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Unusual electric birefringence transients from sepiolite suspensions
Unusual Electric Birefringence Transients from Sepiolite Suspensions B. L. BROWN 1 Physics Department, Queen Elizabeth College, London, United Kingdom AND
B. R. JENNINGS Physics Department, Brund University, Uxbridge, United Kingdom Received September 6, 1972; accepted September 13, 1972 Following a brief summary of the nonregular responses in the literature from pulsed electric birefringence experiments, results are presented of a study on aqueous sepiolite suspensions by this experimental method. The study revealed novel transient responses which varied with repetitive pulsing of the suspension. The behavior indicated that the repetitive field promoted association of the rodlike particles, probably due to the field induced displacement of mobile charges associated with the solvent or surface properties of the particles. The study indicates the value of not neglecting unusual traces, the need for a suitable and complete electric birefringence theory in terms of mobile charge effects, and the need for cauti6n when studying particle suspensions in conducting media.
In recent years, electric birefringence studies have provided much information on the physical properties of synthetic, biological and inorganic macromolecules in both aqueous and organic dispersions (1,2). Because of their large particle size, highly dipolar character and particle rigidity, the aluminum and magnesium silicate clays have been particularly well studied (3-7). These minerals often assume the form of thin rod or disk like particles in dilute suspension. In the majority of electric birefringence studies, a pulsed dc electric field is applied to a cell which contains the sample suspension. A beam of linearly polarized light simultaneously traverses the cell and meets a "crossed" analyzer. The particle orientation, which accompanies the field application, changes the linearly polarized light into
elliptically polarized. Changes in the light intensity penetrating the analyzer are displayed on an oscilloscope and photographed. The conventional, transient response (Fig. la) may be considered in three regions: I, during which the response builds up; II, the region of the steady equilibrium amplitude and; III, after termination of the field, when the response decays back in a continuous exponential manner to the original value. The amplitudes and rates of these regions are generally interpreted (1,2,8) to yield the dipole moment and rotary diffusion constants (D) of the particles in suspension. Unusual birefringence transients occasionally have been presented in the literature, often without satisfactory explanation. From our work and discussions, we believe that "anomalous" transients are more common than the literature suggests. In the hope that results and discussions might be forthcoming in the future,
1Present Address: University College of North Wales, Bangor, Wales, United Kingdom. 170
Journal of Colloidand InterfaceScience, Vol. 43, No. 1, April 1973
Copyright ~ 1973 by" Academic Press. Inc. All rights of reproduction in arty form reserved,
ELECTRIC BIREFRINGENCE
i71
we wish to classify the previously reported applied f i e l d anomalous traces and report another which we have seen with aqueous sepiolite suspensions. The first class is when region I does not rise continuously to region II, but reaches a maximum and "droops" to its steady value (Fig. lb). Region III is then regular. This effect is not strictly an anomaly since it is predicted in the theories of Benoit (8) and Tinoco (9). It is attributed to permanent and induced dipoles along perpendicular geometric axes. It has been observed alone (without the (c) after-mentioned anomalies) for such materials as ~-lactoglobulin (Ref. 10 and Fig. 6.4. of Ref. 11), bentonite (Fig. 20, d and e of Ref. 6) and '°' laponite (Fig. 6 of Ref. 7) in water. The second effect is less