Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy

COLLOIDS AND ELS EVI ER

Colloids and Surfaces A: Physicochemicaland Engineering Aspects 131 {1998 ) 77 87

A

SURFACES

Interactions between dissimilar surfaces in high ionic strength solutions as determined by atomic force microscopy S. Veeramasuneni, M.R. Yalamanchili 1, j. D. Miller * Department o/Metallurgical Engineering, 412 William C. Browning Buihling, College qf Mines am/Earth Sciences, UniversiO' o[Utah, Salt Lake City, UT84112, USA Received 17 May 1996: accepted 31 July 1996

Abstract

The recent non-equilibrium electrophoretic mobility measurements by laser-Doppler electrophoresis coupled with the stability and prevalence of collector colloids in soluble salt flotation systems suggest that the selective flotation of alkali halides is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Previously reported flotation results confirm this surface charge/collector colloid adsorption model. However, the nature of the interparticle forces responsible for heterocoagulation of such oppositely charged particles at high ionic strengths remains to be determined. In this regard, atomic force microscopy was used for interparticle force measurements at high ionic strengths. Model systems such as polystyrene/quartz and silica/sapphire were studied to measure the particle interaction forces prevalent at high ionic strengths. Results from this study indicate that while repulsive hydration forces exist between similarly charged hydrophilic particles, attractive forces exist between oppositely charged hydrophilic particles in high ionic strength solutions. The repulsive forces between similarly charged hydrophilic surfaces at high ionic strengths (2 4 M) are described by a double exponential function with decay lengths of 0.17 and 1.4 nm, depending upon the surfaces involved. On the other hand, attractive forces were observed between oppositely charged hydrophilic surfaces at high ionic strengths ( 2 4 M) and were described by a single exponential function with decay lengths of up to 9 nm, depending upon the surfaces involved. © 1998 Elsevier Science B.V. Kcvwordv." Alkali halides: Forces between dissimilar surfaces: High ionic strength; Hydration l\~rces

1. Introduction

The interactions of soluble salt particles in their saturated solutions is of interest from both a fundamental and practical perspective. For example, the selective separation of alkali halide salts such as KC1 from NaC1 by flotation in saturated brines lionic strength ~ 4 N) is of great industrial * Corresponding author. ~Present address: MEMC Electronic Materials Inc., 501 Pearl Dr., P.O. Box 8, St. Peters, MO 63376, USA 0927-7757/98,$19.00 ,g 1998 ElsevierScience B.V. All rights reserved. PII SI)927-7757(96)03929-5

significance and has been studied by flotation chemists for many decades [1 6]. A phenomenological description of particle dispersion,' aggregation has not been possible due to the high ionic strengths involved in these systems. Nevertheless. some progress has been made and the sign of the surface charge of soluble salt particles in their saturated brines has been established from non-equilibrium laser-Doppler electrophoresis measurements. Results for alkali halides are generally as expected from the simplified lattice ion hydration theory [7]. This recent electrokinetic

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S. Veeramasuneni et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 77 87

information coupled with the stability and prevalence of collector colloids in such soluble-salt flotation systems suggest that the selective flotation of alkali halide minerals is due to the adsorption of oppositely charged collector colloids by heterocoagulation. Experimental flotation results confirm this surface charge/collector colloid adsorption model for a variety of soluble-salt flotation systems including alkali halides, double salts, and alkali oxyanions [8-10]. However, the nature of the interparticle forces responsible for heterocoagulation of oppositely charged particles at these high ionic strengths remains to be described. Importantly, it has been shown that the surface charge developed by these alkali halide particles in saturated brines influences the stability of such particulate suspensions and the extent of particle interactions [11]. The DLVO theory is only of limited utility in describing suspension stability [12,13]. This is especially true at high ionic strengths such as the case for soluble-salt particulate suspensions. The extent of particle interaction has been studied by optical microscopy, which clearly shows that when oppositely charged alkali halide particles are examined as a binary mixture, significant aggregation occurs relative to the behavior of the individual salt particles at the same size and particle concentration, in which case they tend to remain dispersed [11,14]. The photographs presented in Fig. 1 show the extent of interaction between KC1 (negatively charged) and NaCI (positively charged) particles in their saturated brine. It can be observed from these photographs that the mixed particles are distinctly aggregated as compared to a suspension consisting of only one salt, which is clearly dispersed. Similar observations were made with quartz/NaC1 and a variety of other alkali halide systems [11]. These observations show that the DLVO theory is inadequate to describe particle interaction in these systems. Specifically, the stability observed in systems containing only one salt (for example, KC1 alone or NaCI alone, as shown in Fig. 1) can not be explained by the DLVO theory, which predicts aggregation of these salt particles at smaller separation distances due to van der Waals attractions. Israelachvili and others [ 13, 15,16] showed that the

