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An electrophoretic investigation of the effect of chloride and of silanol groups on the properties of the surface of rutile
An Electrophoretic Investigation of the Effect of Chloride and of Silanol Groups on the Properties of the Surface of Rutile G. D. P A I ~ F I T T , 1 J. R A M S B O T H A M , 2 A~n C. H. R O C H E S T E R Department of Chemistry, University of Nottingham, Nottingham, U.K.
Received August 9, 1971; accepted January 7, 1972 Electrophoretic studies with particular reference to isoelectrie point values have been made with futile prepared by hydrolysis of titanium tetrachloride. Comparison of the rutile with that treated with hydrogen chloride shows the chloride impurity remaining after hydrolysis to be ionic in nature and distributed both on the surface and in the buIk of the material. Hydrogen chloride gas reacts predominantly with surface Ti-OH and Ti-O-Ti sites producing Ti-C1 species which differ from those associated with the impurity. Silanol groups are formed on rutile by treatment with silicon tetrachloride followed by water vapor giving a surface which, from electrophoretic data, is shown to be dissimilar to the mixed oxide silica-titania, but similar to rutile onto which hydrated silica is precipitated. INTRODUCTION The existence of hydroxyl groups on ruffle is now well established (1, 2) and in common with other oxides, the amphoteric dissociation of these groups in aqueous solution is thought to establish a surface charge which varies with p H (3, 4). F r o m published d a t a the isoelectric point (i.e.p.) of ruffle is found at p H 4.0-6.0, and is altered b y ionic impurities (3, 4). Chloride impurity introduced into rutile prepared b y the hydrolysis of titanium tetrachloride has been shown to impair the resolution of the infrared bands arising from surface hydroxyl groups (2). One of the aims of the present work was to investigate the nature of this chloride impurity and compare it with chloride introduced by reacting rutile with dry hydrogen chloride. It has recently been shown that the interaction of the futile surface with silicon tetrachloride followed by water vapor results in the removal of hydroxyl species
i Present address: Tioxide International Ltd., Billingham, Teesside, U.K. 2 Present address: Koninklijke/Shell Laboratorium, Amsterdam, Netherlands. Copyrighb (~ 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.
and the introduction of silanol groups (5). This modification forms BrCnsted acid sites of the t y p e found on cracking catalysts and led to the tentative conclusion t h a t the modified surface was similar to t h a t of a "mixed oxide." Parks has demonstrated t h a t the i.e.p, of a mixed oxide is the weighted average of the values for its comportent oxides (6). Since the dissociation of hydroxyl groups on silica results in an i.e.p. at ~ p H 2.0 (3), electrophoretie measurements allow us to characterize the modified surfaces by studying the change in i.e.p. associated with the replacement of rutile hydroxyl groups b y silanol groups. EXPERIMENTAL Rutile samples (code Nos. C L / D 4 2 8 and CL/D338) supplied b y British Titan Products Co. Ltd. prepared by the hydrolysis of redistilled titanium tetrachloride, were freed from surface chloride b y alternate soxhlet extraction in water (24 hr) and heat t r e a t m e n t in air (24 hr, 400°C), and have been described previously (5, 7). Silicon tetraehloride (B.D.H.) was distilled under v a c u u m before use. Hydrogen chloride was
Journal of Colloidand InterfaceScience, VoI.41, No. 3, December1972
437
438
PARFITT, RAMSBOTHAM, AND ROCHESTE~
prepared in situ by the dehydration of Analar hydrochloric acid using concentrated sulfuric acid and was dried by passing it through anhydrous sulfuric acid. The water used was triply ion exchanged, distilled from alkaline permanganate and finally doubly distilled. Analar potassium nitrate was used to prepare 0.02 M solutions, whose pH was varied by the appropriate addition of either nitric acid or potassium hydroxide solution, for use in the electrophoretic measurements. Oxygen (Grade X, British Oxygen Corporation) was used as supplied, Rutile was compressed into self-supporting discs which were mounted in a Pyrex infrared cell fitted with an internal heater (8). The discs were evacuated to 1.3 X 10-4 N m-~, then heated in oxygen ( ~ hr, 40°C, 6.7 kN m -~) to remove organic impurities (2), and evacuated (> 1~ hr, 400°C) before cooling prior to equilibration with saturated water vapor and evacuation at the appropriate temperature. The reactants were stored on a conventional grease-free vacuum system fitted with Rotaflo stopcocks. Reactions were monitored speetroscopieaUy using a Perkin-Elmer 125 Grating Infrared Spectrometer as before (2, 5). Spectra were recorded with the sample at ambient beam temperature (ca. 45°C). The rutiIe was dispersed in the potassium nitrate solutions using 40 ke see-~ ultrasorties, before transfer to the microeleetrophoresis cell which was constructed from a
~2 +
o/° FIG. 1. E f f e c t of ionic s t r e n g t h on t h e e l e c t r o p h o r e t i c m o b i l i t y of rutile. P o t a s s i u m n i t r a t e c o n c e n t r a t i o n 0.01 M ( O ) , 0.025 M (-}-), 0.07 M
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FIG. 2. Change in isoeleetrie point of rutile in 0.02 M potassium nitrate solution as a function of washing and heating to remove chloride impurity. 1 mm silica spectrophotometer cell with silica side arms glass blown onto it, and similar to that repoited earlier (9). In an electrophoresis experiment the particles were observed by dark field illumination and the cell was mounted in a perspex thermostat, tank attached to the stage of the microscope. About ten particles were timed over 54 ~, at each stationary level in both directions,, with a coefficient of variation of about 5 %. The apparatus was installed in a constant: temperature room maintained at 25 ~ I°C, RESULTS AND DISCUSSIONS Figure 1 shows the eleetrophoretic toobility of unmodified futile as a function of pH, and of ionic strength using potassium nitrate as an indifferent electrolyte. The data establish the i.e.p, of the futile at pH 4.6 and indicate the indifference of the surface to the presence of potassium nitrate in terms of specific effects in the inner regions of the electric double layer, although the change in ionic strength is reflected in the data. The plots are similar to those obtained for 7-alumina by Yopps and Fuerstenau (10). Figure 2 illustrates the change in the i.e.p. of rutile as a function of the pretreatment designed to remove the chloride impurity (2). Following the washing procedures there is a steady rise in the i.e.p., consistent with the proposal that the impurity anion is being removed (2). The "saw-tooth" nature of the plot supports the suggestion that the heating procedure causes anionic impurities to diffuse from the bulk to the surface where they influence the i.e.p. ; the surface chloride
