Dispersion properties of silica particles in nonaqueous media with a non-ionic surfactant, dodecyl hexaethylene glycol monoether, C12E6

Colloids and Surfaces A: Physicochem. Eng. Aspects 331 (2008) 162–174

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Dispersion properties of silica particles in nonaqueous media with a non-ionic surfactant, dodecyl hexaethylene glycol monoether, C12 E6 Justice M. Thwala a,∗ , Jim W. Goodwin b , Paul D. Mills c a

University of Swaziland, Private Bag 4, Kwaluseni M201, Swaziland Department of Physical and Colloid Chemistry, Bristol University, Bristol BS8 1TS, United Kingdom c ICI Films, Wilton Centre, PO Box No. 90, Middlesbrough, Cleveland TS6 8JE, United Kingdom b

a r t i c l e

i n f o

Article history: Received 28 April 2008 Received in revised form 8 July 2008 Accepted 1 August 2008 Available online 19 August 2008 Keywords: Electroacoustic sonic amplitude (ESA) Nonaqueous Globular aggregation Non-ionic Critical flocculation Bridging

a b s t r a c t An experimental programme is described that evaluated the role of charge and steric stabilisation in glycol and dodecane dispersions of colloidal silica particles on addition of trace water and acid. The surfactant used for steric stabilisation was a non-ionic surfactant, Dodecyl hexaethylene glycol monoether, C12 E6 with a geometric head-to-tail length of 3.85 nm. The study showed using the self-consistent field theory of adsorption (SCFA), electrophoresis and electroacoustic sonic amplitude (ESA), that the C12 E6 molecules adsorbed with the ethoxy group attached to the silica surface and the alkyl group in the bulk glycol solution. The thickness of the adsorbed layer as estimated from electrophoretic and viscosity measurements was found to be 8 and 10 nm, respectively, indicative of globular aggregation. The dispersions were stable at low levels of C12 E6 concentrations due to electrostatic repulsions as deduced from the zeta potentials of silica which were of the order of about −30 to −50 mV in monoethylene glycol (MEG). Instability on further addition of C12 E6 to the silica particles, a phenomenon normally obtained with high molecular weight polymers, was observed in MEG. Critical flocculation concentrations of C12 E6 , floc , increased with decrease in volume fraction of the silica particles in MEG. Instability is suggested to be from bridging interactions. Restabilisation observed at high surfactant concentration was due to steric repulsions of ethoxy groups of micellar aggregates adsorbed on silica particles. The study also revealed that the presence of trace water introduced charge repulsion which moderated rheological measurements in glycol media and introduced reversal of charge of silica particles in dodecane. © 2008 Elsevier B.V. All rights reserved.

1. Introduction There are many dispersions of industrial importance (ranging from inks, paints, electrorheological fluids, fire retardancy in aircrafts to ceramics) consisting of inorganic particles dispersed in media of polarity lying between that of water and that of pure hydrocarbon that have been widely studied. Some of these systems have been shown to reverse the sign of their charge with small addition of small amounts of water and, in general, the charging mechanism is the result of specific chemical interactions between solution species and those attached to the surface. The acid/base character of the underlying surface, adsorbed species and the species in solution all play a part in defining the sign and the magnitude of the surface charge and hence the dispersion stability and subsequent handling properties. In this study silica particles were investigated. Silica particles were prepared in monodisperse

∗ Corresponding author. Tel.: +268 6036616; fax: +268 5285276. E-mail address: [email protected] (J.M. Thwala). 0927-7757/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2008.08.008

form using the Stober technique and some were supplied by Imperial Chemical Industries (ICI). Glycols have always been of great industrial importance, especially in the polymer industry. For example, the production of polyethylene terephthalate, PET, uses MEG. The choice of glycols offers great latitude in the design of polymer backbones. They offer varied structural properties to polymers such as crystallinity, chemical resistance, surface cure, fire retardancy, low viscosity, heat stability and so on. The addition of fillers to these gylcol-polymer melts has also been widely used to improve the structure of a variety of polymer products. The efficiency of such productions, however, is sometimes, accompanied by side products such as water, methanol and acids resulting in polymer degradation and filler aggregation. The stability of these systems has not been widely studied. This is probably due to their high viscosity and low dielectric constants. A variety of stabilisers are used to maintain the stability of inorganic suspensions in these intermediate polarity media. They range from inorganic materials such as potassium tripolyphosphate, through polyelectrolytes such as sodium polyacrylate to

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non-ionic glycol surfactants typified by dodecyl hexaethylene glycol monoether, C12 E6 . The latter material and its effect on the electrical double layer has been investigated in other interfaces, i.e. at the mercury/solution interface by Watanabe [1], at the polystyrene/solution interface by Partridge [2] and the AgI/solution interface by Ottewil [3]. None has been so far investigated at the silica/glycol interface. Furthermore, studies of dispersion stability using low molecular weight surfactant in nonaqueous media are few. Extensive publications on steric stabilisation are for aqueous or mixed aqueous/nonaqueous media [4–14]. These systems exhibit steric stabilisation and instability at high concentration of the stabilising polymers due to depletion and bridging flocculation resulting in phase separation. Whether these factors leading to instability of polymer stabilised particles are responsible for instability for low molecular non-ionic surfactants such as dodecyl hexaethylene glycol monoether, C12 E6 , in nonaqueous systems typified by monoethylene glycol and diethylene glycol is subject to investigation. The experimental programme was based on studying the effect of a non-ionic surfactant C12 E6 on the electrokinetic, adsorption and rheological properties of silica particles in nonaqueous media such as monoethylene glycol, diethylene glycol and dodecane. The study also shows the effect of addition of small quantities of soluble species of varying acid/base behaviour such as water and terephthalic acid on the zeta potential and rheological properties of silica particles. 2. Experimental 2.1. Materials Silica particles were prepared using the method by Stober and Fink [15] followed by several cycles of centrifugation at speeds less than 1000 rpm to avoid formation of hard sediments. The silica particles were then extensively dialysed in thoroughly boiled dialysis tubing for 3 months until the conductivity of the water in the dialysis tubing was equal to that of purite water (3 × 10−6 −1 m−1 ). In addition, samples of colloidal silica particles “KE P500 seahostar” were generously provided by ICI whose diameters were 1100 ± 80 and 553 ± 30 nm. These were redispersed in water/ethanol mixture and dialysed. Particle size distributions were obtained by a Hitachi HS7 transmission electron microscope, in combination with a Karl–Zeiss particle sizer. The Stober silica was of diameter 547 ± 27 and 43 ± 4 nm with densities in the range 1.805–2.050 g/ml as obtained from an Anton Paar DMA 600 densitometer. The solvents used were analar grade monoethylene glycol, analar grade diethylene glycol, BDH grade acetone, analar grade ethanol, BDH grade dodecane and purite water (conductivity of 3 × 10−6 −1 m−1 ). The non-ionic surfactant, hexaethylene dodecyl glycol monoether C12 E6 was obtained from Nikko Chemicals Co. Ltd., Tokyo, Japan. 2.2. Methods 2.2.1. Electrophoretic mobility Electrophoretic measurements were obtained from a Penkem 3000 system with a 2 mW helium–neon laser. The instrument uses a Doppler type of measurement and analyses the motion of the particles simultaneously using Fourier transform of the multiple sensings. The charge of very viscous dispersions was obtained from the newly developed Electroacoustic technique. The instrument used is a Matec MBS-8000 instrument (Matec Applied Science, Hopkin, MA) controlled by Matec SESA software in the single point

