Download pdf - Physico-chemical Properties of Sols and Gels of Na-montmorillonite with and without Adsorbed Hydrous Aluminum Oxide1

DIVISION S-2—SOIL CHEMISTRY

Physico-chemical Properties of Sols and Gels of Na-montmorillonite withand without Adsorbed Hydrous Aluminum Oxide1

BRIAN G. DAVEY AND PHILIP F. Low2

ABSTRACT

Sodium-montmorillonite was prepared with and without ad-sorbed hydrous aluminum oxide and called Na/Al-clay andNa-clay, respectively. The two clays were mixed with 10~4NNaCl solution in different proportions. Measurements of shearstress at different shear rates showed that the clay content atwhich gelation began, as indicated by an increase in viscosityand the initiation of non-Newtonian and hysteretic behavior,was lower for the Na/Al-clay than for the Na-clay (<~ 2% clayvs. 4% clay). Theoretical considerations indicated that bothstrong and weak bonds were present in the gels but only weakbonds were present in the sols.

Specific heat capacities of the mixtures were determined ina Calvet microcalorimeter and were found to differ from thosecalculated from the known proportions and specific heat capa-cities of clay and water. Differences were greater for theNa/Al- than for the Na-clay. A detailed analysis indicated thatthese differences could be attributed to clay-water interaction.

EMF measurements, made with sodium glass and Ag/AgClelectrodes, showed that the relative partial molar free energyof NaCl changed with clay content in somewhat different waysfor the Na/AI- and Na-clay mixtures. The changes were cor-related with changes in other properties of the mixtures. Itwas concluded that adsorbed hydrous aluminum oxide doesinfluence the physico-chemical properties of sols and gels ofNa-montmorillonite.

Additional Key Words for Indexing: homoionic clay.

IT is NOT UNCOMMON for small amounts of adsorbedmaterial to markedly affect the physico-chemical prop-

perties of colloids. Colloidal clays that have been saturatedwith hydrogen and subsequently titrated with a base areknown to contain some adsorbed hydrous aluminum oxide(Paver and Marshall, 1934; Harward and Coleman, 1954;Low, 1955). Therefore, we wondered if, in preparing ahomoionic clay, a preliminary hydrogen saturation wouldaffect its physico-chemical properties. There were indica-tions that it would. Consequently, the present study wasundertaken. Part of this study has already been reportedelsewhere (Davey and Low, 1968).

MATERIALS AND METHODS

Wyoming bentonite (Volclay 200 from the American Col-loid Co.) was suspended in deionized water and the suspension

1 Journal Paper no. 3895, Purdue Univ. Agr. Exp. Sta., Lafay-ette, Ind. Contribution from the Agronomy Dept. Presentedbefore Div. S-2, Soil Science Society of America at Washington,D.C., Nov. 1967. This research was supported by the NationalAeronautics and Space Administration. Received Nov. 29, 1969.Approved Nov. 10, 1970.

was allowed to stand quietly until particles larger than 2 /i,as calculated by Stake's law, had settled out. Then the < 2-nparticles remaining in suspension were saturated with Na+ bytwo different methods. In the first, the suspension containingthem was passed through a Na-saturated cation-exchange resin.In the second, the suspension was passed through a H-saturatedcation-exchange resin, allowed to stand 4 days, and then titratedwith NaOH to pH 7. Unwanted anions were removed fromboth Na-saturated clays by washing them in a centrifuge untilno Q- could be detected in the supernatant solution and theelectrical conductivity of this solution was of the order of 50itimhos cm-1 at 25C. Finally, the clays were freeze-dried, pow-dered in a ball mill for 15-20 min and stored in glass bottles.

Sodium-montmorillonite prepared by the first method wasbelieved to be relatively free of hydrous aluminum oxide. Itwill be called Na-clay. Two different batches of it were pre-pared in different years. These batches will be referred to asbatch I and batch II, respectively. Sodium-montmorilloniteprepared by the second method is known to contain about 0.37meq Al/g (Low and Anderson, 1958). Hence, the hydrousaluminum oxide, calculated as A1(OH)3, amounted to lessthan 1% by weight. This montmorillonite will be called Na/Al-clay.

