WATERn Y CAKBUN integral values of the temperature. The entropy a t 29S.lG'K. is 22.44 e.u., of which 0.41 e.u. is conTABLE I MOLARHEATCAPACITY OF CUPROUS OXIDE Mol. wt. 143.14 g , 0.7844 mole Mean Z', OK.
14.72 16.47 18,99 21.31 23.49 25.22 26.00 28.22 28.75 30,53 31.31 32.84 35.70 38.89 40.87 42.72 43,39 46.69 47, 04 50,34 51.84 54.00 57.01 57.82 60.65 61.65 64.01 69.44 76.39 82 80 I
CP,
tributed by extrapolation below 16'K. Millar3 reported a value of 21.7 f 1.0 e.u., based on his own data from 75 to 291'K. and an extrapolation below --0 I J K.j TABLE I1 THERMODYNAMIC FUNCTIONS FOR CUPROUSOXIDE
CP I
6, NC-1; 7, Graphon. 0.997), Graphon holds only two statistical layers of water vapor, whereas i t can sample cannot be pumped down a t room tempera- hold in excess of 30 statistical layers of ethyl chloture t o its original weight after desorption is com- ride. On the other hand, a charcoal will hold only pleted. When the contact with water vapor is prolonged there is some reaction and oxygen is firmly held by the surface. 4oo Surface Complexes.-It has been previously shown b y several workers' that when carbon is exposed to oxygen a t elevated temperatures some of the oxygen is 300 firmly held as a surface complex. Weller and Younglg for example, find 3-18% 4 oxygen on a charcoal after treating with 5 oxygen a t 400". The C-0 complex has IL been shown to have an effect on water L adsorption. It is difficult t o remove by 200 & pumping, except at high temperature. The effect of oxygen complex on water absorption is shown in Fig. 4,which gives 2 water isotherms for Graphon with and >without oxygen. The sample was heated O0 to 600' then poured in air from one beaker to another as i t cooled. It was then sealed into an adsorption tube and outgassed by gentle flaming until a good vacuum was obtained. The water isotherm 2 4 6 8 P/ p, of the oxygen-treated sample shows much greater adsorption than that Of the un- Fig. 6.-Isotherms for water a t 28.6' and ethyl chloride a t 0' on treated Graphon, and there is pronounced Darco G60: a, water adsorption; Q, water desorption; 0, ethyl chlodesorption hysteresis, even though the ride adsorption ; 0 , ethyl chloride desorption. sample is non-porous. Emmett and Anderson14 have shown similar results for carbon slightly less liquid volume of water than of other VapOrS. (19) s Weller and T.F. young, THISJOURNAL, TO, 4155 (1948).
-
'
CONWAY PIERCE, R. NELSONSMITH, J. w.WILEY
4556
2
Fig. 7.-Water
4
P/ po
6
e
isotherms of Spheron Grade 6 at 28.6': 0, adsorption; 0 , desorption.
Another effect as one passes from non-porous to porous surfaces is shown in Fig. 5, which gives isotherms for several carbon samples. To facilitate comparison these are plotted as fraction of saturation, V/Vi. The isotherm for the fine pore charcoal S 600 H shows a sharp rise starting a t about 0.55 Po. For S 84 with larger pores, the isotherm rises a t 0.75 Po. Graphon and NC-1 isotherms show practically no adsorption below 0.95 PO.
