3550
P. T. D A W O NG. , T. RICH,A N D 11. A. HAYDOS
Some Factors Influencing a High Energy Adsorption of Polar Molecules from Hydrocarbons onto a Solid Surface
by P. T. Dawson, G. T. Rich, and D. A. Haydon Departwrit of Colloid Science, Cniuersity of Cambridge, Cambridge. England
(Receired March 26, 1964)
A study has been made of the adsorption of formic acid frorn benzene and of stearic acid and stearyl alcohol from benzenc and aliphatic hydrocarbons onto three different speciinens of rigorously dried pure rutile. The rutile was well characterized as to surface area, particle size, and pore distribution by electron microscopy and gas phase adsorption of nitrogen and formic acid. The nitrogen results indicated that the samples had porous structures arising from the void volume between aggregated particles. Vapor phase adsorption of formic acid showed a high energy (-80% irreversible) adsorption onto lattice points, giving monolayer coverages effectively the same in all specimens. From benzene, identical coverages were obtained, showing that the solvent did not effectively compete for sites. However, stearic acid and stearyl alcohol from benzene and aliphatic hydrocarbon, although giving an adsorption similar to each other, gave coverages smaller than those of formic acid, and which wcre smaller the smaller the particle size of the adsorbent. Results on adsorption a t the hydrocarbon-mater interface indicated that the increase in chain length alone is not responsible. I t has been concluded that in all three samples the packing or association of the solid particles imposes a space restriction which limits the adsorption of CIS but not C1 molecules.
Introduction Adsorption from solution onto polar solid surfaces is known to be complicated by a numbcr of factors. I n particular, we should like to know more about how the adsorption is influenced by the size and shape of the solvent molecules, the chain flexibility of long-chain organic adsorbates, the pore and aggregate struct,ure, particle sizes of the solid, and the extent of localization of the adsorbate molecules a t the surface. Previous investigations into adsorption onto metal oxides have revealed results of incrcasing complexity. One clear point arising from studies of adsorption of longchain acids from hydrocarbons is that an apparently close-packed acid film is not usually obtained.2 Furt,hermore, adsorption from nonaqueous solvents3 and from the vapor phase is very sensitive to the amount of water present. In this paper an attempt has been made to determine experimentally how some of the above factors irifluericc adsorption onto titanium dioxide, by studying the adsorption of carboxylic acids from the vapor phase and froin hydrocarbons under The Journal of Physical Chemastry
rigorously dry conditions. The results have been interpreted with thc aid of data from a general investigation into the properties of the titanium dioxide used.
Experimental Adsorbent. Rutile (TiOz) specimens were kindly provided by t,he British Titan Products Co. of Billingham, County Durham. They were prepared by the hydrolysis of pure TiCL and the calcination of the resulting products. The calcination temperatures were approximately 600, 300-400, and 250' for the low, medium, and high area specimens, respectively. The (1) ( 8 ) K'. D. Harkins and D. M. Gans, J. Am. Chem. Soc.. 53, 2804 (1931): (b) W. D. Harkins and D. M. Gans, J . Phys. Chem.. 36, 86 (1932): ( c ) C. M. Hollabaugh and J. J. Chessick, ibid.. 65, 109 (1961). (2) €I. E. Ities, M . F. L. Johnson, and J. 5. Melik, ibid., 53, 638 (1949). (3) W. Hirst and J. K. Lancaster, T r a m . Faraday Soc., 47, 316 (1951). (4) P.T. Dawson, Thesis, Cambridge University, 1963.
