Ansorimros
OF
ATXOHOLSox RUTILEF R O M ~ - X Y I , E N SOLUTIONS E
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Adsorption at the Solid-Liquid Interface. 11. Alcohols on Rutile from Solutions in p-Xylene by G. D. Parfitt and I. J. Wiltshire Department of Chemistry, IJniaeraity of R'ottingham, Nottingham, England
(Received June 96, 1961)
The adsorption by a pure rutile of alternate members of the homologous series of aliphatic straight-chain primary alcohols (ethanol to octadecanol) from solutions in p-xylene has been studied at 25'. The amount of each alcohol adsorbed a t low relative coriceritratioiis reached a minimum value at hexanol and octanol, indicating the importance of solutesolvent interactions to the adsorption at the solid-liquid interface. Adsorbed water on the rutile surface caused a significant reduction in the extent of adsorption.
Introduction From the significant amount of data that have been reported on adsorption at the solid-liquid interface, it is clear that the process is complicated by a large number of factors. Both adsorbate-adsorbent interactions and those between the components in solution, as well as the surface structure of the adsorbent, are important in determining the extent of adsorption. Furthermore, adsorption from nonpolar media onto polar surfaces is affected by the amount of water present in the system. The present paper is concerned with the adsorption by a pure rutile of alternate members of the homologous series of aliphatic straight-chain primary alcohols (ethanol to octadecanol) from solutions in pxylene at 25 '. Ethanol to dodecanol inclusive form binary liquid mixtures with p-xylene at 25' over the entire range of mole fraction, whereas for the remainder, the range of concentration available for adsorption studies is restricted by solubility.
Experimental Materials. Two samples of pure rutile (>99.90/,), supplied by British Titan Products Co. Ltd., of Billingham, County Durham, were prepared by hydrolysis of purified 1itanium tetrachloride and the product calcined at about 400". Surface areas of 24.4 and 21.0 ni.2 g.-l were determined by the B.E.T. method using nitrogen at - 195 '. In the adsorption experiments, the sample of area 24.4 nx2 g.-l was used with ethanol to ocianol, respectively, and the other sample with the remaining
alcohols. Before use, the rutile samples were outgassed to an ultimate pressure of mm. and twice treatcd with spectroscopic oxygen at a pressure of about 10 cm. for 30 min., the residual oxygen being pumped off after each treatment. The temperature of the sample was maintained at 400 ' throughout and a liyuid nitrogen trap isolated the sample from stopcock grease. The gray coloration in the samples outgassed a t 400' was irreversibly removed by the oxygen treatment. This activation procedure is similar to that used by Hollabaugh and Chessick.2 p-Xylene (B.D.1-I. Ltd.) was distilled from phosphorus pentoxide and the fraction boiling at 139' was collected. From Karl Fiseher ineasurenients the pxylene was found to contain 0.005% by weight of water. Absolute commercial alcohol was refluxed with silver nitrate, distilled, and allowed to stand over calcium sulfate, finally distilled, and the fraction boiling at 78.5' collected. n-Butyl alcohol (R.D.H. Ltd.) was left to stand over molecular sieves Type 4A for 2 days, decanted, and distilled, the fraction boiling at 119 being collected. The remainder of the alcohols wcrc Puriss grade from Fluka of Switzerland and werc usrd as supplied. From analysis by g.l.c., irifrarcd, and n.1n.r. spectroscopy, all the alcohols were found to bc of very high purity. O
(1) W . Hirst and J. K. Lancaster, T r a m . Faradail Soc., 47, 315 (1951).
(2) C. .If. Hollabsugh and J. J. Chessick, J . Phys. Chem.. 6 5 , 109 (1961).