To examine the physico-chemical properties of the Na- andNa/Al-clay, the freeze-dried clays were mixed in different pro-portions with a KHN solution of NaCl to make sols and gels.The NaCl solution was used instead of pure water so thatEMF measurements involving the Ag/AgCl electrode could bemade. Before any property was measured, the sols and gelswere extruded through an 18-gauge hypodermic needle. Thiswas done so that, in the gels at least, the particles would tendto be oriented parallel to each other. Also, extrusion stabilizedthe gels so that their properties were essentially independentof time. Thus, variations in physico-chemical properties due todifferences in particle arrangements and time were minimized.

Data on shear stress versus shear rate were determined onthe different sols and gels at room temperature by means of aFann Viscometer (Model 35) using the R1-B1 rotor-bob com-bination and appropriate torsion springs. The reproducibilityof the data was within ± 3 %.

Specific heat capacities were measured at 25C in a Calvetmicrocalorimeter by the procedure of Calvet and Prat (1963).Voltage signals from'the microcalorimeter were measured bya Keithley (Model 150AR) microvoltammeter and recorded ona Sargent MR recorder equipped with a Model 204 Disc chartintegrator. The specific heat capacity was determined betweentwo and eight times on each sample. The maximum differencebetween determinations was 5 x 10~3 cal "C-1 g-1. In severalcases, specific heat capacities were determined on duplicatesamples. The difference between duplicates did not exceed0.1% of the mean. _ _

The relative partial molar free energy of NaCl, (F — F°)Naci>was determined in the different sols and gels at roomtemperature by measuring the electromotive force, E, betweena Ag-AgCl electrode and a Beckman sodium glass electrodeand using the following equation:

2 Formerly Post-Doctoral Research Associate and Professorof Agronomy, respectively. Present address of the senior authoris Dept. of Soil Science, University of Sydney, Sydney, N.S.W.,Australia.

230

DAVEY & LOW: PHYSICO-CHEMICAL PROPERTIES OF SOLS AND GELS OF Na-MONTMORILLONITE 231

I5OO-

1000

1>t</>o«>

500

...........A No/AI-cloy

• No-clay (batch I)

— —o No-clay (batch H)

,*'

1 3 3 4 5 6CLAY CONTENT (percent by wt.)

Fig. 1—Effect of clay content on the viscosity of Na/Al- andNa-clay systems (determined at a shear rate of 5.112 sec"1).

E ~(F - F«)NaCi

[1]

where E° is the standard potential and F is the faraday. Sincethe sodium glass electrode ages and E° changes, the electrodecouple was calibrated periodically in a NaCl solution of known(F — F0)NaCj. By this procedure, the electromotive forcesmeasured in replicate extruded samples differed by less than2 mv.

RESULTS AND DISCUSSION

Plotted in Fig. 1 are curves of viscosity, ^, versus claycontent calculated, assuming Newtonian behavior, from therelationship between shear stress, /, and shear rate, ds/dt,viz,

[2]

o 2OOOin

~ I6OO

m I20O

UJ

5 80O

.....i Na/AI-cloy

o Na-clay (batch I)

.........—-••>••••;;.'.'.'.'......•<•••.....a

....-a

.- ->-&•••""

....--•-A-""

__•»-

u 2 4 6 8SHEAR RATE (sec"1 X IO'Z)

Fig. 3 — Relationship between shear stress and shear rateNa/Al- and Na-clay systems at a clay content of 4%weight.

forby

240-

40

4.............A Na/Al -clay

o————o Na-clay (batch I)

Na/Al-clay (theoretical)

SHEAR RATE (sec"'x IO"Z)

Fig. 2—Relationship between shear stress and shear rate forNa/Al- and Na-clay systems at a clay content of 2% byweight.

Although (as shown in Fig. 2, 3, and 4) the assumptionof Newtonian behavior is not strictly valid at shear ratesas low as 5.112 sec-1, data for this shear rate were usedin the calculations because it was the smallest one availableand disturbed the system the least. Observe that the curvesfor the two batches of Na-clay are nearly the same. Com-parison of these curves with the one for Na/Al-clay showsthat the viscosity of the Na/Al-clay was greater than thatof the Na-clay at all clay contents. Further, the viscosityof the Na/Al-clay increased monotonically with clay con-tent, whereas, the viscosity of the Na-clay increased verylittle until a clay content of 4% was reached; then itincreased suddenly.