Journal of Colloid and Interface Science, Vol. 41, No. 3, December 1972
ELECTI~OPHOI~ETIC~STUD
IES OFr RUTILE
439
is removed either as hydrogen chloride gas fore, the i.e.p, of rutile when present both on during the heat treatment or as chloride the surface and in the bulk. ions during the subsequent washing. The In a previous infrared investigation of the former becomes important at temperatures interaction of hydrogen chloride with ruffle above 400°C ( 2 ) . Repeated treatments (7) it was found that reaction occurred reduce the effect as the bulk chloride con- either with isolated hydroxyl groups, or with centration decreases. Ti-O-Ti sites which arose from the condenBoth surface chloride and chloride in the sation of adjacent hydroxyl groups to form bulk apparently have the effect of lowering water. These reactions may be summarized, the i.e.p. A surface containing hydroxyl together with the frequency of the infrared groups exchanged with chloride has a lower bands, using a covalent notation as below, basieity than that of a completely hy- although an ionic representation could be droxylated surface, hence the effect of reduc- adopted as in previous papers (5, 7). ing the i.e.p. The general trend of an increasOH--- -CI ing i.e.p, as the bulk concentration is Ti-O-Ti + HCI " _ I I [1) reduced by heat treatment is shown in Ti Ti Fig. 2; eventually the value reaches that 3 6 6 0 cm q (pI-I 4.6) for the pure material (Fig. 1). The failure of the i.e.p, to rise immediately to a value of pH 4.6 after any particular soxhlet extraction is an indication that either chloride is not completely removed from the T i - O H + HCI • T i +/H t2) surface or that bulk chloride affects the i.e.p. _ - O\H In addition an individual heat treatment 3 7 0 0 cm -1 causes a reduction in i.e.p, which must be associated with an increase in surface Cl-~_~_Ti - Cl + H20 chloride since on further extraction the i.e.p, is raised to a higher level than before. 1605 cm -1 Recently Honig (11, 12) published experi3 3 6 0 ond 1565 cm -1 mental data on the silver halides which support the theory of Grimley and Mort (13) Comparison of Figs. 3a and 3b shows that dealing with the influence of lattice defects on a surface outgassed at 250°C then treated on colloidal properties. In silver halides the with hydrogen chloride, hydroxyl groups point of zero charge (p.z.e.) is that concen- exist predominantly as molecular water tration of silver ions in solution (Ag, +) for associated with the complex, giving rise to an which the crystals carry no net charge. infrared absorption band at 3360 era-~. Grimley and Mott postulate that the net Hydroxyl groups adjacent to chloride ions charge in the crystal depends on an equilibformed via reaction [1] must have reacted rium Ag, + ~ Agi+, or the p.z.c, depends on further according to reaction [2]. Evacuation the concentration of interstitial silver ions at 25°C caused reversal of both reactions [1] (Ag~+). ttonig has shown that the inclusion and [2] and the reappearance of the infrared of S~- and Pb ~+ into silver bromide, respec- bands (3660 and 3700 cm-1) characteristic of tively, raises and lowers the p.z.c, by increashydroxyl groups on futile (Fig. 3c). Howing and decreasing [Agi+]. Berube and de ever, their intensities were lower than those Bruyn (4) demonstrated that the p.z.c, of for the starting surface showing that some rutile is determined by [H~+]. If an equilib- dehydroxylation occurred during the hydrorium between It~+ ~-~ I-I~+ exists in rutile, the gen chloride treatment. The final surface had inclusion of interstitial chloride in the crystal an eleetrophoretic mobility similar to that of lattice would result in the raising of [H~+] the starting material and an i.e.p, at pH 5.0 and in a shift in p.z.c, to lower pH. There- (Fig. 3). The slight change in the i.e.p. fore, it would seem feasible that the chloride towards a higher pH is the reverse of what impurity could affect the p.z.c, and, there- would be expected had the chloride diffused Journal Qf Colloid and Interface Science, Vol. 41, No. 3, D e c e m b e r 1972
440
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FIG. 3. (1) Effect of adsorbed hydrogen chloride on the isoclectric point of rutile. (a) rutile evacuated (20 hr, 250°C); (b) in contact with excess hydrogen chloride ( ~ hr, 45°C, 10.6 kN m-2) and evacuated (1 inin, 45°C); (c) further evacuated (1 hr 250°C). (2) Electrophoretic mobility of rutile from (c) dispersed in 0.02M potassium nitrate solution. /o~O
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the amphoteric behavior of hydroxyl groups. However, the mobility of the reacted rutile is greater t h a n t h a t of pure material at p H < 4, inferring an increased surface charge density. Inflared spectra indicate t h a t a hydrogen chloride treated surface is covered with chloride ions formed via reactions [1] and [2]
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FIG. 4. Electrophoretic mobility of l~atile treated with excess hydrogen chloride (~ hr, 45°C, 26.6 kN m-2) followed by evacuation at 45°C for 3 hr. Dispersed in 0.02 M potassium nitrate solution. to the surface from the bulk, and suggests t h a t the interaction of hydrochloric acid with the dry surface leads to chloride in a different environment.
