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Journal of Colloid and Interface Science211,252–263 (1999)Article ID jcis.1998.5951, available online at http://www.idealibrary.com on

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The Electrochemistry of Nonaqueous Copper PhthalocyanineDispersions in the Presence of a Metal Soap Surfactant:

A Simple Equilibrium Site Binding Model

Paul Jenkins,*,1 Subhayu Basu,*,2 Roland I. Keir,† John Ralston,* John C. Thomas,† and Brigette M. A. Wolffenbutte,3

* Ian Wark Research Institute and†School of Physics and Electronic Systems Engineering,University of South Australia, Mawson Lakes, Adelaide, SA 5095, Australia

Received May 4, 1998; accepted October 24, 1998

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The electrophoretic mobilities of copper phthalocyanine parti-les, dispersed in isoparaffin solutions containing zirconium oc-anoate, have been determined using phase-analysis light scatter-ng. All the samples studied contained trace concentrations ofater. The mobility values were converted to zeta potentials using

he Huckel equation. All the systems studied exhibited a pro-ounced maximum in zeta potential as the zirconium octanoateoncentration increased. The maximum occurred at a bulk zirco-ium octanoate concentration equivalent to that required for com-lete coverage of the particles. The zeta potential data were con-erted to surface charge density values through the use of theoisson–Boltzmann equation. The latter were in the range 0.4 to.5 mC m22. A simple two equation site binding theory, whichonsidered the dissociation of zirconium octanoate and the subse-uent adsorption of ions at a generic surface site, was successfullypplied to the surface charge data. It is proposed that the maxi-um in the zeta potential and surface charge as a function of

irconium octanoate concentration was observed due to the pref-rential location of ZrO21 ions at the particle surface, followed byharge neutralization with octanoate anions. It is suggested thatater facilitates the dissociation process of the zirconium octano-te, although it does not directly contribute to the surface chargetself. Two plausible qualitative mechanisms are described. Therst involves the presence of water at the particle–solution inter-ace, whilst the second considers the formation of micelles in theulk isoparaffin phase. © 1999 Academic Press

Key Words: nonaqueous dispersions; copper phthalocyanine;irconium octanoate; electrochemistry of pigments; nonaqueouslectrochemistry.

1. INTRODUCTION

The stability of colloidal particles in nonaqueous mediaow dielectric constant is of considerable interest, particu

1 To whom correspondence should be addressed. E-mail: paul.jennisa.edu.au. Facsimile161 8 8302 3683.

2 Current address: Mobil Technology Company, 13777 Midway Road,as, TX 75244-4390, USA.

3 Current address: Faculty of Chemical Technology, University of TwO Box 217, 7500 AE Enschende, The Netherlands.

252021-9797/99 $30.00opyright © 1999 by Academic Pressll rights of reproduction in any form reserved.

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o the printing, petroleum, and ceramics industries. Whehe role of electrostatic interactions in the stabilization pron aqueous media is well known, particularly if the partoncentration is low, there is a paucity of informationonaqueous media (1). It is possible to estimate the eletatic interaction in the latter by measurement of the elehoretic mobility and subsequent calculation of the zeta

ential. van der Hoeven and Lyklema (2) stated that marticles possess a positive zeta potential in nonpolar liqhe zeta potential of a particle may be controlled by adsorpf a surfactant at the surface–solution interface. A numbuthors (3–16) have published studies investigating the ef adding surfactants to particles dispersed in nonaquedia in order to vary the zeta potential.Morrison (17) and van der Hoeven and Lyklema (2) rece

eviewed mechanisms for electrical charge generation inolar media. From their analyses, it is clear that no unequiview of the mechanism(s) exists. One of the few attempropose a semi-quantitative charging mechanism is thitahara and co-workers (6). These authors used a simplquation site binding theory, similar to those often employequeous systems (18). The equilibrium site binding thdopted here is a variant of the Kitaharaet al.model. The rolef any adventitious water contained in nonaqueous disper

s another important consideration. Kitaharaet al. ignoredater in their theory. However, other authors (11, 17) hiscussed the possible influence of water on the extearticle charging.In this paper, the electrophoretic mobilities of copper ph

ocyanine pigments dispersed in isoparaffin solutions conng zirconium octanoate have been determined using theively new technique of phase-analysis light scattering (0). The technique is particularly suited to the measureme

ow electrophoretic mobilities. Adsorption isotherm measents were also carried out. The experimental data gatere used to modify the site binding theory of Kitaharaet al.nd provide an insight into the role of both surfactant and w

n the charging process of the practical dispersions invated. Copper phthalocyanine is widely used as a blue

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253NONAQUEOUS COPPER PHTHALOCYANINE DISPERSIONS

igment in the printing industry. The current study of thigments in nonaqueous media is therefore of great praenefit in the successful formulation of printing inks.

