Effects of humic acid release from sepiolite on the interfacial and rheological properties of alkaline dispersions

Applied Clay Science 102 (2014) 1–7

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Research paper

Effects of humic acid release from sepiolite on the interfacial and rheological properties of alkaline dispersions B. Benli ⁎ Mineral Processing Engineering Department, Istanbul Technical University, Istanbul 34469, Turkey

a r t i c l e

i n f o

Article history: Received 17 July 2013 Received in revised form 30 September 2014 Accepted 10 October 2014 Available online xxxx Keywords: Alkaline dispersions Humic acid Rheology Sepiolite Surface energy components

a b s t r a c t Humic acid (HA) is a major extractable and soluble component of soil organic matter that can play an important role in the stability of environmental colloidal dispersions. This study suggests that HA content of sepiolite is one of the essential reasons for non-Newtonian behaviour of sepiolite dispersions in alkaline media. Releases of HA from sepiolite matrix dramatically changed their rheological properties (thixotropy, apparent viscosity and yield stress), and interestingly, the level of sepiolite surface hydrophobicity. Alkaline environments also increased the release of Mg2+ ions from the sepiolite matrix and, thus, caused to a stable complex formation with HA macromolecules. This colloidal solid phase was directly observed and imaged by atomic force microscopy (AFM) which revealed a spherical shape of complexes between Mg2+ ions and HA in the size range of 18–63 ± 8 nm at pH 11.5. Alkaline environments also led to significant changes in the surface properties of sepiolite; after HA release, acid/ + base components of the surface free energy of sepiolite were determined as γ− S /γS = 1.49 and defined as having a highly hydrophobic character like natural hydrophobic minerals, e.g. talc. The major soil organic matter component, HA, was also found to be responsible for the changes in the sepiolite surface hydrophilicity. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Sepiolite (Sep) is one of the most important industrial magnesiumrich 2:1 clay minerals, and it differs from other layer silicates because of the lack of continuous octahedral sheets (Brigatti et al., 2013). Discontinuous octahedral sheets extend only in one direction and tetrahedral sheets divided into ribbons form a chain-layer molecular structure with 3.6 × 10.6 Å of very large open channels or tunnels and 320 m2/g of corresponding high surface areas (Sabah et al., 2002). A unique fibre structure exactly determines the hydrophobicity and anisotropic characteristics in half-cell formula Mg4Si6O15(OH)2.6H2O. Electron acceptor and donor components of Sep fibres were previously − calculated as γ+ S = 0.149 and γS = 20.09 (Benli et al., 2012). Sep was also identified as either hydrophobic or hydrophilic, and then it was found that hydrophilic repulsion is dominant according to van Oss's (1994) classification. However, computational molecular simulation results of the same study clearly showed that Sep basal surfaces, even talc-like surfaces, are hydrophobic (Benli et al., 2012). The results were also compared with contact angle measurements via the capillary rise technique. The implications behind hydrophobic material are not enough to explain why the electron donor component of Sep surfaces is much greater than its electron acceptor component. A possible explanation for the larger γ− S value of Sep could be attributed to ⁎ Tel.: +90 212 285 6267. E-mail address: [email protected].

http://dx.doi.org/10.1016/j.clay.2014.10.004 0169-1317/© 2014 Elsevier B.V. All rights reserved.

extractable components such as soil organic matter because HA have a large number of free and bound phenolic –OH and carboxylic –COOH groups arising from the natural environment and groundwater (Arnarson and Keil, 2000; Wall and Choppin, 2003; Tombacz et al., 2004; Feng et al., 2005). Soil organic matter adsorption on clay mineral surfaces and their influence on the solution chemistry (e.g. ionic strength, pH, and soluble cations) have been extensively explored (Arnarson and Keil, 2000; Feng et al., 2005). However, direct measurements focusing on the relationships between viscosity and HA content are scarce; only a study on the effect of grinding on the rheology of Sep dispersions in acid and alkaline media (Çınar et al., 2009) and a few observations on the macromolecular structures of HA (Caceci and Billon, 1990; Pokorna et al., 2001; Zhao et al., 2013) are available. Çınar et al. (2009) showed that Sep dispersions exhibit viscosities practically close to zero at pH 12. This kind of change in viscosity was ascribed to the disruption of the particle network. On the other hand, Pokorna et al. (2001) suggested that carboxylic groups in HA with acidic pH are largely nonionic (R-COOH), whereas with alkaline pH carboxylic R-COO− groups are more dominant. Various groups ionised in HA and relatively more negative charges could lead to more hydrophilic Sep surfaces. Such interesting behaviour led us to investigate the role of HA in the flow behaviour of Sep with alkaline and highly alkaline pH values. The aim of this study was therefore to show if HA content in the Sep matrix can modify the rheological and interfacial properties of