HIGHENERGY ADSORPTIOSOF POLAR MOLECIJLE~
3651 ,-
latter had to be recovered from its aqueous suspension after the hydrolysis by freeze-drying the suspension. The purity of the samples was 99.95%, the principal inipurities being silica and chloride. The surface areas were determined by nitrogen adsorption using a conventional B.E.T. method. These were, respectively, 115, 44.8, and 158,O mq2/g. The particle size distxibutions were found from electron microscope studies. The size (diameter) ranges of the primary particles in the specimens were, respectively, 0.01 to 0.3, 0.005 to 0.1, and 0.005 p (almost monodisperse). A pore size distribution analysis based on a nonintersecting cylindrical pore model (described in more detail elsewhere) yieldedo the results shown in Fig. 1. The sharp peak a t 25 A. for the high area monodisperse sample indicates that the pores were between the particles in this specimen rather than in them. A more detailed exarnination of the low and intermediate area samples suggests that in these cases also the particles themselves, as opposed to aggregates of the particles, were not porous. For the adsorption studies described below, the oxides were heated to 250 + 2' and degassed through liquid nitrogen traps for 50 hr. a t mm. Under these conditions the oxides did not noticeably change colIDr, and they were not treated with 0zn1' The adsorption behavior of the oxides, prepared as described above, was accurately reproducible. For liquid phase ndsorption measurements, the degassed samples were sealed off in thin-walled glass bulbs and transferred to a drybox for further manipulation. Adsorbates. Formic acid of A.R. grade was used, from which dissolved gases were removed just prior to adsorption by evacuating the space above the sample. The acid had a saturated vapor pressure of 32.4 mm., which from the work of Coolidge5 suggests the presence of approximately 1% water. The stearic acid and stearyl alcohol were of the highest grade available from Eastman Kodak and were recrystallized twice from acetone, twice from benzene, and then vacuum dried over phosphorus pentoxide. The benzene and peti-oleum ether (80-100' fraction) were A.R. grade solvents. They were first dried by standing over sodium wire, which was extruded directly into the solvent at tntervals, until it remained clean and bright. The solvent was then passed through a 1-in. long column of Fluka active alumina (5OlGA) which, it is claimed, reduces the mater content to less than 10 p.p.m. The outlet of this column was inside the drybox. Vapor Phase Adsorptzon Apparatus. The adsorption of formic acid vapor was carried out by a conventional volunletric method. The formic acid dosage and equilibrium pressures were measured with a
Pore radius,
A.
Figure 1. Pore size distribution curves for three rutiles (A, 13, and C ) prepared by liydrolysis of Ticla and calcination at 600, 300-400, and 250°, respectively: 0,rutile A (11.5 m.Z/g.); m, rutile B (158 m.z/g.); 0,rutile C (44.8m.z/g.).
wide bore (25-mm. internal diameter) mercury manometer in which the heights of the menisci were determined to 0.01 mm. by means of a cathetometer. Adsorptions were carried out at 20'. Liquid Phase Adsorption Apparatus and Procedure. Carefully dried solvents were used to prepare, in l,he drybox, suitable solutions for adsorption. The bulbs containing adsorbent were broken under the surfaces of these solutions in the drybox and the containing bottles immediately stoppered and sealed. The bottles were then shaken for several hours in a thermostat at 20'. Samples of the supernatant liquid were withdrawn in the drybox and analyzed either by titration with alcoholic potassium hydroxide under nitrogen, in the case of formic acid, or by spreading on a Langmuir trough, in the cases of stearic acid and stearyl alcohol.
Results Adsorption of Formic Acid Vapor. It is known from the work of Coolidge that formic acid forms di'mers in the vapor phase Consequently, in the volumetric procedure adopted, a correction for nonideality was necessary. For i,he present investigation, however, Coolidge's results did not extend to sufficiently h Igh relative pressures of formic acid and his experiments were repeated and e ~ t e n d e d . ~ This work will be described elsewhere, but the results agreed accurately with those of Coolidge in the overlap region, and at relative pressures, >0.95 showed that trimers and possibly higher polymers begin to form. ( 5 ) A. S.Coolidge, J . Am. Chem. Soc., 50, 2166 (1928)
Volume 68, rumber 19
December, 1964
3552
G . '1'. I ~ I P I - T ,
1'. '1'. I ~ A W S O N ,
AND
I ) . A . HAPDON
4
3
d
2
1
1
0
0.2
0.6
0.4
0.8
1.o
Relative pressure.
