Electrophoretic Phenomena as Applied to the Investigation of Interaction Between Clays and Anionic Polyelectrolytes.

465

ELECTROPHORETIC PHENOMENA AS APPLIED TO THE INVESTIGATION OF INTERACTION BETWEEN CLAYS AND ANIONIC POLYELECTROLYTES. D. RIOCHE and B. SIFFERT Centre de Recherches sur la Physico-Chimie des Surfaces Solides 24, Avenue du President Kennedy, 68200 MULHOUSE - France. (with the collaboration of Elf-Aquitaine laboratories).

ABSTRACT The Interaction between various anionic polyelectrolytes (ferrochromelignosulfonate, carboxymethplcellulose,chromium lignite) and different clay minerals (montmorillonite and kaolinite) has been investigated, using an electrophoretic mass-transport analyzer. The data confirm the existence of two electric double layers at the surface of the clay micelle. The results corroborate, in particular, the existence of a

strong positive double layer around the kaolinite particles dependant upon the pH, and located on the Al-faces and on the edge surfaces of the clay layer. The same double positive layer located solely on the edges of montmorillonite particles displays a very low intensity.

INTRODUCTION Drilling fluids are generally made up of aqueous bentonite suspensions with diverse added products which allow the different properties, in particular the rheological properties to be controlled. The use of these chemicals is often empirical. There is a lack of understanding of the mechanism involved therein. Hence, an investigation was undertaken to study the interaction between two model clays (kaolinite and montmorillonite) and the three polyelectrolytes, commonly used in drilling muds

:

- a ferrochrome lignosulfonate (F.C.L.) used for its fluidizing properties and its filtrate reducing ability

-

a chromium lignite (L.C.) used for its synergic action towards lignosulfo-

nate, notably at high temperature

- a carboxyrnethylcellulose (CMC). The CMC are used to enhance the viscosity and to reduce the filtrate of drilling muds. Electrokinetic measurements provide convenient means for characterizing the

466

materials and the adsorption processes of surface active minerals. Chemical reactions occuring at the surface of compounds such as oxides and clays can be traced (OTTEWILL and HOLLOWAY, 1975). With this end in view, the electrophoretic mobilities of clay suspensions have been measured in the presence of the above mentioned polyelectrolytes, using an electrophoretic mass-transport analyzer. THEELECTROPHORETIC MASS-TRANSPORT ANALYZER The measurements on concentrated clay suspensions have been performed using an electrophoretic mass-transport analyzer of the type 'Micromeritics" model 1202 (fig. 1 A). The apparatus consists of a reservoir which may contain 100 ml of suspension. The measuring cell proper contains approximately 6 ml. Before any determination, the cell containing the experimental suspension is accurately weighed

(+ 0 , l

mg)

after level adjustement with the filling - tube. The measuring cell, fastened to the reservoir, is isolated from the later by a shutter during reservoir filling, closing of the shutter prevents particle sedimentation or diffusion before and after each measurement. While the electrophoretic mobility is being determinated, the sedimentation effects are eliminated by continuously rotating the measuring cell (about 3 0 r.p.m.1.

reservoir

Fig. 1. The electrophoretic mass-transport analyzer (A), sketch of the measuring cell (B). Reservoir and cell are made of plexiglass

;

the electrodes of zinc. The appa-

ratus with its chamber ofelectrophoretic mass-transport is attached to a steady intensity generator. Under the action of the electric field, the particles migrate from the reservoir to the cell or in the opposite direction depending on the polarity of the field

467

applied. Provision should be made to ensure a weight increase after performance of the various operations. Upon completion of a test - the experimental time being known to within a hundredth of a second - the measuring cell was once again weighed after adjusting the level with the filling-tube. The weight increase is directly related to the electrophoretic mobility of the particles. The theory of mass-transport has been described by OLIVIER and SENNET (1965). The electrophoretic mobility (V ) is given in terms of the different parameters by the general formula

v = E

:

Aw.