Infrared spectra indicate that evacuation at 45°C is not sufficient to remove either the complex shown in Eq. [2] or the surface chloride formed in Eq. [i]. The i.e.p, of hydrogen chloride treated rutile (Fig. 4) is similar to t h a t of pure rutile indicating t h a t the charging process is sti]l associated with
and with T i - 6
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hydroxyl groups which were either on the starting surface or products of reaction [1]. Immersion of the surface in water will cause reversal of reaction [2] and regeneration of the hydroxyl groups on the starting surface. Hydroxyl groups adjacent to chloride ions will also reappear and are considered to be of two types depending upon the nature of the T i - O - T i sites originally involved in reaction [1]. First]y, on a rutile surface heated in v a c u u m at temperatures above about 150°C there are stained oxide sites (oxygen bridges) which are able to add water to form adjacent hydroxyl groups or to add hydrogen chloride to form hydroxyl groups adjacent to chloride ions. Immersion of rutile in water after hydrogen chloride t r e a t m e n t causes the formation on these sites of adjacent hydroxyl groups which are iden-
Journal of Colloid and Interface Science, Vol. 41, No. 3, D e c e m b e r 1972
ELECTROPtIORETIC STUDIES
OF RUTILE
441
tical in amphoteric, and therefore, electrophoretic behavior to those on pure rutile. Secondly, it is proposed that there are also Ti-O-Ti sites on futile which react with hydrogen chloride (reaction [1]) to form hydroxyl groups and chloride ions on adjacent titanium ions but which cannot react with the weaker base water to form pairs of adjacent hydroxyl groups. Some evidence for the existence of two types of oxide sites has been reported previously (7). Plausible acid-base reactions for these sites in the presence of liquid water following hydrogen chloride treatment are as follows:
2.0. In a r.eview dealing with the i.e.p, of complex oxides, Parks (6) concludes that the i.e.p, is the weighted average of the individual values for the component oxides and gives data on the alumina-silica and iron (III) oxide-silica systems as evidence. In both these systems there is an approximate linear relationship between the i.e.p, and the percentage composition of the coprecipitates. Parfitt and Ramsbotham (14) have shown that the i.e.p, of a rutile coated with a silicaalumina mixture is a function of the percentage of alumina in the coating. While the alumina content of the coating was increased with the silica content constant, the i.e.p. increased linearly to a value of pH 4.5, as Hx~/H C H+ OH CI predicted by Parks (6). When the silica conI I ~ I I Ti Ti Ti Ti tent was progressively decreased keeping the alumina level constant, the i.e.p, remained at pH 4.5, revealing an apparent anomaly. The OHi.e.p, behavior was paralleled by amine adsorption and this was taken as indicating that the number of BrCnsted sites was conf CII zO\ stant when the percentage of alumina was Ti Ti " Ti Ti + CInot changed. The contact of a rutile surface containing In alkaline solution the oxide sites are reformed and are not hydrated by water. This similar numbers of isolated (3700 cm-1) and reaction makes no contribution to the elec- adjacent (3660 cm-I) hydroxyl groups (Fig. 5a) with silicon tetrachloride for 1 rain retrophoretic mobility and therefore for pH > 4.5, the experimental variation of mobility sults in an incomplete removal of the adjawith pII is similar for the hydrogen chloride cent species and a perturbation of some treated and pure rutile surfaces. In aqueous unreacted hydroxyl groups (Fig. 5b). Equilisolutions for which pH < 4.5, however, bration of the surface with water vapor protonation of the hydroxyl groups occurs (Fig. 5c) did not give any bands at 1565 or and increases the total positive charge den- 1580 cm-1 characteristic of complex formation with hydrogen chloride, but peaks at sity on the surface above that for untreated rutile. The eleetrophoretic mobility of the 3700 and 3730 cm-1 were observed indicative of silanol and isolated rutile hydroxyl groups. hydrogen chloride treated surface should therefore be higher than that for the un- The presence of a borad band at 3400 em-1 together with a shoulder at ~-~3520 cm-1 is treated surface in accord with the observed due to water moleeu]es hydrogen bonded to results (Figs. 1 and 4). In a recent paper (5) it was shown that the adjacent hydroxyl groups. This latter obhydroxyl groups on rutile could be progres- servation is confirmed when evacuation at sively removed by interaction with silicon 250°C (Fig. 5d) leaves a peak at 3660 cm-1 tetrachioride. The subsequent hydrolysis of after removing the broad band at lower wave untreacted Si-CI bonds resulted in surface numbers. The peak resulting h'om isolated silanol groups which were characterized by hydroxyl groups at 3700 em-1 is decreased in an infrared band at ~3730cm -i, and the intensity presumably by condensing with silanol groups to form Ti-O-Si sites. Howmodified rutile surface showed Br~nsted ever, an overall intensity increase in the acidity in the presence of adsorbed ammonia and pyridine (5). Parks (3) has discussed the band at 3730 cm-I suggests that the silano! i.e.p, values for pure oxides and that charac- groups were also perturbed by water vapor teristic of silica is to be found at about pI-I at lower temperatures. The presence of a
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Journal of Colloid and Interface Science, VoI. 41, No. 3. December 1972
442
PAI~FITT, RAMSBOTHAM,
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FIo. 5. Interaction of rutile with silicon tetrachloride vapour. (1) Infrared spectra of rutile after (a) evacuation (2.5 hr, 250°C); (b) in contact with silicon tetrachloride (1 min, 45°C, 65 N m-2) and evacuated ( ~ hr, 45°C); (c) in contact with saturated water vapour (2 hr, 45°C) and evacuated (11.5 hr, 45°C) ; (d) further evacuated (1 hr, 250°C). (2) Electrophoretic mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
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FIG. 6. Interaction of rutile with silicon tetraehloride vapour. (1) Infrared spectra of (a) starting surface (see text); (b) in contact with silicon tetrachloride (1/~ min, 45°C, 2 kN m-2) and evacuated ( ~ hr, 45°C) ; (c) equilibrated with saturated water vapour and evacuated (11/~ hr, 45°C) ; (d) further evaucated (11/6 hr, 250°C). (2) Electrophoretie mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
band due to isolated rutile hydroxyl groups and the absence of any bands arising from reactions involving hydrogen chloride after the equilibration with water vapor are indications that the interaction with the silicon tetraehloride was incomplete. The behavior of the band at 3730 em -1 suggests that a large number of Ti-O-Si(OH)3 species were present to prevent the perturbation of isolated futile hydroxyl groups b y water vapor at ambient temperatures. The i.e.p, is similar J o u r n a l of Colloid and Interface Science,
to that of pure rutile, and indicates t h a t the remaining rutile hydroxyl groups are sufficient to dominate the surface charge. A more extensive reaction of silicon tetrachloride was carried out with an hydroxylated rutile surface (Fig. 6a) which was initially equilibrated with saturated water vapor at ~150°C for i rain after oxygen treatment at 400°C. At first the individual bands characteristic of rutile hydroxyl groups were replaced by a broad band (Fig.
