Rheological behavior of dilute imogolite suspensions

Colloids and Surfaces A: Physicochem. Eng. Aspects 435 (2013) 109–114

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Rheological behavior of dilute imogolite suspensions Y. Tsujimoto, A. Yoshida, M. Kobayashi ∗ , Y. Adachi Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8572, Japan

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 We analyzed the rheology of imogolite suspensions based on colloidal stability.  Shear thinning at dispersed state can be attributed to the alignment of the imogolite.  Imogolite flocs are repeatedly subjected to breakup and re-coagulation.  The coagulated imogolite suspensions have weak yield stress.

a r t i c l e

i n f o

Article history: Received 12 October 2012 Received in revised form 13 December 2012 Accepted 14 December 2012 Available online 5 January 2013 Keywords: Imogolite Rheology Yield stress

a b s t r a c t Imogolite is one of the most important clay minerals contained in volcanic ash soil. The morphology of imogolite is very unique and comprises thin fibrous tubes with inside and outside diameters of approximately 1 and 2 nm, respectively. The chemical structure of imogolite features SiOH groups along the inner walls of the tubes and AlOH groups on the outer surfaces of the tubes. As a result of this morphology and the chemical structure, imogolite coagulates at high pH. In the present study, we focus on the detailed differences of the non-Newtonian behavior of the dilute imogolite suspension in the dispersed and coagulated states. Experiments on the viscosity as a function of volume fraction were performed using a concentric cylindrical viscometer. The applied shear rate was increased from 6.45 to 258 s−1 and then decreased to the initial value; this procedure was repeated three times. The flow curves for the dispersed imogolite suspensions showed slight hysteresis. Additionally, shear thinning, which is a decrease of the viscosity with increasing shear rate, emerged; this is thought to be due to the change of the orientation distribution of the imogolite particles. For the coagulated suspensions, the area of the hysteresis loop increased with increasing volume fraction. The flow curves tended to shift upwards and became constant as the shearing cycles increased. Moreover, shear thinning at high pH was more significant than that at low pH. The yield stress of the coagulated state was determined from the intercept of the flow curve. We propose that imogolite at high pH forms flocs that are repeatedly subjected to breakup and re-coagulation by the given shear stress; thus, a stronger structure is formed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Imogolite is one of the most important clay minerals contained in volcanic ash soil. The morphology of imogolite is very unique and comprises thin fibrous tubes with inside and outside diameters of approximately 1 and 2 nm, respectively [1]. Aomine explained

∗ Corresponding author. Tel.: +81 29 853 4861; fax: +81 29 853 4861. E-mail address: [email protected] (M. Kobayashi). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.12.041

the structure of imogolite particles on the basis of X-ray diffraction analysis. The results showed that imogolite is intermediate between an amorphous-like allophane and a crystalline clay mineral [2,3]. Kajiwara and co-workers have investigated that the liquid crystal structure of acidic imogolite suspension as an ideal lyotropic system. They have revealed that the pleated sheets consist of a nematic organization of imogolite tubes on the basis of the observation by the polarized light and the electron microscopy for freeze-dried samples. They have also demonstrated the phase transition from isotropic to anisotropic of acidic imogolite sample

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and revealed that the polydispersity of rod lengths can shift the phase boundary concentration lower than that expected from theory [4–6]. The chemical structure demonstrates that SiOH groups are located along the inner walls of the tubes while AlOH groups coat the outer surfaces of the tubes. Gustafsson have inspected the structural charge behavior of imogolite by using a basic Stern model approach [7]. Also, on the basis of the electrophoresis measurements, it was argued that the charge properties of imogolite depend on the deprotonation and protonation of AlOH and SiOH; the electrophoresis of imogolite was zero above pH 9 or 10 regardless of SiOH was negatively charged [8]. From titration and electrophoresis measurements, Tsuchida et al. proposed that the counter-ions of SiO− contribute to the net surface charge while those of AlOH2 + influence the electrokinetic phenomena of the imogolite suspension [9]. As a result of its morphology and chemical structure, imogolite coagulates at high pH [10,11]. Karube et al. investigated the flocculation–dispersion properties of imogolite as a fundamental clay property based on changes in the relative turbidity and relative viscosity [11]. They revealed that the change of the viscosity of imogolite with pH corresponds to a transition from the dispersed state to the coagulated state. Thus, they showed that the micro colloidal properties affect the macroscopic properties. Although the rheology of imogolite suspensions is a relatively undeveloped discipline, numerous studies have been investigated for rod-like or spheroid particle suspensions. In the dilute limit, volume fraction  much less than 1, the effective viscosity of suspensions with rigid, uncharged, and non-interacting spheres is assumed to obey the Einstein relation r =

