electrolyte interface — Parameters of the electric double layer

ADSORPTION

OF

INTERFACE

SODIUM

AND

- PARAMETERS

CHLORIDE

OF

THE

IONS

AT

ELECTRIC

THE

RUTILE/ELECTROLYTE

DOUBLE

LAYER

V. JANUSZ Department

of Radiochemistry

Maria Curie-Sktodowska

and Colloid

University,

Chemistry,

Institute

Sq.M.Curie-Sklodowska

of Chemistry,

3, 20-031 ILublin

iPoland) Received

February

20, 1989; accepted

March

31, 1989

ABSTRACT The surface

rutile/water tration

charge

and the adsorption

interface

has been studied.

data the surface

reaction

of background

constants

theory of the electric

of estimation

potentral

of surface

binding

taking

double

layer

the surface

between

on the basis of the site binding

of electrolyte

were calculated

at the TiC$/electrolyte

into consideration

model of the EDL. Comparison

calculated

ions at the

On the basis of the potentiometric

equilibrium

ding to the site binding

ted in this paper

electrolyte

ti-

accor-

(EDL). An attempt

interface

is presen-

charge data and the site

concentratron

of surface

species

model of the EOL and the adsorption

ions is made.

INTRODUCTION Surface

charge

the effect

at the solid/aqueous

of redistribution

of ions of opposite forming

[l]. For oxides,

as the binding

by the surface

sites

adsorption

of oppositely

the formation

of surface

of the H’ or OH- ions and supporting

(surface

hydroxyl

ding theory for the oxide/aqueous ming reactions

systems may be created

due to unequal

charge and unequal dissolution

the solid phase

considered

of charge

electrolyte

groups) [l-lo]. According

electrolyte

take place on the solid surface

system,

as

from solution charged

ions

charge may be

electrolyte

ions

to the site bin-

the following

charge

for-

[l-10]:

K11

-SOH

+

H+

(1)

K a2 -SOH c---)

-SO-

+

H+

(2)

-SOH;,

0254-05841891s3.50

0 Ekevicr

Sequoia/Printed

in The ?Jetherlands

KA

-SOH+A-2 -SOH

-SOH

On the

basis

of the

constants

K

$I

(6)

r~

potential,

yp

and $i ,

to the

& =b

besides

[-SOH;]

site +

charge

The above complexation

+

set

species,

A- = anion,

T = temperature,

coefficients

6,

fi,

surface

of anion

charge

K+ = cation,

% = mean surface

F’, fi,

r~ are the sur-

and cation

(eqn. 9) and the

main parameters

binding

of the

electric

respectively.

total

double

number of layer

(EOL)

theory.

factor

[-so-]

-

-

from moles/dm’

constants

from the

acidity

of the

+

[-SOH]

of equations

also [I].

by the

used

charge

+

[-SO-K+]

(9)

to uC/cm* of charge,Gb

system ionic

[-SO-]

+

to calculate

surface

= surface

is

the

controlled

However

thus

on the

basis

not

and the

proposed

[-SO-K+]

method of the

is

only

surface

calculation not

obtained

(10)

surface

ionization

The surface

potential.

strength

Some authors

quotients. surface

is

and the

electrolyte

of H+ ions, but

lation

the

[ SOH;A-]

constants

oxide/aqueous

xation

(8)

density.

Ns = [-SOH;]

tal

or surface

in the p plane.

[-SOHGA-]

where_: b = conversion

)

(7)

of -SOH, -SOH;, -SO-, -SOH;A-, -SO-K+ respectively,

if-~ are the activity

according

exp ( -e(F;W)

constant,

= mean potential

10) are

)

of i ions

coefficients

Ns (eqn.

exp ( -ecrT-yP'

rc

k = Boltzman’s

These constants,

sites

J-H

x0

= concentration charge,

activity

and com-

(5)

rk

[H+l

e = proton

face

ionization

b’o fi fi

[.-SOH] [K+]

Ii/

intrinsic

Ffi

[-so-K+]

where:

appropriate

as follows:

+,

[-SOHIb+] [A-l

=

(4)

$t~ x-

[-SOH]

C

+

reactions

[-SOHGA-J K

H

(3)

x-+

[H+] r-so-]

KA =

A-

+

above

[-SOH;]

a2 =

+

can be defined

[H+] [-SOti] al =

H+

-+ -SO K

H+ A

+

plexation K

+

potential by the

correct, results

me-

concentration

ionization of the

and for

and comple-

surface because leads

potential recalcuto values

41

different method this

from those

previously

calculation

of the

for

method

Another model

were symmetrical

method of the

technique

by Westall

double

and Parks

potential,

in relation

was introduced electrical

James

used. surface

layer

Adsorption

of background

rutile/NaCl

sis

to the

point

of zero

basis

of a constant

(EDL) and using

a graphical acquired

on the

of the

aqueous

electrolyte

solution

potentiometric

ions

system

titration

charge

a nonlinear

in

(pHpzc). capacitance

least

data

the

estimate

surface

potential

was performed.

