Salt partitioning in Winsor Type II systems

Salt Partitioning in Winsor Type II Systems J. BIAIS,* M. BARTHE,* M. B O U R R E L , t '1 B. CLIN,*

AND

P. LALANNE*

*Centre de Recherche Paul Pascal, Domaine Universitaire, 33405 Talence CFdex, and ~fSNEA (P) - C R L B.P. 34 L A C Q - 64170 Artix, France

Received January 8, 1985;acceptedJune 28, 1985 In systems in which an excess aqueous phase is in equilibrium with a miceltar solution, so-called microemulsion, i.e., Type II in Winsor's nomenclature, a nonuniform partitioning of sodium chloride has been observed. This partitioning has been investigated under a variety of surfactant and salt concentrations with both nonionicand anionic surfactants. In both cases,the salt concentrationin the excess brine phase has been found higher than in the brine contained in the micellar phase. The magnitude of the effectis, however,lowerin the caseof nonionics. With nonionicsurfactants, the results areinterpreted by considering that some salt-free water molecules in the micellar phase are devoted to hydrate the surfactant heads; in the remainingwater of the micellarphase, the salt concentrationis assumed identical to that in the excess aqueous phase. With anionic surfactant, beside the hydration of the polar heads by some salt-free water molecules, the partial surfactant ionization has also to be taken into account through a dissociation constant. For both systems, despite the large variety of experimental conditions, the calculated excess aqueous phase salinities are found to agree well with the experimental resuits.

© 1986 Academic Press, Inc.

1. INTRODUCTION The effect of inorganic salts on the phase behavior of mixtures of water, oil, and ionic surfactant is well known since the pioneering work of Winsor (1) and has more recently attracted a great deal of interest because of the potential application of micellar systems to enhanced oil recovery. In particular a special attention has been devoted to the transition between type I, III, and II systems (in original Winsor's nomenclature) which can generate the low interfacial tensions required for efficient oil recovery. Such a transition can be achieved through a number of ways (1-3), but changing the salt concentration has been especially investigated because of its application in the salinity gradient technique (4, 5). The effect of added salt on the micellar properties of surfactants and the mechanism of its interaction with ionic micelles have been widely investigated (6-14). In the case of oil containing systems in which an excess aqueous To whom all correspondenceshould be addressed.

phase is in equilibrium with an oil-rich micellar solution, so called microemulsion (Winsor type II system), a nonuniform partitioning of the salt has been observed; namely, the concentration and composition of the inorganic electrolytes may be different in the excess brine phase and in the brine contained in the micellar phase (15-17, 26). Such a partitioning yields difficulties in representing and interpreting the phase behavior of surfactant-alcohol-brine and oil mixtures by considering, as it is usually done, the brine as a pseudocomponent. In particular, we have previously proposed a model (so-called pseudophase model) which allows, with the aid of a few thermodynamic partitioning constants, to calculate the composition of the water, oil, and interfacial phases or pseudophases coexisting in equilibrium in micellar systems (18). In several cases, this model has been proved successful to choose appropriately the pseudocomponents able to draw simplified and tractable two-dimensional phase diagrams (19) from three-dimensional experimental phase

576 0021-9797/86 $3.00 Copyright © 1986 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 109, No. 2, February 1986

