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IV. Components of Surface Charge
SURFACE CHARGE AND SOLUTE INTERACTIONS
area. Conceptually, diffuse-layer H+ is not included in the definition of uH The
values of uH can be negative, zero, or positive, depending on pH, ionic strength,
Inner-sphere complex surface charge density is contributed by the net total
charge of the ions, other than the potential determining ions (PDIs), such as H+or
OH-, which are bound into inner-sphere surface complexes. Outer-sphere surface
complexes are contributed by the net total charge of the ions, other than the PDIs,
which are bound into outer-sphere surface complexes. The inner-sphere and outer-spherecomplex charge components are also known as specifically adsorbed and
nonspecifically adsorbed charge components, respectively (Bowden et al., 1980).
V. SOLUTION-SURFACE INTERFACE
Most particles in aqueous media are charged for various reasons, such as the
ionization of surface groups and specific adsorption of ions. In a solution, the distribution of ions around a charged particle is not uniform and gives rise to an electric double layer (Hunter, 1981).The behavior of charged soil and colloidal particles in soil water suspension is similar to that of charged particles in an electric
field (Lee,similar to that of electrophoresis).During electrophoresisthe charge distribution in soil solution relative to the immobile capillary surface leads to the formation of an electrical double layer (Li, 1992). Similarly, in a soil with predominantly negatively charged particles there is an accumulation of cations and a deficit
of anions in the vicinity of the solid surfaces relative to the equilibrium solution.
The thermal motion of the ions counteracts the electrostatic interaction. Thus, as
cations are being attracted and anions repelled, the cation concentration increases
as the surface is approached, whereas the anion concentration decreases (Fig. 6).
The concentration of ions near the soil particle surface is high and it decreases with
increasing distance from the surface. This diffuse character of the counter ion
“atmosphere” was first noticed by Gouy ( 1910, 1917) and Chapman ( 1913), who
presented a theoretical relationship describing the diffuse layer.
According to the Gouy model, if the double layer is created by the adsorption
of PDIs such as H+or OH-, the electric potential at the double-layer surface is
solely determined by the concentration (or activity) of these ions in solution since
the particles act as a reversible electrode toward these ions. If this was so then the
potential is given by the Nernst equation:
Qo = (kT/ve)Zn(c/co)
where Q is the electric potential at the surface, k is the Boltzman constant, Tis the
absolute temperature, e is the electric charge, v is the valence of the PDIs, c is the
N. S. BOLAN ETAL.
High electrolyte concentration
Figure 6 The Gouy double-layer model.
concentration of these ions in solution, and co is the concentration at the point of
zero charge when a0= 0.
Despite the initial success of the Gouy model, deviations from the general conclusions of the theory were frequently encountered with specific colloidal systems.
Some of the difficulties with the model were attributed to the assumptions which
consider ions as point charges and because specific effects related to ion size are
neglected. Given these deficiencies, Stem (1924) proposed a double-layer model
which, unlike the Gouy model, considers that the closest approach of a counter ion
to the charge surface is limited by the size of these ions. Stem suggested that the
ion is separated from the surface charge by a layer of thickness 6 in which there is
no charge; this separation was later described as “Stern layer.” In this layer the
electric potential drops linearly with distance from a value a0at the surface to a
which is called the Stem potential (Fig. 7). Beyond this point the electric potential decreases approximately exponentially with distance similar to the
Gouy model. Unlike the Gouy model, however, the interaction between charged
particles is governed predominantly by the overlap of diffuse layers, so the potential most relevant to the interaction is that at the boundary between the Stem and
diffuse layers (the Stem potential) rather than the potential at the particle surface.
As discussed previously, this boundary (the Stem plane) is generally considered
to be at a distance of 0.3-0.5 nm from the particle surface and corresponds to the
diameter of a hydrated counter ion. The Stern potential cannot be measured directly, although the electrokinetic or zeta potential is often used as the closest approximation.
