Jan., 1961
ADSORPTION OF WATER AND POLAR PARAFFINIC COMPOUNDS ONTO RUTILE
109
ADSORPTION OF WATER AND POLAR PARAFFINIC COMPOUNDS ONTO RUTILE' BY C. M. HOLLABAUGH' AND J. J. CHESSICK Surface Chemistry Laboratory, Lehigh Unwersity, Bethlehem, Pa. Received June 90, 1060
The heats of immersion of temperature activated rutile have been measured in water, n-propyl alcohol and n-butyl chloride as a function of the amount of wetting liquid preadsorbed. Vapor phase adsorption isotherms were Type I1 for water and n-butyl chloride and Type I for n-propyl alcohol. Isosteric heats of adsorption were calculated. These data showed the futility in comparing values of the heat of immersion of a clean solid, hI(sL),immersed in several liquids without further data. The assumptions made too often in the past in studies where adsorption and calorimetric data were incomplete that physical adsorption predominates, orientation and monolayer capacities can be estimated, and that hI ( S L ) near monolayer coverage is equal to hL, etc., in order to estimate heats of adsorption were shown to be invalid for some and perhaps many other systems reported in the past.
Introduction Many investigators have reported heats of immersion of several evacuated solids in water and a variety of organic liquids. In most cases, no adsorption data were available to determine the amounts adsorbed. As a consequence, assumptions have been made regarding the monolayer capacity, nature of the energetics of the adsorbed film and orientation, the heat of immersion value of the monolayer covered surface and others. The present investigation was initiated to evaluate the validity of these assumptions by obtaining detailed data and descriptive models for the adsorption of water, n-propyl alcohol and n-butyl chloride on rutile (TiOz) activated in vacuo a t about 500".
Experimental Adsorbates.-The adsorbates used in this study were nbutyl chloride, n-propyl alcohol and water. Water was purified by distillation and had a specific conductance of 2 X 10- ohm-' cm.-l. The two organic liquids, obtained from Matheson, Coleman and Bell, East Rutherford, New Jersey, were fractionally distilled. Middle fractions of nbutyl chloride and n-propyl alcohol boiling a t 77.477.5' and 97.5-97.8", respectively, were used. The boiling points by Stull2 are 77.8 and 97.8' for pure n-butyl chloride and npropyl alcohol. Trace amounts of water remained in n-butyl chloride even after distillation. This water was removed by storing over activated silica in a dry box. Adsorbent.-The rutile (Ti02) sample was furnished by the New Jersey Zinc Company, Palmerton, Pennsylvania. This sample was prepared from purified TiClr and water. After calcining a t 850", the powder was ball-milled in methanol, dried and remilled in the dry state. The results of chemical analysis showed the fol1oP;ing per cent. impurities: Si, 1.0; Ca, 0.01; Al, Sn, Mg, 0.001; B, 0.001; Cu, Fe, Pb, 0.0001. X-Ray analysis showed the sample to be almost entirely in the rutile modification. A surface area of 13.4 m.*/g. was determined from nitrogen adsorption data by the conventional BET method. Three groups of samples were activated a t a number of different conditions by evacuation to a final pressure of 104 mm. on a high vacuum apparatus. Heats of immersion in water were measured next to determine the effect of varying activation conditions and to define the conditions necessary to yield a reproducible surface. The first group was activated a t 450' without further precautions. All samples in this group turned slightly grey when heated in uamo. Since evidence3 exists to show that the surface titanium ions are reduced when heated in the presence of organic contaminants, this discoloration probably results from the reduction of the sample by the alcohol (1) Presented at the 4. C. S. hleeting, Colloid Division. Boston. Mass., 1959. This matkrial was used as part of the thesis submitted by C. M. H. in partial fulfillment of the requirements for the degree of Doctor of Philosophy. (2) D . R. Stull, Ind. Eng. Chem.. 39, 517 (1947). (3) J. Gebhardt and K. Herrington, THISJOURNAL, 62, 120 (1958).
