Adsorption of surfactants on mineral oxide surfaces from aqueous solutions

Adsorption of surfactants on mineral oxide surfaces from aqueous solutions

Adsorption of Surfactants on Mineral Oxide Surfaces from Aqueous Solutions I1: Binary Mixtures of Anionic Surfactants J. F. S C A M E H O R N , 1 R. S...

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Adsorption of Surfactants on Mineral Oxide Surfaces from Aqueous Solutions I1: Binary Mixtures of Anionic Surfactants J. F. S C A M E H O R N , 1 R. S. S C H E C H T E R , ~ A N D W. H. W A D E * * Department o f Chemical Engineering and **Department o f Chemistry, The University o f Texas, Austin, Texas 78712 Received December 5, 1980; accepted April 22, 1981 Individual surfactant adsorption was measured for well defined mixtures of two isomerically pure alkylbenzene sulfonates adsorbing from aqueous solutions onto both alumina and kaolinite. An adsorption model, which includes two-dimensional phase transitions resulting from lateral interactions, is shown to describe the observed isotherms. The phase transitions result in the formation of mixed hemimicelles (high-density aggregates of surfactant mixtures on the surface). The environment for methylene groups in the mixed hemimicelle is slightly less favorable than that in either pure surfactant hemimicelle. For solution compositions below the mixed Critical Micelle Concentration (CMC), the surfactant having the least tendency to adsorb exhibits greatly increased adsorption because of the presence of the mixed hemimicelles. The adsorption of the surfactant having the greatest tendency to adsorb is not greatly different from the pure component adsorption. Since only monomer adsorbs significantly, monomer-micelle equilibrium had to be considered simultaneously with the adsorption model to predict results above the CMC. Idea/ solution theory was shown to be a good approximation for this equilibrium. Above the CMC, conditions are described which result in minima and maxima in total adsorption isotherms. INTRODUCTION

The vast majority of surfactant adsorption studies have been made using commercially available surfactant mixtures of unknown composition rather than with well-characterized mixtures. Moreover, almost none of these studies measured the adsorption of each individual surfactant in the mixture; generally only total adsorption has been measured. Interpretation of the results has often been based on considerations of a pure adsorbate and has largely ignored special isotherm features which may be entirely attributable to the fact that the adsorbate is a 1 Current address: School of Chemical Engineering and Material Science, University of Oklahoma, Norman, Oklahoma 73019. 2 To whom all correspondence should be addressed.

mixture. The few studies, which have reported individual component adsorption, involved greatly dissimilar adsorbate constituents. The recent application of high-pressure liquid chromatography to the analysis of surfactants allows the simultaneous determination of individual concentrations in a mixture of components differing by as little as one methylene group. Therefore, the measurement of the adsorption of each member of a homologous series in a surfactant mixture is now possible. In this study, the adsorption of a binary mixture of isomerically pure alkylbenzene sulfonates was measured on alumina and kaolinite from low concentrations to well above the CMC. Many of the concepts presented here represent an extension of work on single-surfactant adsorption de-

479

Journalof Colloidand Interface Science, Vol.85, No. 2, February 1982

0021-9797/82/020479-15502.00/0 Copyright© 1982by AcademicPress, Inc. All rightsof reproductionin any formreserved.

480

SCAMEHORN, SCHECHTER, AND WADE

scribed in a previous paper (1). To avoid redundancy, extensive reference will be made to that work. Few attempts have been made to model multicomponent surfactant adsorption. A model, which predicts adsorption isotherms of the observed shape, has not been reported. There are several complexities in the derivation of such a model. For example, one must understand the nature of the mixed hemimicelles in order to predict individual surfactant adsorption. Also, because surfactant incorporated into micelles tends to adsorb very differently from that in the monomer form, monomer-micelle equilibrium must also be quantified. This paper presents a model, which attempts to take into account these, and other complexities. EXPERIMENTAL

A detailed description of the materials and methods used here is given in a previous paper (1). It should be noted that the surfactants used were sodium 4-[(3')decyl]benzene sulfonate and sodium 4-[(4')dodecyl]benzene sulfonate and are designated as 3-&C10ABS and 4-&C12ABS, respectively. These may be considered as members of a homologous series with constant degree of branching. THEORETICAL This paper will only consider binary mixtures. Extension to more components is straightforward. Monomer-Micelle Equilibrium In a previous paper, we demonstrated that only monomer and not micelles adsorb from aqueous solutions containing surfactants of the type used here (1). It was also shown that the pseudo-phase separation model provides an adequate representation of micelle formation for these same surfactant systems. It may be anticipated, therefore, that the adsorption of binary surfactant.mixtures Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

will depend only on the monomer concentration and composition. If it is assumed that both the monomer and micellar phases are ideal for a mixture of surfactants A and B (2): Yi

