Clay Minerals (1967) 7, 19.

C H E M I S O R P T I O N OF M E T H Y L E N E B L U E

BY K A O L I N I T E *

F. A . F A R U Q I , S U S U M U O K U D A AND W. O. W I L L I A M S O N

West Regional Laboratories, Pakistan Council of Scientific and Industrial Research, Lahore, Pakistan, Kyoto Technical University, Japan, and Department of

Materials Science, Pennsylvania State University, University Park, Pa., U.S.A.

(Received 20 January 1967)

A B S T R A C T : The chemisorption of methylene blue by kaolinite crystals in- creased as the aqueous suspensions changed from acid to alkaline because, at high pH values, not only the basal pinacoids but the edge-faces were negatively charged. The inability to calculate acceptable specific surfaces or cation exchange capacities from the chemisorption data is discussed, with special reference to the orientation of adsorbed dye cations, the covering of more than one exchange site by a monomer or polymer, the different concentrations of exchange sites on the basal pinacoids and edge-faces respectively, the possibility that such sites occur on the tetrahedral rather than on the octahedral basal pinacoid, and the incomplete replacement of inorganic cations.

I N T R O D U C T I O N

7(he nature ol methylene blue

T a y l o r (1935a, b) ascr ibed to methylene b lue (MB) molecules the pa ra -qu inono id structure shown in Fig. 1 and de te rmined their a r rangement in the crys ta l l ized com- pound. The molecule , a l though la th- l ike, is no t cer ta in ly fiat. I t can be accompan ied by water of crystal l izat ion.

Warwicke r (1955) ob ta ined results agreeing genera l ly wi th those of T a y l o r

I- N 7Ct'

l_cH< FIG. 1. Structure of the methylene blue molecule. In the crystal the chlorine probably

has methyl groups as nearest neighbours (Taylor, 1935a, b).

* Contribution No. 66-33 from the College of Earth and Mineral Sciences, The Pennsylvania State University, University Park, Pa., U.S.A.

20 F. A. Faruqi, Susumu Okuda and W. O. Williamson

(1935a, b) but found that several hydrates exist, each of which is presumably stable within a certain humidity range.

The adsorption oJ methylene blue by clays Chemisorption of MB by clays has been employed to estimate the cation ex-

change capacity (e.g. by Robertson & Ward, 1951; Fairbairn & Robertson, 1957; Loughnan & See, 1958; Ramachandran, Kacker & Petwardhan, 1962) and the specific surface (e.g. by Worrall, 1958; Orr & Dallavalle, 1959; White & Cowan, 1960; Boardman & Worrall, 1965, 1966). Chemisorption of the dye has been used to predict the approximate plasticities and dry strengths because these properties tend to be enhanced when the cation exchange capacities and specific surfaces are greater (Gaskin & Samson, 1951; Cox et al., 1956). It may be increased or decreased if organic matter is present in the clays (Cox et al., 1956).

Plesch & Robertson (1948) confirmed the observations of earlier investigators that the MB initially adsorbed replaces the exchangeable cations, It is not sub- stantially desorbed when its concentration in the ambient solution is decreased and thus chemisorption is indicated. Worrall (1958) and others agreed with these conclusions and, indeed, assessments of cation exchange capacity (c.e.c.) and specific surface depend on their validity.

If the chemisorbed MB forms a complete monolayer, the specific surface of the substrate can be estimated if the area occupied by a single MB cation can be surmised. This area is much greater than that of the actual contact between the cation and the substrate if the calculations of Ramachandran et al. (1962) are acceptable. These calculations assume that the decrease in the surface area avail- able to argon, after chemisorption of MB on kaolinite, is the area occupied by the MB cations and is not, for example, the result of enhanced aggregation of the kaolinite crystals, perhaps promoted by the adsorbed MB.

Cation adsorption by kaolinite is commonly assumed to occur on discrete exchange sites rather than to result from a 'smeared charge' on the surface of the substrate. This concept receives support from studies of illite and montmorillonite by Edwards, Posner & Quirk (1965a, b).