adsorbed in the preparation of the rutile or organic contaminants such aa stopcock greaae. The second group of samples was activated a t 450' on a vacuum apparatus employing a liquid nitrogen trap to condense stopcock grease vapor and facilities for treating the sample with oxygen to remove organic contaminants present on the Burface. The oxygen treatment oxidized the organic contaminants and reoxidized the sample to the white form. The following procedure was used. A sample was evacuated for 30 minutea at 450', then oxygen was admitted to a pressure of 1 atmosphere. After 5minutex3, the sample was evacuated for 2 hours. Oxygen was again added and allowed to remain in contact with the sample for 5 minutes. The sample was re-evacuated for 1.5 hours. Oxygen wm added for a third time and allowed to remain in contact with the sample for 10 minutes. Final evacuation was carried out to a pressure of 104 mm. The temperature of the sample was maintained at 450' throughout this process. These conditions are the standard activation conditions adopted for this investigation. The third group of samples was activated using conditions similar to those for the second group except that the liquid nitrogen trap was not used. Table I lists heat of immersion values for samples act~ivated under these three conditions. The heat values for Group I samples are manifestations of the reduced-state of the surface. Oxygen treatment a t elevated temperatures lowers significantly the heat of immersion in water.
TABLE I EFFECTOF ACTIVATION CONDITIONS ON THE HEATOF JMMERSION OF RUTILE IN WATER Group no.
1 2
Sctivation conditions
hicsw (ergs/cm.')
No 0 2 No trap
-708 f 19
0 1
-588i
9
Trap
3
0 2
-628 f 16
No trap Group 3 samples were not protected from stopcock grease vapor by a liquid nitrogen trap. Although the samples were not grey after activation, the high heat value of 628 ergs/ cm.2 clearly indicates the influence of organic vapors in the vacuum system on the final oxidation state of the surface, even though oxygen treatment was used. The results point out the care necessary when t,emperature activated samples are to be studied by calorimetric or adsorption techniques. Adsorption Apparatus.-A sensitive Bourdon-type pressure gauge4 was constructed and incorporated into a volumetric vapor adsorption apparatus specially designed6 to eliminate dissolution of organic vapors by stopcock grease or manometer oil used with conventional adsorption systems. The simplicity of operation in conjunction with the sensitivity of the pressure gauge makes this apparatus very useful in obtaining adsorption isotherms of organic vapors which (4) S. Dushman, "Scientific Foundations of Vacuum Technique." John Wiley and Sons, Inc.. New York, N . Y.. 1949. p. 273. ( 5 ) Yung-Fang Yu, J. J. Chessick and A. C. Zettlemoyer, THIS JOURNAL, 63, 1626 (1959).
C.M. HOLLABAUGH AND J. J. CHESSICK
110
have room temperature vapor pressures from 1 to 10 cm. The dosing and equilibrium pressures could be determined to &0.003 cm. Adsorption isotherms for water, n-propyl alcohol and nbutyl chloride were determined at 26" on the activated sample. Amounts irreversibly adsorbed were determined by evacuating a sample which had previously been exposed to vapor at relative equilibrium pressure greater than 0.5. After evacuating a t a selected temperature until a pressure of 10" mm. wa.s obtained, the amount of vapor readsorbed at 26' was measured as a function of equilibrium pressure. The difference in the volume adsorbed at a relative prewure of 0.2 under these conditions and those used to obtain the original isotherms was calculated. This difference is the volume of vapor retained a t the degassing temperature employed. Evacuation temperatures used for desorption were 26 and 90" for both n-propyl alcohol and water, and 26 and 50" for n-butyl chloride. The degassing times necessary to obtain a final pressure of 10- mm. varied with adsorbate and temperature. Table I1 lists the times used in these experiments to obtain a fmal pressure of 10" mm. EVACUATION
TABLE I1 TIMEUSED TO DESORBVAPORS -Time
Adsorbate
26"
n-Butyl chloride n-Propyl alcohol Water
120 20 96
of evacustion(hr.) 50' 900
24
.. ..