CMCM -

xi

,

[11

CMCi

where i can be either surfactant A or B, y~ is the mole fraction of surfactant i in the micellar phase, xi is the mole fraction of i in the monomer phase, CMCM is the mixture CMC, and CMCi is the CMC of surfactant i. The mole fraction is the fraction of the total surfactant in the phase, which is composed of that individual species. If the pure component surfactant and mixture CMCs are known in the presence of the same, large concentration of added electrolyte (electrolyte concentration ~> surfactant concentration), the counterion concentration, and so, CMCi, is independent of the mixture composition. The term composition refers to the proportional makeup of the surfactant by the individual surfactant species. By definition, Y~ +yB = 1.0 and XA + X B = 1.0. [2] Solving Eqs. [1] and [2] simultaneously, CMCM = CMCACMC# (XACMCB +

xsCMCa).

[3]

Because of the large salt concentration restriction, Eq. [3] is the same as the relationship describing the CMC of mixtures of nonionic detergents (3). Equations [ 1] and [3] permit calculation of the mixture CMC and micelle composition at any monomer composition. Adsorption as a Function of Monomer Composition We have proposed a model to describe the adsorption of a single surfactant onto a mineral surface (1). This model was shown to provide an accurate description of the

ADSORPTION

OF SURFACTANTS,

adsorption for the surfactants and substrates used in this study (1). That model will be extended to a binary system of surfactants here. Since this is a patchwise adsorption model, the adsorption isotherms on a homogeneous patch will first be derived. The patch energy distribution will then be combined with this to generate the adsorption isotherms for the entire surface. The two surfactants are labeled A and B. There are six types of adsorption possible: (I) first layer adsorption o f A , or (II) of B; (III) second layer adsorption of A, onto A in the first layer, or (IV) A on B, or (V) B on A, or (VI) B on B. On a given patch, the adsorption of each of these types can be described by an extension o f the Fowler adsorption isotherm (4). The resulting six coupled equations, which correspond to these adsorbate states in the same order as listed above, are, 01, A -- 02,AA -- 02,BA

1 -

01,~

-

01, B

= K~,Ae--A~°,A/RTCAe--(~AO1,A+~aa°I'È)/RT,

[4]

01, B -- 02,Z B -- 02,BB

1 -

OLA--

01,~

I¢'ABe--A~°•B/RT['-' ~--(OJBO1B+OJAB01A)/RT =,Xl, t.~Bt; • , ,

[51

02,AA 01, A -- 02,AA -- 02,B A = K A 2,A e - a ~ "AA/RTC A e--(OJAOl ' A+OJABO1"B)/RT

[6]

02,BA 01, A -- 02,AA -- 02,B A = KA2,Be A~O,BA/RTCB e --(O)BOI'B+¢OABO1"A)/RT ,

[7]

2,AB 01, B -- 02,AB -- 02,B B = KA2,A e--AIz°'AB/RTCAe--('OAO~'A~-O)ABO1 "B)/RT, [ 8 ] 2,BB 01, B -- 02,AB -- 02,BB = KA2,B e--A~'BBIRTCB e --(OJBO1"B-L°)ABO1"A)/RT,

[9]

481

II

where K~,a =

1 x 10 -4 -- 2 × 1 0 - 8 )

)

× e l×l°-4(~°a+°~AB)/RT,

K,% = ( 1

× 10 -4

-

-

2 ×

[10]

10-8 t

× e I×IO-'(~°B+~A~)/RT,

[11]

1 × 10 -8 K A2,A =-

1 × 10 - 4 -

2 x 10 -8

J

× e l×~°-'('°A+~,)/nz, 1 x 10 -8 KA'B

=

1

X

10 -4 --

2 x 10 -8

[12]

\

)

x e lx1° '(~8+,oA~)/nr, [13] where 01,i is the fraction of patch covered by surfactant i in the first layer, 0z,~j is the fraction of the patch covered by surfactant i in the second layer adsorbed onto surf a c t a n t j in the first layer, C~ is the m o n o m e r concentration of/, A/2°~ is the standard state chemical potential change of surfactant i upon adsorption in the first layer, A/z2,i~ -0 is the standard state chemical potential change of surfactant i upon adsorption in the second layer onto s u r f a c t a n t j in the first layer, o~ is the lateral interaction energy of surfactant i in either layer interacting with surfactant i in the first layer, o~aB is the lateral interaction energy for surfactant A in either layer interacting with surfactant B in the first layer or vice versa, R is the ideal gas constant, T is the absolute temperature, and K f a , K f x , KLA, and KA,9 are constants. The standard states are arbitrarily defined as: 01, A ---- 01, B = 1 × 1 0 - 4 , 02,An = O2,AB -~ O2,BA = 02x. = 1 x 10-8; Ca = C . = 1 ~mol/ liter. The total adsorption of each surfactant on a patch is OH, A = 01, A ~" 02,AA -}- 02,AB ,

[14]

Orl,~ = 01,8 + 02,8A + 02,~,

[15]

Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

482

SCAMEHORN, SCHECHTER, AND WADE MOLE FRACTION 4 - ~ b - C i z ABS

1.0

0.8

0.6

0.4

0.2

0

800. 25* C 0.171 M NaCI

700.

pH 6 . 6

600. "' .J 0

500.