2. EXPERIMENTAL MATERIALS AND METHODS

.1. Materials Preparation and Characterization

2.1.1. Isoparaffin. Isoparaffin (under the tradenameIso-ar G) was obtained from Exxon Chemicals (USA). The waoncentration of this isoparaffin was measured to be 1.8 mm23 (“as supplied” isoparaffin), using a Denver Instrumodel 150 Karl–Fischer Potentiometric Coulometer. A porf the isoparaffin was dried over 3 Å molecular sieves. Thater concentration was determined to have decreasedmol dm23 (“dry” isoparaffin). A second portion of the isaraffin was taken and mixed, in a separating funnel, witqual quantity of high purity water (with a surface tension2.8 mN m21 and a conductivity of 0.5mS m21) dispense

rom an Elga UHQ system. The mixture was vigorously shaor 5 min. After standing to allow phase separation, thearaffin was isolated from the water. The water concentra

n the isoparaffin was measured to be 3.3 mmol dm23 (“wet”soparaffin). All three samples of isoparaffin were storeightly sealed glass bottles until use.

2.1.2. Zirconium octanoate.Zirconium octanoate, deoted as ZrO(Oct)2, was chosen as the surfactant of interes

he current study. Hattrick Chemicals (Australia) suppliedample used as a 26 wt% solution (under the trade nuxtra). It was used without further purification. Microanaly

esults indicated that the structure of the ZrO(Oct)2 in theuxtra was that originally proposed by Mack and Parkernd is shown in Fig. 1.

2.1.3. Pigment concentrates.The pigment concentratsed in this work were practical systems kindly providedesearch Laboratories of Australia (Adelaide). Varymounts of copper phthalocyanine particles (see Fig. 2),vinyl acetate) latices (PVA) and isoparaffin were milledether for 72 h using ceramic media in a ball jar. Table 1

he preparation details of the various pigment concentroduced. During the milling process, the PVA particlesome physically “smeared” onto the surface of the cohthalocyanine particles. The pigment dispersions were t

erred to tightly sealed containers after milling.Dynamic light scattering measurements, using a Brookh

FIG. 1. The structure of zirconium octanoate (from Ref. 21).

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nstruments 2030 correlator and an Argon ion laser, warried out on the concentrates supplied. All three pigmoncentrate dispersions were quite unstable and it provecult to measure the particle size distributions. To promispersion stability and allow determination of the particle ssmall aliquot of ZrO(Oct)2 was added prior to each lig

cattering experiment. Measurements were taken twicach sample using an integration time of 5 min. Using stanecond-order cumulants analysis, the average particle diaetermined was 6006 100 nm.Nitrogen BET surface area measurements were carried

sing a Coulter Omnisorb 100 instrument, on samples ohree pigment dispersions, from which the isoparaffin hademoved by filtration followed by gentle heating in a vacuven. The surface areas determined are given in Tabvidence for significant macroporosity (pore size 40–60as seen for pigment concentrate CuPc. Electron microshowed that the particles were formed from an ensembtrongly aggregated primary particles of a diameter commurate with the size of the macropores. Conversely, noence for macroporosity was seen for the pigment concenuPc-PVA1 and CuPc-PVA2, even though electron mic

FIG. 2. The structure of copper phthalocyanine (from Ref. 22).

TABLE 1Preparation Details for the Pigment Concentrates

Used in This Work

Pigmentconcentrate

code

Copperphthalocyanine

powder (g)

PVA latex(20% w/w in isoparaffin)

(g)Isoparaffin

(g)

uPc 40 0 358uPc-PVA1 40 103 255uPc-PVA2 20 103 275

c omp evd ninp

ala pet rome toa omc

2

-p ry“ PcP svw wt hab riuw

at telm ALtT lacep ghc solt ld o1 the .M 5°CT retm

wd di

reticm is thep elec-t th isi iumw lec-t thisg arge.T un-d ave-l andt 10V thee par-t laseren n thee thei per-s urfacec nt byp tento ressa

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254 JENKINS ET AL.

opy again indicated that the individual particles were cosed of aggregated smaller particles. The PVA laticesently “block” the macropores of the copper phthalocyaigment.Dried samples of the three pigment dispersions were

nalyzed by the surface sensitive X-ray photoelectron srometry (XPS) technique, using a Phi ESCA 2600 spectter. The take-off angle used was 45°, which correspondsnalysis depth of less than 3 nm. Table 2 shows the atoncentrations of copper detected.

.2. Experimental Methodologies

2.2.1. Electrophoresis measurements.Samples were preared by addition of the appropriate mass of isoparaffin (“das supplied,” or “wet”), pigment concentrate (CuPc, CuVA1, or CuPc-PVA2), and ZrO(Oct)2 solution to clean glasials. The total volume of each sample was 20 cm3. The vialsere capped and sealed using an epoxy resin before they

umbled end-on-end for 72 h to ensure that equilibriumeen established. Preliminary tests indicated that equilibas reached in shorter times.The water concentration of the sample was determined

he electrophoretic mobility of the particles was immediaeasured using the phase analysis light scattering (P

echnique, developed and described by Milleret al. (19–20).he sample cell consisted of parallel-plate platinum blectrodes, with a separation of 1.57 mm, housed in a 4 mmathlength rectangular quartz cuvette. The cell was thorouleaned with ethanol, dried, and then rinsed with sampleions prior to each measurement. A sinusoidal electric fie060 V cm21 at a frequency of 30 Hz was applied acrosslectrodes and the electrophoretic mobility (UE) determinedeasurements were carried out at room temperature (2he Huckel equation was used to convert the electrophoobilities to zeta potentials (z). The relationship is

z 53hUE

2eoer, [1]

hereh is the viscosity (0.8 mPa s for isoparaffin),er is theielectric constant of the solvent (2.005 for isoparaffin), aneo

s the permittivity of a vacuum (8.8543 10212 F m21).