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measurement mode, and using a cell geometry with a steel electrode attached to the transducer. It also has a Wavetek model 23 waveform generator and a SSP-60 cell with a piezo electric transducer. The electrokinetic sonic amplitude (ESA) signals were measured at 0.96–1.78 MHz. ESA is related to dynamic mobilities using the following equation: ESA = c(ω)G(ω, )

(1)

where c is the speed of sound in the liquid, (ω) is the dynamic mobility and G(ω, ) is the geometric coupling constant [16,17]. Static electrophoretic mobilities were converted to zeta potential using Henry’s equation as follows: E =

2εr ε0 f ( a) 3

(2)

where  is the solvent viscosity and the correction for retardation effects is given by f ( a) =

9 75 330 3 − + − 3 3 2 2 a 2 2 a2

a

(3)

The ESA signals from the Matec are reported as dynamic mobilities which are then converted to static mobilities using O’Brien’s treatment [18]. Electroacoustics overcomes contamination problem, normally encountered in traditional methods (such as electrophoresis, streaming potential and electroosmosis), by allowing measurements at significant particles concentrations [17,18]. 2.2.2. Adsorption measurements The adsorption isotherm was obtained using surface tension measurements from a Kruss Du Nuoy tensiometer at 25 ◦ C. The surface tension of supernatant solutions decanted from the silica dispersions of different concentrations were used to calculate the amount of surfactant adsorbed. Centrifugation has been reported to cause desorption of adsorbed polymers from adsorbates [19,20] and was therefore avoided. The dispersions were allowed to settle under gravity for about 3 weeks before the supernatant was decanted and analysed. Simulated adsorption isotherms were also generated using the SCFA theory [21]. In the SCFA theory the molecules are placed in a lattice, which facilitates the counting of conformations of the molecules which are represented as step weighted random walks on a lattice. In the SCFA theory each molecule is seen as a sequence of segments of equal size. The various segments are allowed to interact with each other and with the solvent. The magnitude of the contact interaction between segment types w x and y is expressed by the Flory–Huggins interaction parameter xy . The Gibbs energy of a free segment x in layer z, relative to that in the bulk solution is given by ux (z) = u (z) + kT



xy (y (z) − yb )

(4)

y

where y (z) is a weighted volume fraction accounting for the fact that segment y in layer z also has contacts in layers z − 1 and z + 1 and u (z) is the potential independent of the segment type and accounts for the fact that the sum of the volume fractions in layer z has to equal unity. This finally yields the segment density profiles and the excess adsorbed amount, iexc required in the generation of the adsorption isotherms of the surfactant. The use of the Flory–Huggins interaction parameters, AB , allows alteration of the conditions for different situations. With the interaction parameters chosen, the properties of non-ionic surfactants, such as their capability to form aggregates such as micelles and bilayers, can be reproduced. The thermodynamic parameters, [Flory–Huggins parameter] and the net adsorption interaction parameters, some of the s that were used in the theoretical calculations for the polyether used in this work are given in Table 4. The Flory–Huggins

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parameter CG for the interaction between a segment of type C and the solvent G should reflect the poor solubility of segments C in the solvent G. To achieve this, CG = 1.5 was used. The C segments are hydrophobic, and the interaction C and G can be compared to the interaction between alkyl group and water. The interaction parameter EG between segments of type E and solvent of type G was given the value 0.2, indicating that the solvent is moderately good solvent for E. This value compares with the interaction parameter between polyoxyethylene and water derived by Boomgaard and co-worker [22,23]. The interaction between the hydrophilic segments E and the hydrophobic segments C should be repulsive. To mimic moderately strong repulsion between segments C and segments E, a value of CE = 1.5 was used. According to Bohmer et al. such a high value ensures spatial separation of headgroup segments and tail segments, which is necessary for the formation of micelles. At low CE values, below 1.5, no stable micelles can be formed [23]. 2.2.3. Rheological measurements The rheological behaviour was characterised using capillary viscometry, constant stress viscometry (CS), continuous steady shear viscometry and shear wave rigidity modulus. A Bohlin rheometer (Bohlin Reologi, Lund, Sweden) interfaced with a Viglen microcomputer was used for the continuous steady shear viscometry studies, which were performed at 25 ± 0.01 ◦ C. Capillary viscometry was used for dilute dispersions (volume fractions, ϕ, of 0.01–0.1) whereas CS and the Bohlin (CSS) were used for concentrated dispersions of ϕ = 0.1–0.5. Concentric cylinders, cone and plates and double gap geometries were utilised. CS and CSS have the disadvantage of breaking the structure of colloidal formulations and thus insensitive to small structural formations in dispersions. This problem was solved by using Shear Wave Rigidity. Shear wave rigidity modulus, G∞ , was measured using a pulse shearometer (Rank Bros, Bottisham, Cambridge, UK) based on the model originally described Olphen [9]. G∞ is measured at high frequency, 1200 rad/s, and low strain amplitude (10–4 rad) for volume fractions above 0.4. In the first procedure steady-state shear stress ()–shear rate () ˙ curves were obtained and the Bingham model applied, where  = ˇ + pl ˙

