Characterization of solid surfaces by electrokinetic measurements

Progress in Organic Coatings, 17 (1989)

CHARACTERIZATION MEASUREMENTS

115

115 - 133

OF SOLID SURFACES BY ELECTROKINETIC

HANS-JGRG JACOBASCH Institut fiir Technologie Hohe Str. 6, DDR-8010

der Polymere der Akademie Dresden (G.D.R.)

der Wissenschaften

der DDR,

Contents Introduction. ............................................. Theoretical background. ...................................... Measuring methods. ......................................... Inter-relation between zeta potential and the composition of the solid and liquid 4.1 Dissociation of acidic or basic functional groups ................... 4.2 Adsorption of ions. ...................................... 4.3 Electron acceptor-donator mechanism. ......................... Investigation of technological processes and technologically relevant material properties. ............................................... Conclusions .............................................. References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

115 116 117 120 120 124 127 128 132 132

1 Introduction Measurements of the electrokinetic or zeta potential have long been used in colloid chemistry mainly for the calculation of electrostatic interaction forces in lyophobic colloid systems. In addition, this method allows the characterization of solids relative to the chemical composition of the surface region and to the interaction of such solids with liquids. Both the chemical composition and interaction forces influence many technological processes and properties of coatings, polymeric materials, composites, etc. This article describes the theoretical background of surface characterization by electrokinetic measurements and the progress in developing methods for the measurement of electrokinetic phenomena, and gives some examples of the application of zeta potential measurements in technologically relevant fields. The surface characterization of plate or foilshaped organic polymers will be considered in particular. It should be mentioned that an extraordinarly large number of articles concerning the theory of electrical double layers and electrokinetic phenomena have been published in the past (cf. refs. 1 and 2). For this reason, it is not possible to give a review of these topics within this article. On the basis of a simple model involving electrical double layers, the inter-relationship 0033-0655/89/$7.15

@ Elsevier Sequoia/Printed

in The Netherlands

116

between electrokinetic strated.

phenomena and surface parameters will be demon-

2 Theoretical background The origin of electrokinetic phenomena and the inherent opportunity to apply electrokinetic measurements for characterizing solid surfaces can be deduced from the scheme of electrical double layers given in Fig. 1.

layer

distance -

Fig. 1. Schematic representation of the electrical double layer.

Generally, the charge distribution at the interface of a solid with a liquid phase differs from that inside the phases. There is an excess of positive or negative charges at the interface. According to Lyklema [ 31, the origin of charges at interfaces may be attributed to (i) the dissociation of surface groups, (ii) the preferential adsorption of cations or anions, (iii) the adsorption of polyelectrolytes, (iv) the isomorphic substitution of cations and anions, and (v) to the accumulation or depletion of electrons. For most nonmetallic surfaces, particularly organic polymers in contact with aqueous electrolytes, surfactants or polyelectrolyte solutions, the first three of the above mechanisms predominate. The isomorphic substitution of ions, e.g. of Si4+ ions by A13+ions, is commonly found in clay minerals. The accumulation or depletion of electrons is the main charging mechanism of metal-solution interfaces in the absence of Faradaic currents, and may play an important role in charging insulator-organic liquid systems. Generally, descriptions of the charge distribution at the solid-liquid interface are based on the model developed by Stern [4]. According to Stern, the charges at the solid surface are compensated by charges attributed partly to ions strongly adsorbed at the interface and partly to ions situated at a greater distance from the interface due to their thermal movement. The double layer is divided into a fixed or Stern layer and a diffuse or Gouy layer. Hence, we may write

117

where u‘, is the charge of the solid wall, u, is the charge of the Stern layer and cd is the charge of the diffuse layer. Information about improvements to the Stern model is available in the specialized literature [l - 31. The surface (wall) potential &, and the surface charge density u,,, as defined in Fig. 1, cannot be determined experimentally. However, the potential of the ‘shear plane’ in electrokinetic experiments {, can quite easily be measured by electrokinetic methods. The electrokinetic or zeta potential is frequently assumed to be equal to that at the boundary between the Stern and the diffuse layer, e.g. (2)

{"J/d

Whereas the potential decreases linearly with distance inside the Stern layer, the potential of the diffuse layer decreases according to an exponential function, i.e. (3)

K(K-d) ti@)