TABLE 2Characterization Details for the Pigment Concentrates

Pigmentconcentrate code

Copper atomicconcentration

(%)Surface are

(m2 g21)

CuPc 2.066 0.05 76.36 0.3CuPc-PVA1 1.536 0.05 16.26 0.2CuPc-PVA2 1.056 0.05 6.46 0.1

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One issue that must be considered when electrophoobilities are measured using a light scattering techniquehotoelectrophoresis effect. Gairns (23) stated that photo

rophoresis can occur if light of an appropriate wavelengncident onto particles dispersed in a highly insulating medhen an electric field is applied between two electrodes. E

rons are ejected from particles in the bulk solvent andives rise to an enhancement of the positive surface chulagin (24) showed that copper phthalocyanine particlesergo photoelectrophoresis if the incident light has a w

ength in the red region of the electromagnetic spectrumhe field applied is above a critical value of approximately5

cm21. The large magnitude of this field may reflectnergy needed to overcome the Brownian motion of the

icles as well as that needed by photoelectrophoresis. Themployed in the PALS system did produce red light (l 5 632.8m) and photoelectrophoresis was expected to occur ixperimental system studied in this work. However, as

ntensity of the laser light was high and dilute pigment disions were measured, it has been assumed that the sharge of each particle was enhanced to the same extehotoelectrophoresis. Investigations to quantify the full exf this effect in the current experimental system are in prognd will be reported in a later publication (25).

2.2.2. Adsorption isotherm determinations.Samples werrepared as for electrophoretic mobility measurements.2 h of tumbling at room temperature (25°C), the water centration in the samples was measured. The particleshen separated from the liquid medium at 12,000 rpm inppendorf 5416 centrifuge. The supernatants were ana

or zirconium using a Spectroflame M Spectro Inductivoupled Plasma emission spectrophotometer. It was ass

hat the amount of zirconium octanoate (measured as zium) depleted from solution had been adsorbed at theent–solution interface, so that the following relations

olds,

@ZrO(Oct)2#S 5 @ZrO(Oct)2#bulk,i 2 @ZrO(Oct)2#bulk,f, [2]

here [ZrO(Oct)2]S, [ZrO(Oct)2]bulk,i, and [ZrO(Oct)2]bulk,f arehe surface, initial bulk, and final bulk concentrationsrO(Oct)2, respectively.The presence of zirconium on the pigment surfaces

onfirmed using X-ray photoelectron spectroscopy, usingollowing procedure. The pigments separated by centrifugaere redispersed in clean isoparaffin and tumbled for 30hese dispersions were then centrifuged again and the pras repeated to remove any residual ZrO(Oct)2. No zirconiumas detectable in the supernatant. The pigment residueried in an oven overnight at 100°C and the resultingigment cake was analyzed. Significant atomic concentraf zirconium were observed on the surface of the pigmarticles.

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3. THEORY

.1. The Determination of Particle Surface Charge fromElectrophoresis Measurements

For a spherical particle, the Poisson–Boltzmann equatiiven in spherical coordinates by

¹2c 51

r 2

d

dr S r 2dc

dr D 5 21

eoerO

i

en8izi expS2zieco

kT D , [3]

here, in the absence of a compact (Stern) layer,co is theurface potential of the particle,c is the potential at a distanfrom the surface,zi andn8i are, respectively, the valence aulk number concentration of ionic speciesi , k is the Boltz-ann constant, andT is absolute temperature.An analytical solution to this equation cannot be obtainowever, for small values of the potential, application ofebye–Hu¨ckel approximation,uziecu ! kT, and subsequexpansion of the exponential on the right hand side of Eqields

¹2c 5 e2 Oi

n8izi2c

eoerkT5 k2c. [4]

ere k21 is the Debye length. The solution of Eq. [4] fopherical particle of radiusa is shown in Eq. [5a] and thifferential form in Eq. [5b].

c 5 co

a

rexp~2k~r 2 a!! [5a]

dc

dr5 2co

a

r 2 ~1 1 kr !exp~2k~r 2 a!! [5b]

he total charge per unit surface area of particle surfacurface charge density,sS, is given by

sS 5 2Ea

`

eoer¹2cdr. [6]

ssuming the charge to be uniformly distributed overarticle surface and, for spherical symmetry,¹2c 5 d2c/dr2,q. [6] may be evaluated using Eq. [5b] to yield

sS 5 eoerSdc

dr Dr5a

5 eoer

co

a~1 1 ka!. [7]

n solvent media of low dielectric constant, the Debye lengery large. Hence,ka ! 1 and the difference in magnitude

is

.

]

or

e

is

he surface potential and the zeta potential is small, so thco

z and Eq. [7] simplifies to

sS 5 eoer

z

a. [8]

n important implication of Eq. [8] is that, for sphericarticles in nonaqueous solvents, the charge on the paurface is independent of the (large) Debye length and hhe ionic strength of the solution.