¥ol. 41, No. 3, December 1972
ELECTROPttORETIC
6o c 50
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443
OF ]~UTILE
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Fro. 7. Interaction of rutile with silicon tetrachloride vapour. (1) Infrared spectra after (a) evaucation (}i hr, 250°C); (b) in contact with silicon tetrachloride (1 hr, 250°C, 2 kN m-J), cooled and evacuated (1/~ hr, 45°C) ; (c) equilibrated with saturated water vapour (2 hr, 45°C) and evacuated (1/~ rain, 45°C) ; (d) further evacuated (2 hr, 250°C). (2) Electrophoretic mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
6b), which finally resolved into a peak at 3730 em-~ with a shoulder at 3660 em-~ (Figs. 6c and d) after the hydrolysis of Si-C1 to silanol groups. The more extensive replacement, but incomplete removal, of futile hydroxyl by silanol groups is reflected in the i.e.p, plot. At pH > 8 the mobility is higher than that of pure rutile indicating that replacement of potential Ti-O- species by similar silicon-oxygen groups is greater than 1:1. The strong inflection in the curve at pH 4.0 corresponds approximately to the i.e.p, of hydroxyl groups on futile while the overall i.e.p, comes at pH 2.5 and is characteristic of that for silica (3). The interaction of excess silicon tetrachloride at 250°C with a dry futile surface (Fig. 7a) results in replacement of most, if not all, of the ruffle hydroxyl groups (Fig. 7b) by silanol species (Figs. 7e and d). This is reflected in the i.e.p, of the surface which is very similar to that of silica as might be expected by the extensive coating of silanol groups. A small inflection (dotted line Fig. 7) may be an artifact but could arise from some adjacent rutile hydroxyl groups formed by the dissociation of water on Ti-O-Ti sites (Fig. 7d). The form of the plot of electrophoretie mobility as a function of pit where the rutile hydroxyl groups have largely been replaced by silanol groups (Fig. 7) is similar to that for
a pure silica surface (15). This is reasonable in view of the fact that the surface species are predominantly Si-OIt groups, and infrared spectra indicate that they are similar to those on silica. Previously (5) it was shown that the modification of futile by interaction with silicon tetraehloride resulted in a surface having similar acid sites to those on a mixed oxide catalyst with respect to adsorbed ammonia and pyridine vapor. If the surface behaves similarly to that of a silica-titania, Parks' (6) investigations predict that the i.e.p, of a rutile surface only partly coated with silanol groups should produce a single i.e.p, on a smooth mobility curve between pH 2.5 and 4.5. Partial modification (Fig. 6) produces a curve which has the distinctive characteristics of both silica and titania surfaces and resemble the composite plots published by James and Healy (15) for the adsorption and precipitation of hydrolyzed cations onto titania and silica. They show, for example, that at and above the i.e.p, of titanium dioxide cobalt (II) ions are adsorbed from solution, and at pH > 8 the cobalt precipitates as the hydroxide. The electrophoretie mobility of rutile as a function of pit in 10-4 M cobalt (II) salts indicates that from pH 3 to 5 the surface behaves similarly to titania until at pH > 6 the adsorption of the cobalt reverses the sign of the surface charge, and at pH > 8
Journal of Colloid and Interface Science, Vol. 417 No. 3, December 1972
444
PARFITT, RAMSBOTHAM, AND ROCHESTER
the electrophoretic behavior is similar to t h a t of cobalt (II) hydroxide. Similar p h e n o m e n a are reported for quartz in solutions of cobalt (II), copper (II), lead (II) and zinc (II) salts. These systems are different f r o m the mixed oxides reported b y Parks (6) since there is an actual change in composition at the oxide/ solution interface as the p H varies when h y d r o x y complexes a n d finally hydroxides are precipitated onto the surface. T h e mixed oxides h a v e a c o n s t a n t composition no m a t t e r w h a t t h e p H of the supporting electrolyte. ACKNOWLEDGMENT The authors thank British Titan Products Co. Ltd. for rutile samples and for a fellowship to J.R. REFERENCES 1. YATES,D. J. C., J. Phys. Chem. 65,746 (1961). 2. JACKSON, P., AND PARFITT, G. D., Trans. Faraday Soc. 67, 2469 (1971). 3. PA~KS, G. A., Chem. Rev. 65, 177 (1965).
4. BERUBE, Y. G., AND DE BRUYN, P. L., J. Colloid Interface Sci. 27, 305 (1968). 5. PARFITT, G. D., RAMSBOTHAM, J., AND ]:~OCI-IESTER, C. It., J. Chem. Soc. Faraday Trans. 1, 17 (1972). 6. PARKS,G. A., Advan. Chem. Ser. 67,121 (1967). 7. PARFITT, G. D., RAMSBOTItAM,J., AND ROCHESteR, C. H., Trans. Faraday Soc. 67, 3100 (1971). 8. BUCKLAND,A., RAMSBOTH.~M,J., ROCHESTER, C. I-I., AND SCURRELL,M. S., J. Sci. lnstrum. 4,146 (1971). 9. PAR~ITT, G. D., J. Oil Colour Chem. Ass. 51, 137 (1968). 10. ¥oPPS, J. A., AND FUERSTENAU, D. W., J. Colloid Sei. 19, 61 (1964). 11. HoNI~, E. P., Trans. Faraday Soc. 65, 2248 (1969). 12. HONIG, E. P., Nature 225,537 (1970). 13. GRI1VILEY,T. B., AND MOT% N. F., Discuss. Faraday Soc. 1, 3 (1947). 14. P~RFITT, G. D., ANn RAMS~OTHA~,J., J. Oil Colour Chem. Ass. 54, 356 (1971). 15. JAMES, R. O., ~-NDHEALY, T. W., J. Colloid Interface Sei. 39 (1972).