=

J/de-

where x is the distance, ‘reciprocal Debye length’:

d the thickness of the Stern layer and

K

is the

(4) where e is the charge of an electron, n the number of anions or cations per unit volume, 2 the valence, e. the influence constant, e the relative permittivity and k the Boltzmann constant. The surface charge at the distance d is given by Us = (~.e~nkZ’)~‘~ sinh For more details, see the specialized literature [ 1 - 41. Inter-relations between the zeta potential or surface charge and the chemical constitution of the solid surface, as well as the composition of liquids, is described in Section 4 below. 3 Measuring methods It is a characteristic of electrokinetic phenomena that an externally applied electrical field causes a relative movement of solid and liquid phase in solid-liquid systems (electrophoresis, electro-osmosis), or that the relative movement of solid and liquid phase generates an electrical potential or produces an electrical current (streaming potential/streaming current, sedimentation potential). Electrokinetic measuring methods based on these phenomena are shown schematically in Fig. 2.

118 flowing

liquid

IAp)

diaphragm

liquid

/ electrode

/ electrode

\ electrode

electrode

(a) microscope

It

(d) Fig. 2. Electrokinetic measuring methods. (a) Electra-osmosis; (c) electrophoresis; and (d) sedimentation potential.

(b) streaming potential;

The correlations between zeta potential and the parameters obtained by these various methods are given by eqns. (6) to (9), respectively. Elec trophoresis :

Streaming potential

Streaming current I, Electra-osmosis

reme&R {e. e,U D = =

vl

771

(9) , ,

where V’, is the particle velocity, E the field strength, u the electrophoretic mobility, a the particle radius, US and I, the streaming potential and streaming current, respectively, D the electro-osmotic volume flow, U the externally applied voltage, I the electrical current, R the electrical resistance, 1 and q the capillary length and cross-sectional area, and x the electrical conductance of the measuring solution. The measuring method most frequently employed is particle electrophoresis. The electrophoretic mobility can be measured within a wide range of particle diameters. Whereas the observation of particle movement by a standard microscope permits the characterization of particles with diameters ranging from 0.5 pm to 5 pm, the application of the lastest equipment (laser Doppler electrophoresis, rotating grating method) enables the determination of the zeta potential of particles with diameters between 10 nm

119

and 10 pm (cf. ref. 2). For concentrated dispersions, mass transport analysis (cf. ref. 1) or acoustophoresis 131 can be applied. The zeta potentials of coarse powders, fibres, plates, sheets, membranes, etc. have been determined mainly by streaming potential measurements. According to eqns. (7) to (9), the determination of the zeta potential depends on the existence of single capillaries or capillar bundles. Their length to cross-sectional ratio has to be determined, and the streaming potential (or streaming current) measured, as a function of the pressure decay in the capillary (capillar bundle). In addition, q and E, and if necessary R or K, must be measured. Figure 3 is the schematic representation of an automatic microprocessor-controlled measuring device which allows the rapid and reproducible measurement of the zeta potential by streaming potential/ streaming current meas~ements (cf. ref. 6). Recently, massing cells suitable for plate- or foil-shaped materials have been described in various publications [6 - 81. Electra-osmosis has predominantly been used for zeta potential measurements of porous plugs with small pore diameters. Systematic work has shown that the zeta potential of polymers can be determined unambigously, Le. independent of the measuring method and geometric parameters of the capillary system [9, lo]. It can be concluded from these results that the zeta potential is a well-defined parameter accounted for by the chemical constitution of the solid and the composition of the liquid.

re:iTder

plotter

-.--I-. I

r

computer

basic device

EKM

electronic

I

measuring

constant current/constant voltage supply source

3 measurement

conductwty

centrhfugal

\presrure

--%measuring

and

of potential,

sensor

pump

transducer

cell, elektrodes

Fig. 3. Schematic representation of the Electrokinetic Austria).

Meter EKM (Anton Paar KG, Graz,

120

4 Inter-relation between zeta potential and the composition of the solid and liquid 4.1 Dissociation of acidic or basic functional groups If N, acidic groups AH are present at the solid-electrolyte solution interface, the dissociation equilibrium may be described by eqns. (10) to (14) (cf. refs. 1 and 11) -AH +-A-+H+

(10)

[-A1[H+ls =

KA =

e-2.3B~A

[---AH]

N, = [-A-] (lI_ =

+ [-AH]

(11) (12)

[---A-l

(13)

NS a_ + a, = 1

(14)

where KA is the dissociation constant, pKA = log KA, [H’ls is the activity of the protons at the solid surface, and CL and (Y, are the fractions of -Aand -AH groups, respectively. The correlation between the activities (= concentrations) of the protons and functional groups, and the electrical potential and charge are given by eqns. (15) to (17).