DLVO theory fails to explain interaction forces between particles when they approach closer than a few nanometers in aqueous solutions at high salt concentrations. The basic reason for the failure of the DLVO theory is that the bulk water properties differ in the interracial region corresponding to small separation distances and consequently the interaction forces are quite different from those predicted by DLVO theory. The non-DLVO forces that occur at short ranges are known as structural or hydration forces. The hydration forces, which are now known to determine the interaction of surfaces at small distances of separation, were unrecognized until recently. In the past decade, however, there has been significant progress in measuring and characterizing these hydration forces, which have been found to be repulsive between like surfaces. It is believed that the hydration force arises due to the ordering of interracial water molecules. The ordering of water molecules depends on many factors such as the geometry and coordination number of aqueous species, the type of interface involved, and the physical and chemical nature of the surfaces involved. Generally, hydration forces have been considered to be repulsive in nature [121. Several researchers in the past decade have measured the hydration forces in different systems by using Israelachvili's surface force apparatus [13,15,16]. Israelachvili and Paschal [17] measured a short-range repulsive force between mica surfaces at high salt concentrations, which they attributed to the work required to dehydrate the adsorbed ions at small distances of separation. Pashely [18] reported a repulsive hydration force of approximately 10 raN/m( ~ 10 mJ/m 2) between curved mica surfaces (at a separation distance of < 1 nm) immersed in 1M KC1 solutions. Similar values were reported by Claesson for curved mica surfaces in NaI solutions [13]. Recently, Butt [19] measured these hydration forces between mica and a silicon nitride tip in aqueous solutions containing very high salt concentrations(>3M) using an atomic force microscope. The results from particle interaction experiments (Fig. 1) at high ionic strengths suggest that van der

S. I2'eramasuneni et al. / Colloids Sur[aces A: Physicocheol. Eng. A.~pects 131 ( 199~¢1 77 ,~'7

79

\

f,

' l),

"k7--~

r(L

Fig. 1. Particle interaction photographs for the KCI (negatively charged )NaCI {positively charged) system. (a) KCI 135 jam) particles dispersed in saturated brine. (b) NaCI (20 jam) particles dispersed in saturated brine. Ic) Aggregation of particles of KCI (40 jam) and NaC1 (20 jamt in saturated brine. (d) Aggregation of particles of KCI (35 jaml and NaC1 (200 jam) ill saturated brine [11,14].

Waals attractive forces are not particularly significant and that n o n - D L V O or structural forces must be considered in order to explain the state of interaction at these high ionic strengths. Further,

in the case of heterocoagulation, it is important to note that the sign of the surface charge appears to have a very significant effect at these high ionic strengths.

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S. Vceramasuneni et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 77 87

2. Research objective It is clear from previous research findings that measurement of interparticle forces involved in high ionic strength solutions may provide for improved understanding of the interfacial phenomena occurring in soluble-salt flotation systems. Also, knowledge of these forces should be relevant and beneficial to the study of biological/ physiological systems dealing with saline solutions, including phenomena such as protein adsorption processes. In this regard, atomic force microscopy was used for interparticle force measurements at high ionic strengths. Model systems such as polystyrene/quartz and silica/sapphire were studied to measure the particle interaction forces prevalent at high ionic strengths. Microspheres of hydrophilic silica and hydrophilic polystyrene (with R NH2 surface functional groups) were glued to an AFM cantilever in order to measure the surface forces involved in these systems as a function of pH and ionic strength. In addition, force measurements were conducted for the silica/sapphire system in the presence of LiC1 and CsCI saturated solutions in order to examine the effect of cation type on the measured interparticle forces.

3. Experimental 3.1. Materials Quartz (SiO2) and sapphire (AlzO3) single crystal windows (13x 1 ram) were purchased from Harrick Scientific Corporation. Tipless cantilever probes (V-shaped) were purchased from Digital Instruments for the AFM experiments. Polystyrene and silica microspheres, with a mean diameter of 9.7 and 4.8 p,m, respectively, were purchased from Bangs Laboratories, Inc. c~-alumina (Alfa Aesar, 99.99%, ~ 1 lam) and silica (Sigma, 99.9%, 1-5 gin) in powder form were purchased from Sigma Chemicals for transmittance studies. The chemicals used in the present study include reagent grade KC1, LiCI, and CsC1 with purity greater than 99% (Mallinckrodt, Inc.), reagent grade acetone (Fisher Scientific), reagent grade

methanol (EM Science), reagent grade sodium hydroxide (NaOH, Mallinckrodt, Inc.), and reagent grade hydrochloric acid (HC1, EM Science). A Milli-Q water system (Millipore) supplied with distilled water, provided high purity water with a resistivity of + 18 M~, and a surface tension of 72_+ 0.2 mN/m at 22°C. The pH of the high purity water was stabilized at pH 5.8+0.1 after equilibrating with the atmosphere.