common. Here, regions I and II are regular, but region III F]o. 1. Unusual electric birefringence transients, experiences an "upshoot" after the applied to be found in the literature. (a)-(d) represent a field has terminated (Fig. lc). The decay regular trace, a drooping response, an afferfield uprises rapidly before falling away to the pre- shoot and the symmetric blips, respectively. field value. Examples can be found in the literature for bentonite (Fig. 20, f and g of decay process may be possible however with Ref. 6), ¢Mactoglobulin (Fig. 6.2 and 6.3 of flexible molecules as invoked for RNA (13) Ref. 11), collagen (Fig. 1 of Ref. 12), R.N.A. and collagen (12) solutions. (13) and sonicated DNA (14). We note two The third effect we refer to as "blips." A things here. First, all of these studies were in small jump occurs in region I. This is then aqueous media where mobile charges probably mirror imaged in region III. Region II is play an important role. Second, this "up- regular (Fig. ld). The effect appears in an shoot" effect is more pronounced when the early electric birefringence paper on aqueous aforementioned "droop" phenomenon has TMV solutions (15) but without explanation. reduced the birefringence to near-zero values From this work, it appears to be concentration in region II. Shah and Hart (6) attributed both dependent. It was observed recently by one upshoots and droops to the quadrature nature of us (16) in electric field light-scattering of the dipoles for rigid bentonite particles. In experiments where it was shown that it can be our opinions, this explanation is not generally eliminated if pulsed sinusoidal fields are used. tenable for the upshoots. Whereas it may be Because of this, mobile charges were suggested possible for a rigid particle to rotate in the as being of importance. The effect has also field by way of two orientations (one fast and been observed with fibrinogen solutions (17,18). the other slow) as the particle responds to the Against this brief survey of unusual electrotwo quadrature dipoles, the decay would optic transients, we report some observations always follow a single compounded motion. on aqueous suspensions of sepiolite clay that This is because the decay is in the absence of exhibited a modified upshoot effect in which any field and corresponds to brownian dis- there is clear evidence that the electric field orientation forces alone. Furthermore, the induces particle associations. droop effect has been observed and predicted Dr. Neumann of Laporte Industries, Redhill, (8,9) independently of the upshoot. A twofold U.K. kindly provided the sample of the
(a)
!
~
(b)
Journal of Colloid and Interface Science, VoL 43, No. 1, April 1973
172
BROWN AND jENNINGS
sepiolite, rodlike clay. One gram was placed in 10 ml of distilled water and left to stand for 1 month, whence it was diluted to 50 ml and centrifuged at 2000g for 20 min in order to remove coarse impurities. The supernatant was then filtered through a 5-#m filter to leave a concentration of 8X10 -5 g/ml. This was diluted as required for further experiments. The suspensions were not dialyzed as this caused precipitation. This factor shows the importance of the local charge distribution upon particle association. The electric birefringence apparatus has been described elsewhere (7,19). Square pulses of 150 V amplitude and 50 msec duration were applied to the suspensions in a cell of 10-cm path length with 0.5 cm electrode separation. The temperature was kept constant at 25 (+1)°C. Figure 2 shows transients for a suspension of 4X10 -5 g/ml concentration. It is seen that regular, repetitive pulsing greatly affected the response. This was true of all suspensions at higher concentration. Dispersions of lower concentration than about 2X10 -5 g/ml were always regular and similar to that of Fig. 2a. They did not exhibit this behavior, even after 15 successive pulses. The response is thus dependent upon both concentration and the
b
/
c .
\
.
.
.