If a monolayer of MB forms before the c.e.c, is satisfied, a single MB cation or molecule could cover more than one exchange site, particularly if the sites were close together (Boardman & Worrall, 1965). Thus, the c.e.c, estimated from MB adsorption may be lower than that found by techniques involving small inorganic cations.

When the concentration of MB in the ambient solution is sufficiently increased, chemisorption is followed by physical adsorption (Plesch & Robertson, 1948). The latter adsorption is reversible and may involve molecules and possibly dimers or higher polymers, rather than cations.

The surfaces of kaolinite crystals The basal pinacoids of kaolinite crystals have negative charges which persist

Chemisorption by kaolinite 21

despite changes in pH, but the edge-faces are charged positively or negatively in solutions of low or high pH respectively. Thus only the edge-faces display an iso-electric point. Street & Buchanan (1956) surmised that this is at ca. pH 6"5 but Quirk's studies of chloride adsorption (1960) suggest that it occurs in alkaline solutions. Other explanations of Quirk's results are, however, conceivable (Hunter & Alexander, 1963b).

The permanent negative charges on the basal pinacoids are commonly ascribed to a slight degree of substitution within the crystal structure, probably A13+ for Si ~+, while the charges on the edge-faces arise from broken bonds, as shown schematically by Hunter & Alexander (1963a). They depict M--O--M bonds (M = Si or AI) as breaking to give M- -O- and M +. As the M--OH acid is very weak, the M- -O- group becomes neutral by adsorbing a proton. Thus the positive edge-charge in acid solutions arises from the M +, and becomes neutralized as the - -OH con- centration in the solution increases. The hypothesis of Schofield & Samson (1953), however, requires a more elaborate, but perhaps more plausible, scheme.

Cashen (1963) suggested that kaolinite crystals become positively charged near their peripheries by an excess of complex aluminium-containing cations in the Stern layer.

Van Olphen (1963) remarked that the broken tetrahedral sheets at the edge-faces are comparable to the surfaces of silica particles and such particles are known to become positively charged, if, as is likely, there are small amounts of aluminium in the ambient solution. Further, if the tetrahedral sheets tend to break preferentially where A1 *+ is substituted for Si ~+, the resulting surfaces become comparable in behaviour to alumina surfaces, and thus almost their entire area could develop a positive charge at low pH values.

Despite various opinions as to the mechanisms which cause the development of negative or positive charges on the surfaces of kaolinite crystals, it may be concluded that the maximum chemisorption of the MB cation should occur from alkaline suspensions where all these surfaces are negatively charged. Indeed, the chemisorption of MB may be expected to resemble that of cetyl-trimethylammoniurn bromide (Cashen, 1961) and to increase as the pH of the suspension is progressively raised. This possibility does not appear to have been investigated systematically prior to the present study, although White & Cowan (1960) suggested that the edge- faces, if positively charged in acid suspensions, are unlikely to adsorb MB, while Boardman & Worral (1966) postulated that acid s may interfere competitively with the adsorption of acidic dyes at very low pH values.

In summary, the best estimates of specific surface should be obtainable when the chemisorption of MB occurs from suspensions that are sufficiently alkaline for the edge-faces as well as the basal pinacoids of the kaolinite to be negatively charged. This pH condition should provide also the maximum values for c.e.c., calculated from MB adsorption data, although it can be objected that the condition might be particularly favourable for the covering of more than one exchange site by a single cation.

22 F. A. Faruqi, Susumu Okuda and W. O. Williamson

E X P E R I M E N T A L

Materials used

Chemicals. The dyes and other chemicals were of laboratory reagent grade. The methylene blue hydrochloride was the trihydrate (water found by drying at 115 ~ C, 14.32%; calculated, 14.45%).