3.2. Atomic force microscopy Force measurements were conducted using a Nanoscope E atomic force microscope (Digital Instruments, Inc.). Measurement of interaction forces at high ionic strengths involved two model systems, viz. hydrophilic polystyrene microspheres containing R-NH2 surface functional groups/ hydrophilic quartz plate, and hydrophilic silica microspheres containing Si-OH surface functional groups/hydrophilic sapphire plate. Both the quartz and sapphire single crystal windows used in this study were cleaned by washing with water/ methanol/water/acetone/water followed by drying and plasma cleaning. Plasma cleaning was carried out in a Tegal plasma chemistry reactor and the crystals were subjected to argon plasma for 30 to 40 min to remove any residual organic contaminants. The polystyrene microspheres suspended in distilled water were first dried by taking a few drops on to a glass plate. The silica microspheres were received in the dry state. These microspheres were cleaned using the same cleaning procedure described above for the single crystal windows. After cleaning, a single spherical particle (polystyrene/silica) was glued to the AFM cantilever tip using a speed bonder and an activator ( Loctite Corporation) by means of a micromanipulator and a CCD camera/monitor system. Extreme care was taken to prevent spreading of the glue on the cantilever tip. Fig. 2 shows a micrograph of a silica particle mounted on a V-shaped AFM cantilever. The standard contact AFM technique can be used to measure the interaction force between a smooth flat surface and a pyramidal tip (usually silicon nitride). However, the resulting force-separation distance curves can not be analyzed

X l'eeramasuneni et al. / Colk~ids Surjaces A: Physicochem. En~. A.~pects 131 : 1998; 77 b;7

81

",,,",, \

7"

3'

/

/ /

L

"

Fig. 3. Characleristic dimensions of the A F M cantilever,

Fig. 2. SEM photograph of a 4.8 pm hydrophilic silica particle glued to the V-shaped A F M cantilever. Extreme care was taken to prevent spreading of the glue on the cantilever tip.

theoretically due to the ill-defined geometry of the tip, which does not allow normalization of the measured forces with respect to the radii of the objects involved. Only when the force measurements are conducted between two crossed cylinders [20--23], two spheres [24], or between a sphere and a plate [25] can the data be analyzed theoretically and converted into force using the Derjaguin approximation [26]. For example, the surface force apparatus (SFA) is used for force measurements between two crossed cylinders. Other techniques such as internal reflection microscopy use sphere--sphere and sphere plate geometry for force measurements [27,28]. Atomic force microscopy has only recently been used to measure interaction forces with the sphere-plate geometry [19,29, 30]. For these A F M measurements, microspheres of radius of about 3-10 lam were used as probes in place of the pyramidal tips for the cantilever. Spring constants for the cantilevers used were calibrated from the dimensions of the cantilever and Young's modulus (E) of the cantilever material. The cantilever dimensions were determined by scanning electron microscopy. For the cantilevers (tipless probes) used in the present work, the spring constants/, values were calculated from the following expressions [31 ]. D'=Et'~"12

(1)

k = 6 D ' b s d i l L 3 ( 4 d 3 + t~3 )]

{2 )

where L, h, d. and t are as shown in Fig. 3. Since the cantilevers are made of silicon, a Young's modulus value of E = 1 . 5 x 1011 N/m e was used. There may be some error associated in estimating the k values by this method due to the microscopic measurements of the cantilever dimensions, particularly the thickness, d. Similarly, taking the value of E from a handbook may also introduce some error since E is prone to changes in the sample purity, crystallinity, and the method of sample preparation. In any case, a reasonable agreement between the k values calculated by this method and those supplied by Digital Instruments was obtained as shown in Table 1. 3.3.

Tr~llLS'ltlJlf~ltlgC 1tl~'~l,~71r('lll('lll.'~

Transmittance measurements were conducted using a Bausch & Lomb Spectronic 20 spectrophotometer with the light wavelength set at 580 nm. Carbon black and the respective saturated solutions (CsC1 and LiC1) were used to calibrate the instrument. In these experiments, silica and zalumina colloidal particles (I%) were added simultaneously to the saturated solution in a 50cm "~ Table 1 Calculated spring constants (k values) for the cantile,mrs used Cantilever No.