--/
TABLE I ROTARY DIFFUSION CONSTANTS (D) THE ULTIMATE Fig. 2 frame
D (sec-1)
a
DECAY b
RATES c
OBTAINED FROM
OF FIGURE d
2 e
8.3 4.8 3.3 2.4 2.4 (4-2.0) (4-0.7) (4-0.6) (4-0.3) (4-0.3)
rate and number of applied pulses• If the more concentrated suspensions were repetitively pulsed and then left to stand for a period of 20--30 min, they returned to their original condition. The complete behavioral cycle was then reproducible. Table I lists the rotary diffusion constants for the transients of the figures. The gradual fall in D through the sequence a-e indicates an increase in particle size with successive pulses. The alternative explanation that D indicates a particle conformation change is not feasible for such rigid rods (Fig. 3). The electron micrograph was obtained by evaporating some of the most concentrated solution onto the microscope grid. The sample was not stained. It is seen that the dry clay particles are long rods of just less than 1 ~m in length and have an axial ratio (p) of approximately 40. Using this ratio in Burger's (20) equation, which is relatively insensitive to p, a value of D=8.3 sec-I for the most dilute suspension corresponds to a rod length of L = 0 . 8 vm, which is in good agreement with the electron microscopic value. We summarize the observations as follows:
d
:__7_ Fla. 2. Consecutive responses for a sepiolite suspension. A dispersion of 4 X l0-~ g/ml in water was repetitively pulsed with a field of 300 V cm-1 and 50 msec duration at 30-sec intervals. Frames (a)-(e) represent the first fiveresponses. Journal of Colloid and Interface Science, Vol. 43, No. 1, Apr il 1973
(i) Changes in D can be induced with repeated pulsing of the suspensions• These changes are indicative of particle size increases. (ii) Prior to this, the rod lengths from the transient decay are in concord with those from electron microscopy. (iii) This effect is concentration dependent• These three factors suggest an aggregation phenomenon• (iv) The upshoot effect is only observed when the aggregation is present. (v) These aqueous suspensions are highly
ELECTRIC BIREFRINGENCE
173
Fio. 3. Electron micrographof the sepiolitesample. The rodlike nature of the particles is evident.
conducting, with conductivities of the order of 10-3 ohm cm -1. A possible mechanism for the behavior is as follows. Recent studies of the electrical properties of solutions and suspensions have indicated the inadequacy of just the permanent dipole and bulk polarizability of the solute to account for all of the observed phenomena. The intervention of counterions and other mobile charges have been invoked (16,21-23). Such charges are abundant in aqueous and conducting media. Hence, as the sepiolite particles orientate owing to one or more permanent or induced dipole moment, the mobile charge distribution in the bound solvent or particle surface layer becomes distorted. Such behavior of a counterion sheath in an electric field has been substantiated (24,25) together with the fact that the diffusion of the ions is very much faster in the field than after it has terminated (22). Furthermore, theoretical considerations have predicted (26-29) and experimental observations confirmed (30-33) that for orien-
rated rods, aggregation is strongly favored by distortion of the counterion cloud. This seems to be appropriate to the sepiolite case. Hence, in the field, the particles are held in dynamic equilibrium in region II of the transient response and the counterion cloud is distorted. When the field ceases, the individual particles find themselves in a temporary state which highly favors their aggregation. Because of the relatively long time required for the ions to redistribute (22), the clay particles aggregate and the upshoot of region III of the transient response is observed. A lower rotary diffusion constant is then observed in the transient decay as this is now a property of the partially aggregated material. Upon standing, the displaced charges slowly rearrange themselves, the aggregate disassociates and the system returns to its original state. This behavior is consistent with the observation (22) that the diffusion rate of the ions is much slower after the termination of the field than in the establishing field. JournM of Colloid and Interface Science, Vol. 43, No. 1, April 1973
174
BROWN AND JENNINGS
