Mechanisms of Alkyl Sulfoate ... Water Interface

8/10/2019 Mechanisms of Alkyl Sulfoate ... Water Interface

1/7

y

00

P. SOKASUNDARAN AND

D. W. FUEBSTF.~A

C>r)re f --4 D./A. However, care should be taken

sincewe nitially assumed hat 'P'

.

a.ndP'B are orthogonal

a.nd n this case he homopolar dipole vanishes. If we

had included the overlap integral, then the primary

moments would have changedsomewhat.

Ackr&OtDlIgmentB.

e wish to thank Proiessor

Harrison Shull for his interest and hospitality. Our

tha.nksa.re&Isodue to Mr. John P. Chandlerand ~Ir.

Robert Fern for their help with computational pro\>-

lems.

Mechanisms of Alkyl Sulfonate Adsorption at the Alumina-Water Interface

by P. Somasundaran nd D. W. Fuerstenau

~ 0/ MtII-z T~. U 0/ Cal~ B.-~. Calt/omiA ~ J",.. S. 19f1)

Adsorption iaothermsof sodium dodecyl sulfonateon alumina were determined at con-

trolled ionic strength, pH, and temperature. Electrophoreticmobilities of the alumina

werealsom~ for the sameconditions. The ~ts indicate hat at a certain critical

concentrationor critical pH, adsorption ncreasesmarkedly through 88SOOiationf the

hydrocarbon chains of the adsorbed ulfonate ons to form hemimicellesat the solid-

liquid interface. The dependencef the isothermson surfactantconcentrationand on pH

is interpreted n terms of the Stern-Grahamemodelof the electricaldouble ayer.

Indications of interactions at the interface can be

found in the work by Wayman and co-workers~7on

the adsorptionof surfactantaon clay minerals. Ad-

sorption sothennsof surfactantson silica and alumina

determined by other workers'-lo do 'exhibit marked

changes t certain critical concentrations. However,

these nvestigationshave not been extensive and no

Introduction

In previouspapers"a t was shown hat such nter-

facial parameters sr potential and ftotation ~ponse

dependupon the number of caroon atoms n the hy-

drocaroonchain of a surfactant and upon the concen-

tration of the surfactant n solution. Correlation of

the various nterfacial parameters howed hat nuked

changesn eachof the parameters ccurat somecritical

concentration of the surfactant. It was postulated

that these abrupt changes re due to the association

of adsorbedsurfactant ions to form two-dimensional

aggregates r hemimicellesat the solid-solution in-

terface. From the dependence f the flotation and

electrokineticbehavioron the chain ength of the sur-

factant, the van der Wa.aJs ohesiveenergy ~pon-

sible for latera. nteraction between urfactant ons at

the interface was calculatedon a molar basis to be

-0.6 kcal for eachCH, group n the chain. A direct

test of the proposedadsorption mechanismss best

obtained through the measurement f the adsorption

of the surfactant tself and that is the purpoeeof this

study.

(1) Baaed in part on the Ph.D. m-tation of P. Somuundalan,

University of California, Berkeley, Calif., 1960&.

(2) D. W, Fuerstenau, T, W. Healy, and P. Somaaunduan, Tram.

AlAtE. 129, 321 (1964). .

(3) P. Somuundalan, T. W. H.I7, and D. W. Fueratenau, J.

Ph.-. C1I8m,. 8, 3562 (1964).

(4.) C. H. Wayman, Proc. I~ CZII~Coni" I, 329 (1963).

(6) C. H. Wayman, J. B. Robert8OD,and H. G. Pap, C. S. Geo-

IoIic81 8uney Prof~ Paper 47&-B (1968).

(8) C. H. Wayman, ~., 45()..C 1962).

(7) C. H. Wayman, H. G. Pap. and J. B. Robertson, ib1'd., 4.76-0

(1968).

(8) L W. Wark and A. B. Cox, ~. AlMB', liZ, 189 (19M).

(9) K. Shinoda, d aI., "CoIloidal Surfactant.." Academic ~

New York. N. Y., 1963, Chapter 3,

(10) M. J. J&)'COCk nd R. H. Otte1ril1, Btdl.lf&1t. MimiIg JIet., n.

497 (1968).

TM J--Z of p~ ~


2/7

j

.