Calculated fi'om Eq. ( 2 ) (N m )

Supplied by Digital I nst rmnents, Inc. t N ml

I 2 3

0.126 0.101 0.121

ILl2 ILl2 11.12

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S. Veeramasuneni et al. / Colloids Surfaces A: Physicochem. Eng. Aspects 131 (1998) 77-87

beaker. The suspension was then conditioned for 10 min and was allowed to settle for 30 rain before transmittance measurements.

4. Results and discussion

For all force measurements only the approach curve was considered. First, raw data was obtained for both systems (polystyrene sphere/quartz surface and the silica sphere/sapphire surface) at different pH values and also at different ionic strengths. These deflection-Z (sample displacement in the z direction) curves were obtained by moving the piezo crystal on which the crystal plate was mounted along the z-direction toward the sphere at the end of the cantilever while monitoring the cantilever deflection. The initial non-linear portion of the force-Z curve represents the changes in attractive or repulsive forces with distance. After contact is established between the sample surface and the sphere, the force curve again becomes linear since the piezo and the sphere glued to the AFM cantilever move together. All the force measurements were conducted in a liquid cell in an aqueous environment as a function of pH and ionic strength. It should be mentioned here that excellent reproducibility was observed in the force measurements. Curves such as the force/radius-separation curve shown in Fig. 4 were obtained from the raw data by using an AFM analysis program [32]. In this analysis the zero points for both force and separation must be defined, of course the force is

i

0.5

Silica Sphere/Sapphire 2 M KCI pH 3.5

o

3O ~ -0.5 o -1

Separation [nm] Fig. 4. Force/radius - separation curve obtained between the silica sphere and sapphire substrate for a 2 M KC1 solution at pH 3.5.

calculated based on the the cantilever deflection [33]. The zero point of force was chosen where the deflection was constant (where the sample and sphere were far apart) and the zero point of separation distance was chosen when the cantilever deflection became linear with respect to sample displacement at large force values. Then the actual force-separation distance plots were obtained by using appropriate values for the spring constant of the cantilever and the radius of sphere glued to the cantilever. As discussed earlier, two different systems (polystyrene sphere/quartz plate and silica sphere/ sapphire plate) were used for AFM force measurements. It may be noted that the materials used in the present study namely, polystyrene, quartz, and sapphire have iep values of pH 9.0, 1.8, and 9.1, respectively [34-36]. In both systems, repulsive forces were observed at a pH value of 11.5, when the interacting surfaces are similarly charged and attractive forces were observed at a pH value of 3.5, when the interacting surfaces are oppositely charged. Similar results were obtained at different ionic strength values (2 M and 4 M KC1). As discussed earlier, the DLVO theory is inadequate to explain the forces observed in these systems at high ionic strengths. As a consequence, non-DLVO forces (hydration forces) at small separation distances are considered in order to explain these observed forces. The hydration forces, which are now known to determine the interaction of surfaces at small separation distances, were unrecognized until recently. In the past decade, however, there has been significant progress in measuring and characterizing these repulsive hydration forces between hydrophilic surfaces of similar charge. However, in the present work, attractive forces have been measured between oppositely charged particles at these high ionic strengths. These results suggest that the sign of the surface charge plays an important role and that attractive non-DLVO forces can be present between hydrophilic surfaces of opposite charge at these high ionic strengths, as shown in Fig. 4.

4.1. Repulsive forces Several researchers have recently measured the hydration forces in different systems (between like

83

s. I'eeramasuneniet al. Colloids Surfaces A: Physicochem. Eng. A~'pects 131 ( 1998~ 77 ,~7