(5)

From which the Bingham yield stress,  ˇ was obtained by extrapolation of the linear portion of the shear stress ()–shear rate () ˙ curve to ˙ = 0. In the second procedure, the shear modulus, G∞ , is calculated from the following equation: G∞ = u2

(6)

where ‘’ is the density of the suspension and ‘u’ is the velocity of the sinusoidal wave through the dispersion. 3. Results and discussion 3.1. Particle physicochemical characterisation The particles as examined by TEM yielded particle monodisperse distributions illustrated for the 553 and 42 nm particles size in Fig. 1. Some of the physical properties and porous characteristics of the silica particles studied are presented in Tables 1 and 2. The distributions are normalised particle density distributions. The average diameters were 42 ± 4, 553 ± 30, 547 ± 27, and 1101 ± 80 nm. The particles produced were fairly monodisperse with polydispersities, Pd , 1.015, 0.913, 0.983, and 0.991, respectively. Table 1 shows the elemental analysis results

Fig. 1. Particle size distribution for silica particles: (a) 500 nm ICI silica particles; (b) 43 nm Stober particles.

Table 1 Particle sizes and elemental analysis (w/w) for silica particles Particle code

Diameter, d (nm)

Standard deviation (nm)

Polydispersity, Pd

%C

%H

%N

ICI5 ICI1 STOB5 STOB4

553 1101 547 42

30 80 27 4

0.913 0.991 0.983 1.015

4.35 5.41 0.02 0.29

1.42 1.66 0.76 0.59

0.04 0.00 0.04 0.00

of the silica particles. The percentage carbon, C, hydrogen, H, and nitrogen, N compares favourably with those of Stober and Fink [15]. Fig. 2 shows the pore size distribution obtained from a quantachrome autoscan mercury porosimeter for the 553 and 42 nm particles size. The bulk unesterified particle densities shown in Table 2 are in the range 1.80–2.06 g/ml and the particle porosity gave values of 3.2–14.3%. Most of the particles exhibited monodisperse pore size distributions except the 43 nm Stober silica particles, which had a bimodal pore size distribution. Table 2 Pore characteristics of silica particles from quantachrome autoscan mercury porosimetry and density values from an Anton Paar 600 densitometer Particle code

Particle density (g/cm3 )

Surface area (m2 /g)

Pore volume (cm3 /g)

Porosity (%)

ICI5 ICI1 STOB5 STOB4

1.802 1.891 2.057 1.938

30 80 27 4

0.913 0.991 0.983 1.015

4.35 5.41 0.02 0.29

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Fig. 2. Pore size distribution D(r) for silica particles: (a) 500 nm Stober silica particles; (b) 43 nm Stober particles.

3.2. Electrophoretic mobilities 3.2.1. The effect of added C12 E6 on charge of silica particles in MEG Fig. 3 illustrates the variation of particle mobility obtained from the Penkem 3000 system and the calculated zeta potentials with added C12 E6 concentration covering the range studied in preparing the adsorption isotherms. An initial increase in the particle mobility (zeta potential) at low concentrations of C12 E6 followed by a gradual decrease at high C12 E6 concentrations is observed. ESA measurements were also taken for silica particles with varying C12 E6 concentration to confirm the data from the Penkem 3000 system. Initially, An ESA- linearity test was done to determine the linear range for the ESA signal. Fig. 4 shows the plot of ESA against volume fraction of silica particles in ethylene glycol. The data exhibit a linear dependence on  at low  and deviate significantly from linearity at  > 0.10. A linear least-squares fit to the data in the linear region (0 <  < 0.12) yields a slope d[ESA]/d of −343.6 ± 10.8 ␮Pa mV−1 with an intercept of −2.8 ± 1.3 ␮Pa mV−1 . ESA/G(ω, ) is independent of sensor geometry, but depends on properties of the colloid in the limit of  due to the variation of acoustic properties with . The curvature of the ESA plot is due to particle–particle interactions, particle effects on the speed of sound propagation and the higher order volume-conservation effects (particle boundary layer backflow effects) at high  [16,18]. ESA measurements were therefore made inside the linear region. Particle mobility data made using a Matec 8000 show good agreement with the Penkem 3000 system. An initial increase in the ESA signal at low concentrations of added C12 E6 followed by a gradual decrease at high C12 E6 concentrations is observed (Fig. 5). Microphoresis measurements in Fig. 3(a) shows an increase of the electrophoretic mobility of silica particles in MEG (at a volume frac-

Fig. 3. (a) Dynamic mobility vs. C12 E6 concentration for 0.1% (v/v) 500 nm silica particles dispersed in glycol. (b) Zeta potential vs. C12 E6 concentration for 0.1% (v/v) 500 nm silica.

tion, p , of 0.001 (w/v)) from −1.0 × 10−9 to −1.4 × 10−9 m2 V−1 s−1 with the surfactant concentration increasing from 0 to 7 × 10−4 M at monolayer coverage. The corresponding zeta potential as shown in Fig. 3(b) range from −47 to −67 mV. Electroacoustic measurements showed an increase in the electroacoustic amplitude from −2.9 to −4.0 ␮Pa mV−1 for silica particles (at p = 0.01) which translates to zeta potentials of −25 to −30 mV. The difference in zeta potential between the two techniques is due to differences in the volume fractions used as required by the techniques. Zeta potentials of the

Fig. 4. ESA-volume fraction behaviour of silica particles dispersed in glycol (MEG).