The structural charge, discussed previously, is balanced by cations or anions at
SURFACE CHARGE AND SOLUTE INTERACTIONS
Charge reversal due to specifically adsorbed counterions
Figure 7 The Stem model.
or near the mineral surface. The spatial distribution of these ions, often described
as counter ions, controls the colloidal behavior of clay minerals. Mattson (193 la,b)
was one of the first scientists to study the role of such charges in the colloidal behavior of clay minerals. Since then, a great deal of effort has been directed toward
the development of models which describe the interfacial chemistry of the colloidal particles; the best known of these models is the diffuse double-layer theory
(DDL). van Olphen (1977) suggested that the equilibrium distribution of exchangeable cations can quantitatively be described by the Boltzmann equation:
n+(x) = n: exp[-z+e+(x)/kr]
where n+ (x) is the concentration of cations, +(x) is the electric potential at a distance x from the surface, n: is the concentration of cations in the bulk solution, z+
is the cation charge, and e, k, and Tare as defined earlier.
Although Eq. (7) does not account for the effect of entropy on the spatial distribution of ions, the model illustrates that the electric potential develops as a result of the thermal energy of the counter ions which diffuse away from the surface to an extent limited by the higher energy states of ions further from the
surface. For real systems, such as colloid-water suspensions, the charge distribution on colloidal surfaces should also include the hydration energy because the
hydration water of the counter ions affects the distribution of ions at the surface
(Shainberg and Kemper, 1966).Shainberg and Kemper calculated cation distribution near a mineral surface, demonstrating that the presence of cations in the
Stem layer was dependent on their degree of hydration. For example, relatively
N. S. BOLAN ETAL.
few of the strongly hydrated lithium ions (Li+) are strongly adsorbed in the Stern
layer; most Li+ are in the diffuse layer. The opposite trend was evident for the
weakly hydrated potassium ions (K+).
However, the nature of interaction between
the ions in solution and the colloidal particle surface is dependent on the origin
and distribution of particle surface charge and potential. In the constant surface
potential systems, maintenance of surface chemical equilibrium is assumed during particle interactions but this may not be a realistic assumption because of the
very short time of the encounter between colloidal particles (typically about 10
ps for a Brownian collision; Verwey and Overbeek, 1948). Constant charge
interactions may be expected when the particles have a fixed surface charge density, such as with latex particles with bound ionic groups or clays with certain ionexchange capacity. Although much effort has been directed toward the development of theories of DDL interactions, the results are of limited use due to the lack
of information on real systems encountered in field environments. Moreover, the
dynamics of double-layer interactions in complex systems, such as soil colloids,
are not fully understood and it is for this reason that most calculations are based
on constant potential or constant charge models, neither of which apply directly
to real situations.
The net surface charge density (0) at any point near the charged surface is a result of the distribution of positive and negative charges over a unit surface area.
Such a distribution varies with the distance from the colloidal particle surface:
where e, z,, and n+ are as defined previously and z- and n- are the anion charge
and concentration, respectively, at distance x.