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B. Benli / Applied Clay Science 102 (2014) 1–7

Table 1 Mineralogical analysis of natural sepiolite samples. Samples

SiO2 mass%

MgO mass%

Sepiolite content %

Mineral

Formula

SepTT

49.85

20.15

85

SepKS

52.66

21.76

60

SepSG

51.84

20.06

50

Sepiolite Dolomite Albite Quartz Calcite Sepiolite Quartz Dolomite Calcite Albite Sepiolite Dolomite Albite Quartz Calcite Montmorillonite

Mg4 Si6 O15 (OH)2⋅6H2O CaMg (CO3)2 (Na, Ca) Al (Si, Al)3O8 SiO2 CaCO3 Mg4 Si6 O15 (OH)2⋅6H2O SiO2 CaMg (CO3)2 CaCO3 (Na, Ca) Al (Si, Al)3O8 Mg4 Si6 O15 (OH)2⋅6H2O CaMg (CO3)2 (Na, Ca) Al (Si, Al)3O8 SiO2 CaCO3 Na0.3(Al, Mg)2Si4O10(OH)2⋅4H2O

Sep dispersions in highly alkaline media. The understanding of the mechanisms controlling clay mineral response to pH changes is important for evaluating the rheological and interfacial behaviour of the surrounding media: in this case, controlling the viscosity of Sep dispersions depending on the extent of hydrophobicity or wettability in industrial applications.

2. Materials and methods 2.1. Materials All Sep samples were obtained from the AEM Company in the Sivrihisar region of Turkey and labelled SepTT, SepKS and SepSG for Turktaciri, Kurtşeyhi and Sıgırcık, respectively. Quantitative chemical analyses were carried out by ICP (inductively coupled plasma) spectrophotometry in the ACME Analytical Lab., Canada. Table 1 represents the main constituents of chemical analysis and mineralogical characterization of the Sep samples with a Shimadzu XRD-6000 (Shimadzu Corp., Tokyo, Japan) equipped with a Cu X-ray tube (λ = 1.5405 Å). Analytical grade sodium salt of humic acid (HA) was purchased from SigmaAldrich Inc., MO, USA. The stock solution of HA was prepared by dissolving 25 mg of dry HA in 1000 mL of deionised water. After stirring overnight, the prepared solution was then pre-filtered through 0.22 μm nitrocellulose filter membranes (Millipore Corp., MA, USA) under the applied pressure of an N2 gas cylinder. Systematic studies on pH were enabled with the addition of dilute HCl and 0.01 M KOH. Bi-distilled water was also used throughout the study.

2.3. Preparation of alkaline modified sepiolite Sep dispersions (5 mass%) were shaken for 30 min under alkaline conditions (pH 8.5–11.5) and centrifuged at 6000×g for 2 min. The supernatant was then filtered using with 0.45 μm syringe membrane filters (Millipore, Corp., MA, USA) and analysed according to method 5910B (Eaton and Franson, 2005) by a UV-visible spectrophotometer (UV-160A, Shimadzu, Japan) at 254 nm ultraviolet wavelength (UV254). UV254 also known as a water quality test parameter, which utilizes light at the 254 nm wavelength to be able to detect organic matter in water and wastewater. This is due to the fact that most organic compounds absorb light at the UV wavelength 254 nm. Mohammadi et al. (2012) shows that there is a significant linear relationship between humic acid concentration and UV254 absorbance. Hence, the corresponding UV254 absorbance values can be taken as an indication of the level of HA concentration. When UV254 increases, the humic acid content increases. Changes in the level of free Mg2+ ions within the supernatant were directly measured by an Atomic Absorption Spectrophotometer (Varian AA240FS, Agilent Technologies, CA, USA). After phase-separation of supernatant in centrifugation, alkaline modified Sep samples were washed several times with fresh bi-distilled water, re-centrifuged and decanted until their pH reached to the initial pH; finally, all modified samples were dried at 60 °C overnight. These samples were also used for HA loading studies. After 50 ppm of HA was added to the 5 mass% of Sep dispersion, HA loaded samples were shaken for 24 h under natural pH. For the mechanical activation of HA loaded samples, they were re-agitated for 2 min at 20,000×g with an Arcelik brand mixer. 2.4. Contact angle measurements and surface free energy determinations

