alkyltrimethylammonium bromide

Colloids and Surfaces A: Physicochemical and Engineering Aspects, 76 (1993) 251-265 Elsevier Science Publishers B.V., Amsterdam

257

Fluorescence-probe study of anionic/cationic surfactant systems 2. Alkylsulfonate/alkyltrimethylammonium bromide* Gan-Zuo

Li”, Fang Li”, Li-Qiang

Zhengb and Hong-Li

Wangb

“Shandong University, Jinan 250100, People’s Republic of China bInstitute of Chemical Physics, Chinese Academy of Sciences, Lanzhou

730000, People’s Republic of China

(Received 2 October 1992; accepted 8 April 1993) Abstract Steady state fluorescence and time-resolved fluorescence techniques have been used to study the properties of two kinds of anionic surfactant systems and four kinds of mixed anionic/cationic surfactant systems. The two anionic surfactant systems are sodium dodecyl sulfate and sodium dodecylsulfonate (C,,As) aqueous solutions. The four alkylsulfonate/alkyltrimethylammonium bromide systems consist of C,,As/cetyltrimethylammonium bromide (CTAB), sodium octylsulfonate (C,As)/CTAB, sodium hexadecylsulfonate (C,,As)/CTAB and C,,As/dodecyItrimethylammonium bromide (DTAB). We discuss mainly the variation of the micellar aggregation number of these systems with the NaCl concentration and temperature. Both the addition of anionic surfactant to cationic surfactant and of cationic surfactant to anionic surfactant result in a large increase in the micellar aggregation number. In addition, by a steady state fluorescence technique, the critical micelle concentration (CMC) for the &A&TAB and &As/DTAB systems and the second CMC for the C,,As, Cr2As/CTAB and C,,As/DTAB systems at which the micelles changed from spherical to rod shaped were determined. At the same time, the solubilization position of the fluorescence probe (pyrene) and the polarity of the probe microenvironment in the mixed anionic/cationic micelles were studied. Keywords: Anionic/cationic

surfactant

systems; fluorescence

techniques

Introduction

Mixed cationic/anionic surfactant micelles are of considerable interest from both practical and fundamental viewpoints. In general, the surface activity of the mixed systems is much greater than those of the pure surfactant solutions [l-4]. It is very important to know the aggregation number N which determines to a large extent the micelle properties. However, from a purely fundamental viewpoint the study of the aggregation number of Correspondence to: G.Z. Li, Shandong University, Jinan 250100, People’s Republic of China. * For Part 1, see G.Z. Li, F. Li, L.Q. Zheng and F.I. Bai, Proc. Int. Semin. Surfactants and Detergents, Shanghai, China, 1992, p. 439. 0927-7757/93/$06.00

0

1993 -

Elsevier Science Publishers

mixed systems can help us to understand better the process of micellization. So far, many studies of mixed anioniclcationic systems have focused mainly on the composition, the degree of ionization and the CMC values of the combined systems [5,6]. There were no reports on the aggregation number of the mixed micelles until Zana and co-workers first studied the aggregation number of mixed micelles of dodecyltrimethylammonium (DTAB) sodium dodecyl sulfate (SDS) [7] in 1986. In 1989 and 1990, Kato and co-workers [8,9] published two papers on the number of the study of the aggregation SDS/octyltrimethylammonium bromide (OTAB) mixed micelle. To our knowledge, there have been no reports on mixed systems in which alkylsulfoB.V. All rights reserved.

G.Z.

258

Li et al./Colloids

nate is used as an anionic surfactant. Chinese oil belongs to the paraffin wax type; therefore alkylsulfonate is of great importance for enhancing oil recovery. Fluorescence ties of surfactant have [lo-l

methods

in the study of the proper-

systems are new techniques

which

been used increasingly in recent years 33. Fluorescence techniques include steady

state fluorescence

for the study of the variation

of

Surfaces

A: Physicochem.

