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8 Force and Potential Energy of theDouble-Layer Interaction BetweenTwo Charged Colloidal Particles

8.1 INTRODUCTION

When two charged particles are approaching each other in an electrolyte solution,

the electrical diffuse double layers around the particles overlap, resulting in electro-

static force between the approaching particles [1–8]. The theory of the double-layer

interaction between two charged particles has been developed by Derjaguin and

Landau [1] and Verwey and Overbeek [2] and their theory is called the DLVO the-

ory. In this theory, the balance between the electrostatic force and the van der

Waals force (see Chapter 19) acting between two particles determines the behavior

of a suspension of charged colloidal particles, as will be discussed in Chapter 20.

That is, the DLVO theory assumes that the behavior of a colloidal suspension is

determined by the interaction potential between two particles, that is, the pair

potential. In the DLVO theory it is also essential to assume that the sizes of colloi-

dal particles are very large compared with those of electrolyte ions. Because of this

size difference, the rates of motion of the particles are negligibly small compared

with those of the electrolyte ions. This makes it possible to regard the motion of

the electrolyte ions as being about fixed particles placed at given distances from

one another. As a result, the interaction force and energy between the particles are

expressed as a function of the interparticle distance. This assumption corresponds to

the adiabatic approximation employed in quantum mechanics to describe diatomic

molecules. The pair potential is thus the adiabatic pair potential. In this chapter,

we give general expressions for the force and potential energy of the electrical

double-layer interaction and analytic approximations for the interaction between

two parallel plates.

8.2 OSMOTIC PRESSURE ANDMAXWELL STRESS

Hydrostatic pressure po is acting on a single uncharged particle in a liquid in the

absence of electrolyte ions. When the particle is immersed in a liquid containing

Biophysical Chemistry of Biointerfaces By Hiroyuki OhshimaCopyright# 2010 by John Wiley & Sons, Inc.

186

N ionic species with valence zi and bulk concentration (number density) n1i (i¼ 1,

2, . . . , N), in addition to the hydrostatic pressure po, the osmotic pressure is acting

on the particle, which is obtained by integrating the osmotic pressure tensor over an

arbitrary surface surrounding the particle. The osmotic pressure is given by the bulk

osmotic pressure tensorPo,

Po ¼ kTXNi¼1

n1i ð8:1Þ

If, further, the particle is charged, the particle charge and electrolyte ions

(mainly counterions) form an electrical double layer around the particle, as shown

in Chapter 1. The osmotic pressure becomes

P(r) ¼ kTXNi¼1

ni(r)

¼ kTXNi¼1

n1i exp � ziec(r)kT

� � ð8:2Þ

The electrolyte ions in the electrical double layer thus exert an excess osmotic

pressure DP(r) on the particle, which is given by

DP(r) ¼ P(r)�Po

¼ kTXNi¼1

n1i exp � ziec(r)kT

� �� 1

� � ð8:3Þ

At the same time, the coulomb attraction acts between the charges on the particle

surface and the counterions within the electrical double layer, which is obtained by

integrating the Maxell stress tensor over an arbitrary surface surrounding the parti-

cle. The Maxwell stress tensor is given by

T(r) ¼ ereo EE� 1

2E2I

� �ð8:4Þ

where I is the unit tensor and E¼� gradc is the electrostatic field vector with mag-

nitude E. Thus, the electrical double layer exerts two additional forces on the

charged particle: the excess osmotic pressure and the Maxwell stress (Fig. 8.1).

When two charged colloidal particles approach each other, their electrical dou-

ble layers overlap so that the concentration of counterions in the region between the

particles increases, resulting in electrostatic forces between them (Fig. 8.2). There

are two methods for calculating the potential energy of the double-layer interaction

between two charged colloidal particles [1,2]: In the first method, one directly

calculates the interaction force P from the excess osmotic pressure tensor DP and

OSMOTIC PRESSURE ANDMAXWELL STRESS 187

the Maxwell stress tensor T (Section 8.3). The potential energy V of the double-

-layer interaction is then obtained by integrating the force P with respect to the par-

ticle separation. In the second method, the interaction energy V is obtained from the

difference between the Helmholtz free energy F of the system of two interacting

particles at a given separation and those at infinite separation (Section 8.4).