2.2. Rheological measurements First, 5 mass% of Sep dispersion was stirred for 15 min via a magnetic stirrer at the desired pH. Prepared dispersion was then reagitated for 2 min at 20,000×g with an Arcelik brand mixer to obtain highly dispersed fibres. Flow behaviour (thixotropy, viscosity and yield stress) was measured by the R/S plus Soft Solids Tester in a controlled stress rotational rheometer featuring a CC458 model R/S coaxial spindle (Brookfield Engineering Laboratories, Inc., MA, USA) under laboratory conditions. All rheological measurements were conducted at 25 °C. Collection and evaluation of rheological data were accomplished with Brookfield Rheo‐2000 V2.8 software (Brookfield Engineering Co., MA, USA) with a three-step program contains: The shear rate was increased from 0 to 1000 s − 1 over for 400 s, held at 1000 s − 1 for 1 min and then ramped down from 1000 to 0 s− 1 for another 400 s. Finally, each sample was performed a hysteresis loop.

Water contact angles of Sep fibres were measured by a Sigma 701 tensiometer (KSV Instruments Ltd, Helsinki, Finland) with the capillary rise method (Benli et al., 2012). A Sep sample of 0.5 g (−150 μm particle size) was placed in a standard tube of 9 mm internal diameter; the bottom was replaced with a filter paper of 16,000 cm2 mesh density, the tube was tapped approximately 50 times and finally it was compressed tightly with a metal rod until the bed height no longer changed. During the experiments, once the gauze was in contact with the surface of the wetting liquid, the mass of absorbed liquid was automatically recorded. The relation between the rate of capillary rise and penetration time in such a porous system is given by a modified Washburn equation (Parry et al., 2008):

2

m ¼ cos θ

C w :ρL 2 :γ L t ηL

ð1:aÞ

B. Benli / Applied Clay Science 102 (2014) 1–7

3

Fig. 1. Flow curves of 5 mass% dispersions at different pH values, (a) SepTT, (b) SepKS and (c) SepSG. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Cw ¼

1 2 RðL:e:f Þ 2

ð1:bÞ

where CW is the Washburn or capillary constant, m is the mass of liquid absorbed by the sample in time t, θ is the contact angle between the wetting liquid and the solid of the powder, ρL is the liquid density, γL is the liquid surface tension and ηL is the liquid viscosity. For the

calculation of surface free energy components, polar and apolar liquids were selected: hexane, decane, water, formamide and diiodomethane. According to van Oss's theory (1994), the surface free energy of solid, γs, can be expressed as the sum of two components, non-polar or Liftshitz–van der Waals (LW) and polar or acid–base (AB) interactions: LW

γs ¼ γs

AB

þ γs

ð2Þ

Table 2 Variations in the rheological behaviours of highly alkaline dispersions. Herschel–Bulkley model: τ = τ0 + κ. γn Yield stress

Plastic viscosity

Yield exponent

Regression coefficient

Samples

pH

το (Pa)

κ (Pa.s)

n

R2

Thixotropy (Pa/s)

SepTT

8.5 9.5 10.5 11.5 8.5 9.5 10.5 11.5 8.5 9.5 10.5 11.5

24.533 16.077 4.2684 2.3124 1.6721 1.1166 0.1142 0.0326 7.1024 2.5029 0.3686 0.1961

18.718 11.858 0.0946 0.0003 0.0079 0.0006 0.0003 0.0001 1.2617 0.1094 0.0006 0.0001

0.1545 0.7712 0.1784 1.5159 1.066 1.411 1.635 1.493 0.397 0.485 1.382 1.541

0.9279 0.9084 0.9827 0.9999 0.9898 0.9889 0.9999 0.9999 0.9848 0.9852 0.9989 0.9999

16,744.19 10,354.77 1551.32 516.51 881.58 742.45 44.08 41.68 1529.94 1100.04 627.05 36.49

SepKS

SepSG

B. Benli / Applied Clay Science 102 (2014) 1–7

Absorbance at 254 nm wavelength,U254

4

with regression coefficients. The results were similar to those for other viscoplastic systems such as raw cement (Lachemet et al., 2008) and drilling mud (Coussot and Piau, 1995). However, when pH was increased to highly alkaline conditions (pH 10.5 and pH 11.5), since individual fibres can repel each other and move independently under flow, the flow behaviour of all samples changed to Newtonian. Similar results were also obtained for palygorskite dispersions (Neaman and Singer, 2000) and Na-montmorillonite (Keren et al., 1988).