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Procedure. In the majority of the adsorption experiments, samples of rutile after oxygen treatment (ca. 1 g.) and solution (ca. 5 g. made up by weight) were weighed into clean dry Pyrex adsorption tubes, which were then sealed by fusion and rotated in a water thermostat at 25.0 f 0.2” for a t least 12 hr., although adsorption equilibrium was found to be established in a shorter time. The tubes were then centrifuged a t 2500 r.p.m. for 10 min. and returned to the thermostat before removing the clear supernatant liquid for analysis. The solutions were analyzed using a differential refractometer of the type designed by Brice and Halwer3 with a cell obtained from the Phoenix Instrument Co. of Philadelphia, Pa. The original and equilibrium solutions were placed in the two compartments of the cell and the concentration of the unknown was found from a calibration using known solutions of concentration in the range under examination. I n the experiments carried out under very dry conditions using octanol and octadecanol as adsorbates, each sample of treated rutile in an evacuated thinwalled Pyrex bulb (similar to those used for heat of immersion experiments) was placed in an adsorption tube containing a known weight of the alcohol, which was then sealed to a vacuuni apparatus. A tube containing a known weight of p-xylene over degassed rutile, used as a “getter” for water (previously filled and allowed to stand in a drybox), was attached to the apparatus, the contents of both tubes were frozen, and the apparatus then pumped down to IO+ mni. The p-xylene was cold-distilled into the adsorption tube which was then sealed off under vacuum and the bulb was broken under the solution by ultrasonics. The calorimeter used to measure heats of imniersion consisted of a silvered dewar flask (capacity 350 ml.) fitted with a B55 Quickfit joint and included a heater, stirrer, sample holder and breaker, and a “Stantel” thermistor (resistance 1800 ohms and temperature coefficient 56 ohnis/deg. at 20’) for temperature measurement. Resistance changes in the thermistor were measured using a Cambridge vernier potentiometer and a Tinsley Type V.S. 645 galvanometer, and the accuracy of temperature measurement was f1 X The calorimeter was immersed in an oil thermostat a t 25.0 f 0.005’. Sample bulbs were blown from Pyrex glass and an average value of 0.150 cal. for the heat of bulb breaking was used to correct the measured heats of immersion.
Results and Discussion The following heat of immersion values were obtained for samples of the activated rutile (three to five measurements in each case): (a) in water, 626 f 13 ergs The Journal of Physical Chemistry
G. D. PARFITT AND I. J. WILTSHIRE
(b) after exposure to the atmosphere, in water (c) in dry p-xylene over “getter,” 282 f 6 ergs 131 13 ergs (d) in dry p-xylene after exposure of the xylene to the atmosphere, 166 -f 16 ergs cm.-z, (e) in p-xylene saturated with water, 257 f 12 ergs The large difference between (a) and (b) is presumably the result of adsorption of water vapor from the atmosphere onto the activated rutile. Only a small increase in the heat of immersion in p-xylene resulted from exposure of the liquid to the atmosphere. For adsorption from a two-component system the composite isotherm obeys the relation
*
noAx/m
=
nls(l - x ) - n:x
(1)
where Ax is the decrease in mole fraction of component 1 when no moles of original solution are brought into contact with m grams of adsorbent, and nls and n: are the numbers of moles of components 1 and 2, respectively, which are adsorbed per gram of solid. To calculate the individual isotherms for the two components, it is usual4 to assume that the adsorbed layer is one molecule thick as expressed in the relation
where (nls),. and (n:),. are the corresponding numbers of moles required to cover 1 g. of adsorbent to the extent of a monolayer. Choice of values for (nls), and (nz’),. is often difficult, although where adsorption measurements from the vapor phase are possible, monolayer values derived from vapor isotherms have been used successfully5 with the assumption that the molecules adopt the same orientation at both the solidliquid and solid-vapor interfaces. With less volatile adsorbates, cross-sectional areas based on molecular models are usually used assuming a specific orientation at the surface and close-packing of the molecules. However, in the case of polar molecules adsorbed on a crystalline polar surface it is probable that localized adsorption occurs over specific sites which may not correspond to a close-packed monolayer. In this work it has been assumed that p-xylene molecules are adsorbed with their major axis parallel to the surface of the adsorbent and an area of 45 A.z from a molecular model used in the calculation of (n:),.. The choice of an area for an adsorbed alcohol molecule has been based on the octadecanol coniposite isotherms as shown in Fig. 1. Since the octadecanol solutions are very dilute, eq. 1 reduces to noAx/m nls and the composite isotherm may therefore be taken as equiva-
-
(3) B. A. Brice and M. Halwer, J . Opt. Soc. Am., 41, 1033 (1951).