Figure 2. T h e adsorption and desorption at 20" of formic wid o n rutile 13 (158 m.*/g.) which had been activated a t 250 f 2" a t -10-6 mm. for 50 hr.: 0 , first cycle; 0 , second cycle; 0, third cycle; A, after treatment of the activated material with formic acid vapor and evacuation a t 20".
Thc adsorption and desorption of formic acid on rutilc B is shown in Fig. 2. The reproducibility is illustratcd by the agreement of three succcssive cycles. The total pore volume calculated from the hysteresis loop (0.227 cc./g.) agrees closely with the value obtained from nitrogen adsorption. If, after adsorbing foriiiic acid a t 20' onto a saniple which had been degassed a t 250°, the sample is thcn evacuated at 20' arid a second isotherm determlncd, the lower curve The Journal of Phusical Chemistrlj
is obtained. Thc difference between the two adsorption curves is approximately thc same a t all the cyuilibrium acid pressures. Rutiles A and C showcd similar behavior to H, although the differences between adsorption onto 250'activated specimens arid onto thc samc specimens after evacuation at 20' were not, in terms of surface coverage, exactly the same. The adsorption curves for rutile A are shown In Fig. 3. The broken curve
3553
HIGHEKEHUY ADSOR~WON OF POLAR MOLECULES
Table I-: A Comparison of the Plateau (or Monolayer) Adsorption Values a t 20" for the mrn. Three Rutiles. The Oxidee were Degstssed for 50 hr. at 250 f 2' and _.___ _ _ _ . . . . I -
____ Specific surface area, m.z/g.
Rutile A
11.5
Rutile C
44.8
Rutile I3
158'
Formic acid---Vapor phase Benzene
3.85 (3.60 irr.) 3.75 (3.12 irr.) 3.75 (2.40 irr.)
Adaorption, molcculea/cm.l X 10--'IStearic wid---Petroleum Benzene ether
4.04
3.12
3.75
1.93
3.95
2.31
3.32
Octadecyl alcohol---Petroleum Benzene ether
3.35
3.30
in this figurr shows the formic acid adsorption onto the rutile after it had bcen treated with water vapor and evacuatrd a t 20'. The e plotted on the right-hand axis of Fig. 2 and 3 is the surface coverage and was calculated from the expression
=
no. of acid molecules adsorbed/g. of oxide X area of acid molecule _____ specific surface area determined by nitrogen adsorption I __ _ _ I _ _ _ _ I _
Thc cross-sectional area of the formic acid molecule was assumed to be similar to that of other normal chain carbozylic acids in the vertical orientation, namely 20.8 A.2.6 Adsorption f r o m Hydrocarbons. Adsorption and desorption results for formic and stearic acids from benzene to rutiles A, R, and C are shown in Fig. 4. For convenience the corresponding coverages are given on the right-hand axis. The extents of adsorption on the plateaus of the isotherms have been collected in Tablr I. In calculating the coverages, the molecules have been assumc.d to be vertically oriente! and therefore to have a cross-sectional area of 20.8 A.2 per molecule. The highly polar and nonpolar nature of the titanium dioxide and benzene, respectively, make this assumption rrasonable and, moreover, experimental evidence of the .vertical orientation has been found.' It can be seen from the desorption results that the acid molecules were extrcmely strongly adsorbed; in fact, no suggestion of desorption could be detected in any of the systems examined. As the bulk concentrations of acid w c w vrry low, the number of molecules adsorbed was given directly by the change in the bulk concentration. The rise in the adsorption of stearic acid onto rutile C a t low concentrations is thought to be due to the systems having had insufficient time for equilibrium a t the higher concentrations. The adsorptions of
Figure 3. The adsorption at.20' of formic acid vapor on rutile A (11.5 m.*/g.) activated 88 in Fig. 2: 0, directly after activation; D, after treatment with formic acid vapor and evacuation a t 20"; 0, after treatment with water vapor and evacuation a t 20'.
stearic acid from petroleum ether and of octadecyl alcohol from benzene and petroleum ether were measured only in the plateau region of the adsorption isotherm. The results are shown in Table I.