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Fig. 6. Zeta potential–C12 E6 concentration profile for 500 nm ICI silica particles in dodecane.

potential through Eqs. (7) and (8): =

d

exp(− x)

(7)

Thus, 2 = exp(− d2 ) 1

(8)

where the thickness of the steric barrier is given by dh = d2 + d1 Fig. 5. (a) The ESA and dynamic mobility vs. C12 E6 concentration for 1% 500 nm ICI silica in glycol. Diamonds: ESA; circles: mobility. (b) Zeta potential vs. C12 E6 concentration for 1% 500 nm ICI silica in glycol. (c) The zeta potential vs. C12 E6 concentration for 500 nm ICI silica stirred for 24 h.

order of −80 to −100 mV in water have been previously reported [24]. The initial increase in the zeta potential is attributed to the ionisation of the silica surface and is indicative of adsorption of the C12 E6 with the polar ethoxy group with the alkyl group dangling in the bulk glycol media. This is evidence of adsorption of the surfactant molecules on silica particles in MEG and implies that the electrostatic contribution to stability cannot be neglected in nonaqueous media of intermediate polarity. The observed decrease at high surfactant concentration is due to the shift in the Stern plane on adsorption of the non-ionic C12 E6 molecules [5–11]. Depression of the electrical double layer could not be accountable for the decrease in the zeta potential as confirmed by conductivity measurements which show a constant specific conductivity value of 2.80 × 10−5 −1 m−1 in the whole C12 E6 concentration studied. The surfactant does not offer any significant electrostatic stabilisation normally offered by ionic surfactants. The trend in zeta potential with added surfactant was reported previously by several authors. Tadros reported a gradual increase in zeta potential with increase in the extent of adsorption of polyvinyl acetate on ethirimol in aqueous media followed by a decrease at higher concentrations [5]. Siffert noted the same behaviour for the adsorption of amines on TiO2 powder in aqueous media [25]. The trend in zeta potential of silica particles in dodecane with increase in C12 E6 concentration, as shown in Fig. 6, is different from that of silica particles in MEG. Fig. 6 shows an increasingly positive zeta potential of silica particles with increase in C12 E6 concentration. This is attributed to a shift in the shear plane towards solution as reported by Siffert et al. [25]. The approximate thickness of the steric barrier of C12 E6 on silica particles in MEG can be found from the decrease in the zeta

(9)

where d1 is the Stern layer thickness in the absence of the adsorbed surfactant and is approximately 0.3–0.4 nm. The estimated thickness of the steric barrier was 8.05 ± 1.16 nm which compares well with the extended length of two C12 E6 molecules arranged end to end (7.70 nm). A value of 10.2 nm was obtained from viscometry as discussed in the section on rheological measurements. Similar studies by Tiberg concluded by null ellipsometry, that the adsorbed layer of C12 E6 on silica dispersed in water is built up of surface aggregates, or micelles, with dimensions resembling those observed in bulk solution [26]. Further, principal conclusions resulting from several sorption microcalorimetric studies on related systems suggested non-ionic systems form globular aggregates at the silica/aqueous interface [27,28]. Atomic force microscopy images (AFM) clearly indicated that globular surface aggregates are the preferred configuration on hydrophilic silica, not only for cationics [29] but also for Cx Ey non-ionics [30]. However, bilayer formation of Cx Ey surfactants in hydrophobic systems was reported [31–33]. In the system studied further work using AFM or microcalorimetry is required to ascertain the exact configuration in the glycol media studied. However, the steric barrier thickness obtained in this study is indicative of micellar aggregation. 3.2.2. The effect of added acid on the charge of silica particles The effect of added acid on the electrophoretic mobility, the ESA and consequently the zeta potential of the silica particles stabilised with surfactant was studied using terephthalic acid. Terephthalic acid is normally used in the polymerisation condensation reactions of glycol to produce polyethylene terephthalate (PET) polymers used in the preparation of PET films. Fig. 7 shows the effect of added acid on the particle mobility and zeta potential. The isoelectric point determined with added terephthalic acid was at pH ∼ 4.3 which is much less than that in aqueous media of about pH ∼ 3. The addition of surfactant tended to shift the isoelectric point to lower values at levels near the second plateau of the adsorption isotherm and increased it at levels of the added surfactant above the saturation level. The slight decrease in the isoelectric point at surfactant concentration below the saturation level

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Fig. 7. Zeta potential of 500 nm ICI silica in glycol with and without C12 E6 vs. pH due to added terephthalic acid. () C12 E6 = 5.0E−2 M, (♦) C12 E6 = 0 M and ( ) C12 E6 = 2.25E−3 M.

of the particles was due to the slight increase in the zeta potential of the particles due to ionisation of the silica particle on adsorption of surfactant molecules. The slight increase in the isoelectric point is due to the shift in the Stern plane. The decrease in the zeta potential on the addition of acid would be expected to result in instability of the silica particles in the glycol media. The change in charge on addition of acid can be illustrated by the following equation by Hunter [24]: H+

− OH

SiOH2 + ⇔SiOH ⇔ SiO− + H3 O+ similar to the equilibrium that would be observed in aqueous systems. Acid addition shift the equilibrium to the left resulting in a decrease in the zeta potential and consequent change in the sign of the charge. Fig. 8 shows the change in zeta potential for silica particles in MEG with increase in surfactant concentration. The data shows a shift in the minimum from a C12 E6 concentration of 1.25 × 10−3 M at pH 7.6 to 5.0 × 10−3 M at pH 4.0. This data proves that adsorption of C12 E6 is through hydrogen bond formation. At pH 7.6 the silica surface is deprotonated and has few hydroxyl groups for bonding and thus the minimum occurs at low C12 E6 concentration. At pH 4.0 the silica surface is protonated and has more hydroxyl sites for adsorption of C12 E6 and thus the minimum occurs at a higher concentration than at pH 7.6. 3.2.3. The effect of added water on charge of silica particles in nonaqueous media-MEG and DEG In this work, by measuring the surface charge of silica particles, it is shown how moisture affects the surface chemistry of silica particles. Figs. 9–11 show the dependence of electrophoretic mobility, ESA, dynamic mobility and zeta potential on added bulk water in mono ethylene glycol, diethylene glycol and dodecane, respectively. All the experiments for silica dispersed in MEG and DEG show a steady increase in the zeta potential with addition of water. Zeta potentials in monoethylene glycol are more negative than in diethylene glycol on account of the differences in dielectric constants. In MEG zeta potentials range from −45 to −95 mV for water content of 0 to 10%. In DEG zeta potentials range from −22 to −35 mV. These values are obtained from 0.1% (w/w) silica dispersions using microelectrophoresis. Zeta potential values obtained from ESA measurements at 1% (w/w) silica dispersion are less than those obtained from microelectrophoresis due to the differences in volume fractions used. The DEG zeta potential values from ESA measurements are still less than those of MEG as shown in Fig. 10. The mechanism by which the particle gain a charge is explained in terms of the equation below. The particle is represented by P and

Fig. 8. (a) Zeta potential from electrophoretic mobilities against bulk concentration of C12 E6 concentration for 500 nm ICI silica particles (0.1%, v/v) at pH 4. (b) Zeta potential from ESA data against bulk concentration of C12 E6 concentration for 500 nm ICI silica particles (1%, v/v) at pH 4.

the solvent represented by S. The solvent will function as a proton donor or acceptor. The sign of the charge depends on the direction of transfer [34]: PH2 + + S− ⇔ PH + SH ⇔ P− + SH+ The relative acid–base character of the solid surface and of the liquid determines the sign of the charge. The addition of water leads to a competitive proton transfer between water or the solvent and the particle. In the case of protic solvents like MEG and DEG the equilibrium is shifted to the right where the particle loses its proton to the adsorbed water molecules.

