Adsorption from nonelectrolyte solutions onto Cab-O-Sil

Notes Adsorption from Nonelectrolyte Solutions onto Cab-O-Sil I Adsorption at the liquid-solid interface is important in understanding such areas as liquidsolid chromatography and the stability of colloidal dispersions. Far less emphasis has been placed on experimental studies of adsorption at the liquid-solid interface (1) than for such studies at the gas-solid interface (2, 3). This disparity is due to a fundamental difference in the measurement of adsorption in the two cases. The adsorption of a single component from the gas phase onto a solid surface can be measured unambiguously by either vohunetric or gravimetric techniques. On the other hand, there is no direct experimental method to measure the adsorption of a single component from the liquid phase onto a solid surface. However, progress has been made in the measurement of adsorption on solid surfaces from binary solutions (1). The systems chosen for study in this work represent several intermolecular interactions between the adsorbed liquid molecules and the surface of the solid. Silica, specifically a flame hydrolyzed silica, Cab-O-Sil, was chosen as the adsorbent. Cab-O-Sil has been used in gas phase adsorption studies (4--9) and thus some aspects of its surface chemistry are known. Bassett et al. (9) have concluded that the surface of Cab-O-Sil is approximately 25% hydrophilic based on a comparison of water and argon surface areas. Hair and Hertl (6) reported a value of 1.7 OH groups for 100 ~ based on reaction of Cab-O-Sil with methanol which corresponds to approximately 25% of a fully hydroxylated silica surface (7.9 OH groups/100 ~2) (10). Thus the surface of Cab-O-Sil consists of both siloxane ( - - S i - - O - Si--) as well as silanol (--Si--OIt) groups. The present work is the first phase of a thermodynamic study of solution adsorption and this paper reports the results of adsorption from ethyl alcohol-cyclohexane, cyelohexane-benzene, and benzene-ethyl alcohol solutions onto Cab-O-Sil. The use of Cab-O-Sil as an adsorbent in studies from the above solutions has not been reported. 1 The support of the National Science Foundation is gratefully acknowledged under Grant GP-8448. The authors dedicate this paper to the memory of Dr. J. J. Kipling.

However, adsorption on a different silica from ethyl alcohol-benzene solutions has been reported by Kipling and Peakall (11) and by Schay et al. (12). EXPERIMENTAL METHODS The surface area of the adsorbent is needed in the calculation of the amounts of liquid components adsorbed. The surface area of Cab-O-Sil was determined to be 223 =t= 2 m~/gm from low temperature nitrogen adsorption. Duplicate samples (0.1 gm) of Cab-O-Sil were outgassed at ll0°C for 2-4 hr under vacuo prior to surface area measurements. Benzene (Fisher: 99 mole % grade) and cyclohexane (Fisher: 99 mole % grade) required no pretreatment and were used as received. It was necessary to dry the ethanol over Cab~-O-Sil prior to its use in solution adsorption measurements. Cab-O-Sil (0.5 gm) was oven dried in custom tubes at 120°C for at least 15 hr. The tubes were capped in the oven and placed immediately in a dry box to cool. Solutions were prepared volumetrically at 0.1 intervals over the entire mole fraction range and 20 ~rd were introduced into the Cab-O~Sil tubes in the dry box. Solutions in tubes without Cab-O-Sil were used as references in the differential refractometer measurements. All tubes were sealed and equilibrated at 30°C for at least 48 hr. The tubes were then centrifuged, opened, and a sample of the supernatant liquid was withdrawn and compared with the reference sample (not equilibrated with Cab-O-Sil) using a Brice-Phoenix differential refractometer. The differential refractometer was calibrated using solutions of known concentration prepared gravimetrically. The change in mole fraction of the reference and equilibrated solutions is directly related to the amount of both liquid components adsorbed. RESULTS AND DISCUSSION A plot of the experimental data for the adsorption from ethyl alcohol-cyclohexane solution onto Cab-O-Sil at 30 ° is shown in Fig. 1 where the product of the total number of moles of both components (No) and the change in mole fraction

Journal of Coll~d and Interface Science, Vol. 35, No. 2, February 1971

354

NOTES

18

355

Q

% 09 12 O

\ H <3

Z°

6

0

0

I

I

I

0.2

0.5

0.8

1.0

XA

FIG. 1. Net isotherm for adsorption on Cab-0-Sil from ethanol (A)-eyelohexane (B) solutions at 30°C.

