Mechanistic Origin of Transient Electric Birefringence Anomaly of Clay

Mechanistic Origin of Transient Electric Birefringence Anomaly of Clay Mineral .... Pharmaceutical companies employ teams of lawyers and intellectual ...
2 downloads 0 Views 135KB Size
Download PDF
4412

J. Phys. Chem. B 2002, 106, 4412-4418

Mechanistic Origin of Transient Electric Birefringence Anomaly of Clay Mineral Dispersion S. Holzheu and H. Hoffmann* Department of Physical Chemistry I, UniVersity of Bayreuth, D-95440 Bayreuth, Germany ReceiVed: July 3, 2001; In Final Form: NoVember 29, 2001

The transient electric birefringence (TEB) anomaly is known for more than 40 years. Despite many experimental results, there is up to now no model describing the TEB anomaly on a molecular level. In this paper, we will present a systematic analysis of the effect of external parameters such as adsorption, solid or ion concentration on the anomaly. For the experiments, we use a special inversed charge clay mineral. This material shows, compared to pure clay minerals, the advantage of enhanced insensitivity to salt-induced aggregation. The results reveal that there is no indication of multiparticle processes. For the existence of the anomaly, both presence of salt and suitable surface conditions are necessary. The disappearance of the anomaly upon adsorption of nonionic substances is due to a change of permanent-like dipole direction from perpendicular to parallel to the clay mineral surface. The explanation model proposed is based on permanent-like dipole moments originating from statistical ion fluctuations leading to different numbers of condensed counterions in the Stern layers of the two sides of the clay disk. Calculated values agree well with experimental quantities. When the mobility of the counterions is high, the mechanism produces a dipole with a dominant perpendicular component. A decrease in counterion mobility leads to a larger parallel component.

Introduction There is a long tradition in investigating clay mineral dispersions through transient electric birefringence (TEB).1-11 Early studies concentrated on the determination of mean diameters using the time constants evaluated from the decay of the TEB signal.1 However, more detailed studies have shown that the orientation behavior of clay particles is more complicated. One characteristic feature is the observation of a sign inversion of the steady-state birefringence with increasing DC field strength.2,8 Simultaneously, the occurrence of an anomalous TEB signal (anomaly) composed of two processes of different signs and time constants is detected.2,9 We will denote the initial increase of the TEB signal as first effect and the decrease as second effect. The first theoretical explanations of these anomalous signal patterns were based on the assumption of a fast induced dipole moment parallel and a permanent dipole perpendicular to the clay mineral surface.2 However, this explanation somehow implies that the properties of the clay minerals are fixed and that there is no influence of external parameters such as clay particle concentration or ionic strength. Experimentally, one observes an increase of the amplitude of the second effect with increasing clay mineral concentration2,5,22 as well as with increasing ionic strength.11 It is not clear at present whether these are two different mechanisms resulting in the same effect or just one mechanism. One drawback of the pure clay mineral dispersions is their tendency to aggregate. This makes it difficult to study precisely the influence of salt or clay concentration on the anomaly as aggregation may blanket some changes.11 Recently while studying the surfactant and polymer adsorption on clay minerals, we have found that in addition to salt and clay mineral concentration, the surface properties also play a crucial role in the appearance of the anomaly.12,13 The adsorption * To whom correspondence should be addressed.

