The application of surfactant-selective electrodes to the study of surfactant adsorption in collodial suspension

389

~~a~8a~Su~~e~4(1982)38'~396 Bfsevier Scientific PubfirhlngCampany, Aauterdam - Frintedin The Ne4herknds

THE APPLICATIONOF SURFACTAHT-SELECTIVE ELECTR(IOESTO THE STUDYOF SURFACTANT ADSORPTION IN COLCOOIAL SUSPEHSIaH KATWITUHAYAKAWA, A. CATIFF AYUB and JAN C. T. WAR Uepartm@nt of Chemfstry, Dal%usfe Unfversfty, Halifax, WI 433 (Aeceived2November1981; acceptedinfiilform

Uova Scotia,

Canada

1 April 1982)

ABSTRACT Surfactant-selective electrodes in which the active agent is a plastfcfzed poly(vfnyfchlarfde) mmnbrane contafnfng a dfsrolved complex of dudecyltrfmmethylamonfm and dodecylsulfate ions have &en appl fed to the study of surfactant adsorption from dilute aqueous solution by colloidal or suspended particles. AS examples of the utflfty of the surfactant selecti9.e electrode in such studfes. Isotherms for the adsorption ;I dodecyltrGaethylamon~um (WA+) ions on hauiofanfc and #g’* farm in dilute aqueous suspension with bentanfte clay fn the Ha+. Ca and without added electrolyte, and,on coal fines from a wash plant fines circuit fn addition. binding curves for the interactfan bctueen DTA+and are reported. the anionic potyelectrolyte dextransulfate. again in very dilute aqueur solutions. are reported. . IHTWWCTJOH The adsorption of surT’actants frun dilute aqueous solution by dfssoJv@1 pol$nters. colloidal partfclas or suspended partfctes is an important area oF A nunber of experimental techniques, study fn basic and applied colloid scGme. including dfalysfs caubfned with a-variety of analytical methods (spectr%hotocIetry. titration, cartductance etc.) hare been used, hut such techniques all becolre increasingly mare dafficult at loner surfactant cancentratfans. In this paper ne will demanstrate the applfcatfan of surfactant selective elect_rodes to the study of surfactant adrorptfan, at v&-y ion surfactant cancentrations, and an a variety of substrates fnctudfng dissolved polyelectrolytes. bentanfte clay in dflute suspensfan, and coal fines fn aqueous suspension, Surfactant selective electrodes have beeri successfully applied to study surfactant adsorptfw by polymers (t-3), and to study of the solution chemistry of surfactants (Q-10). Advantages of the use of surfactant selectfve electrodes fn adsorptfan studfes include excel lent sew? tfvf ty and reproducfbfl f ty (nom1 ly far super?&* to

results

obtained from eauilibrim

ease of

measurement(usually

suspension. for

without prior

t+

~pp?fcatlon

surfactants

into

In this

catfon (DTA*),

reporting

to a

paper we examine

convenient solid state type electrode

dwtransulfatem

the

and tolerance

Electrodes selective

have been develaped,

for the dadecyl trimethylamanium fn aqueous solution

by centrifugatfon).

in large excess.

of a partfcolarly

DTA* on the polyelectrolyte all

kan be insefi*ted directly

removal of turbidity

the presence of electrolytes

number caf different

experiments). small sample volume.

dialysis

the electrade

selective

adsorptIon studies of

on bentonite clay,

and on coal fines,

or suspeoslon with ar without added electrolyte,

EXPEAIHENT~ Ptaterialr Dodecyltrfmethylaumnium repeated recryrtall taut-selective

fzaMon

bromide(Kodak Laboratory Chemicals) was purSfied by

fraQI ethanol.