Plotted in Fig. 2, 3, and 4 are curves of shear stressversus shear rate for systems of both clays at different claycontents. Examination of these figures discloses that theyield value (shear stress required to produce a finite shearrate) increased with clay concentration in both clays butwas always higher in the Na/Al-clay. Also, non-Newtonian

8000

4000

5UJ

5

.............a Na/AI-cloy

o————o No-clay (botch I)

SHEAR RATE (sec" X IO" )

Fig. 4—Relationship between shear stress and shear rate forNa/Al- and Na-clay systems at a clay content of 6% byweight.

232 SOIL SCI. SOC. AMER. PROC., VOL. 35, 1971

behavior and hysteresis began at lower clay contents in theNa/Al-clay than in the Na-clay, i.e., the Na/Al-clay tendedto gel first.

According to the relaxation theory of viscosity (Powelland Eyring, 1944), if two kinds of bonds exist in the sys-tem, namely, one kind of strong bonds and one kind ofweak bonds, the equation relating shear stress and shearrate is given by

ds ZkT ,f - •>?„ — + ——— In (

" dt A2A3A vAi ds_.

~dt- [3]

where ^2 is the coefficient of viscosity due to the weakbonds, k is the Boltzmann constant, T is the absolute tem-perature, A2A3 is the cross-sectional area of the flowingunit on which the shear stress acts, AX is the distance be-tween neighboring flow units in the direction normal toshear, A is the distance between adjacent equilibrium posi-tions in the direction of shear, and kr is the frequency withwhich the flowing unit crosses the energy barrier betweenthese positions at zero stress. Dahlgren (1958) has shownthat this equation can be applied to a system containing anynumber of strong and weak bonds. Thus,

i — + S« Indt dt [4]

Either equation [3] or equation [4] indicates that the shearstress depends exponentially on the shear rate at smallshear rates (when the term for strong bonds, i.e., the sec-ond term, predominates) and depends linearly on the shearrate at large shear rates (when the term for weak bondspredominates). This is what we observed in the non-Newtonian systems of Fig. 2, 3, and 4.

To make a more quantitative comparison between theoryand experiment, the theoretical curve for the Na/Al-clayis included in Fig. 2. This curve was obtained by meansof equation [3]. The value of ̂ in the equation was deter-mined from the slope of the linear portion of the experi-mental curve; the value of 2kT/\2\3\ was determinedfrom the slope of the straight line obtained by plotting(/ — T)2 ds/dt) against In ds/dt, using experimental valuesfor / and ds/dt; the value of Ai/A^r was determined fromthe intercept, In (Aj/A^ r) • 2kT/\2\3\, of the same lineon the ordinate. These values were 0.10 poise, 21.71 dynecnr2, and 0.2148 sec, respectively. It is evident that equa-tion [3] describes the experimental results quite well.

On the basis of Fig. 1-4 and equations [3] and [4] itappears that, in the time allowed, weak bonds predominatein the Na/Al- and Na-clay systems until clay contents of2 and 4%, respectively, are reached. Then relatively strongbonds, causing rapid gelation, are formed. (Strong bondswill form and cause gelation at lower clay contents if suffi-cient time is allowed. The clay contents noted above arethe critical ones for rapid gelation.) These bonds arestronger in the gels of Na/Al-clay than in the gels of Na-clay. The presence of hysteresis in the shear stress-shearrate curves at the higher clay concentrations indicates that

the strong bonds take time to reform after being broken,i.e., thixotropy exists.

At the present time, the nature of the weak and strongbonds is unknown. There are many long- and short-rangeforces that could cause interparticle bonding (e.g., see Low,1968). It may be supposed that the different forces com-bine to produce at least two minima in the curve of inter-action energy versus particle separation and that the energyof each bond is that required to move the particles out ofthe corresponding energy minimum. When hydrous alumi-num oxide is adsorbed on the particle surfaces, the magni-tudes of these forces are altered. For example, the electro-static repulsive force would be reduced, especially if thehydrous oxide is positively charged. As a result, the depthsand locations of the two minima (and, hence, the strengthsand lengths of the interparticle bonds) are also altered. Evi-dently, hydrous aluminum oxide adsorption deepens theminima corresponding to the strong bonds and causes theseminima to occur at greater interparticle distances.