of nonionic substances reduces significantly the amplitude of the second effect and in some cases it disappears totally. At the moment, this observation is not explainable with the available models. Experimental Section Materials. The synthetic clay mineral hectorite (SKS 21) was obtained as a gift from Clariant (Germany). It has a cation exchange capacity (cec) of 0.88 meq/g and was already used in former studies.12 The polyelectrolytes (Luviquate) are copolymers of cationic polyimidazole and nonionic polyvinylpyrollidone (Figure 1) and were provided by BASF (Germany). Three polyelectrolytes with different compositions were used (Table 1). The blockcopolymere Pluronics F127 has an average composition EO97PO69EO97 and is also a product of BASF (Germany). The cationic dye basic violet is a triphenylmethane dye and was provided by Wella (Germany). All chemicals were used as received without further purification. Sample Preparation. Solutions of hectorite and polyelectrolyte were prepared in double-distilled water. The solutions were quickly mixed and ultrasonicated for 5 h at 60 °C. The solutions were diluted and mixed to achieve the final concentrations. The samples were ultrasonicated again for 1 h and kept at rest for at least 1 day. Measurements. Zeta potential of the clay particles was determined using Malvern Zetasizer 3000 and the Smoluchowski approximation. Electric conductivity was measured through a platinated electrode. 10 mM KNO3 was used to determine the cell constant of the electrode. Electric linear dichroism (ELD) measurements were carried out through a modified T-jump apparatus. The light source of the apparatus is a high-pressure mercury lamp. The white light passes a monochromator. After the monochromator, the beam is split and a fraction is routed to a reference photomultiplier. The rest of the light is linearly polarized in front of the sample cell. A photomultiplier after the sample cell detects the light transmitted by the sample cell. Before each measurement, the

10.1021/jp012531v CCC: $22.00 © 2002 American Chemical Society Published on Web 04/04/2002

Transient Electric Birefringence Anomaly

J. Phys. Chem. B, Vol. 106, No. 17, 2002 4413

Figure 1. Molecular structure of the used polyelectrolyt (Luviquate).

TABLE 1: Molecular Mass, Chemical Composition, and Specific Charge of the Polyelectrolytes Used in This Study; Data from BASF, Germany molecular mass [g/mol] weight percent Imidazol charge [meq/g]

FC370

FC550

FC905

100 000 30% 2.48

80 000 50% 3.91

40 000 95% 6.66

photocurrent of the reference and the detector are balanced. The change in amplifier output is directly proportional to the relative change of light intensity at the detector. A high-voltage pulse generator (Cober 606) delivering rectangular pulses up to 2.5 kV with a duration of 3 ms was used to produce the electric field. Transient electric birefringence (TEB) was measured through a self-built instrument. The principal operation of the apparatus was already described earlier.14 The light source of the instrument is a He-Ne laser (λ ) 632.8 nm). High-voltage rectangular pulses were produced by the same high-voltage pulse generator (Cober 606) also used in the dichroism apparatus. Signals were recorded using the linear detection mode with a quarter-wave plate. The primary signal of both TEB and ELD is light intensity which is also influenced by light-scattering effects. However, light scattering of hectorite dispersions in the concentration range studied is extremely low, for example, the transmission of a 2 g/l hectorite dispersion in a 2-cm cuvette is over 97%. Hence, there is no need to correct for light-scattering contributions. During all measurements, the temperature was kept constant at 25 °C. Results Characterization of Inversed Charge Hectorite. Positively charged polyelectrolyte strongly adsorbs on negatively charged surfaces. The adsorption is irreversible, and adding excess polyelectrolyte typically leads to surface charge overcompensation. As shown by several groups,15-18 it is even possible to build up multilayers by alternative adsorption of positively and negatively charged polyelectrolyte. On this background, it is not surprising that also the charge of clay particles can be reversed using polyelectrolyte adsorption. Figure 2 shows the ζ-potential of hectorite particles in the presence of different polyelectrolytes. With all polyelectrolytes, it was possible to invert the ζ-potential from initially -40 mV to about +50 mV. The concentration needed to reach the point of zero charge (pzc) increases in the order FC905, FC550, FC370 and coincides well with the calculated values using the specific charge of the polyelectrolytes (Table 1) and the cation exchange capacity of hectorite of 0.88 meq/g. In the vicinity of the pzc, the samples were turbid because of flocculated hectorite particles. In a sufficient distance from the pzc, the samples became transparent almost like water. TEB signals of the transparent samples after the charge inversion are presented in Figure 3. For comparison, also the signal of the pure hectorite with the same concentration is shown. It is possible to reach the stationary birefringence. The

Figure 2. ζ-potential of the hectorite particles (2 g/l) with increasing concentration of polyelectrolyte (FCxxx).