Discussion Formic Acid Adsorption. In Fig. 2 and 3 the adsorption of formic acid has been plotted against the relative pressure of the acid. The isotherms do not show a well-defined linear region or a very distinct knee, and no clear estimate of the monolayer adsorption is possible by conventional means. However, by comparing the adsorption isotherm for the oxide degassed a t (0) 9.J. Gregg, Trana. Inat. Chem. Engre. (London), 2 5 , 40 (1947). (7) J. J. Kipling and E. H. M. Wright, Nature, ZOO, 1167 (1963).
Volume 68,Number 18 December, 1964
P. T. DAWSON, G . T. RICH,AND D. A, HAYDON
3354
8.0
- 1.5
0.6
i
0.4 1 0 . 5
0.2
0
1
2
3
4
Partial pressure of monomer, mm.
00 2
0.02
0.04
0.06
0
Bulk concentration, mole/l.
Figure 4. The adsorption a t 20” of formic and stearic acids from dry benzene onto the three rutiles, activated as in Fig. 2 : 0, rutile A (11.5 m.”g.); E, M, rutile C (44.8 m.”g.); A, A, rutile B (158 m.”g.). Open points indicate the desorption and filled points the adsorption isotherms.
250’ with the isotherm for a formic acid-treated specimen degassed at 20°, we can infer that sufficient acid is irreversibly adsorbed a t 20’ to give approximately half-coverage. The precise values of the adsorption of irreversibly adsorbed acid are given in Table I. In order to attempt an estimate of the monolayer adsorption of the acid, we shall use the fact that the equilibrium vapor phase over the lower part of the isotherm contains monomeric and dimeric molecule^.^^^ From the chemical nature of titanium dioxide and formic acid we may conclude that by far the strongest bonds between the acid and the surface will be either the hydrogen bond or the ionic bond resulting from the formation of a formate. Only the monomeric acid would form such bonds. We therefore conclude from rough calculations that since the first layer is evidently complete at relative pressures of -0.1 (ie., where the monomer concentration is still of the same order of magnitude as that of the dimer) the adsorption of dimer in the first layer is likely to be negligible. If we now plot the adsorption against the partial pressure of the monomer, we obtain the curves shown in Fig. 5 . As can be seen, there is a well-defined linear portion and a knee, from which a fairly accurate monolayer adsorption can be read. These values are shown in Table I. The question now arises as to why the monolayer adsorptions are all about the same per unit area, but The Journal of Physical Chemistry
Figure 5 . The adsorption of formic acid vapor at 20” onto the three rutile specimens (activated as in Fig. 2 ) plotted as a function of the partial pressure of the monomeric acid: A, rutile A (11.5 m.*/g.); m, rutile B (158 m.Z/g.); 0,rutile C (44.8 m.2/g.).
correspond to coverages of less than unity. One obvious reason is that as the adsorption is clearly a very strong one, over half the molecules being irreversibly adsorbed, the film is no doubt highly localized on specific sites. If the spacing of the localized molecules is a little larger than the diameter of the formic acid molecule, and it is energetically unfavorable to displace the molecules laterally from their sites to make room for the completion of the monolayer, then the observed coverages of less than unity for the monolayer would be expected. While this may be the principal explanation, there may be another contributing factor. It is shown in Fig. 3 that in the first stages, the adsorption of formic acid onto oxide previously treated with formic acid and degassed at 20°, is almost the same as the adsorption onto oxide previously treated with water vapor and degassed a t 20’. Thus, irreversibly adsorbed water competes with, and is not displaced by, formic acid. [The complete explanation for the course of this curve at higher pressures is not entirely obvious, although it is very probable that ultimately some of the residual adsorbed water is displaced by the formic acid. This is consistent with the eventual merging with the upper isotherm.] If, therefore, after degassing the original oxide a t 250°, about one-fifth of the surface was covered by adsorbed water, we should not expect to obtain a complete monolayer of formic acid. I n fact, detailed studies on the water adsorption4 suggested that a t the most only onetenth of the surface mas originally covered by mater. It is therefore concluded that both the above mechanisms probably contribute to the results.