8.3 DIRECT CALCULATION OF INTERACTION FORCE

The interaction force P can be calculated by integrating the excess osmotic pressure

DP and the Maxwell stress tensor T over an arbitrary closed surface S enclosing

either one of the two interacting particles (Fig. 8.3), which is written as [8]

FIGURE 8.1 Electrical double layer around a charged particle exert the excess osmotic

pressure DP and the Maxwell stress T on the particle.

FIGURE 8.2 Overlapping of the electrical double layers around two interacting particles.

188 FORCE AND POTENTIAL ENERGY OF THE DOUBLE-LAYER INTERACTION

P ¼ZS

DPþ 1

2ereoE2I

� �� n� ereo(E � n)E

� �dS ð8:5Þ

The potential energy V of the double-layer interaction is then obtained by inte-

grating the force P with respect to the particle separation.

Consider several cases.

(i) Two parallel plates. Consider two parallel dissimilar plates 1 and 2 of thick-

nesses d1 and d2, respectively, separated by a distance h between their sur-

faces (Fig. 8.4). We take an x-axis perpendicular to the plates with its origin

FIGURE 8.3 Calculation of the interaction force between two particles by integrating the

excess osmotic pressure DP and the Maxwell stress T over an arbitrary surface S enclosing

one of the particles.

FIGURE 8.4 Two parallel plates 1 and 2 at separation h. The interaction force P between

the plates can be calculated from {DP(x2)� T(x2)}� {DP(x1)�T(x1)}, where x1 is an arbi-

trary point in region I (�1< x� 0) and x2 is an arbitrary point in region II (0� x� h).

DIRECT CALCULATION OF INTERACTION FORCE 189

0 at the surface of plate 1. We denote the regions (�1< x��d1), (0� x�h), and (h + d2� x<1) by regions I, II, and III, respectively. As an arbi-

trary surface S enclosing plate 1, we choose a plane x¼ x1 in region I (�1< x��d1) and a plane x¼ x2 in region III. Here we have assumed that the

plates are infinitely large so that end effects may be neglected. The interac-

tion force P(h) per unit area between plates 1 and 2 per unit area can be

expressed as

P(h) ¼ [T(x2)þ DP(x2)]� [T(x1)þ DP(x1)]

¼ [T(x2)þP(x2)]� [T(x1)þP(x1)]ð8:6Þ

with

T(x) ¼ � 1

2ereo

dcdx

� �2

ð8:7Þ

P(x) ¼ kTXNi¼1

n1i exp � ziec(x)kT

� �ð8:8Þ

DP(x) ¼ kTXNi¼1


� �� 1

� �ð8:9Þ

where T(x) is the Maxwell stress and DP(x) is the excess osmotic pressure

at position x. By substituting Eqs. (8.7) and (8.8) into Eq. (8.6), we obtain

P(h) ¼ kTXNi¼1

n1i exp � ziec(x2)kT

� �� 1

2ereo

dcdx

��x¼x2

!224

35

� kTXNi¼1


� �� 1

� �� 1

2ereo

dcdx

��x¼x1

!224

35ð8:10Þ

We assume that the potential c(x) in the region outside the plates obeys

the one-dimensional planar Poisson–Boltzmann equation:

d2c(x)dx2

¼ � 1

ereo

XNi¼1

zien1i exp � ziec(x)

kT

� �ð8:11Þ


On multiplying dc/dx on both sides of Eq. (8.11), we have

dcdx

� d2cdx2

¼ � 1

ereo

XNi¼1

zien1i exp � ziec(x)

kT

� �dcdx

ð8:12Þ

which is rewritten as

d

dx

1

2ereo

dcdx

� �2" #

¼ d

dxkTXNi¼1


� �" #ð8:13Þ

By noting Eqs. (8.7) and (8.8), Eq.(8.13) can be rewritten as

d

dx[P(x)þ T(x)] ¼ 0 ð8:14Þ

or

d

dx[DP(x)þ T(x)] ¼ 0 ð8:15Þ

That is,

DP(x)þ T(x) ¼ C ð8:16Þ

or

kTXNi¼1


� �� 1

� �� 1

2ereo

dcdx

� �2

¼ C ð8:17Þ

where C is an integration constant independent of x but the value of C in the

region between the plates (region II) is different from that in regions I and

III. We denote the values of C in regions I, II, and III by CI, CII, and CIII

(=CI) respectively. We thus see that P(h) is given by

P(h) ¼ CII � CI ð8:18Þ

It is convenient to set x1¼�1, since c(x)¼ dc(x)/dx¼ 0 at x¼�1 so

that DP(�1)¼ 0 and T(�1)¼ 0 (Fig. 8.5). The left-hand side of Eq.