Figure 3. TEB signals of pure hectorite (2 g/l) and hectorite (2 g/l) charge inversed with different polyelectrolytes (E ) 0.75 kV/cm).

overall time constants of the inversed charge hectorite are in the same order of magnitude as those of the pure hectorite. This is a clear indication that we were able to redisperse the hectorite particles after the charge inversion to a comparable extend as the unmodified hectorite. There is no indication of extensive cross-linking of the hectorite particles through polyelectrolyte bridges. Under such conditions, TEB signals reflect a slow buildup, and it is not possible to reach the stationary birefringence. We want to call attention to the differences between the three polyelectrolytes. Regardless of similar ζ-potentials and net surface charge, the sample with FC905 shows a pronounced anomaly whereas the FC370-inversed charge hectorite exhibits a normal signal. Qualitatively, this is comparable to the findings on poly(vinylpyrrolidone) and blockcopolymere adsorption on hectorite. At a certain amount of these compounds, the anomaly also disappeared. Obviously increasing the percentage of poly(vinylpyrrolidone) from 5% in FC905 to 70% in FC370 makes the polyelectrolyte more “nonionic”. Effect of Clay Concentration Versus Ionic Strength. In pure clay minerals, the second effect shows an increase in amplitude with both increasing clay mineral and salt concentration. The same observation can be made with the FC905 inversed charge hectorite. As presented in Figure 4, diluted

4414 J. Phys. Chem. B, Vol. 106, No. 17, 2002

Holzheu and Hoffmann

Figure 4. TEB signals of inversed charge hectorite (ratio hectorite/FC905 constant 1/0.4); first row: series with increasing hectorite/FC905 concentration, concentrations refer to the concentration of hectorite; second row: series with constant hectorite/FC905 concentration (0.2 g/l hectorite) and increasing salt concentration; E ) 0.75 kV/cm.

independently. Coupled rotations of several particles should yield larger time constants and hence larger apparent diameters. The quantitative analysis of the buildup was performed using an empirical approach decomposing the measured TEB signal into two processes with positive and negative amplitudes. The parameters ∆n+, ∆n-, τ+, and τ- of eq 1 were adjusted using a numerical fitting procedure. +

∆nB(t) ) ∆n-(1 - e-t/τ ) + ∆n+(1 - e-t/τ ) -

Figure 5. Mean diameter calculated from the decay of the TEB signal through extrapolation to infinitively high field strengths; the stars denote the series with increasing hectorite/FC905 concentration, the triangles the series with constant hectorite/FC905 (0.2 g/l hectorite) and increasing NaCl concentration. The scale of the two x-axes was adjusted to represent approximately the same electric conductivity. Ratio hectorite/FC905 constant 1/0.4.

FC905/hectorite dispersions show normal signals. One can obtain an anomalous signal by increasing the total concentration or just by adding salt. In principle, it is possible that these are two different mechanisms resulting in the same effect. To clarify this point, we will carry out a detailed quantitative analysis of the TEB signals. The time constant of the field off decay of the birefringence is directly related to the rotational diffusion constant of the particle. In polydisperse samples, the calculated diameter of the disks shows a field strength dependency. The volume averaged mean diameter will be measured at infinitively high-field strength. A reasonable estimation for this value can be calculated by linear extrapolation in a E-2 - d plot.10 In Figure 5, the mean diameters are almost constant and there is no significant difference between the series with increasing salt and the series with increasing clay mineral concentration. The constant diameters imply that there is no appreciable aggregation of the clay particles and further thatsin the case of increasing clay mineral concentrationsthe particles relax

(1)

