MOLECULAR INTERACTION OF MXTAT, OXIDESAND ADSORBEDVAPORS
Fch., 1963
251
ADHESION OF LIQUIDS TO SOLIDS. MOLECULAR INTERACTION BETWEEN METAL OXIDES AND ADSORBED VAPOkS'
'
BY EDWARD H. LOESER, WILLIAM D. HARK INS^ AND SUMNER B. TWISS Chrysler Corporation of Detroit, Michigan, and the Department of Chemistry, University of Chicago, Chicago, Illinois Received July 17, 1062
The adsorption isotherms at 25" of the vapors of n-heptane, n-propyl alcohol and water on TiOz, SnOz and Sn and of npropyl acetate on Sn and SnOz have been determined. The calculated values of re,the decrease of free surface energy of a solid caused by the adsorbed film of vapor and of WA, the work of adhesion between the adsorbed liquid and a solid, are given. re and WA values for adsorbed liquids on the solids increase in the order: n-heptane, n-propyl acetate, n-propyl alcohol and water. reand W Avalues for n-propyl acetate, n-propyl alcohol and water on SnOz are markedly larger than those for the same liquids on Sn. A linear relationship appears to exist between refor a given solid and the inverse of the molecular volume of the adsorbate, provided the temperature is constant and the adsorbates have permanent dipole moments of approximately equal value. More data are needed to test the validity of this relation. Company and was purified by the method of Lund and Bjerrum .4 Double distilled water was treated with an excess of manganous sulfate-sodium hydroxide reagent6; the water was distilled from the reaction mixture into a reservoir on the line. Each of the liquids was thoroughly degassed by repeated solidification and ebullition in vacuum maintained a t 10-6 mm. Experimentally determined vapor pressures of the degassed liquids in mm. a t 25" are: n-heptane 45.77, n-propyl acetate 33.63, n-propyl alcohol 20.86, and water 23.76. The anatase was a sample known to be oil-free and not surface treated. The X-ray parameters a. and co were both 0.01 b. lower than those listed by Wyckoff.6 The stannic oxide powder, of Baker analyzed grade, contained the following impurities: soluble salts 0.23%, C1 O.OOl%, SO4 O;Ol%, free alka!i (as NaOH) 0.01% and Fe 0.0003%. The tin powder contained the following impurities: approximately 0.1% lead, traces of copper, antimony and indium and very slight traces of bismuth and silver. The surface of the tin particles is probably covered with a thin coating of oxide formed during exposure of the tin to air. The surfaces of the solids were cleaned by heating the solids in vacuo. The outgassing conditions and the values for the areas of the solids are listed in Table I. These areas were calculated from adsorption data by means of the Brunauer, Emmett and Teller method and the Harkins and Jura relative method.'
Introduction One of the most important quantities involved in the adhesion of liquids to solids is the free energy or work of adhesion. The method for the determination of the work of adhesion of volatile liquids adsorbed from the vapor phase on the surface of solids in powder form has been described by Jura and H a r k k a These workers define work of adhesion by the simple relation WA =
r e
+
YL (1
+
COS
0)
where reis the decrease in free surface energy of the solid caused by the adsorption of a vapor a t its saturation pressure. The present work was undertaken to discover the order of magnitude of the difference in energy of interaction of polar and non-polar liquids adsorbed on the surfaces of two inorganic oxides and a metal. The non-polar liquid is n-heptane; the polar liquids were chosen because of their similarity in dipole moment and wide difference in molecular size. These are water, n-propyl alcohol and n-propyl acetate. As expected, the present results show that energy of interaction between solids and liquids depends not only upon the properties of the adsorbed molecules, but also on the nature of the solid surface. When polar liquids are adsorbed, higher energies of adhesion are obtained with tin oxide and titanium dioxide than with untreated tin surface.
TABLE I PRETREATMENT AND AREASOF SOLIDS Solid
Area, per g.
m.2
150 60 0.274" Tin 450 16 6.84" Tin oxide, SnOz Anatase, TiOl 500 16 13.Sb B.E.T. or H.J. method using heptane adsorption data. B.E.T. or H.J. method using nitrogen adsorption data.