Minerals. These were artificial cryptocrystalline low-quartz, artificial micro- crystalline a-alumina, the dickite previously studied by Okuda & Williamson (1964), and four Georgia kaolinites. Three of the latter were described by Murray & Lyons (1956, 1960) and by Kitahara & Williamson (1964). As in the publications cited, they were designated G, E, and I. They were well, moderately, and poorly ordered respectively. Kaolinite G contained stacks, while E and I had agglomerates of plates.

Kaolinite Z was a size-fraction obtained from the fourth sample by sedimenta- tion. The electron microscope showed eumorphic crystals between 0-1 and 2"5 /~ across, most being 1-2 ~ across. The edge-faces constituted ca. 20% of the surface area.

No montmorillonite was detected in the samples by X-ray techniques but electron microscopy suggested its presence in kaolinite Z (cf. Bundy, Johns & Murray, 1966).

The exchangeable cations in the 'as received' kaolinites appear in Table 1.

TABLE 1. Partial analyses of exchangeable cations in kaolinite samples (m-eq/100 g)

Sample c.e.c. Ca Mg A1 Na K

G 2"27 0"80 n.d. 0"41 nil nil E 3-01 0-43 n.d. 0.84 nil nil I 4.30 0.75 n.d. 0"72 nil nil Z 4"64 0"17 0.92 1 "65 0"16 0"07

Results for G - I quoted from Kitahara & Williamson (1964).

Procedures. The specific surfaces of the kaolinites were found by low temperature nitrogen adsorption (B.E.T. technique). Those of G, E, and I have already been reported (Kitahara & Williamson, 1964).

Cation exchange capacities were estimated by leaching the kaolinites four times with 1"0 N calcium acetate, washing them successively with distilled water, aqueous ethanol (50% by volume), and absolute ethanol, displacing the calcium by four treatments with 1-0 N ammonium chloride solution and then determining it flame- photometrically.

Hydrogen kaolinites were prepared by rapidly and repeatedly leaching the samples with 1.0 N hydrochloric acid (Harward & Coleman, 1954) and then washing them with distilled water until the filtrate was free from chloride. The preparations

Chemisorption by kaolinite 23 were dried at 100~ They may be expected to contain some exchangeable aluminium.

The following technique was used to estimate the chemisorption of MB at various pH values. A 0"2% by weight solution of the dye was prepared. Hydrochloric acid or aqueous sodium hydroxide were added to aliquots of this to obtain suitable volumes (3-5-6"5 ml) having the pH values plotted in Fig. 2. No colour change or precipitation followed these additions. The MB solutions remained in contact with

% x

?,

g

13--

12--

I I - -

I0--

9--

B

l:n 7 - -

6 -

5 - -

. . . .o________. .o- - I

o ~ ~ Z o

G

I I I I I I I I I I. 3 4- 5 6 7 8 9 I0 II 12

pH

FIG. 2. Methylene blue ehemisorbed by kaolinites G, E, I, and Z at different pH values, expressed as weights of the hydrochloride.

0"5 g of H-kaolinite for 24 hr with occasional shaking. This method was found adequate for equilibration. The kaolinite was then washed without intermission with aqueous hydrochloric acid or sodium hydroxide, of the same pH values as the corresponding suspensions of kaolinite in dye-solution originally prepared, until no blue colour was detectable in the filtrate. The MB in the accumulated filtrate was determined electro-photometrically and hence the weight of dye retained by 1 g of kaolinite.

In addition, the above technique was used to estimate the chemisorption of MB by kaolinite G at pH 6 where the initial material was in the Ca- and not the H, Al-form.

Similar techniques were used when dyes other than MB were involved, al- though electrophotometric determinations of the adsorbed dyes were not made (Table 2).

24 F. A. Faruqi, Susumu Okuda and W. O. Williamson TABLE 2. Staining of dickite, s-alumina, and quartz

Material MB M e A L

pH3 p H l l pH3 p H l l pH3 p H l l

Dickite 7 10 4 0 3 0 s-alumina 0 5 10 0 10 3 Quartz 1 10 4 0 8 0

MB = methylene blue (coloured cation), Me = methyl orange (coloured anion); AL = sodium alizarin monosulphonate (coloured anion). The anion is CI' in MB and the cation is Na in Me and AL. The numerals represent the approximate intensity of the staining of the bulk material, as estimated by eye. A zero implies no perceptible staining.