In Figure 6, three of the parameters do not show any difference between the samples. All points fall on a master curve independent from salt or clay concentration. The only parameter which does exhibit a concentration behavior is the amplitude of the second effect ∆n-. Taking these results, there is no indication of a real concentration dependency of the anomaly in a sense of interparticle interactions. Especially, the invariance of the time constants indicates that the anomaly is a one-particle process. The primary effect leading to an increase of anomaly with increasing clay concentration is not the clay but the simultaneously enhanced salt concentration. Clay minerals typically have ionic impurities of the order of 1 mmol/g. In the inversed charge hectorite, there is additional salt originating from the exchange process. Altogether, one can roughly estimate a salt content of about 2 mmol per gram inversed charge hectorite. This is also confirmed by conductivity measurements. Figure 7 summarizes the concentration and salt dependency of the anomaly. A reasonable quantity measuring the anomaly is the ratio between the two amplitudes ∆n- and ∆n+. By rescaling the x-axis by the factor mentioned above, the two series fall on the same curve. The shape of the curve resembles a square root dependency. This could be important while developing a mechanistic model explaining the anomaly. Estimation of Dipole Moments. The orientation behavior of colloidal particles is typically described by the interaction of dipoles with the electric field.19,20 Taking the measured field strength dependency of the stationary birefringence and a model assigning different dipoles to a particle, one can estimate dipole moments. When fitting a model to experimental values, one should always remember that the model describing the data reasonably well is not a proof for the validity of the model. The aim of this subsection is to get a rough estimation of dipole

Transient Electric Birefringence Anomaly

J. Phys. Chem. B, Vol. 106, No. 17, 2002 4415

Figure 8. Best fits of the field strength dependency of the stationary birefringence using the classical theory (µ * 0, solid lines) and the PD-SUSID theory (∆σ * 0, dotted lines); the points were weighted by E-1.

Figure 6. (i) Parameters τ+ and τ- and (ii) ∆n+ and ∆n- derived from the fitting procedure of eq 1 of the buildup of the TEB signal as a function of the field strength; concentration notation like in Figure 4.

Figure 7. Ratio of the parameters ∆n- and ∆n+ at constant field strength (0.75 kV/cm) and increasing clay mineral or salt concentration; the scale of the two x-axes was adjusted to represent approximately the same electric conductivity.

moments needed to exhibit the measured field strength dependency of the stationary birefringence. Such values are of great help while judging whether a mechanistic model is reasonable or just out of range. There are several different models assuming different dipole moments. We will use the PD-SUSID theory developed by Yamaoka et al.10 PD-SUSID stands for permanent dipole, saturable, unsaturable induced dipole. The PD-SUSID theory

is especially suited for disks and assumes three dipoles: an unsaturable induced dipole parallel to the disk surface (E∆R) and a saturable dipole (E∆σ for E e E0, E0∆σ for E g E0) and a permanent dipole (µ) both perpendicular to the disk surface. The quantities ∆σ and ∆R represent the saturable and unsaturable polarizability anisotropies. To reduce the number of fitting parameters, the value of ∆nEf∞/c was extrapolated for one concentration using the method proposed by reference 10 and then was used as a constant in all fitting procedures. By setting ∆σ to zero, the PD-SUSID theory reduces to the classical theory of Shah.19 Figure 8 shows exemplary fits of the field strength dependency of the stationary birefringence. The solid lines represent the classical theory with µ * 0. The dotted lines are the fits of the PD-SUSID theory with µ set to zero. In the case of the 1 g/l sample, the fit of the PD-SUSID theory is better than the classical theory. However, this is not surprising, as the PD-SUSID theory uses three free parameters and the classical theory uses only two. In principle, one would have to take polydispersity into account too. With additional parameters for polydispersity, it should be possible to obtain a fit of comparable quality with the classical theory. For this reason, we want to regard the values summarized in Table 2 as a rough estimation only, which gives an idea of the order of magnitude of dipole moments involved in the orientation process. Surface Properties. In previous work12 and also in the first subsection presenting the inversed charge hectorite, it was evident that the adsorption of nonionic substances leads to a disappearance of the anomaly. On the other hand, addition of salt promotes the anomaly. At this point, the question rises what is necessary and what sufficient. FC370 inversed charge hectorite shows a normal TEB signal. It could be possible that the salt concentration is just too low. However, this is not the case. As shown in Figure 9, the signal stays normal upon an addition of 10 mM NaCl. FC905 inversed charge hectorite in contrast exhibits such as the pure hectorite a disappearance of the anomaly through blockcopolymere adsorption (Figure 10). Obviously, both conditions are necessary: There has to be enough salt in the system but at the same time there must be suitable surface conditions. One can gain more insight into the nature of the dipole moments by using reversing pulse electric birefringence (RPEB). Figure 11 presents two RPEB signals of two modified hectorite

