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Effect of the surface-active substance cetylpyridinium chloride on the electric light scattering of bentonite
Colloids and Surfaces, 17 (1986) 229-239 Elsevier Science Publishers B.V., Amsterdam -Printed
in The Netherlands
229
EFFECT OF THE SURFACE-ACTIVE SUBSTANCE CETYLPYRIDINIUM CHLORIDE ON THE ELECTRIC LIGHT SCATTERING OF BENTONITE
Ts. RADEVA,
S.P. STOYLOV
and T. SUONG
Institute of Physical Chemistry, Bulgarian Academy
of Sciences,
(Received 6 August 1985; accepted in final form 4 September
1040 Sofia (Bulgaria)
1985)
ABSTRACT The changes in the electric properties and the stability of an aqueous suspension of sodium bentonite on addition of the cationic surface-active substance cetylpyridinium chloride (CPC) is studied by electric light scattering. The CPC concentration varies from 10 es to 10e3 mol l-l, and the suspension weight concentration is 5 * 1O--6 kg l--l. The dimensions of the bentonite particles are followed by electron microscopy and the decay of the electrooptic effect. The particle dimensions are greatest at the isoelectric point (lo-’ mol 1-l CPC), determined microelectrophoretically. The greater value of the electrooptic effect in the kHz range for suspensions to which CPC is added in comparison with untreated suspensions is explained by the increase of the particle dimensions, which leads to an increase in the particle electric polarizability. The electric polarizability per unit interfacial area of the particles decreases, which is related to CPC adsorption on the particles. The greater dispersion for suspensions with CPC at frequencies below 100 Hz, also manifest in large negative effects, is explained by the increase of the ratio of permanent dipole moment to induced dipole moment.
INTRODUCTION
In this paper the effect of the surface-active substance cetylpyridinium chloride (CPC) on the light scattering of an aqueous suspension of sodium bentonite in an electric field is studied. The changes in electro-optic effects and in stability after the addition of surface-active substances have been studied for other suspensions: palygorskite [ 1,2], kaolinite [3], sepiolite [4], silver iodide [ 51, etc. In these studies, a correlation was found between the surface electric properties of the respective colloid particles and the stability of their suspensions. However, in these studies no systematic study was made of the frequency dependence of the electro-optic effects. It is well known in electro-optics [6--121 that the frequency dependence of electro-optic effects provides information about the particles’ dimensions and electric properties. The nature of electro-optic effects in the kHz range, for most colloid suspensions, is more or less predominantly connected with the induced dipole moment of the particles, mainly due to the polarization of the diffuse part of the electric double layer (EDL) of the particles [lo].
0166-6622/86/$03.50
0 1986
Elsevier Science Publishers B.V.
230
On the other hand, the nature of the low-frequency (10 Hz--l kHz) dispersion of electro-optic effects is much less clear. Most frequently this dispersion is explained in terms of the permanent dipole moment of the particles [6,9-121. It has been shown that the low-frequency dispersion could depend on the type of counterion, compensating the charge of the particle [13], on the type of dispersion medium [14], on the magnitude of the electric field strength [ 151, etc. These factors, which favour the possible interfacial origin of the low-frequency effect, in most cases also influence the electro-optic effect in the kHz range. They may be explained by changes in the EDL of the particles. It has also been suggested that the low-frequency dispersion of the electro-optic effect may be explained by the formation of “domains” in the more concentrated suspensions [ 161. Another explanation has been put forward by Radeva and Stoimenova [13] for the case of rodlike palygorskite particles, where the polarization of the particles due to displacement of ions in the dense part of the EDL was found to be essential. The study of the effect of CPC on the electric properties of palygorskite suspensions [ 21, however, showed that the influence of the ions in the dense part of the EDL manifests itself in a much more complicated manner. The change of particle dimensions in the presence of surface-active agents (due to coagulation) also makes the interpretation of the results more difficult. The low-frequency dispersion of electro-optic effects of bentonite suspensions has, at presept, no explanation accepted by all researchers in this field. Together with the explanation based on the suggestion for the existence of two types of particles, possessing different electric properties, most of the explanations favour the postulation of a permanent dipole moment of the disc-like bentonite particles, transverse to the plane of the particles [6,12, 17-191. The basic suggestion is that, at low electric field strengths and low frequencies (or in d.c. electric fields), the bentonite particles are oriented with their long axis perpendicular to the direction of the electric field. This leads to lower or even to negative electro-optic effects. THEORY
Light scattering in an electric field is defined through the change in the intensity of the scattered light (Yafter the application of an electric field to the suspension: IE -1,~ Ly=-----=Ill