Journal of Colloid and Interface Science, Vo[. 41, No. 3, December1972
Received August 9, 1971; accepted January 7, 1972 Electrophoretic studies with particular reference to isoelectrie point values have been made with futile prepared by hydrolysis of titanium tetrachloride. Comparison of the rutile with that treated with hydrogen chloride shows the chloride impurity remaining after hydrolysis to be ionic in nature and distributed both on the surface and in the buIk of the material. Hydrogen chloride gas reacts predominantly with surface Ti-OH and Ti-O-Ti sites producing Ti-C1 species which differ from those associated with the impurity. Silanol groups are formed on rutile by treatment with silicon tetrachloride followed by water vapor giving a surface which, from electrophoretic data, is shown to be dissimilar to the mixed oxide silica-titania, but similar to rutile onto which hydrated silica is precipitated. INTRODUCTION The existence of hydroxyl groups on ruffle is now well established (1, 2) and in common with other oxides, the amphoteric dissociation of these groups in aqueous solution is thought to establish a surface charge which varies with p H (3, 4). F r o m published d a t a the isoelectric point (i.e.p.) of ruffle is found at p H 4.0-6.0, and is altered b y ionic impurities (3, 4). Chloride impurity introduced into rutile prepared b y the hydrolysis of titanium tetrachloride has been shown to impair the resolution of the infrared bands arising from surface hydroxyl groups (2). One of the aims of the present work was to investigate the nature of this chloride impurity and compare it with chloride introduced by reacting rutile with dry hydrogen chloride. It has recently been shown that the interaction of the futile surface with silicon tetrachloride followed by water vapor results in the removal of hydroxyl species
i Present address: Tioxide International Ltd., Billingham, Teesside, U.K. 2 Present address: Koninklijke/Shell Laboratorium, Amsterdam, Netherlands. Copyrighb (~ 1972 by Academic Press, Inc. All rights of reproduction in any form reserved.
and the introduction of silanol groups (5). This modification forms BrCnsted acid sites of the t y p e found on cracking catalysts and led to the tentative conclusion t h a t the modified surface was similar to t h a t of a "mixed oxide." Parks has demonstrated t h a t the i.e.p, of a mixed oxide is the weighted average of the values for its comportent oxides (6). Since the dissociation of hydroxyl groups on silica results in an i.e.p. at ~ p H 2.0 (3), electrophoretie measurements allow us to characterize the modified surfaces by studying the change in i.e.p. associated with the replacement of rutile hydroxyl groups b y silanol groups. EXPERIMENTAL Rutile samples (code Nos. C L / D 4 2 8 and CL/D338) supplied b y British Titan Products Co. Ltd. prepared by the hydrolysis of redistilled titanium tetrachloride, were freed from surface chloride b y alternate soxhlet extraction in water (24 hr) and heat t r e a t m e n t in air (24 hr, 400°C), and have been described previously (5, 7). Silicon tetraehloride (B.D.H.) was distilled under v a c u u m before use. Hydrogen chloride was
Journal of Colloidand InterfaceScience, VoI.41, No. 3, December1972
437
438
PARFITT, RAMSBOTHAM, AND ROCHESTE~
prepared in situ by the dehydration of Analar hydrochloric acid using concentrated sulfuric acid and was dried by passing it through anhydrous sulfuric acid. The water used was triply ion exchanged, distilled from alkaline permanganate and finally doubly distilled. Analar potassium nitrate was used to prepare 0.02 M solutions, whose pH was varied by the appropriate addition of either nitric acid or potassium hydroxide solution, for use in the electrophoretic measurements. Oxygen (Grade X, British Oxygen Corporation) was used as supplied, Rutile was compressed into self-supporting discs which were mounted in a Pyrex infrared cell fitted with an internal heater (8). The discs were evacuated to 1.3 X 10-4 N m-~, then heated in oxygen ( ~ hr, 40°C, 6.7 kN m -~) to remove organic impurities (2), and evacuated (> 1~ hr, 400°C) before cooling prior to equilibration with saturated water vapor and evacuation at the appropriate temperature. The reactants were stored on a conventional grease-free vacuum system fitted with Rotaflo stopcocks. Reactions were monitored speetroscopieaUy using a Perkin-Elmer 125 Grating Infrared Spectrometer as before (2, 5). Spectra were recorded with the sample at ambient beam temperature (ca. 45°C). The rutiIe was dispersed in the potassium nitrate solutions using 40 ke see-~ ultrasorties, before transfer to the microeleetrophoresis cell which was constructed from a
~2 +
o/° FIG. 1. E f f e c t of ionic s t r e n g t h on t h e e l e c t r o p h o r e t i c m o b i l i t y of rutile. P o t a s s i u m n i t r a t e c o n c e n t r a t i o n 0.01 M ( O ) , 0.025 M (-}-), 0.07 M
(o).