577

SALT PARTITIONING IN MICELLAR SYSTEMS

uene by Merck, and sodium chloride by Prolabo. All these chemicals are of>99.5% purity. A number of mixtures have been investigated; all of them are Type II in Winsor's nomenclature, i.e., a microemulsion is in equilibrium with an excess aqueous phase. For the NOP case, two systems, the weight composition of which is given in Table I, have been investigated at three salinities Sw: 10, 20, and 40 g/liter. All of them have the same water-oil ratio (1 in weight) and the same surfactant-alcohol ratio (1.5 in weight). For the SBT system, three salinities have been considered: 20, 40, and 60 g/liter NaC1. The volume compositions of the mixtures investigated are given in the four first columns of Table II, where VA, Vo, Vs, and Vw refer to the volume fractions of alcohol, oil, surfacrant, and water respectively. The other col2. MATERIALS AND EXPERIMENTAL umns deal with the phase compositions and PROCEDURES will be discussed later. All the mixtures have Two systems have been investigated: one the same surfactant-alcohol ratio (0.5 in contains a nonionic surfactant (ethoxylated weight). They differ by the surfactant + alcohol nonyl phenol with eight ethylene oxide units, concentration or by the water-oil ratio. Mixtures were sealed in graduated glass pinoted ENP8), n-pentanol, octane, water, and pets, shaken several times, and then stored 2 sodium chloride; the other contains an anionic days at 21 _ 0.2°C. Phase volumes were resurfactant (sodium dodecyl sulfate, noted corded and are given in Tables I and II: p SDS), n-butanol, toluene, water, and sodium chloride. Throughout this paper, these systems = V w ~ , / V w , , • : V M I c / V w r,, where V~,, Vw,, will be referred to as NOP and SBT, respec- and V~c are, respectively, the volume of the excess aqueous phase W", the volume of the tively. ENP8 exhibits the usual distribution of eth- water W' solubilized in the microemulsion ylene oxide units.SDS (Touzart et Matignon) phase (see Fig. 4) and the volume of the miis 99.9% pure. N-pentanot and n-butanol were croemulsion. The excess aqueous phase was supplied by May and Baker, octane and tol- then sampled and analyzed for the C1- condiagrams exhibiting quite complex topologies (20). Such simplified phase diagrams, which turned out to exhibit the overall features of ideal Winsor's diagram, have recently proven convenient to incorporate the phase behavior information into numerical simulators for enhanced oil recovery applications (21). The pseudophase model, however, has so far considered the brine as a pseudocomponent, an approximation not always justified which has led to some difficulties. It is the purpose of this paper to investigate the partitioning of salt between the phases of Winsor type II systems, in the presence of anionic or nonionic surfactants, and to propose a simple model allowing us to account quantitatively for the results.

TABLE I NOP System Characteristics Sw (~li~r)

A (wt%)

0 (wt%)

S (wt%)

W (wt%)

p

3

Sw" (g/lit~)

Sw' (g/liar)

a (moleSts)

10 20 40 10 20 40

8 8 8 12 12 12

40 40 40 35 35 35

12 12 12 18 18 18

40 40 40 35 35 35

0.395 0.691 0.827 0.215 0.367 0.633

9.26 6.37 5.86 17.93 11.6 7.73

11.50 22.09 43.10 12.44 24.38 46.54

9.41 18.55 35.86 9.48 18.39 35.86

0.887 1.075 1.16 1.324 1.49 1.78

Journal of Colloid and InterfaceScience, Vol. 109, No. 2, February 1986

BIAIS ET AL.

578

TABLE II SBT System Characteristics Sw (g/liter)

Va (vol%)

Vo (vol%)

Vs (vol%)

Vw (vol%)

20

8.26 16.14 25.86 30.1 18.34 19.84 22.12 25.88

5.86 11.32 18.70 22.8 40.76 39.67 38.01 35.26

2.89 5.62 9.02 10.50 6.40 6.92 7.71 9.02

82.99 66.92 46.42 36.60 34.50 33.57 32.16 29.84

40

30.11 25.93 16.19 8.28 19.93 22.20 25.96 15.37 30.47 20.12 19.

22.79 18.74 11.36 5.81 40.91 39.8 38.13 35.36 43.15 32.05 11.30 45.33

10.50 9.05 5.64 2.88 6.42 6.95 7.74 9.06 5.36 10.62 7.02 6.63

30.39 26.48 16.39 8.41 18.47 20.07 22.19 26.12 15.47 20.35 19.11

22.95 19.13 11.51 5.94 41.24 40.06 38.49 35.58 43.43 11.47 45.56

10.59 9.23 5.71 2.93 6.44 7. 7.74 9.11 5.39 7.09 6.66

18.41

60

~

See" (g/liter)

a (mole/liter)

b (mole/liter)

1.30 1.49 0.526 0.262 0.195 0.23 0.19 0.1

1.13 1.5 5.3 12.10 16.71 14.95 18.42 35.