O

O

3

2 S

o 1

-

o

"

I

0.2

,.

I

0.5

t

0.8

1.0

XA

FIG. 2. Net isotherm for adsorption on Cab-O-Sil from benzene (A)-cyelohexane (B) solutions at 30°C. Journal of Colloid and Interface Science, V o l . 35, N o . 2, F e b r u a r y 1971

NOTES

356

solid surface. Such a curve is t e r m e d a net adsorption i s o t h e r m since it reflects t h e adsorption of b o t h components. M e a s u r e m e n t s were also made at 25 and 35 ° and no significant differences in the net adsorption isotherms were observed. Thus, the t e m p e r a t u r e dependence of adsorption from cyclohexane-ethyl alcohol on silica is negli-

of ethyl alcohol (AXA) is plotted against the equil i b r i u m mole fraction of ethyl alcohol (Xx). T h e value of the NoAX• product passes t h r o u g h a m a x i m u m at a mole fraction of about 0.2. T h e rationale for such a m a x i m u m in solution adsorption contrasted to gas adsorption is t h a t b o t h components m a y be adsorbed from solution at the

16 0

-g

. •

12

o

o

o

~

o


~

4

i

00'

,,,

0.2

I

I

0.5

0.8

.0

XA

FIG. 3. N e t i s o t h e r m for adsorption on Cab-O-Sil from ethanol (A)-benzene (B) solutions at 30°C

O ETHANOL @ CYCLOHEXANE

o

O

'.~ 20

O

Y @

i0

J

0

,

O.2

A

A

0.5

•

I

|

0.8

•

1.0

xA FIG. 4. Individual isotherms for adsorption on Cab-O-Sil from ethanol (A)-eyclohexane (B) solu. tions at 30°C. Journal of Colloid and Interface Science, Vol. 35, No. 2, February 197i

NOTES gible, which is in m a r k e d c o n t r a s t to t h e t e m p e r a ture dependence of adsorption from the gas phase. The net adsorption i s o t h e r m for t h e adsorption from cyclohexane-benzene solution onto Cab-OSil at 30°C is shown in Fig. 2. T h e shape is similar to t h a t for the preceding case except for the inflection at XA ~ 0.6. T h e net adsorption i s o t h e r m for the adsorpt i o n from b e n z e n e - e t h y l alcohol solution onto Cab-O-Sil at 30 ° is shown in Fig. 3. Again, the shape is similar to those for the preceding cases. T h e presence of trace w a t e r in the b e n z e n e - e t h y l alcohol solutions has a m a r k e d effect on t h e shape of the net adsorption isotherm. T h e net i s o t h e r m t u r n e d upwards sharply at high mole fractions of ethyl alcohol if trace w a t e r were present. Removal of trace water from ethyl alcohol as indicated in t h e E x p e r i m e n t a l Section produced t h e e x t r a p o l a t i o n to zero as shown i n Fig. 3. T r a c e w a t e r has been shown to have a m a r k e d effect on o t h e r interracial properties (13, 14). T h e net adsorption isotherms are not very ins t r u c t i v e b y themselves. T h e resolution of the net isotherms into individual isotherms has been discussed b y Kipling and Tester (15) and b y E l t o n (16). E q u a t i o n s (1) and (2) express NA s and NB s, the moles of components A and B adsorbed on the surface, respectively, in terms of the experimenN S = z X A + No ~XXA ~B

~A XA + ~B XB

(1)

357

N~ =

~X~

- No

AXA ~B

zA XA + zB XB

(2)

t a l l y accessible values of Z (surface area) X • , X n , and NoAXA and l i t e r a t u r e values for ¢~ and o'B . T h e assumptions made in the resolution of the net isotherms have been discussed b y Kipling (17). T h e cross-sectional areas (z) of benzene, cyclohexane, and ethyl alcohol adsorbed on CabO-Sil were t a k e n as 32, 46, and 18 ~2/nlole, respectively. The resolved isotherms for the ethyl alcoholcyclohexane-Cab-O-Sil system are shown in Fig. 4. E t h y l alcohol is preferentially adsorbed on CabO-Sil over a wide range of concentration. T h e adsorption of cyclohexane is minimal over the same range. T h e dipole-dipole i n t e r a c t i o n between ethyl alcohol and the polar silica surface appears to predominate over the dispersion interaction between the surface and cyelohexane. T h e resolved isotherms for the cyclohexanebenzene-Cab-O-Sil system are shown in Fig. 5. N e i t h e r component is strongly preferentially adsorbed on the silica surface, although benzene is adsorbed to a somewhat greater extent t h a n cyclohexane. Thus, the ~r electrons of benzene do not i n t e r a c t exclusively w i t h the Cab-O-Sil surface compared to t h e nonspeeific dispersion i n t e r a c t i o n of cyclohexane. T h e resolved isotherms for the b e n z e n e ethyl alcohol-Cab-O-Sil system are shown in Fig. 6. E t h y l alcohol is preferentially adsorbed on the silica surface, although a finite adsorption of benzene is also observed. Again, however, the