IDEAL

chemical potentials of molecules in both the high-density and the low-density states are equal. Secondly, the surface tension between the solution and both of the surface phases must be equal. Application of the Gibbs equation to this second criterion yields:

I OH,adIn C A

=Z 4oo.

surface

(D "5

(~ 500.

+ I

OHxdIn C8

= 0.

[16]

d

surface

200. I00.

0

0.2

0.4

0.6

0.8

1.0

MOLE FRACTION 5-(~-CIoABS

FIG. 1. Effect of composition of 3-~b-C10ABS and 4-¢b-C12ABS mixtures on CMC.

where On,~is the total fractional coverage of patch by surfactant. Simultaneous solution of Eqs. [4]-[9], [14], and [15] yields the individual component and total adsorption on a given patch at a specified monomer concentration and composition. If the total adsorption on a patch is calculated using the high lateral interaction energies observed experimentally for the surfactants used in this study, there will exist solution conditions for which three different values are predicted. This indicates the existence of a two-dimensional phase transition occurring on the surface (1). Therefore, a region of low adsorption density is in equilibrium with one of high adsorption density at some specific concentration on the S-shaped adsorption isotherm surface. To establish the solution concentrations at which this phase transition from the low adsorption density to the high one occurs, two criteria must be satisfied. First, both surface phases must be in equilibrium with a solution having the same concentration and composition. This ensures that the Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

The integrals are evaluated along the calculated S-shaped isotherm between lowdensity surface phase and the high-density one in equilibrium with it. From the first criterion of equilibrium, the values of Ca and C, must be the same at the end as at the beginning of the integration. The path taken during the integration will not affect the concentration at which the phase transition is predicted to occur because it is a state thermodynamic property. For example, one could generate an isotherm at constant CA, allowing only CR to vary. A constant composition (CA/CB = constant) was found to be most convenient in this work. As the total concentration increases, the resulting adsorption isotherm on a patch generally shows a slow increase in adsorption, a vertical step at a low adsorption density, followed by a slow approach to bilayer adsorption. A Gaussian distribution is used to describe the distribution of patch area associated with A~L10Aand A/2°,8:

da

= exp(-(A/2°'i- A/2M~)2/2cr~) ~ri(27r)"~ x

dA/2°l,i, [17]

where A is the fraction of the total surface area assigned to patch associated with A/2°i, - M is the mean value of A/2°,i, and o-~ A/~l,i is the standard deviation of the distribution for surfactant i. All second-layer thermody-

ADSORPTION OF SURFACTANTS, II

483

[ ~

MOLEFRACTION

0.5 0.8 I.O .20oc 0.171MNQCI

I0,

~L

CM CMC CMC

CMC

I ,R _OT,ON I lI Cl I~-,-c,°A~sl**-0,~A-V// /

CMC

)

MOLE FRACTION IN SOLUTION

o

T

, 0.9

,

0

0.8

pH 4.3

A

SOLUTION/SOLID

i

C1~1~l / I

!/

1 - -C,oABS

05 0.2

,0.

Lid

1.0

-

3o*c

o =E

0.171 M NoCI pH4.3

_ OJ

o

CMC ~/

0.2[---/'// O. I d #

/// ///

/// ///

. I.O

m

0.01

L i i llllll

1,0

,

i ,li,HI

,

i ,liHi

I0. I00, TOTALCONCENTRATION ~ccMOI_E/L)

1(300.

FIG. 2. Simulation of effect of solution composition on total adsorption of 3-&C~0ABS and 4-&C~2ABS mixtures on kaolinite.

namic parameters (A[£O,AA, Ai.Z2,A0B,AI.L2,BA , 0and A/k° aa), and lateral interaction energies (~Oa, ~08, and COAB)were a s s u m e d to be the same for all patches. This is a very reasonable approximation. From Eq. [17], in cal500C 0171MNaCI pH45 SOLUTION/SOLIDRATIO00~ IL/G)

/ /

CI~C

:0/]/ c#c

Io.

c[c

m•l.0 g

4 0,I

,

1.0

/

~

0.5 0.8

0,5 0.2

.