20.14 23.24 30.21 41.3 28.7 29.9 32.15 37.74

0.32 0.84 1.19 1.46 0.89 1.02 1.15 1.34

0.61 0.575 0.99 1.65 2.10 1.84 2.12 3.70

36.60 46.28 66.81 83.03 34.26 33.32 31.93 29.62 36.12 26.86 61.56 29.04

0.377 1.38 4.07 12.08 1.22 1.06 0.94 0.84 1.34 0.33 2.66 0.89

9.0 2.73 0.86 0.3 4.33 4.93 5.51 6.46 3.82 14.12 1.27 6.36

63.2 53.8 44.6 40.50 50.9 52.8 56.06 62.6 47.5 72.5 46.5 53.9

1.59 1.88 1.73 1.83 1.68 1.73 1.89 2.26 1.39 2.11 1.68 1.74

2.51 1.18 0.85 0.74 1.25 1.34 1.42 1.51 1.20 2.79 0.94 1.45

36.07 45.16 66.39 82.72 33.85 32.87 31.58 29.19 35.71 61.09 28.67

0.76 2.06 6.18 22.7 2.06 1.93 1.68 1.11 2.16 3.08 1.34

5.40 2.28 0.75 0.26 3.41 3.68 4.08 5.56 3.11 1.16 5.08

85.7 75.5 64.6 60.5 72.5 75. 78.6 86.2 69.75 68. 76.2

2.08 2.51 2.49 3.53 2.32 2.55 2.62 2.61 1.90 1.93 2.19

2.37 1.52 1.19 1.07 1.52 1.56 1.64 1.95 1.5 1.36 1.79

centration by p o t e n t i o m e t r y , using the Mettler D L 40 M e m o t i t r a t o r (silver electrode in A gN O3 a q u e o u s solution; reference electrode: Ag/AgC1).

3. RESULTS

3.1. NOP System T h e results o f the salinity Sw,, (g/liter NaC1) o f the excess a q u e o u s phase m e a s u r e d experi m e n t a l l y are g i v en in T a b l e I. T h e y are also plott e d against the initial salinity Sw in Fig. 1. Journal of Colloid and Interface Science, VoL 109, No. 2, February 1986

p

Interestingly, it can be seen that, although the surfactant is n o n i o n i c type, Sin, is higher t h a n Sw, a n d that the difference increases with the a lc oh o l + surfactant c o n c e n t r a t i o n . Sw, is the salinity o f the water solubilized in the micellar phase. It has been d e t e r m i n e d by material balance.

3.2. SBT System T ab l e II gives the salinity Sw,, o f the excess a q u e o u s phase as d e t e r m i n e d experimentally. T h e results are also sh o w n in Figs. 2 a - c corr e s p o n d i n g to the initial salinities Sw o f 20,

579

SALT PARTITIONING IN MICELLAR SYSTEMS

ENP 8

= 1.5

/

n-pENTANOL <

6(

WATER OCTANE'

/ 1

2/;'/~

:

/"

T : 21oc x

<

"0//~ 'r

w

" 2|

i

/

*

ENP 8

+ : 12 % ENP 8

o~//'/

magnitude of the effect is however quite higher than in the former case, as shown by Fig. 3 which provides a comparison of the results obtained for both systems at 40 g/liter NaC1 and water-oil ratio = 1, when the surfactant concentration varies. The scattering of the data in Fig. 2 is not explainable by the accuracy of the measurements. It indicates therefore that the A + S concentration is not the only parameter to rule the salt partitioning, and that the water-oil ratio, embedded in the results presented in Fig. 2, as well as the relative volumes of the phases, are also involved in this phenomenon. 4. THEORETICAL CONSIDERATIONS-DISCUSSION

Sw = SALINITY g/I (INITIAL)

FIG. 1. Salinity of the excess aqueous phase as a function of the initial brine salinity for the NOP system.

4.1. Introduction--Model

40, and 60 g/liter, respectively. S ~ , is plotted against the overall alcohol + surfactant concentration A + S in weight percent. It can be seen again that in all cases Sw,, is higher than Sw and that the difference increases with the salinity and the A + S concentration. The

The structure of the micellar phase in a Winsor II system is generally viewed as droplets of water IV' surrounded by a surfactantalcohol membrane M' in an oil phase, or pseudophase O' containing some alcohol molecules (Fig. 4). It has been shown in the literature (22-24) that the mean activity coefficient 3' of sodium

S w " lexp)

Sw,,(exp)

so

(~ .y/S /

/

o/

®

60 ..,~l~.~.~ . . . . . .

Sw (INITIAL)

T = 21 °C

..z 7~

/

!

40 . . ~ f..~ . . . . . . . . .

O

O/S.D.S.