EOL the

calculated

concentrations

the

pH were compared

calculated

with

and potentiometric presented

constants

of the

were

are

complexation

versus

results

square

[13].

for

model

[3] described

but the

using

experimental

titration

in this

paper.

appropriate

surface

the

methods.

various

Using

tne

data

On the

ionization

and

An attempt

to

ionization-complexation

of -SO-Na+ and -SOH$ladsorption

ba-

species

data.

MATERIALS The surface method ments

charge

of the

of the

potentiometric

were performed

Ti02 suspension. in order

titania

in NaCl solutions

titration

of a suspension.

simultaneously

NaCl solutions

to determine

with were

adsorption

was determined

the

labeled

densites

using

The adsorption

the

experi-

potentiometric titration of the 24 radioisotopes with Na and C136

cf Na+ and Cl-

ions,

respectively.

RESULTS AND DISCUSSION The dependence 0.1,

0.01

of surface

The pH observe Pzc ture and it is the impurities

[14].

for

this

modified

On the

Schwarzenbach

the

surface

constants methods

it was assumed

and Parks

[3] ),

as 12.5

ionization

and complexation

the

obtained

values

different

from the

to judge

calculations adsorption

by the

which for data.

values set

the

groups

constants

titania

titration

were calculated

[15], and method

pH + 0.5) is equal to 1.23 Pzc were made. Total density of surface

James

of the

of potentiometric

range

ions

der

that

basis

In the case of the method

(in

for concentration -vs. pH of NaCl solution in Fig.1. As is seen, pH occurs at 5.1. Pzc agrees with values reported in the litera-

sample

evidence

and complexation

zation

charge

M is presented

and 0.001

hydroksyl square

not

contain

surface

ioni-

using Davis [5] et.

F/m* and for

are

data,

II described

that the capacity

per

does

this

groups nanometer.

collected

by Westall

range,

pH Pzc calculat-

the

was taken

(after

Values of

in Table

I.As

it

surface is

Davis -et al. method [5] are significantly obtained by the modified Schwarzenbach method.

of equilibrium presented

set

constants of constants

[13].

of the EOL near

is more reliable, were compared

the with

seen, In or-

model

experimental

42

Fig.

1. Surface charge of TiO (rutile) in aqueous 1 - O.lM, 2 - il.06, 3 - O.OOlM.

pH; curves

Table

I. Surface

different

ionization

and complexation

solutions

constants

of NaCl as a function

of ii02

calculated

methods.

Constants

Method Davis et al.

Schwarzenbach

Method

1151

[5J-----

2.04

2.16

2.23

PKa2

8.48

8.26

7.96

pKcl

1.72

1.18

pKNa

7.56

7.04

Adsorption

electrolyte characteristic

for

adsorption

according

densities

of Na+ and Cl-

concentration

are

an oxide

depicted

system,

of Na+ ions increases

to the

eqns.

(4)

and (3)

ions

on TiO2 as a function

in Fig. i.e.

2. The course

when the

pH of the

whereas adsorption respectively.

II

Cl31

pKal

the

using

of pH

and

of adsorption

is

solution

increases

of Cl- ions decreases,

43

One method of calculation

where: a = activity Using eqn.(9) +

>) war

ntrated

of electrolyte,

calculations vz

is based on eqn. (11)

aH = activity

whereas the second,

Of course,

the present

but it reflects

the boundary conditions

of calculations

are presented of ionic

conditions: i e -2-2 to the systems in conce-

in Fig.3

strengths

to the systems in dilute

so-

model of the EDL is oversimplified,

which occur in such systems. forw>>yeand

seen from these Figs the second assumption yOZ and increase

[3]:

of Hi ions.

assumption corresponds

solutions,

respectively.

potential

were performed for two different

yr? The first

electrolyte

lutions,

of surface

Fig.4

+leads

causes a decrease

foryv%

The results @.As

is

to lower values of go,

of the values of

F.