0

,

,,,,I

,

i

I I I I LJ

I0. 4-~b-CB2 ABS CONCENTRATION (IzMOLE/L)

I00.

culafing the adsorption isotherm for a h o m o g e n e o u s patch, A~L0,A and A/k°,B must be the same number of standard deviations from their respective mean values for both quantities to be applying to the same patch. The individual surfactant adsorption on the entire surface is the area weighted sum of the adsorption on each patch under the same conditions: 0TOTAL,i

M~:' EFiAOTION IN SOLUTION 3-q~- ABS 4-9~-C~2ABS

,

F I G . 4 . Simulation of effect of solution composition on 4-&C12ABS adsorption on kaolinite in a mixture with 3 - & C 1 0 A B S .

$ /

,

=

f:OH,flA F =

0n,i e x p ( - ( A # ° , i - A#Mi)~/2o'~) X

o-i(27r)"2 x dAft°i,

[18]

with the total surfactant adsorption being

O.Ol I0. I00 3-qb-CioABS CONCENTRATION(t~MOLE/L)

I000.

FIG. 3. Simulation of effect of solution composition on 3-&C10ABS adsorption on kaolinite in a mixture with 4-&C12ABS.

OTOTAL,A B =

0TOTAL,A

..L 0 T O T A L , B '

[19]

w h e r e 0TOTAL,~ is the total fractional coverage of the entire surface for surfactant i (range of 0 to 2) and OWOTAL,AB is the total Journal of Colloid and Interface Science, Vol.

85, N o . 2, F e b r u a r y 1982

484

SCAMEHORN, SCHECHTER, AND WADE

fractional coverage of the entire surface arbitrarily assumed that (range of 0 to 2). A ~2,AB -0 = ~°,BA = (A~0,AA + A ~ ° ~ ) / 2 " [20] Since the standard states chosen here are the same as those used for the single- The only remaining, undetermined paramadsorbate case (1), the following variables eter, which can be adjusted to fit mixture can be evaluated f r o m application of the data, is tOAB. model to single-surfactant adsorption isotherms: A/2~.A, A~'L1,B, - M A[£2,AA, -0 -0 A~L2,BB, O)A, Prediction of Static Adsorption Isotherms Experimental adsorption isotherms for ~OB, O'A, and O'B. AS with the single-component case, it was found here that the ad- mixtures are normally obtained at a constant sorption isotherms are fairly insensitive to feed (solution before adsorption) composisecond-layer parameters. Therefore, it was tion. However, the equilibrium composiCMC BILAYER COVERAGE

CMC

0 I00.

~,1 ._1 0 X

z 0 I-n.

I0.

0 o~ o

I.0 I.-

MOLE FRACTION IN FEED 3-O-CIoABS 4 - 0 - C i 2 ABS

1.0

A

0 O.I

E]

0

Q

0

#

<>

0.2

0.8

,0

I.o

o

~,0"

[

O.I

[ [[ll[I]

O. TOTAL

i

IO0. CONCENTRATION

C

0.171 M NoCt pH 7.7 SOLUTION/SOLID THEORY i

I

I:O 0.9

I llllll

RATIO 0,1 ( L / G

I

I

I I

1000, ( //.MOLE / L )

FIG. 5. Effect of feed composition on total adsorption of 3-&C~0ABS and 4-&CzzABS mixtures on alumina. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

485

ADSORPTION OF SURFACTANTS, II

tion will generally vary with the total equilibrium concentration. Material balances and monomer-micelle equilibrium must be considered to predict these constant feed composition adsorption isotherms from the adsorption model isotherms, which were obtained here at constant equilibrium composition. Below the CMC, a component material balance in a static adsorption experiment results in w i C T : x i C D + OTOTAL,iFMS [21]

and above the CMC, wiCv = X~CD + yiCM + OVOTAL,iFMS, [22]

where w~ is the mole fraction of the feed solution, which is composed of surfactant i, Ca- is the total surfactant concentration in the feed solution, CD is the total equilibrium concentration of monomer, CM is the total equilibrium surfactant concentration in micelles, FM is the monolayer coverage on the surface, and S is the weight of substrate/volume of solution.

,.•'

CMC

OMC

I00.

0

~k ag o IL w. 0 Q O q[

,o. L_

QO

m

O 0

MOLE FRACTION IN FEED

,.o~-

i-i

o

rl

3- O-CIoABS 1 4 - Q-C,~AB,~ 0. I 0.9

<~ ,0

0.2 t.o

o.e o

30 ° C

O, I T l M NoCl pH 7.7 SOLUTION/SOLID RATIO O.I ( L / G ) THEORY

0.1

I O.