Sw

(INITIAL)

15

WOR = 1

.J

o/ /

3'0 (A +iS) %

N ~- 60.

/ O

/o/

50.

100! Sw"(exP)

o 80

.

i

/

/ 0

ENP8

.Jr*"

. . . . 4-- o . - *

oao

Sw

/

(INITIAL)

....;"~:°~;"° (INITIAL) sw l~

3'*(A +'st

,/,

FIG. 2. Salinity of the excess aqueous phase as a function of overall alcohol + surfactant concentration for the SBT system. Initial brine salinities: (a) 20 g/liter; (b) 40 g/liter; (c) 60 g/liter.

SURFACTANT

CONCENTRATION (%)

FIG. 3. Comparison of salt partitioning between anionic (SDS) and nonionic (ENP 8) systems. The salinity of the excess aqueous phase is plotted against the overall surfac/ant concentration. Journal of Colloid and Interface Science, Vol. 109, No. 2, February 1986

580

BIAIS ET AL.

choride in water decreases rapidly within the low range salt concentration and then remains approximately constant (3" = 0.69) up to concentrations higher than 150 g/liter. Since we are dealing with salinities within the range 1560 g/liter, it will be a good approximation to take 3' constant. A phenomenological approach could consist in considering: (i) The activity coefficient of salt in the W' pseudophase is different from the one of the salt in the excess aqueous phase W". (ii) The sodium associates to the micelles when anionic surfactants are used. Alternatively, the salt partitioning may be discussed in a more physical manner by considering that: (i) The salt activity coefficient is the same in the W' pseudophase and W" phase. (ii) The hydration of surfactant heads involves some water molecules which are likely salt-free. (iii) The dissociation of surfactant (when anionic) may be treated within a quasichemical approximation. Both approaches have been investigated. In what follows, we only report the latter which yields the best results and which is furthermore in line with the pseudophase model (18). 4.1.1. Hypothetical initial state and notations. To calculate the NaC1 concentration in W", it is convenient to consider first an hypothetical initial state described by Fig. 4 in

O'

/

OIL + ALCOHOL PSEUDOPHASEO"

[ = MICELLAR PHASE IMICROEMULSlON}

(~/~ ~t~'~ ~ ~

L~SURFACTANT + ALCOHOL I = PSEUDOPHASEM'

~_+

EXCESS AQUEOUS PHASE

W"

WATER+ SALT ALCOHOL = PSEUDOPHASEW' (Vw') WATER ~ SALT(Sw'} + ALCOHOL PHASEW" (Vw"}

FIG. 4. Model of a Type II system with salt partitioning-definition of the pseudophases W', 0', M'. When surfactant hydration is taken into account, W' and M ' become, respectively, IV* and M*. Journal of Colloid and Interface Science, Vol. 109, No. 2, February 1986

which all the surfactant (located in the micellar phase) is nondissociated (its concentration is a in mole/liter of W'), and all the salt is entirely contained in W" (its concentration is b in mole/liter of W"). This is summarized in Table III together with the concentrations of the various species in phases W' and W" at equilibrium. p is the ratio Vv/,/Vv¢ of the volumes of IV" and W' (Fig. 4) as measured experimentally, x is the amount of NaC1 (in mole/ liter of W n) which diffuses from IV" towards W r.

4.1.2. Hydration of polar heads. The pseudophase W' in Fig. 4 is divided into two subvolumes Vn and Vw, = V ~ - Vn, which correspond, respectively, to the salt-free hydration water (bound water) and to the free water. The surfactant pseudophase M' in Fig. 4 can be viewed now as incorporating some water molecules and therefore will be noted M*. The water pseudophase W' in Fig. 4 is actually reduced to the volume Vw.; it will be noted W* and has the same properties as W". In particular, the activity coefficient 3"* of the salt in W* follows the same trend as 3'" in IV", that is, 3"* is constant over a wide range of salinity, as stated above. Thus: 3"* = 3," = 0.69.