It should be noted here that numerous supplementaryexperiments showed that manipulation of the clay-watersystems by different techniques (e.g., tapping, stirring, orremoulding) produced different viscous behavior. And, ex-cept after extrusion, this behavior changed with time aftermanipulation. It can be concluded that particle arrange-ment and bonding are dependent on the history of thesystem.

Figure 5 shows curves of the specific heat capacity ver-sus clay content for the Na/Al- and Na-clay systems. It isclear that the two clay systems differ in this property.

According to thermodynamic theory, the specific heatcapacity, c, of a mixture of clay and water is given by

r CCXC [5]

where cw and cc are the partial specific heat capacities ofthe water and clay, respectively, and xw and xc are the cor-

i.oo

t .90

-o No-cloy (botch n)

••«• No/AI-cloy

Theoreticol (non-interacting components)

"0 5 10 15 2OCLAY CONTENT (percent by wO

Fig. 5—Effect of clay content on the specific heat capacity ofNa/Al- and Na-clay systems.


responding gram fractions. // there is no interaction be-tween clay and water, i.e., if the clay-water system is ideal,c is given by

[6]

where cw is the specific heat capacity of the pure water andcc is the specific heat capacity of the pure clay. Equation[6] was used to obtain the theoretical line in Fig. 5. A valueof 0.191, from the work of Oster and Low (1964) wasused for cc and the handbook value of 0.998 was used forcw. Obviously, there was pronounced interaction betweeneach of the clays and water. We will now consider thenature of this interaction.

Let us suppose that the water in the clay-water systemis composed of a mixture of species which differ fromeach other in molecular arrangement. Possible moleculararrangements are those found in ice I and its high-pressurepolymorphs (Kamb, 1968), in clathrates (Pauling, 1959;American Chemical Society, 1961), and in polywater (Lip-pincott et al., 1969; Low and White, 1970). It is often sup-posed that pure bulk water is such a mixture (Marchi andEyring, 1964; Wicke, 1966; Kamb, 1968). In addition tothe water there are undissociated clay, i.e., clay in whichthe exchangeable cations are essentially a part of the claycrystal, dissociated clay, and dissociated cations. The heatcontent, H, of this system is given by

perature, we have

If =

humu [7]

where hv h2, h3, + . . . hj are the partial specific heatcontents of the water species 1, 2, 3, . . . /, respectively;hu, h-, and h+ are the partial specific heat contents ofthe undissociated clay, dissociated clay, and dissociatedcations, respectively; and mv mz, ma, . . . mj; mu, m~, andm+ are the respective masses in grams. Now let

m =mu = yumc, m_ = 7_mc,

and

where mw is the total mass of water and mc is the totalmass of clay in the system. Then

• hjyj)m w

[8]

The specific heat content of the system, h, is obtained fromthis equation by dividing through by the total mass, mw +mc. Thus,

+ (huyu + h_y_ + ~h+y+)xc. [9]

If equation [9] is differentiated with respect to the tem-

c = [~Stciyi + Zht (d7{/8T)]xw

+ [$ckyk + $hk (dyk/dT)]xc [10]

in which cj is the partial specific heat capacity of any waterspecies, z, and c~k is the partial specific heat capacity of anyspecies, k, that is derived from or related to the clay. Ifwe compare this equation with equation [5] we see thatthe summations in the square brackets are equivalent toc~w and cc, respectively.

When clay-water interaction occurs, water species maybe formed that do not exist in bulk water, or the relativeproportions, partial specific heat capacities, and/ or ther-mal stabilities of the various species may be altered. Also,ions dissociate from the clay surface. Consequently, thereare many ways in which the values of c^ and ~cc may beaffected. When there is no clay-water interaction, i.e., whenthe system is composed of an ideal mixture of bulk waterand undissociated clay, equation [10] becomes

[ll]c = EW + 2/V> (dy?/dT)}xw + ccxc

where the zero superscripts indicate the values of therespective quantities for pure bulk water. In this case, thesummation in square brackets is equal to cw. We will callthe difference between equations [10] and [11] the excessspecific heat capacity of the system and designate it by ce.Note that ce is the difference between the observed andtheoretical curves in Fig. 5. Clearly, ce is a measure of thedegree of interaction of the components of the system.