1.0

n

-A

I

G a

-

n

_O-qj-O-_O”-

'I03

4,

51

6

7 PH

fl

n

nnnM

8

9

IO

II

Fig. 2. Adsorption densities of Cl- (open symbols) and Na+ (solid symbols) A - 0.1 M, q - O.OlM, O- 0.001 M. -vs pH of the rutile suspensions:

ions

44

-150

-2oc

Fig. 3. Estimated variation of pH for Ti02/NaC1 system:

surface

I

4

I

I

7

(for

conditiony,>>vt%)

0.001 M, -s-

I

I

5.~6 \\ :\

potential

-..-

ction

8

0.01 M, -

-

as a fun-

0.1 M.

4

I

1

1

9

IO

11 PH

K.\\

v.:+ .\ \ \’ \’ ._i.’

/

‘-4

‘:‘\ \ :‘\ ‘\ \

‘\

‘\

\

:\

*-

a/

‘\ _)

Fig. 4. Estimated variation of ction pH for Ti02/NaC1 system:

surface -..-

potential

(for

0.001 M, -.-

conditiony/,g%> 0.01 M, --

as a fun-

0.1 M.

45

Both sets of surface sults.

ionization

The values of mean surface

by the modified

Schwarzenbach

tained using the constants Another

method

wing eqns.

and complexation potential

method

lead to similar

using constants

are by a few millrvolts

by Davis et al.

of calculation

obtained

constants

re-

calculated

lower than those ob-

[5].

of mean surface

potential

is based on the follo-

[13]:

Kd e Ns srnh (12)

G,=

d cash

l+K where: o(is

defined

as follows:

(13)

where:

aH = activity

Nonlinear of d

eqn.(lZ)

of H+ ions at the pzc, F = Faraday's

can be solved using Raphson-Newton

we can obtain

y0 from eqn.(l3).

Results

in Fig. 5. As is seen from comparision

constant.

method.

of the values of mean surface

In Fig. 5 with data from Figs 3 and 4, these values

depicted

\&

0

4 -5o> A -lOOs--

I

5F.q

Knowing

of these calculation

1

I

a

7

8

9

the values

are depicted potential

are very simrlar

1

10

11 PH

"*., ..\\ '\ ..? \\\ ..\ kj \I

-150-

:"\, b

-200.

Fig. 5. Estimated variation of surface 1'2 - 13) as a function pH for TiO#NaCl 0.1 M.

'\

potential (calculated on the basis eqns system: 0.001 M, 0.01 M,

to

46

the

values

than

for

hod are

obtained

assumptiony6

lower

when the

Concentration the

balance

Figs

3-5.

of the

values

calculations

using

of surface

species.

density

at the

between

calculated

sets

interface

these

in tnis

using

fixed

values

6-8 as concentration

NaCl,

respectively.

see

in Fig.

Figs.

9. The

6-8,

is

[16] and Ti02 using is

above

from Fig.

pHpzc is

-SO-Na+ groups. EOL during urces.

On the

pHpzc are is

may be caused

centrifugation other

higher

depicted

hand,

than

adsorption

binding adsorption

sorption

density

pH, so the binding

ions

theory.

centrifugation

the

ption

can be too point

for

low.

of zero

of sodium

above However,

ions.

from the

of ion pairs

chloride

because

adsorption

As is

seen

destabilization

case

when the

of chloride

from Fig.

2 this

seems

that

predicted

exchange

of ions, place

at

increase

process

results of the

is

not

should the

of from

adsorbed

and our adsorption ions

cause

range

from supernatant

part

adsorption

inthe

of ad-

should whole

calculated

above,

As

calculated

to an equal

than

below

reaction

were prepared

as mentioned

EDL during

charge

the

ions.

be greater

of the

of radioso-

[20] can take

This

of

by theory.

pHpzc but in the

radiosources

part

of pH. It

by the

of ion pairs,

below

[18]. inter-

obtained

only

suspension, from the

inner

than

range

can lead

on pH,

as Zr02/KN03,

Zr02/NaCl

and sodium

pHpzc should

Apart

groups

preparation

higher

M

versus

concentration

of Na+ ions

sites

in

and 0.1

rutile/electrolyte

from the

only

were con-

systems

surface

studied not

on neutral

of Na+, not

of the

can be removed

sults at

equally

whole

adsorption

or adsorption

of adsorption adsorption

in the

can be caused

process

such

before

is

agreement

0.01

of surface

of ions

ions

disagreement

presented

of -SO- and Na+ predicted of Cl-

Generally,

[19]

The adsorption

an increase

ions

theory.

surface.

after

groups

are 0.001,

[17] and for

sample

charge

in the /3 plane

at the

of

and adsorption better

for

of adsorption

6-8 adsorption

of -SOH;Cl-

ternal

by removal

values

the

species

calculated

suspension

concentration

of electrolyte

by site

site

the

the

on Figs.

concentration

of the

the

surface

to attain

constants

was

of concentrations the

pH for

in

species

of surface

concentration

of Na+ ions than

results

calculations

versus

for

Consequences

reconstituted,

calculated

James and Parks

lower

surface

of mean potential

of the

6, adsorption

slightly

This

of these groups

results

met-

presented

or Fig.5.