[1]

If ns is the number of moles of surfactant in the micellar phase, v the number of moles of water hydrating one surfactant head, and w the water molar volume, then VH can be written Vii = nsvo~. [21 4.1.3. Salt partitioning. By equating the chemical potential of the salt in the W* and W" phases, one can write for the partition constant since the standard chemical potentials are identical:

[Na+]"[C1-]" Kp = 1

[Na+],[CI_],

[3]

where [X]" and [X]* represent the concentration of the ion X in W" and W*, respectively. 4.1.4. Surfactant ionization. It is recognized that direct micelles (aqueous solutions) can associate a fraction of the surfactant counter-

SALT PARTITIONING IN MICELLAR SYSTEMS

581

TABLE Ill Concentrations in W' and W" Phases Phase

IV'

W II

Volume Species Initial state

Vw'

vw"

Equilib. state

RNa a

R0

Na ÷ 0

CI0

[RNa]

[R-]

[Na+]'

ox

ions, that is to say the surfactant is not completely dissociated (6-14). The remaining part of the surfactant counterions is, however, free and distributed within the aqueous phase W*. In the case at hand, the fraction of free sodium ions brought by the surfactant will have to be taken into account to calculate the concentration of the salt in the brine W* constituting the core of the inverted micelles. In other words, the surfactant is seen here as only partly dissociated, which differs from Adamson's theory (15, 16). Adamson considers that: The surfactant is completely dissociated. The mean activity coefficient of salt is not the same in the micellar aqueous phase and in the excess brine. On the other hand, in his theory, the electrolyte concentration difference is interpreted as due to the osmotic phenomenon, the osmotic pressure being balanced by Laplace forces in case of curved interface. Since it is now recognized that the interfacial tension (at the drop interface) is very low, in the order of 1 0 -2 m N / m or less, the pressure difference between the drops and the excess brine will be correlatively sufficiently low in order not to play any important role in salt partitioning. Concerning the mechanisms ruling the dissociation equilibrium, it is indeed recognized that the long-range electrostatic forces play a major role (6-14). However, for the sake of simplicity, we use in this paper the quasichemical approximation. The surfactant ionization is thus described by: RNa ~- R - + Na + where RNa and R - are, respectively, the associated and dissociated surfactant species. We

Na ÷

CI-

b b - x = [Na+]"

b b - x = [C1-]"

have seen above that the mean activity coefficient of salt is a constant ('t* = 0.69); for the sake of simplicity, we will assume that the ratio of the activity coefficients of R - and RNa is also constant. The ionization constant ki can then be defined by ki =

[R-]*[Na]* [RNa]*

[4]

It has to be pointed out here that [R-]* and [RNa]* are surface concentrations since they refer to the interfacial pseudophase M*. However, since Eq. [4] deals with their ratio, we are allowed to express them, for convenience, in mole per liter of the pseudophase M*. The surfactant adsorbed at the interface between the micellar phase and the excess aqueous phase IV" (Fig. 4) is neglected. In the general case at equilibrium, the concentrations must satisfy Eqs. [3] and [4], as well as the material balance and electroneutrality conditions [5] and [6] below: a* = [RNa]* + [R-]* [Na+] * = [R-I* + p*x

[5] [6]

where a* is the surfactant concentration in mole/liter of W*. p* is the ratio V w ~ , / V w . . a* and O* can be expressed as functions of a and p which are the quantities actually measurable (see Section 4.1.1.). Using Eq. [2] one can show that a * = ~ a 1 -

a~,o~

,

o*=~P. 1 -

[7] ap¢o

The set of Eqs. [3]-[6] finally leads to Ax 4 + Bx 3 + Cx 2 + Dx + E = 0

[8]

Journal of Colloid and Interface Science, Vol. 109,No. 2, February 1986

582

BIAIS ET AL.

where A = 1 - (/0*)2 B = 2b[(p*) 2 - 2]

ENP8 n-PE-~'~NOL -/q[(p.)2

_

1]

C = b2(6 - (o*) 2) -/q-p*(p*a * + 2b) D = bZk~o* - 4b 3

i

/

15

6(

WATER OCTANE' = 1

50

T = 21 °C

/

/

E = b 4.