G. D. Parfitt and E. Willis, J . Phys. Chem., 68, 1780 (19G4). (6) C. G . Gasser and J. J. Kipling, ibid.,64, 710 (1960).
(4)
ADSORPTIOX OF ALCOHOLS ON RUTILEFROM P-XYLENE SOLUTIONS
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' 1 ---I
I
16 ?..
I
1
I
9 14 X
ab 12
1
s
e" 10
v
E
\
d 8 c'
6 2.
4 0
1
2
3
4 2
5 6 x 108.
7
8
9
1
0
Figure 2a. Composite isotherm for adsorption of ethanol.
Figure 1. Composite isotherms for adsorption of octadecanol on rutile from p-xylene solutions: 0, under normal conditions; 0 , under dry conditions. Saturation mole fraction of octadecanol in p-xylene, 16 X
lent to the individual isotherm for the solute. The lower curve was obtained with samples of rutile which had been handled in the atmosphere after oxygen treatment, as was the case in the majority of the experiments reported here (called normal conditions), while the upper curve corresponds to the very dry conditions in which the bulbs containing the activated samples were broken under the solution. At maximum coverage the areas occupied by an adsorbed octadecanol molecule, based on the B.E.T. area of 21.0 m.2 g.-l, are 23.2 and 28.2 in the dry and normal cases, respectively. We are inclined to the view that the 23.2 per molecule represents the closest possible packing of the adsorbed molecules corresponding to localized adsorption over specific sites. Hollabaugh and Chessick2 have suggested that the surface of rutile after outgassing a t high temperature consists of Ti-0-Ti linkages and Ti-OH groups. Ih-om vapor adsorption isotherms for water and npropyl alcohol, indicating both chemical and physical adsorption, they propose mechanisms which associate each adsorbed molecule with two titanium sites, either by reaction with a Ti-0-Ti group or localization over two Ti-OH in the case of physical adsorption. On this basis, an area of 11.4 per Ti site was obtained using the B.)EE.T. area and the number of Ti-OH groups on a fully hydroxylated surface as determined froin water vapor adsorption data. Such good agreement with our value of 23.2 would make it seem reasonable to associate each octadecanol molecule with two Ti sites on the dry surface. Hollabaugh and Chessiok, however, found a rather larger area of 26.6 per n-propyl alcohol moliecule in the monolayer from their
2.
Figure 2b.
Composite isotherm for adsorption of octanol.
2
Figure 2c.
Composite isotherm for adsorption of dodecanol.
vapor adsorption data. The reason for the discrepancy is not apparent. On exposure of the activated rutile to the atmosphere, both chemisorption and physical adsorption of water molecules are likely to occur and the higher area per adsorbed oct,adeca,nol molecule is assumed to result from part of the fully hydroxylated surface being covered by physically adsorbed water. Comparison of our heat of immersion data with those of Hollabaugh Volume 68, Number 1.9
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G . D. PARFITT AND I. ,J. WILTSHIRE
1 0
0.01
0.02 0.03 0.04
0.05
0.06 0.07 0.08
0 0
0.1
0.2
0.3
0.4
0.4
0.6
0.7
0.8
0.9
1.0
2.
0.09
2.
Figure 2d. Composite isotherm for adsorption of hexadecanol. Saturation mole fraction of hexadecanol in p-xylene, 0.11.
Figure 3c. Individual isotherms for adsorption of p-xylene ( X ) and dodecanol ( 0 ) . I
0
f 0 - - " - O
0
j 7 0
0
0.01
0.02 0.03 0.04
0.06
0.06 0.07 0.08 0.09
2.
0
0.1
0.2
0.3
0.5
0.4
0.6
0.7
0.8
0.9
1.0
2.