The carrier

cmplex

elmztrude uds prepwed by disso1vSr.g equivalent

and sodium dudecyisulfale

trimethylanmPnium bromide dodecylalcohpl

purified

by fractfanal

chlorosul phonic acid [Ill.

dlstil

nhkh

latfon

usml 318the surfacamunts of Uodecyl-

uas pregarmI frm

through esterficatioc

wfth

Scientific) nas size fractionat

Bentonite clay (fW~er

ed and the r2u fraction uds conwertp.d into the sodium.nagnesftan, and calcfum form by disgersdng and decanting the clay in the corresponding metal chloride of 1 ~1 cfme3 followed by dialysis

and ul trafiltratton

Sodfm deKtransulfate.

[12].

u~lecular weight 50Q.000 was obtained from Pharmacia, Upysala. Sueden, by ultraftltratlan Coal fines

and analysis

have all

been described earlier

used were from the -28 atest fSnes circuit

Wash Plant of Cape Ereton Development Corfflratfon. coal tre;lt& Seam. for

in this

plant

is a IiIgh volatfle

to the surfactant

(PVC) and bis(Z-ethylhexyl)phthalate without

further

of the VictorG# JuncfIon

Glace Bay, H.S,*

studies.

as solvent

fr attached to the bottm

C4).

The

Chenkal

teolpetakwe

Poly(uinylchtoride)

Co.* IN.)

uere used

purificatIan.

Potent*-etric measurements Thz functional membraneof the surfactant-selective 253 PVC, 742 bis(2-ethylhexyi)phthalate as plasticircr, The mubrane

Canada,

bftuminous coal from the Harbour

adsorption (Aldrich

Purfffcation

[13].

The fines were dry ground to -2IlQ Eesh and evacuated at ram

four days prior

average

electroda consists and 0.8% carrier

Of

CtwpleX.

of a hard PVC tube using tetrahydrofurdn

In the measurements the electrode

is part of the following

concentr stian ccl 1: f..lelUel Electrode

I

3’

I

Ueference Solution

where the reference solutIan

I

functionat

PVC Mmbrane I

is 1 x 10m3 aml kg”

Test SoluQan

OTAEr.

I %’

Cd14me1 1 Electrode

Ibe electrade was

391 pretreated In the reference solution overnfght. In order to elfmfnate any effect due to leakage of potdssfcrm chloride frm the agar bridge. a double junction was used by inserting a small glass tube, pfnhoied at the top, Into the reference solution Inside the PVC electrode. The el=tromotlve farce of the cell uas measured with a Kefthley 616 digital elecrrameter. with an accuracy of 20.1 m’l. Ths potentfal was wnftored with a recordrr. RESULTSAND DISC~:~gIDN _* The catfan’c -urfactant-selective eliqtrode shows Kernstfan response from the crftfcal mfcelle formation concentration (cmc) down to 2 x 10” mol kg’l eYen In the presence of an excess sodium chlor:de (0.1 mol kg”). The excellent refwoducibflfty of the potential allows US to use a plot of emf vs. log UYD(m is the DTABr molar concentratfon) a5 calftratfon curve. Calibration solution! with added electrolyte (HaCl or HgCl2) at the same concentration as In the unknown solutions were used, The observed potentfometrfc curves for the solution mixtures under study deviate from the calibration curve due to adsorp+.fon by sodium dextran

sutfate,

clay

suspensfon,

or coal

fine

suspensfan

as shown in Fig.

where dm is the amount of DTA+ bound to the collofdal

partfcles,

titratfon

to contamination

1,

and m: the correspondfog equilHwfran molal cancentratlon of the DTA+ ion (i.e. the molal concentration of the remaining free DTA’ ions). From the data as presented fn Fig. 1. binding Isotherms can he constructed as sham in Figs. 2-4. The electrode response Is recheckti Gfter qeasurln.1 each series of unknaun solutions, showing potentZa1 shifts of less than 2 mV, aml rlnce again excellent Hernstfan betivior. TraditIonally, fn surfactant adsorption studies, a variety of analytical technfques have been used to determine the surfactant concentration in the For instance, in the case of adsorptfon by suspended clay equi 1 ibrium solution. partfcles. titration Cl% 161. spectrophotcmetry [I71 and surface tension measurements [I81 fwe been used. Each of these methods suffers from experimental problems and uncertalntfzr. fncludfng turbidity removal by centrffugatfon for surface