The above coefficients (through the relations [7]) involve p, a, b, ki, and v. a and b are calculated from overall compositions and experimental results Vw, and Vw,, (see Tables I-III). Values of/q and v are arbitrarily chosen to fit the experimental salinities of the excess aqueous phase.


f

o : 18% ENP8

/

+: 12%ENP8

O

10 / ' ~ ,;

io

io

+;~

,4

do

Sw" SALINITY g/I (EXPERIMENTAL)

4.2. S t u d y o f N O P S y s t e m

FIG. 5. Calculated vs experimental excess aqueous phase

For this system the set of equations is reduced in Eq. [3] (and relation [2]). The salinity Sw,, is simply given by Sw,, = S w

Vw Vw-

VI~

= Sw

Vw Vw-

nswo

[9]

where Vw is the total volume of water in the system. From Table I, Sw+ can be calculated according to l+p S~, = Sw 1 + p - aw "

[10]

Figure 5 shows the comparison of experimental values of Sw,, with those calculated from Eq. [9] using u = 8. This corresponds to a number h of water molecules per ethylene oxide unit of 1, a realistic number, in agreement with those reported in the literature from N M R measurements (31), although it is recognized that some compensation may occur: a fraction of the nonionic surfactant is dissolved into the O' pseudophase (25), and a fraction of the alcohol is present at the interface. Both effects have been neglected. It has to be pointed out, however, that this simple approach allows us to calculate the salinity of the excess aqueous phase without requiring the knowledge of its volume. For the nonionic system, Sv~, is identical to the salinity of the water pseudophase W*. Journal of Colloid and Interface Science,

Vol. 109,No. 2, Febraary 1986

salinity for the NOP system at two surfactant concentrations. "Hydration" model, assumingone moleculeof salt free water bound to each ethyleneoxide group.

4.3. S t u d y o f S D S S y s t e m

This system was studied with the aid of Eq. [8]. A number of combinations have been tried to test the influence of the two parameters/q and u on the value calculated for Sw,,. It is found that for/q = 0 it is not possible to fit the results with only the hydration parameter v. For v = 0 and ki = 50, the correlation between calculated and experimental values of the excess phase salinity appears satisfactory (see Fig. 6). However, in this case, the ionization degree ri of the surfactant, defined by ri -

[R-] a

[11]

is almost independent of the salinity and is close to 1. This is unlikely since it has been shown in the literature that, for aqueous solutions of surfactants above the CMC, the ionization degree varies with the salinity and is in the order of 0.4 to 0.8, depending on the alcohol concentration (27-30). A more realistic picture is thus given by intermediate values of ki and v.

583

SALT PARTITIONING IN MICELLAR SYSTEMS

100 T = 21 °C

~

~

I

< <

~- 6c z .,,i

20 40 50 80 Sw,, SALINITYg/I (EXPERIMENTAL)

100

FIG. 6. Calculated vs experimental excess aqueous phase salinity for the SBT system, ki = 50; p = 0.

Figure 7 shows the comparison of experimentally measured salinities of the excess aqueous phase W" with salinities calculated from the model, taking ka = 0.9 and ~ = 6 (6 water molecules per surfactant head). The agreement is excellent over the entire range of

10(

compositions investigated. The value ~ = 6 must be viewed as an average since alcohol is also present at the interface. Its concentration has been calculated with the aid of the pseudophase model and the alcohol/surfactant molecular ratio has been found close to 2 for all the systems investigated. The ionization degree calculated for/q = 0.9 and ~ = 6 is shown in Fig. 8. It displays the same trend as expected from literature data (27-30). Beside these results, it must be pointed out that the model offers the advantage to be in line with the pseudophase model: the calculation of the salt partitioning is possible based on a simple change of the definition of the M ' pseudophase which has to include an appropriate amount of salt-free water (the dissociation of the surfactant, when ionic, has also to be taken into account). The pseudophase model can then be utilized without further modification. 5. IONIZATION AND SOLUBILIZATION

As recalled in the introduction, an increase in the salt concentration produces a transition from Type I system (in Winsor's nomencla-

/

SDS = O.S n-BUTANOL

/

SDS =0.5 n-BUTANOL OIL : TOLUENE T = 21 *C

OIL: TOLUENE T = 21 °C <

i,o

1 : 3 / 1 : 3

k I = 0.9

[]

2O

o

k i = 0.9 V=S

V=6

2o

'

~'O

6'o

s'o

1~o

Sw" SALINITYg/I (EXPERIMENTAL) FIG. 7. Calculated vs experimental excess aqueous phase salinity for the SBT system. "Hydration" model, ~ = 6 molecules of salt-free water are assumed to be bound to each surfactant head./q- = 0.9.