If the pulse should be reapplied before the aggregate has had time to disperse, further ionic migration might take place. This might itself favor an increase in the tendency of the clay particles to cluster. Larger aggregates could form with resulting decreased rotary diffusion constants upon termination of the field. This is consistent with the data (Table I). A consideration of D indicates that the particles aggregate neither discretely end-toend nor side-by-side. The former type of association would result in drastic reductions of D as this parameter is approximately proportional (20) to L -a for a rod of length L. A simple end-to-end dimerization would change D by a factor close to eight. This is not the case with sepiolite. Moreover, D is insensitive to the rod diameter, so that side-by-side associations would not change the rotary diffusion constant by as much as four times (Table I) as observed in this study. Some form of tess discrete clustering would satisfy the data. A n y association is likely to impose greater order on the particle array and thus account for the increasing birefringence of the upshoot prior to the disorientation of the aggregate to a random array. I n conclusion, we wish to draw attention to the study of unusual transient electrooptic responses as we believe that they contain valuable molecular information. I n particular, our data concern novel traces which seem to demonstrate that repetitive application of direct-current, rectangular electric fields can cause association of the rod particles of sepiolite clay when dispersed in water. The role of mobile counterion charges is apparently important and hence indicate the need for suitable electrooptic theories which include contributions from such charges. When studying molecules and particles in conducting solvents, care should be taken that the system is not unnecessarily pulsed prior to the observations being recorded. ACKNOWLEDGMENTS Thanks are expressed to Messrs. Imperial Chemical Industries Limited, who gave a grant for the original
purchase of equipment; to Mr. R. Webb of Queen Elizabeth College (Q.E.C.) for assistance with the electronics and to both physics departments. One of us (B.L.B.) thanks Q.E.C. for a Student Demonstratorship during the tenure of which this study was commenced. REFERENCES 1. YOSHIOKA, K., AND WATANABE, H., "Physical Principles and Techniques of Protein Chemistry," Academic Press, New York, 1969. 2. O'KONSKI,C. T., "Encyclopedia of Polymer Science and Technology," Interscience, New York, 1969. 3. NORTON,F. J., Phys. Rev. 55, 668 (1939). 4. MUELLER,H., Phys. Rev. 55, 792 (1939). 5. KAHN,A., AND LEWIS, D. R., J. Chem. Phys. 58, 801 (1954). 6. SHAH, M. J., AND HART, C. M., I. B. M. J. Res. Dev. 7, 44 (1963). 7. JENNINGS,B. R., BROWN,B. L., AND PLU~MER,H., J. Colloid Interface Sci. 32, 606 (1970). 8. BENOIT, H., Ann. Phys. (Paris) 12, 651 (1951). 9. TINoco, I., J. Amer. Chem. Soc. 77, 4486 (1955). 10. INGRAM,P., ANDJERRARD,H. G. Nature (London) 196, 57 (1962). 11. RIDDIFORD, C. L., AND JENNINGS, B. R., Biopolymers 5, 757 (1967). 12. KHAN, L: D., AND WITNAUER, L. P., Biochim. Biophys. Acta 243, 388 (1971). 13. GOLUB,E. I., ANDNAZARENKO,V. G., Biophys. J. 7, 13 (1967). 14. HOUSSlER, C., COLSON, P., AND FREDERICQ, E., private communication. 15. O'KoNsKI, C. T. ANDZIMM,B. H., Science 111, 113 (1950). 16. SCHWEITZER,J., AND JENNINGS, B. R., 3. Phys. D: Appl. Phys. 5, 297 (1972). 17. HASCHEMEYER,A., ANDTINOCO, I., Biocher~istry 1, 996 (1962). 18. tlASCHEYIEYER,A., Biochemistry 2, 851 (1963). 19. BROWN,B. L., JENNINGS,B. R., AND PLU~MER, H., Appl. Opt. 7, 2019 (1969). 20. BURGERS,J. M., Verk. Kon. Ned. Akad. Wet. Sec. 1. No. 4 16, 113 (1938). 21. O'KoNsI~, C. T., AND HALTNER, A. ~., J. Amer. Chem. Soc. 79, 5634 (1957). 22. EIGEN, M., AND SCHWARZ, G., J. Colloid Sci. 12, 181 (1957). 23. STOYLOV,S., in "Chemistry, Physics and Application of Surface Active Substances" (J. Overbeck, ed.), Vol. 11, p. 171, Gordon and Breach, New York, 1967.
Journal of Colloid and Interface Science, Vol. 43, No. 1, April 1973
ELECTRIC BIREFRINGEN CE 24. TAKASH1MA, S., J. Phys. Chem. 70, 1372 (1966). 25. POLLAK, M., J. Chem. Phys. 43,908 (1965). 26. OHmsI~I, T., IMAI, N., AND OOSAWA,F. J., J. Phys. Soc. Yap. 15, 896 (1960). 27. OSAWA,F., Biopolymers 9, 677 (1970). 28. I~AT., N., J. Phys. Soe. Jap. 16, 746 (1960). 29. CASSEL,J. M., Biopolymers 4, 989 (1966).