R E S U L T S

Staining of dickite, a-alumina, and quartz

The ability of certain acidic or basic dyes to stain dickite, a-alumina or quartz is indicated in Table 2. Dickite was examined because, like kaolinite, it develops positive charges on the edge-faces at low, and negative charges at high, pH values (Okuda & Williamson, 1964), but, as its crystals are relatively coarse, preferential staining of the edge-faces by acidic dyes at p H 3 might be discernible. Only the degree of staining of the bulk material could, however, be assessed with confidence.

The staining of a-alumina and quartz was examined because of its possible analogy to dye-adsorption by the edge-faces of kaolinite, if these faces are regarded as equivalent to the surfaces of alumina and silica particles.

Adsorption o/methylene blue by kaolinite

The weights of MB adsorbed by 1 g of kaolinite, initially in the H, Al-form, remained approximately constant between pH 3 and 4.5, increased at higher p H values, and became again approximately constant between p H 9"5 and 11 (Table 3, Fig. 2). The maximum difference in duplicate experiments was 0"0004 g /g but most differences were 0"0002 o r less.

At pH 6, kaolinite G, originally in the Ca-form, adsorbed 0-00692 + 0-00002 g /g in four replicates, corresponding to a mean c.e.c, of 1"85 m-eq/100 g.

D I S C U S S I O N

The variations in the staining behaviour of a-alumina and quartz with the acidic or basic character of the dye, and the pH of its solution, accord with the develop- ment of positive and negative surface charges at low and at high p H values res- pectively, as reported by Yopps & Fuerstenau (1964) for a-alumina and by Verwey (1941) for quartz (Table 2).

d e4

•

<

o~

0 x

" 0

d~

o

�9 .~ ~- .~ e, I ~ .

0 U~ ~ N

o

0

0

, .0 o

0

i

. ~ . ~ ' ~ " 0

~ , B , B x o o

~ ~

26 F. A. Faruqi, Susumu Okuda and W. O. Williamson The staining behaviour of dickite (Table 2) and, by analogy, that of kaolinite

accord with the presence of edge-faces which develop positive or negative charges, in response to low or high pH values respectively, in a manner comparable to that exhibited by the surfaces of a-alumina or quartz.

When the kaolinite suspensions changed from acid to alkaline, the weight of MB increased by c a . 23-28% of that held at pH 11 (Table 3, column G). This percentage is presumably similar to the percentage of total surface area of the crystals contributed by the edge-faces because, for kaolinite Z, the percentage found electron-microscopically, and that appearing in Table 3, column G, were 20% and 26"74% respectively. The fact that the latter percentage is the higher might indicate exchange sites which are closer together on the edge-faces than on the basal pina- coids, thus permitting the edge-faces to retain more MB per unit area. This hypo- thesis becomes doubtful when the results given by kaolinite G are examined. The value in Table 3, column G, is 22"86 %, yet this kaolinite contains numerous stacks, the edge-faces of which could be reasonably expected to contribute a considerably higher percentage of the total surface area. At least some stacks, however, are composed of flakes, which are not in complete contact (cf. Murray & Lyons, 1960, Plate 4), and thus basal pinacoids within the stacks would be available to the MB, and the contribution of the edge-faces to the total surface area, as assessed by MB adsorption, correspondingly reduced.

It cannot be assumed that all the surfaces available to nitrogen are also avail- able to the relatively large MB cation. However, when the c.e.c., calculated from the chemisorption of MB at pH 11, is plotted against the specific surface estimated by nitrogen adsorption, the curve is essentially linear (Fig. 3). At this pH, optimum conditions for the complete coverage of the kaolinite surface by a MB monolayer may be expected, as the whole surface is negatively charged.

o Z (c.e.c. Ca 2+)

0

0 4-- 0 ~" .c. co ~

c .e

o c.e -c" ~ o~ 9 g ~ d 3

6

2

Z E Z I ; I I I ; t0 II 12 13 14- 15

Surface area (rn2g - t)

FIG. 3. Cation exchange capacities (m-eq/100 g) of kaolinites G, E, I, and Z, determined by calcium and by methylene blue saturation plotted against the corresponding specific

surfaces found by the B.E.T. technique.