s = 1 + 2.5 0

(1)

where r , s , and 0 denote the relative viscosity, viscosity of the suspension, and viscosity of the solvent, respectively. The so-called intrinsic viscosity [] is given by []

r − 1 lim →0 

(2)

Using Eq. (2), we can rewrite Eq. (1) as follows; r = 1 + []

(3)

It can be formally equation for the suspensions of anisometric particles. The intrinsic viscosity depends on the aspect ratio of the dispersed spheroid or rod-like particles [12–21]. Therefore, the form of the suspended particles can be estimated from the intrinsic viscosity. With this theory and the viscosity of imogolite suspensions, Egashira inspected the structural units of these clay minerals in an aqueous system and reported that the imogolite structural unit is a suspended tube with a diameter of several hundred angstroms [22]. His result agreed with that suggested by Gustafsson [7]. Since Egashira did not measure the viscosity as a function of pH, the change in the intrinsic viscosity with flocculation–dispersion state was not elucidated. To analyze the effects of shear deformation and particle orientation on the applied shear stress, it is desirable to determine the flow curve, which shows the relationship between shear rate and shear stress. Wells et al. studied the flow properties of the concentrated imogolite gel under shear. They revealed that the gel structure distorted and ruptured via shearing and that imogolite gels behave similar to pseudoplastic materials, which undergo irreversible association/breakdown [23]. Based on shear response data, their results showed that the pH influences gel formation. The previous rheological measurements of the imogolite suspensions were used to estimate flocculation-dispersion state or the variation of imogolite particle/gel structure. However, there have been few reports on the investigations for the difference

of flow properties between the dispersed and coagulated state, and the analysis based on both flow properties and the structural change. Though the flow properties of the coagulated imogolite suspensions is considered to be obviously different from that of the dispersed suspensions, such as the presence of yield stress, data for coagulated suspensions at high pH is lacking. In the present study, we performed rheological measurements as a function of volume fraction and shear rate for the imogolite suspensions at different pH values. By systematically organizing the information obtained from the flow curves and the intrinsic viscosities, we determined the variation in the rheological behavior in terms of the colloidal stability. 2. Material and methods 2.1. Preparation of materials The imogolite sample was hand-separated from the surface of pumice beds at Kitakami in Iwate Prefecture, Japan. We purified this sample according to the method by Shimura et al. [24]. This clay formed a gel-like coating, which was dispersed by crushing with a colloid-mill in aqueous media. To remove the organic matter, these clay samples were treated with 30% H2 O2 and left for more than two days. The suspensions were then dispersed in 10−3 M HCl solution and flocculated by adding 10−3 M NaOH solution to collect the imogolite. The surface ions were replaced by sodium and chloride ions by dispersing the clay particles into a 4.0 M NaCl solution at a 2:1 ratio. Salt-free stock dispersions were prepared by repeatedly dialyzing against distilled water until the electric conductivity reached 10 ␮S/cm. After these treatments, the sample was freezedried and stored in glass bottles. The imogolite samples were yellow as a result of contained iron. TEM picture of acidic imogolite samples is shown in Fig. 1. This picture shows that imogolite formed cross-linked network and purified imogolite suspensions contained small amounts of allophane. 2.2. Measurements Viscometric measurements were performed using a concentric cylindrical viscometer (DV-II Pro., Brooke Field Ltd.) with a gap of 0.71 mm between the inner bob and outer cup. This instrument controls the shear rate and measures the resulting shear stress (or torque). The viscosity is determined using the constitutive equation, as follows: () =

 

(4)

Fig. 1. TEM picture of imoglite at pH 5.23.