.2. An Equilibrium Site Binding Model to Describe theCharging of Pigments

Kitaharaet al.(6) proposed a model to describe the chargf particles in nonaqueous media. In essence, the associainding of ionic species to a generic surface group onarticle surface is proposed, in a similar fashion to the tent of metal-oxide particles in aqueous systems. A gen

zed two equation site binding model to describe the charehavior of particles, based on the work of Kitaharaet al., isow outlined.Consider a generic surface group, S, on a particle sur

he particle is dispersed in a nonaqueous solvent that contrace amount of water; it is experimentally difficult to red

he concentration of water in any solvent to levels belowmol dm23 (ca. 10 ppm). For the purpose of this analysis,resence of water is ignored, although it is recognizedater may play an intermediary role. In the presenceimple CA surfactant, which may dissociate into a C1 cationnd an A2 anion, the following reactions may occur atarticle surface:

S1 C1 NKC1

SC1; KC1 5@SC1#eq

[S]eq[C1]eq

[9]

SC1 1 A2 NKA2

SCA; KA2 5@SCA#eq

[SC1]eq[A2]eq

. [10]

he surface density ofchargedsites per unit volume,rcharge, inhe region of the surface of the pigment particles created beaction in Eq. [9] can be represented as

rcharge5 @SC1#eq. [11]

he total surface site density,rsites, is given by

rsites5 @S#eq 1 @SC1#eq 1 @SCA#eq. [12]

he fraction of charged sites,ucharge, may be expressed as

ucharge5rcharge

rsites5

@SC1#eq

[S]eq 1 @SC1#eq 1 @SCA#eq. [13]

T df

w eru intoE

3

-m iriab tioi

w or-bLm lid.p hev epta

c or ap nergyo

w re.I viori 27),e equalm no nets

mole-c

wA

on ofZ at an vese ct)c zetap

256 JENKINS ET AL.

he surface charge density per unit area,sS, can be determinerom

sS 5 eNSucharge, [14]

heree is electronic charge andNS is the number of sites pnit area of surface. Substitution of Eqs. [9], [10], and [13]q. [14] yields the following relationship:

sS 5 eNSS KC1[C1]eq

1 1 KC1[C1]eq 1 KC1KA2[C1]eq[A2]eq

D . [15]

.3. The Langmuir Adsorption Isotherm

All the adsorption isotherms for ZrO(Oct)2 onto the pigents measured in this study exhibited pseudo-Langmuehavior. The classical Langmuir (26) monolayer adsorp

sotherm expression is given as

Ceq

G5

1

KadsGmax1

Ceq

Gmax, [16]

hereCeq is the equilibrium solution concentration of adsate, G is the amount of adsorbate adsorbed,Kads is theangmuir adsorption equilibrium constant, andGmax is theaximum amount of adsorbate adsorbed per mass of solot of Ceq/G againstCeqshould yield a linear relationship. Talues ofKadsandGmax can be determined from the intercnd slope of such a plot.The Langmuir adsorption equilibrium constant,K , can be

FIG. 3. Zeta potential (z) of CuPc particles, dispersed in isopara

ads

nn

A

onsidered to represent the affinity of the adsorbate farticular surface. It can be related to the standard free ef adsorption,DG8ads, from the expression

DG8ads5 2RT ln Kads, [17]

hereR is the gas constant andT is the absolute temperatut is pertinent to note that adherence to Langmuirian behas often due to a mutual compensation of nonidealities (.g., the two assumptions that the solute and solvent haveolecular cross-sectional surface areas and that there is

olute–solvent interaction in the surface or bulk phases.The effective substrate area occupied per adsorbate

ule, A8, can be calculated using

A8 5Mw

GmaxNA, [18]

here Mw is the molar mass of the adsorbate andNA isvogadro’s number.

4. RESULTS AND DISCUSSION

Figure 3 shows the change of zeta potential as a functirO(Oct)2 concentration for the CuPc pigment dispersionumber of different water concentrations. All three curxhibit positive zeta potentials across the range of ZrO(O2oncentrations studied. In each case a maximum in theotential of the CuPc particles was seen at a ZrO(Oct)con-

as a function of ZrO(Oct)2 concentration (Error inz-potential' 3 mV).
ffin,
2

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tec ialt -c asu tercu thc n thp

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entration of approximately 2.5 mmol dm23. Similar behavioas been reported by many authors, who have studied syuch as carbon black and tetra-iso-amylammonium piispersed in benzene (3), copper phthalocyanine and AeT in xylene (4), and iron oxide or titanium dioxide in xyleolutions containing Aerosol OT (7, 9).The adsorption isotherms for ZrO(Oct)2 adsorbing onto thuPc pigment from isoparaffin solution are given in Fig. 4an be seen that maximum coverage of the CuPc pigurface occurs at equilibrium ZrO(Oct)2 concentrations of aproximately 2.5 mmol dm23, i.e., the same concentrationhich a maximum in the zeta potential of the CuPc iserved. Clearly, the amount of ZrO(Oct)2 adsorbed dictates thxtent of charge generation at the CuPc surface.It is interesting to note, from Fig. 3, that as the wa

oncentration in the isoparaffin is raised, the zeta potenthe CuPc pigment, at a certain ZrO(Oct)2 concentration, inreases. Figure 4 shows that this is not due to an increptake of the ZrO(Oct)2 by the CuPc particles at higher waoncentrations. The adsorption of the ZrO(Oct)2 is virtuallynchanged by the variation in water concentration overoncentration range examined. Water must be involved iarticle charging process in some other way.Using Eq. [8], the zeta potential values for the CuPc, give

ig. 3, were converted into surface charge densities,sS. Figure 5hows the change ofsS as a function of ZrO(Oct)2 concentrationor the CuPc pigment dispersion at the three different woncentrations investigated. The values ofsS determined aretween 0.4 and 2.5mC m22. This is in good agreement with tange of values (0.1 to 10mC m22) for other nonaqueous dispe