4416 J. Phys. Chem. B, Vol. 106, No. 17, 2002

Holzheu and Hoffmann

TABLE 2: Parameters of PD-SUSID Fitsa µ ) 0 E [kV ∆σ ) 0 0 ∆R[Fm2] cm-1] ∆σ[Fm2] E0‚∆σ[Asm] ∆R[Fm2] 0.2 g/l 0.4 g/l 0.7 g/l 1.0 g/l 1.3 g/l 1.6 g/l 2.0 g/l

2.5‚10-31

1.9‚10-31 1.6‚10-31 1.3‚10-31 1.3‚10-31 1.4‚10-31 1.4‚10-31

1 mM 2 mM 3 mM 4 mM 5 mM 6 mM 7 mM 8 mM 9 mM 10 mM

1.8‚10-31 1.7‚10-31 1.5‚10-31 1.5‚10-31 1.3‚10-31 1.4‚10-31 1.1‚10-31 1.2‚10-31 1.1‚10-31 1.0‚10-31

µ[Asm]

0.00 0.00 0.03 0.10 0.19 0.21 0.29

1.6‚10-31

0.00‚10-26

2.5‚10-31 1.8‚10-31 2.5‚10-31 2.6‚10-31 3.0‚10-31 3.0‚10-31 3.3‚10-31

0.07‚10-26 0.11‚10-26 2.5‚10-26 3.2‚10-26 3.8‚10-26 3.9‚10-26 4.3‚10-26

0.04 0.18 0.18 0.18 0.27 0.30 0.24 0.32 0.36 0.36

11‚10-31 4.1‚10-31 4.4‚10-31 5.0‚10-31 3.4‚10-31 3.5‚10-31 3.8‚10-31 3.3‚10-31 3.1‚10-31 2.9‚10-31

0.44‚10-26 0.73‚10-26 0.80‚10-26 0.90‚10-26 0.91‚10-26 1.1‚10-26 0.92‚10-26 1.0‚10-26 1.1‚10-26 1.0‚10-26

3.2‚10-31 3.5‚10-31 3.3‚10-31 3.4‚10-31 3.0‚10-31 3.1‚10-31 2.9‚10-31 2.9‚10-31 2.8‚10-31 2.6‚10-31

3.2‚10-26 3.9‚10-26 3.9‚10-26 4.2‚10-26 4.0‚10-26 4.1‚10-26 4.0‚10-26 4.0‚10-26 4.0‚10-26 3.9‚10-26

8.7‚10-31 10‚10-31 5.2‚10-31 4.1‚10-31 4.0‚10-31 3.8‚10-31

0.04‚10-26 0.33‚10-26 0.53‚10-26 0.76‚10-26 0.86‚10-26 1.1‚10-26

a The first four columns show parameters with µ ) 0; in the last two columns ∆σ was set to zero; for all fits ∆nEf∞/c was set to 6‚10-6 l/g hectorite.

Figure 9. TEB signals of hectorite (2 g/l) charge inversed through adsorption of FC370 (2 g/l) in the absence and presence of added NaCl; E ) 0.75 kV/cm.