Chemisorption by kaolinite 27 Ormsby, Shartsis & Woodside (1962) found that the c.e.c, of kaolinite, estimated

by manganese-saturation, increased linearly with the specific surface; size-fractions of samples E and I were used in their study. In the present investigation the linear increase was confirmed for the c.e.c, found by the calcium-saturation of kaolinites G, E, and I but kaolinite Z gave anomalous results (Fig. 3).

The cation exchange capacities estimated by calcium retention were ca. 18-24 % greater than those calculated from MB chemisorption, except for kaolinite Z where the c.e.c, was ca. 52% greater (Table 3, column K). If the calcium retention were measured at pH 11, where the whole kaolinite surface is negatively charged, instead of at near neutrality, the percentage differences would probably be still greater.

Murray & Lyons (1960, Fig. 1) plotted the c.e.c, of a series of kaolinite samples against the corresponding crystallinities. Samples G, E, and I are presumably among those represented by unlabelled points in the figure, inspection of which suggests that the c.e.c, plotted were lower than those found by calcium-saturation in the present experiments. A personal communication from the authors confirmed this and reported 1.40, 1.15, and 3"55 m-eq/100 g for G, E, and I respectively, as deter- mined by ammonium saturation according to the technique of Schollenberger & Simon (1945). Unlike the values obtained by calcium saturation (Fig. 3) or by manganese saturation (Ormsby, Shartsis & Woodside, 1962) these c.e.c, are not linearly related to the specific surfaces of the l~aolinites. Ormsby et al. (1962) reported the c.e.c, only of size-fractions of E and I and not those of the unfrac- tionated samples. However, size-fractions comparable in specific surface to the unfractionated samples gave c.e.c, similar to those found in these laboratories for the latter.

By treating the unfractionated samples once only with hot N ammonium chloride solution, and estimating the ammonium retained, Kitahara & Williamson (1964) obtained c.e.c, which, like those found by Murray & Lyons (1960), were relatively low, i.e. 1.3, 1"4, and 2.2 m-eq/100 g for G, E, and I respectively. They remarked that complete replacement of the original cations had probably not been obtained.

The c.e.c, reported by Murray & Lyons (1960) give 108"5, 158.9, and 71-1 A 2 for the calculated areas of occupation of the substrate by a monovalent cation, where the areas refer to G, E, and I respectively. These areas show a large and irregular variation when compared to the 66.9, 60"7, and 58-7 A 2 of Table 3, column D.

The present authors prefer to base their conclusions on their own c.e.c, data. They realize that certain workers (e.g. Helmy, 1963) claim that saturation with a divalent, as compared with a monovalent, cation may lead to higher c.e.c, values, sometimes because the supposedly divalent cation has changed to a monovalent basic cation (Mackenzie, 1951). The problem is discussed further by Okazaki, Smith & Moodie (1964) who showed that there is not necessarily any difference in the c.e.c, determined by the retention of monovalent and divalent cations respectively, if appropriate techniques are used. In the present study, the lack of accord between the c.e.c, found by retention of calcium and of MB respectively is thought to result essentially from causes other than the different valencies of the cations.

28 F. A. Faruqi, Susumu Okuda and W. O. Williamson Table 3, columns D and H, shows the calculated areas of occupation of a kaolinite

surface by a monovalent inorganic cation and by a monovalent MB cation res- pectively. The latter cation occupies a larger area. This suggests that an MB cation would be more apt to cover more than one exchange site and the reported c.e.c. would be correspondingly lowered. It is assumed, however, that the exchange sites are equally distributed over the whole surface of a kaolinite crystal. This is unlikely to be true, as a higher concentration may be expected on the edge-faces, especially at pH 11 where these have developed their maximum negative charge according to Schofield & Samson (1953; Fig. lc). Thus, on these faces, covering of more than one exchange site by a single MB cation is especially likely, with a consequent lowering of the measured c.e.c.