and spectrophotametric active

materfals

for

methods . sensitivity

surface

by other

tortsfan measurements. and long equflfbratfnn

times when dialysis Is used to separate tollofd phase and eauilfbrfum soTutfcn. In comparison. the emf method wfth a surfactant selective electrode is far ltmre canvenfent. Electrodes can be Inserted dfrectly fnto the clay suspension, and electrode resganse Is Independent of the flocculatfon of the clay, and of added Ffg. 2 shows adsorptfon Isotherms of DTA* on homa-fonfc electrolytes, bentanfte clay fn the Da*. Dg2+ and Ca2+ lanfc for-or (0.2% by uefght aqueous The exchange capacfty of th*s clay is 8.5 x lOa equfv/g clay [19], suspension). Clay-surfactant equflfbrfrm times were f&t excess of 24 hrs, no time dependence

392

uas notfced far epullibratfon that

Hams lrr the razqe of 16-48 hrs.

fn 0.01 m HaCf. the DTA+ adsorptfm

temlned

Uhe$athe ior+

the added salt

de&e&s

strength

1s fncreased to 0.1 m In HaCl (Ffg.

2a)

the amaunt of WA+ adsorbedat law concentrations,

These observations

suggest

.

-1

de-

DTA+ adsw’bed/g clay at rn: I lD.3

but at high solutionconcentrations the amount adsorbed exceeds exchange capacity.

Zb shows

Is very close to the am ;,Wcally

exchmge capxcSty (8.5 x lD*4 equIv.

mwkg)*

Ffg,

the explanation

the

fan-

that at ion

I

a

oc

c -1 oc

% ir’

c

E -1oc

0

-4 Fig.

1

-2

-3 fag m&tdeJ

1

Respmse of the DTA+ electrodeto change 1~ OTPSr concentration sodtm &xtransulfate cmtrlfugates,

solution

canttfniE?0.02 m &Cl.

and (c) C+bentonite

foot explanationof

suspmsion. 0:

syatbols see text.

in

(a)

(b) coal fine

Calibration

curve.

393

DTA* equilibrium electrostatfc surfactant adsorbed

phobic

cqncentratfons

forces

*:oncentratfon organic

cations,

interacttan

The adsorption

(FSg. 2c.d).

surface

allowing

for mre surface

decreases

and catfonfc

the added electrolyte

with the clay

shun a trend compatible

5 fIl;

2

the added electrolyte

clay

the attractive

surfactant,

shields surfactant

while

the repuls4ve adsorptGm

ar with already

tsotherms of DTA* on Mg- and Ca-bentonite

0

Fig.

betwee:

adsorbed without

at high

forces

through

betneen

hydra-

surfactant. added electrolyte

with the assumption that counterion

and

10

(W4mlfkg)

isotherms for fbe OfA+-bentonite systear at 2!PC, (a) Nabentonite, 0.1 m NaCl. (b} Ha-bentmite, 0.01 m DaCl. (c) Ca-be’ntonite, (d) Mg-bentoni te. Clay ctncentracttan 2.0 g/kg t120.

Adsarption

394 surfactant

catlon compete for

is mm-e difficult Binding

the same exchange sites

to remve fmm auter or

isotherms for

inner clay surfaces than &J’+.

the s&stem NABr-sodfm

presence of WC1 or HgCl, are shown fn Fig. binding

is defined as the ratio

ions t.3 the mlal

unity with Increasing binafng sites

of anionic

equiirbrfum

dioxrde Fig.

nith

interaction

3 also indkates

(i.e.