AI 10
where I_g and IO are the intensities of the scattered light in an electric field of strength E and with no field, respectively. When the energy of orientation of the particles in the field is low with respect to the energy of thermal motion, the electro-optic effect changes linearly with the square of the electric field strength, E* [20,21]. In this case, a solution of the diffusion equation was suggested in Ref. [22], which
231
gives the dependence of the electric light scattering effect on the angular frequency of the sinusoidal field, o . If the orientation of the particles is due only to the interactions of the induced dipole moments of the particles with the electric field, the electrooptic effect is represented by the sum of a constant term (Ye, equal to the time average value of the effect and one alternating with frequency 2 w . The amplitude of the latter decreases with increasing frequency from 2a, to 0, and for a given frequency o changes from CY( 1 + l/d1 + c..?/4.D’) to a(1 - l/d1 + wz /40z) [lo], where D is the particle rotational diffusion constant. When the orientation is due only to the permanent dipole moment of the particle, the time-independent term depends on the frequency, its amplitude changing from ‘Ye (at w = 0) to 0 (at w >> l/D). The time-dependent term also changes with frequency 2 w , its amplitude varying from 2a, to 0 [lo]. The decrease of the electro-optic effect after switching off the electric field is due to the particle disorientation, due to the effect of the thermal motion. The effect depends on the viscosity of the medium q, and the particle form and dimensions. At low degrees of orientation, the decay of the electro-optic effect is described by the equation [ 10,201: cy
=
(y,,
e-6Dt
(2)
where CQ is the effect at the moment of switching off the electric field,D is the rotational diffusion coefficient and t is time. By measuring the electrooptic effect of the particle’s disorientation, at moment t = 1/6D after switching off the electric field, it is possible to determine the rotational diffusion coefficient of the particles. From the latter the particle diameter B can be calculated following the expression of Perrin [23] given for very thin discs: D = 3kT/4qB3
(3)
where k is the Boltzmann constant, T the absolute temperature and B the particle diameter, the thickness being assumed to be much smaller than the wavelength. At complete particle orientation (saturation), an expression for A1, was suggested in Ref. [24] for infinitely thin discs: Ar, =2
5
G-l)n+lW)2n
n=l
(n + I)! (n + 2)!
1 _ (2n + l)!!
t
(2n - l)!!
(2n)!! (2n)!!
)
(4)
where K = (2n/h) sin 0’12, X is the wavelength in the medium and 8’ is the angle of observation. The decay of the electro-optic effect after switching off the electric field is given in Ref. [24] by a complicated expression. From it, for small values of the time (i.e. t + 0) when the effect changes linearly with time, an expression has been suggested [25] :
232
AIt =2Llt
C II=1
(-l)n+l(I@n(2n (n +
l)!
(n + 2)! (2n)!!
From Eqns (4) and (5) it follows the decay curve: A(1 -Art/AI,) fi=[
At
+ l)!!
(2n - 3)!! (2n - 2)!!
(5)
that, at known h , from the initial slope of
1
t-o
so that it is possible to determine D [ 251. EXPERIMENTAL
Materials An aqueous suspension of sodium bentonite with a weight concentration 4.5 g 1-l was fractionated by differential centrifugation following a procedure described in Ref. [26]. The particle dimensions were controlled by electron microscope and by the decay curves, Table 1. The form of the particles was approximated by circular discs. The weight concentration of the suspension was 5 * 10m6 kg 1-l and its specific conductivity was 3 * 10W4 R-l m -I. The critical mice& pon:e?fration of CPC is 9. 10v4 mol 1-l [27]. The apparatus used and the application of the electric light scattering are described in detail in Ref. [lo]. RESULTS
AND DISCUSSION
In Fig. 1 the dependence of the kHz electric light scattering cy on E2 is shown for a suspension of sodium bentonite in water (curve 1) and in the presence of CPC (curves 2-4). It is seen that, at the field strength of 6. lo3 V m-l at which all dispersion curves presented on Figs 2-4 have been studied, the electro-optic effect is proportional to E2. This means that the condition for low energies of orientation and, consequently, for low degrees of orientation, is fulfilled and no complications should be expected in the dispersion curves. In the frequency range l-10 kHz, the orientation of the particles has no time to follow the changes in direction of the electric field and, therefore, the electro-optic effect is independent of the permanent dipole moment of the particles. On the other hand, the frequencies are sufficiently low for bentonites so that no relaxation of the electric polarizability could be expected. The addition of the cationic surface-active substance CPC to the suspensions of negatively charged particles diminishes their charge (which is confirmed electrophoretically). On the other hand, the increase of CPC con-
233
Fig. 1. Dependence of the relative variation in electric field strength. Frequency: 1 kHz. bentonite; curve 2, bentonite + :LO-5 mol 1-l CPC;and curve 4, bentonite + 5~10~’ mall-’
the intensity of the scattered light on the Curve 1, aqueous suspension of sodium CPC; curve 3, bentonite + 10e4 mol 1-l CPC. (1 CGSE = 300 V cm--‘).