Fig. 9. Zeta potential vs. bulk water for 500 nm ICI seahostar (0.1%, v/v) in MEG from static electrophoretic mobility measurements.

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Fig. 10. Zeta potential vs. bulk water for 500 nm ICI seahostar (1%, v/v) in MEG and DEG from ESA measurements.

This line of thought, however, fails to explain the existence of charge for the aprotic dodecane medium. An electron transfer electron transfer through an ionisation mechanism involving trace adsorbed water suggested by Williams is used in explaining charging in aprotic solvents like dodecane [40]. Silica is known to be a strong proton donor and water to be a good proton acceptor [11,24]. Dodecane is a poor proton acceptor; the above equilibrium will lie to the left, due to trace impurities of water, with silica having a positive surface [25]. Fig. 11 shows the zeta potential plot for dodecane against water content. Adding water makes the bulk solvent increasingly efficient in proton abstraction and shifts the equilibrium to the right with a negative silica surface. This leads to a reversal of charge of silica particles from a positive of 3 mV to negative charges of −4 mV on addition water trace water in dodecane. The point of zero charge occurs at water contents of 0.05%.

Fig. 12. Surface tension–C12 E6 concentration profile in MEG.

namely: one clearly defined at 4.9 × 1018 molecules/m2 and one at 18 × 1018 molecules/m2 which corresponds to an area per molecule of 0.2 and 0.088 nm2 /molecule, respectively. The geometric area of the alkyl chain and the ethoxy group are 0.072 and 0.286 nm2 /molecule, respectively [2]. This implies that the adsorption result in a micelle like aggregate formation of hydrophobic tails of the surfactant with the second layer (plateau) occurring at 1.1% C12 E6 . The first layer is due to hydrogen bonding of the ethoxy group

3.3. Adsorption isotherms The variation of surface tension with surfactant concentration for C12 E6 is shown in Fig. 12. The break in the curves corresponds to the critical micellar concentration, cmc, which occurs at 2.30 × 10−2 M in MEG. The area occupied by each C12 E6 molecules at the MEG air interface was calculated to be 0.66 nm2 . This compares well with values obtained by Lange [35] who reported a value of 0.60 nm2 at the water–air interface. Corkill et al. [36] reported a value of 0.55 nm2 on graphon. Adsorption isotherms of C12 E6 from MEG onto silica particles plotted against equilibrium concentration are shown in Fig. 13(a). The adsorption isotherm shows two plateau regions,

Fig. 11. Zeta potential vs. bulk water for 500 nm ICI seahostar (0.1%, v/v) in dodecane from static electrophoretic mobility measurements.

Fig. 13. (a) Adsorption isotherm for C12 E6 on 553 nm ICI silica at 25 ◦ C. (b) BET fitted adsorption isotherm for C12 E6 on 553 nm ICI silica at 25 ◦ C at low coverage.

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to the surface and the second layer is a result a van der Waals chain to chain interaction previously reported by Rubio and Kitchener [37]. The data was fitted using the BET isotherm as shown by the solid lines in Fig. 13(b). The BET isotherm used is given by  =

BC0 (Cs − C)[1 + (B − 1)C/Cs ])

(10)

 represents amount adsorbed (molecules/m2 ), C is the equilibrium concentration, Cs is the saturation adsorbate concentration,  0 is the adsorption density at monolayer coverage and B is a constant (=Kads /(1/Cs )) where Kads is the equilibrium constant for adsorption. Values for B,  0 , Cs and log Kads obtained are 869, 2.01 × 1018 molecules/m2 , 2.15 × 10−2 M and 4.61. The BET fit to the experimental data is shown in Fig. 13(b). It shows a good fit at low concentrations below the CMC value for C12 E6 in MEG. Similar studies were reported by Ghiaci et al. [38] on the adsorption isotherms of non-ionic surfactants on Na-bentonite (Iran) and evaluation of thermodynamic parameters. Ghiaci reported values of log Kads ranging from 4.4 to 5.4 for temperatures of 298–330 K. Adsorption of the surfactants was proposed to be in a flat configuration with the ethoxy group attached to the surface of Na-bentonite with silanol groups. The concentration ranges of the surfactants studied by Ghiaci were, however, below the critical micellar concentrations of the surfactants studied. The Dubinin–Radushkevich (D–R) isotherm was also used to quantify the mean free energy of sorption, ε. The D–R equation is given by the following relationship: ln qe = ln qm − Kε2

(11)

where qe is the amount of analyte sorbed at equilibrium, K is the constant related to mean free energy of sorption, qm is the theoretical saturation capacity, and ε is the Polani potential, equal to RT ln(1 + (1/Ce ). The mean free energy of sorption, E, is given by (2K)−1/2 . Plotting as shown in figure ln qe against ε2 gave a qm value of 6.83 × 1018 molecules/m2 and an energy value, E, of 12.91 kJ mol−1 indicating that chemisorption through hydrogen bonding plays a significant role in the sorption process. Fig. 14 shows the adsorption data obtained using the SCFA theory. The theory shows a higher adsorption density profile than the experiment data. The Gibbs free energy of adsorption of the adsorbate is shown in Fig. 15(a). Above the CMC the chemical potential of the adsorbate hardly changes and hence the adsorption remains constant at the CMC concentration and above. In Fig. 15(b), the volume fraction profiles, x (z), for C12 E6 are shown at a bulk volume fraction just before the phase transition at the surface occurs and