20 40 60 80 Sw" SALINITYg/I (EXPERIMENTAL)

100

FIG. 8. Surfactant ionization degree as a function of excess aqueous phase salinity for the values of v and ki used in Fig. 7. Journal of Colloid and Interface Science, Vol.

109, N o . 2, F e b r u a r y 1986

584

BIAIS ET AL.

ture) to Type III, then to Type II. According to Winsor (1), this is due to the decrease in the interaction energy Acw of the surfactant with water while the interaction with oil Aco remains constant. Winsor defined the ratio R

SOS

+

n-BUTANOL

.= e~

OIL =

TOLUENE

T = 21 ° C

i

as

Aco g = -Acw"

[12]

Types I, III, and II correspond therefore to R < 1, R ~ 1, and R > l, respectively. Winsor also originally suggested that the cosolvent power of an amphiphile for oil and water is enhanced when Aco is close to Acw and when both interaction terms are simultaneously high. This concept has been proved very useful for optimizing the microemulsion formulation (32, 33). In the case at hand, dealing with Type II systems, R is > 1. Since the oil nature and the surfactant tail are fixed, Aco is constant; changing the salinity entails a change in Acw by modifying the tendency of the polar groups of the surfactant to disperse in water. More precisely, the interaction Acw is likely related to the charge density of the interface, that is to say to the ionization degree. An increase in the ionization degree will tend therefore to increase Acw and to make it closer to Aco. The solubilization of water in the micellar phase is thus expected to rise. Figure 9 shows the variation of 1/a with the ionization degree for all the systems investigated. 1/a represents the volume of water (of pseudophase IV') solubilized per mole of surfactant, since a is the concentration of surfactant per liter ofpseudophase IV'. A strong correlation between 1/a and ri is observed, whatever the composition of the system. As expected, the solubilization increases with the ionization degree. It must be emphasized that T i has been calculated in Fig. 9 using a constant hydration number (v = 6) whatever the salinity. This assumption should likely be revisited if the surfactant dissociation were very small, that is to say at very high salinities or/and in the presence of divalent ions which are known to asJournal of Colloid and Interface Science, Vol. 109, No. 2, February 1986

E ._=

.J

Ii

4-

at .1.4`.I

o.'a

'

0:4

ol, oi. o'7 IONIZATION DEGREE

FIG. 9. Correlation between the amount of water solubilized in the micellar phase of SBT systems and the surfactant ionization degreeas Calculatedin Fig. 8.

sociate strongly to the micelles (14). In the case at hand, the lowest ionization degree attained is 0.3, a value still compatible with the assumption v = 6. 6. CONCLUS~-)NS In Winsor Type II systems, involving anionic or nonionic surfactants, sodium chloride has been found to partition preferentially into the excess aqueous phase in equilibrium with the oil-rich microemulsion phase. The magnitude of the effect is however lower in the case of nonionic surfactant. The salt partitioning has been calculated using a model which considers that some saltfree water molecules in the micellar phase are devoted to hydrate the surfactant heads; in the remaining water of the micellar phase, the activity coefficient of the salt is assumed identical to that in the excess aqueous phase. For sodium chloride, its value is found in the literature constant over a wide range of concentrations. The partial dissociation of the surfactant, whenever ionic, is taken into account within a quasichemical approximation.