centration leads to a thinning of the particles’ diffuse electric layers, which are shown in some cases to be strongly connected with the electric polarizabilities of the particles. So it may have been expected that, as in the case of other colloids such as the palygorskite [1,2], the electro-optic effect should decrease with increasing concentration of CPC before the isoelectric point. However, the results in Fig. 1 show the opposite. The effect increases with increasing CPC concentration. One of the most trivial explanations of this increase could be the increase of particle dimensions by coagulation, which overcompensates the decrease due to the factors mentioned above. In Table 1 the dimensions of the particles, determined by three independent methods, are compared. It shows how dimensions increase with the increase of CPC concentration. The particle dimensions are greatest at the isoelectric point, which is at, 1O-4 mol 1-l CPC. At still greater concentration, the particles’ dimensions slightly decrease. The dimensions of the particles were determined by several independent methods since the different preparations were rather polydisperse. The divergence of the values for the dimensions obtained by the different methods gives an idea of the polydispersity of the suspensions, since the different methods (or the same method under different conditions) provide different types of averages. In the first four columns of Table 1 the values of the particle diameters are calculated from the time of relaxation of the effect after switching off the electric field at 6, 15, 30 and 450. lo3 V m-‘, respectively. In the fifth and sixth columns the dimensions were determined respectively by electron microscope statistics and from the initial slope of the curve of relaxation of the electrooptic effect after switching off the electric field after an orientation by 450 - lo3 V m-l (i.e. after full particle orientation). There is good agreement between the dimensions but not in the way the
1
540 600 640 570 570
680 680 700 -
10V5 1O-a 5.10-4 10-S
15*103
570
Vm-’
Aqueous suspension
6*103
B (nm) V m-’
of the particles of the sodium bentonite
CPC concentration (mall-‘)
Dimensions
TABLE
420 470 470 560 570 560
400
510
V m--l
of CPC
450~10~
510
V mm-’
and after addition
30*103
before
260 340 330 -
160
Electron microscopy
240 320 310 -
270
From initial slope of f~( t)
235
change with CPC concentration in columns five and six. The only considerable difference is between the dimensions of the bentonite particles in the suspension where no CPC has been added. It could be that this difference is due to the formation of T-like aggregates [28], caused by the presence of charges of different sign on the surfaces and the peripheries of the particles. This is also assumed to be one of the causes for the electrooptically obtained greater dimensions in the first four columns. The electron microscopy techniques which were used in this study make it possible, through shadowing with carbon under 45”, to determine with sufficient accuracy the dimensions of single particles, Table 1 shows that the dispersity of the bentonite suspensions changes in different ways at different CPC concentrations. At a CPC concentration of 10V5 mol 1-l the dimensions of the large particles increase; however, simultaneously the dimensions of the intermediate particles decrease. As a result the dimensions of the latter particles prove to be of lowest average dimension. At higher CPC concentration, particles of more uniform dimensions are obtained. The frequency behaviour of the electric light scattering in aqueous suspension (curve 1) and in the presence of CPC is shown on Fig. 2. It is seen that the electrooptic effect decreases with the lowering of the frequency in the range 100-300 Hz. This low-frequency decrease is stronger when CPC is added. At 10e4 mol 1-l (i.e. at the isoelectric point determined microelectrophoretically), and in the more concentrated suspensions at low frequency, the suspension shows a negative effect, reaching a magnitude two times greater than the maximum positive effect at higher frequencies. Dependence of another electro-optic effect, the electric birefringence, on particle dimensions was observed for bentonite by Shah and Hart [19]. In order to exclude in our experiment the influence of the different dimen-
Fig. 2. Frequency dependence of the electro-optic effect of sodium bentonite at different CPC concentrations: curve 1, no CPC; curve 2, 10e5 mol 1-l CPC; curve 3, lo-’ mol 1-l CPC; curve 4, 5.lo-’ mol I-’ CPC;and curve 5,10-I mol 1-l CPC. Electric field strength: 6010~ V m-l.
236 TABLE
2
Dimensions
of the particles
CPC concentration
of sodium
bentonite
after centrifugation
B (nm) 6.103
Aqueous suspension
570
10-5 lo-’ 5.10-4 1o-3
570 570 570 570
Vm-’
15.10’
520 490 520 520
V m-l
30.10’
V mm-’
460 490 500 -
sions, an attempt was made to prepare suspensions of equal particle dimensions by eliminating the larger particles obtained after the coagulation induced by CPC. This was done by centrifugation at 4000 rpm for 40 min. The particle dimensions so obtained are very similar (Table 2). However, as is seen from Fig. 3 the differences in the low-frequency behaviour of the electrooptic effect at different CPC concentration remain practically the same. The smaller particle dimensions merely cause a decrease of the electrooptic effect. This indicates that the electrical surface properties are essential for the observed low-frequency behaviour of the bentonite suspensions in the presence of CPC. It seems unlikely that this behaviour can be explained by the presence of two types of particles in the suspension (one having a transverse permanent dipole moment and the other being- _polarized predominantly along the long a.& [6,17]). In our case the frequently observed negative electro-optic effect for bentonite suspensions at low frequencies arises only on the addition of CPC.