1

j

i

1

MECH.~"'ISMS OF ALKYL SULFONATE ADSORPTION

91

quantitative d{',.rription of the adsorption process has

becn presented In :\,'conunt nr the breaks in the iso-

tbcnlls or for tht: ,;i: [UII.::~rll~",j thc slope of the iso-

thl'n\l:;, T:~.I1:lrllu,.lli :1/111 :~m:lkil' attemptal to

e.~pl:lill sucll i.~IIII1'r\II'; ill t('rlll ' oj:\ Bnmaucr-Emmctt-

Teller (B,E. T.) tY11C f equ:Ltion. but the va.lidity

of tllcir :lpl,roach i" flU{': l ollablc I~su:;c of their neglect

of clcctri(':\I,'ffi.,'"". .\1 ), ;Id-"'()rplioll wthc~ gCllcr-

ally have not been detemlined over a sufficiently wide

range or with a precise control of such variables 88

ionic ~lrell~lh, i,ll, ;111.1t'lllr,,'r:llI.n ",lli,'11 :ItT..." tIll.'

ad.-~rptionprocf':;~.

The objective of tllis ImI~r is to present the re1'ult,-s

of detailed studies of the adsorption isothem1S of s0-

dium dodecyl sulfonate on alumina in aqueous media,

These isothenns together with the electrophoretic

behavior of alumina suggest mecha lLR DSf adsorp-

tion of surfactant ions at the solid-liquid interface.

where is the dielectric onstant f the medium,R

is the gas constant, T is the absolute emperature,z

is the valence of counterions ncluding the sign, F

is the Faraday constant,and C is the molesof ions/

cm.' n the bulk solution. Based n the Stem-Grahame

treatmentof the double ayer, expressionor the ad-

sorptionof counterionsat the planea s given by the

equation

(2)

il

where , is the adsorption density in moles/cm.' at the

Plane &, r is the effective radius of the adsorbed ion in

centimeters, C is the concentration of ions in the bulk

in moles/em. , and W, is the work required to bring

the ions to the plane &.

Vol- 10,N-- 1 J~ 1966

Equation 2 can be modified o take into account he

interaction of surfactant ions at the interface by

scparotingW. into electrostaticand interaction terms


3/7

92

P. SOM:A8UNDARAN AND D. W. FUEBSTE.."i411

-c

10""'

*-~~A;._~~_A ~~~

...~

=..

...,

G...

00

~>

+1 "c

-..

::...

+2 ooj

%..

Go.,.

+3 ~~

~~

.

2

U

...

oJ

~

>-"

..

Ui

z

...

0

Z

0

~

Go

~

0

III

0

4 /

ALUMINA

pH 7.2

25.C

IOMC STREfGTH 2aI0-a..

(WITH HoCI)

-

ADSORPTION

- LECTROPHORESIS

+5

10"'-

,I

10"

10-5 0-4 io-.

EQUILIBRIUM CONCENTRATIONOf' SOOIUM OOOECYL

SULFONATE. MOLE PER LITER

Figure 1. Adsorption density of dodecyl suJfonateons

on alumina and the electrophoreticmobility of alumina as a

function of the concentrationof sodium dodecyl sulfonate at

pH 7.2 and at constant emperature and ionic strength.

culated by taking the averageof the area occupied

by cubic packing and by hexagona.l lose-packing f

iOllSof 5.9-:\. diameter (the diameterof the sulfonate

head). Sodium dodecyl sulfonate usedwas prepared

from high-purity dodecyl sulfonic acid by treating it

,,-ith -"Odium y,lroxide, and the resulting sodium salt

,,-;'-.; lurifi"ll by rt"'rY8tallization rom hot absolute

ethyl alcohol. Triply distilled conductivity water

wasused or adsorption nd electrokinetic tudies.

.\l~"or Jti"ll oj :;odiumdodecylsulfonateunder var-

ious conditions was measured y determining he dif-

ference n I'onl','ntration of the surfactant n solution

before and after adding alumina. Experimentswere

carried out at a. constant emperatureof 25 = 0.2,

and an atmosphereof purified nitrogen was main-

tained n the adsorptioncell. Total ionic strengthwas

also kept constant at 2 X 10-1 M by additions of

decinormal solutions of ~dium chloride and hydro-

chloric acid and sodiumhydroxidewhich wereused or

pH adjustment. The: adsorption cell, which was

stirred magnetica.1ly, onnaUy contained 200 ml. of

solution and 4 g. of al~ for ~h experiment,

but for caseswhere he adsorption was ower, 8 g. of

alumina. and 150 mi. of solution were used. The

lowest point was obtained using 16 g. of alumina and

100 mi. of solution. Although equilibrium was at-

tained in much less han 1 hr. under the conditions

used, each sample was agitated for 2 hr. (but more

vigorous agitation was found to decrease he equi-

librium time appreciably). After the equilibrium was

attained, sampleswere collected or detennination of

the equilibrium concentration f sodiumdodecylsulfo-

nate and for electrophoresis xperiments. Determin-

ations of pH were made after a.1lowinghe suspension

to settle or at least30min.