It has recently been claimed that the c.e.c, of the basal pinacoids is manifested especially by those paved with silica tetrahedra, rather than with aluminous octahedra (Amain Weiss & Russow, 1963). Thus the exchange sites on the basal pinacoids would be closer than usually assumed, though they might remain less crowded than on the edge-faces. However, covering of more than one exchange site by a single MB cation would become more likely. This is sugested by the areas tabulated in Table 3, column I. These were calculated on the assumption that the exchange sites were evenly distributed over the edge-faces and only one pinacoidal face of each crystal.

In summary, the measured c.e.c, could be lower than that obtained from the retention of inorganic cations because the relatively large organic cation is more likely to cover more than one exchange site. This situation could be aggravated if part of the adsorbed dye is retained as polymers. It is noteworthy that Bergmann & O'Konski (1963) found that, on montmorillonite, the adsorption of MB as dimers or other low polymers began to occur before the c.e.c, was satisfed. These polymers could form on the substrate even from monomeric solutions. Further, the adsorbed dye molecules tended to occupy adjacent sites in preference to distributing them- selves randomly over all the sites available. These findings, if relevant also to kaolinite, render still more uncertain the assessment of specific surface by assigning an appropriate area of occupation to an adsorbed RIB cation.

The large discrepancy between the cation exchange capacities of kaolinite Z measured by MB and by calcium adsorption (Fig. 3 and Table 3) probably results from the presence of montaori l lonite, the internal surfaces of which might be available to calcium rather than to nitrogen (Greene-Kelly, 1964) or to MB. Similarly, the results of Ramachandran et al. (1962) can be interpreted as revealing the in- accessibility of the intracrystalline surfaces of aontmoril lonite to argon in the B.E.T. technique and to MB, in contrast to NH4 +, in determinations of c.e.c, in which the dye gave lower values. The lower values were ascribed to a fiat dis- position of the MB cations, on the interior surfaces of the montmorillonite crystals, which allowed a single cation to cover several exchange sites. This effect also could contribute to the present results if montmoriIlonite were present.

Another factor contributing to the smaller c.e.c, found by MB chemisorption (Table 3, Figs. 2 and 3), could be the inability of the dye to displace completely

Chemisorption by kaolinite 29

the inorganic cations already occupying exchange sites, e.g. H + or complex aluminium ions. This is suggested by the decreased adsorption of MB that followed the treatment of silica with acid (Gibb & Ritchie, 1954), and by the fact that kaolinite G, in the Ca-form, treated with MB at pH 6 gave a c.e.c, of 1"85 as compared with ca. 1"60 for the H, Al-form (the latter figure calculated from data in Fig. 2).

The dye used was not specially purified and a source of error could arise if trimethylthionine were present as an impurity. This dye appears to be neutral at high pH values which, indeed, favour its formation from MB (Bergmann & O'Konski, 1963). It is unlikely to be chemisorbed by the kaolinite and thus could cause errors in the colorimetric estimates of MB adsorption.

Accurate determinations of the surface area of kaolinites from MB adsorption data require that a mono-layer of the dye be obtained and an appropriate area of occupation assigned to the dye-cation. This area will vary according to the orienta- tion ascribed to the cation and, moreover, if a significant amount of dye is adsorbed as polymers, even before the c.e.c, is satisfied (Bergmann & O'Konski, 1963), a meaningful area of occupation cannot be deduced.