3,555
HIGHENERGY ADSORPTION OF POLAR MOLECULES
The similar first-layer adsorption values for the three rutiles suggests thak either the site spacing or the amount of water remaining on the surface after degassing a t 250°, or both, is very similar for each oxide. On the other hand, the variation in the amount of formic acid irreversibly adsorbed a t 20' reveals that the proportion of the sites with a high energy decreases with the increase in specific surface area. I n fact, the larger the surface area, the lower the temperature a t which the oxide was calcined. It is, therefore, quite possible that the initial heat treatment of the rutile was responsible for the number of high energy sites. The maximum adsorption of formic acid from dry benzene gives values which agree t o within experimental error with the monolayer vapor phase values. It is nlot known to what extent this adsorption was effectively irreversible a t 20'. All that can be said is that no desorption could be detected for bulk phase concentrations down to 5 x IOuh mole/l. It seems reasonable to suppose that the irreversible adsorption was similar in extent to that found for the vapor phase systems. By comparison of the benzene results with those for the vapor phase, we conclude that the benzene does not effectively compete for space in the monolayer. This is easy to understand in the present systems, since the benzene, having no polar group, would be expected to have a relatively small energy of interaction with the surface.* If, co~iversely,we assume that the benzene would have no influence on the adsorption, the results of adsorption from benzene (which do not Involve any suppositions regarding the relative adsorption of monomers and dimers) confirm our interpretation of the vapor phase data. T h e Adsorption of C I S Molecules. The adsorption isotherms of stearic acid from benzene showed the same qualitative features as the formic acid adsorptlon. The maximum adsorption obtained, however, was not only much lower than that for formic acid, but was considerably different for the three rutile specimens. This lower adsorption evidently was not related t o the particular combination of carboxyl group and long chain, since the results for octadecyl alcohol from benzene are very similar. It is probable that, owing t o the high energy of the adsorption, the size and shape of the hydrocarbon solvent is of very minor importance. This is borne out t o some extent by the results of adsorption from petroleum ether (Table I). Investigations into the adsorption of normal aliphatic alcohols from benzene and aliphatic hydrocarbons to a water surfaceg have shown that the effective cross-sectional area of the molecules is, to a first approximation, independent of chain length. There
does not, therefore, appear to be any possibility that there is a space liimitation in the plane of a flat surface for the Cl8 chains. It seems that the explanation of the low adsorption of CIp relative to C1 molecules is that the packing or aggregation of the oxides results in a space limitation for the former, but not for the latter. This conclusion is supported by the fact that the stearic acid adsorption per unit area is largest for the oxide with the largest pores and [smallest for the oxides with the smallest pores. It can be shown that the London-van der Waals forces between the particles are very unlikely to be sufficiently large to prevent adsorption occurring between particles held together by these forces. As the pore volume analyses indicate that the pores are almost certainly between rather than in the particles, it is possible that the pores which are significant in limiting the adsorption are formed by adjacent particles which have partially coalesced or grown together a t qujte localized points. T h e Adsorption Bonds. It is not possible to say anything definite from this investigation about the nature of the bonds between the acids and the oxides, but there are some implications in the experimental results. Smithlo has concluded from infrared adsorption studies of adsorbed carboxylic acids on various specimens of titanium dioxide that both ionic and liydrogen-bonded acids may be found on the surface of one specimen. Those molecules bound ionically were shown to be irreversibly adsorbed, while the hydrogenbonded molecules could be washed off in excess solvent, I n the present rutile specimens it is concluded from the formic acid adsorption that both reversilbly and irreversibly adsorbed acid is present. It is possible that these molecules may be hydrogen- and ionically bonded, respectively, but evidence of water adsorption4 is in some ways difficult to reconcile with this. It was pofnted out above that water molecules may become irreversibly adsorbed and so limit the adsorption on the first layer of formic acid. There is some evidence4 that the water which does this is hydrogen-bonded and not in a dissociated, chemically bonded state. If this is so, hydrogen bonding cannot be entirely ruled out in the present systems for the irreversibly adsorbed acids.