surfaces) using the surface force apparatus [21,2Y37-40]. Earlier, Derjaguin and Zorin [41] and Pashley and Kitchener [42] measured the disjoining pressure at the quartz-water-air interface to be much larger than expected from electrostatic and dispersion forces, both of which are repulsive. It is also well-known that strongly hydrophilic colloids such as silica are anomalously stable and their stability can not be predicted by the DLVO theory [43]. Pashley [37] showed that the hydration force for mica immersed in various 1:1 electrolyte solutions is best described by a double-exponential function, the first (D1) and the second (O2) decay lengths being 0.17 0.3 and 0.6-1.1 nm, respectively. The repulsive forces observed in the present study at a pH value of 11.5 for both systems (polystyrene with surface R-NH2 functional groups/quartz and silica/sapphire) were also best described by a double-exponential function as shown in Fig. 5. In the case of polystyrene/quartz system, the first (D1) and second (O2) decay lengths were found to be 0.04 and 0.65 nm, respectively, at an ionic strength of 2M KC1 and 0.04 and 0.68 nm, respectively, at an ionic strength of 4 M KC1. Similarly in the case of sapphire/silica system the first (D~) and second (D2) decay lengths were found to be 0.17 and 1.4nm, respectively, at an ionic strength of 2 M KC1 and 0.1 and 1.42 nm, respectively, at an ionic strength of 4 M KC1. The decay lengths for the repulsive forces observed in the present work agree reasonably well with the reported decay lengths for repulsive hydration forces observed in symmetric systems [37]. 4.2. ,4 ttractive Jbrces

The attractive forces measured at a pH value of 3.5 in both systems (polystyrene/quartz with surface R - N H 2 functional groups and silica/sapphire) were described by an exponential function used by Israelachvili and Pashley [44] for the case of hydrophobic solids, which is of the form F/R = - (" e x p ( - H/Do)

(3)

where R is the radius of curvature of the mica surface, Do a parameter referred to as decay length, and C is the pre-exponential parameter. Since

Table 2 Force parameters obtained from Eqs. (3) and ~4) Force Parameter Polystyrene/Quartz Silica/Sapphire (mN/m)

2 MKCI

4 MKCI

2 MKCI

4 M KCI

C Cj D tnml

0.28 0.33 4.8

0.28 0.32 4.6

1.113 1.31 S.q

1.113 1.3l S.9

atomic force microscopy (sphere interacting with a flat substrate) was used for force measurements in the present work, R was considered to be equal to the radius of the sphere glued to the AFM cantilever. Table 2 presents the best-fit parameters of the exponential function for the data shown in Fig. 6. Also shown in Table 2 are the force parameters obtained using the so-called jump (gradient) method [21]. In this method, the jump distance (Hi) is determined as the point at which the slope ( d F / d H ) of the force curve becomes constant. This point represents the distance (Hi) at which the sphere jumps into contact with the sample surface [21,25,26]. This distance (Hi) is used to calculate the pre-exponential parameter. The two interacting surfaces jump into contact when the lbrce gradient ( d F / d H ) exceeds the spring constant k. From this information, the following relationship for the exponential force law can be derived [45] Cj = ( k D / R ) exp(Hj/D)

t4)

where the subscript j refers to the values obtained using the jump method. Eq. (4) was solved by using the D wtlues determined from the experimental data. It is clear from the results presented in Table 2 that the force parameter obtained by the jump method, i.e. Cj, is comparable to that obtained by using the equilibrium method (fitting the exponential function to the experimental data). 4.3. N o n - D L VO./orces

It is evident both from optical and atomic force microscopy studies that there exists both repulsive and attractive forces between hydrophilic surfaces at high ionic strengths, depending on whether the surfaces are similarly or oppositely charged. The

S. Veeramasuneni et al. / Colloids Surfaces A. Physicochem. Eng. Aspects 131 (1998) 77 87

84

1.2

,~ 0.4 E

E

E. 0.3

0.8

i U)

~

0.2 n-

P o l y s t y r e ~ 2 M KCI pH 11.5

0.1 uo 0 0

Silica Sphere/Sapphire 2 M KCI pH 11.5

0.4

~1~

I

I

2

4

0 [J. 6

0

0

I

0

I

2 4 Separation [nm]

Separation [nm]

6

1.2

0.4 E

E

~E 0.3

.E. 0,8

W

.:3 '1o 0.2

o~ "O

0,1 o

M. 0

I

0

Silica Sphere/Sapphire 4 M KCI pH 11.5

0.4

P o l y s t y r e ~ 4 M KCI pH 11.5

P

o 14.

v I

2 4 Separation [nm]

(a)

6

0

I

0

I

2 4 Separation [nm]

6

(b)

Fig. 5. AFM Force/radius separation distance curves obtained at pH 11.5 in 2 M and 4 M KCI. Solid lines represent the best-fit parameters for a double exponential function. (a) Negatively charged polystyrene sphere (with R-NH,, surface functional groups)/negatively charged quartz substrate. (b) Negatively charged silica sphere/negatively charged sapphire substrate.