(8.16) (or Eq. (8.17)) becomes zero for region I, namely,

CI ¼ 0 ð8:19Þ


Consequently, Eq. (8.18) reduces to

P(h) ¼ CII

¼ DP(x2)þ T(x2)

¼ kTXNi¼1


� �� 1

� �� 1

2ereo

dcdx

��x¼x2

!2ð8:20Þ

where x2 is an arbitrary point between plates 1 and 2 (region II).

For low potentials, Eq. (8.9) can be linearized to give

DP(x) ¼ kTXNi¼1


� �� 1

� �

¼ kTXNi¼1

n1i 1� ziec(x)kT

þ 1

2

ziec(x)kT

� �2

þ � � �( )

� 1

" #

¼ �XNi¼1

n1i ziec(x)þ kTXNi¼1

n1i1

2

ziec(x)kT

� �2

þ � � �

ð8:21Þ

Here we use the condition of electroneutrality

XNi¼1

zin1i ¼ 0 ð8:22Þ

and introduce the Debye–Huckel parameter

FIGURE 8.5 Two parallel plates 1 and 2 at separation h. If x1¼�1, then DP(x1)¼ 0 and

T(x1)¼ 0.


k ¼ 1

ereokT

XNi¼1

z2i e2n1i

!1=2

ð8:23Þ

Then, Eq. (8.21) reduces to

DP(x) ¼ 1

2ereok2c

2(x) ð8:24Þ

where higher-order terms of c(x) has been neglected. Thus, Eq. (8.20)

becomes

P(h) ¼ 1

2ereo k2c2(x2)� dc

dx

��x¼x2

!224

35 ð8:25Þ

For further calculation of P(h), one needs to solve the Poisson–Boltz-

mann equation (8.11) under appropriate boundary conditions on the plate

surfaces to obtain the value of CII or c(x2) at an arbitrary point x¼ x2 inregion II between the plates, as shown in the next chapters.

If we approximate c(x) in region II (0� x� h) as the simple sum of the

unperturbed potentials of the two plates (see Eq. (1.25)), namely,

c(r; �) ¼ co1e�kx þ co2e

�k(h�x1) ð8:26Þ

This is only correct in the limit of large particle separations, since

Eq. (8.26) does not always satisfy the boundary conditions on the particles

surface. The interaction force P(h) per unit area between two parallel plates

at separation h is thus given by

P(h) ¼ 2ereok2co1co2e�kh ð8:27Þ

The potential energy V(h) of the double-layer interaction per unit area

between two parallel plates at separation h is given by

V(h) ¼ �Z h

1P(h)dh

¼ 2ereokco1co2e�kh

ð8:28Þ

Equations (8.27) and (8.28) are correct in the limit of large separations.

(ii) Two spheres. Consider two interacting dissimilar spheres of radii a1 and a2at separation R between their centers (Fig. 8.6). As an arbitrary surface

S enclosing sphere 1, we choose the surface of sphere 1. We consider the


low potential case and use the low potential form of Eq. (8.3)

DP(r) ¼ P(r)�Po

¼ 1

2ereok2c

2ð8:29Þ

It follows from Eq.(8.5) that the interaction force f is given by

P ¼ R p0

DPþ 1

2ereo(E2

r þ E2�)

� �cos�

�� ereoEr(Er cos�� E�sin�)

�r¼a1

�2pa21sin� d� ð8:30Þ

with

DP ¼ 1

2ereok2c

2 ð8:31Þ

Er(r; �) ¼ � @c(r; �)@r

ð8:32Þ

E�(r; �) ¼ � 1

r

@c(r; �)@�

ð8:33Þ

where P> 0 corresponds to repulsion and P< 0 to attraction. The poten-

tial distribution is derived by solving the spherical Poisson–Boltzmann

equation. Here we consider the simple case where the potential is as-

sumed to be simply given by the sum of the unperturbed potentials of

FIGURE 8.6 Two spheres 1 and 2 at separation R.