Eng. Aspects

and to a good approximation CMC. According ln[Z’(t)/l’(o)] From

76 (1993)

257-265

can be taken as the

to Eqn (1)

= N([St] - CMC)/[Q]

a plot of ln[l’(o)/l’(t)]

against

(3) [Q],

N can

be easily calculated from the slope of the straight line. Equation (3) is the steady state fluorescence quenching equation which is used to determine N. When

the

micelle

is large,

the quenching

or

the micellar aggregation number N. In 1978, Turro and Yekta [I41 proposed this method on the basis

excimer formation might be controlled by diffusion within the micelle. This effect results in limiting

of fluorescence quenching to determine the value of N of SDS micelles. The following interparticle reactions must be considered. (i) Excitation decay: M, @ M,_ 1*P* where M, and M, 8 M,_ 1-P* refer to micelles containing P probes in the ground state, and one excited probe and P - 1 probes in the ground state respectively. (ii) Excimer formation: M, 1 - P* + M, 2 - Pz where M,_, * Pz refers to a micelle containing one excimer and P-2 probes in the ground state. (iii) Excimer decay: M,- 2 - PT -+ M, (iv) Quenching: M, * P* + M 1_ 4+ heat where M; P* refers to a micelle containing one excited probe and q quenchers, and M, ~4 refers to a micelle containing one ground state probe and q quenchers. In addition, there are still other pathways such as the intermicellar exchange of fluorophotons, but this process is slow compared with the decay of the excited probes. If some assumptions [15] hold, N can be determined by measuring the total intensity of fluorescence with and without quencher, I’(t) and I’(o) respectively. The following relationship exists:

the rate and thus the extent

I(t) = Go) exp( -

CQIICMI)

(1)

where [Q] is the concentration of quencher and [M] refers to the micelle concentration, which is related to N by

CM1= (CW - CSflYN

(2)

where [St] is the total concentration of surfactant, [Sf] is the concentration of monomer surfactant,

of excimer

formation

and subsequently in underestimating the surfactant aggregation number, so that Eqn (3) cannot be used. In other words, accurate N values cannot be obtained when the concentration of added salt is very large or the surfactants are not ionic [16]. In practice, precise time-resolved measurements using a single-photon counting fluorometer are required. The following conditions must also be fulfilled. The rate of quenching in a micelle containing II quencher molecules is n times larger than in a micelle with just one quencher. With this assumption, the expression for the fluorescence decay is given by I’(t) = I’(O)expf - t/z, + ii[ exp( - k,t) - I]}

(4)

where z, is the unquenched lifetime, or the lifetime of the excited monomeric fluorophoton in the absence of quencher, I’(t) and I’(0) are fluorescence intensities at time t and t =0 respectively, ti = [Q]/[M] is the mean number of quencher molecules per micelle, and k, is the first-order rate constant for fluorescence quenching in a micelle with one quencher, or the formation constant of the excimer when no quencher is added. Experimental Materials The following surfactants were used: sodium dodecylsulfonate (C,,As) (originally chemically pure) was recrystallized twice from ethanol (A.R.);

G.Z. Li et al.lColloids Surfaces A: Physicochem. Eng. Aspects 76 (19931 257-265

cetyltrimethylammonium nally analytically

bromide

(CTAB) (origi-

pure) was also recrystallized

from a mixed ethanol(A.R.)-acetone(A.R.) (CsAs), (C,,As)

CTAB and sodium were all purchased

twice

solution;

hexadecylsulfonate from Tokyo Kasei

259

9-MeA, we obtained a straight line with a linear relationship between ln CWMt)l and CQI. According to Eqn. (3), N can be easily obtained from the slope of this line (Fig. 1). Varying the content

of added NaCl in C,,As

Kogyo Company (gold label) and were used without further purification. There was no minimum in the surface tension vs concentration plot for any

the variation of N (Table 1) is obtained.

of these surfactants.

the surface tension

The fluorescence probes, which were pyrene and Ru(bpy): + > and the quencher 9-methylanthracene (9-MeA) (gold label, from Sigma) were also used without further purification. Instruments The instruments used were a Hitachi MRES-4 fluorometer and a Horiba NAES-1100 nanosecond fluorometer. Preparation

of the solution

The accurate amount of pyrene or 9-MeA was solvated in methanol. The non-aqueous solution of pyrene or 9-MeA was transferred to a dry flask and the solvent evaporated in a stream of nitrogen. The pyrene or 9-MeA was obtained as a thin layer distributed over the bottom of the flask. Subsequently, the surfactant solution was added and the solubilization of pyrene or 9-MeA occurred on stirring for several hours. We then obtained a transparent micellar solution. All samples were deoxygenated with nitrogen for 30 mins.

aqueous

solution,

vs the salt concentration Here the CMC of C,,As

micelles is 8.25 - 10m3 M, which was determined

by

method.