SALT PARTITIONING IN MICELLAR SYSTEMS

585

The model yields a good agreement with 13. Jonsson, B., and Wennerstrom, H.,J. Colloid Interface Sci. 80, 482 (1981). the experimental results. Furthermore: 14. Hirasaki, G. J., and Lawson, J. B., SPE Paper 10921, It gives some insight into the mechanisms presented at the 57th Annual Technical Conference responsible for the nonuniform partitioning and Exhibition of SPE, New Orleans, Louisiana, of the salt; for nonionic surfactants, especially, Sept. 1982. the salt partitioning can be predicted simply 15. Adamson, A. W., J. Colloid Interface Sci. 29, 261 (1969). from the polar head hydration without know16. Tosch, W. C., Jones, S. C., and Adamson, A. W., J. ing the volume of the excess aqueous phase. Colloid Interface Sci. 31,297 (1969). It is consistent with the pseudophase model 17. Glover, G. J., Puerto, M. C., Maerker, J. M., and which needs only a slight change in the defiSandvik, E. L., Soc. Pet. Eng. J. 3, t83 (1979). nition of the interfacial pseudophase and can 18. Biais, J., Bothorel, P., Clin, B., and Lalanne, P., J. Disp. Sci. Technol. 1, 67, (1981). then be utilized in the usual way. 19. Biais, J., Barthe, M., Clin, B., and Lalanne, P., J. ColFinally, a correlation has been observed beloid Interface Sci. 102, 361 (1984). tween the calculated ionization degree of the 20. Bellocq, A. M., Biais, J., Clin, B., Gelot, A., Lalanne, interfacial pseudophase and the solubilization P., and Lemanceau, B., J. Colloid Interface Sci. 74, 311 (t980). of water into the microemulsion phase. This has to be confirmed, however, on other sys- 21. Prouvost, L. P., Satoh, T., Sepehrnoori, K., and Pope, G. A., SPE Paper 13031 presented at the 59th Antems, especially containing divalent cations. nual Technical Conference and Exhibition of SPE, Houston, Texas, Sept. 1984. 22. KieUand, F., J. Amer. Chem. Soc, 59, 1675 (1937). The authors wish to thank the management of Elf- 23. "Handbook of Chemistry and Physics," 48th Ed., p. Aquitaine for permission to publish this work. One of the D93. CRC Press, Boca Raton, Fla., 1967-1968. authors (M. Barthe) gratefully acknowledges Elf-Aquitaine 24. Harned, H. S., and Owen, B. B., "The Physical for financial support. Chemistry of Electrolytic Solutions," p. 557. Reinhold, New York, 1950. REFERENCES 25. Graciaa, A., Lachaise, J., Sayous, J. G., Grenier, P., 1. Winsor, P. A., Trans. Faraday Soc. 46, 762 (1950). Schechter, R. S., and Wade, W. H., J. Colloid In2. Winsor, P. A., "Solvent Properties of Amphiphilic terface Sci. 93, 474 (1983). Compounds." Butterworths, London, 1954. 26. Salter, S. J., SPE paper 12036 presented at the 58th 3. Salager, J. L., Vasquez, E., Morgan, J. C., Schechter, Annual Technical Conference and Exhibition o f R. S., and Wade, W. H., Soc. Pet. Eng. J. 19, 107 SPE, San Francisco, California, Oct. 1983. (1979). 27. Phillips, J. N., Trans. Faraday Soc. 51, 561 (1955). 4. Pope, G. A., and Nelson, R. C., Soc. Pet. Eng. J. 5, 28. Lindman, B., and Wennerstrom, H. in "Solution Be339 (1978). havior of Surfactants" (K. L. Mittal, Ed.), Vol. 1, 5. Nelson, R. C., and Pope, G. A., Soc. Pet. Eng. J. 5, p. 10. Plenum, New York, 1982. 325 (1978). 29. Zana, R., Yiv, S., Strazielle, C., and Lianos, P., J. 6. Shinoda, K., in "Colloidal Surfactants" (K. Shinoda, Colloid Interface Sci. 80, 208 (1981). B. Tamamushi, T. Nakagawa, and T. Isemura, 30. Jain, A. K., and Singh, R. P. B., J. Colloid Interface Eds.). Academic Press, New York, 1963. Sci. 81, 536 (1981), 7. Stigter, D., J. Colloid Interface Sci. 47, 473 (1974). 31. Rendall, K., and Tiddy, G. J. T. J. Chem. Soc. Faraday 8. Stigter, D., J. Phys. Chem. 78, 2480 (1974). Trans. 180, 3339 (1984). 9. Stigter, D., J. Phys. Chem. 79, 1008 (1975). 32. Bourrel, M., and Chambu, C., Soc. Pet. Eng. J. 2, 327 10. Stigter, D., J. Phys. Chem. 79, 1015 (1975). (1983). 11. Gunnarsson, G., Jonsson, B., and Wennerstrom, H., 33. Bourrel, M., Verzaro, F., and Chambu, C., SPE paper J. Phys. Chem. 84, 3114 (1980). 12674 presented at the 4th SPE/DOE Symposium 12. Lindman, B., and Wennerstrom, H., Top. Curr. Chem. on Enhanced Oil Recovery, Tulsa, Oklahoma, April 1984. 87, 1 (t980). ACKNOWLEDGMENTS

Journal of Colloid and Interface Science, Vol. 109, No. 2, February 1986