0.35 0.30 0.25 0.20 SepKS

0.15

SepTT SepSG

0.10

3.2. Effects of release humic acid from sepiolite 0.05 0 8.5

9.5

10.5

11.5

pH Fig. 2. The changes of UV254 absorption by humic acid (which is proportional to the humic acid concentration) dissolved from sepiolite in alkaline and highly alkaline media.

γLW can be obtained from the measured contact angles (θ) of three s wetting liquids and solve the extended form of the Young's equation (van Oss, 1994) as below: AB

γs ¼ 2

qffiffiffiffiffiffiffiffiffiffiffiffiffiffi − γþ s :γ s

ð3Þ

+ Using the electron–donor (γ− s ) and electron–acceptor (γs ) parameters of the material, polar or acid–base (AB) interactions (γAB s ) can be calculated as follows:

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffi þ LW − þ γþ ð1 þ cos θÞγ L ¼ 2 γ− γ LW s :γ L s :γ L þ s :γ L

ð4Þ

2.5. Atomic force microscopy (AFM) AFM samples were prepared according to several procedures (Benli et al., 2012) and those on montmorillonite and smectite-type clay minerals (Bickmore et al., 1999; Plaschke et al., 2001). The measurements were performed in AFM contact mode with 910 M-NSC36/Cr-Au-type cantilevers and a scan rate of 0.5 Hz (AFM, XE-70E; Park Systems Corp., Suwon, Korea). AFM tips were cleaned in a UV ozone cleaner chamber (UV/Ozone ProCleaner, Bioforce Nanosciences Inc., IA, USA) under ultraviolet light for 15 min. In order to quantify the size of HA particles, all AFM images were also processed by XEI software (Park Systems Corp., Suwon, Korea). 3. Results and discussion 3.1. Rheological properties of sepiolite dispersions at alkaline media The rheological measurements of three different Sep-water dispersions at different pH values are shown in Fig. 1. It is obvious that Sep dispersions behave very similarly to those in other studies of natural environments (~pH 8.5) (Tunç et al., 2008; Çınar et al., 2009). However, their flow behaviours and also their shear stresses sharply decreased when pH conditions were changed from natural environment to pH 11.5 (Fig. 1a–c). In alkaline media, the flow type of Sep dispersions was nonNewtonian and pseudoplastic as represented by the Herschel–Bulkley equation (Herschel and Bulkley, 1926): τ ¼ τ0 þ κ:γ

n

ð5Þ

where τ and τo are the shear stress and the yield stress, respectively; γ is the shear rate; Herschel–Bulkley plastic viscosity, K and Herschel– Bulkley yield exponent, n are indexes for consistency and flow behaviour. The parameters of this equation are listed in Table 2 together