.. 20 12
Calorimeter.--The thermister calorimeter used to measure the heats of immersion has been described previously.6 Since any electrical current carried by the liquid in which the thermistor is immersed causes erratic behavior of this element, the thermistor and leads were coated with a thin layer of an epoxy resin (Bakelite Company, Resin BV 1600) and cured foi 12 hours a t 150" to prevent electrical contact with the liquid. This resin was inert to the liquids used. The entire calorimeter was coated with this resin when n-butyl chloride was used as the wetting liquid to reduce the amount of water adsorbed on interior surfaces. It was found that trace amounts of water greatly affected the heat of immersion values obtained with this liquid. Heats of immersion of rutile into each of the three liquids were measured as a function of the amount of preadsorbed vapor.6 In order to dry completely both the calorimeter and n-butyl chloride, a technique used by Bartell and Suggitt7 was adopted. This consisted of adding one gram of activated silica by breaking an evacuated glass bulb directly into the calorimeter immediately prior to final assembly. A dry box was used for transferring the liquid and for assembling the calorimeter. The heat of immersion values for rutile in n-propyl alcohol were not altered by this drying procedure. Isosteric heats of adsorption were calculated from both the heat of immersion data and the adsorption data measured a t 32 and 18'. Heat of Bulb Breaking.-The heat of breaking of sealed sample bulbs w v , ~determined s in each liquid. All the bulbs were initially evacuated a t 450" for 6 hours. Measurements made with completely evacuated bulbs and with bulbs containing saturated vapor of the liquid. The results of these measurements are presented in Table 111. The values in the second column represent the total heat envolved in breaking a sealed evacuated bulb and include the strain energy For cracking the glass, a PA V work term, and the heat effects due to evaporation and condensation. The latter effects are a consequence of such processes a8 desorption from the walls of the calorimeter resulting from lowering of the liquid level and resaturation of the vapor which has been diluted by incoming air. The values in column three result from a combination of the heat evolved with an evacuated bulb and the heat evolved in condensing the saturated vapor contained in the bulb. Differences in the values of columns 2 and 3 represent the heat of condensation of 6 ml. of saturated vapor at 26" which are tabulated in column 4. Theoretical heat of (6) .4. C. Zettlernoyer, G . J. Young, J. J. Chessick and F. H. Healey,
J Phys. Chem., 6'7, 649 (1953). (7) F. E. Bartel' and R . A I . Suggitt, ibid.,68, 36 (1954).
Vol. 65 TABLE I11
HEATOF BULBBREAEINQ(CALORIES) Liquid
Relative pressure P/PQ = 1
P/Po = 0
n-Butylchloride -0.25 n-Propyl alcohol - .41 Water - .16 Exptl. values f 0.07 cal. Bulb volume 6.0 f 0.5 mi.
Difference
-AHY
-0.32
-0 27 .07 - 08
-0.57
-
-
-
.42 .I6
-
.01 .OO
vaporization values calculated for these vapors are shown for comparison in column 5. The significant difference found for n-butyl chloride is due to its high vapor pressure (10.5 cm.) at 26". At this pressure a large n number of molecules are present in the vapor phase. Condensation of the small number of vapor molecules of n-propyl alcohol and water contained in those bulbs yields insignificant contributions t o the correction terms. Heat of immersion values were corrected by subtracting the heat of breaking from the total heat evolved in each run. Since the difference in the heat of breaking for the evacuated bulb and the bulb containing saturated vapor of n-propyl alcohol and water was within the limits of experimental error, a constant value was used with these adsorbates. The heat of breaking in n-butyl chloride varied de ending on the vapor pressure in the bulb. It, was assumefthat the variation in the heat of breaking was a linear function of vapor pressure and the correction applied depended on the equilibrium pressure above the sample.
Results Adsorption of %-Butyl Chloride, %-Propyl Alcohol and Water on Rutile (TiOz).-The adsorption isotherms for n-butyl chloride, n-propyl alcohol and water determined a t 26" are shown plotted in Fig. 1. Curves I represent the amount initially adsorbed as a function of equilibrium relative pressure on the activated and oxygen treated rutile surface. Thereafter, the sample was evacuated a t a specified temperature, 50" for n-butyl chloride and 90" for both water and n-propyl alcohol, to an ultimate vacuum of 10-6 mm. to remove as much of the adsorbate as possible. Curves I1 represent the amount a t 26" after evacuation. The differences in curves I and I1 of the isotherms of Fig. 1 can be taken measure of the volumes of vapor irreversibly adsorbed a t the temperature used for desorption because of the parallelism of the isotherms for a given adsorbate. The calculated Vm values and amounts irreversibly adsorbed are listed in Table IV. Heat data in combination with the values in Table IV will be used to distinguish between the processes of physical and chemisorption. T.4BLE
VOLUME
Iv
VAPORS ON RUTILEUNDER CONDITIONS ml. (S.T.P.)/g.