For the sake of convenience, the specific heat capacityof the clay-water mixture will be expressed in another way.If we assign to the water in the system the thermal prop-erties of pure bulk water, we have

+ <t>cxc [12]

in which <t>c is the apparent specific heat capacity of theclay. Thus all interaction effects are arbitrarily included in4>c- Utilizing equations [11] and [12], the expression forc, is

[13]

Now let us attempt to calculate the value of ce thatwould be obtained if, in an aqueous suspension of clay, thewater interacts with the dissociated cations in solution butnot with the particle surfaces. To do this we will adopt aconvention used with electrolytes in aqueous solution anddivide the apparent specific heat capacity of the clay intoits components. Thus,

<£c = $iiYu + <t>-y~ + 4>+y+ [i4]where (f>u, <f>~, and <£+ are the apparent specific heatcapacities of the undissociated clay, the dissociated clay,and the dissociated cations, respectively. Equation [14]


does not account explicitly for any change in the degree ofdissociation of the clay with temperature. If such a changeoccurs, it could only make a positive contribution to <f>cbecause the dissociation of cations requires energy. How-ever, we will assume that thermal dissociation is insignifi-cant. Also, we will assume that undissociated cations onclay particle surfaces oscillate in simple harmonic motionin three directions about the negative charge sites. Accord-ing to classical theory (p. 416, Glasstone, 1946), ionicoscillators of this kind contribute 3R cal deg"1 mole"1,where R is the molar gas constant, to the heat capacity ofthe clay. Therefore, if 7 + xc grams of exchangeable cationsare dissociated from the clay, its heat capacity is dimin-ished by 3Ry + xc/M+, where M+ is the gram atomicweight of the cations. As a result,

[15]

Equations [13], [14], and [15] can be combined to yield

c, = M+(*+ - 3R) [16]

if <£+ is replaced by a> + /M+ , where $+ is the apparentmolar heat capacity of the cations, and 7+xc is replacedby x+, the gram fraction of dissociated cations.

Several investigators (Rossini, 1931; Ackermann, 1957;Lewis and Randall, 1961; Noyes, 1964; Criss and Cobble,1964) have calculated the apparent or partial molar heatcapacity of Na+ in solution. Their data range from —9.3 to+37 cal/deg/mole at infinite dilution, depending on theassumptions used in dividing the apparent molar heat

1.016

1.012

1.008

> I.OO4623 i.oooi

.996i

.992

I

.984

.980,

o.———o No-cloy (batch n)

A............A Na/AI-clay (data from this study)

A...........A No/AI-clay (data from Oster ft Low)

\ 'av\

28 324 8 12 16 2O 24CLAY CONTENT (percent by wt.)

Fig. 6—Effect of clay content on the apparent specific heatcapacity of water in Na/Al- and Na-clay systems.

capacity of a reference electrolyte into the contributionsof its component ions. But regardless of the value adopted,the value of ce calculated from equation [16] is only asmall fraction of the observed maximum value of this quan-tity. For example, if we assign $+ a value of —9.3 cal deg-1

mole"1 (which is the maximum negative value that has beenreported) and x± a value of 0.00276 (which can be calcu-lated by assuming that there is 1 mmole of exchangeableNa+ per gram of clay and that, at a clay content of 12%,the Na"1' is completely dissociated) we find that the calcu-lated value of ce is 0.0018 cal deg"1 mole"1. The observedvalue (Fig. 5) is 0.014. Thus, it appears that the dissoci-ated cations in the intermicellar solution cannot accountfor the observed values of ce. And, as already noted, thethermal dissociation of cations can only increase oic and,thereby, ce. Consequently, the particle surfaces must inter-act with water to alter its contribution to the heat capacity.Reference to equation [10] suggests that this interactionalters the relative proportions and thermal properties of thespecies present in the water.

Since attention has been focused on alteration of thewater by the particles, it is of interest to examine the rela-tionship between <f>w, the apparent specific heat capacity ofthe water, and the clay content of the system. This relation-ship is presented in Fig. 6, which was obtained by usingthe following equation

c — ccxc [17]

Evidently, particle-water interaction must be extensive,even in dilute sols. The curve derived from the data ofOster and Low (1964) represents an extension of that forthe Na/AI-clay, because these investigators passed theirclays through a H-exchange resin and then titrated themwith NaOH.