Gast (1977) provided a detailed mathematical treatment of the distribution of
electrical potential from the clay surface. According to his model, the total excess
charge of the diffuse double layer per unit surface is obtained by integration from
the surface (x = 0) into the bulk solution (x = 03):
eo = - 6 e a x
The double-layer model prediction of surface electric potential for permanentcharge and variable-charge surfaces varies substantially.For permanent-charge
surfaces and for small surface potentials there is an exponential decrease in
potential with distance from the surface (van Olphen, 1977):
= $0 exp( - a)
According to Figs. 6 and 7 and Eqs. (8) and (9), high counter ion charge (z), high
electrolyte concentration, or low dielectric constant of the solvent (E) should reduce the thickness of the double layer, thereby flocculating colloidal suspensions
SURFACE CHARGE AND SOLUTE INTERACTIONS
by reducing interparticle electrostatic repulsions. Flocculation occurs only if particles (i) collide with each other and (ii) can adhere when brought together by collision (Gregory, 1989). These predictions are well illustrated by Norrish (1954),
who demonstrated a reduction of interlayer spacings of montmorillonite at higher
electrolyte concentrations consistent with the double-layer model which predicts
a reduction in the thickness of the double layer with increasing electrolyte concentration. Other examples include the studies by Schofield and Samson (1953,
1954) and Quirk and Schofield (1955), who showed coagulation of soil colloidal
particles above a critical electrolyte concentration at which the thickness of the
double layer is reduced beyond the critical level, leading to destabilization of the
There are, however, many examples that show flocculationof colloidal particles
in electrolytes of weakly hydrating cations which is not predicted by doublelayer theory. One example of such a phenomenon includes the flocculation of clays
in the presence of quartenary ammonium (NH,+)salts, implying that in the absence
of strong cation hydration forces the Boltzmann distribution of cations near the
surface of layer silicates is nondiffuse (McBride, 1989). Such difficulties with the
DDL model led to the development of the Stem model (Stem, 1924), which reduces the predicted electric potentials near the surface to realistic values by recognizing that the closest approach of an ion to the surface is limited by the hydrated ionic radius (Shainberg and Kemper, 1966).
As discussed previously, regardless of the origin of the charge, electrical neutrality in the system must be maintained. This requires ions of charge opposite to
the surface (counter ions) to accumulate in a diffuse cloud around the particle. The
diffuse cloud is composed of an excess of ions of opposite charge to the surface
and a deficit of ions of similar charge (co-ions).This diffuse cloud of ions, together
with the charged surface, constitutes the electrical double layer. The behavior of
the variable-charge surfaces can be described qualitatively by the Gouy-Chapman
equation (van Olphen, 1977):
uo = ( ~ ~ & T / I T sinh(ze/2kT)+,
where uo, n, E, k, T z, e, and Jl0 are as defined earlier.
The variables which can easily be controlled in Eq. (1 1) to determine the variable surface charge and n and z of the counter ion. Because in variable-charge systems +o is constant, Jlo is a direct function of these variables. Thus, if the electrolyte concentration or valence of counter ions is increased, surface charge
increases. Because the surface area of the system remains constant, increasing the
N. S. BOLAN E T A .
charge on the surfaces increases the surface charge density. Because H+ and OHare the PDIs, surface charge is governed by pH and hence the frequently used but
technically incorrect term “pH-dependent charge.” From the Nernst equation,
which relates +o to pH:
+o = (kT/e) In (aH+/&+) = (2.303kT/e)@H0 - pH)
where pHo is the pH at which IJ.I~= 0, indicating that the surface has an equal number of positive and negative charges. pH, is identical to the PZC of the variablecharge surfaces. Combining Eqs. (11) and (12), one obtains
uo = (2nekT/n)”* sinh [1.15 z(pHo - pH)]
which shows that at some point on the pH scale, uo = 0, indicating that the variable-charge surface has a net zero charge. According to Eq.(13),one can readily
manipulate the surface charge by changing the values of (pH, - pH), n, and z.
When the relationship in Eq.(13) is plotted (Fig. 7), charge varies with pH, n, and
z. The latter two parameters define ionic strength (0in the relationship
I = 1/2 xtcizi’
where ci = (ni X 10-3)lN, and N is Avogadro’s number (6.02 X
Increasing n, z, and pH and decreasing pHo increases net negative charge in the
system and vice versa. In systems with mixed permanent- and variable-charge surfaces, the charge distribution takes the shape as shown in Fig. 7 as a result of
the addition of a constant value for the permanent negative charge (a,). In such
cases, one must redefine the point of zero net charge (PZNC), at which the entire
system (permanent plus variable charge) contains equal numbers of positive and
negative charges. Thus, the more permanent-charge minerals are present in a soil,
the lower the pH at which the soil reaches an overall net zero charge. For those
readers interested in a more complete description of these phenomena, see Uehara
and Gillman (1980).