Unpredictable changes in the UV254 absorption data by humic acid at pH 8.5–11.5 are shown in Fig. 2. UV254 absorbance in alkaline conditions proportional to the dissolved humic acid concentrations of SepSG, SepTT, and SepKS, increased, respectively 6, 4 and 3.5 times more than their initial UV254 absorbance at pH 8.5, in contrast to the previous rheological measurements in Table 2. After 24 h conditioning time at pH 11.5, the appropriate amount of Mg2+ ions was released from the Sep matrix such as 59.61 ppm from SepTT, 10.62 ppm from SepKS and 6.18 ppm from SepSG. A higher amount of Mg2+ ion released from the SepTT sample could be a reason for its extra high thixotropy and unexpected rheological behaviour for 16744 Pa.s of thixotropy and 12417 Pa.s of viscosity (Fig. 1). Such Sep/ water organisations were previously verified by MD simulations in order to determine the role of release Mg2+ ions from the Sep matrix. Plenty of active sites led to hydrogen bonding on the basal/edge surfaces and kept Sep fibres together (Benli et al., 2012). According to time-dependent measurements of SepTT (Fig. 3a), HA release curves were nearly similar below pH 9.5, whereas considerable changes were only observed for higher alkaline solutions over pH 10.5. Similar results were also seen in the alkaline treatment of HA in the absence of clay or soil solutions (Pokorna et al., 2001). On the other hand, time-dependent measurements of Mg2 + ions released from SepTT revealed that free Mg2 + ions declined over pH 8.5 (Fig. 3b) because of the complex formation between HA macromolecules and free Mg2 + ions which was observed as a colloidal solid phase over pH 9. Colloids were slightly resized over pH 10.5, because of depletion of free Mg(OH)+ in the solution and probably formation of insoluble solid Mg(OH)2, according to the pH-pC diagram of Mg2+ in the solution concentration of 1.10−4 M (Butler, 1998). The colloidal solid phases, HA–Mg2+ complexes, were also directly observed by high-resolution AFM images (Fig. 4) and spherical HA particles were determined at different mean sizes in the range of 18–63 ± 8 nm at pH 11.5. These particles formed through coagulation of HA in the presence of Mg2 + ions. Increases in turbidity and colour are also seen in coagulation of humic substances and contribute to the formation of coloured organic complexes like HA-Fe(III) salts (Wall and Choppin, 2003) and HA-Na+ ions (Chen et al., 2007). Previous studies show that there is a relation between the particle size of HA colloids and the pH values of their dispersions such as 10–50 nm (Namjesnik-Dejanovic and Maurice, 1997; Balnois et al., 1999; Assemi et al., 2004), less than 100 nm at pH 4 (Chen et al., 2007), 130 nm at pH 9 (Pinheiro et al., 1996), 100 nm at pH 10 (Plaschke et al., 2001; Chen et al., 2007) and 70–160 nm at pH 11.3 (Caceci and Billon, 1990). 3.3. Rheology and stability of alkaline modified sepiolite dispersions Rheological behaviours of Sep dispersions were observed in Fig. 5 after alkaline treatment at pH 11.5 with two alternative suggestions: 50 ppm of HA loading and mechanical activation of HA-loaded Sep dispersions. After alkali modification and continuous washing processes, all released ions were removed from the system. Although 50 ppm of HA was loaded on the dispersions, the rheology of alkaline-modified Sep was not affected. However, re-mechanical activation of HA-loaded and alkaline-modified Sep caused the dispersion of fibres and the release of Mg2 + ions from new dispersed surfaces. This is caused by

B. Benli / Applied Clay Science 102 (2014) 1–7

5

(a)

(b)

Fig. 3. Time-dependent ion releases from SepTT at alkaline and highly alkaline pH, (a) HA and (b) Mg2+ ions. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

the restructuring of Sep matrix in the dispersion. It shows that the behaviour of release ions is important in the Sep system, especially Mg2 +, which is a known prerequisite for gel formation (Çınar et al.,

2009), and for restructuring of the Sep network (Santaren, 1993). Moreover, Sep is known to be highly chemically stable in neutral and alkaline media, the changes in their rheological behaviour could be owed to decrease in length of fibres (Golden et al., 1985; Martínez-Ramírez et al., 1996) and aggregation of HA complexes into finite-sized clusters that moved independently under applied stress (Neaman and Singer, 2000). 3.4. Role of faces and edges in the presence of humic acid

Fig. 4. 3D view of AFM image for HA colloids adsorbed on mica at pH 11.5. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

Sep water contact angles and surface free energy components of three Sep samples (Table 3) were calculated from Liftshitz–van der Waals interactions and van Oss theory (Eqs. (2) to (4)) under two alkaline conditions, pH 8.5 and pH 10.5, respectively. Interestingly, Sep water contact angles measured were bigger than previous ones: 72° (Benli et al., 2012) and 70.9° (Helmy and Bussetti, 2008). Increased pH in alkaline media slightly improved the hydrophobicity (Table 3) probably due to the dissolution of silanol groups. On the other hand, other indicators of the level of hydrophobicity are − electron–acceptor (γ+ S ) and electron–donor (γS ) components. Rela− tively large differences between γS values (Table 3) were determined at pH 8.5 and pH 10.5 as a consequence of the dissociation of carboxyl groups from the anionic HA-covered surfaces (Lopez-Duran et al., 2003). This effect was reversed at higher alkaline pH because of the higher ionisation and continuous release of humic molecules at

6

B. Benli / Applied Clay Science 102 (2014) 1–7

(a)

80

soils that can be expected to foster enhancement of the overall soil wettability (Ramos-Tejada et al., 2003).