O F ADSORBED
Volume v m
Vol. retained at 26" 90O 50 "
\'ARIOUS
n-Butyl chloride
n-Propyl alcohol
Water
1.08
1.80
3.31
0.18
..
1.42 1.20
1.66 1.20
0.18
..
..
The type I1 isotherms found for the adsorption of n-butyl chloride and water on rut'ile are typical of multimolecular adsorption. The shape of the isotherm for n-propyl alcohol is quite unusual for these types of systems. The insignificant increase in the amount adsorbed over a wide range of relative
Jan., 1961
ADSORPTION OF WATERAND POLAR PARAFFINIC COMPOUNDS ONTO RUTILE
pressure is chaxacteristic of monomolecular adsorption. BET plots for n-butyl chloride and water were used to estimate the monolayer capacities of these two adsorbates. The V , value for n-propyl alcohol was calculated from the slope of a Langmuir plot of these data. Heats of Immersional Wetting.-Corrected heat of immersion values obtained for n-butyl chloride, n-propyl alcohol and water are plotted in Fig. 2 as a function of the amount of vapor preadsorbed on the sample before immersion. These plots show the characteristic large decrease in heat values a t small volumes adsorbed. The initial change in the heat of immersion with volume adsorbed is linear for both n-propyl alcohol and water up to a coverage of 1.2 ml./g. With n-butyl chloride, however, an exponential decrease in the heat values with increasing adsorption is observed. The heat of immersion of activated samples into freshly distilled n-butyl chloride was 621 f 31 ergs/cm.2. After rigorous drying of the liquid, the heat of immersion decreased to 280 f 12 ergs/ cm.2. The large change in heat values due to small amounts of waler made imperative the use of a completely dry system. X o changes were found after drying n-propyl alcohol in a similar manner, probably because the alcohol shows a larger heat of adsorption than water and is preferentially adsorbed a t the very low concentrations of water present as an impurity in the wetting liquid. Heats of Adsorption.-Differential heats of adsorption were calculated in two different manners. First, the conventional isosteric heat values, qst, yere obtained from equilibrium vapor adsorption data at 18 and 32" for 6 values near unity. The low equilibrium pressures at 18" precluded the extension of the calculation of these heat values to lower surface coverages. Second, graphical differentiation of the heat of immersion curves yielded differential heat values, Qd, not only near the monolayer but a t very low surfaces coverages as well. These results are shown plotted in Fig. 3. Discussion Surface of Rutile (TiOz).--Although the composition and array of constituents in a surface cannot be determined exactly. the spacing of surface ions can he estimated from crystal lattice parameters. The bulk rutile structure consists of units with a titanium ion surrounded by six oxygen ions in the form of an octahedron. Two of the Ti-0 bond distances Stre 2.01 A. and the other four 1.92 A.8 Using an average value of 1.95 A. a model of the surface was constructed which contained 8.8 X 1019titanium ions per gram of sample of area 13.4 m.2/g. With the further assumption that an unactivated rutile surface is completely hydroxylated, a model can be presented which explains adequately the experimental adsorption and calorimetric data. When a sample of rutile is evacuated most physically adsorbed gases are removed near room temperature when an ultimate vacuum of 10-6 mm. is achieved. As the temperature is increased, neighboring hydroxyls are assumed to interact to form (8) W. Huokel, "Structural Chemistry of Inorganic Compounds," Vol. 11, Elsevier Publishing Co., New Pork, N. Y . , 1951,p . 683.
111
e 6.0 P,
$. 5.0
3 v
-0
-ea 4.0 8
m 6
3.0 2.0
1.0
0.2
0.4 0.6 0.8 1.0 PPO. Fig. 1.-Adsorption isotherms for the adsorption of vapors on rutile (TiOz) at 26': 0,water; A, n-butylchloride; a, n-propyl alcohol. Upper curve of a given set determined after activation at 450'; lower curve obtained with the mme sample after evacuation a t 90'.
