Journal of Colloid and Interface Science 300 (2006) 788–794www.elsevier.com/locate/jcis

Temperature and pressure effects on zeta potential valuesof reservoir minerals

Karina Rodríguez a, Mariela Araujo b,∗

a Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas, Venezuelab Earth Science and Engineering Department, Imperial College, London, United Kingdom

Received 19 January 2006; accepted 6 April 2006

Available online 6 May 2006

Abstract

An experimental study of the effect of temperature and pressure on zeta potential of typical reservoir minerals, including quartz, kaolinite,and calcite, is presented. Experiments included the design and construction of an electrophoretic cell for zeta potential measurements at variablepressure and temperature. Electrolyte concentration was varied in the range from 0.0001 to 0.1 M in the pH range from 2 to 9. For all the mineralsit is found that the zeta potential decreases with temperature at a rate characteristic of each mineral; values are around −2.3 mV/◦C for quartz,−0.96 mV/◦C for kaolinite, and −2.1 mV/◦C for calcite for pressure values less than 45 psi. The effect of pressure is found to depend on themineral nature and pH of the electrolytic solution. In the case of quartz, a systematic increase in the value of the zeta potential with pressure isobserved, whereas a decreasing trend is measured for the kaolinite. In the case of calcite, a decreasing trend is observed for pressures up to 45 psi,whereas the experimental data suggest an increasing trend for higher pressure values.© 2006 Elsevier Inc. All rights reserved.

Keywords: Reservoir minerals; Zeta potential; Quartz; Kaolinite; Calcite; Pressure

1. Introduction

Understanding the electrokinetic response of mineralspresent in natural porous structures is essential for the descrip-tion of interfacial processes in such formations. In particular,rock–fluid interactions in reservoir rocks are of primary inter-est to the oil industry since they affect the fluid distribution,and therefore the macroscopic flow properties of reservoir flu-ids, information required for the prediction of future reservoirperformance [1–3].

Minerals such as quartz, kaolinite, and calcite are presentin many different forms of porous media, including reservoirrocks. In general, reservoir rock composition includes severaltypes of clay minerals besides quartz and other metallic com-ponents [4].

In the literature, very little information is available on in-terfacial properties such as zeta potential at high temperatures,

* Corresponding author.E-mail address: m.araujo@imperial.ac.uk (M. Araujo).

0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2006.04.030

and even less is known about the effect of pressure on suchproperties. Interesting industrial processes such as flotation andenhanced oil recovery normally occur under pressure and tem-perature conditions different from ambient or lab conditions(20 ◦C and 1 atm); thus any knowledge in this direction is ofparticular scientific, technical, and industrial interest.

Electrokinetic response refers to the phenomena that takeplace at a solid/liquid interface as a result of an applied elec-trical potential gradient. They derive from interactions betweenmacroscopic motion and diffuse electric charge. The electroki-netic response can be measured by several different methodsaccording to the experimental conditions, such as electrophore-sis, electroosmosis, streaming potential, or sedimentation po-tential [5].

Zeta potential measurements are commonly performed us-ing electrophoresis and streaming potential techniques. Johnsonused quartz samples to demonstrate that the zeta potential asdetermined using both methods is equivalent within experimen-tal error [6]. The streaming potential method has been com-monly used since the technique is most easily adaptable formeasurements at nonambient conditions. In this work we use

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the electrophoresis technique in a new specially designed cellthat allows exploring a limited temperature and pressure range,avoiding the presence of convectional currents and nonuniformcell expansion effects.

Previous work reporting zeta potential measurements ofquartz, kaolinite, and calcite was mostly performed under am-bient conditions, i.e., at normal pressure (1 atm/14.7 psi) andtemperature (20–25 ◦C). High-temperature experiments weresuccessfully performed by Kulkarni and Somasundaran [7] andRamachandran and Somasundaran [8] using the streaming po-tential technique on quartz and kaolinite samples. Moulin andRoques studied the effect of calcite concentration under three-phase (gas, liquid, and solid) conditions [9]. Hussain et al. stud-ied the zeta potential on clay samples, including kaolinite, andthe effect on coal flotation [10].

In this work, the zeta potential is studied as a function ofthe pH of the electrolytic solution for three different minerals,quartz, kaolinite, and calcite, in the temperature range from 20to 45 ◦C and pressures from 14.7 to 74.5 psi. As electrolyticsolution, NaCl brine, at different concentrations ranging from0.0001 to 0.1 M was used.