Ha*)

indicating

ton binding

that dhalent

that shaws

pracess,

with cationPc chloride

and

b
betueen adjacent ~lyion-bawd

the binding of DTA* by the polyanim,

of this

approaches

It

Figure 3 clearly

of decylrulfate

irr sutfactant

counterions

(i.e.

Hg’*)

more so than ImMVdlent

fs

surfactant in&wfere caunterfons

even though the ionSc strength of both systems Is the same.

the surflctmt

fo:;~ still

bSnd rt.wngIy

in the

of bound surfactant

of surfactant.

of the polyiatt-surfactant

This coaperatgvity

[3].

(Ha&G)

the degree of

C23 or with a copolymer of dimethytdfallylamon~m

caused by the hydraphobic chafns.

ffgure

on the palmer.

groups on polymer.

to what is observed Tar the interaction

polypeptIdes sulfur

sites

concentratWI

are mainly the anionfc

the hfgh degree of cooperativity similar

3.

dextransulfate

IR thrs

of the molal cmcentrat?on

concentration

2+ nhere Ca

on the clay,

to the DxS pulyanion,

In spite

even in

the presence of HgC12. Ths bindfng

of DTA* ions Is cooperativk

DTABr-bmtonIte dcxtransulfate

Fig.

3

in the DTABr-HnDxS systm,

clay system as cm be seen from d cmparison used here has

8CndIng isotherms far HaCl (b) &a067

an

averaye limzar

chrge

the DTA+-dentransuifatc

m Hgc12.

sepratfan

4.9 x 16

2 and 3.

on the

system at 3Wt.

Polymer concentratfon

but not in

of Ffgs.

(a) 0.020

&l/kg

Hfl_

39s

tthiln for

polymer of 2.5 ;i [13], clay

platelets

to Tanford [.?I]

Since according at 17 ii,

the tta-bentonfte

at an average

are located

used surface

distance

charges

from each other

on the

of I1 i

[ZOI.

the maximrrm chain length of DTA* may be estfmated

interaction betneen neiqhfmurfng bound surfactants is in the OxS polymer because of the long contact length and absence

hydraphobIc

preferable

hydrophobicfty

the polymer backbone.

in

This effect

may be absent

of

on the clay

surface. Fig. this

4 shows an adsarptfon the, coal fines

case,

before

inserting

isotherm far

the electrado

of coal particles

in the equflibrfun

tg the membrane.

membranemore often

systems, reproducibtl

ftfer

Fig.

concentration

4 is about I@

are easityxuffieient diffwent uben

applied

4

and the need to

or wlyelectrolyte

study are sl fghtly

can be performed (the towest surfactant

m, or about

2 pw

in OTA*).

inferior

of

the coal,

influence

the surfattant-selective

fEothenn far

in

between different of pH, salintty,

coals. etc.

electrode method has many advantages

studies of surfactant

66 hours of shakfrrg.

concentration

Accuracies and reprdducfbfl ftiec

to allow for comparative studies

to adsorptfon

Adsorption

In

as can be seen frclm the scatter In the binding even for coal fSncs bindfltg studies at very lan cationic

.

Fig.

system,

by ccntrifugatfon

due to the adherence

procedure,

than in the clay-

in the coal adsorption

degrees of oxrdation

In conclusion1

fine

two systems,

Heverthaless,

isotherms. surfactant

solution

Because of this

reneu the functiunal compared to the other

the DTfWr-coal

have to be removed froar suspension

ions in co1lofdal

solutions.

.

the jlTA+-coal fines

system at pii 7 and 25% after

396 The method can be used in the presence of excess electrolyte, obtained

are morr sens?tive

and reproducible

and the results

can be achieved with

than what

other measurener.t techniques.

This council

research

Moore and Peter

by the Natural

is supparted

Canada, T~xhnical

of

assIstaRc@

Fciewzer

was provided

Thomsor~. al 1 undergraduate

and Engineering by Ctwmaine

Research David

Cooke,

sldamer asslstaatr.

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