N h
-2
600
2 500
.il
.-E
400
Lc
300 6
m 200 100
0
~~
~
1.0 2.0 3.0 Vol. preadsorbed (ml. STP/g.). Fig. 2.-Heat of immersion of activated rutile (Tion) in several liquids a t 26".
water vapor and reactive Ti-0-Ti groups. Isolation of some hydroxyls occurs because of the random nature of the interaction of neighboring OH groups. The surface after activation at 450" would then be composed of active Ti-0-Ti linkages and isolated hydroxyl groups if precautions were taken t o prevent reduction of Ti+4ions by organic contaminants or if a reduced surface were reoxidized according
C. M. HOLLABAUGH AND J. J. CHESSICK
Vol. 65
TABLEV NUMBER OF ADSORPTION SITESAND MOLECULES ADSORBED PERGRAMOF RUTILE ( X 10-19) Adsorbate
Water n-Propyl alcohol n-Butyl chloride
1.0 2.0 3.0 Vol. adsorbed (ml. STP/g.). Fig. 3.-Diffsrential heats of adsorption of vapors on activated rutile (TiOz) at 25": 0, n-propyl alcohol; A, n-butyl chloride; 0, water.
0
to the procedure adopted here to yield a reproducible surface. Adsorption of Water.-The adsorption isotherms for water are Type I1 showing multilayer adsorption. The calcuiated area per water molecule on the activated surface is 15.1 As2based on a V m of 3.34 $I. (ST:P)/g. This value is higher than the 10.8 A.2 usually obtained for physical adsorption of wateroon polar surfaces. A cross-sectional area of 15.1 A.2 is further surprising when it is considered that the Vm value on which it is based contains mm. water irrevemibly adsorbed at 90" and vacuum. The adsorption process can be explained conveniently by considering surface dehydroxylation to occur as a result of thermal treatment at 450". For comparison, Table V lists the number of sites on rutile based on the proposed model and experimental resultr;. After high temperature evacuation the surface consists of active Ti-0-Ti linkages formed by condensation of adjacent hydroxyl groups as well as isolated Ti-OH group^.^ A water molecule is chemisorbed by reaction with one Ti-0-Ti to form two hydroxyl linkages. The number of such active sites based on the amount of water irreversibly adsorbed a t 90" is 3.2 X 10lg per gram of solid. Physical adsorption over the hydroxylated surface, judged by the large C.S.A. of 23.5 k2/water molecule in the monolayer, apparently occurs by localization of a water molecule over two surface hydroxyl groups. Using the BET Vm and subtracting out the initial strong adsorption, a value of 5.7 >(: 1019water molecules was calculated as the amount of wi3ter adsorbed in this manner. This amount physically adsorbed corresponds to the (9) No independent data such a8 infrared or n.m.r. measurements have been obtained which support the view that an unactivated rutile surface is hydroxylated. Nevertheless, the adsorption processes described here for rutile parallel remarkably the results found for silica which is known to contain surface silanol groups.
iMoleoules ads. C Surfaoe site-Chemi- PhysiAotivated After chemisorption oal oal Ti-0-Ti TiOH Ti-0-Ti TiOH
3.2
5.7
3.2
4.9
0.0
11.3
3.2
1.6
3.2
4.9
0.0
8.1
0.5
2.4
3.2
11.4 X 10-19 surface hydroxyl groups and was assumed to be the OH population on a fully hydroxylated surface. From this total population and the amount chemisorbed it was calculated that 4.9 X 1019OH groups remained on the surface after thermal treatment a t 450". These values for Ti-OH and Ti-O-Ti population based on water data are important since results consistent with these findings should be observed for n-propyl alcohol adsorption. The interatomic spacings used in constructing the model Ti02 surface gives a total TiOH population of 8.8 X 10lg/gram. The larger amount taken up determined experimentally suggests the values assumed for the interatomic spacings were too large. The presence of hydroxy groups on rutile appears to be substantiated by treating the solid with dimethyldichlorosilane. A sample was first evacuated a t room temperature to remove the physicaily adsorbed water. After saturating with liquid dimethyldichlorosilane, the sample was heated to 100". It was washed with alcohol and then with water. Drying at either 26, 100 or 200" left a powder that was hydrophobic. The well known fact that (CH&SiC12 reacts readily with OH groups has been utilized by Kiselevlo and others to methylate the hydroxyl groups on silica. Adsorption of n-Propyl Alcohol.