It is found that for the three minerals, the zeta potential de-creases with temperature at rates on the order of −2.3 mV/◦Cfor the quartz, −0.96 mV/◦C for the kaolinite, and −2.1 mV/◦C for the calcite for pressure values less than 45 psi. Thedecreasing trend of the zeta potential with temperature wasobserved in previous reported experiments for the quartz andkaolinite in Refs. [7,8]. In terms of the pressure response, a sys-tematic increase in zeta potential values with pressure is ob-served for the quartz at all pH values, whereas a monotonicdecreasing trend is measured for the kaolinite. The calcite re-sponse is variable with pressure. For pressures less than 45 psi,a decreasing trend is observed, whereas the behavior seems toincrease for higher pressure values.

In this paper we first present in Section 2 the strategy fol-lowed in the experimental procedure, including the design andconstruction of the electrophoretic cell. Section 3 contains theexperimental results of the zeta potential measurements withtemperature and pressure and their discussion.

2. Experimental methods

2.1. Sample preparation and characterization

Quartz, kaolinite, and calcite samples were obtained fromcommercial vendors. Solid characterization included elemen-tal analysis by X-ray photoelectron spectroscopy (XPS) using aLeybold LH-11 with a monochromatic X-ray source, and sur-face area measurements performed through a BET isotherm.Elemental composition as derived from the XPS analysis isgiven in Table 1. Table 2 summarizes the corresponding spe-cific surface areas of the chosen samples.

Samples were submitted to a careful cleaning process pre-vious to their preparation. The quartz samples were leachedwith concentrated nitric acid and repeated washing with dis-tilled water. The kaolinite was subjected to repeated washingwith NaCl using the procedure of Hollander et al. [11]. The

Table 1XPS elemental composition of the used samples (% atomic)

Quartz Kaolinite Calcite

C–O – – 15.99C–H 26.58 6.88 39.14O 40.65 40.62 32.15Al – 24.25 –Si–O 32.77 17.00 –Si–H – 11.25 –Fe – – –Ca – – 12.72

Table 2Specific surface area of selected reservoir minerals

Mineral SSA (m2 g−1)

Quartz 1.25 ± 0.05Kaolinite 21.33 ± 0.06Calcite 1.40 ± 0.15

sample was washed several times with distilled water and thenwith triply distilled water until a constant pH of the supernatantwas obtained. The samples were ground using conventional me-chanical methods and sieved in a 400 mesh.

As electrolytes, solutions with different NaCl concentrations(0.0001, 0.001, 0.01, and 0.1 M) were used. All glass materialused was cleaned with H3NO3 5 M, followed by a 50%–50%mixture of H2SO4 and H2O2 (3%), and finally washed with dis-tilled water.

Different solid/electrolyte suspensions were prepared usingthe selected set of minerals and the different electrolyte con-centrations. Each suspension contained 100 mg of solid in 1 Lof fluid. The solutions were let equilibrate for 24 h. Then ex-perimental measurements including particle size distribution,solution conductance, and surface charge density were per-formed as part of the characterization process and are given inAppendix A.

2.2. Solid/electrolyte suspensions properties

Solid/electrolyte suspensions were characterized by severalmeasurements including density, viscosity, and surface tension.The typical expected behavior of these properties was observedas a function of temperature. Surface charge density was alsodetermined as a function of the pH of the electrolytic solu-tion in the range from pH 2 to 9 by potentiometric titrationsat 20, 35, and 45 ◦C. Potentiometric titrations were done witha Metron 683 unit with a Dosimat 665 dosification unit for afast exchange of liquids. For the titrations, NaOH and HCl ofknown concentrations were used. Calibration buffers at pH 4and 7 were used.

2.3. Zeta potential measurements

The measurements were performed at room conditions us-ing a conventional commercial electrophoretic cell (Zeta Me-ter 3.0). The cell setup contained three main elements: a high-quality microscope for particle observation, an electrophoretic

790 K. Rodríguez, M. Araujo / Journal of Colloid and Interface Science 300 (2006) 788–794

cell where the colloidal particles are introduced, and a powersource that provides the electric field to the cell. A new cell wasdesigned and built for measurements at variable pressure andtemperature.

The new electrophoretic cell was built from Teflon and Plex-iglas. Material testing determined that the cell resist up to130 ◦C without bending. In terms of pressure the cell was ableto support 110 psi at room temperature. Several resistant rubberrings were placed on the cell boundaries to avoid any leakagewhen it was operated under pressure.