The sulfonate concentrationwas determinedby a

variation of the methyleneblue method described y

Jones.20 t is basedon the principle hat the anionic

dodecylsulfonate n water combineswith the cationic

methyleneblue to give a blue complex hat is soluble

in chloroform. Hence, the concentrationof sodium

dodecyl sulfonate n any aqueous olution can be de-

termined by extracting the methyleneblue sulfonate

complexwith chloroformand evaluating he concentra.-

tion colorimetrica.1ly y comparisonwith the color

produced by known quantities of sodium dodecyl

sulfonate. The possible xperimental rrorshave been

examined by Wayma.n.1 Since the readings could

vary from one dye to another, he same ype of dye

was used through each set of analyses. Absorption

by the methylene blue sulfonate complex was de-

termined rom 640 o 660mp n a Cary Model 14 spec-

trophotometer. The mAximum optical density ob-

tained was used for evaluation of the sulfonate

centration. The lowest concentration hat could

measured as 10-6M in the aqueous olution.

The electrophoreticmobility of the alumina

was determined by means of a Zeta-Meter

method was necessaryf the measurement f

kinetic behavior was to be determinedwith the

small particles that were used n the adsorption ex';

periments. In addition, this method has the advan-

tage over the streamingpotential technique n that it

givesan ndication of the distribution of charges mong

the particles rather than only a net value for the

charge f the particles n the system.

Results

The adsorption sothermof sodiumdodecylsulfonate

by alumina at pH 7.2 and 25 is given in Figure 1.

Electrophoretic mobilities of the particles under the

same conditions are also given in Figure

1.

The adsorption sotherm consistsof t~ regions:

region 1, characterized y a low increasen adsorption

with surfactant concentration; egion 2 by an abrupt

increase n the slope of the isotherm; and region 3

again by a lower dependence f adsorption on sur.

factant concentration. The electrophoreticmobility

exhibits an almost negligibledependence n sulfonate

addition n region 1, but it decreasesbruptly through

region2. The end of region 2 is characterized y the "

(20) J. H. Jon_. J. A_. Agr. ClIemw.. 28, 398.409 (1945).

(21) C. H. Wayman, U. B. GeoIocicalSurvey ProfessionalPaper

4ro-B (1962).

TA. JwnIGl 01 Ph1/aicalChemi#)JI

10"i'

10-12


4/7

~{ECHA.NISMS OF ALKYL SULFON.~TE ADSORPTION

93

mobility becoming ero. In region3, the mobility i5

opposite n sign. Moreover, n region3 a number of

particles were observed o have a positive ch~\rge,

and hoseparticleswith a negati\',~ h:irgehave a wide

distribution of electrophoreticmobilities. This prob-

ably results rom a distributionof particleswith varying

surfaceactivities, thereby giving rise to a distribution

of adsorption densities. However, the net elect oo

phoretic mobility is characterized y a slower change

in region 3.

10-t01-

fr I

..

.

..2

r

0 :;~

-..

-..

00

/

+12>

~ + 2 ~ ~

%".

Go...

+3 O~

~o

,- 'M ~~

;: .\' \.4 a i

J

I

.

I

...

... .

'W ;

... ,

0 '

2 I

..- 10."L

... ,

in 1

z '

w

0

z

l ' ALUMINA

;: pH 6.9

Go 10-'1

U 25.C,/

-:..,c STAENGTM 2.

~ i

/' - .. (WITHQt.,

-

DSORPTION

-ELECT~OP"CAES.S S

10"

EQUILIBRJUM CONCENTRATION OF SODIUM DOOECYL

SULFONATE, MOLE PER LITER.

Figure 3. Adsorption density of dodecyl sulfonate ons on

alumina and the electrophoreticmobility of alumina as a

function of the concentrationof sodium dodecylsulfonate at

pH 6.9 and at constant temperature and ionic strength.

fl"

'5

...

...

0

2

..

..

~

...

0

..

..

~

~

i

.- 5

~

8

9

Discussion of Results

In interpreting these results, we shall consider that

sulfonate ions are adsorbed at the alumina-water

interface. as counterions in the electrical double layer.