FIG. 4. Isotherms for the adsorption of

mstesol

tnt

-

rof

ed

ee

n

r

ions that have been reported in the literature (see, for exaefs. 12, 13). The general trend observed in the data, iaximum in surface charge density as the zirconium octa

oncentration increases, has been discussed by Kitaharaet al. (6)or similar systems. These authors attribute the trend to therential adsorption of positive ions at the pigment surface aurfactant concentrations. However, as the surfactant concion increases, negative ions are electrostatically attractedositive ions adsorbed on the pigment surface and charge n

zation occurs.In view of this proposed charging process, the equilibr

ite binding model described in Section 3.2, based oneneralized dissociable surfactant C1A2, has been adaptedonsider the current experimental systems. It is assumerO(Oct)2 undergoes the following partial dissociation re

ion, with the degree of dissociation represented bya.

ZrO(Oct)2 Na

ZrO21 1 2Oct2 [19]

he following reactions may occur at a generic site S onarticle surface:

S1 ZrO21 NKZrO21

SZrO21; KZrO21 5@SZrO21#eq

[S]eq[ZrO21]eq[20]

ZrO21 1 2Oct2 NKOct2

SZrO(Oct)2;

KOct2 5@SZrO(Oct)2#eq

[SZrO21]eq[Oct2]eq2 . [21]

(Oct)2 from isoparaffin onto CuPc particles.
ZrO

A gene od seno

s

E ee[

G met et

s

w ntsd -

tb finedi 20]a

inedsd cop-p itc ce, avp east-s kageI ares iumc

lateda ular,t um,

Kt

013

258 JENKINS ET AL.

n analogous treatment to that previously given for theralized surfactant C1A2 yields the following equation tescribe the surface charge density of a particle in the pref zirconium octanoate:

S 5 eNS

3 S 2KZrO21[ZrO21]eq

1 1 2KZrO21[ZrO21]eq 1 2KZrO21KOct2[ZrO21]eq[Oct2]eq2 D .

[22]

quations [23a] and [23b] give the relationships betwZrO21]eq, [Oct2]eq and [ZrO(Oct)2]eq, respectively.

@ZrO21#eq 5 a@ZrO(Oct)2#eq [23a]

@Oct2#eq 5 2a@ZrO(Oct)2#eq [23b]

iven that the Debye length is very large, it may be assuhat [ZrO(Oct)2]eq 5 [ZrO(Oct)2]bulk. Equation [22] can bhus be rewritten as

S 5 eNS

3 S 2K9ZrO21[ZrO(Oct)2]bulk

1 1 2K9ZrO21[ZrO(Oct)2]bulk 1 8K9ZrO21K9Oct2[ZrO(Oct)2]bulk3 D ,

[24]

hereK9ZrO21 andK9Oct2 are the reduced equilibrium constaefined byK9 21 5 aK 21 and K9 2 5 a2K 2, respec

FIG. 5. Surface charge (sS) of CuPc particles, dispe

ZrO ZrO Oct Oct

-

ce

n

d

ively. It is important to note thatK9ZrO21 andK9Oct2 are affectedy both the extent of zirconium octanoate dissociation de

n Eq. [19] and the site binding equilibria given in Eqs. [nd [21].Equation [24] was applied to the experimentally determ

urface charge densities plotted in Fig. 5. Kirklandet al. (28)etermined that the maximum cross-sectional area of theer phthalocyanine unit cell structure is 5 nm2 and each unell contains two copper phthalocyanine molecules. Henalue of 43 1017 sites m22 was chosen forNS. The fittingrocedure was carried out using an iterative, nonlinear, lquares method available in the commercial software pacgor Pro 2.01 (Wavemetrics, USA). The fits obtainedhown in Fig. 5 and the optimized values for the equilibronstantsK9ZrO21 andK9Oct2 are given in Table 3.There is generally good agreement between the calcu

nd experimental surface charge density values. In partiche maximum in the data is well described. The equilibri

d in isoparaffin, as a function of ZrO(Oct)2 concentration.