The cell was placed inside an external jacket for heating,which was performed by a continuous flow of water at the de-sired temperature. The jacket is made of Plexiglas and has twooutlets, allowing its connection to a heat bath for temperaturecontrol of the system. Sensitivity to flow and electrical mea-surements were performed on the cell to verify the presence orabsence of convection currents when the cell was operated un-der various temperature and pressure conditions. Fig. 1 presentsdifferent views of the cell.

For the injection of the suspension samples, a syringe pumpat the desired working pressure was used (constant-pressureinjection mode). Due to the nonconductive nature of the celljacket, temperature values were verified using two methods:(a) direct readings of temperature strips attached to the innersurface of the jacket close to the cell, and (b) a PT100 ther-mocouple connected to one of the ends of the glass capillarycell. Pressure was monitored through a gauge connected to theinjection line. The sample was preheated to the same cell tem-

Fig. 1. Different views of the electrophoretic cell designed for measurements atvariable pressure and temperature.

perature while being maintained under constant agitation priorits placement in the electrophoretic cell.

From each suspension, five samples of 100 ml were taken.The samples were set to five different pH conditions (2, 4, 6, 8,and 9) using HCl and NaOH solutions at 0.1 M concentration ata constant agitation. This gave 20 suspension samples for eachmineral except for the calcite, which suffers dilution at low pHvalues. The solutions were let equilibrate for 24 to 48 h.

Once a solution was inside the electrophoretic cell, the twoelectrode terminals were connected, and the voltage was ap-plied. The mobility of the particles was monitored through anoscilloscope. Between 30 and 50 measurements of zeta poten-tial were taken for each suspension sample. Getting triplicatemeasurement sets for each mineral sample controlled experi-mental reproducibility. Handling of the large number of datafrom zeta potential measurements was possible by automaticcapture and computer acquisition.

The new electrophoretic cell was calibrated using a com-mercial Zeta Meter 3.0 as a reference for measurements underambient conditions (20 ◦C, 14.7 psi). Additional measurementswere performed at 35 and 45 ◦C and four more pressures, 30,45, 60, and 74.7 psi.

3. Results and discussion

3.1. Quartz

The results for quartz zeta potential measurements as a func-tion of the pH of the electrolytic solution for 20, 35, and 45 ◦Cdegrees are shown in Fig. 2 for 0.01 M concentration. It is ob-served that the zeta potential becomes more negative as thetemperature is increased (i.e., the zeta potential absolute mag-nitude increases). Solid lines in the figure are just a guide tothe observed data trend. This behavior is consistent with exper-imental results from previous authors [7,8].

At alkaline pH, elevation of the temperature caused signifi-cant changes in the final zeta potential values, partly due to thechange in the pK of water and also due to the mineral solu-tion equilibria. This is reflected as an almost linear dependencebetween the zeta potential value and the pH. Somasundarannoticed the same effect on streaming potential measurementsup to 75 ◦C [8]. Tewari and McLean [12] observed similar pHchanges at elevated temperatures for the alumina–water system.

Fig. 2. Zeta potential as a function of pH for several temperatures. Diamondscorrespond to data at 20 ◦C, squares at 35 ◦C, and triangles at 45 ◦C for. Mea-surements are for electrolyte concentration of 0.01 M and 1 atm of pressure.Solid lines are just a visual guide for the observed trend.

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Fig. 3. Zeta potential as a function of pH for differential pressures. Symbolsare diamonds (14.7 psi), squares (30 psi), triangles (45 psi), crosses (60 psi),and stars (74.7 psi). Solid line is just a visual guide to show trend of data.Measurements were performed at 20 ◦C with a 0.01 M electrolyte solution.

The main cause for the quartz surface charge is the disso-ciation of the silanol groups at the interface. To understandthe temperature dependence of the zeta potential of quartz itis necessary to analyze the mineral solution chemical equilib-rium of the system at different temperatures. The temperaturedependence of the solubility of crystalline quartz [13] can bedescribed by the following equations:

SiO2 + 2H2O = H4SiO4,

log(H4SiO4) = 0.151 − 1162/T .