I00. 3 " 0 - C 1 0 AB$

I000. CONCENTRATION

( F MOLE/L)

FIG. 6. Effectof feetl composition on 3-~b-C10ABSadsorption on alumina in a mixture with 4-~b-C12ABS. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

486

SCAMEHORN, SCHECHTER, AND WADE

The adsorption model predicts 0 T O T A L , i a s a function of xi and CD. Below the CMC, simultaneous solution of this model with Eq. [21] results in xi, CD, and OTOTAL,i" Above the CMC, Yi and Co (which is equal to CMCM) is obtained from Eqs. [1] and [3] as a function of x~ for ideal monomermicelle equilibrium. Simultaneous solution of Eq. [22] with these relationships and the adsorption model results in x~, y~, Co, CM, and 0TOTAL,i. The CMCi used here was that concentration for which the calculated adsorption intersects the measured plateau adsorption.

It was necessary to use this instead of the CMC measured by surface tension techniques to accurately predict adsorption above the CMC since the two CMCs are not exactly the same (1). RESULTS AND DISCUSSION

Critical Micelle Concentration of Mixtures The CMC of mixtures of the sulfonates used in this study is shown in Fig. 1 as a function of composition at the same added NaCI concentration as used in the adsorption experiments. This mixture deviates

CMCI *CMC

O

MOLE FRACTION IN FEED

I00,

~

l.O 0.9

0.2

I

O.8

$OLU TION / SOLID RATIO 0.1 (L/G) TH[ORY

Ft•I <>

0 Z

J J

30" C 0. 17 I M NoCI

F'I o

O O. I

[]

[]

z

9p L

I0.

El

E] m u ,it I.O

0.1

I IIIIJ

I

I0.

l i J~JlJl

i

i i iltlll

I DO. 4-0-CI2

I

I ~

IOOO,

ASS CONCENTRATION (F" MOLE/L)

FIG. 7. Effect of feed composition on 4-~-C12ABS adsorption on alumina in a mixture with 3-~-C10ABS. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

487

ADSORPTION OF SURFACTANTS, II

slightly from Eq. [3] and, therefore, approximates ideal solution theory. The effect of the difference in temperature and pH between this experiment and the adsorption experiments on CMC is negligible for these compounds (1).

Each individual component adsorption isotherm in these graphs has that component's concentration as the independent variable to allow comparison to pure component adsorption. If there were no interactions of any kind between components, these isotherms would be independent of Predicted Adsorption Isotherms composition. This always occurs at suffiat Constant Composition ciently low concentrations. Figures 2 - 4 present the predicted total From Fig. 3, below the CMC, the adsorpand individual component adsorption iso- tion of 3-6-C10ABS is higher at a given 3-~btherms for mixtures of 3-6-Ca0ABS and C10ABS concentration, as its mole fraction 4-qb-C12ABS on kaolinite. The value of OJaB in solution decreases. This is because the used was obtained from experimental ad- 4-6-CI~ABS tends to adsorb much more sorption data, as described later. than the 3-qS-C10ABS at a given component The total adsorption isotherms in Fig. 2 concentration (see pure component curves vary monotonically with composition. This in Fig. 2), resulting in the formation of hemiis because OJA~is roughly the same as OJA micelles (composed mainly of 4-6-C12ABS) and o~8. at a lower 3-q~-C10ABS concentration than

CMC

CMC

O 0 PO

MOLE FRACTION IN FEED [email protected] ABS

0 0.1 0.2 I.O

O

O O

IO

4-c~-C~2ABS I.O O.9 0.8 0

30"C 0.171 M NaCI z

o

pH 4.3

in n~ 0

--

.-J

SOLUTION/SOLID RATIO O.OI(L/G) THEORY

0.1

0

OOI LO

I0.

I00. TOTAL

I000.

CONCENTRATION ( / ~ M O L E / L )

FIG. 8. Effect of feed composition on total adsorption of [email protected] and 4-~b-C12ABS mixtures on kaolinite. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

488

SCAMEHORN, SCHECHTER, AND WADE

would occur for the pure component. The intercomponent interaction energy causes the 3-~b-C10ABS to have increased adsorption in this situation. Conversely, the 3-6Ci0ABS tends to adsorb so little, compared to the 4-~b-C12ABS, that large mole fractions of the former must be present in solution to significantly affect the adsorption of the latter, as shown in Fig. 4. Thus, the effect of mixture composition on adsorption is profoundly different for strongly and weakly adsorbing individual surfactants. STATIC ADSORPTIONISOTHERMS Adsorption below the Mixture CMC The pure component and mixture adsorption isotherms for 3-~b-C10ABS and 4-~bC12ABS are shown in Figs. 5 - 7 on alumina, and Figs. 8-10 on kaolinite. The same qualitative conclusions reached about the effects of equilibrium composition

below the CMC in the previous section also apply to the effects of feed composition under the conditions used here. The total adsorption varies monotonically with feed composition on both substrates. The adsorption of the 3-~b-CloABS below the CMC is affected greatly by feed composition, while that of the 4-~b-C12ABS is largely unaffected. This effect is both predicted and observed. The value of toAB, which results from this data, is compared to to for each pure component in Table I. The smaller negative value of toAs, as compared to to, indicates that the methylene group environment in the mixed hemimicelle is slightly less favorable than that in a pure component hemimicelle, perhaps due to less efficient packing of the hydrocarbon chains of different lengths. It is interesting to note that the same shape of total adsorption isotherm is preCMC 0

10.