AND
w.CORDES
VOl. 73
Po are very narrow, these capillaries may already be filled by ethyl chloride when the isotherm levels out a t point B. (These capillaries might be but two or three molecular diameters in width.) To fill a capillary volume of 0.072 cc. requires about 1 millimole or 22.4 cc. (S.T.P.) of ethyl chloride vapor (assuming a density of 0.93 a t 0" for liquid ethyl chloride). Therefore the free surface in capillaries not ailled a t point B is covered by 60 22 = 38 cc. (S.T.P.) of ethyl chloride vapor. *is pressure is increased, the multilayer film of ethyl chloride now builds up on this free surface only, since the surface in the small capillaries has presumably disappeared with the filling of the small capillaries. The ethyl chloride isotherm shows that there are also still larger capillaries which are filled a t high relative pressure and which appear to be too wide to be filled completely by water. The saturation adsorption, measured after allowing the sample bulb to equilibrate with bulk liquid for many hours, amounts to about 0.35 cc. per gram for water and 0.94 cc. per gram for ethyl chloride. Capillaries in Carbon Blacks.-The line of reasoning of the last section may be extended to an interpretation of the water isotherms of carbon blacks. The water isotherm for Spheron 6, Fig. 7, shows a n upturn between 0.55 and 0.7 Po, which we interpret as indicating the presence of small capillaries which fill with water in this region, since Graphon has in the same region practically no adsorption. The nitrogen isotherm and the ethyl chloride isotherm both fail to show the
On the basis of this comparison, we believe that the existence of a sharp rise below 0.9 POin the water isotherm of a carbon sample provides 1 I I I I I I 1 indication for the presence of capillaries. 70 0 Further, i t is indicated that the smaller the capillary width, the lower the relative pressure a t which the sharp rise begins. If this interpretation is correct, the shape of a water isotherm for a clean carbon sample may be used to detect the presence of capillaries and the amount of adsorption a t the nearly vertical portion of the isotherm permits one to compute the volume held in these capillaries. The water isotherm for Darco G GO, Fig. 6, illustrates such a computation. -4n upturn begins a t about 0.55 $0 and a t about 0.7 Po the isotherm flattens again. The adsorption in this region amounts to some 90 cc. of vapor (S.T.P.). .2 .4 6 8 P/ p. If our interpretation is correct this indicates that there are present small capil- Fig. S.--Water isotherms of Carbolac I a t 28.6": 0, adsorption; 0 , laries which fill a t 0.55-0.7 PO. The desorption. liquid volume held in these capillaries is equivalent to 90 cc. of water vapor, or 4 millimoIes. presence of these capillaries. Presumably the Assuming a density of one for the liquid water, this pores are so narrow that the carbophylic adsorbates gives a liquid volume of 4 X 0.018 = 0.072 cc. in fill them in the low-relative pressure region at which these capillaries. the first layer is formed on the exposed surface. It is of interest to compare the water isotherm If so, the point B adsorption includes the amount for Darco G 60 with that for ethyl chloride, Fig. 6. of vapor held in these narrow capillaries as well as A t "point B" about 60 cc. (S.T.P.)of ethyl chloride that held as a monolayer on the surface not within is adsorbed. If we assume, as appears logical, that the capillaries. Emmett and Anderson l 4 have previously ohthe capillaries which thv water fills at 0.55-0.7 I
Oct., 1951
MOLECULAR WEIGHTOF SALTS IN BENZENE
served the same type of water isotherm for deg s s e d Spheron 6 that we find, but their isotherm indicates about twice as much water adsorption as we find in the range 0.5 to 0.7 PO. This is not disturbing, in view of the fact that their sample and ours were treated differently before the isotherm was determined. If we are correct in assuming that Spheron 6 particles have small cracks, the nature of the heat treatment will affect the porosity. After strong heating, in preparation of Graphon, all evidence of pores has disappeared. For most of the Cabot carbon blacks that are thought to be essentially non-porous, the size of particles as computed from nitrogen areas is in good agreement with electron microscope values. Carbolac I is an exception. The electron-microscope arithmetic-mean diameter of the particles is 106 A. The nitrogen area of about 1000 sq.m./g. gives a computed mean diameter of 26 A. The discrepancy would correspond to a roughness factor of 4 for the nitrogen area. The water isotherm for Carbolac I, Fig. 8, and the previously published water isotherm determined by Emmett and Anderson14 both show the type of rise we have attributed to the effects of capillaries. Comparison of the Carbolac I isotherm with that for Spheron 6 shows however an important difference, The rise for both starts a t about 0.55 PO but for Carbolac I there is no flattening a t or about 0.7 pa as is the case for the Spheron isotherm. This suggests that the capillaries to which we ascribe the rise are wider in the Carbolac particles than in the Spheron particles, and that there is a greater range of capillary widths. If these capillaries are filled by nitrogen a t point B they may account for the large roughness factor. [CONTRIBUTION FROM