the two spheres (Eq. (1.72)) (see Fig. 8.7).

c(r; �) ¼ co1

a1re�k(r�a1) þ co1

a1r0e�k(r0�a1) ð8:34Þ

with

r0 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ R2 � 2rRcos�

pð8:35Þ

where co1 and co2 are, respectively, the unperturbed surface potentials of

spheres 1 and 2, respectively. Equation (8.34) is only correct in the limit of

large particle separations, since Eq. (8.34) does not always satisfy the

boundary conditions on the particles surface. The interaction force P(R)between two spheres 1 and 2 at separation R is thus given by

P(R) ¼ 4pereoco1co2a1a2

R2(1þ kR)e�k(R�a1�a2) ð8:36Þ

The potential energy V(R) of the double-layer interaction between two

spheres at separation R is given by

V(R) ¼ � R R1 P(R)dR

¼ 4pereoco1co2a1a2R

e�k(R�a1�a2)ð8:37Þ


FIGURE 8.7 Spheres 1 and 2. r is the distance from the center O1 of sphere 1 and r0 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ R2 � 2rRcos�

pis the distance from the center O2 of sphere 2.


For two identical spheres 1 and 2 of radius a carrying unperturbed sur-

face potential co separated by R, one may choose the intermediate plane at

z¼ 0 between the spheres as an arbitrary plane enclosing sphere 1 (Fig. 8.8).

Here we use the cylindrical coordinate system (r, z) and take the z-axis to bethe axis connecting the centers of the spheres and r to be the radial distance

from the z-axis. Equation (8.34) can be rewritten by using the cylindrical

coordinate (r, z) as

c(r; z) ¼ co

affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ (zþ R=2)2

p exp �kffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ (zþ R=2)2

q� a

� ��

þ co

affiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ (z� R=2)2

p exp �kffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ (z� R=2)2 � a

q� ��

ð8:38Þand Eq. (8.30) as

P ¼Z 1

0

DPþ 1

2ereo

@c@r

� �2

� @c@z

� �2( )" #

z¼0

2prdr

¼ 1

2ereo

Z 1

0

k2c2(r; 0)þ @c@r

� �2

� @c@z

� �2( )" #

z¼0

2prdr ð8:39Þ

for the low potential case. By substituting Eq. (8.38) into Eq. (8.39), we

obtain

FIGURE 8.8 Two identical spheres 1 and 2 with the intermediate plane S.The integral

taken on the surface represented by a dashed line (far from the sphere 1) may be neglected.


P(R) ¼ 4pereoc2oa

2

R2(1þ kR)e�k(R�2a) ð8:40Þ

(iii) Two cylinders. Consider two interacting infinitely long dissimilar cylinders

of radii a1 and a2 at separation R between their axes. As an arbitrary surface

S enclosing cylinder 1, we choose the surface of cylinder 1 (Fig. 8.9). We

consider the low potential case and use the cylindrical coordinate system (r,z), where the z-axis is taken to be parallel to the cylinder axes and r is thedistance measured from the cylinder axis. In this case, the interaction force

P(R) per unit length becomes

P ¼Z 2p

0

DPþ 1

2ereo(E2

r þ E2�)

� ��cos�� ereoEr(Er cos�� E�sin�)

�r¼a1

� a1d�

ð8:41Þ

with

DP ¼ 1

2ereok2c

2 ð8:42Þ

Er(r; �) ¼ � @c(r; �)@r

ð8:43Þ

E�(r; �) ¼ � 1

r

@c(r; �)@�

ð8:44Þ

FIGURE 8.9 Two cylinders at separation R.


where P> 0 corresponds to repulsion and P< 0 to attraction. We consider

the simple case where the potential is assumed to be simply given by the

sum of the unperturbed potentials of the two cylinders (Eq. (1.143))

c(r; �) ¼ co1

K0(kr)K0(ka1)

þ co2

K0(kr0)K0(ka2)