Figure 3a. Individual isotherms for adsorption of p-xylene ( X ) and ethanol (0).
0
a 0
0
I
0
0.1
I
1
0.2
0.3
0
; ; ;\ 0.5
0.4
0.6
0.7
X i X d
0.6
0.9
1.0
2.
Figure 3b. Individual isotherms for adsorption of p-xylene ( X ) and octanol ( 0).
and Chessick would appear to confirm this view. The area of the surface which is available for adsorption of the two components in solution is therefore less for t,he normal than for the dry surface. Since eq. The Journal of PhVaical Chemistry
Figure 3d. Individual isotherms for adsorption of p-xylene ( X ) and hexadecanol ( 0).
2 assumes the adsorbed layer to be one molecule thick, it seeins justifiable to use the lower surface area for the calculation of (nls)m and (n;),, assuming an area occupied by a physically adsorbed alcohol molecule of 23.2 Asz. On this basis, the area covered by octadecanol molecules a t saturation in the normal case is 17.2 m.2 g.-l and the corresponding values of (n18)m and (nzB)mhave been used in the calculation of the individual isotherms, with the appropriate correction for the higher (B.E.T.) area sample used with ethanol to octanol, inclusive. The composite isotherms for the adsorption of ethanol, octanol, dodecanol, and hexadecanol onto rutile, which had been exposed to the atmosphere after oxygen treatment, are shown in Fig. 2, and the corresponding individual isotherms are shown in Fig. 3. Those for the remaining alcohols show intermediate behavior and have been omitted to save space. The composite isotherms for the alcohols up to dodecanol show long linear portions which, under certain conditions16would correspond to an adsorbed layer of
ADSORPTION OF ALCOHOLSON RUTILEFROM p-XYLENE SOLUTIONS
1
I
I
,
1
Number of carbon atoms in alcohol chain
Figure 4. Adsorption of alcohol a8 a function of chain length a t a 0.3 equilibrium mole fraction of alcohol in solution.
constant coniposition over a large concentration range. This occurs with butanol to dodecanol inclusive, the range of concent ration over which the adsorbed layer shows constant composition decreasing with increasing chain length. It is perhaps surprising that this situation arises in the case of physical adsorption of alcohols on rutile when, purely on thermodynamic grounds, an increase in adsorption with concentration would be anticipated. Similar results were observed in the adsorption of a series of alkylbenzenes onto Graphon from solutions in n-heptane reported in part I of this series.4 No adequate explanation has yet been proposed to account for this behavior. The most striking results of this work is the variation of alcohol adsorption with chain length. At the lower end of the concentration range the amount of alcohol adsorbed at equilibrium mole fractions corresponding
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to the flat portions of the individual isothernis decreases froni ethanol to hcxariol and octanol and then increases up to the saturation value found with the long-chain alcohols. This effect is shown in Fig. 4 and clearly indicates the importance of solution intcractions to the adsorption at the solid-liquid interface. At present the appropriate thermodynamic data are not available for these systems but it is of interest that the molar volumes of hexanol and p-xylene are similar, suggesting that a mixture of the two might represent the most ideal of all the alcohol-xylene mixtures used in this work. With the alkylbenzene~~ no such minimum was observed, thc adsorption of the alkyl benzene increasing with chain length. Although adsorption experiments under dry conditions have been carried out with only onc of the lowcr alcohols, namely octanol, it is particularly significant that in this case the amount adsorbed rcaches saturation at a mole fraction in the rcgion 0 . 2 4 . 3 suggesting much stronger solute-adsorbent interactions on the partially hydroxylated rutile, reducing the effect due to solution properties. Another possibility is that trace water in the solutions might contribute to the minimum. Further work on these aspects is in progress. Acknowledgments. The authors are grateful to British Titan Products Co. Ltd. for preparing the samples of pure rutile and to Shell Research Ltd. for a grant to I. J. W. (6) P. V. Cornford, J. J. Kipling, and E. H. M. Wright, Trans. Faraday Soc., 5 8 , 1 (1962).
Volume 68, Number 18 Beeember, lQ&