175
30. VAN OLPHEN,]7I., J. Colloid Inte~jace Sc,. 19, 313 (1964). 31. BANIN, A., AND LAHAV,N., Israel f . Chem. 6, 285 (1968). 32. 0To~I, H., AND SHAINBERG,I., Israel J. Chem. 6, 251 (I968). 33. STEINBERG,I., ANDKAISERMAN,A., Soil Sci. Amer. Proc. 33, 550 (1969).
Journal of Colloid and Interface Science, VoL 43, No. 1, April 197.3
B. R. JENNINGS Physics Department, Brund University, Uxbridge, United Kingdom Received September 6, 1972; accepted September 13, 1972 Following a brief summary of the nonregular responses in the literature from pulsed electric birefringence experiments, results are presented of a study on aqueous sepiolite suspensions by this experimental method. The study revealed novel transient responses which varied with repetitive pulsing of the suspension. The behavior indicated that the repetitive field promoted association of the rodlike particles, probably due to the field induced displacement of mobile charges associated with the solvent or surface properties of the particles. The study indicates the value of not neglecting unusual traces, the need for a suitable and complete electric birefringence theory in terms of mobile charge effects, and the need for cauti6n when studying particle suspensions in conducting media.
In recent years, electric birefringence studies have provided much information on the physical properties of synthetic, biological and inorganic macromolecules in both aqueous and organic dispersions (1,2). Because of their large particle size, highly dipolar character and particle rigidity, the aluminum and magnesium silicate clays have been particularly well studied (3-7). These minerals often assume the form of thin rod or disk like particles in dilute suspension. In the majority of electric birefringence studies, a pulsed dc electric field is applied to a cell which contains the sample suspension. A beam of linearly polarized light simultaneously traverses the cell and meets a "crossed" analyzer. The particle orientation, which accompanies the field application, changes the linearly polarized light into
elliptically polarized. Changes in the light intensity penetrating the analyzer are displayed on an oscilloscope and photographed. The conventional, transient response (Fig. la) may be considered in three regions: I, during which the response builds up; II, the region of the steady equilibrium amplitude and; III, after termination of the field, when the response decays back in a continuous exponential manner to the original value. The amplitudes and rates of these regions are generally interpreted (1,2,8) to yield the dipole moment and rotary diffusion constants (D) of the particles in suspension. Unusual birefringence transients occasionally have been presented in the literature, often without satisfactory explanation. From our work and discussions, we believe that "anomalous" transients are more common than the literature suggests. In the hope that results and discussions might be forthcoming in the future,
1Present Address: University College of North Wales, Bangor, Wales, United Kingdom. 170
Journal of Colloidand InterfaceScience, Vol. 43, No. 1, April 1973
Copyright ~ 1973 by" Academic Press. Inc. All rights of reproduction in arty form reserved,
ELECTRIC BIREFRINGENCE
i71
we wish to classify the previously reported applied f i e l d anomalous traces and report another which we have seen with aqueous sepiolite suspensions. The first class is when region I does not rise continuously to region II, but reaches a maximum and "droops" to its steady value (Fig. lb). Region III is then regular. This effect is not strictly an anomaly since it is predicted in the theories of Benoit (8) and Tinoco (9). It is attributed to permanent and induced dipoles along perpendicular geometric axes. It has been observed alone (without the (c) after-mentioned anomalies) for such materials as ~-lactoglobulin (Ref. 10 and Fig. 6.4. of Ref. 11), bentonite (Fig. 20, d and e of Ref. 6) and '°' laponite (Fig. 6 of Ref. 7) in water. The