Experimental The apparatus and procedure for the determination of the adsorption isotherms have been described in an earlier paper.3 The temperature of the water-bath around the bulb which contained the adsorbent was maintained at 25.0 f 0.05'. The n-heptane was Bureau of Standards grade obtained from Westvaco Chlorine Products Company. It was dried over sodium wire for one month and then distilled into a reservoir on the vacuum line. The n-propyl acetate, obtained from Eastman Kodak Co., was refluxed with acetyl chloride and the unreacted acetyl chloride was distilled off. The propyl acetate was washed, dried and distilled through a Podbielniak column. The fraction which had a constant boiling point and an index of refraction, n%, of 1.3843 was dried over Drierite and then distilled into a reservoir. The n-propyl alcohol was obtained from the Coleman and Bell (1) Presented at the Twelfth International Congress of Pure and Applied Chemistry. (2) Deceased. (3) 0.Jura and W. D. Harkins, J . A m . Chem. Soc., 66, 1356 (1944).
Outgassing conditions Temp., O C . Time, hr.
Results and Discussion The adsorption isotherms at 25" of n-heptane, n-propyl alcohol and water on anatase (TiOz) are exhibited in Fig. 1. The data for heptane and water have appeared in an earlier paper.a Figures 2 and 3 show the adsorption isotherms a t 25' of n-heptane, ,n-propyl alcohol, n-propyl acetate and water on tin and stannic oxide. Each isotherm consists of approximately thirty-five experimental points measured over the relative pressure range of
,
(4) € Lund I. and J. Bjerrum, Ber., 64, 210 (1931). (5) W. D. Scott, "Standard Methods of Chemical Analysis," 5th ed., D. Van Nostrand Co., Inc., New York, N. Y.,1939, P. 2079. (6) R. W. G. Wyokoff, "The Structure of Crystals," 2nd ed., The Chemical Catalog Co. (Reinhold Publ. Corp.), New York, N.Y.,1931, p. 239. (7) E. H . Loeser and W. D. Harkins, J . A m . Chsm. Soc., 72, 3427 (1950).
,
35r EDWARD H. LOESER,WILLIAMD. HARKINS A N D SUMNER B. TWISS
252
Vol. 57
30. I
.
0 X
t
0
0.2
04
06
0.8
IO RELATIVE PRESSURE,
RELATIVE PRESSURE,
Fig. 1.-Adsorption isotherms on anatase (TiOz) a t 25": 0, n-heptane and -A-, n-propyl alcohol. The isotherm of Harkins and Jura for water on anatase (-) is given for purposes of comparison.
0.0002 to 0.98. Phase changes* at very low pressures were found in many of these isotherms. They are not shown in the figures and will be discussed in a subsequent paper. The choice of units in the figures, relative pressure ( p l p o ) for the abscissa and molecules adsorbed per unit solid area for the ordinate, allows comparison of the data. At any given relative pressure, the number of water molecules adsorbed per unit of area of each of the solids is considerablv larger than the number of propyl alcohol, prop9 ace&te or heptane molecules. The number of heptane molecules adsorbed at any given p / p o below 0.8 is smaller than that for the other liquids. The decrease of free surface energy (R) of the solid which accompanied the adsorption of the vapor was calculated by the method described by Jura and hark in^.^ The lowering of free surface energy of tin oxide as a function of the relative pressure of the various adsorbed vapors is given in Fig. 4. The curves in Fig. 4 indicate the manner in which the R value of a given solid is influenced by the type of molecules adsorbed on its surface. The 7~ values at any given relative'pressure increase in the order: n-heptane, n-propyl acetate, n-propyl
%.a
Fig. 2.-Adsorption isotherms on tin a t 25": -G, n-heptane; -A-, n-propyl alcohol; -e-, n-propyl acetate; -A-,water.