THE
4557
An alternative explanation might be made for the capillary-like nature of the water adsorption by Carbolac I. This is that the capillaries might not be within the rather small granules but that they might be in the spaces between the granules. The ultimate particles of Carbolac I are very small and it might be that they fit closely enough together that the voids between particles are sufficiently narrow to act as capillaries in which vapor may condense. There is some indication for such condensation of vapor. When the sample bulb is removed for weighing after equilibration a t high relative pressure with any of the vapors we have studied, i t has been noted that the particles are agglomerated and stick together as a solid ball. This must be due to a film of liquid between the particles which acts as a binder. As soon as the relative pressure is lowered this clumping disappears. Desorption Hysteresis-When water vapor is desorbed from a carbon surface there is always a pronounced hysteresis, regardless of whether the sample is porous or not. Some speculation regarding the possible source of hysteresis in absence of pores has been previously p ~ b l i s h e d . ~ In addition to “reversible hysteresis” in which the isotherm loop closes a t a relative pressure near 0.4 PO water desorption isotherms also sometimes fail to rejoin the adsorption branch, even a t zero pressure. This is, we believe, due to chemisorption or reaction with water during the period in which the vapor is in contact with carbon. Such an effect seems to be more pronounced when one starts with a surface which has some oxygen complex than with an initially clean surface. CLAREMONT, CALIF.
RECEIVED JANUARY 16, 1951
METCALF RESEARCH LABORATORY OF BROWNUNIVERSITY!
Properties of Electrolytic Solutions. LIII. Molecular Weight of Salts in Benzene by the Cryoscopic Methodl B Y DAVIDT. COPENHAFER~ AND CHARLES A. KRAUS The molecular weights of triisoamylammonium picrate, n-amyltri-a-butylammoniumiodide, n-octadecyltri-n-butylammonium formate, tetraisoamylammonium thiocyanate and tetra-n-butylammonium thiocyanate have been determined in benzene up to the limit of their solubilities by a cryoscopic method. The molecular weights of equimolar mixtures of the last two salts have likewise been determined. All these salts exhibit marked association in benzene solution, the degree of association being dependent on structural and constitutional factors. For the two thiocyanates and the iodide, the association number, n (ratio of observed molecular weight to formal weight) passes through a maximum a t concentrations ranging between 0.12 and 0.28 molal; the association numbers, at the maximum, range between 21.9 and 31.9. The association number of tri-isoamylammonium picrate varies between 1.01 at 0.001 m and 1.58 a t 0.1 m. Of the salts measured, octadecyltri-n-butylammonium formate exhibits the highest degree of association; a t 0.001 m, n = 10.7; a t 0.0033 m, n = 21.5. The association number for the mixture of the two thiocyanates at a given total concentration is the mean of the value for the two components alone a t the same concentration. The results of this investigation are discussed and correlated with the results of conductance measurements.
I. Introduction and Rothrock6 with solutions of salts in benzene Cryoscopic measurements by Kraus and Vingeea have shown that, in these solvents of low dielectric with solutions of salts in dioxane and by Batson4 constant, association of ion pairs takes place to a measurable extent a t concentrations as low as 0.001 (1) This paper is based on a portion of a thesis submitted b y David m. At higher concentrations, the degree of associaT.Copenhafer in partial fulfillment of the requirements for the degree tion increases markedly, particularly with salts of of Doctor of Philosophy in the Graduate School of Brown University, May, 1942. Paper L I I , THISJOURNAL, 78, 3293 (1951). large cations. However, these earlier measure(2) University Fellow in Brown University. 1940-1941. Metcalf ments were not carried t o concentrations above a Fellow, 1941-1942. few hundredths molal. It seemed of interest, (3) C A Kraus and R. A. Vingee, THISJOURNAL,16, 511 (1934). (4) F M Batqon and C A. Kraus. ihid.. 16, 2017 (1934).
( 5 ) D. A. Rothrock, Jr., and C. A. Kraus, {bid., 19, 1699 (1937).
