R. A. YOUNT
326
Vol. 66
ADSORPTIOX AND DIELECTRIC STUDIES OF THE ALUT\/1CTNA--ETHYL CHLORIDE SYSTE111: AT 3501 BY R. A. YOUNT Department of Chemistry, University of North Carolina, Chapel HiEE, I?. C. Received October 8. 2961
The adsorption isotherm of ethyl chloride on alumina was determined a t 35.0'. The shape of this isotherm resembled that classified by S. Brunauer as Type IV. In conjunction with the isotherm determination, dielectric studies were made on the system. The dielectric constant and loss were measured a t frequencies between 100 e. and 100 kc. for various amounts of adsorbed ethyl chloride. The dielectric loss exhibited an increase until monolayer completion; then it became relatively constant upon further amounts adsorbed. The dielectric constant increased with increasing amount adsorbed. For any given amount adsorbed, both the dielectric constant and loss decreased with increasing frequency.
Introduction This work constitutes a determination of the adsorption isotherm of ethyl chloride on powdered alumina. At various points along the isotherm, dielectric studies were made to see if there is a possible correlation between dielectric properties and the amount of ethyl chloride adsorbed. The alumina-ethyl chloride system was chosen for several reasons. First of all, the alumina used has a very large surface area. Because of this and the ionic nature of the alumina, one would expect the dipolar ethyl chloride to be adsorbed readily. Secondly, ethyl chloride was chosen as an adsorbate with a relatively large dielectric constant, so that the heterogeneous system would exhibit measurable changes in dielectric properties upon increasing amounts adsorbed. Finally, study of this system affords an opportunity to contrast results with those obtained for the system alumina-water,2 and with ethyl chloride on adsorbents other than alumina.3-6 Experimental Materials.-The alumina used was Merck and Co. reagent grade aluminum oxide for use in chromatographic adsorption. Before use, the alumina was vacuum degassed a t 300" for six hr. and cooled under vacuum. After heating a t 300' for 16 hr. more in the adsorption cell, constant values of the capacitance of the cell were obtained. The ethyl chloride used was Matheson Co. U. S. P. grade dried over calcium chloride before use. Apparatus.-The dielectric cell consisted of concentric nickel cylinders held rigidly in two concentric sections of lar e diameter Pyrex tubing. The capacitance of the empty celfin vacuo at 35.0" was 57.1 f 0.1 ppf. between 50 c. and 100 kc. At the same temperature and frequencies the capacitance was 131.75 0.25 ppf. when the cell was filled with alumina. The gas buret was kept at 31.0" and the cell a t 35.00 f 0.03". Dielectric measurements were made with a General Radio Type 1610-A Capacitance Measuring Assembly. Using the Substitution Method? measurements could be made t o an
accuracy of i0.1% or f 0.8 ppf., whichever is larger, in the dielectric constant. The dissipation factor could be measured within f 2y0. Procedure.-Using both adsorption and desorption procedures, the 35' isotherm was established for values of P/Pobetween 0 and 0.825, where P = equilibrium pressure, and PO = vapor pressure of liquid ethyl chloride a t 36.0". For various amounts of ethyl chloride adsorbed, the dielectric constant, e', and dielectric loss, E", were determined at frequencies between 100 e. and 100 kc.