Acknowledgments. P. T . Dawson and G. T . Rich are indebted to the Department of Scientific and [ndustrial Research and t o Imperial Chemical Industries (8) A. V. Kiselev and D. P. Poshkus, Dokl. A k a d . N a u k S S S R , 120, 4 (1958). (9) G. T. Rich and D. A. Haydon, unpublished results. (IO) I. T. Smith, Nature, 201, 67 (1964).
Volume 68; Number l d
December, 196Q
(Paints Division), respectively, for maintenance grants. The authors also wish to thank Dr. I. T. Smith of the
Pairit Research Station, Teddington, for discussions on severalpoints.
The Influence of Detergents on the Dewetting of Calcium Palmitate
by F. van Voorst Vader and H. Dekker Iinilever Research Laboratory, Mercatorweg 2 , Vlaardingen, The .Vetherlands
(Received A p r i l 19, f M / t )
The changes in surface composition of a solid because of replacement of one adj oiriing fluid by another can be derived from measurements of the contact angle between thcsc two fluids and the solid interface and of the interfacial tension between the fluids. The changes in the solid surface are calculated from these data by means of formulas combining Young’s equation with the Gibbs adsorption equations valid a t the two solid-fluid interfaces considered. This method was applied to the system calcium palmitate-aqueous buffered detergent solution-air in the pH range 7.5-9.2. I t was fourid that the calcium palmitate becomes nearly completely covered by a monolayer of acid soap on replacing the buffer solution by air in the absence of detergent. The presence of potassium N-lauroyl-Nmethyltaurate, ethoxylated lauryl alcohol, or bis(methylsulfiny1)dodecane in the solution markedly reduces the increase in fatty acid adsorption during dewet,ting. I t was found by direct titration that the presence of these detergents increased the fatty acid adsorption a t the interface of calcium palmitatesolution.
Introduction The hydrophylic character of calcium soaps can be greatly enhanced by adding detergents (so-called “scum dispersants”) to their aqueous suspensions. In order to explain this behavior, the influence of a number of dispcrsants on the surface composition of calcium palmitate against, dispersant solution and against air was invw t igated. The values of the differences between the adsorption at the interfaces of calciuni palmitate against these two phascs were derived from contact angle and surface tension measurements. In addition, the values of thcsc adsorptions at the calcium palmitate solution interface itself were dctermined by titration and electrokinetic measurements. In this way, a fairly complctc picture of the surface phenomena connected with the phase change a t the calcium palmitate surface during dewetting is obtained. The Journal of Physical Chemistry
Thermodynamics Consider a three-phase system consisting of an aqueous suspension of calcium palmitate (Cap,) and air, where the aqueous phase contains Ca2+, H f , K f , 1’- (palmitate), C1-, and D- (dispersant) ions and HP (palmitic acid) as solutes. Changes in the surface tension (dy) a t each of the three interfaces solid-air (SA), solid-liquid (SW), and air-liquid (AW) are related to the adsorption of these solutes at the interfaces by means of the relevant Gibbs adsorption isotherm -dr
=
rpdpp
+ rcadma +
FrlpdCrHp f
rKdp=
-k rDdpD (1)
where r is the adsorption relative to water, and p is the thermodynamic potential. In formula 1 the adsorptions of €I+, OH-, and C1ions a t the calcium palmitate interface have been