second effect is less common. Here, regions I and II are regular, but region III F]o. 1. Unusual electric birefringence transients, experiences an "upshoot" after the applied to be found in the literature. (a)-(d) represent a field has terminated (Fig. lc). The decay regular trace, a drooping response, an afferfield uprises rapidly before falling away to the pre- shoot and the symmetric blips, respectively. field value. Examples can be found in the literature for bentonite (Fig. 20, f and g of decay process may be possible however with Ref. 6), ¢Mactoglobulin (Fig. 6.2 and 6.3 of flexible molecules as invoked for RNA (13) Ref. 11), collagen (Fig. 1 of Ref. 12), R.N.A. and collagen (12) solutions. (13) and sonicated DNA (14). We note two The third effect we refer to as "blips." A things here. First, all of these studies were in small jump occurs in region I. This is then aqueous media where mobile charges probably mirror imaged in region III. Region II is play an important role. Second, this "up- regular (Fig. ld). The effect appears in an shoot" effect is more pronounced when the early electric birefringence paper on aqueous aforementioned "droop" phenomenon has TMV solutions (15) but without explanation. reduced the birefringence to near-zero values From this work, it appears to be concentration in region II. Shah and Hart (6) attributed both dependent. It was observed recently by one upshoots and droops to the quadrature nature of us (16) in electric field light-scattering of the dipoles for rigid bentonite particles. In experiments where it was shown that it can be our opinions, this explanation is not generally eliminated if pulsed sinusoidal fields are used. tenable for the upshoots. Whereas it may be Because of this, mobile charges were suggested possible for a rigid particle to rotate in the as being of importance. The effect has also field by way of two orientations (one fast and been observed with fibrinogen solutions (17,18). the other slow) as the particle responds to the Against this brief survey of unusual electrotwo quadrature dipoles, the decay would optic transients, we report some observations always follow a single compounded motion. on aqueous suspensions of sepiolite clay that This is because the decay is in the absence of exhibited a modified upshoot effect in which any field and corresponds to brownian dis- there is clear evidence that the electric field orientation forces alone. Furthermore, the induces particle associations. droop effect has been observed and predicted Dr. Neumann of Laporte Industries, Redhill, (8,9) independently of the upshoot. A twofold U.K. kindly provided the sample of the
(a)
!
~
(b)
Journal of Colloid and Interface Science, VoL 43, No. 1, April 1973
172
BROWN AND jENNINGS
sepiolite, rodlike clay. One gram was placed in 10 ml of distilled water and left to stand for 1 month, whence it was diluted to 50 ml and centrifuged at 2000g for 20 min in order to remove coarse impurities. The supernatant was then filtered through a 5-#m filter to leave a concentration of 8X10 -5 g/ml. This was diluted as required for further experiments. The suspensions were not dialyzed as this caused precipitation. This factor shows the importance of the local charge distribution upon particle association. The electric birefringence apparatus has been described elsewhere (7,19). Square pulses of 150 V amplitude and 50 msec duration were applied to the suspensions in a cell of 10-cm path length with 0.5 cm electrode separation. The temperature was kept constant at 25 (+1)°C. Figure 2 shows transients for a suspension of 4X10 -5 g/ml concentration. It is seen that regular, repetitive pulsing greatly affected the response. This was true of all suspensions at higher concentration. Dispersions of lower concentration than about 2X10 -5 g/ml were always regular and similar to that of Fig. 2a. They did not exhibit this behavior, even after 15 successive pulses. The response is thus dependent upon both concentration and the
b
/
c .
\
.
.
.