Results and Discussion Adsorption Studies.-For the P/Poranges 0.8250.600 and 0.200-0.000, adsorption and desorption curves (Fig. 1) were coincident. Values of W/g, the number of milligrams of ethyl chloride per gram of alumina, obtained on desorption are greater than those obtained on adsorption in the PIP0 range 0.600-0.200, the maximum discrepancy being about 5% at PIPo = 0.350. For values of PIP0 less than 0.05, the desorption points fell on a smooth curve, whereas the adsorption points had been distributed very erratically. Additional adsorption poir?-ts, determined after completion of the desorption isotherm, fell on the desorption branch of the isotherm, thus defining the hysteresis as irreversible. This probably is due to impurities, mostly permanent gases, originally adsorbed on the adsorbent .8 The isotherm has the same general shape as that classified by Brunauers as Type IV. This isotherm represents multimolecular adsorption with capillary condensation at high pressure. Brunauer, Deming, Deming and Teller,10 discussing a model system with the above features, obtained the isotherm ,
..-
v = v m {m ii +
+
+
-
2 ( c - l)X + 2(C - 1)"2 ( a 0 ah - 2 a c ii2c2)x; 211 + 2(C - 1 ) X ( C - l)ZX2 (GZ h - 2 c aC"x + (2G + 82f.72 + 2ac 2G2 - Zf.72 - 2h - 26h)X."' (ECZ 2c - 2G2 - 2h)X,+'
+
+
+
-
+ +
(1) Based on a thesis by the author submitted to the Graduate School, University of North Carolina, in partial fulfillment of the requirements for the degree of Master of Arts, 1960. (2) M. G. Baldwin, Doctoral Thesis, Dept. of Chemistry, University where FZ = maximum number of layers that can of North Carolina, 1958. fit into the capillary; h = (aC2 - 2C2 2C)g, (3) E. Channen and R. McIntosh, Cun. J . Chem., 33, 172, 341 if the a layers do not exactly fill the capillary; or (1955). layers exactly fill the capillary; (4) R. McIntosh, E. Rideal and J. A. Snelgrove, Proe. Rou. SOC. 12 = C2g, if the (London), A208, 292 (1957). C = constant; g = constant; X = P/P0; V = (5) J. A. Snelgrove, H. Greenspan and R. McIntosh, Can. J . Chem., (8) R. Zsigmondy, Z. anorg. u. allgem. Chem., 71, 3.56 (1911). 31, 72 (1953). (9) S . Brunauer, "The Adsorption of Gases and Vapors," Val. 1 , (6) J. A. Snelgrove and R . McIntosh, i b i d . , 31, 84 (1955). Princeton University Press, Princeton, N. J., 1943. (7) This method is described fully in the instruction booklets ac(10) S. Brunauer, L. S.Deming, W. E. Demingand E. Teller, J . Am. companying the Measuring Assembly. Forms 681-G, 844-B, 785-B Chem. SOC.,62, 1723 (1940). and 661-F.
+
Feb., 1962
A\nSORPTION ANI)
DIELECTRIC 8'lTJI)IES
327
O F ALUMINA-ETHYL &LORIl)E
volume adsorbed; and V , = volume that must be adsorbed to cover the surface with one com100 plete unimolecular layer. Equation 1 gives a fair representation of the ex80 perimental isotherm for ethyl chloride on alumina. From the linear portion of a BET" plot, there was 60 obtained V , = 48.51 mg./g. and C = 27.74. W -. D Using these values, a and h were obtained from 40 equation 1, the best fit being produced with a = 6, and h = 30,000 cal. Typical points are: V = 62.4 at X = 0.25, V = 82.6 at X = 0.36, V = 20 104.3 a t X = 0.50, and V = 130.9 at X = 0.75 (Fig. I). The discrepancy above X = 0.50 may 000 PW 0 arise from the use of a model capillary consisting of open-ended parallel walls. If the capillary walls Fig. 1.-The adsorption isotherm of ethyl chloride on are not parallel, a fifth constant will appear in the alumina at 35.0": solid line, adsorption; broken line, adsorption isotherm. If the capillaries are com- desorption; circles, points calculated from Brunauer, pletely enclosed, and if V , is taken as the amount Deming, Ueming and Tcller equation. of adsorbate to cover the walls of all the capillaries with one unimolecular layer, V, will he a function 2.9 0 of a and will decrease with amount adsorbed. The calculated values of X suggest that enclosed capil2.8 c laries may exist in the alumina used in this work. It is instructive to compare the above BET area for ethyl chloride with that for nitrogen on the 2.7C same adsorbent. The BET area using nitrogen a t e' 77OK. has been determined to be 270 sq. m./g.2 260 Comparing this with the value of V , for ethyl chloride, we see that an area of 63 A.2 is available 2 c for each molecule of ethyl chloride, assuming one molecule adsorbed on each site. Using a relation given by Emmett and Bru2,4( nauer,12 the area actually needed by each molecule can he obtained. The average cross section of the Fig. 2.-The frequency dependence of E' and of E" a t adsorbed molecule is assumed to be the same as various amounts of ethyl chloride adsorbed: a, 27.48 that corresponding to the plane of closest packing mg./g.; b, 46.17 mg./g.; c, 73.52 ing./g.; d, 90.15 mg./g.; in the solidified gas. solid lines, E'; broken lines, E".