W+l, = [H’lt+ -W,IRT

(15)

-(I~ = e[-A-]

(16)

(x-=

-

(Jo

eN,

= 1 +

[H+lb

e-F&,/RT

KA

(17)

where [H+],, is the proton concentration in the solution. If the Stern layer is neglected, i.e. assuming that $0 = 3‘

US)

and --(7, = 0,-J

(19)

the pK, value of the acidic groups, according to Fig. 4, results in eqn. (20) (cf. ref. 11).

pK, = pHb + 0.4343

where pH,, is the pH of the bulk solution.

(20)

121

basic

groups

Fig. 4. The pH dependence of the zeta potential of solids with acidic or basic functional groups at the surface.

The number of acidic groups capable of dissociating is given by eqn.

(21) (21)

eNs=2dmsinh

where y is the mole fraction of ions in the solution. The pK, value can be determined by inserting a pair of values into eqn. (20), i.e. pH, and c along the ascending part of the { uersus pH curve and the plateau value fplateau. Since the value of cr,iateaudepends on the ionic strength, the plot of { uersus pH has to be examined at different ionic strengths. Both the calculated values of pK, and N, must be extrapolated to the zero ionic strength. A simplified procedure for the determination of pK, values has been developed by Ottewill and Shaw [ 121. It can be applied to zeta potentials with values <25 mV. From this procedure, it may be shown that: PKA =

PH/s=s,,~~~~~~,~ + 0.4343

Frplatea”

2RT

(22)

The dissociation constant K, for basic groups B is given by eqns. (23) and (24). B + Hz0 e

BH+ + OH-

K = [BH+l[OH-Is z--z_[BH+] K, B PI F3201 PI W+ls

(23) [ BH+] KweFSjRT PI

[H+lb

(24)

where K, is the ionic product of water. Equation (20) gives a PKA’ value for basic groups that corresponds with PKB via eqn. (25) PKA +

PKB = P&

where pK, is the PKA value of water.

(25)

122

Approaches for the determination of pK* and pKn values of amphoteric polymers have been derived by Rendall and Smith [ 131, and Biirner (cf. ref. 11). Shortcomings of such approaches arise especially from the assumption that the number of dissociating groups can be determined from the charge associated with the diffuse layer. Since this assumption only holds true in exceptional cases, other methods (e.g. potentiometric titrations) generally give higher values for the dissociable groups (cf. ref. 1). On the other hand, the pK, values determined by zeta potential measurements agree very well with the dissoc~tion constants derivable from the chemical constitution of the groups [ 161 (see Table 2 below). The acidity or basicity of solid surfaces can be determined qualitatively from the pH which corresponds to a zero zeta potential (zero point of charge, often not quite correctly called isoelectric point, IEP). At this pH, the number of negative charges equals the number of positive ones. If spec~i~~y adsorbed ions are lacking from the Stern layer (cf. Section 4.2 below), these charges may be attributed to the dissociation of acidic or basic groups. In the case of low ZPC values, the number of acidic groups dominates. Provided the ZPC lies in the alkaline pH range, basic groups dominate. Table 1 lists the IEP values (=ZPC) of some inorganic oxides. According to Parks (cf. ref. 14), IEP is a function of the ratio of cation valence over effective cation radius. TABLE 1 isoelectric points of inorganic oxides (cf. ref. 14) Oxide

IEP (PI-I units)

SiOz TiOz

2 4.5

Cr203

7

Fe203

8.5

A1203

8

Fe0 WO

12 12

Figures 5 and 6 show, as examples, the influence of acidic or basic groups on the plot of c uers’suspH for polymers. While the presence of basic groups drastically changes the zero point of charge, the introduction of carboxylic groups into the PTFE surface causes a weak plateau over the pH range 4 to 5. Obviously, the dissociation of carboxylic groups occurs in competition with the specific adsorption of OH- ions. The pKA value of 4.2 as determined by eqn. (21) from the plateau in Fig. 6 corresponds well with the p& values of carboxylic groups listed in Table 2. Further examples concerning the determination of the kind and number of acidic or basic groups by zeta potential measurements have been

PUP. coated with a basic acrylate -2

3

4

5

d

PTFE.

i

lrradrated

sheets before and after coating with a basic acrylate.

lm?l

+I0

0

-10

PTFE. unmodified

before and after irradiation with &rays.

terephthalate)

Fig. 6. Plot of { uersus pH for poly(tetrafluoroethylene)