Furthermore, since sulfonate ions are not chemisorbed

they are adsorbed appreciably only below the zero-

point-of-charge of alumina, namely below pH 9.

Effect of Surfactant Coocentration 00 Adsorption.

The shape of the adsorption isotherm supports the

adsorption mechanism mentioned earlier. In region

1 of Figure 1 or in region A of Figure 2, at low concen-

trations of surfactants or at low surface potentials,

the surfactant ions :)ore dsorbed individually and ad-

sorption results primarily from electrostatic forces

between surfactant ions and the charged solid surface.

However, when the concentration of the reagent reaches

the hemimicelle concentration or hemimicelle pH,

that is the pH at which hemimicelles start forming

at the solid-liquid interface at constant equilibrium

concentration of surfactant, the ions begin to associate

with each other. This associationenhances he adsorp-

tion of sulfonate ions considerably and, hence, he rapid

rise in the adsorption isothenns result. Thus, in

region 2, the adsorption is due to the electrostatic at-

traction between the ions and the charged solid surface

and hemimicelle association of hydrocarbon chains.

When sulfonate ions equivalent in number to the surface

sites have been adsorbed, the contribution due to the

electrostatic attraction disappears, and the further

increase in adsorption will be only due to association

between the hydrocarbon chains. Thus, in region 3,

pH

Figure

2. Adsorption density of sodiwn dodecyl sulfonate on

alumina and the electrophoretic mobility of alumina as a

function of pH at constant residual sodium dodecyl sulfonate

concentration, ionic strength, and temperature. The ionic

strength or the point at pH 1.98,however,exceeds X 10-1 N

The adsorption isotherm for sodium dodecyl sul-

fonateon alumina.s given n Figure 2 as a function of

pH at constant onic strength of 2 X 10-1 N and an

equilibrium concentrationof approximately3 X 10-6

mole/I. This curve also consists of three parts:

region A showing a low increase n adsorption with

lowering of pH, region B with a greater ncrease n

adsorption,and region C with a constantly decreas-

ing.rateof increase f adsorptionwith loweringof pH.

F.igure3 presents he adsorptiondensity and elec-

tro.phoreticmobility 88 a function of sulfonateconcen-

tration at pH 6.9underconditions f constant empera-

ture an~ constant onic strength. This set of curves

is siriillar to that given in Figure 1 for pH 7.2. The

adsorption sothermsat pH 6.9 and 7.2 are now put

togethern Figure4-forcomparative urposes.

Volum. 70. N-.b..1 JGmA477/986


5/7

9-1

P. SOMASUNDARAN AND D. W. FUERSTENA.1

",lope n the second egion is due to this specific adsorp

tion. Equation 5 can be modified in the following

manner to take this into account. At any particula

value of 1ft.

001-

Asswning the specific adsorPtion >Qtential f chlorid

ions negligible compared to that of alkyl sulfonat

ions and differentiating eq. 7 in the logarithmic form

with respect o In (JR-yields

the slope of the adsorJ)tinn i.o:ntherm dcCTe:L",cs. Thc

rc:~on melltiont.-d above for tht: d~cre~~~ in ~lopc from

n'~ion 2 to rt gion :~ ,; ~\IPI)(lrt~'(1by the el('ctrophoretic

mobility ("In'c \\'hi


6/7

MEcHANISMS OJ' ALKYL SULJ'ONATm ADSoRPTION 95

(23) D. W. Fuerstenau and H. J.Modi, J.Bl.ceroeAem.8oc..106.336

(1959).

Volu_10. Number 1 JGAVIJrV 966

In regionA, dn/d pH is nil and he change f adsorp-

tion densitywith pH should hen be determinedby the

change f

1/-. with pH. Sincehe potentials directly

proportional o the electrophoreticmobility at constant

temperature, ""a/dpH will be determinedby the slope

of the mobility-pH curve. In region A, d1/-a/d H

is negativeand d In ra/ d pH must also be negative,as

canbeseenn Figure2.

The transition point from regionA to B corresponds

to the hemimicellepH for that particular equilibrium

concentration and association between the hydro-

carbon chains begins to take place at that point.

Hence, n region B, the second erm (dn/d pH) also

becomeseffective. The term dn/d pH is negative,

whereasd1/-a/d H is positive in region B. It can be

seen hat the magnitudeof the seconderm, dn/d pH,

due o hemimicelleassociations very high so that the

slopeof the adsorption sothermd In ""./d pH is nega-

tive. The drop in r potential in region B is indicative

of the strong effect of this rapid hemimicelleassocia..-

tion.