For purposes of discussion the projected areas of the dye molecule given by Kipling & Wilson (1960) are used, viz. 135 A 2 (adsorbed fiat), 75 A 2 (adsorbed edgewise) and 39-5 A 2 (adsorbed on end). Othel estimates in recent literature, e.g. by Allingham et al. (1958), are of similar orders. The areas listed in Table 3, column H, are too small to accommodate MB cations adsorbed fiat but are similar to the projected areas of these cations adsorbed edgewise; alternatively the cations might be on end. If, however, the exchange sites are closer on the edge-faces than on the basal pinacoids, the cations could be arranged edge- or end-wise on the former and at least some of them might lie flat on the latter, but, if only the tetrahedral basal pinacoids have exchange sites, the adsorbed cations on all surfaces would be arranged end-wise (Table 3, column I). It may be noted that the areas cited in columns H and I of Table 3 are too large if, as suspected, the original cations are only partially replaced by MB.

Further complexities arise if it is postulated that the edges and coigns of the crystal adsorb MB more effectively than the faces, or that the faces are not fiat. For example, the pinacoids might have steps, the lateral part of which could have the adsorption characteristics of edge-faces. Further, disorganized quartz-surfaces adsorbed more MB per unit area than unabraded ones (Gibb & Ritchie, 1954). Comparable phenomena might be manifested by kaolinite.

In summary, the unresolved problems cited above make it impossible to assign an area to the adsorbed MB molecule which could be used with confidence to calculate specific surfaces. This accords with the statement of Orr & Dallavalle (1959)--'the conclusion that the sorption of dyes is not a satisfactory general measure of surface area is nevertheless inescapable at present."

C O N C L U S I O N S

(1) The maximum chemisorption of the methylene blue cation by H, Al-kaolinite

30 F. A. Faruqi, Susumu Okuda and W. O. Williamson occurred in alkaline suspensions, i.e. where both the basal pinacoids and edge-faces were negatively charged.

(2) Even at pH 11 the cation exchange capacity determined by methylene ad- sorption was ca. 18-24% less than that estimated f rom calcium saturation near neutrality.

(3) The low cation exchange capacities found were ascribed to the covering of more than one exchange site by a single MB cation, monomer , or polymer, and to the incomplete displacement of the original exchangeable cations by the adsorbed dye.

(4) The possible differences in the concentrations of the exchange sites on the edge-faces and basal pinacoids respectively, and on the tetrahedral as compared with the octahedral basal pinacoids, make it impossible to assign meaningful areas of occupat ion to methylene blue cations present as a monolayer on the substrate.

(5) A particular kaolin showed 52% difference between the cation exchange capacities estimated f rom calcium and methylene blue retention, the latter adsorbate giving the lower value. The phenomenon was ascribed to the presence of mont- morillonite.

A C K N O W L E D G M E N T S

Gifts of kaofinites by the Georgia Kaolin Company were appreciated. Dr Pa Ho Hsu made helpful criticisms and, together with Mr Gerald Q. Weaver, gave practical assistance. The support of the investigation by the National Science Foundation is gratefully acknowledged.

R E F E R E N C E S

ALLINGHAM M.M., CULLEN J.M., GILES C.H., JAIN S.K. & WOODS J.S. (1958) J. appL Chem., Lond. 8, 108.

BERGMANN K. & O'KONSKI C.T. (1963) J. phys. Chem., Ithaca 67, 2169. BOARDMAN G. & WO~,ALL W.E. (1965) Science o/ Ceramics (G. H. Stewart, editor), Vol. 2,

p. 47. Academic Press, London. BOARDMAN G. & WORRALL W.E. (1966) Trans. Br. Ceram. Soc. 65, 343. BUNDY W.M., JorlNS W.D. & MURRAY H.H. (1966) Clays Clay Miner. 14, 331. C^SHEN G.H. (1961) Chem. Ind. No. 43, 1732. CASI-IEN G.H. (1963) Nature, Lond. 197, 349. Cox R.W., FROSTlCK A.C., GARREYr W.G. & WILLIAMSON W.O. (1956) A Reconnaissance of

the Ceramic and Re/factory Clays of Western Australia. C.S.I.R.O. (Australia), Div. Ind. Chem. Tech. Paper No. 2.