I(
A = 4(0.866)(M/442 N D L ) " ~
(2)
-
OD0 2.9 where DL is the density of the liquefied gas, which ---- - - 7 - M is the molecular weight is 0.003 g . / ~ m a. t~ , 20 / 007 of the gas, and N is Avogadro's number. Substituting these values into equation 2 gives a value 006 of 28 for the area needed by each molecule. Thus each molecule occupies only about half of 405 the area that is available to it. This would seem E: to indicate that the molecules are oriented with 0.0 4 their long axes parallel to the surface since they are 0.03 not close enough together for mutual repulsion to have set in. 0.02 Dielectric Studies.-Plots were prepared of the dielectric constant, e', arid of the dielectric loss, 0 20 40 80 100 VqP 6o e", ZIS. the natural logarithm of the frequency, f, Fig. 3.-The dependence of 6' and of E" on the amount of for various amounts of ethyl Chloride adsorbed ethyl chloride adsorbed at various frequenries: a, 1 kc.; (Fig. 2). Clearly the curves for e" 8s. li~fexhibit~b, 10 kc.; c, 50 kc.; solid lines, e'; broken lines, E". no maxima such as those characteristic of a Dcbye7 = dipole relaxation time, although such maxima like system for which may occur at frequenc+s less than 100 c. €" = (eo - e,)w.r/(l + u272-) (3114 Plots of e' and e" us. W / g (Fig. 3) are consistent where eo = static dielectric constant, em = optical with Ikbye-type behavior. As the amount of dielectric constant , w = angular frequency, arid ethyl chloride adsorbed increases, sites of lower binding energy are progressively occupied, giving (11) S. nrunauer, 1'. H. Emmett and E. Tellci, J Am Chem Soc ,60, 309 (1938). a, system characterized by shorter and shorter (12) P. 11. Einmott and S. T3riinaucr, t b z d . , 69, 2682 (1937). relaxation times. The less strongly the molecules (13) J. Tunmermuns, "Phvsico-Chemical Constank," Elsevier Pubare held to the surface, the easier it will be for them Iirhing Co., New York, N. Y . . 1950. to return to their original random orientation once (14) P D d w r " P d n r hIolrrulrs," Clirniirnl Catslop, Nrw York, the field is removed and e" increases. N. Y..192!1. /
E
328
M. K. RERNETT,N. L. JARVIS AND W. A. ZISMAN
For the low values of W/g,the molecules of ethyl chloride occupy the sites of highest binding energy, and the dipole relaxation time is much larger than the phase interval between reversals of the field. Thus COT is very much greater than 1, and df is immeasurably small. Almost until the completion of the monolayer, c f f remains small. Near the completion of the monolayer, sites of lower binding energy begin to be occupied, and 7 becomes smaller so that COTapproaches unity. Thus a measurable absorption region arises, e l f increasing as amount adsorbed increases because T decreases. This continues until the monolayer is completed. Because of the difference in heat of adsorption for the first and subsequent layers, a rather sharp decrease in relaxation time is to be expected at monolayer completion. (Perhaps even the mechanism of orientation changes.) For such a sudden decrease, COT becomes much less than 1. Thus amounts adsorbed past the monolayer will not contribute appreciably to e", but those molecules already adsorbed before completion of the monolayer retain their characteristic relaxation times,
Vol. 66
producing a plateau in the curve for amounts adsorbed past the monolayer. This is in contrast to the alumina-water system in which Baldwin2 found that e" did not begin to increase until after the monolayer had been completed. The studies of McIntosh and co-worker~~--~ of ethyl chloride on silica gel and on rutile were made in the megacycle region and thus cannot be compared with the alumina-ethyl chloride system. The values of V m as found by identifying the point of monolayer completion from the plateau characteristics are in good agreement with those found from the adsorption isotherm. At a frequency of 1 kc., V , = 51 mg./g.; at 10 kc., V m = 50 mg./g.; and at 50 kc., V , = 49 mg./g. Thus it seems reasonable to identify the beginning of the plateau in the 6'' os. amount adsorbed curve with the completion of the monolayer. The author wishes to thank Professor J. C. Morrow for his valuable suggestions and criticisms. The help of Mr. D. E. Sampson and Dr. M. G. Baldmin in the construction of the vacuum system and dielectric cell is gratefully acknowledged.
