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IV. Components of Surface Charge

IV. Components of Surface Charge

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SURFACE CHARGE AND SOLUTE INTERACTIONS



97



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,

etc.

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

A. DIFFUSE

DOUBLE

LAYER

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)



(6)



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



98



N. S. BOLAN ETAL.



High electrolyte concentration



Distance,d

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

value

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



6



99



DiffuseLayer



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]



(7)



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



100



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



(9)



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)



(10)



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



101



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

suspended material.

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).



B. DIFFUSEDOUBLE-LAYER

MODEL

OF VARIABLE-CHARGE

MINERALS

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



102



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)



(12)



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)]



(13)



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’



(14)



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



103



surface charge density develops mainly to balance the charge developed on the

particle.

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-:

Ui"



= a,



+ UH



(15)



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



(16)



The net total particle surface charge density (a,) then can be defined by

aP = (Ti"



+ a, = a, + U H + ais+ aos



(17)



Thus, the total particle charge density (a,) is balanced by the diffuse layer charge

(ad):



a,



+ OH + ais+ aos= -ad



(18)



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



104



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

chemisorbed ions.

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:

(Tis



+ uos+ ad= (n+ - n - )



uo + U H = - (n+ - n - )



(19)

(20)



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



105



lsble V

Definitions of Some Point of Zero Charges“

Symbol



Name



Definition



PZC or

ISP

PZNPC



Point of zero charge

or isoelectric point

Point of zero net

proton charge

Point of zero salt

effect

Point of zero net

charge



pH at which the total net particle charge

vanishes

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

is zero



PZSE

PZNC



Defining

equation

ud= 0



u,=o

(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-



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