TABLE 3The Optimized Values of the Equilibrium Constants K*ZrO21 and*Oct2 for CuPc Particles Dispersed in Isoparaffin Solutions Con-

aining Zirconium Octanoate (NS 5 4 3 1017 sites m22)

Water concentration[mmol dm23]

103 K9ZrO21

[mol21 dm3]1028 K9Oct2

[mol22 dm6]

.6 6 0.1 (“dry” isoparaffin) 6.906 0.59 5.786 0.88

.8 6 0.1 (“as supplied” isoparaffin) 7.796 0.47 3.786 0.50

.3 6 0.1 (“wet” isoparaffin) 12.066 0.80 3.446 0.46

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byw thp mo hos clec oamO oni s,t ffins tioo noa veS eca n om olut ths bet

fo

m thep rfacec isms,c ta inF Eq.[ . 7.Ts u-p e4a bleH Inc.,U con-s per-f hez m 5,y oateo er-m ofa s ofK ents r ofa h isa Inter-s ryinga

re-p totalr TR-F thata par-a hich

cha-n S is ag


enoted in Eq. [20], involving the formation of positiZrO21 surface sites lies to the left of the equation (K9ZrO21 '022 mol21 dm3). This implies that the creation of the chargurface sites is unfavorable, as would be expected in nonedia. Moreover, the equilibrium involving the interaction

he SZrO21 surface sites with Oct2 ions (Eq. [21]) stronglyavors the formation of neutral SZrO(Oct)2 sites (K9Oct2 ' 108

ol22 dm6). The relative magnitudes of both the equilibrionstants indicates that the initial dissociation of the ZrO(O2s unfavorable, i.e.,a ! 1.

Closer inspection of the values of the equilibrium const9ZrO21 and K9Oct2 reveals thatK9ZrO21 increases andK9Oct2

ecreases as the concentration of water in the isoparaffireases. Hence, the extent of formation of the charged SZ21

urface sites is enhanced with an increased availabilitater. Morrison (17) stated that water itself cannot ch

nterfaces but may have a profound role in the determinatiohe electrochemical properties of particles in nonaqueousia. These effects are related to two factors. First, it isnown that water will adsorb onto the surface of hydropharticles in nonaqueous solutions (2). At the particle–solu

nterface, the water may help to change the acid-base chaf the surface and hence alter the nature of the adsourfactant–surface interactions. Second, the presence ofn the bulk solution greatly enhances the formation of invurfactant micelles. Evidence exists in the literature that evingle water molecule will cause the formation of a micellonaqueous media (29, 30). These micelles consist of aore, surrounded by surfactant chains. The core water proregion where dissociation of molecules may take placebefore

ransport of ions to the hydrophilic particle surface occhether the water exists at the particle interface or in theedia, its role with regard to surface charging may be

cribed as “catalytic” in nature.Mindful of the role of water, two possible mechanismshich the charging of copper phthalocyanine particles, inresence of zirconium octanoate dispersed in isoparaffin,ccur are suggested. The two (related) mechanisms are schematically in Fig. 6. In mechanism (I), the pigment partiarry an outer layer of adsorbed water. Zirconium octanolecules dissociate in this water layer to yield ZrO21 andct2 ions. The ZrO21 ions adsorb at the particle–soluti

nterface to form SZrO21 surface sites. At low concentrationhe Oct2 ions may be transported to the bulk isoparaolution, where they are stabilized, possibly by the formaf micellar species. Alternatively, at higher zirconium octate concentrations, the Oct2 ions may adsorb to the positiZrO21 surface sites and cause charge neutralization. In mnism (II), the presence of water promotes the formatioicelles of zirconium octanoate in the bulk isoparaffin s

ion. The water core of the micelles allows dissociation ofurfactant into ZrO21 and Oct2 ions. These ions can thenransported through the micelle to the pigment surface.

Both mechanisms result in the same outcome, i.e., the

lar

)

ts

in-

ofeofe-ll

nteredterea

teres

.lk-

eaywnste

n-

h-f

-e

r-

ation of ionic species which subsequently adsorb atarticle surface and hence give rise to a change in suharge. Further evidence, which supports these mechanan be gained from analysis of the adsorption isotherm daig. 4. The data were fitted to the Langmuir isotherm (

16]). A typical example of the fit obtained is shown in Fighe values obtained for the maximum adsorbed amount (Gmax),tandard free energy of adsorption (DG8ads), and the area occied per zirconium octanoate molecule (A8) are given in Tabl. In particular, the experimentally determined values forA8 ofround 3.5 nm2 are enlightening. Using commercially availayperchem 5 molecular modeling software (HypercubeSA), a zirconium octanoate molecule (see Fig. 1) wastructed, whilst simultaneous geometry optimization wasormed using the MM1 force field (31). The dimensions of tirconium octanoate molecule, determined using Hypercheielded a maximum molecular area of the zirconium octanf the order 1–2 nm2. Furthermore, the experimentally detined value ofA8 is very similar to the cross-sectional areasingle copper phthalocyanine molecule from the studieirkland et al. (28). Thus, it is proposed that the pigmurface is covered with an almost “close-packed” layedsorbed zirconium octanoate molecules, each of whicssociated with a single copper phthalocyanine center.persed in this layer are very small numbers of sites cardsorbed ZrO21 ions.Differentiation between the two mechanisms will be

orted in a future publication (32). Preliminary attenuatedeflectance Fourier transform infra red spectroscopy (ATIR) studies of the dispersions used in this work revealedsignificant amount of water is present at the pigment–isoffin interface. Sensitive conductance measurements, w

FIG. 6. A schematic representation of the two (related) possible meisms by which charging of copper phthalocyanine particles may occur.eneric surface site.