Time-resolved fluorescence

measurements

Pyrene was used as the probe and the fluorescence decay curve was obtained according to the self-quenching of pyrene (no quencher was added). At the same surfactant concentration, two samples with different pyrene concentrations (lo-’ and 10 - 3 M) were prepared. At the original surfactant concentration and the very low probe concentration (i.e. lo-’ M), the mean number of pyrene molecules per micelle would be much less than 1, so that one pyrene molecule would not coexist with another one in the same micelle. Therefore the excimer could not be formed, and the fluorescence decay in the system was due only to the excited monomeric pyrene. The fluorescence decay followed

the equation

I(t) = Z(0) exp( - t/T,)

or

ln[l(t)/Z(O)]

= - t/z0 (5)

When

the molar

concentration

of pyrene

Ol

I

I

I

2

3

4

was

Results and discussion Determination

of micellar aggregation

numbers

Steady statejkorescence measurement was used as the fluorescence probe Ww): + and 9-MeA as the fluorescence quencher. The excitation of the probe was performed at 467 nm and the emission monitored at 620 nm. The concentration of the surfactant was constant (C= 0.075 M). The concentration of the probe was 7. IO- 5 M. On changing the concentration of

[Ql

x104 (mol

Fig. 1. Variation of In[I(o)/I(t)] in C,,As systems (4O.O”C).

I

5

I-‘)

vs quencher

concentration

[Q]

G.Z. Li et al./Colloids

260 TABLE

Surfaces A: Physicochem.

Eng. Aspects

76 (1993) 257-265

I

Aggregation

number

of C,,As

Concentration

Method

Steady state Time-resolved

at 4O.O”C

fluorescence

of NaCl (M)

0.0

0. I

0.2

54 50

59 58

63

-

0.3

0.4

0.5

0.6

0.8

1.0

70 66

78

86 84

97

147

186

10m2 M, two or more pyrene molecules could coexist in one micelle, so that the excimer of pyrene could be formed by the collision between one excited monomeric pyrene molecule and a ground state pyrene molecule. As a result, the fluorescence intensity 1(t) was related to time t by Eqn. (4). When the time is long enough, the second term (excimer part) in Eqn (4) reduces to 12,so Eqn (4) becomes In [l(t)/I(O)]

= - t/z, - ii

(6)

Equations (5) and (6) have the same part, -t/r,, so ti can be worked out by the parallel difference between the two straight lines, based on the relationship between ln[l(t)/Z(O)] and t. To test the validity of this method, we first measured the aggregation number of SDS micelles at C NaC,=O and 0.3 M. At 3O.O”C, the values of N at C Nacl= 0 and 0.3 M were 56 and 106 respectively, in good accord with results from the literature [16]. Because of the large size of the micelles, accurate N values of the ionic surfactant cannot be obtained by the steady state fluorescence method, We measured the N value of C,,As micelles at different NaCl concentrations by time-resolved fluorescence at 40°C (Fig. 2). The results (Fig. 3) are compared with those obtained by the steady state method in Table 1. The lifetime t, of the excited monomeric pyrene was 332 ns in C,,As micelles in the absence of NaCl, which was almost the same as in SDS

c171. The results showed that the N values of C,,As micelles increased with increasing NaCl concentration. In the presence of NaCl, the electric double layer of the micelles was compressed by the increase

I

I

I

I

I

0

20

40

60

00

Time

I 100

(ns)

Fig. 2. The time-resolved fluorescence spectrum of the 0.075 M C,,As system (in the absence of NaCI) at 4O.O”C.

0,2

0.4

0.6

C (mol Fig. 3. The relationship (4O.O”C).

0.8

1.0

I”)

between

N and C,,,,

in C,,As

system

in ion strength and the repulsion between the ionic heads of two surfactant molecules was decreased so that more surfactant molecules could participate in micelle formation. Considering that these mixed anionic/cationic micelles may have a very large value of N, we

G.Z. Li et al./Colloids

determined

Surfaces A: Physicochem.

N only

Eng. Aspects

by the time-resolved

cence technique for C,,As/CTAB, C,6As/CTAB mixed systems.

fluores-

C,As/CTAB

261

76 (1993) 257-265

and

s

1000

kl

r +

900

n

E

The measurements were performed at a fairly high and constant total surfactant concentration

2

800

t

\

(C = 0.01 M) in order that the CMC of the mixtures could be neglected in the calculation of N. With all three systems, the measurements were performed only in the CTAB molar fraction (x) range from 0 to 0.3 and from 0.7 to 1, because tion

occurred

in the range

precipita-

0.3 < ~~0.7

in these

0.01 M (total concentration) combined systems. The results for these combined systems are shown in Figs 446. In all three systems, N varies with x in the same way. Taking C,2As/CTAB as an example, we can see that both the addition of CIZAs to CTAB and of CTAB to C,,As causes an obvious increase in N. The value of N becomes larger and larger as x approaches 0.5. Clearly these changes are associated with the neutralization of part of the micelle charge by added surfactant of opposite charge, and as a result the surfactants tend to arrange closely with each other in micelles. At the same value of x, the same total surfactant concentration and the same temperature, the following relationship exists: N(C,,As/CTAB) > the Thus N(Ci2As/CTAB) > N(CsAs/CTAB). charge