35
30
*
b'
x
25
f a
E
20
2 K
2 4
15
v)
J
3 Y
0
=
10
5
0 0
02
04
06
08
IO
MOLECULAR INTERACTION OF METALOXIDESAND ADSORBED VAPORS
Feb., 1953
of approximately the same magnitude, the decrease in free surface energy of a given solid caused by the adsorption of these vapors, indicated by curves in Fig. 4 and Re values in Table 11, is not equal. It is of interest to investigate the reason for this and, in
/
/ /’
TABLE I1 FREEENERGY OF INTERACTION BETWEEN SOLIDSURFACES AND ADSORBED FILMS
/’ # 4 A T ER
/’
/-Y-
’YL ALCOHOL/
253
I
/
/ A
e/-
)PYL ACETATE
Relative pressure p / p o . Fig. $.-Lowering of free surface energy ( T ) for films on tin oxide as a function of the relative pressure.
polar n-heptane indicate that the T values are not influenced by the properties of the surface. The value of a for a solid in equilibrium with the saturated vapor is obtained by extrapolation of the ?r us. p / p ~ curve to p/po = 1; it is designated by ?re. Table I1 lists the r e values for the various vapors adsorbed on the solids.
Liquid Adsorbed n-Heptane n-Propylacetate n-Propylalcohol Water
Decrease in free surface energy, r e . in ergs om. -3 a t 25O Tin AnaTin oxide tase 50 64 46 70 104 83 117 108 168 220 196
..
Work of adhesion, WA,in ergs cm.-l a t 2 5 O Tin AnsTin oxide tase 90 94 86 117 151 129 163 154 812 364 340
..
particular, the relationship between the decrease of free surface energy ( r e of ) a solid and the rholar volumes (M.V.) of the adsorbed molecules. Molecular size might be considered to be an important factor in the observed adsorption energy, since it controls the closeness of approach of the adsorbate molecules to the surface of the solid and the number of molecules oriented a t the polar surfaces. The proportionality between r e and (l/M,V.) X loa is illustrated by the curves in Fig. 6 . It will be noted that a linear relationship appears to exist between ae for a given solid and l/M.V., provided the temperature is constant and the adsorbed molecules have approximately the same dipole moment.
,
I
I
3
Fig. 6.-Lo\vcring of free surface energy ( r e )of anatasc, tin and tin oxide as a function of the molecular volume of the adsorbed molecules. Points for non-polar n-heptane fall below the lines.
L 0.5 I .o
OO
Relative pressure p / p ~ . Fig. 5.-Lowering of free surface energies ( T ) of anatase, tin and tin oxide by propyl alcohol films a6 a function of the relative pressure.
Although the liquids n-propyl acetate, n-propyl alcohol and water have permanent dipole moments
In this case the range of dipole moments is 1.70 to 1.86 Debye units. The anatase data consist of only two points. There is no justification, outside of analogy with the behavior found for tin and tin oxide, for assuming a linear relationship for the anatase data. Since the importance of such effects as hydrogen-bonding, dispersion forces and polarizability are not considered in this treatment, it is possible that the observed relationship between ?re and 1,’M.V. is fortuitous. Even if this relation-
OSCARD. BONNXI~ AND VICKXESRHETT
254
ship holds only for adsorbed liquids having similar dipole moments and polarizabilities, considerable information on adhesion mechanisms can be gained from this approach. Obviously more data are needed to test its validity. It is conceivable, also, that the placement of the lines in Fig. 6 may be related to the concentration of polar sites on the solid surfaces, since the line for tin oxide with a higher concentration of ionic sites favorable to adsorption lies above that for untreated tin. The lowering of the free surface energy (7,)
Vol. 57
by a film adsorbed on the surface of a solid is the basis for the calculation of a fundamental quantity, the work of adhesion (WA). The values for W A given in Table I1 were calculated by the method of Jura and hark in^.^ Approximately four times as much work must be expended to separate liquid water from the surface of these solids as is needed to separate liquid heptane. The work required to separate water from these surfaces is about 2.5 times that necessary for the separation of propyl acetate or propyl alcohol.
EQUILIBRIUM STUDIES OF THE SILVER-SODIUM-HYDROGEN SYSTEM ON DOWEX 50 BY OSCARD. BONNER AND VICKERSRHETT~ lhpartinent of Chemistry of the University of South Carolina, Columbia, S. C . Received July d l , 1968
Equilibrium studies of the silver-hydrogen, silver-sodium and sodium-hydrogen exchanges on two samples of Dowex 50 have been made while maintaining a constant ionic strength of approximately 0.1 M . Comparison of these exchanges with another series of sodium-hydrogen exchanges, which have already been reported, shows a definite relationship between the selectivity of the resin and the maximum water uptake. The selectivity values and the selectivity-loading curves are discussed in the light of present theories.