The reaction is independent of pH, and as can be seen, theH4SiO4 formation is favored at higher temperatures. The num-ber of ionizable ions per silicon atom is higher for a silicic acidsurface than for a fresh quartz surface. Thus, the silicic acidsurface is expected to have a larger surface charge density andhence a larger magnitude of the zeta potential. H3SiO−

4 is theonly major ionic species in solution. In alkaline solutions theequilibrium for quartz dissolution is governed by

H4SiO4 = H+ + H3SiO−4 , pK = 9.8,

−9.8 = − log(H4SiO4) + log(H+) + log(H3SiO−4 ).

Thus, as the pH increases while K is constant, log(H3SiO−4 )

must increase. Increases in both temperature and pH favor theformation of H3SiO−

4 , leading to more negative zeta potentialvalues. This approach is consistent with the analysis reportedby Van Lier et al. [14].

Fig. 3 shows zeta potential values at 20 ◦C for different pres-sures as indicated. A systematic decrease in the magnitude ofthe zeta potential value with pressure is observed. From the sta-bility diagram of quartz [15], we see that the a-quartz phasein the temperature and pressure range studied here is stable,thus no phase transformations are occurring in the system, andthat the increase in pressure can only change the suspension or-dering at the solid–liquid interface in a monotonic variation, asseen in the experiments at a given temperature. The solubility ofquartz is increasing with temperature and there is no pH depen-dence for the pH range between 2 and 9. For higher pH values(pH > 9), the solubility rises sharply, but this regime was notaddressed in this study.

Fig. 4. Zeta potential of kaolinite 0.01 M as a function of pH, at 20 ◦C. Line isonly a visual guide to the data trend.

3.2. Kaolinite

Measurement of the zeta potential of kaolinite and other claymineral particles is not so difficult if the particles are relativelylarge (>1 µm) and the electrolyte concentration is reasonablylarge (above about 20 µM) [5,16]. In the case considered herethese conditions were fulfilled, giving us confidence in the re-sults from the experiments, since the Smoluchowski equationcan be used.

Zeta potential values at room temperature are found to de-crease with pH as shown in Fig. 4. This behavior is similar tothose values reported by Hu and Liu for kaolinite samples [17].A similar experimental trend was reported by Ramachandranand Somasundaran [8] in measurements at 25 ◦C. In our exper-iments it was observed that as the temperature increases, themagnitude of the zeta potential increases for both acidic andalkaline pH values.

The kaolinite temperature response can be understood interms of the processes taking place in the system and the differ-ent species active in a given pH range. For alkaline pH values,zeta potential is negative and tends to stabilize for larger pHs.This result can be understood in terms of the activity of Al3+,which is very high in the acidic region and decreases rapidlywith increasing pH, a change of 16 orders of magnitude for thestudied pH range (from 2 to 9). In the alkaline region the majorspecies are H3SiO−

4 and Al(OH)−4 and their adsorption causesthe mineral to be highly negatively charged, resulting in a neg-ative zeta potential. For acidic pH values the mineral surfaceis in fact more positively charged at higher temperatures. Thiscould be attributed to dissolution and readsorption of Al3+ andAl(OH)2+ species, which have a high activity in this pH range.

For pH values near the neutral range the relevant species areAl(OH)3, H4SiO4, and H3SiO−

4 . The net negative potential onthe surface is attributed to the adsorption of H3SiO−

4 , which isthe only charged species that is active.

In terms of the pressure response, data from Fig. 5 corre-spond to zeta potential values versus pressure for pH 4. It isobserved that for pressure values up to 45 psi there is a clearlinear decreasing trend with slope ∼ −0.96 mV/◦C and forhigher pressure values, the trend has a lower slope in the rangeof −(0.5/0.6) mV/◦C.

Note that the effect of temperature is to produce a shift ofthe zeta potential values and that with pressure there is also adecaying trend, almost linear, tending to stabilize for large pres-

792 K. Rodríguez, M. Araujo / Journal of Colloid and Interface Science 300 (2006) 788–794

Fig. 5. Zeta potential versus pressure for kaolinite for three different pressuresat pH 4 and 0.01 M of electrolyte concentration. Symbols correspond to 20 ◦C(diamonds), 35 ◦C (squares), and 45 ◦C (triangles).

sures. The same behavior was observed for other pH values.A similar trend in temperature was reported by Ramachandranand Somasundaran for measurements of Na-kaolinite at 75 ◦Cat 1 atm (14.7 psi) [8]. However, these authors did not report anyevidence of reaching a plateau region, since they did not haveenough points, just reporting measurements for three points.