Fig. 3. Dependence of the electrooptic effect 01 on the frequency v after centrifugation the suspensions. The different curves are at the same conditions as in Figs 1 and 2.
of
237
It is also difficult to explain these results by polarization of the compact EDL [13]. With the increase in CPC concentration, i.e. of the number of ions absorbed in the compact layer, the value of the electro-optic effect decreases instead of increasing. The most attractive explanation of this low-frequency behaviour of the electrooptic effect at different CPC concentrations is the presence and changing of a transverse permanent dipole moment in the bentonite particles. The contribution of these permanent dipole moments could be assumed to increase with the approach of the isoelectric point of the suspension and also because of the decrease of the value of the electric polarizability. This could be expected with the decrease of the interfacial electric charge and of the thickness of the EDL with the rise of the CPC concentration. In Fig. 4 are shown the values of the electric polarizability of bentonite particles in the presence of CPC (curve 3), calculated from the initial slope of the curve of the rise of the electro-optic effect after switching on the electric field. This is done following the procedure suggested in [24,25]. It is seen that the polarizability of the particles rises slightly at the isoelectric point, which was observed earlier for calcium bentonite [29]. The values of y, however, have been calculated per unit length and not per unit area. For the disc-like bentonite particles it seems more reasonable to compare the values of y per unit area of the disc. The values of y obtained in this way (curve 4) decrease with increasing CPC concentration up to the isoelectric point. The considerable electrooptic effect which is observed at the isoelectric point could be due to the mobility of ions both in the diffuse part of EDL, situated between the slipping plane and the particle surface, and in the inter-
Fig. 4. Dependence of the diameter of the particles (curve l), the electrophoretic ity (curve 2) and the electric polarizability (curve 3) on the CPC concentration. represents the electric polarizability per unit area from the particle surface.
mobilCurve 4
238
layer particle spaces. There exists experimental data for the considerable mobility of the ions in the interlayer particle spaces [ 301. Our investigations do not provide evidence as to whether or not y has a minimum at the isoelectric point. This is due to the fact that with further increase of CPC concentration beyond the isoelectric point, the ionic strength was not kept constant. CONCLUSION
The greater electrooptic effect in the kHz range of sodium bentonite suspensions to which CPC has been added in comparison with that of the pure aqueous suspensions seems to be due mainly to the changes of the particle dimensions and not with the electric polarizability per unit area of the interface. Minimum stability of the suspensions has been observed at the isoelectric point. The low-frequency (10 Hz-100 Hz) behaviour of the suspensions and the negative electro-optic effects observed at greater CPC concentration is assumed mainly to be due to surface electric properties of the particles and especially to the increased contribution of the transverse permanent dipole moment of the particles in their orientation.
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
S. Stoylov and I. Petkanchin, J. Colloid Interface Sci., 40 (1972) 159. Ts. Radeva, S.S. Dukhin, V.N. Shilov and A.A. Baran, Kolloid Zh., 44 (1982) 265. I. Petkanchin and R. Bruckner, Colloid Polym. Sci., 254 (1976) 596. R.C. Fairey and B.R. Jennings, J. Colloid Interface Sci., 85 (1982) 205. I. Petkanchin, R. Bruckner and S. Stoylov, Colloid Polym. Sci., 257 (1979) 1121. C. Wippler, J. Chim. Phys., 53 (1956) 316. C.T. O’Konski and A.J. Haltner, J. Amer. Chem. Sot., 79 (1957) 5634. M. Wallach and H. Benoit, J. Polym. Sci., 57 (1962) 41. B.R. Jennings, Thesis, Southampton, England, 1964. S.P. Stoylov, Adv. Colloid Interface Sci., 3 (1971) 45. H.H. KPs and R. Bruckner, Z. Angew. Phys., 26 (1969) 368. G.B. Thurston and D.J. Bawling, J. Colloid Interface Sci., 30 (1969) 34. Ts. Radeva and M. Stoimenova, J. Colloid Interface Sci., 76 (1980) 315. N.A. Tolstoy, A.A. Spartakov and A.A. Trusov, in Investigations in the Field of Surface Forces, Nauka, Moscow, 1967, p. 56. M. Stoimenova, Ts. Radeva and S. Stoylov, Colloid Polym. Sci., 257 (1979) 1226. T. Yamamoto, Y. Mori, N. Ookuno, R. Hayakawa and Y. Wada, Colloid Polym. Sci.,
17 18 19 20 21 22 23 24
260 (1982) 20. B.R. Jennings and H.G. Jerrard, J. Chem. Phys., 42 (1965) 511. S. Stoylov, Izv. Inst. Phys. Khim., 6 (1967) 79. M.J. Shah and C.M. Hart, IBM J. Res. Develop., 7 (1963) 14. H. Benoit, Ann. Phys. (Paris), 6 (1951) 561. C.T. O’Konski, K. Yoshioka, and W.H. Orttung, J. Phys. Chem., 63 (1959) H. Plummer and B.R. Jennings, J. Chem. Phys., 50 (1969) 1033. F. Perrin, J. Phys. Radium, 5 (1934) 497. M. Stoimenova, J. Colloid Interface Sci., 53 (1975) 42.