Figure 10. TEB signals of hectorite (2 g/l) charge inversed through adsorption of FC905 (0.8 g/l) in the absence and presence of blockcopolymere F127; E ) 0.75 kV/cm.

Figure 11. Reversing pulse signals of hectorite/FC370 (2 g/l/2 g/l) and hectorite/F127 (2 g/l/2.4 g/l); E ) 0.75 kV/cm.

dispersions showing a normal signal in the single-pulse TEB experiment. The striking property of both signals is the deep minimum in the second pulse. This has to be attributed to a permanent-like dipole parallel to the surface. Hence, the

disappearance of the anomaly through the adsorption of nonionic substances is not due to the disappearance of the permanentlike dipole. The permanent-like dipole is still present, but it has changed its direction from perpendicular to parallel to the surface. Orientation Function. Measured ∆n(t) is described as a product of the particle number density 1N, the specific anisotropy factor of the particle ∆g, and the orientation function Φ:

∆n(t) ) 1N‚∆g‚Φ

(2)

In this description, only Φ is regarded as a function of time. Taking this interpretation, the positive beginning of the anomaly must be due to a parallel orientation and the negative stationary birefringence must represent a perpendicular orientation. However, in principle ∆g could be a function of time, too. To rule out this possibility one has to probe the orientation function independently of the specific anisotropy factor. One may think of scattering techniques. Unfortunately, these methods do not have the necessary time resolution. A suitable method is electric dichroism. Dichroism is directly related to the orientation function of the transition moments.23,24 Adsorbing some dyes on the hectorite surface and adjusting the other parameters in a way that the anomaly appears one should also see an anomaly in the dichroism signals. In the first row of Figure 12, the concentration of the sample is in a range of a normal TEB signal. Under these conditions, also the corresponding dichroism signals are normal. The different signs of the dichroism signals are a consequence of different wavelength. At 435 nm (second column), a transition moment perpendicular to the clay surface is dominant; at 545 nm (third column), the transition moment is parallel. Row number two shows a dispersion with an anomalous TEB signal. Now also the dichroism signals exhibit an anomaly. Summing up the experimental results, we can state that the anomaly is a one-particle process. It is composed by an initial orientation parallel to the electric field because of an induced dipole parallel to the clay surface and a perpendicular steadystate orientation through a permanent-like dipole perpendicular to the clay surface. Adding salt increases that permanent-like dipole in a square root dependency. Adsorbing nonionic substances changes the direction of the permanent-like dipole from perpendicular to parallel. The aim of the discussion is to give a mechanistic explanation of changes in dipole moments. Discussion Obviously, the ions play an important role in the appearance of the anomaly. This was also recognized by Szabo et al.21 when they introduced the ion-fluctuation theory to explain the anomalous TEB signals. The ion-fluctuation theory assumes temporary ion fluctuations in the diffusive layer leading to dipoles. However, one drawback of this theory is that it uses some ad-hoc assumptions and it is not clear why fluctuations perpendicular to the clay surface should be larger than parallel fluctuations. It is also difficult to explain the influence of nonionic substances. For these reasons, we will develop a new model adopting the idea of the ion-fluctuation theory. As adsorption does have an influence on the Stern layer but hardly on the diffusive layer, it is reasonable to have a closer look at the processes of the Stern layer. A clay mineral platelet possesses two Stern layers divided by the thickness of the clay particle of about 10 Å. In each Stern layer, there is some exchange of counterions with the adjacent diffusive layer, but the exchange between the two Stern layers is blocked by the

Transient Electric Birefringence Anomaly

J. Phys. Chem. B, Vol. 106, No. 17, 2002 4417

Figure 12. TEB and corresponding electric dichroism signals of aqueous hectorite/basic-violet dispersions; first column TEB signals, second column dichroism signals at 435 nm, third column dichroism signals at 545 nm; first row 0.1 g/l hectorite and 30 µM basic violet; second row 1 g/l hectorite and 100 µM basic violet.