Fig. 5. Plot of 5 versus pH for poly(ethylene

+z ImV I

+20 / -

+lO

I 2

-20

-10

0

-30

-40

-20

rmtvi

PETP

-t lmVl

ti

b

with II-rays

;O pH

124 TABLE 2 p& values of acidic functional groups as determined by electrophoresis according to ref. 16 Functional group

p& value Ehctrophoresis

Titration COOH OS08 PO&

and titration,

2.5 - 4.8

2.5 - 4.3 -2

1 - 1.5

1 - 1.5

publish~ for polyamide dispersions and fibres [13,15 3, polystyrene latices [l&16] and the corrosion products of iron 1173. Nitzsche [18] found that a quantitative relationship exists between the oxygen and nitrogen content of silicon nitride and the IEP. 4.2 Adsorption of ions The description of the its-relationship between the adsorption of ions (inorganic, surfactant and polyelectrolyte ions) and the zeta potential is based on the following assumptions: 1. Adsorption occurs in the Stern layer and the relationship between potential and distance changes as shown schematically in Fig. 7. 2. The adsorption can be described by the Langmuir adsorption isotherm. 3. The ions are adsorbed by elec~os~tic and non-electros~tic forces, i.e. the adsorption free energy can be divided into an electrostatic and a non-electrostatic term of the type given in eqn. (27).

-w

-w

‘I

(a) Fig. 7. Schematic representation tial.

of the influence of ionic adsorption on the zeta poten-

125

The charge of the Stern layer, (Td, can be calculated according to these assumptions: (Td= ZeN,y’exp(-AG&d,/RT)

(26) (27)

where NI is the number of adsorption sites, AG,Oad, is the molar standard adsorption free energy of the cations and anions, $I+ is the molar nonelectrostatic adsorption free energy of the cations and anions (adsorption potential), and y is the mole fraction of the ions in solution. The electrostatic term associated with the adsorption free energy can be determined from the zeta potential of the solid in the pure liquid (e.g. water) used for preparing the electrolyte, surfactant or polyelectrolyte solutions. C#I+ can be calculated by several approximation procedures.

Fig. 8. The dependence of the zeta potential of negatively charged polymers on the concentration of electrolytes and ionogenic surfactants.

As shown schematically in Fig. 8, strongly adsorbed anions (e.g. chloride ions in 1: l-valent electrolytes, surfactant, and polyelectrolyte anions) result in a nearly parabolic plot of 5 uersus c. Strongly adsorbable cations (3- and 4-valent inorganic cations, surfactant and polyelectrolyte cations) induce a sign reversal in the zeta potential. According to Stern [4], the adsorption potentials of ions of 1:1-valent electrolytes, as in Fig. 8, may be calculated by means of eqns. (28) and (29):

126

These equations can be used for determining the adsorbability of 1:l valent electrolytes and anion-active surfactants. The adsorbability of polyelectrolyte anions has not yet been investigated quantitatively. The adsorption potential of primarily negatively charged polymers and surfactant cations can be determined from the surfactant concentration at which the sign of the zeta potential is reversed, according to Bijrner and Jacobasch [ 1 l] : #+ = RTln yczO

(30)

The values obtained using this approximation are quite consistent with the results [ 111 determined according to an approximation procedure developed by Ottewill and Rastogi [20] (cf. ref. 1): %O (Jd

0; = ZeJqk, - k,(ofzO - (Tg

(31)

where IJ:~’ and ui are the charges of the diffuse layer in the water and surfactant solution, respectively, and k, = exp(-f%id,/kT)/55.6

(32)

Using eqns. (28) and (29), the adsorbabilities of K+ and Cl- ions on numerous polymers have been determined (cf. ref. 21). The value of frnax in KC1 solutions was found to be lower the higher the hydrophilicity of the polymers. A linear relationship exists between c,,X and the contact angle with water [22]. In addition, the decrease in the zeta potential was found to be proportional to the water sorption of the polymer (cf. ref. 21). The relationship between the hydrophilicity and the zeta potential may be accounted for by competitive adsorption between the ions forming the double layer and water, and by the fact that the electrical double layer is formed in a swelling layer on the polymer. The thickness of the double layer, and as a result the decrease in the potential, is greater the more hydrophilic the polymer. The dependence of surfactant adsorption on solids on the structures of the latter has been investigated by numerous authors (cf. refs. 23 to 26). Generally, the adsorbability increases with increasing interaction energy between the solid and surfactant ions, and with increasing association of the hydrophobic surfactant residues. The dependence of the association energy (which correlates with the magnitude of the hydrophobic interaction energy between the alkyl residues) is described quantitatively by the approach of Schubert and Baldauf [27]. Zeta potential measurements also permit the characterization of adsorption layers of solids with regard to the degree of coverage and orientation. Comparisons between the amount of surfactant taken up by polymers and the change in the zeta potential provide evidence that the concentration which induces reversal in the sign of the zeta potential also causes the formation of a monolayer of adsorbed surfactant in the case of primarily negatively charged polymers and cation-active surfactants [21]. Hence, a negative zeta potential indicates a degree of coverage less than 1 and an orientation of the polar surfactant groups towards the solid. A positive