CMC CMC

1 W Q

MOLE FRACTION IN FEED 3-~-CIoABS

z o

1.0

Q_ no co [] o

°

0.1

0.9

o o

0.2

0.8

1.0

0

3o*c

0.171M

3

0.1

NoCI

SOLUTION/SOLID

[]

,h

ABS

p H 4.3

[]

_o

4-

~~o~

4-~-Ciz

D

--

RATIO 0.01 (L/G)

THEORY

•o []

I 1.0

I

I I• I I l l

I 1(3.

J

i

i I till

J I00.

J

I J f ill

I I0 00.

I

i

i I III

II I0,000.

5-~-Cto ABS CONCENTRATION (/~ MOLE/L)

FIG.9. Effectoffeedcompositionon 3-(b-C10ABSadsorptionon kaolinite in a mixturewith4-d~-C12ABS. Journal of Colloid and Interface Science, Vol. 85, No. 2, February I ~ 2

ADSORPTION

OF SURFACTANTS,

489

II

CMCCMC CMC l MOLE FRACTION IN FEED 4-~b- C~z ABS 3-~" Cio ABS 1.0 0 09 0.1 0.8 0.2

I0.

50"C 0.171M NoCl pH 4.3

ILl

/ o

SOLUTION/SOLID RATIO O.01(L/G)

z _o i-o_ n~ 0 ,,¢

~THEORY

A A

m ,¢ 0.1

+

O.OI 1.0

I0.

I00. 4-~b-CItABS CONCENTRATION (/z MOLE/L)

I000

I0,000.

FIG. 10. E f f e c t o f f e e d composition on 4-qb-ClzABS adsorption on kaolinite in a m i x t u r e with 3-q~-C10ABS.

dicted for the mixture as for the pure surfactants; i.e., the usual three adsorption regions below the CMC (1, 5-10) are predicted to exist. Therefore, isomerically pure surfactants are not necessary to generate this type of isotherm.

is almost independent of concentration in Figs. 5 and 8. This is predicted at all feed compositions for both substrates under the conditions used. To assist in understanding this, Figs. 11 and 12 show the effect of solution/solid

Adsorption above the Mixture CMC TABLEI

F o r a mixture at constant feed composi-

tion, the composition of the monomer will vary with total concentration above the CMC. Therefore, unlike single-component systems, the total adsorption may vary with total concentration above the CMC, without micelles adsorbing or violation of the pseudo-phase separation model. However, the experimental and theoretical adsorption

Comparison o f L a t e r a l I n t e r a c t i o n E n e r g i e s f o r P u r e Components and B e t w e e n D i s s i m i l a r C o m p o n e n t s f o r 3-~b-C10ABS a n d 4-~b-ClzABS Pure component to (kcal/mole) OJAB

Substrate Alumina

Kaolinite

3-~C10ABS - 4.32 - 3.72

4-qS-CI2ABS - 4.97 - 3.70

(kcal/mole) - 4.16 - 3.31

Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

490

SCAMEHORN, SCHECHTER, AND WADE

SOLUTION /SOLID RATIO ( L / G )

50 - - i O . O

o°o',

w 40 CMC

d.

CMC

0.001

:¢ 5 0 o

2O

//

/

~- I 0

I o A' l

/

I I00,

I

o.,~

I

PH, -?-' ~ l l 500.

TOTAL

IN FEED

L4"e'cl2 Ass I 50"C 0,17[ M NaCI

CONCENTRATION

j

i

I

I000. ( ~ MOLE/L)

FIG. 11. Simulation of effect of solution/solid ratio on total adsorption of a 3-~-CloABS and 4-qS/C~ABS mixture on kaolinite.

CMC

1.0 --

CMC

0.9 Z

o

o.8

0.7 LI. LIJ J 0 0.6 =E

ratio on the predicted total adsorption isotherm, and corresponding monomer composition on kaolinite at a constant feed composition. As the solution/solid ratio approaches zero, almost all of the surfactant in the system will be adsorbed, except at very high surfactant concentrations (at least an order of magnitude above the CMC), resuiting in the adsorbed phase having the same composition as the feed. As the total equilibrium concentration approaches very high values, at any solution/solid ratio, almost all of the surfactant in the system will be present in micelles, resulting in the micelles having the same composition as the feed. Since the monomer composition (and therefore adsorption) at low solution/solid ratios is almost independent of concentra-

CMCj.