DLVO theory fails to describe these forces observed at high ionic strengths. Failure of the DLVO theory may be due to the fact that the separating medium (water) is treated as a structureless continuum. While this approach may be applicable at large separation distances, for short distances from the surface the separating medium may have a distinct structure that varies with distance and is significantly different from that of the bulk. Evidence for this is given by the oscillation of repulsive hydration forces measured on mica surfaces [46,47]. Also, molecular simulations of water dipoles near surfaces [48-52] suggest that hydration forces arise due to the ordering of interfacial dipoles. The ordering of water molecules depends on many factors such as the geometry and coordination number of aqueous species and the physical and chemical nature of the surfaces

involved [12,53]. Marcelja and Radic [54] suggested that a polar surface will perturb interfacial water molecules and the propagation of this perturbation via dipole interactions will result in a force that extends over several molecular layers. The strength of the perturbation of the water structure would depend on the surface charge and other characteristics. Such an approach was favored by Rand and Parsegian [55] to describe the attractive hydration forces between phospholipid bilayers. In this regard, the attractive forces observed (optical microscopy) and measured (AFM) between oppositely charged surfaces at high ionic strengths appear to result from an antisymmetric water dipole arrangement near these surfaces. On the other hand, repulsive forces between similarly charged surfaces may be due to the symmetric dipole ordering near the surfaces involved.

S. Veeramasuneni et al. / Colloids Surfaces A. Physicochem. Eng. Aspects 131 ( 1998~ 77 ,~7

4

6

8

10

12

0

4

8

E

-0.05 u} .2 "o -0.10

u

a

-0.6

~ z 2 MKCI pH3.5

O -0.20

~

-0.8

~

-1.0

~

h

i

0

2 m

4

6

i

8

i

!

10

12

(~

,-~ 0.0 E

~ -0.10

._=

a

r

t

e

Separation [nm]

E -O.O5

~

r

pH3.5

Separation [nm]

u. 4).20

20

.m

~

'~ -0.15

o

20

E z -0.2 E ¢e -0.4

W

"~ -O.lS

16

0.0

E o.oo

E o.oo

12

85

pH3.5

8

12

16

=

=

=

v

-0.4

-0.8

~

p

h

i

r

e pH3.5

~. -1.0

Separation [nm]

(a)

4

-0.2

-0.6

z

0

Separation [nm]

(b)

Fig. 6. AFM Force/radius separation distance curves obtained at pH 3.5 in 2 M and 4 M KC1. Solid lines represent the best-fit parameters for a single exponential function. (a) Positively charged polystyrene sphere (with R NH 2 surface functional groups) negatively charged quartz substrate. (b) Negatively charged silica sphere/positively charged sapphire snbstrate.

At this point it is interesting to review the work done by Claesson et al. [56] and Lea et al. [57] who observed an attractive force stronger than that expected from conventional DLVO theory in concentrated electrolyte solutions between dissimilar surfaces. Claesson et al. suggested that the additional attraction necessary to explain the measured interaction (between positively charged hydrophobic mica and negatively charged hydrophilic mica) could originate either from a force similar to hydrophobic attraction between two coated surfaces or from ion-ion correlation effects neglected in the Poisson-Boltzmann approximation. In a more recent study, Lea et al. observed similar long-range attractive forces (at a separation distance of approximately 25 nm) between a negatively charged silicon nitride tip and a positively charged silicon nitride surface (silanized with 3-aminopropyltriethoxysilane). These authors mentioned that these forces could not be predicted either by the largely screened electrostatic force

existing in 0.1 M KNO3 solution, or by the van der Walls force. In any event, it is evident from these results that relatively long-range attractive forces, the origin of which is not known currently, exist between oppositely charged hydrophilic surfaces at high ionic strengths. To further examine the hypothesis of interfacial water being responsible for these observed attractive or repulsive forces at high ionic strengths, force measurements and transmittance studies were conducted for the silica/alumina system in saturated solutions containing LiC1 and CsC1. These two salts (which are at either end of the lyotropic series) were selected because of the difference in their degree of hydration. Fig. 7 presents the force/radius - separation distance diagram for the silica/' alumina system in two different saturated solutions (LiC1 and CsCI). It can be noticed from the figure that the attractive interaction forces are less in LiC1 solution when compared with the CsCI solution, both in magnitude and in the range.

S. Veeramasuneni et al. / Colloids Surfaces A: Physicochem, Eng. Aspects 131 (1998) 77-87

86 0

Z

5

10

15

20

25

-0,2

CsCI

spectroscopy should be used to determine the extent of interfacial water structure near polar surfaces at high ionic strengths in order to further understand the nature of the interparticle forces in high ionic strength solutions.

h5 -0.4 ca

n-

~ -0.6

5. Conclusions

u0

-0.8 Separation (nm)

Fig. 7. Force/radius - separation curves obtained between the silica sphere and sapphire substrate in saturated solutions of LiCI and CsC1.