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Fig. 2. The flow curve of imogolite suspensions (a)HCl 10−3 M (b) NaOH 10−3 M.

where , , and  indicate viscosity (Pa s), shear stress (Pa), and shear rate (s−1 ), respectively. The fluid is Newtonian when the viscosity is constant. The applied shear rate increased from 6.45 to 258 s−1 and then decreased to the initial value. This procedure was repeated three times. The volume fraction of clay particles was adjusted from 1.48 to 0.18%. To compare the rheological behavior of the dispersed and coagulated states, imogolite samples were dispersed in either 10−3 M HCl or 10−3 M NaOH solutions. The pH values of the resulting suspensions were 4.7 ± 0.8 and 9.0 ± 0.6, respectively. According to the electrophoresis data measured by Shimura et al., both imogolite suspensions can be positively charged [24]. Nevertheless, the resultant imogolite suspensions were considered to be dispersed and coagulated, respectively because the studies of Horikawa and Karube et al. confirmed that imogolite coagulates above pH 6–7 regardless of the existence of iron [8,11,25]. All experiments were performed in an air-conditioned room at a temperature of 20.0 ◦ C. 3. Results The flow curves of imogolite suspensions with different volume fractions are shown in Fig. 2. The flow curves for the 10−3 M HCl solution showed slight hysteresis, and the hysteresis loop shifted upward with successive cycles for volume fractions above 0.66%. The flow curve of the coagulated state showed significant hysteresis, and the hysteresis loop shifted upwards with each successive cycle (Fig. 3), which differs from the tendency reported by Wells et al. [23]. The areas of each hysteresis loop tended to remain constant with each successive cycle. Moreover, it is peculiar that the flow curves of the coagulated suspension have intercepts that indicate the yield stress. We calculated the viscosities using Eq. (4) and plotted them against the shear rate in Fig. 4. The magnitude of the relative viscosity at high pH was greater than that at low pH. For the high volume fraction at high pH, shear thinning, which is a decrease of the viscosity with increasing shear rate, emerged. Thus, even at low volume fractions, imogolite suspensions show non-Newtonian flow behavior. As shown in Fig. 4, the viscosity increased with increasing volume fraction. We plotted the relative viscosities against the volume fractions (Fig. 5), which revealed that the relative viscosity of the coagulated state is higher than that of the dispersed state. Also, to compare the

Fig. 3. The flow curves for 0.84% suspension at NaOH 10−3 M.

differences via shear rate, the intrinsic viscosities at 258 and 24.8 s−1 are shown as the higher and lower shear rates, respectively. Each intrinsic viscosity is listed in Table 1. Regardless of the pH and shear rate, the intrinsic viscosities are much higher than 2.5, which is a result of the influence of the particle shape. Also our obtained value of intrinsic viscosity is similar to those reported by Egashira.

Table 1 The data of the intrinsic viscosity, aspect ratio and the increasing rate. Shear rate (/s) Intrinsic viscosity, [](−) in 10−3 M HCl Aspect ratio, f (−) ˛ (−)

258 1192 149.6 2.27

24.8 2406 220.4 4.41

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Fig. 4. The relative viscosity against shear rate (a)HCl 10−3 M (b) NaOH 10−3 M.

imogolite suspensions correspond to 0.039, phase transition can occur even though it is outside of gelation. Shear thinning at dispersed state has been demonstrated for various kinds of non-spherical particle [26–29]. The phenomenon is induced by the change of the oriental distribution resulted from that the balance between randomization caused by rotational Brownian motion and flow-induced anisotropy shift with shear rate. This balance can be expressed by the rotational Peclet number as follows; Per =

˙ Dr

(5)

Dr is the rotary diffusivity, which were given by Brenner [18]. In the case of the rod-like particle, Dr is given by Dr = 3kT

Fig. 5. The relative viscosity against volume fraction of imogolite suspensions (: 10−3 M NaOH at 24.8/s, : 10−3 M NaOH at 258/s, :10−3 M HCl at 24.8/s, 䊉:10−3 M HCl at 258/s, ----:Einstein equation).