ð8:45Þ

r0 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ R2 � 2rRcos�

pð8:46Þ

where co1 and co2 are, respectively, the unperturbed surface potentials of

cylinders 1 and 2, respectively. Kn(z) is the modified Bessel function of the

second kind of order n. Equation (8.45) is only correct in the limit of large

particle separations, since Eq. (8.45) does not always satisfy the boundary

conditions on the particles surface. The interaction force P(R) per unit

length between two spheres 1 and 2 at separation R is thus given by

P(R) ¼ 2pereokco1co2

K1(kR)K0(ka1)K0(ka2)

ð8:47Þ

The potential energy V(R) of the double-layer interaction per unit length

between two cylinders at separation R is given by

V(R) ¼ �Z R

1P(R)dR

¼ 2pereoco1co2

K0(kR)K0(ka1)K0(ka2)

ð8:48Þ


8.4 FREE ENERGY OF DOUBLE-LAYER INTERACTION

In this section, we calculate the interaction energy V from the difference between

the Helmholtz free energy F of the system of two interacting particles at a given

separation and those at infinite separation (Fig. 8.10), namely,

V ¼ F � F(1) ð8:49Þ

The form of the Helmholtz free energy F depends on the type of the origin of

surface charges on the interacting particles. The following two types of interaction,

that is (i) interaction at constant surface charge density and (ii) interaction at con-

stants surface potential are most frequently considered. We denote the free energy

F for the constant surface potential case by Fc and that for the constant surface


charge density case by Fs. The expression for the interaction force (Eq. (8.5)), on

the other hand, does not depend on the type of the double-layer interaction.

8.4.1 Interaction at Constant Surface Charge Density

The first case corresponds to the situation in which the surface charge densities of

the interacting particles 1 and 2 remain constant during interaction. If the particle

surface charge is caused by dissociation of ionizable groups, and further, if the

dissociation is complete, then the free energy Fs for the interaction between two

particles 1 and 2 with constant surface charge densities s1 and s2, respectively,can be given by the electric part Fel of the double-layer free energy (Eq. (5.48)),

namely,

Fs ¼ Fel ¼ZS1

Z s1

0

c1(s0)ds0dS1 þ

ZS2

Z s2

0

c2(s0)ds0dS2 ð8:50Þ

where surface integration is carried out over the surface Si of particle i (i¼ 1, 2) and

integration with respect to s0 is the electric work of charging the surface of each

particle and ci(s0) is the surface potential of particle i at a stage at which the surfacecharge density is s0 during the charging process. For the low potential case where

c(s0) is proportional to s0,

FIGURE 8.10 Difference between the free energy F(R) of two interacting particles at

separation R and that for infinite separation gives the potential energy V(R) of the double-

-layer interaction between the particles.

FREE ENERGY OF DOUBLE-LAYER INTERACTION 199

Z s

0

ci(s0)ds0 ¼ 1

2sicoi (i ¼ 1; 2) ð8:51Þ

and thus Eq. (8.50) reduces to

Fs ¼ 1

2s1

ZS1

co1dS1 þ1

2s2

ZS2

co2dS2 ð8:52Þ

Here coi is the actual surface potential of particle i (i.e., the surface potential atthe final stage) and may differ at different points on the particle surfaces.

8.4.2 Interaction at Constants Surface Potential

In the second case, the surface potentials of the interacting particles remain constant

during interaction. Consider two interacting particles 1 and 2 whose surface charges

are due to adsorption of Ni ions (potential-determining ions) of valence Z adsorb

onto the surface of particle i (i¼ 1, 2). If the configurational entropy Sc of the

adsorbed ions does not depend on Ni, then the surface potential coi of particle i isgiven by Eq. (5.10), namely,

coi ¼kT

Zeln

n

noi

� �ð8:53Þ

where noi is the value of the concentration n of potential-determining ions at which

coi becomes zero, and the double-layer free energy is given by Eq. (5.13).