127

zeta potential points to a degree of coverage greater than 1 and an orientation of the polar group towards the solution. Similar statements can be made for primarily positively charged solids and anion-active surfactants. Zeta potential measurement can be applied beneficially to the investigation of the diffusion of ionic surfactants into polymers [28]. The change in the zeta potential of polymers in solutions of ionic surfactants, which arises from thermal treatment, may be attributed to diffusion. The investigation of pigments modified by adsorption layers has recently been described in detail by Schrijder [29]. Chibowski et al. (cf. refs 30 and 31) have reported the characterization of apolar adsorption layers at solids by zeta potential measurements. 4.3 Electron accep tar-dona tor mechanism According to Fowkes [32], the electrokinetic phenomena of solids in contact with liquids of low relative dielectric constants are caused by an electron acceptor-donator mechanism. Labib and Williams [33, 341 have shown that the sign of the zeta potential of solids depends on the donor number of the liquid. Thus, the donicity of solids equals the donicity of that liquid which causes a zero zeta potential. In this way, Labib and Williams have determined the donicity of a number of inorganic solids (Table 3). Table 3 also contains the donicity of polymers as determined by Biirner [35]. Simon [36] has investigated the adsorbability of triaryl methyl halogenide compounds from organic solvents onto SiO? particles which is due to an electron acceptor-donator mechanism, and has demonstrated excellent agreement between the donor properties of the adsorptive and the zeta potential.

TABLE 3 Donicities of polymers and inorganic materials as determined by electrophoresis Material

Donicity (kcal molP1)

CaC03 TiOz diamond Alz(SO&xHzO (heat-treated at 250 “C!) A12(S0&.xHz0 mica mica, (heat-treated at 250 “C) CaO polyamide poly(viny1 alcohol) poly(viny1 chloride) polypropylene poly( ethylene terephthalate)

0 -2.7 0 - 2.7 0 - 2.7 29-55 0 0 17 27.8 0 0 0 0 0

2.7 2.7 20 2.7 2.7

[34, 351

128

Results concerning the inter-relationship between zeta potential and the electron acceptor-donator properties of polymer solutions in contact with inorganic fillers have been published by Fowkes et al. [32, 381. The sign and magnitude of the zeta potential are given by the electron acceptor-donator properties of polymer, solvent and filler.

5 Investigation of technological processes and technologically relevant material properties As mentioned in Section 3 above, the zeta potential of solids of any shape can be measured reproducibly, and from this it has been shown that the course of technological processes is influenced by the chemical composition of the respective materials and the adsorption of different substances. Hence, the electrokinetic measuring method is very suitable for the scientific elucidation and control of technological processes. Some examples are given below which serve to demonstrate the applicability of zeta potential measurements to the investigation of adhesion processes and to describing some selected material properties. It is well known that the adhesion forces measured in real systems are determined by a number of geometrical factors such as contact area and distance, and entanglements of the adherends, as well as by the magnitude of the interaction forces between the adherends. If covalent chemical bonds, which only act in certain cases, are neglected, adhesion is found to be caused by dispersion and acid-base interactions. Dipole forces are negligible, and the hydrogen bond may be considered as a sub-set of acid-base interactions [32], The magnitude of such interactions depends on the chemical constitution of the surface regions of the adherends and the existence of adsorption layers, provided that their distance is known. Adsorption layers may be due to impurities, modifiers, processing agents, or to water which is always present. If solids are dispersed in a liquid medium, long-range electrical double-layer forces and structural forces occur in addition to the above-mentioned interaction forces (cf. ref. 39). The characterization of interaction forces by zeta potential measurements has been tackled using a number of approaches. Thus, in adhesion systems with mainly unspecific interaction forces, i.e. dispersion forces, a correlation has been demonstrated between the maximum zeta potential in KC1 solutions, as illustrated in Fig. 8, and adhesive forces as determined by practical measuring methods. The soiling tendency of textile fibres, and the adhesive strength of unsaturated polyester resins on glass fibres (without coupling agents), are both proportional to the fmax value of the fibres [21]. The decrease in the adhesive strength through adsorption of water or highly disperse oxides (SiO*; Ti02; Al,O,) may be correlated with a decrease in the tmax value. The influence of adsorbed substances on the adhesive strength can be described quantitatively on the basis of the thermodynamics of chemical equilibrium as given by eqns. (33) and (34).