4. Discussion 4.1. Dispersed state In the dispersed state, the flow curves showed slight hysteresis and shear thinning. Wells et al. have reported the flow curve of imogolite gel have the hysteresis loop regardless of pH. Also, Kajiwara et al. have demonstrated that acidic imogolite suspension showed thixotropic gel-like property if its concentration is above the boundary concentration corresponding to the anisotropic regime. Therefore, the hysteresis phenomenon at dispersed state was induced by gelation. The area of the hysteresis loop was smaller than that reported by Wells for imogolite suspensions at pH 3; this is because the volume fraction was sufficiently low for complete gelation. According to the investigation by Kajiwara et al., the phase boundary concentration from isotropic to isotropic and anisotropic is corresponding to weight fraction 0.01 ∼ 0.035 depending on the aspect ratio. As the highest concentration of our

lnf − 0.8 0 L3

(6)

where f indicates the aspect ratio which is the ratio of the major axis, L, to the minor axis. Boltzmann constant k = 1.38 × 10−23 J/K and absolute temperature T, respectively. Dhont et al. have shown the shear thinning of rod-like particles suspensions as a function of rotational Peclet number Per in which the rotary diffusivity Dr is used [26] Also, Leal and Hinch have calculated the oriental distribution and shear viscosity for the regime Per » f3 and 1 « Per « f3 [15,17]. As mentioned above, the intrinsic viscosity can be used to obtain information regarding the aspect ratio. Using Eq. (3), we calculated the intrinsic viscosities from the slope of the plots in Fig. 5. We calculated the aspect ratio according to the Onsager equation, as the previous study showed that the relation between the intrinsic viscosity and the aspect ratio of imogolite particle agreed with his theory [20,30]. [] =

4 f2 15 lnf

(7)

We listed the values of aspect ratio in Table 1. According to the Eq. (7) with the assumption that imogolite’s diameter is more than several dozen angstrom, the imogolite particles is regarded as an ellipsoid with several ␮m length. Moreover, using Eq. (5) and Eq. (6), the given rotation Peclet number is about 3.5 < Per < 15.6. From the Dhont’s article, we found that the shear thinning occurs at rotational Peclet number between 0.5 and 100. Though the regime of Per in our investigation is narrow, shear thinning can be induced by the change of the orientation distribution with shear rate.

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4.2. Coagulated state We compared the differences in the rheological behavior of suspensions in dispersed and coagulated states. The hysteresis loop is significant for the coagulated state, which indicates that an irreversible change occurred for the imogolite flocs in the process of increasing and decreasing the shear rate. For the coagulated suspensions, the particle structure can be changed by shear. Moreover, this loop shifted upwards with successive cycles, which means that the viscosity increases. As shown in Fig. 3, the changes eventually stabilize. Ordinarily, the flow curves of the coagulated suspensions tend to shift downwards with successive time because the flocs are broken by shear, however, imogolite showed the opposite tendency; that is stronger structure imogolite can form, more cycling time proceed [23]. Meanwhile, it has been reported that weak shear induced flow aggregation in carbon nanotube suspensions [31]. Thus, the hysteresis and the upward shifts of the flow curves could be explained by picturing the occurrences in the suspension: imogolite particles formed much stronger flocs due to weak shear and, which were repeated with successive cycles. Moreover, the shear thinning at high pH that is evident from Fig. 4 suggests that the decreasing viscosity with increased shear rate resulted from breakup of the imogolite floc. Thus, the hysteresis and shear thinning could be explained by picturing the occurrences in the suspension: Once the imogolite is broken with increasing shear rate, imogolite flocs are repeatedly subjected to breakup and coagulation due to the shear stress and then, a stronger structure is formed. The relative viscosity of the coagulated state is larger than that of the dispersed state. This is because the effective volume fraction increases by retaining immobile water in the flocs, as demonstrated by the following equation: r = 1 + [](˛)