The free energy Fc of two interacting particles carrying constant surface poten-

tials co1 and co2 can thus be expressed by

Fc ¼ Fel �ZS1

s1co1dS1 �ZS2

s2co2dS

¼ZS1

Z s1

0

c1(s01)ds01 � s1co1

� �dS1 þ

ZS2

Z s2

0

c2(s02)ds02 � s2co2

� �dS2

¼ �ZS1

Z co1

0

s1(c0)dc0dS1 �

ZS2

Z co2

0

s2(c0)dc0dS2

ð8:54Þ

where surface integration is carried out over the surface Si of particle i (i¼ 1, 2) and

si(c0) is the surface charge density of particle i at a stage at which the surface

potential is c0 during the charging process.For the low potential case, Eq. (8.54) reduces to

Fc ¼ � 1

2c1o

ZS1

s1dS1 � 1

2c2o

ZS2

s2dS2 ð8:55Þ


where soi is the actual surface charge density of particle i (i.e., the surface charge

density at the final stage) and may differ at different points on the particle surfaces.

If the thermodynamic equilibrium with respect to adsorption of potential-determin-

ing ions does not attain, the constant surface charge model (instead of the constant

surface potential model) should be used [9, 10].

8.5 ALTERNATIVE EXPRESSION FOR THE ELECTRIC PART OF THEFREE ENERGY OF DOUBLE-LAYER INTERACTION

The electric part Fel of the free energy of double-layer interaction (Eq. (5.4)) can be

expressed in a different way on the basis of the Debye charging process in which all

the ions and the particles are charged simultaneously from zero to their full charge.

Let l be a parameter that expresses a stage of the charging process and varies from

0 to 1. Then Fel can be expressed by

Fel ¼ 2

Z l

0

1

lE(l)dl ð8:56Þ

with

E(l) ¼ 1

2

ZV

r(l)c(l)dV þ 1

2

ZS1

ls1c(l)dS1 þ 1

2c2o

ZS2

ls2c(l)dS2 ð8:57Þ

where E(l), r(l), and c(l) are, respectively, the internal energy, the volume charge

density and the electric potential at stage l in the charging process, and volume

integration is carried out over both regions outside and inside the particles.

8.6 CHARGE REGULATIONMODEL

If the dissociation of the ionizable groups on the particle surface is not complete, or

the configurational entropy Sc of adsorbed potential-determining ions depends on N,then neither of co nor of s remain constant during interaction. This type of double-

-layer interaction is called charge regulation model. In this model, we should use

Eqs. (8.35) and (5.44) for the double-layer free energy [11–13].

Namely, if there are Nmaxi binding sites for ions of valence Z on the surface of

particle i (i¼ 1, 2) and we assume 1:1 binding of the Langmuir type, then the dou-

ble-layer free energy is given by

F ¼ �RS1 R co1

0s(c0

o1)dc0o1dS1 �

RS1

Nmax1kT ln(1þ Ka1ne�yo1 )dS1

�RS2 R co2

0s(c0

o2)dc0o2dS2 �

RS2

Nmax2kT ln(1þ Ka2ne�yo2 )dS2

ð8:58Þ

CHARGE REGULATION MODEL 201

with

yoi ¼Zecoi

kT(i ¼ 1; 2) ð8:59Þ

where coi and Kai are, respectively, the surface potential and the adsorption constant

of ions onto the surface of particle i (i¼ 1, 2).

If, on the other hand, there are Nmaxi dissociable sites on the surface of particle i,then the double-layer free energy is given by

F ¼ �ZS1

Z co1

0

s(c0o1)dc

0o1dS1 �

ZS1

Nmax1kT ln 1þ nHe�yo1

Kd1

� �dS1

�ZS2

Z co2

0

s(c0o2)dc

0o2dS2 �

ZS2

Nmax2kT ln 1þ nHe�yo1

Kd1

� �dS2

ð8:60Þ

yoi ¼ecoi

kT(i ¼ 1; 2) ð8:61Þ

where coi and Kdi are, respectively, the surface potential and the dissociation con-

stant of dissociable groups on the surface of particle i (i¼ 1, 2).

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3. B. V. Derjaguin, Theory of Stability of Colloids and Thin Films, Consultants Bureau, NewYork, London, 1989.

4. J. N. Israelachvili, Intermolecular and Surface Forces, 2nd edition, Academic Press, New

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12. D. Y. C. Chan, T. W. Healy, and L. R. White, J. Chem. Soc., Faraday Trans. 1, 72 (1976)2845.

13. H. Ohshima, Y. Inoko, and T. Mitsui, J. Colloid Interface Sci. 86 (1982) 57.


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