Fig. 15. (a) Gibbs free energy of adsorption for C12 E6 on silica. (b) Volume fraction profiles of C12 E6 at very low to high concentrations covering the lower and upper parts of the plateau of the isotherm on silica in glycol at 25 ◦ C from the SCFA theory. % C12 E6 ∼ : 0.5%, : 0.8%, : 1% & : 2% Alkyl group and : 0.5 %, : 0.8 %, : 1% & : 2 % Ethoxy group.

at a bulk volume fraction corresponding with the plateau of the isotherm. The distance from the origin to the ethoxy peak represents the adsorbed layer thickness. The figure shows the volume fraction profiles vs. layer number obtained using the SCFA theory at different surfactant concentrations in nonaqueous media. The SCFA predicts the adsorption of the ethoxy group of C12 E6 with the alkyl chain directed towards the bulk ethylene glycol solution. A thick asymmetrical surfactant micellar aggregate is formed. Increasing the concentration increases the adsorbed layer thickness and the intensity of the ethoxy peaks due to the increase in association of the ethoxy groups. 3.4. Rheological measurements 3.4.1. Adsorbed layer thickness Fig. 16 shows the plot of relative viscosity r vs. volume fraction of the silica dispersion in MEG. The results are those expected of concentrated dispersions, namely a gradual increase in r followed by a rapid increase with increase in volume fraction. For comparison, the curve represented by the open symbols in Fig. 1 shows the theoretical curve for hard sphere dispersion, calculated using the Krieger–Dougherty equation [12]:



 r = 1 − p

Fig. 14. Calculated adsorption isotherms for C12 E6 on a lyophilic silica surface in glycol using the SCFA theory.

−[]p

(12)

where [] is the intrinsic viscosity. According to the hard sphere approximation by Krieger the intrinsic viscosity is equal to 2.5 and p is the maximum packing fraction which is 0.64 for random packing [39]. All data show higher r at any given  when compared with

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Fig. 18. Rheological parameter, rel against C12 E6 concentration for silica ( = 0.10) in MEG.

Fig. 16. Relative viscosity–volume fraction plots for silica in glycol without surfactant.

the hard-sphere diameter. This is not surprising since the particles are coated with surfactant layers which will make a contribution √ to . From the plot of these data as 1/ r vs.  using the empirical Krieger equation [13]: K  = m + √ r

(13)

where K is a constant and m is the apparent maximum packing fraction. The intrinsic viscosity obtained is 2.7 and m was 0.65 in the absence of the C12 E6 surfactant and 0.61 with the adsorbed layer. The value of 0.61 in the presence of C12 E6 is much less than maximum packing fraction p due to the contribution of the adsorbed layer. From m and p one may calculate the adsorbed layer thickness (ı) according to the following equation:

 ı=R

p m



1/3 −1

(14)

where R is the particle radius. The ı value obtained was 10.2 nm which is close to the value obtained from electrophoretic measurements of 7.8 nm indicating a micelle like aggregate formation on adsorption of the surfactant.

Fig. 19. (a) BingHam yield stress, /Pa, vs. C12 E6 concentration for 0.30 (v/v) silica in MEG. (b) BingHam yield stress, /Pa, vs. C12 E6 concentration for 0.50 (v/v) silica in MEG.

3.4.2. The effect of C12 E6 on dispersion The effect of surfactant concentration on viscosity was studied up to as high as 300 s−1 as shown in Fig. 17. The shear stress, , and

correspondingly the viscosity, , increased with increase in surfactant concentration. Figs. 18 and 19 show the measured relative viscosity and Bingham yield stress vs. C12 E6 concentration. The dispersions indicate a maximum degree of dispersion at 0.05–0.4 vol.% C12 E6 concentration for 50.0 vol.% solids. Similarly, G∞ plots given in Fig. 20 show a high degree of dispersion below 0.5 vol.% C12 E6 . The dispersions showed a rapid increase in viscosities and rigidity values above 0.4% C12 E6 (referred to as the critical flocculation

Fig. 17. Viscosity–shear rate plots for silica particles in MEG at various concentrations of C12 E6 . () 3.6%; () 1.8%; ( ) 0.88%; () 0%.

Fig. 20. High frequency limit shear modulus for 500 nm silica in glycol vs. added C12 E6 .  = 0.50.

J.M. Thwala et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 331 (2008) 162–174

concentration, floc ) for 50.0 vol.% solids, indicative of the onset of flocculation. Clearly the r ,  ˇ and G∞ parameters offer a sensitive means of detecting weak flocculation in concentrated dispersions, as observed previously [5]. The trends in the variation of the rheological parameters r ,  ˇ and G∞ indicate that at the onset of flocculation the dispersions show marked viscoelasticity [7]. This is clear from the sudden and sharp increase in r ,  ˇ and G∞ above the flocculation point, floc . The flocculation observed is due to the formation of a gel network as a result of interaction between particles and free surfactant. Phase separation due to selective segregation, bridging flocculation and charge neutralisation has been reported in other systems as the cause of flocculation [10,12]. Furthermore, instability reported in literature is mainly for particles with nonadsorbing high molecular weight polymers with large radius of gyration in aqueous or mixed aqueous/nonaqueous systems. Depletion flocculation is reported to be the main cause of instability. Flocculation following adsorption of a low molecular weight steric stabiliser in nonaqueous media is seldom reported in literature. Snowden reported depletion flocculation of silica particles in aqueous media by an adsorbing polymer (hydroxyethyl cellulose) using wavelength dependence of turbidity at volume concentrations of 0.5% (w/w) [40]. Beattie observed depletion flocculation of poly(methylmethacrylate) (PMMA) lattices by hydroxyethyl cellulose (HEC) [11]. Poly(ethylene) glycol showed instability of Nabentonite suspensions at high concentration [14]. Cambia reported depletion flocculation of poly(12-hydroxystearic acid) coated poly(methylmethacrylate) (PMMA) by cis-poly(isoprene) polymers of molecular weight ranging from 28,000 to 130,000 g/mol. He noted that low molecular weight added polymers failed to induce depletion flocculation. Low polymer weight polymers acted as diluents [41]. However, the nature of instability for low molecular weight surfactants such as C12 E6 require careful investigation of concentrated dispersions, using freeze-fracture techniques for example; such experiments will be attempted in future. Another important feature of the rheological results which is difficult to explain is the reduction in r and  ␤ at higher surfactant concentration values while in contrast G∞ continue to increase. Whether this reduction in the values of r and  ˇ may be taken as an indication of the reduction in the extent of flocculation, or not, is difficult to say. Only some theories predict reduction in flocculation at higher free-polymer concentration. The Vincent theory [10,42] predicts restabilisation at much higher free-polymer concentrations than those observed experimentally. Thus the above trend in r and  ˇ is extremely difficult to account at present, particularly since G∞ continue to increase. Direct comparisons between rheology and zeta potential data is not possible as the amount of surfactant adsorbed onto silica particles in the rheology measurements is unknown. On the basis of the increase in zeta potential at low surfactant concentration, it is proposed that at the dispersions were stabilised by electrostatic repulsions between the particles. At moderate surfactant concentrations the dispersions flocculate (at low surface coverage). The destabilisation of dispersions is thought to be due to bridging interactions. Depletion interactions are discarded here since it is mainly observed with nonadsorbing species. A comparable situation is the flocculation of silica particles by non-ionic surfactant TX100 [27], reversible flocculation of silica by poly(oxyethylene glycol) polymers [47] and poly(acrylic acid) (PAA)–poly(ethylene) comb polymer on silica suspension [48]. At intermediate surface coverage the silica particles has bare patches and most of the adsorbed surfactant are in surface micelles [48]. The flocs are imagined in a simple vision as large silica spheres bound by smaller spheres of surface micelles and can thus be considered as a simple phenomenon of bridging flocculation. Theoretically, flocculation before full coverage is indicative of bridging interactions

