a matrix of hy-
drophobic sites.“ In a further work (4) the
me author has concluded that water clus-
ten; are more easily formed on surfaces that
appear mildly hydrophobic The above picw
tun: annealed to us that the umtsition from
liquid water to ice under supercooled 00n-
ditions could he favored by the preemoe of
hydrocarbon on the nucleating surface. In this paper we shew that silicas contain-
in; methyl residues act as nucleation agents
for the transition from liquid waterto ice and ' On leave from Unidnd de Qinmiea. Univentdad de
la Fromm, Temueo.C1Iile. ’Aodlortouhomallcorrewondenceslumldbend-
dressed. - 1'0 002 I #79733 53.00
Comvwc Imhmm In.
All rigor crane-admin in any [win luerwd. that the nucleation capability is a function
of the degree of methylation. EXPERIMENTAL
Materials Aeroail 200 from chusa. Germany. of
200 m1! (BET) with an average particle size
of 120 in diameter and with three-SiOH
groups per 100 A’ ofsurfaoe (S) was cleaned
with concentrated nitric acid at 60°C, washed
with distilled water. and dried at [15°C dur-
ing 24 hr. Total methylation of Aeroeil 200
was obtained by refluxing with a substantial
excess of trimethylchlorosilane (TMCS)
Hoploin and Williams for 5 hr. This neat-
ment. however, gave the same degree of
methylation as one done in the same con-
ditions for no more than 5 min, suggesting
that the ailylation reaction is that and goes to
completion in a short time. Hence, different
degrees of methylation were obtained by
treating a known amount ofAerosil 200 with
diï¬â€˜erent amounts of 'IMCS in benzene. The
obtained methylated Aerosil (MA) samples
were washed with benzene and dried under
vacuum at 30°C. Inflow-ed Spectra PelletsofMA werepreparedatabout ll
tons of creature per sq. inch and their in-
frared spectra recorded in a Perkin—Elmer
62] infrared spectrophotometer in at least
four positions. Jamaal qcmm rm am Vol. at. Ha I. Jul! rm NUCLEATION or WATER 7] Contact A ngtes The pellets used for [R studies were also
used for measuring contact angles with water.
A small drop of distilled water was delivered
with a microaylinge on the surface of the
pellet and its image projected an a screen on
which heighthandbasebofthedropwere
measured. The contact angle ll wascalcn lated
by use of the equation lg 612 = 2m. A contact angle of [05" was obtained with
this technique for water placed on a was sur»
face in agreement with reported values (6). Heels of Immrsim A sealed glass ampul containing the decio
cated MA sample was placed in a 250-ml
Dewar flask ï¬tted with a thermistor sensor
connected to a KeithleglI multimeter model
lTZ-A. A known volume of water was added
and the resistance of the thermistor recorded
aflerhmakingthcampulwithaglass rod.
The enuipment was calibrated by mixing in
it two known volumes ofwater at two dif-
ferent temperatures close to 25°C and re-
cording thc changes in resistance of the
thermistor. Adsorption of Water The adsorption of water by the dilt‘erent
samples ofMA was measured by introducing
a weighed amount of each MA sample in a
desiccator at 25 1 O.l°C which contained at
the bottom a solution of a given concentra-
tion of sulfuric acid corresponding to a cer-
tain known value of water vapor pressure.
The samples were weighedevery 12 hr until
a constant Widtt was obtained. The ad-
sorbed water per gram of MA was then cal»
ctdated. Nucleation Temperatures The nucleation from drops of liquid water
to ice was followed-by measuring the tem-
perature at which the phase transition from liquid water to ice occurred in presence and
absence of the samples of MA with different
degrees of methylation. A cooled nucleation
chamber ï¬tted with a light source and a cal-
ibration then'nistor connected to an elec-
tronic thermometer ISl model 8502-20 was
used. The chamber. 26 cm in depth and 15
cm in diameter and whose temperature var-
ied from -4l°C at the bottom to about 0°C
at the too. had three holes on its glass stopper.