2

1200

neutralization

does not appear

CTAB

fraction

Fig. 5. The aggregation number of As/CTAB mixed micelles at different temperatures (50°C in the absence of NaCI): + ,5O"C; 0, 65°C; 0, 80°C.

e

1000

.-5 %

800

F g m

600

b aI

400

g

2001 1.0

I 0.9

I 0.8

to be the

I 0.7

CTAB

’ ’

molar

I 0.3

0.2

0.1

I 0

fraction

Fig. 6. The aggregation number N of As/CTAB mixed micelles (50°C) at different concentrations of NaCI: 0, 0.1 M NaCI; 0, 0.3 M NaCI; A, 0.5 M NaCI; + , 0.8 M NaCI.

r

only

I.0

molar

0.9

0.8

+

0.7

CTAB

molar

I

I

0.3

0.2

I

0.1

I 0

fraction

(l ), number of As/CTAB aggregation Fig. 4. The C,,As/CTAB ( + ) and C,As/CTAB (A) mixed micelles at different molar ratios of CTAB (x) (50°C in the absence of NaCI).

factor

which

determines

the changes

in N

upon mixed micelle formation. During the formation of mixed anionic/cationic micelles, the interattraction between the anionic and cationic surfactant is caused not only by the opposite charges but also by the hydrophobic hydrocarbon chains. There are the same polar heads (-SO:and -N+(CH,),) in all the three systems. The only difference among them is the length of the sulfonate’s hydrophobic chain. The sum of the lengths of the anionic and cationic surfactants in Ci6As/CTAB micelles is the largest among the three types of system; moreover, the difference in

G.Z. Li et al./Colloids Surfaces A: Physicochem. Eng. Aspects 76 (1993) 257-265

262

length

of the hydrophobic

chains

between

cationic

The intensity I of these peaks is closely related to the polarity of the solvent. The intensities of the

and anionic surfactants is the smallest. Therefore the strongest “hydrophobic interaction” in the

first (I) and third (III) peaks are different.

C,,As/CTAB

peak (III) is not sensitive

micelles

results in its micellar

aggre-

The third

to the environment,

but

gation number being the largest in all the three systems. Therefore the packing of chains in the micelle interior and of the headgroups at the

the intensity of the first (I) peak became very weak in a non-polar environment. When pyrene was dissolved or solubilized in a surfactant solution,

micelle

the environment

surface

probably

contributes

to

these

changes in N. From Fig. 5 we can see that N decreases with the rise of temperature. This result agrees with the general rule. The rise of temperature can result in an increase in the entropy values in this system. The arrangement of the surfactant molecules tends to become irregular and the surfactant molecules move from the micelle into the bulk solution. The micellar aggregation numbers in all the mixed systems were larger than 200, indicating that the micelles may be rod shaped. Because the molar ratio (anionic/cationic) is far from 1 : 1, the charge neutralization of the micellar charge is not complete and the electric double layer still exists. Therefore the increase in NaCl concentration results in an increase in N. The maximum value of N is larger than 1000, and a large value of N has also been obtained by Kato et al. [9] using the light-scattering method in the SDSjOTAB system. The variation of N us the concentration of added NaCl studied by the polarity change of the pyrene fluorescence microenvironment The monomer of pyrene has five peaks in its steady state fluorescence spectrum [ 181 (Fig. 7).

for pyrene

the ratio 1,/Z,,,. The molar ratio

of the two kinds

of surfactant

systems was 1 : I for the Ci2As,

in all the mixed anionic/cationic and the total concentrations

C,As/CTAB, Ci2As/DTAB, As/CTAB systems 5.0*10m3 M, 5.0*10-3 M and were 0.1 M, 7.5~10-~ M respectively. For the mixed systems, the total concentration was low enough and phase separation did not occur. In all samples the molar concentration ratio of pyrene to total surfactant was lo-‘. The Ci2As/DTAB system was kept at 5O.O”C whereas the others were kept at 4O.O”C. The relationship between 1,/I,,, and the concentration of NaCl is shown in Fig. 8. We can see from Fig. 8 that for the C,,As system, 1,/I,,, decreased with increase in NaCl concentration; in other words, the polarity of the pyrene microenvironment decreased with increase in NaCl concentration. When pyrene was incorporated into the micelles, its solubilization site was in the palisade layer near the polar headgroups of the surfactant, where it senses a rather polar environment [18]. Partly responsible for this may be the presence of water molecules which have permeated

1.05r

0.90:,

.. -

1 .