-The Type I isotherm obtained for the adsorption of n-propyl alcohol on rutile has not been observed previously for systems of this type. The calculated area per molecule in the monolayer of 26.6 A.z is larger than the value of about 21 A.z often found for long chain but similar organic molecules physically adsorbed on many substrates. This large C.S.A. is likely caused by localized adsorption over specific sites. Similarities of adsorption results for water and npropyl alcohol suggests the same type of adsorption mechanism; chemisorption of an alcohol molecule could take place by reaction with Ti-0-Ti group to form Ti-OR and Ti-OH groups. Indeed the same number of water and alcohol molecules (3.23 X lOl9/g.) were chemisorbed on reactive Ti-0-Ti groups. Physical adsorption of n-propyl alcohol occurs by the formation of hydrogen bonds with the surface hydroxyl groups also. Some of these surface hydroxyl groups remain after activation and the remainder (one-half those formed by chemis0t.ption of water) are formed during chemisorption (10) 8 . V. Kiselev, "Proceedings of the Tenth Symposium of the Colston Research Society," Butterworths Scientific Pub. London, 1958, p. 19s.
Jan., 1961
ADSORPTION OF WATERAND POLAR PARAFFINIC COMPOUNDS ONTO RUTILE
of the alcohol. ‘Table V lists the number of molecules adsorbed and the number of sites available. The number of OH groups on the surface after completion of alcohol chemisorption is approximately 8.1 X 1019 per gram. Some of these hydroxyls are not available for bonding with the physically adsorbing alcohol molecules because of steric hindrance of neighboring Ti-OR groups and the necessary presence of some isolated OH groups now. Hence, the number of physically adsorbed molecules is 1.6 X lOI9 per gram. Formation of two hydrogen bonds with each of these molecules requires only 3.2 X l O I 9 of the 8.1 X 1019 surface hydroxyls. Although the surface presented after completion of the monolayer is not a close packed film, it has properties that are essentially of hydrocarbon character onto vvhich further adsorption does not occur. This type of autophobic surface typified by the type I isotherm for this adsorbate has not been found previously for the adsorption of short chain, polar, pa,raffinic compounds onto a solid surface. The autophobic surface is a direct consequence of both chemisorption and strong physical adsorption in which the molecules are oriented with the polar :snd toward the surface. Similar surfaces are found when long chain, polar molecules are adsorbed onto glass and platinum because of the lateral interaction energy.” The small increase in volume adsorbed near saturation is probably due to condensation in the capillaries between particles. Adsorption of n-Butyl Chloride.-Adsorption of n-butyl chloride onto rutile takes place principally by physical forces. Unlike n-propyl alcohol, multilayer adsorption occurs as indicated by the Type I1 isotherm. The calculated area per molecule is 46.2 A.2 a t monolayer coverage. Orientation of the molecules must be “flat” on the surface since an area of 20 to 25 A.2would be expected if vertical orientation occurred. An area of 43.8 1 1 . 2 per molecule for flat-wise adsorption is calculated from liquid density by assuming that the molecule is a rigid rod with a cross-sectional area of 20 A.2. The length of the molecule based on these calculations is 8.7 A. compared to 8.3 A. calculated using Pauling’s12 values for bond angles and bond lengths. Area values for similar molecules adsorbed on solid surfaces are in good agreement. For example, areas of 49.3 A. for 1-pentene adsorbed on anatazreI3 and 42 A. for n-butyl alcohol on graphon from aqueous solutions14 have been reported. The surface states or rutile after activation consists of 3.2 X 1019active Ti-0-Ti groups and 4.9 X 1019 OH groups per gram of solid. A small volume of n-butyl chloride is chemically adsorbed probably by dissociative reaction with the most active Ti-OTi linkages forming Ti-C1 and Ti-OR groups. At the completion of the chemisorption process, ( 1 1 ) E. F. Hare and W. A. Zismsn, THIS,JOURSAL, 59, 335 (1955). (12) L. Pauling, “Nature of the C:heinical Bond,” Cornell University Prem, Itnaca. N. Y., 1940, Char). :