i rei iuo f tc ghc trib deu oai

A1C texp affia aint ircn nct tia

o ion ofz ptioni iumo Pc-P

s byt uredz ationd t thiso niumo PVAc zir-c opperp d fort rdf erz s.[

ensi-t dingmC1 oft sump-t pperp nsu dis-c -l A2w

eCP

0

1

3

260 JENKINS ET AL.

nvestigated zirconium octanoate in xylene/methanol mixtundicated that micelles were present above a critical zirconctanoate concentration. The actual charging process oopper phthalocyanine pigments probably occurs throuombination of both mechanism (I) and (II). The exact conution of each of these mechanisms is probably depenpon the concentrations of both water and zirconium octan

n the actual dispersion.Figure 8 shows the zeta potential of CuPc, CuPc-PVuPc-PVA2 particles (which differ in the amount of PVA laresent on the surface), dispersed in “as supplied” isopars a function of zirconium octanoate concentration. Ag

here is a pronounced maximum in zeta potential as the zium octanoate concentration increases. Moreover, the co

ration of surfactant, at which the maximum in zeta poten

FIG. 7. Langmuir fits to the adsorption of ZrO(O

TABLE 4The Maximum Adsorbed Amount (Gmax), Standard Free En-

rgy of Adsorption (DG°ads), and Area Occupied per Surfactanthain (A°) for the Adsorption of Zirconium Octanoate onto CuPcarticles Dispersed in Isoparaffin

Water concentration[mmol dm23]

Gmax

[mmol m22]DG8ads

[kJ mol21]A8

[nm2]

.6 6 0.1(“dry” isoparaffin) 0.46 6 0.05 25.2 6 0.4 3.65 6 0.30

.8 6 0.1(“as supplied” isoparaffin) 0.48 24.4 3.47

.3 6 0.1(“wet” isoparaffin) 0.45 26.0 3.69

s,mhea

-ntte

,

n,,

o-en-l

ccurs, corresponds to the attainment of plateau adsorptirconium octanoate onto the pigment particles. The adsorsotherms that were determined for the adsorption of zirconctanoate from “as supplied” isoparaffin onto CuPc, CuVA1, and CuPc-PVA2 particles are shown in Fig. 9.As the coverage of the copper phthalocyanine particle

he PVA particles increases, the magnitude of the measeta potential at a particular zirconium octanoate concentrecreases markedly. The adsorption isotherms confirm thabservation is not due to a decrease in uptake of the zircoctanoate as the coverage of PVA increases. In fact, theovered copper phthalocyanine particles adsorb far moreonium octanoate per unit surface area than do the bare chthalocyanine particles. Table 5 lists the values obtaine

he maximum adsorbed amount (Gmax) and also, the standaree energy of adsorption (DG8ads) and the area occupied pirconium octanoate molecule (A8) after analysis using Eq16]–[18].

The zeta potentials were converted to surface charge dies using Eq. [8]. These values were fitted to the site binodel represented by Eq. [24]. The values ofNS used foruPc, CuPc-PVA1, and CuPc-PVA2 were 4.03 1017, 3.0 3017, and 2.03 1017 sites m22, respectively. The choice

hese values was made after consideration of the dual asions that the important surface sites form part of the cohthalocyanine structure and the PVA latex surface containoseful sites. The validity of these assumptions will beussed later. The value ofNS for CuPc was taken from Kirkandet al. (28), whilst those for CuPc-PVA1 and CuPc-PVere established by reducing the CuPcN value in line with

om “as supplied” isoparaffin onto pigment particles.
ct)2 fr
S

t dT ntas so -i uc P

p PVAl ech-a perp

ta t the


he relative surface atomic concentrations of copper listeable 2. The fits of the site binding model to the experimeurface charge data are shown in Fig. 10 and Table 6 giveptimized values ofK9ZrO21 andK9Oct2 obtained. Within exper

mental error, the values of the equilibrium constants arehanged as the coverage of the copper phthalocyanine by

FIG. 8. Zeta potential (z) for pigment particles, dispersed i

FIG. 9. Isotherms for the adsorption of ZrO(Oc2

inl

the

n-VA

articles increases. This observation indicates that theatex particles do not participate in the surface charging mnism other than to “block” potential sites on the cophthalocyanine surface.The assumptions regardingNS in the site binding model fi

re obviously crude. Of course, it might be assumed tha

s supplied” isoparaffin, as a function of ZrO(Oct)2 concentration.

m “as supplied” isoparaffin onto pigment particles.

n “a

t)fro

P s tc refi VAafi PVp actr eno ord thep eau n oz ticli thP zirc ill br

oa

m of thec ses.T nt oft n in-t (33,3 solids “flats f am s oft thatt theh g ah ce.O rfacea

pperp trace

eCm

CCC

Kp

CCC

262 JENKINS ET AL.

VA particles contain as many potential surface sites aopper phthalocyanine itself. The site binding model wastted to the experimental surface charge data for CuPc-Pnd CuPc-PVA2 using a constant value of 4.03 1017 sites m22

or NS. The optimized values ofK9ZrO21 decreased andK9Oct2

ncreased as the coverage of copper phthalocyanine byarticles increased. This suggests that the PVA particlesetard dissolution of the zirconium octanoate; i.e., the presf PVA on the copper phthalocyanine particles makes it mesirable forundissociatedzirconium octanoate to reside atigment–solution interface. However, this scenario appnlikely given that the standard free energy of adsorptioirconium octanoate onto bare copper phthalocyanine pars significantly more favorable than for adsorption ontoVA covered pigment particles. The precise form of theonium octanoate on the bare and PVA coated pigment weported in a future publication (32).

