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ISQ-Paper-FQSuperficies - Nucleation of Water by...

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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 difi‘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 fitted 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 fitted 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 film ol'a detergent solution (Triton X- 100) on which the transition of the water droplets to ice was clearly detected. The ring with the film 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 - ”.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 fitnction 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 configuration 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 fire 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 fit 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 fit 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: (filing...) 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 usfunuiondwenluhewcrmwfi. Our results are consistent with the findings 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- ification. ACKNOWLEDGMENTS Support ofthis work by the W6: Dwar- mllu dc la Invesfila'l‘jfin 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. H”. 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- fime 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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