Natural sepiolite Alkali modified Alkali modified and HA loaded Alkali modified and mechanical activated Alkali modified, mechanical activated and HA loaded

70

4. Conclusions

Shear stress (Pa)

60

Rheological properties of Sep dispersions indicate that alkaline solutions cause a sharp decrease in viscosity values as a result of the disruption of the particle network, and dissolutions of Sep surfaces as well as contribution of HA release from natural Sep matrix. Increasing the pH of the dispersions from alkaline to highly alkaline, their rheological behaviours changed from non-Newtonian and pseudoplastic represented by the Herschel–Bulkley viscoplastic behaviour to Newtonian. Released Mg2 + ions are some of those most responsible for viscoplastic behaviour and formation of strong hydrogen bonding which keeps the fibres together. Highly alkaline pH caused an increase in the total amounts of HA and free ions in relation to the formation/precipitation of both hydrophobic HA aggregate and HA-Mg2 + complexes. High-resolution AFM images determined a spherical shape of these colloidal solid particles in the size range of 18–63 ± 8 nm at pH 11.5. Finally, contact angle measurements also showed an enhancement in hydrophobicity with increasing pH in alkaline media, i.e. overall hydrophilic character decreased, and the wettability of the Sep changed + from the anisotropic to the higher hydrophobic level. The γ− S /γS ratio reached a minimum value of 1.49 for SepTT samples when pH was increased to 11.5.

50 40 30 20 10

SepTT

0 0

200

400

600

800

1000

Shear rate (1/s)

(b) 20

Natural sepiolite Alkali modified Alkali modified and HA loaded Alkali modified and mechanical activated Alkali modified, mechanical activated and HA loaded

18

Shear stress (Pa)

16 14 12 10 8 6

References

4 2

SepKS

0 0

200

400

600

800

1000

Shear rate (1/s) Fig. 5. Comparison of the flow curves for alkali modified dispersions, (a) SepTT and (b) SepKS. (For interpretation of the references to colour in this figure, the reader is referred to the web version of this article.)

pH 10.5; the subsequent decrease was even seen in the electron–donor capacity of the Sep surfaces (Table 3). In addition, maximum hydrophobicity is well defined for naturally hydrophobic minerals like talc surfaces with a balance between electron donor/acceptor sites + and γ− S /γS = 1.1 approaching unity (van Oss and Giese, 1995). Increasing pH up to 10.5 meant this ratio reached a minimum value + of γ− S /γS = 1.33/0.89 ≈ 1.49 for SepTT. AB According to Eq. (2), when γLW S is larger than γS , acid–base interactions do not occur and the non-polar or Liftshitz–van der Waals contribution is dominant at alkaline and highly alkaline pH up to 10.5. Sep surfaces should be covered because of HA adsorption since acid–base contribution at natural pH is higher than that at pH 10.5 (Table 3). In contrast, a significant decrease in acid–base repulsion was found in the pH range studied (i.e. γAB S = 3.46 to 1.55 for SepKS). Interestingly, the overall hydrophilic characteristics decreased, and the wettability of the Sep fibres changed from the anisotropic to the higher hydrophobic range. A similar result was found at alkaline pH in the case of natural Table 3 Surface free energy components in mJ/m2 and water contact angles of sepiolites measured by capillary rise method. Samples

γLW

γAB

γ+

γ−

γsolid

θwater

SepKS at pH 8.5 SepSG at pH 8.5 SepTT at pH 8.5 SepKS at pH 10.5 SepSG at pH 10.5 SepTT at pH 10.5

20.16 32.43 22.19 39.25 35.07 33.86

3.46 12.99 2.02 1.55 0.26 2.17

0.22 3.06 0.09 0.08 0.01 0.89

13.9 13.8 10.4 6.85 3.26 1.33

23.62 45.42 24.21 40.80 35.33 36.03

82.7 89.8 89.1 88.3 89.9 89.4

± ± ± ± ± ±

0.3 0.2 0.1 0.2 0.3 0.3

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