1558.
239
25 26 27 28 29
30
S. Sokerov, in preparation. T.T. Suong and S. Stoylov, Izv. Inst. Phys. Khim., 15 (1982)542. K. Shinoda, T. Nagakawa, B. Tamamusi and T. Isemura, Colloid Surface-Active Substances, Mir, Moskow, 1966, p. 320. H. Van Olphen, An Introduction to Clay Colloid Chemistry, Wiley, New York, London, 1963, p. 301. Ts. Radeva, Effect of CPC on the Electric Polarizability and Zeta Potential of Kardzhaly Calcium Bentonite, Diplome Work, Sofia, 1972; I. Petkanchin. S. Sokerov and S. Stoylov, Proc. VIth Intern. Congress of Surface Activity, Carl Hanser Verlag, Zurich, 1973, p. 671. R.A. Della Guardia and J.K. Thomas, J. Phys. Chem., 87 (1983) 990.
in The Netherlands
229
EFFECT OF THE SURFACE-ACTIVE SUBSTANCE CETYLPYRIDINIUM CHLORIDE ON THE ELECTRIC LIGHT SCATTERING OF BENTONITE
Ts. RADEVA,
S.P. STOYLOV
and T. SUONG
Institute of Physical Chemistry, Bulgarian Academy
of Sciences,
(Received 6 August 1985; accepted in final form 4 September
1040 Sofia (Bulgaria)
1985)
ABSTRACT The changes in the electric properties and the stability of an aqueous suspension of sodium bentonite on addition of the cationic surface-active substance cetylpyridinium chloride (CPC) is studied by electric light scattering. The CPC concentration varies from 10 es to 10e3 mol l-l, and the suspension weight concentration is 5 * 1O--6 kg l--l. The dimensions of the bentonite particles are followed by electron microscopy and the decay of the electrooptic effect. The particle dimensions are greatest at the isoelectric point (lo-’ mol 1-l CPC), determined microelectrophoretically. The greater value of the electrooptic effect in the kHz range for suspensions to which CPC is added in comparison with untreated suspensions is explained by the increase of the particle dimensions, which leads to an increase in the particle electric polarizability. The electric polarizability per unit interfacial area of the particles decreases, which is related to CPC adsorption on the particles. The greater dispersion for suspensions with CPC at frequencies below 100 Hz, also manifest in large negative effects, is explained by the increase of the ratio of permanent dipole moment to induced dipole moment.
INTRODUCTION
In this paper the effect of the surface-active substance cetylpyridinium chloride (CPC) on the light scattering of an aqueous suspension of sodium bentonite in an electric field is studied. The changes in electro-optic effects and in stability after the addition of surface-active substances have been studied for other suspensions: palygorskite [ 1,2], kaolinite [3], sepiolite [4], silver iodide [ 51, etc. In these studies, a correlation was found between the surface electric properties of the respective colloid particles and the stability of their suspensions. However, in these studies no systematic study was made of the frequency dependence of the electro-optic effects. It is well known in electro-optics [6--121 that the frequency dependence of electro-optic effects provides information about the particles’ dimensions and electric properties. The nature of electro-optic effects in the kHz range, for most colloid suspensions, is more or less predominantly connected with the induced dipole moment of the particles, mainly due to the polarization of the diffuse part of the electric double layer (EDL) of the particles [lo].
0166-6622/86/$03.50
0 1986
Elsevier Science Publishers B.V.