In one hole was the thermistor, in another,
the injector of air saturated with water at
about 60°C, and in the last. a ring ofcopper
with a ï¬lm ol'a detergent solution (Triton X-
100) on which the transition of the water
droplets to ice was clearly detected. The ring
with the ï¬lm was raised or lowered to reach
the zone of maximum temperature at which
the nucleation takes place. Tile thermistor
was then located at this zone in order to re-
cord the temperature ofthe phase transition. Samples of MA were sprayed into the
chamber containing a supercooled cloud and
the temperature of nucleation measured. At
least eight determinations were carried out
for every sample. The apparatus described
was checked with a silver iodide solution in
acetone which was sprayed into the cham-
ber. The nucleation temperature determined
(-S'C} was in agreement with reported val-
ues (7). RESULTS AND DISCUSSION The IR spectra of Aerosil 200 show a char—
acteristic band at 3740 em‘l corresponding
to the stretching of the —SiOI-l groups. This
hand gradually disappears in the MA samples
as the amount ofTMCS used for methylation
was increased and the appearence of a new
band at 2940 cm" wasohserved. The degree
of methylation preach sample was calculated
on the basis that the intensity of the band at
2940 cm" is proportional to the number of
the methylated groups and that the sample
treated under drastic conditions of methyl-
ation has [00% methylation. An attempt was
made to obtain difl‘erent degrees of methyl- Mqtcmmrnm aim V190.No.h]ul1l0‘ll T2 samurai: AND SEt'LZILvem client by stopping the reaction or a concen-
trated solution of TMCS and Aerosil by ad-
dition of water at different times. but the
degree of methylation found with the IR
method showed that a loose. methylation was
observed at no more than [0 min suggesting
that the reaction is fast and the degree of
methylation better controlled by mixing
known amounts of TMC‘S and Aerosil. The contact angles of the different samples
of MA increase from zero for a nonmethyl-
ated Aer-psi! to [36° for the sample corre—
sponding to a 100% methylation indicating
that the rnclhylation process is accompanied
by a concomitant increase in the hydropho—
bicity of the MA samples. (Table I}. Table I also shows that the nucleation tem-
perature of water to ice in the presence of
MA samples increases as the degree oft-neth-
ylation increases, as would be expected ac-
cording to our hypothesis that hydrocarbons
promote the transition from water to ice. It
is noteworthy in the cable that the hilly meth-
ylated sample has no nucleation properties.
This sample is also nonwettable and floats
when it is put on water. It follows that a
maximum nucleation capability is obtained
at a certain degree of methylation (about
16%) strongly suggesting that nucleatirtg sur-
faces require both hydrophilic and hydro-
phobic properties as has been pointed out by
Zettlemoycr e! at. (3, 4, 8]. A mechanism of nucleation in which wa-
ter drops already existing in the cloud collide
with the nucleating particles giving rise to the transition ofwater to ice drops was discarded. on the basis that hilly methylated silica has
no e‘ec't on the nucleation temperatures. In-
stead. our data support a mechanism in
which the supercooled vapor water con-
denses on the surface ofthe particles and the
watcrdropsao formed undergoarapidtran-
sition to ice. Adsorption of water vapor on
hydrophilic —SiOH groups followed by a
multilayerl‘onnation similartocondensation
would lead this liquid water to acquire ice
structure when the onndensated layergrowths
reach the neighbor hydrophobic methyl MJCMMIWW.V1% No. Infill! rm TABLE [ Nucleation Tempeflture Tend Contact Angles & for
the [3than Samples of Mc‘lhylated Aaosil 1" Mill! Mini-Ilia {Tl I
0.0 -20.0 0.0
2 . 'l' - la . 2 3]
5.6 - I SA 50
85 — MA 60
19. Z - â€.0 62
3?. I - I23! 66
52.4 - I L4 1'0
63.0 — 10.6 87
35.3 -9.5 91
34.8 - 10.5 I 26
I00 ('40? l3b ‘ Nucleation temperature ol'purc water (l0). groups. The condensation of water vapor
only in the -SiOH groups is supported by the
fact that the heats of immersion presented
in Fig. l decrease with the degree of meth-
ylation and that the average heat of immer-
sion per every free silanol group is a constant
(T able ll) indicating that in the MA surfaces
the methylated groups do not contribute to
the heats of immersion and therefore are not
hydrated. A similar conclusion can be ob-
tained from the adsorption isothenns of wa-
ter on the MA samples which are shown in
Fig. 2. It can be seen that the water adsorption
drastically decreases with an increase of the
degree ofrncthylation and that the isotherms
are Langmuir type at low vapor pressures.
However. deviations of this behavior is ob-
served at higher vapor premres suggesting
the formation of multilayers induced by the
alreadyr existing monolayer formed around
the hydrophilic silanol groups. Similar results
have been reported by Cortes at a}. (9). The
monolayers in the Langmuir region are
formed at low relative water vapor pressure
for samples of high methylation degree while
higher water pressures are required for the
formation of a monolayer in Surfaces with low methylation coverage or high content in Hears or IMMERSION cal/gr
.- In! Flt
of mi. free sila
sorbcd it
easily in
methylat
water in
therefor: process. The I
obtained
is close '
packing
pletely r
groups ‘
age dista
ylatod or
means 11
ylation r
ceulcs at
between
which iii In su
molecuil
silent)! 5
layer ext
group at nuct angles a for
tilted Aemsil 0.0 SSBSSB 8?
9!