EDWARDS D.G., POSER A.M. & QUIRK J.P. (1965a) Trans. Faraday Soe. 6I, 2808. EDWARDS D.G., POSER A.M. & QUIRK J.P. (1965b) Trans. Faraday Soc. 61, 2816. FAIRBAIRN P.E. & ROBERTSON R.H.S. (1957) Clay Miner. Bull. 3, 129. GASKIN A.J. & SAMSON H.R. (1951) Ceramic and Refractory Clays of South Australia. Bull.

geol. Surv. S. Aust. 28. GIBB J.G. & Rrrr P.D. (1954) J. appl. Chem., Lond. 4, 483. GREEtCE-KELLV R. (1964) Clay Miner. Bull. 5, 392. HARWARD M.E. & COLEMAN N.T. (1954) Soil Sci. 78, 181. HELMY A.K. (1963) Soil Sci. 95, 204. HUNTER R.J. & ALEXANDER A.E. (1963a)/. Colloid Sci. 18, 820. HUNTER R.J. & A L ~ E R A.E. (1963b)/. Colloid Sci. 18, 833.

Chemisorption by kaolinite 31 KIPLING J.J. & WILSON R.B. (1960) d. appl. Chem., Lond. I0, 109. AYOA KITAHARA & WILLIAMSON W.O. (1964) I. Am. Ceram. Soc. 47, 313. LOUGHNAN F.C. & SEE G.T. (1958) Aust. J. Sci. 21, 22. MACKENZIE R.C. (1951) J. Colloid Sci. 6, 219. MURRAY H.H. & LYONS S.C. (1956) Clays Clay Miner. 4, 31. MURRAY H.H. & LYONS S.C. (1960) Clays Clay Miner. 8, 11. OKAZAKI R., SMITH H.W. t~ MOOOIE C.D. (1964) Soil Sci. 97, 202. SLISUMU OKUDA & WILLIAMSON W.O. (1964) Clays Clay Miner. 12, 223. ORMSBY W.C., SHARTSIS J.M. & WOOOSIDE K.H. (1962) J. Am. Ceram. Soc. 45, 361. ORR C., JR & DALLAVALLE J . i . (1959) Fine Particle Measurement, Chap. 8, pp. 221-232.

Macmillan, New York. PLESCH P.H. & ROBERTSON R.H.S. (1948) Nature, Lond. 161, 1020. QUIRK J.P. (1960) Nature, Lurid. 188, 253. RAMACHANDRAN V.S., KACKER K.P. & PATWARDHAN N.K. (1962) Am. Miner. 47, 165. ROBERTSON R.H.S. & WARD R.M. (1951) J. Pharm. Pharmac. 3, 27. SCHOFIELD R.K. & SAMSON H.R. (1953) Clay Miner. Bull. 2, 45. SCHOLLENBERGER C.J. & SIMON R.H. (1945) Soil Sci. 59, 13. STREET N. & BUCHANAN A.S. (1956) Aust. J. Chem. 9, 450. TAYLOR W.H. (1935a) Chem. Ind. 54, 732. TAYLOR W.H. (1935b) Z. Kristallogr. 91, 450. VAN OLPnEN H. (1963) An Introduction to Clay Colloid Chemistry, p. 91. Interscience, New York. VERWEY E.J.W. (1941) Recl Tra~,. chim. Pays-Bas Belg. 60, 625. WARWlCKER J.O. (1955) J. chem. Soc. 2531. ARMIN WEISS & RUSSOW J. (1963) International Clay Conference, Stockholm (I. Th. Rosenqvist

& P. Graff-Petersen, editors), pp. 203-213. Pergamon Press, Oxford. WHITE D. & COWAN C.T. (1960) Trans. Br. Ceram. Soc. 59, 16. WORRALL W. (1958) Trans. Br. Ceram. Soc. 57, 210. Yol'PS J.A. & FUERSTENAU D.W. (1964) 1. Colloid Sci. 19, 61.