11 I SOLUnON/SOL,O

~ -

j

-

II II

J

I'

I RATIO L-O.OOI

L//~

~0.1

>

0 0.3 z --

,o.

m

Z

m

--0"4

2

(n <{ o

0

--0.2~

o:o,

n- 0 . 5 LU 0 Z 0 =E

(L/G)

0.4

3-e-C IoABS 4-e-CI2ABS

0.5

30* C 0.171 M NoCI pH 4.3

o - 0.2

!

__ 0

i

0.5 m

MOLE FRACTION IN FEED 0.6 0.4

0.6 ~o -I

_o.7 8 2

--0.8

0.1

0.9

I

I

100.

,

,

I 500.

TOTAL

I,,ll

i

,

i

1.0

1000.

CONCENTRATION (/~ M O L E / L )

FIG. 12. Simulation of effect of solution/solid ratio on monomer molar composition of a 3-4)-C~0ABS and 4-q~-C12ABS mixture adsorbing on kaolinite. Journal of Colloid and Interface Science, Vol. 85, N o . 2, F e b r u a r y 1982

491

ADSORPTION OF SURFACTANTS, II

CMC

o~

600. 500.

0

-

1,0

~.

CMC

400,

MOLEFRACTION 20O

8~

0.9

foo.

30°C 0.171MNoCI pH 4.3 SOLUTION/SOLIDRATIOI0.0{L/G)

CMC

I

i

100.

i

i

i

~ kill

,

I

i

P

i

I000. TOTALCONCENTRATION(~ MOLE/L)

Z

O. [71M

=E

pH

"~

I0,000.

tion above the CMC, that monomer is in equilibrium with an adsorbed phase and micelles, both with almost the same composition (that of the feed). This indicates that a surfactant component has about the same tendency to adsorb as to form micelles, relative to another component, for the ABS isomers studied. The experimental adsorption data were obtained at a low enough solution/solid ratio for this effect to result in flat adsorption isotherms above the CMC. At high solution/solid ratios, the monomer is composed of an increasing proportion of the lesser adsorbing component with increasing total concentration above the CMC, which tends to reduce adsorption.

;

CJ

m 0~

SOLUTION/

IO(L/G)

0.001

i i iiI

o

0,8 -

FIG. 13. Simulation of effect of feed composition on total monomer concentration of a 3-~b-C~0ABS and 4-~b-C~=ABS mixture adsorbing on kaolinite.

A 40.

NO

4.3

:~

L~I~:! O0.OIO.I

. . . . . . .

I00.

0.2 O~ ~

11

,

. . . .

lJll

I000. I0,000. TOTALCONCENTRATIO(~ N MOLE/L)

F I G . 1 5 . Simulation of effect of solution/solid ratio on monomer molar composition of a 3-~b-C~0ABS and 4-qS-C~2ABS mixture adsorbing on kaolinite showing adsorption maxima and minima.

The monomer concentration is simultaneously increasing, as shown in Fig. 13, which tends to increase adsorption. Thus there are two opposing forces--one tending to increase adsorption and the other tending to decrease it. Because of this, adsorption may decrease above the CMC, resulting in a maximum in the total adsorption isotherm. The total adsorption isotherms and corresponding monomer compositions are shown at another feed composition in Figs. 14 and 15. The corresponding monomer concentration for a high solution/solid ratio is shown in Fig. 13. At high solution/solid ratios and concentrations just above the

I.c

700. O

'zL30' ~ -

~

MOLEFRACTION

I

IN FE o

i ,C,oA S I,,C,=ABS

~o 2°

I

I

0.9

I

' I

o.

30"C 0.171M NaCI

IO. ~

o

oO?

,

t

,

% ~ 600.

,,,,,i

,

,

* iiiiii

1000 I0,000. TOTALCONCENTRATION(/.¢MOLE/L)

FIG. 14. Simulation of effect of solution/solid ratio on total adsorption of a 3-q~-C,0ABS and 4-q~-C12ABS mixture on kaolinite showing maxima and minima.

MOLE FRACTION IN FEED

o

o J

~ 5o0.

\'o.ol - - O.001

I00.

d

pH4.3

~,,~SOLU TION/SOLIDRATIOIL/G)

O

w

3-~-Cto

ABS

0.35

4"~-C=z

ABS

0.65

30" C

0.171MNoCI pH8.0 SOLUTION/SOLIDRATIO O.025(L/G) --THEORY i i i i 20,000. 40,000. TOTALCONCENTRATION(~MOLE/L)

FIG. 16. Adsorption of a mixture of 3-qS-CIoABS and 4-~-C,2ABS on alumina showing increased adsorption with concentration above CMC. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