Introduction It is known that when an ion exchange reaction occurs, selectivity is shown in nearly all instances. This is demonstrated by the fact that for a reaction such as A+
+ BR~B
=
B+
+A R ~ ~
the mass action equilibrium quotient is usually different from unity. It is also recognized that this equilibrium quotient is not constant for an exchange betnfeen two given ions but is a function of several variables, not all of which may be Some of the known variables are the capacity of the resin, the percentage divinyl benzene cross-linkage in the resin (as demonstrated by the maximum water uptake of the resin) and the percentage of each ion associated with the resin (ie., loading). There are no exact cation exchange equilibria data in the literature for a thoroughly charactekized resin. . Most of the.earlier work.was of an exploratory nature only. Recent investigators of ion exchange equilibria have reported only the equilibrium data and not the characteristics of the r e ~ i i i . Theoretical ~~~ interpretation of the ion exchange process and quantitative predictions of ion exchange selectivity have thus been very difficult. Experimental These data, which will be presented, represent six series of exchange reactions between three pairs of ions on two different resin samples. Equilibrium data on one resin sample (nominal 16% divinyl benzene) were obtained at Oak Ridge (1) Part of the work described herein was included in a thesis submitted by Vickers Rhett t o the University of South Carolina in partial fulfillment of the requirements for the degree of Master of Science. (2) G . E. Boyd, A n n . Reu. PILUS.Chem., 11, 309 (1951). (3) E. Hogfeldt, E. Ekedahl and L. G. Sillen, Acta Chem. Scand., 4, 1471 (1950). (4) 0. D. Boiiner, W. J. Argcrsinyer a i d A. W . Davidson, J. Am. C‘hsm. Soc., 14, 1044 (1952).
National Laboratory and on the second sample (nominal 8% divinyl benzene) at the University of South Carolina. Preparation of Resin Samples.-For the study of each exchange reaction two “pure” resins were prepared, one containing each of the cations involved in the exchange. The hydrogen form was prepared by passing 6 N hydrochloric acid through a column of the commercial product until the effluent gave no further flame test for sodium ion. For the preparation of pure silver resin, hydro en resin was placed in a column and silver nitrate passed %rough until the pH of the effluent showed no more than a negligible decrease from that of the influent. When radioactive methods of analysis were to be used a trace of AgllO isotope was added to the influent silver solution. A sample of the influent was retained in order to determine the activity per milliequivalent of silver ion. The pure sodium resin was obtained similarly by passing a solution of sodium chloride or sodium nitrate through a column containing pure hydrogen resin. When radioactive methods of analysis were to be used a trace of Na2‘ isotope was added to the influent solution. General Method.-In each exchange experiment, a weighed sample of the resin to be studied, of known moisture content, was placed in a ground glass stoppered flask in contact with an aqueous solution of the appropriate salts of the cations involved at a constant total ionic strength of 0.1 M . Equilibrium was hastened by agitation and was reached, in every case, withip two hours. The temperature was maintained a t 25 i 1 . Equilibrium Solution.-The concentration of each cation in the aqueous solution at equilibrium was determined by means of direct analysis; hydrogen ion by titration with standard sodium hydroxide solution; silver ion by potentiometric titration with standard potassium iodide solution or by measurement of radioactivity; sodium ion by means of measurements with a Beckman DU flame photometer or radioactivity measurements when NaZ4isotope was used as a tracer. When sodium ion concentration was determined by means of the flame photometer, the test solution was closely bracket,ed with two solutions of known sodium ion content. Equilibrium Resin.-The washed equilibrium resin was analyzed for each cation by means of complete exchange of both for a third ion which would not interfere in the determination. For example, nitric acid solutions were used to displace silver and sodium ions from the cquilibrium resin in the silver-sodium exchange experiments. The resulting solution was then analyzed as before. When radiochemical