129

The interaction of the adhesive substance C is given by AB+2C

A and B with an adsorbing

+AC+BC

The equilibrium K _

compounds

constant

K is given by

[AC1WI [ABI[Cl*

and it is valid to write that AGO=-RTlnK=xG-_GO=Gt+G&-G& E

(34)

s

where Gfi and Gi are the standard free energies of the final products and the starting materials; G,$ is equal to zero according to chemical reaction equilibrium, GiB is the adhesive free energy, and Glc and G& are the changes in the standard free energy of substance C on A and B, respectively. This simple approach has been applied successfully to the elucidation of the textile washing process (cf. ref. 21), the influence of moisture on the tensile strength of polymer-glass-fibre composites (cf. ref. 21), and of surfactant-modified polyethylene-chalk composites [ 401. It was found that the loss of tensile strength of polyethylen~chalk composites due to modification of chalk correlates with the adsorption free energy of the surfactants on chalk. The tensile strength of hot-melt polymer joints can be characterized by zeta potential measurements in KC1 solutions of the hotmelt substance as well as of the polymer substrate [41]. According to Bolger and Michaels (cf. ref. 14), the interaction between oxides and polar organic compounds may be accounted for mainly by acidbase interactions in the sense of the Briinsted theory. This also holds true for the interactions between metals and polar polymers, since under real atmospheric conditions the former carry layers of hydrated metal oxides. Such interactions may be described by the following equations: (i) Organic acids H MOH+HXR--,MO~*~~~HXR+

M&H2 . . . ..XR

(II)

(ii) Organic bases MOH+YR+MOH-.*.-YR-

-MO-.....H+R

(III)

where HXR is an organic acid and YR an organic base. The equilibrium constants of reactions (II) and (III) are given by eqns. (35) and (36), respectively.

K = [MO&+1W-l * [MOH] [HXR] K =

[MO-IN’RI

B

[MOH] [YR]

(35) (36)

130

The acidity and basicity of the metal oxides and the organic compounds may be calculated from eqns. (37) to (40): oxid

KA

_

[MO-I

W+l

(37)

-

IMOW oxid KB

=

[MO&+1 [MOHIW+l

(38)

erg_ EH’IW-1 WXRI

KA

(40) By assuming that the number of MOH: groups equals that of the MOgroups at the isoelectric point of the metal oxide, Bolger [14] has provided the following equations to express the strength of the acid-base interaction in systems involving metal oxides and organic compounds: AA = log KA = IEP - pKirg &3 = log KB = --pK;Xid

-

(41) pKrg

(42) The Bolger relationships (41) and (42) were generalized by Biirner (cf. ref. 42) to give: AA = log KA = pKgxid - pKrg m = log KB = -pK;Xid - pKg’8

(43) (44)

According to Fig. 9, published by Bolger, the interaction energy between oxides and organic compounds may be divided into three regions: (I) AA and AB have high and negative values; only dispersion forces occur between oxide and organic compound; acid-base interactions can be neglected. (II) AI? = 0 or slightly positive; acid-base interactions are approximately strong as the dispersion interactions.

as

(III) AB is clearly positive; strong acid-base interactions are observed. As described in Section 4.1, the pK values and isoelectric points of metal oxides, as well as the pK values of acidic and basic polymers can be determined readily by zeta potential measurements. Bolger gives examples of the applicability of his acid-base approach for metal polymer coatings with high strengths at positive AA and Al3 values, for the displacement of oil by water at oxidic surfaces, and the adsorption

131

REGION

I

REGION

II

REGION

III

POLYMER/SURFACE INTERACTION ENERGY TERM

FORCES

I 0

0 NEGATIVE

-

AA OR AB -

LONDON FORCES

POSITIVE

Fig. 9. Acid-base forces predicted by h4 or &3 values according to Bolger [ 141.