The understanding of the detailed mechanism of the pressureresponse of the zeta potential of kaolinite is not simple, sinceseveral species are active in the studied pH range. For example,Al3+ and Al(OH)2+ have a decreasing activity as a function ofpH, but they may reabsorb for pH ∼ 4, which is the case shownin Fig. 5, whereas the solubility of Al(OH)−4 increases with pH,and the increase rate is higher when pH ∼ 4. Other species, suchas H4SiO4, only display activity near pH ∼ 4 and a decreasingtrend in solubility. The case of H3SiO−

4 is interesting, since itshows increasing solubility for pH between 2 and 3, then de-crease around pH ∼ 4, and linear increasing activity for pHvalues higher than 5 (see activity plot in Ref. [8]). The analysisof the phase diagram for kaolinite shows that no phase changesare expected in the pressure and temperature range studied [15].The steadily decreasing trend observed in the zeta potential withpressure could be associated with local rearrangements of theseactive species near the inner and outer Helmholtz planes, andeffect that tends to stabilize at high pressure values. Further de-tailed modeling is required to understand the detailed orderingmechanism.

3.3. Calcite

For the calcite, as stated previously, only measurements athigh pH were performed due to dilution of the mineral [18].Sjoberg [19] noticed that calcite suspension in water is accom-panied by a phenomenon of surface dissolution followed bya reversion to equilibrium through a mechanism of recrystal-lization. Thus, the equilibrium state at the surface depends onthe kinetics of dissolution. The experiments show that calcitein aqueous media displays complex behavior mainly due to itssolubility, which is governed by many chemical equilibria andthe surface electrical charge.

In the case of calcite, even the sign of the surface charge hasbeen strongly disputed in the literature. Some authors, includingFuerstenau et al. [20] and Yarar and Kitchener [21], report apositive value, but others, such as Douglas and Walker [22] and

Fig. 6. Zeta potential versus pressure for calcite at pH 8 and three differenttemperatures. Data symbols are diamonds for 20 ◦C, squares for 35 ◦C, andtriangles for 45 ◦C. Solid lines are only visual guides to the data trend.

Smani et al. [23], indicated that the surface charge is negative.More recent work suggests that the surface charge depends onthe nature of the sample [24]. A similar situation also occurswith the zeta potential measurements, as reported by Moulinand Roques [9] and Amankonah and Somasundaran [25].

The dilution effect was also observed when the temperatureand pressure were changed. An increase in the magnitude ofthe zeta potential was observed with the increase in temper-ature with characteristic curves for this mineral. In the stud-ied pressure range, for pressure values less or equal to 45 psi,a decreasing trend of the zeta potential with temperature wasobserved with a rate factor on the order of −2.1 mV/◦C. Forhigher pressure values, such a trend disappears, and data sug-gest that the zeta potential value tends to increase; however,only one set of experimental points display this last behaviorfor every measured temperature, as shown in Fig. 6.

The pressure dependence of the zeta potential for the cal-cite is also characteristic of this mineral, as observed clearly inFig. 6. At 20 and 35 ◦C, zeta potential values are almost inde-pendent of pressure up to values of 45 psi. Then the behaviorchanges, giving rise to an apparent minimum around 60 psifor 20 ◦C. An increasing trend was observed for higher pres-sure values. From the data at 45 ◦C, there is a curvature in thefunctional form of the zeta potential however the characteris-tic apparent minimum at 60 psi observed for 20 ◦C seems to bepreserved with an increasing trend for higher pressures.

The behavior of the zeta potential with temperature can beunderstood by analyzing the processes and species involved.Regarding the sign of the zeta potential, it is dependent onthe number of excess Ca2+ or CO2−

3 ions available in solu-tion, in agreement with the work of Berlin and Khabakov [26]and Douglas and Walker [22]. With an increase in tempera-ture, the solubility of calcite decreases, as is also the case forCO2. The Ca2+ ions have increased solubility as the tempera-ture increases, allowing them to preferentially leave the calcitestructure, leaving the surface with a negative charge. This is thesimplest plausible mechanism to account for the observed re-sponse. In general, there are six dissolved species (H+, OH−,H2CO3, HCO−

3 , CO2−3 , and Ca2+) that are active in this pH

range, and the observed response is a combination of the be-havior of all of them. In the experiment we did not monitor thespecies concentrations; thus we are unable to model their ki-netic behavior.