--/
TABLE I ROTARY DIFFUSION CONSTANTS (D) THE ULTIMATE Fig. 2 frame
D (sec-1)
a
DECAY b
RATES c
OBTAINED FROM
OF FIGURE d
2 e
8.3 4.8 3.3 2.4 2.4 (4-2.0) (4-0.7) (4-0.6) (4-0.3) (4-0.3)
rate and number of applied pulses• If the more concentrated suspensions were repetitively pulsed and then left to stand for a period of 20--30 min, they returned to their original condition. The complete behavioral cycle was then reproducible. Table I lists the rotary diffusion constants for the transients of the figures. The gradual fall in D through the sequence a-e indicates an increase in particle size with successive pulses. The alternative explanation that D indicates a particle conformation change is not feasible for such rigid rods (Fig. 3). The electron micrograph was obtained by evaporating some of the most concentrated solution onto the microscope grid. The sample was not stained. It is seen that the dry clay particles are long rods of just less than 1 ~m in length and have an axial ratio (p) of approximately 40. Using this ratio in Burger's (20) equation, which is relatively insensitive to p, a value of D=8.3 sec-I for the most dilute suspension corresponds to a rod length of L = 0 . 8 vm, which is in good agreement with the electron microscopic value. We summarize the observations as follows:
d
:__7_ Fla. 2. Consecutive responses for a sepiolite suspension. A dispersion of 4 X l0-~ g/ml in water was repetitively pulsed with a field of 300 V cm-1 and 50 msec duration at 30-sec intervals. Frames (a)-(e) represent the first fiveresponses. Journal of Colloid and Interface Science, Vol. 43, No. 1, Apr il 1973
(i) Changes in D can be induced with repeated pulsing of the suspensions• These changes are indicative of particle size increases. (ii) Prior to this, the rod lengths from the transient decay are in concord with those from electron microscopy. (iii) This effect is concentration dependent• These three factors suggest an aggregation phenomenon• (iv) The upshoot effect is only observed when the aggregation is present. (v) These aqueous suspensions are highly
ELECTRIC BIREFRINGENCE
173
Fio. 3. Electron micrographof the sepiolitesample. The rodlike nature of the particles is evident.
conducting, with conductivities of the order of 10-3 ohm cm -1. A possible mechanism for the behavior is as follows. Recent studies of the electrical properties of solutions and suspensions have indicated the inadequacy of just the permanent dipole and bulk polarizability of the solute to account for all of the observed phenomena. The intervention of counterions and other mobile charges have been invoked (16,21-23). Such charges are abundant in aqueous and conducting media. Hence, as the sepiolite particles orientate owing to one or more permanent or induced dipole moment, the mobile charge distribution in the bound solvent or particle surface layer becomes distorted. Such behavior of a counterion sheath in an electric field has been substantiated (24,25) together with the fact that the diffusion of the ions is very much faster in the field than after it has terminated (22). Furthermore, theoretical considerations have predicted (26-29) and experimental observations confirmed (30-33) that for orien-
rated rods, aggregation is strongly favored by distortion of the counterion cloud. This seems to be appropriate to the sepiolite case. Hence, in the field, the particles are held in dynamic equilibrium in region II of the transient response and the counterion cloud is distorted. When the field ceases, the individual particles find themselves in a temporary state which highly favors their aggregation. Because of the relatively long time required for the ions to redistribute (22), the clay particles aggregate and the upshoot of region III of the transient response is observed. A lower rotary diffusion constant is then observed in the transient decay as this is now a property of the partially aggregated material. Upon standing, the displaced charges slowly rearrange themselves, the aggregate disassociates and the system returns to its original state. This behavior is consistent with the observation (22) that the diffusion rate of the ions is much slower after the termination of the field than in the establishing field. JournM of Colloid and Interface Science, Vol. 43, No. 1, April 1973