4"0 & 3.5
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Pretreatment
FIG. 2. Change in isoeleetrie point of rutile in 0.02 M potassium nitrate solution as a function of washing and heating to remove chloride impurity. 1 mm silica spectrophotometer cell with silica side arms glass blown onto it, and similar to that repoited earlier (9). In an electrophoresis experiment the particles were observed by dark field illumination and the cell was mounted in a perspex thermostat, tank attached to the stage of the microscope. About ten particles were timed over 54 ~, at each stationary level in both directions,, with a coefficient of variation of about 5 %. The apparatus was installed in a constant: temperature room maintained at 25 ~ I°C, RESULTS AND DISCUSSIONS Figure 1 shows the eleetrophoretic toobility of unmodified futile as a function of pH, and of ionic strength using potassium nitrate as an indifferent electrolyte. The data establish the i.e.p, of the futile at pH 4.6 and indicate the indifference of the surface to the presence of potassium nitrate in terms of specific effects in the inner regions of the electric double layer, although the change in ionic strength is reflected in the data. The plots are similar to those obtained for 7-alumina by Yopps and Fuerstenau (10). Figure 2 illustrates the change in the i.e.p. of rutile as a function of the pretreatment designed to remove the chloride impurity (2). Following the washing procedures there is a steady rise in the i.e.p., consistent with the proposal that the impurity anion is being removed (2). The "saw-tooth" nature of the plot supports the suggestion that the heating procedure causes anionic impurities to diffuse from the bulk to the surface where they influence the i.e.p. ; the surface chloride
Journal of Colloid and Interface Science, Vol. 41, No. 3, December 1972
ELECTI~OPHOI~ETIC~STUD
IES OFr RUTILE
439
is removed either as hydrogen chloride gas fore, the i.e.p, of rutile when present both on during the heat treatment or as chloride the surface and in the bulk. ions during the subsequent washing. The In a previous infrared investigation of the former becomes important at temperatures interaction of hydrogen chloride with ruffle above 400°C ( 2 ) . Repeated treatments (7) it was found that reaction occurred reduce the effect as the bulk chloride con- either with isolated hydroxyl groups, or with centration decreases. Ti-O-Ti sites which arose from the condenBoth surface chloride and chloride in the sation of adjacent hydroxyl groups to form bulk apparently have the effect of lowering water. These reactions may be summarized, the i.e.p. A surface containing hydroxyl together with the frequency of the infrared groups exchanged with chloride has a lower bands, using a covalent notation as below, basieity than that of a completely hy- although an ionic representation could be droxylated surface, hence the effect of reduc- adopted as in previous papers (5, 7). ing the i.e.p. The general trend of an increasOH--- -CI ing i.e.p, as the bulk concentration is Ti-O-Ti + HCI " _ I I [1) reduced by heat treatment is shown in Ti Ti Fig. 2; eventually the value reaches that 3 6 6 0 cm q (pI-I 4.6) for the pure material (Fig. 1). The failure of the i.e.p, to rise immediately to a value of pH 4.6 after any particular soxhlet extraction is an indication that either chloride is not completely removed from the T i - O H + HCI • T i +/H t2) surface or that bulk chloride affects the i.e.p. _ - O\H In addition an individual heat treatment 3 7 0 0 cm -1 causes a reduction in i.e.p, which must be associated with an increase in surface Cl-~_~_Ti - Cl + H20 chloride since on further extraction the i.e.p, is raised to a higher level than before. 1605 cm -1 Recently Honig (11, 12) published experi3 3 6 0 ond 1565 cm -1 mental data on the silver halides which support the theory of Grimley and Mort (13) Comparison of Figs. 3a and 3b shows that dealing with the influence of lattice defects on a surface outgassed at 250°C then treated on colloidal properties. In silver halides the with hydrogen chloride, hydroxyl groups point of zero charge (p.z.e.) is that concen- exist predominantly as molecular water tration of silver ions in solution (Ag, +) for associated with the complex, giving rise to an which the crystals carry no net charge. infrared absorption band at 3360 era-~. Grimley and Mott postulate that the net Hydroxyl groups adjacent to chloride ions charge in the crystal depends on an equilibformed via reaction [1] must have reacted rium Ag, + ~ Agi+, or the p.z.c, depends on further according to reaction [2]. Evacuation the concentration of interstitial silver ions at 25°C caused reversal of both reactions [1] (Ag~+). ttonig has shown that the inclusion and [2] and the reappearance of the infrared of S~- and Pb ~+ into silver bromide, respec- bands (3660 and 3700 cm-1) characteristic of tively, raises and lowers the p.z.c, by increashydroxyl groups on futile (Fig. 3c). Howing and decreasing [Agi+]. Berube and de ever, their intensities were lower than those Bruyn (4) demonstrated that the p.z.c, of for the starting surface showing that some rutile is determined by [H~+]. If an equilib- dehydroxylation occurred during the hydrorium between It~+ ~-~ I-I~+ exists in rutile, the gen chloride treatment. The final surface had inclusion of interstitial chloride in the crystal an eleetrophoretic mobility similar to that of lattice would result in the raising of [H~+] the starting material and an i.e.p, at pH 5.0 and in a shift in p.z.c, to lower pH. There- (Fig. 3). The slight change in the i.e.p. fore, it would seem feasible that the chloride towards a higher pH is the reverse of what impurity could affect the p.z.c, and, there- would be expected had the chloride diffused Journal Qf Colloid and Interface Science, Vol. 41, No. 3, D e c e m b e r 1972
440
PAI~FITT, I~AMSBOTHA1VI, AND I~OCtIESTEI~
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FIG. 3. (1) Effect of adsorbed hydrogen chloride on the isoclectric point of rutile. (a) rutile evacuated (20 hr, 250°C); (b) in contact with excess hydrogen chloride ( ~ hr, 45°C, 10.6 kN m-2) and evacuated (1 inin, 45°C); (c) further evacuated (1 hr 250°C). (2) Electrophoretic mobility of rutile from (c) dispersed in 0.02M potassium nitrate solution. /o~O
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the amphoteric behavior of hydroxyl groups. However, the mobility of the reacted rutile is greater t h a n t h a t of pure material at p H < 4, inferring an increased surface charge density. Inflared spectra indicate t h a t a hydrogen chloride treated surface is covered with chloride ions formed via reactions [1] and [2]
0
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FIG. 4. Electrophoretic mobility of l~atile treated with excess hydrogen chloride (~ hr, 45°C, 26.6 kN m-2) followed by evacuation at 45°C for 3 hr. Dispersed in 0.02 M potassium nitrate solution. to the surface from the bulk, and suggests t h a t the interaction of hydrochloric acid with the dry surface leads to chloride in a different environment.