O BENZENE

• CYCLOHEXANE

~2o % l0

O0

0.2

0.5

0.8

1.0

FIG. 5. Individual isotherms for adsorption on Cab-O-Sil from benzene (A)-cyclohexane (B) solutions at 30°C Journal of Colloid and Interface Science, Vol. 35, No. 2, February 1971

358

NOTES

(9 E T H A N O L

•

BENZENE

o1o1911

20 O

~i0

0

0

I

I

0.2

0.5

~

0.8

1.0

XA

FIG. 6. Individual isotherms for adsorption on Cab-O-Sil from ethanol (A)-benzene (B) solutions at 30°C. dipole-dipole interaction between alcohol and the silica surface is the predominant one. Comparison of the present results with the literature (11) can be made only in the ethyl alcohol-benzene case where the shape of both the composite and individual isotherms is the same. The surface area of the silica used is not stated explicitly although a value of 620 m2/g can be obtained from the monolayer capacity of benzene vapor [Table 4 of Ref. (11)] assuming 32 ~2/mole. Thus, the surface area is 2.75 times larger than that of Cab-O-Sil. Hence the difference (4.2 mmoles/g compared to 1.5 mmoles/g) obtained by a Schay extrapolation (19) can be attributed entirely to a difference in surface area. In summary, isotherms have been obtained for the adsorption on a silica surface from three binary solutions. The measured isotherms are consistent with intermolecular forces between the liquid molecules and the polar silica surface. Heats of solution adsorption measurements are now in progress and the adsorption isotherm measurements reported here will be necessary to interpret these heat measurements. 1.

2.

3.

4.

REFERENCES KIPLING, J. J., "Adsorption from Solutions of Nonelectrolytes." Academic Press, New York (1965). YOUNG, D. M., AND CEOWEL~, A. D., "Physical Adsorption of Gases." Butterworths, London (1962). HAYWARD, D. O., AND TRAPNELL, B. M. W., "Chemisorption." Butterworths, London (1964). BASILA, M. R., J. Chem. Phys. 35, 1151 (1961).

5. YATES, D. J. C., J. Chem. (1964). 6. HAZE, M. L., AND HERTL, W., 73, 4269 (1969). 7. MCDONALD, it. S., J . Phys. (1958). 8. HAIR, M. L., AND HERTL, W., 78, 2372 (1969).

Phys. 40, 1157 J. Phys. Chem. Chem. 62, 1168 J. Phys. Chem.

9. BASSETT, D. n . , BOUCHER, E. A., AND ZETTLE-

10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

~OYER, A. C., Prepr. Nat. Colloid Syrup., 44th, 1970. PERI, J. B., AND HENSLEY, JR., A. L., J. Phys. Chem. 72, 2926 (1968). KZPT,ING, J. J., AND PEAKALL,D. B., J. Chem. Soc., London 1957, 4054. ScHAY, G., NAGY, L. G., AND SZEKRENYESY, T., Period. Polytech. 4, 95 (1960). WIGHTMAN, J. P., AND CHESSICK, J. J., J . Phys. Chem. 66, 1217 (1962). YOUNG, G. J., AND CttESSICK, J. J., J . Colloid Sci. 13,358 (1958). KIPLING, J. J., AND TESTER, D. A., J. Chem. Soe., London 1952, 4123. ELTON, G. A. H., J. Chem. Soc., London 1952, 1955. Ref. (1), pp. 40-41. McCLELLAN, A. L., J. Colloid Interface Sci. 23,577 (1967). Ref. (1), pp. 47-52. D. R. MATA~CO J. P. WIGHTMAN

Department of Chemistry Virginia Polytechnic Institute and State University Blacksburg, Virginia 2~061 Received March 31, 1970; accepted October 19, 1970

Journal of Colloidand Interface Science, Vol. 35, No. 2, February 1971