230
On the other hand, the nature of the low-frequency (10 Hz--l kHz) dispersion of electro-optic effects is much less clear. Most frequently this dispersion is explained in terms of the permanent dipole moment of the particles [6,9-121. It has been shown that the low-frequency dispersion could depend on the type of counterion, compensating the charge of the particle [13], on the type of dispersion medium [14], on the magnitude of the electric field strength [ 151, etc. These factors, which favour the possible interfacial origin of the low-frequency effect, in most cases also influence the electro-optic effect in the kHz range. They may be explained by changes in the EDL of the particles. It has also been suggested that the low-frequency dispersion of the electro-optic effect may be explained by the formation of “domains” in the more concentrated suspensions [ 161. Another explanation has been put forward by Radeva and Stoimenova [13] for the case of rodlike palygorskite particles, where the polarization of the particles due to displacement of ions in the dense part of the EDL was found to be essential. The study of the effect of CPC on the electric properties of palygorskite suspensions [ 21, however, showed that the influence of the ions in the dense part of the EDL manifests itself in a much more complicated manner. The change of particle dimensions in the presence of surface-active agents (due to coagulation) also makes the interpretation of the results more difficult. The low-frequency dispersion of electro-optic effects of bentonite suspensions has, at presept, no explanation accepted by all researchers in this field. Together with the explanation based on the suggestion for the existence of two types of particles, possessing different electric properties, most of the explanations favour the postulation of a permanent dipole moment of the disc-like bentonite particles, transverse to the plane of the particles [6,12, 17-191. The basic suggestion is that, at low electric field strengths and low frequencies (or in d.c. electric fields), the bentonite particles are oriented with their long axis perpendicular to the direction of the electric field. This leads to lower or even to negative electro-optic effects. THEORY
Light scattering in an electric field is defined through the change in the intensity of the scattered light (Yafter the application of an electric field to the suspension: IE -1,~ Ly=-----=Ill
AI 10
where I_g and IO are the intensities of the scattered light in an electric field of strength E and with no field, respectively. When the energy of orientation of the particles in the field is low with respect to the energy of thermal motion, the electro-optic effect changes linearly with the square of the electric field strength, E* [20,21]. In this case, a solution of the diffusion equation was suggested in Ref. [22], which
231
gives the dependence of the electric light scattering effect on the angular frequency of the sinusoidal field, o . If the orientation of the particles is due only to the interactions of the induced dipole moments of the particles with the electric field, the electrooptic effect is represented by the sum of a constant term (Ye, equal to the time average value of the effect and one alternating with frequency 2 w . The amplitude of the latter decreases with increasing frequency from 2a, to 0, and for a given frequency o changes from CY( 1 + l/d1 + c..?/4.D’) to a(1 - l/d1 + wz /40z) [lo], where D is the particle rotational diffusion constant. When the orientation is due only to the permanent dipole moment of the particle, the time-independent term depends on the frequency, its amplitude changing from ‘Ye (at w = 0) to 0 (at w >> l/D). The time-dependent term also changes with frequency 2 w , its amplitude varying from 2a, to 0 [lo]. The decrease of the electro-optic effect after switching off the electric field is due to the particle disorientation, due to the effect of the thermal motion. The effect depends on the viscosity of the medium q, and the particle form and dimensions. At low degrees of orientation, the decay of the electro-optic effect is described by the equation [ 10,201: cy
=
(y,,
e-6Dt
(2)
where CQ is the effect at the moment of switching off the electric field,D is the rotational diffusion coefficient and t is time. By measuring the electrooptic effect of the particle’s disorientation, at moment t = 1/6D after switching off the electric field, it is possible to determine the rotational diffusion coefficient of the particles. From the latter the particle diameter B can be calculated following the expression of Perrin [23] given for very thin discs: D = 3kT/4qB3
(3)
where k is the Boltzmann constant, T the absolute temperature and B the particle diameter, the thickness being assumed to be much smaller than the wavelength. At complete particle orientation (saturation), an expression for A1, was suggested in Ref. [24] for infinitely thin discs: Ar, =2
5
G-l)n+lW)2n
n=l
(n + I)! (n + 2)!
1 _ (2n + l)!!
t
(2n - l)!!
(2n)!! (2n)!!
)
(4)
where K = (2n/h) sin 0’12, X is the wavelength in the medium and 8’ is the angle of observation. The decay of the electro-optic effect after switching off the electric field is given in Ref. [24] by a complicated expression. From it, for small values of the time (i.e. t + 0) when the effect changes linearly with time, an expression has been suggested [25] :
232
AIt =2Llt
C II=1
(-l)n+l(I@n(2n (n +
l)!
(n + 2)! (2n)!!
From Eqns (4) and (5) it follows the decay curve: A(1 -Art/AI,) fi=[
At
+ l)!!
(2n - 3)!! (2n - 2)!!