Table 3 presents %transmittance values for the silica and alumina suspension in LiCl and CsC1 saturated solutions. It is clear from the %transmittance values that in LiC1 saturated solution the particles are more stabilized than they are in CsC1 saturated solution. It is evident from these results that cesium ions promote more coagulation between silica and alumina particles than lithium ions at these high ionic strengths. It is a wellestablished fact that the effectiveness of monovalent cations as coagulants usually decreases from cesium to lithium [12]. This is due to the greater extent of hydration of lithium ions. These results suggest that the structure making/breaking tendencies of these ions in saturated solution influence the interaction forces between charged hydrophilic surfaces. Certainly, additional research is warranted. FT-IR and Raman internal reflection spectroscopy studies should be undertaken for depth profiling of interfacial water [58,59]. In addition, sum frequency generation (SFG), a recent non-linear optical spectroscopy technique, should be useful for interfacial water studies [60]. Also, dielectric Table 3 % Transmittance values for silica/alumina system in two different saturated solutions Salt

%Transmittance

CsC1 LiCI

50 10

AFM was used to measure the interaction forces between two model systems, including polystyrene with R-NH2 surface functional groups/quartz and silica/sapphire, in high ionic strength aqueous solutions at different pH values. Short-range repulsive hydration forces were measured between similarly charged hydrophilic surfaces while relatively longrange attractive forces were measured between oppositely charged hydrophilic surfaces at high ionic strengths. These results suggest that surface charge plays an important role in influencing the nature of the interaction forces even at high ionic strengths. The results from the AFM force measurements support the aggregation/dispersion behavior, as studied by optical microscopy, of alkali halide particles in their saturated brines.

Acknowledgment Support by the DOE Basic Science Division, Grant No. DE-FG-03-93ER14315, is gratefully acknowledged.

References [1] J.P. Searls, Minerals Year Book-1989, US Bureau of Mines, United States Department of the Interior, Washington, 1990 p. 801. [2] W. Halbich, Metall Erz. 30 (1933) 431. [3] A.M. Gaudin, Flotation, 2nd ed. McGraw-Hill, New York, 1957. [4] D.W. Fuerstenau, M.C. Fuerstenau, Mining Eng. 8 1957 ~ 302. [5] H. Schubert, Eng. Min. J. 168 (1967) 95. [6] V.A. Arsentiev, J. Leja, in: M. Kerker, (Ed.), Colloid and Interface Science, Vol. V, Academic Press, New York, 1976, p. 251. [7] J.D. Miller, M.R. Yalamanchili, J.J. Kellar, Langmuir 8 (1992) 1464.

X Veeramasuneni et aL / Colloids Surfaces A: Physicochem. Eng. Aspects 131 ( 199& 77 87 [8] M.R. Yalamanchili, J.J. Kellar, J.D. Miller, Int. J. Mineral Process. 39 (1993) 137. [9] J.D. Miller, M.R. Yalamanchili, Minerals Eng. 7 11994) 305. [10] M.R. Yalamanchili, J.D. Miller, J. Colloid Interface Sci. 163 (19941 137. [11] M.R. Yalamanchili, J.D. Miller, in: B.M. Moudgil, P. Somasundaran, (Eds.), Proc. Engineering Foundation Conference on Dispersion and Aggregation: Fundamentals and Applications, Engineering Foundation, New York, 1994 p. 155. [12] J.N. Israelachivili, Intermolecular and Surface Forces, 2nd ed.. Academic Press, New York, 1992. [13] P.M. Claesson, Prog. Colloid Polym. Sci. 74 (1987) 48. [14] R.J. Roman, M.C. Fuerstenau, D. Seidel, Trans. AIME 241 (1968)56. [15] J.N. lsraelachvili, G.E. Adams, J. Chem. Soc., Faraday Trans. 1 74 (1978) 975. [16] R.G. Horn, J.N. Israelachvili, J. Chem. Phys. 75 (1981) 14()0.