Infrared spectra indicate that evacuation at 45°C is not sufficient to remove either the complex shown in Eq. [2] or the surface chloride formed in Eq. [i]. The i.e.p, of hydrogen chloride treated rutile (Fig. 4) is similar to t h a t of pure rutile indicating t h a t the charging process is sti]l associated with
and with T i - 6
\
H C]- groups formed from H
hydroxyl groups which were either on the starting surface or products of reaction [1]. Immersion of the surface in water will cause reversal of reaction [2] and regeneration of the hydroxyl groups on the starting surface. Hydroxyl groups adjacent to chloride ions will also reappear and are considered to be of two types depending upon the nature of the T i - O - T i sites originally involved in reaction [1]. First]y, on a rutile surface heated in v a c u u m at temperatures above about 150°C there are stained oxide sites (oxygen bridges) which are able to add water to form adjacent hydroxyl groups or to add hydrogen chloride to form hydroxyl groups adjacent to chloride ions. Immersion of rutile in water after hydrogen chloride t r e a t m e n t causes the formation on these sites of adjacent hydroxyl groups which are iden-
Journal of Colloid and Interface Science, Vol. 41, No. 3, D e c e m b e r 1972
ELECTROPtIORETIC STUDIES
OF RUTILE
441
tical in amphoteric, and therefore, electrophoretic behavior to those on pure rutile. Secondly, it is proposed that there are also Ti-O-Ti sites on futile which react with hydrogen chloride (reaction [1]) to form hydroxyl groups and chloride ions on adjacent titanium ions but which cannot react with the weaker base water to form pairs of adjacent hydroxyl groups. Some evidence for the existence of two types of oxide sites has been reported previously (7). Plausible acid-base reactions for these sites in the presence of liquid water following hydrogen chloride treatment are as follows:
2.0. In a r.eview dealing with the i.e.p, of complex oxides, Parks (6) concludes that the i.e.p, is the weighted average of the individual values for the component oxides and gives data on the alumina-silica and iron (III) oxide-silica systems as evidence. In both these systems there is an approximate linear relationship between the i.e.p, and the percentage composition of the coprecipitates. Parfitt and Ramsbotham (14) have shown that the i.e.p, of a rutile coated with a silicaalumina mixture is a function of the percentage of alumina in the coating. While the alumina content of the coating was increased with the silica content constant, the i.e.p. increased linearly to a value of pH 4.5, as Hx~/H C H+ OH CI predicted by Parks (6). When the silica conI I ~ I I Ti Ti Ti Ti tent was progressively decreased keeping the alumina level constant, the i.e.p, remained at pH 4.5, revealing an apparent anomaly. The OHi.e.p, behavior was paralleled by amine adsorption and this was taken as indicating that the number of BrCnsted sites was conf CII zO\ stant when the percentage of alumina was Ti Ti " Ti Ti + CInot changed. The contact of a rutile surface containing In alkaline solution the oxide sites are reformed and are not hydrated by water. This similar numbers of isolated (3700 cm-1) and reaction makes no contribution to the elec- adjacent (3660 cm-I) hydroxyl groups (Fig. 5a) with silicon tetrachloride for 1 rain retrophoretic mobility and therefore for pH > 4.5, the experimental variation of mobility sults in an incomplete removal of the adjawith pII is similar for the hydrogen chloride cent species and a perturbation of some treated and pure rutile surfaces. In aqueous unreacted hydroxyl groups (Fig. 5b). Equilisolutions for which pH < 4.5, however, bration of the surface with water vapor protonation of the hydroxyl groups occurs (Fig. 5c) did not give any bands at 1565 or and increases the total positive charge den- 1580 cm-1 characteristic of complex formation with hydrogen chloride, but peaks at sity on the surface above that for untreated rutile. The eleetrophoretic mobility of the 3700 and 3730 cm-1 were observed indicative of silanol and isolated rutile hydroxyl groups. hydrogen chloride treated surface should therefore be higher than that for the un- The presence of a borad band at 3400 em-1 together with a shoulder at ~-~3520 cm-1 is treated surface in accord with the observed due to water moleeu]es hydrogen bonded to results (Figs. 1 and 4). In a recent paper (5) it was shown that the adjacent hydroxyl groups. This latter obhydroxyl groups on rutile could be progres- servation is confirmed when evacuation at sively removed by interaction with silicon 250°C (Fig. 5d) leaves a peak at 3660 cm-1 tetrachioride. The subsequent hydrolysis of after removing the broad band at lower wave untreacted Si-CI bonds resulted in surface numbers. The peak resulting h'om isolated silanol groups which were characterized by hydroxyl groups at 3700 em-1 is decreased in an infrared band at ~3730cm -i, and the intensity presumably by condensing with silanol groups to form Ti-O-Si sites. Howmodified rutile surface showed Br~nsted ever, an overall intensity increase in the acidity in the presence of adsorbed ammonia and pyridine (5). Parks (3) has discussed the band at 3730 cm-I suggests that the silano! i.e.p, values for pure oxides and that charac- groups were also perturbed by water vapor teristic of silica is to be found at about pI-I at lower temperatures. The presence of a
I
Journal of Colloid and Interface Science, VoI. 41, No. 3. December 1972
442
PAI~FITT, RAMSBOTHAM,
AND
ROCHESTER
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FIo. 5. Interaction of rutile with silicon tetrachloride vapour. (1) Infrared spectra of rutile after (a) evacuation (2.5 hr, 250°C); (b) in contact with silicon tetrachloride (1 min, 45°C, 65 N m-2) and evacuated ( ~ hr, 45°C); (c) in contact with saturated water vapour (2 hr, 45°C) and evacuated (11.5 hr, 45°C) ; (d) further evacuated (1 hr, 250°C). (2) Electrophoretic mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
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FIG. 6. Interaction of rutile with silicon tetraehloride vapour. (1) Infrared spectra of (a) starting surface (see text); (b) in contact with silicon tetrachloride (1/~ min, 45°C, 2 kN m-2) and evacuated ( ~ hr, 45°C) ; (c) equilibrated with saturated water vapour and evacuated (11/~ hr, 45°C) ; (d) further evaucated (11/6 hr, 250°C). (2) Electrophoretie mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
band due to isolated rutile hydroxyl groups and the absence of any bands arising from reactions involving hydrogen chloride after the equilibration with water vapor are indications that the interaction with the silicon tetraehloride was incomplete. The behavior of the band at 3730 em -1 suggests that a large number of Ti-O-Si(OH)3 species were present to prevent the perturbation of isolated futile hydroxyl groups b y water vapor at ambient temperatures. The i.e.p, is similar J o u r n a l of Colloid and Interface Science,
to that of pure rutile, and indicates t h a t the remaining rutile hydroxyl groups are sufficient to dominate the surface charge. A more extensive reaction of silicon tetrachloride was carried out with an hydroxylated rutile surface (Fig. 6a) which was initially equilibrated with saturated water vapor at ~150°C for i rain after oxygen treatment at 400°C. At first the individual bands characteristic of rutile hydroxyl groups were replaced by a broad band (Fig.
