h S O I t P T I O N OF ISOPROPYL ALCOHOL ON
GRAPHON
2757
Adsorptioii of Isopropyl Alcohol on Graphon
by D. R. Bassett, E. A. Bowher,' and A. C. Zettlemoyer Canter for Siirface and Coatings Research, Lehigh C'niaersity, Bethlehem, Pennsyltania (Receized Janiia,y 9, 1967)
Adsorption isotherms for isopropyl alcohol on Graphon have been determined :it 0 and 'L,i0. The isotherms and isosteric heat curve indicate that adsorption proceeds in a stepwise fashion for this system. The conclusion reached from these results and from studies by others is that, in addition to the need for a homogeneous surface, the geometry and chemical nature of the adsorbate are of great importance in determining whether or not stepped isotherms will be observed. Low temperature is clearly riot a requirement.
Graphitized carbon blacks, of which Graphon is a well-known example, have been used extensively for studies involving adsorption from the gas phase and from solution. The uniform, medium energy and nonpolar surface of Graphori permits the interpretation of adsorption data, e.g., isotherms and heats of adsorption, in a mnririer riot possible with heterogeneous surfaces such (3s norigraphitized blacks and inorganic oxides. One consequence of the homogeneous surface of Graphon, and other graphitized carboii blacks, is that adsorption isotheriis often show steps corresponding to the filling of successive layers.? Pierce and Ewing3 investigated the requirements for the occurrerice of stepwise isotherms and concluded that in addition to the need for a homogeneous surface, (i) there must be strong lateral interactions between adsorbed molecules, arid (ii) the temperature must be such that thermal agitation of the adsorbed molecules does not erase the discontinuities between the filling of adsorbate layers. l l o r e recent>ly, Davis and Pierce4 concluded that the reduced temperature is more important than the absolute temperature in determiriing whether or not stepwise isotherms occur. They also predicted that stepwise jsothernis might even be found at room temperature for vapors having a saturation pressure in the range 1 to 10 torr at this temperature. The purpose of this article is to report that stepped isotherms can Indeed be found at relatively high temperatures for certain systems, in this case isopropyl alcohol adsorption on Graphon a t 0 arid 2 5 " . In addition, the isotherms, and isosteric heats of adsorption derived from them, are interesting when considered
in connection with the work of Iiiselev arid co-workers5,6 on graphitized carbon surfaces as well as with that of Pierce, et ~ 1 . ~ 8 ~
Experimental Section Materials. The Graphon was a sample freed by the manufacturer (Cabot Corp.) of impurities such as alkali arid alkaline earth sulfides, which had been detected in previous samples, by leaching with dilute hydrochloric acid under conditions which did not lead to oxidation of the surface. Baker Analyzed isopropyl alcohol was dried over anhydrous magnesium sulfate and redistilled : the middle fraction was retained. Dissolved gases were removed by subjecting the alcohol to five freeze-thaw cycles under high vacuum. Appamtus. Adsorption isotherm determinations for organic vapors require a system free from grease and oil. A suitable volumetric adsorption apparatus was constructed using Teflon needle valve stopcocks featuring Viton O-rings (Fischer-Porter) . The stopcocks met the vacuum requirements imposed during outgassing of the sample ( < l O - j torr) and throughout ~~~~~
(1) Department of Physical Chemistry, The University, Bristol, England. (2) T. L. Hill, J . Chem. Phys., 15, 767 (194i); G. D. Halsey, Jr., ibid., 16, 931 (1948); M. H. Polley, W.D. Schaeffer, and W. It, Smith, J . P h y s . Chem., 57, 469 (1953). (3) C. Pierce and B. Ewing, J . Am. Chem. Soc , 84, 4070 (1962). (4) B. W. Davis and C. Pierce, J . Phus. Chem., 70, 1051 (1966). (5) A. A. Isirikyan and A. V. Kiselev, i b i d . , 65, 601 (1961); 66, 205, 210 (1962). (6) N. N. Avgul, A. 5'. Kiselev, and 513 (1961).
I. A. Lygiria, KoZZoidn. Z h . , 2 5 ,
Volume 71 'Vumber 5
August 1967
2758
D. R. BASSETT, E. A. BOUCHER, AND A. C. ZETTLEMOYER
adsorption measurements. The pressure of isopropyl alcohol vapor was determined with a glass Bourdon spoon gauge. Deflections of the pointer from zero were read using a telescope fitted with a vernier eyepiece. The gauge was calibrated against an oil manometer using varying pressures of helium, and the displacement of' the pointer was found to be a linear function of pressure from 0 to 100 torr. Pressure could be determined to approximately hO.01 torr. Liquid nitrogen traps were used between the adsorption system and the vacuum pumps (rotary and mercury diffusion).