171

Table 3 Interaction energy, Esep , between silica flocs in MEG at varying C12 E6 concentrations for random close packing % C12 E6

Yield stress,  ˇ (N m−2 )

Esep /kT (n = 8)

Esep /kT (n = 4)

0.025 0.05 0.3 0.4 0.6 0.8 1 1.3

1.165 0.97 1.263 1.047 3.761 12.41 30.1 40.11

12.30 10.24 13.34 11.06 39.71 131.04 317.83 423.52

24.60 20.48 26.67 22.11 79.42 262.07 635.65 847.04

[49,50]. Stabilisation at high surfactant concentrations deduced from the drop in Bingham yield and relative viscosity values is thought to be due to steric stabilisation from the repulsion of the polar ethoxy groups of the shells of surface micelles of two different particles as proposed by Naper [51]. The only rheological parameter that may be readily analysed in terms of interparticle is the Bingham yield stress,  ˇ . This is equated to the amount of energy needed to totally separate the flocs into single units [8], i.e.  = NEsep

(15)

where N is the total number of contacts between particles in flocs and Esep is the energy required to break each contact. The number of contacts N is related to the particle volume fraction, , and the average number of contacts per particle, n, by N=

1 2

 3n 

(16)

4a3

Where a is the particle radius. The interaction energy is computed from a combination of Eqs. (15) and (16): Esep =

8a3 ˇ

(17)

3n

where “n” is in the range 4–8 for random close packing and 12 for hexagonal close packing. Table 3 shows the Esep values for 50% volume fraction for silica particles with increasing C12 E6 concentration. The energy required to break each contact in a flocculated system, Esep , may be equated to the free-energy minimum, Vmin , in the potential energy–distance curve in the presence of adsorbed and free surfactant. Several theories exist for predicting this curve. Calculations based on these theories will be presented in a communication to follow. Several investigators [38,41–45] propose values in the range 5–20kT for the minimum energy, Vmin , required for the onset of flocculation, a value of 7kT being the most widely used. From Table 3, interaction energy values, Esep , obtained for the onset of flocculation based on the floc model range from 12 to 423kT for C12 E6 concentration ranging from 0.025 to 1.3% for random close packing of silica particles (Table 3).

Table 4 Flory–Huggins interaction parameters used in the SCFA theoretical calculations for C12 E6 Parameter

CH3 CH2• • [C], alkyl chain

CH2 CH2 O [E], ethoxy group

[Glycol] G solvent

s CH3 CH2• • [C] CH2 CH2 O [E] [Glycol] G

0 0 2 1.5

−6 2 0 0.2

1.5 1.5 0.2 0

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J.M. Thwala et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 331 (2008) 162–174 Table 5 Critical flocculation concentration of silica particles and energy minimum, Gmin , of the free energy–particle separation curve at various volume fraction

Fig. 21. Critical flocculation concentration of C12 E6 vs. volume fraction of silica in MEG.

3.4.3. Dependence of critical flocculation concentration (c.f.c.) on volume fraction Fig. 21 shows the dependence of critical flocculation concentration (c.f.c.) on volume fraction. The critical flocculation concentrations varied with volume fraction of solids. At low volume fractions the increase in the rheological parameters is not as obvious as at high volume fractions. The c.f.c. at low volume fraction was higher than at high volume fraction. The difference is attributed to the reduction in the contribution of entropy in the free energy of flocculation, with increase of the volume fraction. This means that flocculation of concentrated dispersions occur at lower interaction energy (low surfactant concentration) when compared with dilute dispersions [13]. This can be explained using the free energy of flocculation, Gfloc , as pointed out by Vincent and co-workers [12] who postulated that Gfloc may be split into two contributions, as given in the following equation: Gfloc = Gi + Ghs



h (nm)

% C12 E6 critical

Gmin /kT

0.02 0.04 0.1 0.2 0.25 0.3 0.35 0.4 0.46 0.5 0.51

1202.7 840.5 473.7 261.9 203.5 158.9 123.2 93.8 64.4 47.4 43.5

1.2 1.09 0.9 1.2 1.3 1.34 1.2 1.3 0.75 0.6 0.4

−0.87 −0.69 −0.46 −0.29 −0.24 −0.19 −0.15 −0.12 −0.08 −0.06 −0.06

needed to flocculate a concentrated dispersion (e.g. d = 0.51) is smaller than that for a corresponding dilute dispersion (e.g. 1.2% C12 E6 for d = 0.02). 3.4.4. The effect of added water on dispersion properties Fig. 22 shows the variation of G∞ with added water for MEG, DEG and dodecane. Variation of G∞ in dodecane was carried out for comparison purposes. G∞ increases with added water in dodecane. This is attributed to water adsorption on the polar silica surfaces. Water being polar has a high affinity for the polar silica particles and a low affinity for the non-polar vehicle, dodecane. At low levels of added water the main factor leading to instability is the van der Waals forces. Water creates an interface between silica/water with the non-polar dispersing medium leading to a high interfacial energy. In order to decrease the high interfacial energy the water molecules move to junction points on the particle resulting