(8)

where ˛ is the increasing rate of the effective volume fraction. Accordingly, we calculated the value of ˛ (Table 1) for the coagulated imogolite suspensions using the values of the intrinsic viscosity at low pH. This value at low shear rate is higher than that at high shear rate, which means that volume fraction at low shear rate increased due to coagulation. This tendency is agreed with our observation based on the flow curve obtained from our experiments. From the TEM pictures at coagulated state, the bundle of imogolite tube becomes thicker [25]. For imogolite tubes, it is difficult to estimate whether imogolite takes immobile water in its flocs or not. Also, it is not always true that the section of the imogolite bundle form fractal structure because imogolite is not a rigid particle. Therefore, we considered that the increment of effective volume fraction was induced by complicated arrangement among imogolite tubes resulting in the increment of the size of imogolite flocs. Further analytical development on the structure of imogolite flocs is necessary to elucidate this discussion. To analyze the rheological behavior of the coagulated suspension, we assessed the intercepts of the flow curves to determine the yield stress, which is defined as the critical shear stress when flow begins. This is an important factor that is related to the strength of the imogolite flocs. Despite the formation of the cross-linked network in both the dispersed and coagulated states, only coagulation resulted in increased strength. We plotted the yield stress against the volume fraction (Fig. 6). The yield stress was determined via extrapolation of the linear part of the flow curves back to a zero shear rate. The dependence of yield stress on the volume fraction can be varied by clay nature or particle size, for instance, it was reported that the yield stress of montmorillonite suspensions is proportional to the 7th power of the volume fraction [32]. In this case, dilute clay suspensions that are below the gelation point have weak yield stress that is slightly dependent on the volume fraction.

Fig. 6. The yield stress of imogolite suspensions.

5. Conclusions In the present study, we performed a detailed investigation of the non-Newtonian behavior of dilute imogolite suspensions in the dispersed and coagulated states. Also, we investigated each macro property on the basis of the information about the micro-scopic change of structure or phase obtained by the previous works. The flow curves for imogolite suspensions at low pH showed slight hysteresis. Additionally, shear thinning emerged, which is due to the change of the orientation distribution of the imogolite particles. While the coagulated imogolite suspensions showed the apparent hysteresis loop, whose area increased with increasing volume fraction. The flow curves tended to shift upwards and become constant as the number of shearing cycles increased. We consider that imogolite forms flocs at high pH, which are repeatedly subjected to breakup and re-coagulation by the given shear stress. Thus, a stronger structure is formed. The clear differences between the rheological behavior of the dispersed and coagulated states was expressed by the appearance of the yield stress. Acknowledgement This study is financially supported by KAKENHI (22248025, 23688027, and 241583) from Japan Society for the Promotion of Science. References [1] P. Cradwick, V. Farmer, J. Russell, C.R. Masson, K. Wada, N. Yoshinaga, Imogolite, a hydrated aluminium silicate of tubular structure, Nat. Phys. Sci. 240 (1972) 187–189. [2] N. Yoshinaga, S. Aomine, Allophane in some ando soils, Soil Sci. Plant Nutr. 8 (1962) 6–13. [3] N. Yoshinaga, S. Aomine, Imogolite in some ando soils, Soil Sci. Plant Nutr. 8 (1962) 22–29. [4] K. Kajiwara, N. Donkai, Y. Hiragi, H. Inagaki, Lyotropic mesophase of imogolite, 1. Effect of polydispersity on phase diagram, Die Makromol. 187 (1986) 2883–2893. [5] K. Kajiwara, N. Donkai, Y. Fujiyoshi, H. Inagaki, Lyotropic mesophase of imogolite, 2. Microscopic observation of imogolite mesophase, Makromol. Chem. 187 (1986) 2895–2907. [6] N. Donkai, H. Hoshino, K. Kajiwara, T. Miyamoto, Lyotropic mesophase of imogolite, 3. Observation of liquid crystal structure by scanning electron and novel polarized optical microscopy, Makromol. Chem. 194 (1993) 559–580. [7] J. Gustafsson, The surface chemistry of imogolite, Clays Clay Miner. 49 (2001) 73–80. [8] Y. Horkawa, Electrokinetic phenomena of aqueous suspensions of allophane and imogolite, Clay Sci. 4 (1975) 255–263.

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