174
BROWN AND JENNINGS
If the pulse should be reapplied before the aggregate has had time to disperse, further ionic migration might take place. This might itself favor an increase in the tendency of the clay particles to cluster. Larger aggregates could form with resulting decreased rotary diffusion constants upon termination of the field. This is consistent with the data (Table I). A consideration of D indicates that the particles aggregate neither discretely end-toend nor side-by-side. The former type of association would result in drastic reductions of D as this parameter is approximately proportional (20) to L -a for a rod of length L. A simple end-to-end dimerization would change D by a factor close to eight. This is not the case with sepiolite. Moreover, D is insensitive to the rod diameter, so that side-by-side associations would not change the rotary diffusion constant by as much as four times (Table I) as observed in this study. Some form of tess discrete clustering would satisfy the data. A n y association is likely to impose greater order on the particle array and thus account for the increasing birefringence of the upshoot prior to the disorientation of the aggregate to a random array. I n conclusion, we wish to draw attention to the study of unusual transient electrooptic responses as we believe that they contain valuable molecular information. I n particular, our data concern novel traces which seem to demonstrate that repetitive application of direct-current, rectangular electric fields can cause association of the rod particles of sepiolite clay when dispersed in water. The role of mobile counterion charges is apparently important and hence indicate the need for suitable electrooptic theories which include contributions from such charges. When studying molecules and particles in conducting solvents, care should be taken that the system is not unnecessarily pulsed prior to the observations being recorded. ACKNOWLEDGMENTS Thanks are expressed to Messrs. Imperial Chemical Industries Limited, who gave a grant for the original
purchase of equipment; to Mr. R. Webb of Queen Elizabeth College (Q.E.C.) for assistance with the electronics and to both physics departments. One of us (B.L.B.) thanks Q.E.C. for a Student Demonstratorship during the tenure of which this study was commenced. REFERENCES 1. YOSHIOKA, K., AND WATANABE, H., "Physical Principles and Techniques of Protein Chemistry," Academic Press, New York, 1969. 2. O'KONSKI,C. T., "Encyclopedia of Polymer Science and Technology," Interscience, New York, 1969. 3. NORTON,F. J., Phys. Rev. 55, 668 (1939). 4. MUELLER,H., Phys. Rev. 55, 792 (1939). 5. KAHN,A., AND LEWIS, D. R., J. Chem. Phys. 58, 801 (1954). 6. SHAH, M. J., AND HART, C. M., I. B. M. J. Res. Dev. 7, 44 (1963). 7. JENNINGS,B. R., BROWN,B. L., AND PLU~MER,H., J. Colloid Interface Sci. 32, 606 (1970). 8. BENOIT, H., Ann. Phys. (Paris) 12, 651 (1951). 9. TINoco, I., J. Amer. Chem. Soc. 77, 4486 (1955). 10. INGRAM,P., ANDJERRARD,H. G. Nature (London) 196, 57 (1962). 11. RIDDIFORD, C. L., AND JENNINGS, B. R., Biopolymers 5, 757 (1967). 12. KHAN, L: D., AND WITNAUER, L. P., Biochim. Biophys. Acta 243, 388 (1971). 13. GOLUB,E. I., ANDNAZARENKO,V. G., Biophys. J. 7, 13 (1967). 14. HOUSSlER, C., COLSON, P., AND FREDERICQ, E., private communication. 15. O'KoNsKI, C. T. ANDZIMM,B. H., Science 111, 113 (1950). 16. SCHWEITZER,J., AND JENNINGS, B. R., 3. Phys. D: Appl. Phys. 5, 297 (1972). 17. HASCHEMEYER,A., ANDTINOCO, I., Biocher~istry 1, 996 (1962). 18. tlASCHEYIEYER,A., Biochemistry 2, 851 (1963). 19. BROWN,B. L., JENNINGS,B. R., AND PLU~MER, H., Appl. Opt. 7, 2019 (1969). 20. BURGERS,J. M., Verk. Kon. Ned. Akad. Wet. Sec. 1. No. 4 16, 113 (1938). 21. O'KoNsI~, C. T., AND HALTNER, A. ~., J. Amer. Chem. Soc. 79, 5634 (1957). 22. EIGEN, M., AND SCHWARZ, G., J. Colloid Sci. 12, 181 (1957). 23. STOYLOV,S., in "Chemistry, Physics and Application of Surface Active Substances" (J. Overbeck, ed.), Vol. 11, p. 171, Gordon and Breach, New York, 1967.
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