[M
[36 water ([0]. F water vapor
ppm-led by the
:ion presented
gree of meth-
eat of immer-
D is a constant
a MA surfaces
contribute to
alone are not
u out be ob-
therms of wa-
are shown in er adsorption
torus: of the
the isotherms
)or mums.
rho ‘ is ob-
es ' 3
thread by the
med around
Limilarresults
't of. {9). The
‘ region are
tpor pressure
degree while
mired for the
airlines with
;h content in NUCLEATION OF WATER 73
i.
ll!
L“
a 5
U
z
9
In
El:
E
E 3
II.
0
an
E
I 1
10 20 30 40 50
$METHYLATION FIG. I. Heals of immersion {in calls] as a ï¬tnction of the dogma of melhylalinn for different samples ‘ or mama amt 2m. free silanol groups. It Follows that the ad-
sorbed water monolayer is completed more easily in the samples with a high degree of methylation and the contact of the adsorbed
water molecules with the methyl groups is
therefore favored givins'rise to the nucleation
process. The maximum nucleation capability is
obtained at about 76% of methylation, which
is close to the 67% expected for hexagonal
packing in which one —SiOI-l group is com-
pletely surrounded by methylated silanol
groups. This conï¬guration leads to an aver-
age distance between a surface gmup (meth—
ylated or not methylated) of about 7 A. which
means that in the sample with T61: meth-
ylation no more than four or ï¬re water mol-
ecules are required for cavering the distance
between a -SiOH and a —Si0Cl-I, group on
which the transition to ice-eon be promoted. In summary. it is proposed that water
molecules are adsorbed on the hydrophilic
silanol groups and that the adsorbed multi—
layer extends apart until it reaches a methyl
group around which the liquid layer is Han-‘5' formed into ice. giving rise to a rapid nu-
cleation procm
The fact that silver iodide continues to be
a better nucleation agent than methylated
silieas means that the ï¬t bemoan the crystal
_ parameters of ice and silver iodide are still
of some importance in spite of the fact that
other substances such as lead iodide have a
better ï¬t with ice crystal but are poorer nu—
cleating agents than silver iodide (8). TABLE II Hunt: of Immersion of the Methylated AerosiJ pet
Gram of Sample (lull Ind per mole of free «Sim-l
groups calculated assuming Three -SiOH Groups per
every IN A: (ï¬ling...) an mm
mum-u- mil-Inn intro txuuml
0.0 6.0 6.0
1.7 5.2 5.8
5.6 41.? 5.?
8.5 4.1 15
31.2 1.0 5.8
52.4 or! 5.? Moat-[CM and rmm \fnl. 9|. No. I. July I9!) 74 SALAZAR AND SEI’ULVEDA .2 '4 .3 .3 I FIG. 2. Adsomionofmincttsmperm “ml matfllegivun dare: nfmlfllylllinn usfunuiondwenluhewcrmwï¬. Our results are consistent with the ï¬ndings
of Zetflemoyer er a1. (3. 8), who demon-
strated that the nuclaation properties of silver
iodide are mainly due to its hydmphobic
characteristics. In this work we have centered our atten-
tion mainly on the mechanism ofnuclaation.
However, the results obtained indicate that
spatially methylaudsilimm beseriously
considered as a lowmst and ecologically safe
ruminant for cloud seeding and weather mod-
iï¬cation. ACKNOWLEDGMENTS Support ofthis work by the W6: Dwar-
mllu dc la Invesï¬la'l‘jï¬n d: In Univenidul dc DIE]: is wfully W. The authors Ilsa than]: M;
Eli-m Vitlmrorhamlllbomlonin mkinsthe
mun-im- WVCMWIWM "(£94. No. thy M: REFERENCES 1. Frank. H. S..md Evans. M. W.. J. Chan. Phw. 13.
50? “9451.
2. Nhueth‘r. 0.. and Schema. H. 6.. J. Hâ€. Chan.
36. MI “9611:36. 3382 [I961].
3. Taleurelndjian. N.. htlhmm. A. C.. and Cheap
sick. J. 1.. J. Phys. Chem. 68. 113 {1964).
4. Klict. K.. and Zettiemoyer. A. C. J. Colloid {mer-
ï¬me Sci. 58. 2I6 (197?).
5. Wm. Industries Techuiml note.
6. Manson. A. W†"Physical Chemistry “Surface-G.“
3rd ed" p. 352. Wilcyhlntcrlcicnce. New York.
I976.
‘3. Mason. B. 1.. "The Physics of Bonds." Oxford
Univ. Hm. London. I957.
8. thtlcnloyer. A. C.. leeunkdjian. N.. and Healer.
C. L. 1 Am. Math. Phys. I4. 496 {£963}.
9. Mammal. l... Contreras. 3.. and Curtis. .1..J. Caf—
a‘oa'd {madam Sci. 50, 503 {WIS}.
ID. She-dirt. V. 1.. Science I“. 45? [[946]. ...
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