SURFACE ACTIVITY OF FLUORINATED ORGAXIC COMPOUNDS AT ORGANIC LIQUID-AIR INTERFACES. PART IV. EFFECT OF STRUCTURE AND HOMOLOGY1 BY MARIANNE K. BERNETT,N. LYNXJARVIS AND W. A. ZISMAN U.S. Naval Research Laboratorv, Washington 86,D. C . Received October 2, I961
Previous investigations have shown that partially fluorinated carboxylic esters adsorb a t organic liquid-air interfaces as monomolecular films, thus depressing the surface tension of the organic liquid. I n this investigation the surface activities of specially designed fluorinated solutes were studied in seven organic solvents of different compositions and surface tensions. By varying the structure and composition of the organophobic the organophilic and the connecting polar groups, their contribution to solubility, adsorptivity, and orientation and pack& of the solute molecules a t the organic liquid-air interface could be investigated. The degree of solubility, as well as the packing of the molecules at the surface, was shown to be dependent upon fluorination, length and number of the organophobic chain, and the structure of the organophilic portion. From the force-area isotherms the lowest area per molecule attainable in each solvent was calculated. The relation of these lowest areas to the corresponding lowest values of surface tension obtained was discussed in terms of solute structure and orientation a t the interface. As indicated by surface tension values and the steep initial slopes of the surface tension us. concentration curves, several of the new fluorinated solutes have much higher surface activity than those previously reported.
Introduction In Part I2of this investigation it was shown that the initial spreading coefficient can be used to rapidly screen a large number of amphipathic compounds for possible surface activity in organic liquids. By using this technique several classes of partially fluorinated organic compounds were shown qualitatively to be promising surface active agents for organic liquids. I n Part IIa it was demonstrated that the partially fluorinated carboxylic esters were surface active in various organic liquids, the surface tension depression in any one organic liquid being a function of the balance between the organophobic and organophilic constituents of the molecule. These fluoroester solutes adsorbed at the organic liquid-air interfaces as (1) Presented before the Division of Colloid and Surface Chemistry, American Chemical Society, a t the 140th National Meeting in Chicago, Illinois, September 4-8, 1961. (2) N. L. Jarvis and W. A. Zisman, J. Phv6. Chem., 68,727 (1959). (3) N. L. Jarvis and W. A. Zisman, ibid., 64, 150 (1960).
unimolecular films whose orientation and packing depended upon the molecular structure, solubility and extent of association of the solute and solvent molecules. From the F os. A isotherms, equations of state were calculated in Part 1114for each adsorbed monomolecular film. It was concluded that the adsorbed molecuIes fail to form close-packed condensed monolayers even a t the highest film pressures; at low film pressures all films are gaseous monolayers. In the present study emphasis is given to the search for fluorinated solutes having the highest possible surface activities in organic liquids. For this purpose selected types of partially fluorinated compounds were designed, synthesized and studied, some in homologous series. The previous studies suggested some primary qualifications for optimum surface activity, such as low volatility, organophobic fluorocarbon chains so situated as to present (4) N. L.Jarvis and W.A. Zisman, i b i d . , 64, 157 (1960).