K. Rodríguez, M. Araujo / Journal of Colloid and Interface Science 300 (2006) 788–794 793

More complicated processes associated with hydrolysis phe-nomena of surface carbonate ions could take place, as suggestedby Ney [27], and Smani et al. [23]. They considered that theCaOH+ ions determine the intensity and sign of the zeta poten-tial and that the charge is governed by hydrolysis phenomenaof surface carbonate ions. Somasundaran and Agar [28] postu-lated the hydrolysis either of the surface Ca2+ and CO2−

3 ionsor of the dissolved ions, the complexes formed being adsorbed.

The electrolyte concentration has a strong effect on the zetapotential value. This can be ascribed to the change in the con-centration of dehydrated calcium ions in the inner Helmholtzplane, due to variations in the dissolution rate. The pH depen-dency and the maximum value observed at pH 9 can be ex-plained by the reactions of Ca2+ ions with hydroxyl ions on theone hand, and bicarbonate ions on the other. For the electrolyteconcentrations used here the solubility limit was not reachedduring the performance of the experiments since no precipita-tion of solids was observed. However, such solubility effectsmay be enough to modify the structure of the electrical doublelayer, leading to the trend changes observed in the zeta poten-tial measurements, as shown in Fig. 6 for high temperatures andpressures.

In the hydrolysis interpretation of the surface behavior, thesurface ions Ca2+ and CO2−

3 obey the reactions [28]

CO2−3 + H2O ↔ HCO−

3 + OH−,

HCO−3 + H2O ↔ CO2 + H2O + OH−,

Ca2+ + OH− ↔ CaOH+,

and

CaOH+ + OH− ↔ Ca(OH)2aq.

For acidic pH, the first two reactions proceed to the right andthe two others to the left, whereas at high pH the opposite istrue. This reflects the change in sign of the surface charge withpH due to hydrolysis and the corresponding behavior of the zetapotential [22,23].

The pressure response of the calcite can be understood fromthe fact that the calcite solubility slightly increases with an in-crease in pressure [15] that is accompanied by an increase inconcentration of Ca2+ at the surface, thus leading to an overalldecrease in the surface charge (becoming less negative) and acorresponding decrease in the zeta potential, as seen in Fig. 6.However, as the concentration of Ca2+ ions increases, they maycombine with hydroxylic ions (Ca2+

surf + OH− ↔ CaOH+surf) and

HCO−3 (through the reaction Ca2+ + HCO−

3 ↔ CaHCO+3 ) both

resulting in a decrease of the zeta potential. The detailed lo-cal arrangements of ions and cations at the surface cannot beinferred from the measurements performed here and requiredetailed modeling, which is beyond the scope of this experi-mental work. The analysis given here is somewhat similar tothat of Siffert and Fimbel [29] for the calcite zeta potential at25 ◦C, where results were associated with the change of ionicspecies superimposed on problems of hydration and positionsof the various cations present in the inner and outer Helmholtzplanes.

Table A.1Particle size distribution of prepared suspensions

Mineral/electrolyte

Particle size (µm)

1.0 × 10−4 M 1.0 × 10−3 M 1.0 × 10−2 M 1.0 × 10−1 M

Quartz 33.4 26.4 28.4 30.6Kaolinite 23.1 25.9 29.6 27.8Calcite 32.5 31.3 30.2 33.2

Acknowledgments

The authors thank Yani C. Araujo and Hector Franco fortheir helpful contributions and encouraging discussions duringthe realization of this work.

Appendix A. Summary of general electrolyte properties

A.1. Particle size distribution

Particle size distribution was determined using a Mastersizerunit from Malvern Instruments. Each measurement was takenthree times and average values are reported in Table A.1. Parti-cle size under normal conditions is in the range between 21 and32 µm.

A.2. Viscosity

The viscosity of the suspensions was measured with a capil-lary viscosimeter. It displayed a monotonic increase with elec-trolyte concentration and pressure, and it tends to decrease withtemperature.

A.3. Surface tension

Surface tension was determined under room conditions bythe Guingeli plate method. A Robal scale for better precisionsupports the experimental unit. It was found that surface tensionincreases with brine concentration as observed for the densityand suspension viscosities.

A.4. Conductance

The conductance of the suspensions was measured witha Methrom conductimeter (686 Metrohm). Reproducibility ofconductance values was ensured by repeated measurements. Itis found that in all cases the conductance increases with tem-perature.

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