K

R.i.t. $(I-@)

(Ps-Pe)

AW = weight increase

R = resistance of the suspension i = current t = experimental time $ = weight fraction of dispersed solid

p,

=

volumic mass of dispersed solid

pe

=

volumic mass of liquid

K =

conductivity constant of the apparatus

The conductivity constant (K) of the apparatus is determinated by the following equation using a N/100 solution of potassium chloride, whose specific conductance ( A ) is well known. K = R x h KC1 KC1 DESCRIPTION OF THE PRODUCTS AND THE MEASURING TECHNIQUE. Two clay minerals have been used

-

a smectite

:

:

MILBEN bentonite

a kaolinite from Ploemeur (Morbihan, France) In order to work on well defined minerals, both minerals were purified and

transformed into sodium minerals according to the well-known method of ion exchange. Before performing the measurements, the suspensions were allowed to stay so All concentrations

that electrical and physico-chemical equilibria were reached. were stated in grams o f dry matter per liter of suspension. concentration ,CFcL, .

CcMc,

C

is the mineral

CLc are the concentrations of lignosulfonate (FCL), carboxy-

methylcellulose (CMC) and chromium lignite (LC) respectively. In all measurements, C s is fixed at 50 g/l for montmorillonite and 100 g/l

46 8

for kaolinite

the ratios C /CFCL, CMC, Lc

;

vary with the contents of added

polyelectrolytes. The mobilities have been measured in the alkaline pH range (this is always the case in drilling muds), the pH of the suspensions being adjusted with concentrated sodium hydroxide. EXPERIMENTAL RESULTS The electrophoretic mobilities of sodium montmorillonite and sodium kaolinite suspensions are represented in Figure 2 as a function of pH.

8

7

o

.. m

9

10

11

PH

Fig. 2. Electrophoretic mobilities of montmorillonite and kaolinite suspensions as a function of pH.

Nd-montmorillonite

. Nd-kaolinita

I -

The electrophoretic mobility of the sodium montmorillonite is constant in the alkaline pH range. On the other hand, the mobility of the sodium kaolinite varies with pH. This phenomena has already been reported by OTTEWILL and HOLLOWAY (1975). The electrophoretic mobility of kaolinite is always higher in absolute value, than that of montmorillonite.

____________--___

The action of CMC on clay minerals seems to be considerably dependant on the pH. Irrespective of the CMC concentration, all the electrophoretic mobility curves versus pH pass through a minimum for montmorillonite and a maximum for kaolinite at a same pH value of approximately 9 (Fig. 3 ) . Zn-the presence of lignosulfomtel the mobility of sodium bentonite suspension is not subject to significant variations as a function of pH, except when the FCL concentration is high (fig. 4). All the mobility curves fall into a mobility domain ranging from

-

-8 2 -1 -1 1,6 to - 3,6 x 1 0 m s V

.

On the other hand, the action of FCL on kaolinite is much more significant (Fig. 4). All the mobility curves fall into a range between - 1,9 t o 6,5 -8 2 -1 -1 10 m s V , i.e. three times broader than the previous case.

469

4

21 M s4

%-I E

6.160

S = Na+-Montmorillonite

I F 0

g f

. -1 S = Na*-Kaolinite

.-2

.-3

.

--4

-

w

+

0 8

W

z x

c

.-5 "

- 5 -

Fig. 3. Electrophoretic mobility of a sodium montmorillonite and a sodium kaolinite suspensions as a function of pH, in presence of carboxymethylcellulose (CMC).

-1

7

8

9

x)

11

PH

'

7

8

9

10

11

z

-1

--3

9 w

--4

-4.

; 9

D l

Y

.-6

Fig. 4. Electrophoretic mobility of sodium montmorillonite and sodium kaolinite suspensions as a function of pH, in presence of ferrochrome lignosulfonate (FCL). The action of chromium lignite (LC) on the two minerals differs significantly _-______-____________-_--____-----_

- with montmorillonite, the mobility decreases smoothly as a function of

a given LC concentration (Fig. 5).

-

for kaolinite, one again flnds a similar behaviour for high LC concentration

and a sufficiently alkaline p H

;

for lower L.C. concentration, however, the

mobility increases as a function of p H (Fig. 5 ) .