36
t
Results and Discussion The isotherms for isopropyl alcohol on Graphon a t 0 and 25' are shown in Figure 1. Isosteric heats of adsorption computed from the isotherms are given in Figure 2 as a function of the amount adsorbed. The precision of the heats is approximately k 150 cal/mole. At low coverages, the isosteric heats are only slightly greater than the heat of liquefaction of isopropyl alcohol at 12.5' reflecting the very weal: temperature dependence of the isotherms in this region and indicating the lack of strong adsorbate-adsorbent attraction. Another indication of only weak attraction of the alcohol for the carbon surface is the fact that both isotherms are initially convex to the pressure axis. IGselev5 also attributes this trend to the presence of relatively strong lateral interactions between adsorbed molecules. It is clear from the isotherms that the temperatures used are near the upper limit for which two distinct steps would be discernible, and the steps are not as well developed as those found, for instance, by Davis and Pierce4 for ethyl chloride on the graphitized black Sterling 1 I T G a t -S9" and below. The completion of the first knee in the isotherms (point B1)is, however, taken to signify the filling of a monolayer, and the completion of the second knee (point B2) is taken to indicate the filling of two layers. The most obvious reasons for believing the two points of inflection are steps are that both are reproducible, BB is roughly twice the height of Bl, and the two maxima in the heat curve correspond reasonably well to the positions of the knees in the isotherms with respect to volume adsorbed. In addition, the cross-sectional area of isopropyl alcohol derived from the isotherms is a reasonable value if point BI is taken to signify the filling of a monolayer. Pierce and Ewing' have presented strong evidence that the cross-sectional area of nitrogen on a homogeneous carbon surface should be taken as 20 A B . Using this value, the surface area of the Graphon used in this The Journal of Physical Chemistry
0
0.3
0.1
0.9
0.7
0.5
Relative pressure, P,'Pa.
Figure 1. Isopropyl alcohol isotherms on Graphon a t Insert shows low-pressure 0" ( 0 )and 25" (0). region. Solid points indicate desorption.
s
12.0
.--_-__-___________-____ CI
I 0
8.
Pi 4
8
12
16
20
24
Volume adsorbed, cc(STP):'g.
Figure 2. Isosteric heats of adsorption for isopropyl alcohol adsorption on Graphon. Dashed line represents heat of liquefaction a t 12.5" (10.35 kcal/mole).
study is 104 mB/g. The values of points B1 and Bz in Figure 1, as determined on a larger scale, are 9.6 and 19.5 cc(STP)/g, respectively, resulting in a co-area for isopropyl alcohol a t B1 of 40 A B . If 19.5 cc is taken as the monolayer volume, the co-area would be 20 A B . The value calculated from the liquid density a t 25" is 27.7 A2, and, since areas computed from liquid densities tend to represent minimum values, we conclude that point B2 cannot indicate the completion of the monolayer. The rather large difference between the co-area and the cross-sectional area computed from liquid density (7) C. Pierce and B. Ewing, J . Phys. Chem., 6 8 , 2562 (1964).
SORPTION
OF
ISOPROPYL ALCOHOL OK GRAPHON
is iiot surprising. The ratio of 40 to 27.7 is 1.45,very close to ratios found earlierEfor other organic molecules adsorbed on graphite. It is likely that the larger values of the molecular area reflect a more planar orientation of the adsorbed molecules due to the influence of the surface than is the case in bulk liquid. The variation in heat of adsorption with coverage shown in Figure 2 is unusual in that the maximum corresponding to the completion of the second layer is exceptionally large compared to the first maximum. One reason for this pronounced second-layer heat is the possibility that isopropyl alcohol molecules adsorbed in the first layer 011 the nonpolar surface become oriented with increased coverage, thereby presenting the second-layer molecules with hydrogen-bonding opportunities. Vertical and lateral hydrogen bonding, plus van der Waals attraction, mould then produce a strong maximum in the region of point B,. Similar reasoning may explain the anomalous effects found for the immersion of graphite in propyl alcohol,E in which the heat of immersion of graphite covered with a monolayer of alcohol was greater than for the immersion of the bare surface. This picture also helps to explain the appearance of the second step in the isotherm immediately after the first. iz high second-layer heat means that the second layer is easy to form and requires little increase in pressure to initiate I n contrast, for systems showing a weak second-layer maximum, a higher pressure would be required to initiate formation of the second layer. Consequently, a more or less flat plateau region between the steps would result, as is usually the case with stepwise isotherms (see, for example, ethyl chloride adsorption on Sterling N T G in ref 4 ) . Avgul, Kiselev, and Lygina6 studied the adsorption of the isomeric butyl alcohols (n-, i-, sec-, and 1-) on a graphitized carbon black a t 20". %-Butyl alcohol gave two well-defined steps in the isotherm, as did isobutyl alcohol. For sec-butyl alcohol, the steps were less pronounced and more like the isopropyl alcohol isotherms reported herein; no steps were evident in the case of t-butyl alcohol. These authors also determined calorimetrically differential heats of adsorption as a function of coverage. The variation in heats for n- and isobutyl alcohol each show two maxima, the second lower than the first, corresponding to the steps in the isotherms. For sec-butyl alcohol, a broad maximum occurs between the first and second knees which is higher in the region of the second layer. -4 single sharp maximum in the heat curve was found for l-butyl alcohol. The heat curve shown in Figure 2 is therefore intermediate between those found for isoand sec-butyl alcohol.
2759
The gradual disappearance of steps in the isotherms and two well-defined maxima in the heat curves in going from n- to t-butyl alcohol were attributed by Avgul, Kiselev, and Lygina6 to increased steric hindrance caused by increased branching of the adsorbate species. Steric hindrance leads to a decrease in lateral interactions which, as mentioned earlier, are one of the requirements for the occurrence of stepwise adsorption. Although Graphon has a much higher area than the graphitized thermal blacks used by I