492

SCAMEHORN, SCHECHTER, AND WADE

CMC, the composition effect causes the total adsorption to decrease with increasing total concentration, while at higher total concentrations, the increase in the m o n o m e r concentration reverses this trend. Therefore, both a m a x i m u m and a minimum are present in the adsorption isotherms. Trogus et al. (11) p r o p o s e d the concept that minima and m a x i m a in total adsorption isotherms for mixtures are predictable from consideration of m o n o m e r - m i c e l l e equilibrium relationships. T h e y concluded that m a n y literature adsorption isotherms showing minima and/or m a x i m a may be explainable from these considerations. In that work, H e n r y ' s law was used to describe adsorption below the CMC. The use of a more realistic adsorption model in this study does not qualitatively alter their conclusions. The higher the solution/solid ratio, the less the adsorption data accuracy, using the concentration change method of measurement. Unfortunately, the high solution/solid ratios, at which minima and/or m a x i m a are predicted to occur, are not experimentally accessible for the systems studied. H o w ever, conditions under which the total adsorption increases monotonically with concentration a b o v e the CMC are accessible.

.J o 1~ 200.

~

0

O0 0

0

0

MOLE FRACTION IN FEED 3-(~-CmoABS 0.55 4-~-C m ABS 0.65

o I00.

o

30" C 0.1'71M Noel pH 8.0

0

i

5000.

/ ,,5

400.

o

0

0

° °

o

[email protected]% AB.' [email protected], ABS

J

.~ 3oo.

VIOLE FRACTION IN FEED

0.35 0.S5

30"C D.17'IM NoCI pH 8.0 SOLUTION/SOLID RATIO O025[L/G)

--THEORY ~0,000

20, OOO

4-~p-Cm ABS CONCENTRATIONI~MOLE/L]

FIG. 18. Adsorption of 4-4~-C~2ABSon alumina in a mixture with 3-qS-CIoABSfor system showing increased adsorption with concentration above CMC. The adsorption of 3-~b-C10ABS and 4-qSCI~ABS on alumina a b o v e the CMC, and at a low solution/solid ratio is shown in Figs. 16-18. The predicted increase of total adsorption and 4-th-C~2ABS adsorption, and relative independence of 3-~b-C10ABS adsorption, with their corresponding concentrations, is experimentally observed. This further supports the validity of the model a b o v e the CMC. One rather obvious, but important conclusion that this w o r k has illustrated is that adsorption can be highly dependent on composition as well as concentration. It is c o m m o n practice to report only total adsorption vs total concentration for complex mixtures. Since the composition is u n k n o w n and p r o b a b l y varies with concentration and solid/solution ratio, use of those isotherms at any but the exact conditions under which they were obtained is not valid. ACKNOWLEDGMENTS

SOLUTION/SOLID RATIO O.025(L/G) --THEORY

.g

500.

i

IO,000.

3-~-C~o ABS CONCENTRATION ( p. MOLE/L )

FIG. 17. Adsorption of 3-~b-C10ABSon alumina in a mixture with 4-~b-C12ABSfor system showing increased adsorption with concentration above CMC. Journal of Colloid and Interface Science, Vol. 85, No. 2, February 1982

This research has received financial support from the following organizations: Amoco Production Company, Ashland Chemical Company, Atlantic Richfield, British Petroleum Company, Ltd., Chevron Oil Field Research, Conoco Inc., Department of Energy, ElfPetroleum Corporation, Exxon Production Research Company, Gulf Research and Development Company, Marathon Oil Company, Mobil Research and Development Corporation, Shell Development Company, Stephen Chemical Company, Suntech Inc., Texaco

ADSORPTION OF SURFACTANTS, II Inc., Union Oil Company of California, The University of Texas Engineering Foundation, and Witco Chemical Corporation. REFERENCES 1. Scamehorn, J. F., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 85, 463 (1982). 2. Mysels, K. J., and Otter, R. J., J. Colloid Sci. 16, 474 (1961). 3. Clint, J. H., J. Chem. Soc. Faraday Trans. 1 71, 1327 (1975). 4. Fowler, R. H., Proc. Cambridge Philosoph. Soc. 32, 144 (1936).

493

5. Somasundaran, P., and Fuerstenau, D. W., J. Phys. Chem. 70, 90 (1966). 6. Wakamatsu, T., and Fuerstenau, D. W., Advan. Chem. Set. 79, 161 (1968). 7. Dick, S. G., Fuerstenau, D. W., and Healy, T. W., J. Colloid Interface Sci. 37, 595 (1971). 8. Somasundaran, P., and Fuerstenau, D. W., Trans. Soc. Mining Eng. A I M E 252, 275 (1972). 9. Fuerstenau, D. W., and Wakamatsu, T., Faraday Discuss. Chem. Soc. 59, 157 (1976). 10. Rosen, M. J., and Nakamura, Y., J. Phys. Chem. 81, 873 (1977). 11. Trogus, F. J., Schechter, R. S., and Wade, W. H., J. Colloid Interface Sci. 70, 293 (1979).

Journalof Colloidand Interface Science, Vol. 85, No. 2, February 1982