Figure 13. (i) Schematic idea of exchange between Stern layer and diffusive layer leading to different numbers of condensed counterions (es1, es2) on the two sides of the disk. (ii) Model geometry for dipole calculation.

clay particle. On the average, the number (e) of ions in each Stern layer is equal. Temporarily, however, there will be fluctuations around this mean value. To estimate the absolute quantity of the fluctuations, we will apply a simple statistical model. The setup of the model is shown in Figure 13. Each counterion has a certain probability p to be condensed. This probability is determined by the number of condensed (econd) divided by the total number of ions being in a sphere (Veff) around the particle. The radius (r ) (Dτ)0.5) of the sphere is set to the distance a ion can diffuse during the rotational relaxation of the particle (τ ) 2/9 ηd3/kT). The number of condensed ions is set to 90% of the exchange capacity. Taking these settings, the probability p can be calculated from known concentrations of the sample.

p)

econd 0.9‚cecdisc ) efree + econd (csalt‚Veff‚NA) + cecdisc

(3)

cecdisc denotes the cation exchange capacity of a single disk expressed in elementary charges. To calculate the distribution of differences between the Stern layers, we applied a monte carlo procedure. Therefore, we produced two arrays of random numbers with the length equal to the number of ions in each

Figure 14. Calculated distributions of differences ∆e between the two sides of a disk for 0 and 10 mM salt; diameter ) 100 nm, clay concentration ) 0.05 g/l, number of samples ) 10000.

hemisphere. The number of condensed ions is just the number of random numbers below the probability p. The distribution was obtained by processing 10000 particles. Figure 14 presents two distributions calculated for different salt concentrations. Adding salt broadens the distribution and the mean value increases from 11 to 49 ions. It is interesting to compare the calculated values with experimental ones. For this reason, we refer to Table 2. The fitting of the PD-SUSID theory gave estimations on dipole moments (E0‚∆σ). These values can be converted into ∆e values using the elementary charge and the distance between the Stern layers of 10 Å. The maximum value 1.1‚10-26 Asm corresponds to a ∆e of 69. This is in the same order of magnitude as the rough estimation of the model. Hence, the model is capable to describe the origin of the permanent-like dipole moment. However, at this stage we are still not able to understand the changes induced through adsorption of nonionic substances. The model is somehow zero dimensional. It assumes a uniform distribution of ions in each Stern layer. As long as the clay surface is smooth, this assumption is probably reasonable as the ions try to minimize free energy by having the maximum

4418 J. Phys. Chem. B, Vol. 106, No. 17, 2002

Holzheu and Hoffmann In the discussion, we presented a simple mechanistic model which is able to explain qualitatively the experimental observations. In the model, the permanent-like dipole originates from ion fluctuations leading to differences in the number of condensed counterions on both sides of the clay mineral. As long as the counterions possess a sufficient mobility in the Stern layer, mainly the perpendicular component of the dipole remains. If an adsorption layer hinders the mobility of the counterions beneath the perpendicular component, a strong parallel component remains. As a promising outlook, TEB measurements may offer a simple method to probe the microenvironment of counterions and by the way get valuable information on the properties of surfaces. The next step should be to refine the statistical model incorporating electric potentials and surface diffusion coefficients of the counterions. Acknowledgment. We want to thank the Deutsch Forschungsgesellschaft (DFG) for the financial support of this work. References and Notes

Figure 15. Schematic difference between clay minerals with and without an adsorption layer; first picture: local in/out diffusion of a counterion; second picture: intermediate occurrence of a dipole moment with a strong longitudinal component; last picture: readjustment of surface charges, longitudinal component disappeared. Clay minerals with adsorption layer stay trapped in the intermediate state.