¥ol. 41, No. 3, December 1972
ELECTROPttORETIC
6o c 50
"
"
443
OF ]~UTILE
j o f
i
4
~
STUDIES
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6
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Fro. 7. Interaction of rutile with silicon tetrachloride vapour. (1) Infrared spectra after (a) evaucation (}i hr, 250°C); (b) in contact with silicon tetrachloride (1 hr, 250°C, 2 kN m-J), cooled and evacuated (1/~ hr, 45°C) ; (c) equilibrated with saturated water vapour (2 hr, 45°C) and evacuated (1/~ rain, 45°C) ; (d) further evacuated (2 hr, 250°C). (2) Electrophoretic mobility of modified rutile from (d) dispersed in 0.02 M potassium nitrate solution.
6b), which finally resolved into a peak at 3730 em-~ with a shoulder at 3660 em-~ (Figs. 6c and d) after the hydrolysis of Si-C1 to silanol groups. The more extensive replacement, but incomplete removal, of futile hydroxyl by silanol groups is reflected in the i.e.p, plot. At pH > 8 the mobility is higher than that of pure rutile indicating that replacement of potential Ti-O- species by similar silicon-oxygen groups is greater than 1:1. The strong inflection in the curve at pH 4.0 corresponds approximately to the i.e.p, of hydroxyl groups on futile while the overall i.e.p, comes at pH 2.5 and is characteristic of that for silica (3). The interaction of excess silicon tetrachloride at 250°C with a dry futile surface (Fig. 7a) results in replacement of most, if not all, of the ruffle hydroxyl groups (Fig. 7b) by silanol species (Figs. 7e and d). This is reflected in the i.e.p, of the surface which is very similar to that of silica as might be expected by the extensive coating of silanol groups. A small inflection (dotted line Fig. 7) may be an artifact but could arise from some adjacent rutile hydroxyl groups formed by the dissociation of water on Ti-O-Ti sites (Fig. 7d). The form of the plot of electrophoretie mobility as a function of pit where the rutile hydroxyl groups have largely been replaced by silanol groups (Fig. 7) is similar to that for
a pure silica surface (15). This is reasonable in view of the fact that the surface species are predominantly Si-OIt groups, and infrared spectra indicate that they are similar to those on silica. Previously (5) it was shown that the modification of futile by interaction with silicon tetraehloride resulted in a surface having similar acid sites to those on a mixed oxide catalyst with respect to adsorbed ammonia and pyridine vapor. If the surface behaves similarly to that of a silica-titania, Parks' (6) investigations predict that the i.e.p, of a rutile surface only partly coated with silanol groups should produce a single i.e.p, on a smooth mobility curve between pH 2.5 and 4.5. Partial modification (Fig. 6) produces a curve which has the distinctive characteristics of both silica and titania surfaces and resemble the composite plots published by James and Healy (15) for the adsorption and precipitation of hydrolyzed cations onto titania and silica. They show, for example, that at and above the i.e.p, of titanium dioxide cobalt (II) ions are adsorbed from solution, and at pH > 8 the cobalt precipitates as the hydroxide. The electrophoretie mobility of rutile as a function of pit in 10-4 M cobalt (II) salts indicates that from pH 3 to 5 the surface behaves similarly to titania until at pH > 6 the adsorption of the cobalt reverses the sign of the surface charge, and at pH > 8
Journal of Colloid and Interface Science, Vol. 417 No. 3, December 1972
444
PARFITT, RAMSBOTHAM, AND ROCHESTER
the electrophoretic behavior is similar to t h a t of cobalt (II) hydroxide. Similar p h e n o m e n a are reported for quartz in solutions of cobalt (II), copper (II), lead (II) and zinc (II) salts. These systems are different f r o m the mixed oxides reported b y Parks (6) since there is an actual change in composition at the oxide/ solution interface as the p H varies when h y d r o x y complexes a n d finally hydroxides are precipitated onto the surface. T h e mixed oxides h a v e a c o n s t a n t composition no m a t t e r w h a t t h e p H of the supporting electrolyte. ACKNOWLEDGMENT The authors thank British Titan Products Co. Ltd. for rutile samples and for a fellowship to J.R. REFERENCES 1. YATES,D. J. C., J. Phys. Chem. 65,746 (1961). 2. JACKSON, P., AND PARFITT, G. D., Trans. Faraday Soc. 67, 2469 (1971). 3. PA~KS, G. A., Chem. Rev. 65, 177 (1965).
4. BERUBE, Y. G., AND DE BRUYN, P. L., J. Colloid Interface Sci. 27, 305 (1968). 5. PARFITT, G. D., RAMSBOTHAM, J., AND ]:~OCI-IESTER, C. It., J. Chem. Soc. Faraday Trans. 1, 17 (1972). 6. PARKS,G. A., Advan. Chem. Ser. 67,121 (1967). 7. PARFITT, G. D., RAMSBOTItAM,J., AND ROCHESteR, C. H., Trans. Faraday Soc. 67, 3100 (1971). 8. BUCKLAND,A., RAMSBOTH.~M,J., ROCHESTER, C. I-I., AND SCURRELL,M. S., J. Sci. lnstrum. 4,146 (1971). 9. PAR~ITT, G. D., J. Oil Colour Chem. Ass. 51, 137 (1968). 10. ¥oPPS, J. A., AND FUERSTENAU, D. W., J. Colloid Sei. 19, 61 (1964). 11. HoNI~, E. P., Trans. Faraday Soc. 65, 2248 (1969). 12. HONIG, E. P., Nature 225,537 (1970). 13. GRI1VILEY,T. B., AND MOT% N. F., Discuss. Faraday Soc. 1, 3 (1947). 14. P~RFITT, G. D., ANn RAMS~OTHA~,J., J. Oil Colour Chem. Ass. 54, 356 (1971). 15. JAMES, R. O., ~-NDHEALY, T. W., J. Colloid Interface Sei. 39 (1972).
Journal of Colloid and Interface Science, Vo[. 41, No. 3, December1972