The effect of concentration and valence of solutes on the thickness of the diffuse double layer is well known. An increase in the concentration of soil solution
(electrolyte) reduces the thickness of the electric diffuse double layer, enhancing
neutralization of negatively charged surfaces. Under this condition, electrostatic
repulsive forces between soil colloids are reduced. Consequently, soil humic
polyanions interact with soil inorganic colloids and organomineral complexes are
formed. Polyvalent cations may act as bridges between the inorganic surface and
organics to form such complexes (Theng, 1982; Oades, 1989). It is possible that
with decreasing ionic strength changes in surface-bound organic matter can complex more cations.
Diffuse layer surface charge density arises from the ions in the diffuse layer
which may move about freely in aqueous solutions while remaining near enough
to particle surfaces to create an effective surface charge density. The diffuse layer
SURFACE CHARGE AND SOLUTE INTERACTIONS
surface charge density develops mainly to balance the charge developed on the
Intrinsic surface charge density reflects particle or structural charge developed
developed from the adfrom isomorphic substitution (a,) and proton charge (aH)
sorption of H+or OH-:
The Stem layer surface charge density reflects particle charge developed from the
specifically (inner-sphere complex) and nonspecifically (outer-sphere complex)
adsorbed counter ions:
as= ais aos
The net total particle surface charge density (a,) then can be defined by
aP = (Ti"
+ a, = a, + U H + ais+ aos
Thus, the total particle charge density (a,) is balanced by the diffuse layer charge
+ OH + ais+ aos= -ad
This equation represents the balance of surface charge and can be applied both to
individual particles in suspension and to an entire suspension (Sposito, 1984).
Soils with active surface constituents, dominated by oxides and hydrous oxides,
do in fact behave differently from those dominated by clays where charges arise
from isomorphous lattice substitution. The majority of hydrophobic colloids acquire their surface charge by the adsorption of PDIs, which by strict definition are
ions common to the colloid and to the aqueous medium. Operationally, PDIs are
defined as those which leave the solution, cross over the real solid-solution boundary, and become part of the solid surface (Sposito, 1992). On this basis any ion
which is associated with Si4+or other metal ions as a ligand may be capable of altering the surface potential and hence fall into the category of PDIs. In constant
surface potential colloids the surface electrical potential is only constant as long
as the activity of the PDIs remains constant.
Surface PDIs should not, after adsorption, be chemically distinguishable from
ions already present in the lattice, and so PDIs should not alter the chemical potential of the surface of the solid. For many oxides surfaces, H+and OH- behave
as PDIs although they are not always constituents of the lattice. It is therefore debatable whether H+ and OH- can, without qualification, be termed PDIs on the
hydroxylated surfaces of insoluble oxides and hydrous oxides and on edges of clay
mineral lattice. It can be concluded that for oxides and hydrous oxides in aqueous
media, the most important pair of PDIs is H+and OH- and the development of
surface charge is measured by their adsorption. In the presence of other PDIs in an
aqueous oxide system, the specific adsorption causes the PZC to move to a lower
N. S. BOLAN ETAL.
pH in the case of anions and to a higher pH in the case of cations. Clearly, the PDIs
are adsorbed into (or lost from) the surface and hence fall into the category of
Most soils contain both variable-charge surface functional groups (ionizable
groups bound to inorganic or organic adsorbents) and permanent-charge surface
groups (created by ionic substitution within the crystal structure of minerals). The
net charge on colloidal particles bearing both variable and structural charges is the
sum of constituent charge densities (Eq. 18) (Madrid el al., 1984).