of surface-active substances by minerals [ 141. According to Fowkes et al. (cf. ref. 32), Bolger’s approach can be extended to acid-base interactions in the sense of the Lewis theory, according to which bases have electron donor properties and acidic materials are electron acceptors. When acidic and basic solids, or solids and liquids, are in contact electron exchange takes place, causing electrostatic attraction. Examples of the applicability of the electron acceptor-donator concept have been given for the mechanical properties of cast films, rubber, filled thermoplastics, and the adhesion of coatings [ 37, 381. Section 4.3 sketched out how it is possible to determine the electron acceptor-donor properties of solids by zeta potential measurements. Acidbase properties can be determined by spectroscopic, adsorption and contact angle measurements, as has been investigated by Fowkes (cf. ref. 32). In addition, zeta potential measurements are highly appropriate for enquiring into the effect of ionic modifiers and processing agents on technological processes, i.e. allowing conclusions to be drawn concerning the ionogenity of auxiliary agents, the degree of coverage, orientation and the occurrence of diffusion processes. Zeta potential measurements are particularly advantageous when investigating samples with small specific surfaces (plates, sheets) where the presence of small amounts of modifier are difficult to demonstrate by other methods. Detailed studies relating to the interaction of auxiliaries with fibreforming polymers [21] and the influence of processing additives on the surface properties of poly(viny1 chloride) [43] have been reported. The investigation of modified pigments by zeta potential measurements has recently been referred to by Schrijder [29]. The correlations observed between the zeta potential of surface-modified solids have been explained mainly in terms of changes in the electrochemical charge. The minimum adhesive force of polymers observed at zero zeta potential has been attributed to minimum polar interaction forces [21]. On the other hand,

132

satisfactory explanations of some published results, e.g. polyethylene-chalk composites, which show minimum impact strength and where modification of chalk with cation-active surfactants resulted in a zero zeta potential [ 441, are still awaited. Westwood et al. [ 451 have reported the so-called zeta correlation, according to which numerous materials show extrema of several mechanical properties (strength, hardness, coefficient of friction) at a zero zeta potential brought about by means of adsorption layers or variations in the pH value. Apart from the minimum polar interactions mentioned above, the influence of the electrical double layer charge on the structure of solids also requires explanation in terms of observed phenomena. Zeta potential measurements also provide interesting results when applied to the effect of coupling agents in the filling and reinforcement of polymers. Pliiddemann [46] has shown that the effect of aminosilanes is strongly influenced by electrostatic interaction between the glass-fibre surface and the amino group of the silane. This interaction may be characterized by zeta potential measurements. According to Fowkes [47] and Ishida [48], the effect of coupling agents is substantially influenced by acid-base interactions towards the reinforcing material. The influence of the treatment solution pH and the IEP of oxides within reinforcing materials may be attributed to the influence of the electrochemical charge (electrokinetic effect). 6 Conclusions Despite uncertainties in modelling the formation of an electrical double layer at a solid-liquid interface, the surface properties of solids can be described satisfactorily with the help of approximate solutions concerning the correlation between zeta potential and dissociation, electron exchange and adsorption processes occurring at the solid-liquid interface. In addition, zeta potential measurements are very appropriate for characterizing the interactions at interfaces, particularly in solid/solid and solid/liquid systems. Largely automatic measuring systems are available; they permit fast and reproducible determination of the zeta potentials of solids of almost any shape. Thus, zeta potential measurements allow the determination of technologically relevant surface properties of solids and their changes by modification or adsorption processes, and provide statements concerning the optimal course of technological processes at solid surfaces. References 1 R. J. Hunter, Zeta Potential in Colloid Science, Academic Press, New York, 1981. 2 A. Kitahara and A. Watanabe, Electrical Phenomena at Interfaces, Marcel Dekker, New York, Basle, 1984. 3 H. Lyklema, in J. W. Goodwin (ed.), Colloidal Dispersions, The Royal Society of Chemistry, London, 1981, p. 47. 4 0. Stern, 2. Elektrochem., 30 (1924) 508. 5 D. Fairhurst, B. J. Marlow and H. P. Pendse, Abstr. Colloid Conf., Graz, 1987.