(5)
that, at known h , from the initial slope of
1
t-o
so that it is possible to determine D [ 251. EXPERIMENTAL
Materials An aqueous suspension of sodium bentonite with a weight concentration 4.5 g 1-l was fractionated by differential centrifugation following a procedure described in Ref. [26]. The particle dimensions were controlled by electron microscope and by the decay curves, Table 1. The form of the particles was approximated by circular discs. The weight concentration of the suspension was 5 * 10m6 kg 1-l and its specific conductivity was 3 * 10W4 R-l m -I. The critical mice& pon:e?fration of CPC is 9. 10v4 mol 1-l [27]. The apparatus used and the application of the electric light scattering are described in detail in Ref. [lo]. RESULTS
AND DISCUSSION
In Fig. 1 the dependence of the kHz electric light scattering cy on E2 is shown for a suspension of sodium bentonite in water (curve 1) and in the presence of CPC (curves 2-4). It is seen that, at the field strength of 6. lo3 V m-l at which all dispersion curves presented on Figs 2-4 have been studied, the electro-optic effect is proportional to E2. This means that the condition for low energies of orientation and, consequently, for low degrees of orientation, is fulfilled and no complications should be expected in the dispersion curves. In the frequency range l-10 kHz, the orientation of the particles has no time to follow the changes in direction of the electric field and, therefore, the electro-optic effect is independent of the permanent dipole moment of the particles. On the other hand, the frequencies are sufficiently low for bentonites so that no relaxation of the electric polarizability could be expected. The addition of the cationic surface-active substance CPC to the suspensions of negatively charged particles diminishes their charge (which is confirmed electrophoretically). On the other hand, the increase of CPC con-
233
Fig. 1. Dependence of the relative variation in electric field strength. Frequency: 1 kHz. bentonite; curve 2, bentonite + :LO-5 mol 1-l CPC;and curve 4, bentonite + 5~10~’ mall-’
the intensity of the scattered light on the Curve 1, aqueous suspension of sodium CPC; curve 3, bentonite + 10e4 mol 1-l CPC. (1 CGSE = 300 V cm--‘).
centration leads to a thinning of the particles’ diffuse electric layers, which are shown in some cases to be strongly connected with the electric polarizabilities of the particles. So it may have been expected that, as in the case of other colloids such as the palygorskite [1,2], the electro-optic effect should decrease with increasing concentration of CPC before the isoelectric point. However, the results in Fig. 1 show the opposite. The effect increases with increasing CPC concentration. One of the most trivial explanations of this increase could be the increase of particle dimensions by coagulation, which overcompensates the decrease due to the factors mentioned above. In Table 1 the dimensions of the particles, determined by three independent methods, are compared. It shows how dimensions increase with the increase of CPC concentration. The particle dimensions are greatest at the isoelectric point, which is at, 1O-4 mol 1-l CPC. At still greater concentration, the particles’ dimensions slightly decrease. The dimensions of the particles were determined by several independent methods since the different preparations were rather polydisperse. The divergence of the values for the dimensions obtained by the different methods gives an idea of the polydispersity of the suspensions, since the different methods (or the same method under different conditions) provide different types of averages. In the first four columns of Table 1 the values of the particle diameters are calculated from the time of relaxation of the effect after switching off the electric field at 6, 15, 30 and 450. lo3 V m-‘, respectively. In the fifth and sixth columns the dimensions were determined respectively by electron microscope statistics and from the initial slope of the curve of relaxation of the electrooptic effect after switching off the electric field after an orientation by 450 - lo3 V m-l (i.e. after full particle orientation). There is good agreement between the dimensions but not in the way the
1
540 600 640 570 570
680 680 700 -
10V5 1O-a 5.10-4 10-S
15*103
570
Vm-’
Aqueous suspension
6*103
B (nm) V m-’
of the particles of the sodium bentonite
CPC concentration (mall-‘)
Dimensions
TABLE
420 470 470 560 570 560
400
510
V m--l
of CPC
450~10~
510
V mm-’
and after addition
30*103
before
260 340 330 -
160
Electron microscopy
240 320 310 -
270
From initial slope of f~( t)
235
change with CPC concentration in columns five and six. The only considerable difference is between the dimensions of the bentonite particles in the suspension where no CPC has been added. It could be that this difference is due to the formation of T-like aggregates [28], caused by the presence of charges of different sign on the surfaces and the peripheries of the particles. This is also assumed to be one of the causes for the electrooptically obtained greater dimensions in the first four columns. The electron microscopy techniques which were used in this study make it possible, through shadowing with carbon under 45”, to determine with sufficient accuracy the dimensions of single particles, Table 1 shows that the dispersity of the bentonite suspensions changes in different ways at different CPC concentrations. At a CPC concentration of 10V5 mol 1-l the dimensions of the large particles increase; however, simultaneously the dimensions of the intermediate particles decrease. As a result the dimensions of the latter particles prove to be of lowest average dimension. At higher CPC concentration, particles of more uniform dimensions are obtained. The frequency behaviour of the electric light scattering in aqueous suspension (curve 1) and in the presence of CPC is shown on Fig. 2. It is seen that the electrooptic effect decreases with the lowering of the frequency in the range 100-300 Hz. This low-frequency decrease is stronger when CPC is added. At 10e4 mol 1-l (i.e. at the isoelectric point determined microelectrophoretically), and in the more concentrated suspensions at low frequency, the suspension shows a negative effect, reaching a magnitude two times greater than the maximum positive effect at higher frequencies. Dependence of another electro-optic effect, the electric birefringence, on particle dimensions was observed for bentonite by Shah and Hart [19]. In order to exclude in our experiment the influence of the different dimen-
Fig. 2. Frequency dependence of the electro-optic effect of sodium bentonite at different CPC concentrations: curve 1, no CPC; curve 2, 10e5 mol 1-l CPC; curve 3, lo-’ mol 1-l CPC; curve 4, 5.lo-’ mol I-’ CPC;and curve 5,10-I mol 1-l CPC. Electric field strength: 6010~ V m-l.