:

p H for

410

7

8

9

10

11 , p H -1

S= Na+-kaolinite

-2

-3 -4

-5

S = Na+-montmorillonite

-6

Fig. 5. Electrophoretic mobility of sodium montmorillonite and sodium kaolinite suspensiorsas a function of pH, in the presence of chromium lignite ( L C ) . INTERPRETATION OF THE PHENOMENA It is well known that the double layers formed at the surface of pure alumina and silica particles are widely different. They have been studied extensively by JOHANSEN and BUCHANAN (1957), FUERSTENAU (1970) OTTEWILL and HOLLOWAY (1975). For alumina particles, the double layer is positive in an acidic medium

;

it

becomes negative in sufficiently alkaline conditions. The position of the point of zero charge depends on the nature and the structure of the aluminous surface. For pure alumina

(a A1203 ) , the isoelectric point is situated at a pH approaching

8.5. For a surface composed only of silicon atons, the double layer is always negative, except for very acidic conditions (pH < 2). Lastly, for silico aluminates, the double layer is generally positive in acidic medium and negative in alkaline medium. The position of the isoelectric point varies with the A1 0 /SiOz 2 3

ratio of the alumino-silicate (MATIJEVIC and al., 1971).

For montmorillonite, the double layer depends mostly on the intrinsic charge

of the mineral layer and the silica sheet of the basal planes

;

the positive

charges on the layer edges (lateral surface) being negligible. The mobility of montmorillonite suspensions is then constant and negative (Fig. 2). In case of kaolinite, on the other hand, half the basal surface of the mineral layer displays a double layer related to an aluminous surface (-Al-OH) mobility therefore varies according to the pH ( F i g . 2 ) .

:

the

471 Action of carboxymethylcellulose (CMC) The comparison of electrophoretic mobility curves of clay suspensions versus pH, in presence of CMC is represented in Fig. 6.

PH

Fig. 6. Comparison of the evolution of electrophoretic mobility of clay suspensions versus pH, in the presence of carboxymethylcellulose (CMC) (cs/cFCL ratio = 4 0 ) .

The differences in variation of the mobility between the montmorillonite and kaolinite suspensions may be explained by taking into account the following

-

:

the existence of two types of electric double layers at the surface of the

clay particles (Van OLPHEN, 1963)

-

the hydrolysis reaction of CMC according to the pH

:

When the OH- ion concentration reaches a given limit, the equilibrium shifts in the direction ( 2 )

:

CMC is adsorbed in anionic form in a concentrated alkaline

medium. Hence, f o r montmorillonite at a pH below 9, CMC is fixed predominantly in its neutral molecular form. The net negative charge of the clay particle decreases through the screening effect on the neutral molecules (protective colloid effect of the molecules) and the mobility diminishes. Around pH = 9, the CMC

-

dissociation equilibrium reverses. The RCOz

anions are not repelled by negative

clay micelles because of the "protective" neutral layer already formed

:

the

net negative charge will thus increase. For pH values above 9, the mobility increases. At a pH below 9, the positive adsorption sites are numerous at the surface of kaolinite (aluminous double layer). The CMC dissociation equilibrium shifts towards the formation of anions which are fixed onto the positive sites. Hence, the net negative charge and the mobility of the clay particles increase. Above pH = 9, the anionic polymer molecules are more abundant, the repulsion between

the polymer molecules and the negative siliceous surface of the clay micelles continues to exist

:

CMC will only be fixed in its neutral form. The screening

412

effect of the intrinsic negative charge begins to appear, resulting in a decrease of mobility. Action of the lignosulfonate (FCL) The action of lignosulfonate on the dispersed clay particles of montmorillonite and kaolinite seems to be identical for

C ratios below 20. For lower @FCL FCL concentration, the action depends on the nature of the clay mineral (Fig. 7 ) .