[l 7] [18] [19] [20] [21]

J.N. lsraelachvili, R.M. Paschal, Nature 300 (1982) 341. R.M. Pashley, J. Colloid Interface Sci. 83 (1981) 531. H.-J. Butt, Biophys. J. 60 (1991) 777. J.N. Israelachvili, D. Tabor, Nature 236 (1972) 106. Ya.I. Rabinovich, B.V. Derjaguin, N.V. Churaev, Adv. Colloid Interface Sci. 16 (1972) 63. [22] B.V. Derjaguin, Ya.l. Rabinovich, N.V. Churaev, Nature 265 ( 1977 ) 520. [23] Ya.l. Rabinovich, M.B. Rizhikov, V.D. Sobolev, N.V. Churaev, Colloid J. USSR 44 11982) 977. [24] B,V. Derjaguin, I.I. Abrikossova, E.M. Lifshitz, Usp. Fiz. Nauk 64 (1958) 493. [25] P.van. Bolkland, J.Th.G. Overbeek, J. Chem. Soc.. Faraday Trans. 1 74 (1978) 2637. [26] B.V. Derjaguin, Kolloid Z. 69 11934) 155. [27] M.A. Brown, A.L. Smith, E.J. Staples, Langmuir 5 11989) t319. [28] D.C. Prieve, N.A. Frej, Langmuir 6 11989) 396. [29] W.A. Ducker, T.J. Senden, R.M. Pashley, Nature 353 1t~91 ) 239. [30] L. Meager, J. Colloid Interlace Sci. 152 (1992) 293. [31] J.E. Sader, L. White, J. Appl. Phys. 74 (1993) 1. [32] D.Y. Chan, AFM Analysis Software, Version 3.0.l, University of Melbourne, Victoria, Australia, 1994.

87

[33] W.A. Ducker, T.J. Senden, Langmuir 8 (1992) 1831. [34] M.R. Yalamanchili, A.A. Atia, J.D. Miller, unpublished results, 1995. [35] M.C. Fuerstenau, J.D. Miller, M.C. Kuhn, Chemistry of Flotation, SME Society For Mining, Metallurgy and Exploration Inc., New York, 1985. [36] S. Veeramasuneni, M.R. Yalamanchili, J.D. Miller, J. Colloid Interface Sci., 184, 594, 1996. [37] R.M. Pashley, Adv. Colloid Interface Sci. 16 (1982) 57. [38] R.M. Pashley, Chem. Scr. 25 11985) 22. [39] Ya.I. Rabinovich, B.V. Derjaguin, Langmuir 3 (1987) 625. [40] R.G. Horn, D.T. Smith, W. Hailer, Chem. Phys. Lett. 162 (1989) 404. [41] B.V. Derjaguin. Z.M. Zorin, Zh. Fiz. Khim. 29 119551 1755. [42] R.M. Pashley, J.A. Kitchener, J, Colloid Interface Sci. 71 11979) 491. [43] R.K. ller, The Chemistry of Silica. Wiley, New York, 1979 p. 375. [44] J.N. Israelachvili, R.M. Pashley, J. Colloid lnterPace Sci. 98 (1984) 500. [45] Ya.I. Rabinovich, R.H. Yoon, Langmuir 10 11994) 1903. [46] J.N. Israelachvili, Surf. Sci. Rep. 14 (1992) I1 I. [47] R.M. Pashley, J.P. Quirk, Soil Sci. Am. J. 53 ( 19891 1660. [48] I.K. Snook, W.J. van Megen. J. Chem. Phys. 72 (1980) 2907. [49] V.G. Dashevsky, G.N. Sarkisov, Mot. Phys. 27 (lq74) 1271. [50] A. Wallqvist. B.J. Berne, Chem. Phys. LeU. 145 t 1988) 26. [51] M. Marchesi, Chem. Phys. Lett. 97 (1983) 224. [52] I.K. Snook, W.J. van Megen. J. Chem. Phys. 75 (1081) 4738. [53] R.M. Pashley, J. Colloid Interface Sci. 80 ( 1981 ) 153. [54] S. Marcelja, N. Radic, Chem. Phys. Lett. 42 11976) t29. [55] R.P. Rand, V.A. Parsegian, Biochim, Biophys. Acta q88 { 1989) 351. [56] P.M. Claesson, P.C. Herder. C.E. Blom, B.W. Ninham, J. Colloid Interface Sci. 118 (1987) 68. [57] A.S. Lea, J.D. Andrade, V. Hlady, Colloids Surfaces A: Physicochem. Eng. Asp. 93 (1994) 349. [58] M.R. Yalamanchili, A.A. Atia, J.D. Miller, Langmuir, 12, 4176, 1996. [59] Z.S. Nickolov, J.C. Earnshaw, J.J. McGarvey, Colloids Surfaces A: Physicochem. Eng. Asp. 76 (1993) 41. [60] Y.R. Shen, Nature 337 (1989) 519.