As the pH is further increased,he effect of sudden

hemimicelle ormation diminishes and the slope of

the isotherm decreases. n region 0, the association

of a new ion going to the surfacewith a hemimicelle

probably still takes place, but its effect will be very

small compared o that of the association f a number

of individually adsorbed ons to form a new hemi-

micelle. In other words, n region 0, dnld p~ dimin-

ishes and, correspondingly, he rate of increase n

adsorption ecreasesontinuously.

Figure 4 showsa very strong dependence f the ad-

sorption of sulfonate ons on the pH of the system.

The nteresting eatureof this set of curv~ is the change

in hemimicelleconcentration with pH. Hemimicelle

concentrationat pH 7.2 is 8 X 10-6 mole/ . and at

pH 6.9 it is 3.5 X 10-6 mole/ . At a lower pH, the


7/7

P. SoKASUNDARANAND D. W. FUEBSTENAU

6

M 10

2

u

...

~ I

0 '

2

>-"

.. 10""

~

z

...

0

z

0

-=

~ la'

0

I/)

0

~

10-11 mole/cm.s, or at a coverage of only about one..

tenth of a monolayer. The much higher coverage

at about pH 2 as compared to pH 7 may result partly

from the presence of and coadsorption of sulfonic

acid molecul~ which occur n appreciable concentration

at low pH valu~ since the pK of sulfonic acid is about

1.5. Extensive adsorption of the sulfonic acid mole..

cul~ would occur through interaction of hydrocarbon

chains, but without electrostatic repulsion because he

polar eads .re ncharged.

Summary

Detailed adsorption measurementsof sodium dodecyl

sulfonate ions on alumina together with electrophoretic

mobility measurements at constant - ionic strength

and pH indicate that sulfonate ions a.re adsorbed in-

ilividually as counterions in the electrical double layer:'

at low sulfonate concentrations. Adsorption under

th~ conditions occurs by an ion-exchange process.

The adsorption of sulfonate ions was found to be ex-

ceedingly dependent upon the pH of the liquid with no

adsorption occurring above the zero-point-of-charge

of alumina (pH 9), which suggests that chemisorp-

tion doesnot exist in this system. At a certain critical

concentration or critical pH, adsorption increases

rapidly through the formation of two-dimensional

aggregates of the surfactant ions or hemimicelles at

the solid-solution interface. At still greater adsorp-

tion densiti~, the increasen adsorptionwith in-

creasing sulfonate concentration decreases because

the sign of the net electrical charge at the interface is

reversed by the adsorbed surfactant ions. The' de-

pendence of the adsorption isotherms on surfactant

concentrationndon pH is interpreted

terms of the

Stem-Grahame model of the electrical double layer.

Acknowledgments. This research was sponsored by

the National Science Foundation. The authors wish

to thank Dr. T. W. Healy of the University of Cali-

fornia, Berkeley, for helpful discussions and Dr. W.

M. Sawyer of the Shell Development Company,

Emeryville, California, for the sample of dodecyl sul-

fonic cid. .

lC1"a

10-- 10-. K)-4 10-S

EQUILIBRIUM CONCENTRATIOff OF SODIUM DODECYL

SULFOffATE. MOLE PER LITER

Figure 4. Comp&riaon f adsorption BOtherma f dodecyl

sulfonate ons on alumina at pH 6.9 and 7.2.

particles are more positive and, hence, there is an in-

crease n adsorption at the lower pH. Greater charge

on the particle at lower pH will need a greater adsorp-

tion density of sulfonate before the r potential is

brought to zero and this can be observed in Figure 4

by the change in adsorption density at the transition

between region 2 and region 3.

Surface Coverage. From surface area.measurements,

aMuming that saturation adsorption of stearic acid

on alumina. corresponds to a' monolayer, maximum

adsorption in Figure 2 approaches a calculated mono-

layer of dodecyl sulfonate on al'1minA. At the lowest

pH value studied (pH 1.98), the ionic strength exceeds

the value used for all the other experiments, that is

2 X 10-8 M. At these low pH values where the sur-

face charge will be high, the amount of sulfonate ad-

sorbed approachesmonolayer coverage.

Data. on adsorption of the sulfonate ions at pH 7.2

show that association of the adsorbed ions begins at a

coverage of about 10-12 mole/cm.S (based on the area

of the sulfonate head) and that zero mobility (the

transition from region 2 to region 3) occurs at about 3 X

M Joumol 01 Pial/tical C1IcmWFV

Documents

Mechanisms of Alkyl Sulfoate ... Water Interface