236 TABLE
2
Dimensions
of the particles
CPC concentration
of sodium
bentonite
after centrifugation
B (nm) 6.103
Aqueous suspension
570
10-5 lo-’ 5.10-4 1o-3
570 570 570 570
Vm-’
15.10’
520 490 520 520
V m-l
30.10’
V mm-’
460 490 500 -
sions, an attempt was made to prepare suspensions of equal particle dimensions by eliminating the larger particles obtained after the coagulation induced by CPC. This was done by centrifugation at 4000 rpm for 40 min. The particle dimensions so obtained are very similar (Table 2). However, as is seen from Fig. 3 the differences in the low-frequency behaviour of the electrooptic effect at different CPC concentration remain practically the same. The smaller particle dimensions merely cause a decrease of the electrooptic effect. This indicates that the electrical surface properties are essential for the observed low-frequency behaviour of the bentonite suspensions in the presence of CPC. It seems unlikely that this behaviour can be explained by the presence of two types of particles in the suspension (one having a transverse permanent dipole moment and the other being- _polarized predominantly along the long a.& [6,17]). In our case the frequently observed negative electro-optic effect for bentonite suspensions at low frequencies arises only on the addition of CPC.
Fig. 3. Dependence of the electrooptic effect 01 on the frequency v after centrifugation the suspensions. The different curves are at the same conditions as in Figs 1 and 2.
of
237
It is also difficult to explain these results by polarization of the compact EDL [13]. With the increase in CPC concentration, i.e. of the number of ions absorbed in the compact layer, the value of the electro-optic effect decreases instead of increasing. The most attractive explanation of this low-frequency behaviour of the electrooptic effect at different CPC concentrations is the presence and changing of a transverse permanent dipole moment in the bentonite particles. The contribution of these permanent dipole moments could be assumed to increase with the approach of the isoelectric point of the suspension and also because of the decrease of the value of the electric polarizability. This could be expected with the decrease of the interfacial electric charge and of the thickness of the EDL with the rise of the CPC concentration. In Fig. 4 are shown the values of the electric polarizability of bentonite particles in the presence of CPC (curve 3), calculated from the initial slope of the curve of the rise of the electro-optic effect after switching on the electric field. This is done following the procedure suggested in [24,25]. It is seen that the polarizability of the particles rises slightly at the isoelectric point, which was observed earlier for calcium bentonite [29]. The values of y, however, have been calculated per unit length and not per unit area. For the disc-like bentonite particles it seems more reasonable to compare the values of y per unit area of the disc. The values of y obtained in this way (curve 4) decrease with increasing CPC concentration up to the isoelectric point. The considerable electrooptic effect which is observed at the isoelectric point could be due to the mobility of ions both in the diffuse part of EDL, situated between the slipping plane and the particle surface, and in the inter-
Fig. 4. Dependence of the diameter of the particles (curve l), the electrophoretic ity (curve 2) and the electric polarizability (curve 3) on the CPC concentration. represents the electric polarizability per unit area from the particle surface.
mobilCurve 4
238
layer particle spaces. There exists experimental data for the considerable mobility of the ions in the interlayer particle spaces [ 301. Our investigations do not provide evidence as to whether or not y has a minimum at the isoelectric point. This is due to the fact that with further increase of CPC concentration beyond the isoelectric point, the ionic strength was not kept constant. CONCLUSION
The greater electrooptic effect in the kHz range of sodium bentonite suspensions to which CPC has been added in comparison with that of the pure aqueous suspensions seems to be due mainly to the changes of the particle dimensions and not with the electric polarizability per unit area of the interface. Minimum stability of the suspensions has been observed at the isoelectric point. The low-frequency (10 Hz-100 Hz) behaviour of the suspensions and the negative electro-optic effects observed at greater CPC concentration is assumed mainly to be due to surface electric properties of the particles and especially to the increased contribution of the transverse permanent dipole moment of the particles in their orientation.
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