In order

values

to the

As it face

fixed

dependence

similar

Fe203

seen

Results

of surface

this

eqn.(lO)

and -SO-Na+ groups

of concentrations

of$.

the

a calculation,

is well

was observe.

Whereas

of the

to similar

of such

system,

using

potential

from Fig.3

of -SOH$l-

calculations

using

of surface

lead

results

concentration

values

was determined values

of values

made and greater

obtained

greater.

potential

Though the

results

of concentration

surface

both

Figs.

pH are

species

approximation

the

are

and the

and Na+, respectively,

between tinued

surface

of the

assumption%>>y$was

As in Fig.4

strengths

sites

The better

when the

2 yS. ionic

of surface

made using

of Cl-

from eqn.(ll)

the

of ions be equal case.

pairs

reoccurs

to adsor-

Adsorption

of

47

-6

Fig.

6. Comparison of the calculated concentration of surface G _ -SOH:Cl-..-SO- . . . . _SO-Na+ ) with the of Naf ( 0 ) &d Cl- ( 0’) ions and krface r%&& ( A ) for

-1oL

species (- - -SOH;, adsorption density Ti02 in O.OOlM NaCl.

\

Fig. 7. $omparison of _the calcul_ate+d concentration _._ -SOH2Cl- _..-SO of Na+ ( 0 )‘and Cl- ( b‘;‘io~,““a~~ iurface %&~8.

of surface ) with the ( A ) for

species (--SOH:, adsorption density TiO, in O.OlM NaCl.

chloride

and sodium

and these

process

ions

by exchange

can influence

that

internal

that

Na+ and Cl-

completely

redistilled

water.

with

adsorption

The disagreement the

adsorption

reaction

(3)

rutile/NaCl

and (4)

site

binding

cess

which

theory, takes

method only

adsorption

of ions

from the

concentrations

the

However

[lo]. surface

also

of the

constants

on the

surface

of the

it

it

of -SOH;Clthe

seems

was found

was found

that

this

constants

describing

that

and Na+ ions

in such

systems method

the

the at the

is well leads

according all

out

and -SO-Na+ and

testimony

of Cl-

of ions

complexation

interrelated It

of Till2 by washing

is

adsorption

The adsorption

not

because

surface

and Na+, respectively, describe

are

from solutions.

to be negligible,

the

but

place

seems removed

interface.

of not

or internal uptake

calculated

of Cl-

by Sprycha’s

an estimation

are

can not

solution

described

of ions

between

density

the

to

to the

adsorption

pro-

solid.

Fig._8. Comparison_of the calculated concentration o$ surface species (--_SOH;, _._,_ -SOH;Cl-, -. ._.._ -SO-, _.-...... -SO-Na _ the adsorption density of Na+ ( 0 ) ions and surface charge ( *-)%?lc’~i~:‘!n 0.1 M NaCl.

CONCLUSIONS Adsorption not

only

the

It

seems

that

nected

with

of electrolyte result

adsorption the

ions

of the

creation

at

reactions

of electrolyte of surface

the

rutile/aqueous

assumed ions charge,

by the

electrolyte site

can be caused i.e.

exchange

binding

interface model of the

by a reaction adsorption

not

process.

is EOL con-

50

\ - ,;\ -

0

‘<‘\ ‘2, --_. \

-y,---_____--_

‘K\ ._

> E

‘\

’

s

‘\

.‘\ “..._. “\ “\

-5 O-

“\

“\

-10 OI

3

4

I

5

6

I

8

7

“\ I

9

10'

1

PH Fig. 9. Fixed values TiCl2/NaCl system: -

of mean potential in the p plane aS a function 0.001 M, --0.01 M, --. - 0.1 - M.

of OH for

’

ACKNOWLEOGEMENT The research Sciences,

reported

Institute

in this

paper

of Catalysis

an

was supported Surface

by the

Chemistry,

Polish

in the

Academy of

Grant

RPBP 01.10.

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