(18)

where Ghs (=−T Shs ) is the entropic contribution associated with the aggregation of hard spheres in the absence of interparticle interactions. Shs per particles is negative, decreasing in magnitude with increasing volume fraction. Ghs is given by Ghs = kT ln

f

(19)

d

where d is the dispersed phase volume fraction and f is the flocculated phase. Gi is the interaction free energy term associated with other interaction between particles such as van der Waals attractive interactions, electrostatic interactions and steric interactions. In concentrated dispersions Gi is assumed to be independent of volume fraction and is given by Gi =

z 2

Gmin

(20)

where Gmin is the energy minimum in the free energy–particle separation curve and ‘z’ is the coordination number in the floc phase (for random close packing f = 0.64 and z = 8). Assuming phase equilibrium, Gfloc = 0. This leads to the relationship between d and Gmin derived by Vincent et al.: d f

= exp

 zG

min

2kT



(21)

Table 5 shows the critical flocculation concentration of silica particles and the energy minimum, Gmin , of the free energy–particle separation curve at various volume fraction. As Gmin increases from −0.062kT, the dispersed phase, the critical flocculation C12 E6 concentration increases from 0.4 to 1.2% and this accounts for the observation that the free surfactant concentration (e.g. 0.4%)

Fig. 22. (a) High frequency limit shear modulus for 500 nm silica in dodecane vs. added water content. (b) High frequency limit shear modulus for 500 nm silica in glycol (MEG and DEG) vs. water content.

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to bridging and subsequent gellation. Added to that fact, the zeta potential of the particles is reversed on addition of water with a zero charge at levels between 0–0.05%. Removing the electrostatic contribution to instability enhances gellation of the fluid. Furthermore, G∞ increases in dodecane to a maximum point at 0.3% added water. The point of zero charge for silica particles in dodecane is at 0.1% added water. It can be reported, for the first time in an aprotic solvent, that the maximum in G∞ corresponds to point of zero surface potential. G∞ is moderated by the positive potential at low trace water and a negative potential at high trace water. The ratio of water to silica is the most important parameter that produced the maximum in G∞ rather than the absolute quantity of water. The weight of water to silica ratio that produced zero electrophoretic mobility was 0.5 (0.05% water to 0.1% silica) and that producing the maximum in the high frequency limit modulus, G∞ , was 0.6 (0.30% water to 0.5% silica). This is evidence that G∞ is moderated by the total interaction potential, VT , as described by Zwanzig and Mountain [46] and Goodwin and Smith [39] who obtained the following expression: G∞ =

3 32 kT + 4a2 8a6





∞

g(r) 0

d dU(r) r4 dr dr

 dr

(22)

where  is the volume fraction, a, the particle radius and kT the thermal energy, U(r) is the interaction pair potential and g(r) is the pair distribution function. The interaction pair potential U(r) can be equated to the pair potential, VT , given by VT = VS + VA + VR

173

ranging from −30 to −90 mV are reported in mono ethylene glycol and values of −20 to −30 mV are reported for diethylene glycol. The glycol media investigated also exhibited lower isoelectric points at pH 4.3 which is different from that in water of pH 3. • Stability of silica particles in glycols is by electric effects at low C12 E6 concentrations. On the basis of rheology and zeta potential instability at moderate C12 E6 concentrations is suggested to be due to bridging interactions. Restabilisation at high C12 E6 concentrations is due to steric repulsions of ethoxy groups of the adsorbed micellar aggregates. • Critical flocculation concentration, floc , varied with volume fraction of the dispersions in MEG and depends on the free energy of flocculation. • The results show that the rheology (G∞ ) of concentrated sterically stabilised silica particles in nonaqueous systems is moderated by the total interaction potential, VT and is greatly affected by the repulsive term, VR . Future work will report the pair potential particle interaction energy for silica in these nonaqueous systems in the presence and absence of non-ionic surfactants using steric interaction theories. It will attempt to explain the flocculation behaviour of these dispersions typified by low molecular and short chain surfactant molecules. Acknowledgements

(23)

where VS is a steep steric interaction term, VA is the van der Waals attractive term and VR is the repulsive term. In the absence of the repulsive term, VR , at the point of zero charge, G∞ , is at its maximum as observed from the rheological data, and decreases when the surface potential is present as trace water is added. Glycols, on the other hand, are very polar compared to dodecane. Adding water also increases the zeta potentials of the particles and thus increasing the electrostatic contribution to stability of the dispersions. Comparing the MEG and DEG, G∞ curves, however, reveals an interesting point. The slopes of the curves are different. The decrease in G∞ with added water in MEG is pronounced, whereas in DEG, G∞ decreases steadily. This difference is attributed to the difference in zeta potentials in these two media and the corresponding repulsive pair potentials. Monoethylene glycol show potentials in the range −30 to 90 mV and DEG gave lower potentials of −20 to −30 mV for water content of 0–3%. 4. Conclusion and future work The following conclusions can be drawn from the ongoing study: • Adsorption of non-ionic surfactant in ethylene glycol media on hydrophilic silica form structures with dimensions that are equal to the extended length of two C12 E6 molecules arranged end to end typical of globular aggregates. The C12 E6 molecules adsorbed with the ethoxy group attached to the surface and the alkyl group in the bulk solution as predicted using the SCFA theory, viscosity and zeta potential data in nonaqueous media. • Addition of C12 E6 to silica in glycol increases zeta potential from −30 to −45 mV indicating adsorption and ionisation of the silica surface at low C12 E6 . Further addition of C12 E6 shows a decrease in zeta potential due to the shift in the stern plane due to formation of micellar like aggregates of the C12 E6 molecules. • Zeta potential plays a role in the stability of for silica particles in nonaqueous media of intermediate polarity. Values

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