The surface area occupied by a single zirconium octan

TABLE 5The Maximum Adsorbed Amount (Gmax), Standard Free En-

rgy of Adsorption (DG°ads), and Area Occupied per Surfactanthain (A°) for the Adsorption of Zirconium Octanoate onto Pig-ent Particles Dispersed in “as supplied” Isoparaffin

ParticlesGmax

[mmol m22]DG8ads

[kJ mol21]A8

[nm2]

uPc 0.486 0.05 24.4 6 0.4 3.47 6 0.30uPc-PVA1 1.71 22.0 0.97uPc-PVA2 3.98 23.3 0.42

FIG. 10. Surface charge (sS) of pigment particles, dispersed

he-1

Atocee

rsfese-e

te

olecule becomes progressively smaller as the coverageopper phthalocyanine particles by the PVA latices increahe reduction in area may be due to a different arrangeme

he zirconium octanoate molecules at the surface-solutioerface as the pigment type varies. Ducker and Wanless4) discussed the adsorption of surfactant molecules atubstrates from aqueous solution. They state that bothheet” (i.e., single layers) and “hemi-micellar” (i.e., halicelle) structures may form depending on the propertie

he underlying surface. In the current work, it is possiblehe zirconium octanoate forms a flat sheet structure onydrophilic copper phthalocyanine particles, whilst adoptinemi-micelle on the lyophilic PVA covered pigment surfarganization of the surfactant into micelles reduces the surea occupied by each zirconium octanoate molecule.

4. SUMMARY

Zirconium octanoate was shown to adsorb to the cohthalocyanine–isoparaffin interface. The presence of

“as supplied” isoparaffin, as a function of ZrO(Oct)2 concentration.

TABLE 6The Optimized Values of the Equilibrium Constants K*ZrO21 and*Oct2 for Copper Phthalocyanine Particles Dispersed in “as sup-lied” Isoparaffin Solutions Containing Zirconium Octanoate

Particles10217 NS

[sites m22]103 K9ZrO21

[mol21 dm3]1028 K9Oct2

[mol22 dm6]

uPc 4.0 7.796 0.47 3.786 0.51uPc-PVA1 3.0 8.196 0.67 2.996 0.58uPc-PVA2 2.0 7.296 0.95 4.006 1.17

in

c tha owe pep ntz , at nic

sio umw occ ivl enp ioi sitm Fut nip dec had

at-i dm thm tst as“ noa eli erei r os to“ aloc rfac

earL ranS l) isw earL oa hilS ng tf ecn on td (IaW ersio

.

.

11 ald,

11 .1 .

1

1 .

11 sity

1

2

22 ress,

2 ys-

22 s, in

222

23

33 mas,

33


oncentrations of water in the system did not alterdsorption of zirconium octanoate onto the particles. Hver, PVA particles physically “smeared” onto the cophthalocyanine gave rise to an increase in the amouirconium octanoate adsorbed per unit surface areahough the standard free energy of adsorption was sigantly less favorable.The measured zeta potentials and surface charge den

f the pigment particles exhibited a pronounced maximith zirconium octanoate concentration. The maximumurred at a bulk zirconium octanoate concentration equent to that required for complete coverage of the pigmarticles. The presence of water in the pigment dispers

ncreased the zeta potentials and surface charge deneasured at a fixed zirconium octanoate concentration.

hermore, as the coverage of the copper phthalocyaarticles by PVA latices was increased, a significantrease in the measured zeta potentials and surface censities was seen.A simple two equation site binding model, incorpor

ng the dissociation of zirconium octanoate, was useodel the surface charge data. The inclusion of water inodel was not necessary in order to obtain successful fi

he experimental data. The water is believed to actcatalyst” that aids the dissociation of the zirconium octate at either the pigment surface or in the core of a mic

n the bulk isoparaffin solution. The PVA particles wndirectly incorporated by an adjustment to the numbeurface sites (NS) used. The role of the PVA particles isblock” some of the available sites on the copper phthyanine surface and hence reduce the extent of suharging attainable.

ACKNOWLEDGMENTS

Financial support from the Australian Research Council and Resaboratories of Australia, through the Collaborative Research Gcheme, is gratefully recognized. Brian Vincent (University of Bristoarmly thanked for access to his PALS system. Olga Ivanova (Resaboratories of Australia) brought the photoelectrophoresis effect tottention. We also thank Olga, as well as Stephen Nicholls and Ptaples (Research Laboratories of Australia) for discussions regardi

ormulation of the various ink concentrates. Simon Johnston (The Tology Partnership, Cambridge) provided certain valuable commentserivation of the site binding model reported in this paper. Roger Hornark Research Institute) made some helpful remarks on an earlier v

f this paper.

e-rofl-

fi-

ties

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nsiesr-

ne-rge

toetoa-le

f

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chts

churipheh-henon

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the electrochemistry of nonaqueous copper phthalocyanine dispersions in the presence of a metal soap...

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