5. Na+-montmorillonite

Fig. 7. Comparison of the electrophoretic mobilities of clay suspensions in the presence of lignosulfonate (FCL) at various pH values. In order to explain the shape of the curves obtained, the existence of the two electric double layers at the kaolinite particle must always be taken into account. It is worth noting that the lignosulfonates retain their anionic form and react with the aluminium atoms irrespective of the pH. In this way, at slightly alkaline pH values (up to 8 . 0 ) , the aluminous double layer is positive. The anionic form of FCL molecules react with the alumina sites and neutralize them

:

the mobility decreases slightly because of a weak screening effect. For

a pH above 8, the aluminous double layer itself becomes negative, but the reactivity towards the aluminium atoms continues to exist (SIFFERT and FERRAND, 1973). The polymeric anions then contribute to an increase of the negative charge of the micelle. Hence a significant mobility increase is observed. For higher lignosulfonate concentration, the behaviour is identical for kao-

linite and montmorillonite. It may be explained as follows

:

- Neutralization of the remaining positive and poorly localized charges takes place on the layer edges and the bulky FCL molecule produces a partial screening of the negative charge

-

;

hence the mobility decreases.

For even higher FCL concentration a physical adsorption of the anionic poly-

electrolyte onto the negative clay surface is observed. The mobility again

473 begins to increase, owing to the additional supply of negative charges. Lastly, starting from a sufficiently high FCL concentration (C /C ratio s FCL approaching l o ) , the clay micelle is entirely surrounded by a film of lignosulfonate molecules. The thickness of this film grows to such an extent that the lignosulfonate acts as a protective colloid and the electrophoretic mobility decreases resulting from a screening of the intrinsic charge of the clay particle. Action of chromium lignite

,

Theelectrophoretic mobilities of montmorillonite and kaolinite suspension in the presence of chromium lignite are compared as a function of pH in Fig. 8 .

'jz75,

266

y3

S: Na+-kaolinite

46

[ 0

9.5

.10

S =Na+-montmorillonite

I

-6 "

Fig. 8 . Comparison of the electrophoretic mobilities of clay suspensions in presence of chromium lignite (LC) at various pH values. The curves of both minerals are identical to those obtained in presence of lignosulfonate. The phenomena may be interpreted in the same way. It must however be appreciated that the maxima and minima of the mobilities occur for ratios. For an identical interaction, Cs/CLc ratios different from the C /C S FCL ratio, i.e. half the quantity the Cs/CFcL ratio is generally twice the C /C s LC of chromium lignite is required to bring about the same effect. This result may be interpreted by assuming either steric hindrances or different charges for each kind of molecule. CONCLUSIONS These results may account for the "change-over" (SCROFIELD and SAMSON, 195;) in the positive electric double layer charge carried by the clay micelles, as a function of pH. The results discussed above support the existence both of a strong negative double layer independant of pH, located on the siliceous surfaces and of a

414 positive double layer, dependant on pH, on the kaolinite micelles, located on the Al-faces and on the edge surfaces of the layers. They also demonstrate the weak intensity of the positive double layer, located only on the edge surfaces of the layers in the case of montmorillonite particles. $

ACKNOWLEDGEMENTS The authors are grateful to ELF-AQUITAINE Society for its laboratory and financial assistance. REFERENCES Fuerstenau, D.W., 1970. Interfacial processes in mineral water systems. Pure and Appl. Chem., 24 : 135-164. Johansen, P.G. and Buchanan, A.S., 1957. An Application of the microelectrophoresis method to,the study of the surface properties of insoluble oxides: Aust. J. Chem. 10 : 398-403. Matijevic, E., Mangravite, F.J. and Cassell, E.A., 1971. Stability of Colloidal silica, IV : silica-alumina system. J. Colloid and Interface Sci. 35 : 560-568. Olivier, J.P. and Sennett, P., 1965. Electrokinetic effects in kaolin-water systems : the measurement of electrophoretic mobility. Fifteenth Conf. Clay Minerals, Pittsburgh. Ottewill, R.H. and Holloway, L.R., 1975. Electrokinetic properties of particles. Phys. Chem. Sci. Res. Rep. 1 : 599-621. Schofield, R.K. and Samson, H.R., 1953. The defloculation of kaolinite suspensions and the accompanying change-over from positive to negative chloride adsorption. Clay Min. Bull. 2 ( 9 ) : 45-51. Siffert, B. and Ferrand, C., 1973. Contribution & l'dtude du mecanisme d'interaction des argiles et des lignosulfonates. Bull. Gr. Fr. Arg. 25 : 135-148. Van Olphen, H., 1963. An Introduction to Clay Colloid Chemistry. Interscience Publishers, John Wiley, New-York, 301pp.