We have shown that the apparent specific heat capacityof water in clay-water systems is affected by hydrous oxidefilms and clay content. It has also been shown that the heatof compression [B. D. Kay, 1969. The heat of compressionof Na-montmorillonite systems. Ph. D. Thesis. Purdue Uni-versity, Lafayette, Ind.} and the thermal expansion [D. M.Clementz, 1969. Thermal expansion of water in Na-ben-tonite systems. M.S. Thesis. Purdue University, Lafayette,Ind.] of water in clay systems are affected by these factors.In fact, curves of apparent specific heat capacity, heat ofcompression, and thermal expansion versus clay contenthave essentially the same shape. Hence, it appears thatthese properties are interrelated. This is as expected, be-cause they are mutually dependent on the structure of thewater.

The structure of pure bulk water is still unknown anda subject of controversy. Obviously, therefore, it is impos-sible to specify how the structure of the water in the claydiffers from that of bulk water. However, recent evidence,obtained during the writing of this paper, indicates stronglythat epitaxy exists between the clay and water lattices andthat the necessary adjustment in the water lattice affectsthe structure-sensitive properties of the water. If this is


-8.5

-9.0 -

-, -9,5JUO

X"3S. -io.o

o

r*I h.ill -10.5

-12.0

.*'...•••#

,<>i°'/

i A

7Jt

VA............* Na/Al -clay

o---—o Na-clay (batch n)

4 6 8 10 12CLAY CONTENT (percent by wt.)

Fig. 7—Effect of clay content on the relative partial molar freeenergy of NaCl in Na/Al- and Na-clay systems.

so, it is not difficult to see how an adsorbed film of hydrousaluminum oxide would alter these properties. Although thehydrous oxide may have lattice dimensions comparable tothose of the clay (this would be the case if it resemblesgibbsite), the composition of its surface would be differ-ent. Probably, the surface of the hydrous oxide is composedlargely of hydroxyl groups, whereas, the surface of the clayis composed of oxygen atoms. The strength of the bondsbetween water molecules and hydroxyl groups should bedifferent than those between water molecules and oxygenatoms.

The curves in Fig. 7 show the relationship between(F-F°)NaC1 and clay content for the two kinds of claysystems. Note that there is a pronounced minimum in bothcurves at about 0.5% clay, a distinct break in the curvefor Na-clay at about 4% clay, and a less distinct break in thecurve for Na/Al-clay at about 7% clay. One-half percentis the clay content near which there is a marked change inseveral other properties. For instance, it is the clay contentnear which the weight conductance of the system, the meanionic activity coefficient of NaCl within it, and its specificviscosity per unit of clay concentration are at a minimum(Davey and Low, 1968); </>„, is at a maximum (as shownin this study); a levelling-off begins, following a rapid fall,in the temperature of spontaneous nucleation of the water[J. C. Davidtz, 1968. Effect of isomorphous substitutionin montmorillonite on the properties of associated water.Ph.D. Thesis. Purdue University, Lafayette, Ind.]', and alevelling-off begins, following a rapid rise, in the apparentdensity of the clay (Davey and Low, 1968). As regards theNa-clay, 4% is the clay content near which the tension ofthe water in the system begins to increase rapidly (Daveyand Low, 1968). Now, as indicated by studies of opticaldensity (M'Ewan and Mould, 1957; van Olphen and Wax-

man, 1958; Davey and Low, 1968), clay particles beginto cluster when the clay content reaches about 0.5%. Andthis and an earlier study (Leonard and Low, 1964) showthat rapid gelation begins when the clay content reaches4%. Therefore, it is evident that changes in the propertiesof the ions and water occur concomittantly with changes inparticle bonding and arrangement. This has been observedbefore (Yamaguchi, 1959; Kolaian and Low, 1962; Ander-son et al., 1963; Leonard and Low, 1964; Davey and Low,1968; Baker and Low, 1970).

Evidence presented in this and the other part of thisstudy (Davey and Low, 1968) shows that interactions in-volving Na-montmorillonite, water, and ions are markedlyaffected by the presence of adsorbed hydrous aluminumoxide. Supporting evidence has been provided by Millerand Brown (1969) in their study of the effect of exchange-able aluminum on the activation energies of D2O and Na+

transport. Thus, it can be concluded that hydrous alumi-num oxide affects the physico-chemical properties of solsand gels of Na-montmorillonite.