In general, when the charge densities are expressed on a unit mass basis the sum
of inner sphere, outer sphere, and dissociated surface charge is equal to the net adsorbed charge:
+ uos+ ad= (n+ - n - )
uo + U H = - (n+ - n - )
where n+ and n- are the adsorbed positive and negative charges, respectively. This
provides a basis for relating ion adsorption measurements to structural and variable surface charge densities. Equation (20) shows that the net adsorbed ion charge
may be pH dependent in soils which contain a mixture of permanent-charge clay
minerals and variable-charge adsorbents, such as organic matter and oxides and
hydrous oxides of Al, Fe, Mn, and Si.
In summary, diffuse double-layer models are capable of explaining the effects
of solution composition (e.g., pH, ionic strength, and valence of ions in solution)
on the nature and the amount of surface charge of homogeneous materials, such
as metal oxides and hydrous oxides. In heterogeneous media, such as soils which
contain both permanent- and variable-charge components, the double-layer models are less effective in predicting net surface charge. Nevertheless, these models
are useful in explaining the effects of surface charge on some soil properties, such
as flocculation and deflocculation, and the adsorption and desorption of inorganic
cations and anions (discussed in Section IX).
VI. CONCEPTS OF POINT OF ZERO CHARGE
The concepts of point of zero charge are only concerned with minerals exhibiting variable-charge behavior. Such minerals include the oxides and hydrous oxides of Al, Fe, Mn,and Si. These minerals generally do not exhibit permanentcharge behavior. As discussed previously, the ion-exchange capacity of these
minerals results from the adsorption of PDIs (H+ and OH-). These mineral surfaces exhibit amphoteric behavior and their surface charge varies with both pH and
electrolyte concentration. When the pH charge curves are plotted in suspensions
SURFACE CHARGE AND SOLUTE INTERACTIONS
Definitions of Some Point of Zero Charges“
Point of zero charge
or isoelectric point
Point of zero net
Point of zero salt
Point of zero net
pH at which the total net particle charge
pH at which the net proton charge is
equal to zero
The pH value that shows no change
with ionic strength
pH at which the total of dissociated and
the outer surface complex charges
(aU,/al), = 0
uos ud= 0
From “The Surface Chemistry of Soils” by Garrison Sposito. Copyright 0 1984 by Garrison Sposito.
Used by permission of Oxford University Press, Inc.
of varying electrolyte concentrations,the curves intersect at a common pH value.
This pH value is often defined as the PZC. However, since the introduction of this
concept, many PZCs have been identified and defined for variable-charge surfaces
(Table V), including PZC or zero point of charge (ZPC), PZNC, point of zero net
pristine charge (PZNPC), isoelectric point (IEP), and point of zero salt effect
(PZSE). According to Polubesova et al. (1993, one of the challenges for soil and
colloid chemists is to understand and apply these myriad “zero point” terminologies. Parker et al. (1979) insisted that terms such as ZPC and IEP were too vague
and preferred terms such as PZSE and PZNC. Sposito (1981) also suggested the
term PZNPC. Bowden et al. (1977) used the term isoelectric point of the solid and
pristine point of zero charge, and Hendershot (1978) used the zero point of titration. Furthermore, the abbreviations ZPC and PZC are used interchangeably.
Parfitt (1980) observed that “isoelectric weathering” (Mattson, 1932) may also
take place in that ZPC approaches the soil pH with time.
Sposito (1 984) indicated that PZCs are pH values at which one or more of the
individual components of the surface charge density specified are equal to zero
(Table V). The PZNPC, which is a pH value at which the net proton surface charge
density is zero (aH= 0), depends on the concentration of the ionizable surface
functional groups and on the composition of the solution phase. The PZNPC is the
most important PZC for soils which contain both permanent and variable charge
because it is the only PZC in which the contribution of aH is considered separately from that of cro. Many others have simply assumed that PZNPC is equal to PZSE
(Bolan er al., 1986b). Equality between PZPNC and PZSE requires the special
condition that the net adsorbed ion charge at the PZNPC is independent of ionic
strength and that a0 is equal to zero. Clearly, there is a need for chemists to iden-