distance from each other. When an ion moves in or out, the other ions react on these changes by moving a little on the surface (Figure 15 upper half). However, in a situation when an adsorption layer hinders these movements, a strong parallel component of the permanent dipole arises which may overcome the perpendicular component. The experimental evidence is a minimum in the TEB signal in the reverse pulse. Conclusions A special inversed charge hectorite has been used to study the TEB anomaly. The great advantage of this material is its insensitivity to salt-induced aggregation. A detailed study of the concentration and ionic strength dependency of the TEB anomaly revealed that there is no indication of a multiparticle mechanism. The increase of the second effect is an effect of the mostly inevitable increase of ion concentration with increasing solid concentration. However, salt is not the only precondition for the anomaly. At the same time, there must be suitable surface conditions. The disappearance of the anomaly upon adsorption of nonionic substances is also observable for the inversed charge hectorite. Reversing pulse electric birefringence detected that the disappearance is not due to a vanishing permanent-like dipole but a change in direction of this dipole from perpendicular to parallel to the clay mineral surface. Using electric dichroism of dye-coated hectorite, it could be nonambiguously shown that the anomaly is a real change in particle orientation and is not due to some unexpected changes in the specific anisotropy factor.

(1) Kahn, A.; Lewis, D. R. J. Phys. Chem. 1954, 58, 801-804. (2) Thurston, G. B.; Bowling, D. I. J. Colloid Interface Sci. 1969, 30, 34-45. (3) O’Konski, C. T.; Krause, S. Electric Birefringence and Relaxation in Solutions of Rigid Macromolecules. C. T. O’Konski Molecular Electrooptics: Part 1 - Theory and Methods; Marcel Dekker: New York, Basel, 1976; Chapter 3, pp 63-120. (4) Fredericq, E.; Houssier, C. Electric Dichroism and Electric Birefringence; Claredon Press: Oxford, 1973. (5) Hoffmann, H.; Kra¨mer, U. Electric Birefringence Measurements in Micellar and Colloidal Solutions. The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Netherlands, 1990; pp 385-396. (6) Peikov, V.; Sasai, R.; Stoylov, S. P.; Yamaoka, K. J. Colloid Interface Sci. 1998, 197, 78-87. (7) Yamagishi, A.; Soma, M. J. Phys. Chem. 1981, 85, 3090. (8) Shah, M. J.; Thomson, Cd. C.; Hart, C. M. J. Phys. Chem. 1963, 67, 1170-1178. (9) Yamaoka, K.; Tanigawa, M.; Sasai, R. J. Chem. Phys. 1994, 101, 1625. (10) Sasai, R.; Yamaoka, K. J. Phys. Chem. 1995, 99, 17754-17762. (11) Yamaoka, K.; Sasai, R. J. Colloid Interface Sci. 1999, 209, 408420. (12) Holzheu, S.; Hoffmann, H. Prog. Colloid Polym. Sci. 2000, 115, 265-269. (13) Hecht, E.; Hoffmann, H. Tenside Surf. Det. 1998, 35, 185-199. (14) Schorr, W.; Hoffmann, H. J. Phys. Chem. 1981, 85, 3160-3167. (15) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249-1255. (16) Schlenoff, J. B.; Li, M. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 943-947. (17) Sukhorukov, G. B.; Mohwald, H.; Decher, G. M.; Lvov, Y. Thin Solid Films 1996, 285, 220-223. (18) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (19) Shah, M. J. J. Phys. Chem. 1963, 67, 2215. (20) Konski, C. T. O.; Yoshioka, K.; Orttung, W. H. J. Phys. Chem. 1959, 63, 1558. (21) Szabo, A.; Haleem, M.; Eden, D. J. Chem. Phys. 1986, 85, 74737479. (22) Yamaguchi, Y.; Hoffmann, H. Colloids Surf., A 1997, 121, 6780. (23) Yamaoka, K.; Sasai, R.; Takata, N. Colloids Surf., A 2000, 175, 23-39. (24) Yamaoka, K.; Sasai, R. J. Colloid Interface Sci. 2000, 225, 8293.