133 6 J. Schurz, Ch. Jorde, V. Ribitsch, H.J. Jacobasch, H. Kiirber and R. Hanke, GZTZ. Laboratoriumstechnik, 30 (1986) 98. 7 R. A. van Wagenen and J. D. Andrade, J. Colloid Interface Sci., 76 (1980) 305. 8 H. Kaden, R. Trauzeddel, J. Bander and N. Kuehn, Wiss. 2. Tech. Hochsch. Leipzig, 8 (1984) 309. 9 H,.-J. Jacobasch, G. BaubSck and J. Schurz, Colloid Polym. Sci., 263 (1985) 3. 10 A. van der Put, Thesis, Agricultural University Wageningen, 1980. 11 M. BSrner and H.-J. Jacobasch, in Electrokinetische Erscheinungen ‘85, Institute of Polymer Technology Dresden, 1985, p. 227. 12 R. H. Ottewill and J. N. Shaw, Kolloid 2. 2. Polym., 218 (1967) 4. 13 H. M. RendaIl and A. L. Smith,J. Chem. Sot., Faraday Trans. 1, 74 (1978) 1179. 14 J. C. Bolger, in K. L. Mittal (ed.), Adhesion Aspects of Polymeric Coatings, Plenum Press, New York, London, 1983, p. 3. 15 H. Baumann, W. Schempp and J. M. Marzinkowski, Acta Polymerica, 31 (1980) 475. 16 G. Kretzschmar and K.-H. Lerche, Acta Polymerica, 31 (1980) 518. 17 U. Kiinzelmann, Thesis, Technical University, Dresden, 1989. 18 R. Nitzsche, private communication, 1989. 19 H.-J. Jacobasch and J. Schurz, Prog. Colloid Polym. Sci., 77 (1988) 40. 20 R. H. Ottewill and M. C. Rastogi, Trans. Faraday Sot., 56 (1960) 880. 21 H.-J. Jacobasch, OberfZachenchemie faserbildender Polymerer, Akademie-Verlag, Berlin, 1984. 22 N. Kuehn, H.-J. Jacobasch and K. Lunkenheimer, Acta Polymerica, 37 (1986) 394. 23 M. von Stackelberg, W. Kling, W. Benzel and F. Wilke, Kolloid Z., 135 (1954) 67. 24 Y. Iwadare, Bull. Chem. Sot. Jpn., 43 (1970) 3364. 25 T. Suzawa and M. Yuzawa, Yukagaku, 15 (1966) 20. 26 J. B. Kayes, J. Colloid Interface Sci., 56 (1976) 426. 27 H. Schubert and H. Baldauf, Tenside, 4 (1967) 172. 28 H.-J. Jacobasch and I. Grosse, Textihechnik, 37 (1987) 266,316. 29 J. SchrBder, Prog. Org. Coat., 16 (1988) 3. 30 E. Chibowski, B. Janczuk and W. Wojccik, in K. L. Mittal (ea.), Physicochemical Aspects of Polymer Surfaces, Vol. 1, Plenum Press, New York, London, 1983, p. 217. 31 E. Chibowski, in Electrokinetische Erscheinungen ‘85, Institute of Polymer Technology, Dresden, 1985, p. 119. 32 F. M. Fowkes, in K. L. Mittal (ed.), Physicochemical Aspects of Polymer Surfaces, Vol. 2, Plenum Press, New York, London, 1983, p. 583. 33 M. E. Labib and R. Williams, J. Colloid Interface Sci., 97 (1984) 356. 34 M. E. Labib and R. Williams, J. Colloid Znterface Sci., 115 (1987) 330. 35 M. Biirner, private communications. 36 F. Simon, Diploma Paper, Friedrich-Schiller-Universitiit Jena, 1988. 37 J. A. Manson, Znt. Conf Interface-Znterphase Comp. Mater., Liege, 1983. 38 M. J. Marmo, M. A. Mostafa, H. Jinnal, F. M. Fowkes and I. A. Manson, Znd. Eng. Chem., Prod. Res. Dev., 15 (1976) 206. 39 J. N. Israelachvili, Intermolecular and Surface Forces, Academic Press, London, 1985. 40 M. Ratzsch, H.-J. Jacobasch and K.-H. Freitag, J. Adhes. Sci. Technol., in press. 41 B. Stiehl, R. Hiissler, T. Morgeneyer and H.-J. Jacobasch, unpublished results. 42 H.-J. Jacobasch, in Elektrokinetische Erscheinungen ‘85, Institute of Polymer Technology, Dresden, 1985, p. 33. 43 M. E. Lahib and R. Williams, Colloid Polym. Sci., 262 (1984) 551. 44 M. Ratzsch, H.-J. Jacobasch and K.-H. Freitag, in B. Sedlacek (ed.), Polymer Composites, de Gruyter, Berlin, New York, 1986, p. 413. 45 A. R. C. Westwood, J. S. Ahearn and J. J. Mills, Colloids Surfaces, 2 (1981) 1. 46 E. P. Pliiddeman, S&me Coupling Agents, Plenum Press, New York, London, 1982. 47 F. M. Fowkes, J. Adhes., 4 (1972) 155. 48 H. Ishida, in K. L. Mittal (ed.), Adhesion Aspects of Polymeric Coatings, Plenum Press, New York, London, 1983, p. 45.