0.1 1

0.2 I

0.3 I CNpC,

330

gives a large value for

0.4 I (mol

A

05I

0.6I

I-9

460 A (rim)

Fig. 7. The five peaks of the monomer

of pyrene

Fig. 8. The variation of 1,/l,,, against NaCl concentration in different systems: A, C,As/DTAB; X, C,,As; E, C,,AqCTAB; 0, C,As/CTAB.

G.Z. Li et al./Coiloids Surfaces A: Physicochem. Eng. Aspects 76 (1993) 257-265

263

between the headgroups in the palisade layer. When NaCl was added to the C,,As system, the electric double layer of the micelle was compressed

figure. The obtained CMC values, which were 3.0. lo-4 M and 3.5. lop4 M for C,As/CTAB and C,,AS/CTAB respectively, were in good agreement

and the repulsion between the headgroups decreased; therefore surfactant molecules

with values measurement.

more closely packed

in the micelle.

was were

The increased

determined

Below the CMC,

by our

surface

tension

there are no micelles

present

N values forced water molecules out of the palisade layer and into the bulk solution, and caused the

and the pyrene fluorescence spectrum corresponds to that in water with an I,,,/I, ratio of about 0 7.

pyrene molecules to locate deeper so that the polarity of the pyrene ment was decreased.

in the micelle, microenviron-

However,

For the three mixed systems, the electric double layer hardly exists because of the charge neutralization at 1 : 1 molar ratio, and the addition of NaCl had no influence on the repulsion among the headgroups of the surfactant. The aggregation number of the micelles did not change, and therefore the value of 1,/Z,,, did not vary with the addition of NaCl.

ronment increased

Determination method

of CMC by steady statefluorescence

Many methods can be used to determine CMC values, but there are few reports on CMC determinations using the fluorescence method. In the C,As/CTAB system, the molar ratio of the two surfactants was also 1 : 1, and the [pyrene]/[surfactant] ratio was equal to lo-*. The relationship between the I,,,/I, value in the pyrene fluorescence spectrum and the total surfactant concentration C, is shown in Fig. 9. The CMC values were equal to the CT value at the abrupt turning point in the

X-X-

1.0 -

X-x

/ X

w ?

X-*x-x-

0.9 X

J-Y Oe8 -,J,, -X -10

X -9

-8

-7

-6

,

,

-5

-4

In Cr (mol I-‘) Fig. 9. The determination system by the fluorescence

of the CMC method.

of the C,,As/DTAB

at detergent

concentrations

above

the

CMC, pyrene is solubilized in the palisade layer of the micelle. Therefore the polarity of its microenvidecreased abruptly.

sharply,

and

the Z,,,/I, value

Determination of second CMC by the steady state jluorescence technique With the increase in surfactant concentration, the shape of the micelles will change from spherical to rod like. The main advantage of pyrene as the fluorescence probe is that its excited singlet state has a long lifetime, so that the excited molecule can collide with another ground state probe and form an excimer. An amount of pyrene was added to the micelle solution so that every micelle could contain two or more probe molecules and the excimer could be formed. Two peaks in the fluorescence spectrum were studied, the monomer peak F, (at 394 nm) and the excimer peak F, (at 475 nm) (Fig. 10). The intensity ratio of the two peaks can be used to determine the second CMC. In all the combined systems, the molar ratio of the two surfactants was 1: 1 and [pyrene]/C,= lo-*. Figure 11 shows the variation of F,/F, values as a function of the concentration of surfactant (C,,As). The curve shows a sharp increase in the F,/F, value at the second CMC. Table 2 summarizes second CMC data for the four investigated systems. When the micelles become rod-like, the aggregation numbers increase and the micelles are larger than the spherical ones. More micelles may contain enough pyrene (two or more pyrene molecules) to

G.Z. Li et al./Colloids

Surfaces

A: Physicochem.

Eng. Aspects 76 (1993) 257-265

F, (394 nm) 1

F2 (475nm) 1

,:::

0

Cc,2As Fig. 11. Variation

h(nm) Fig. 10. The steady

state fluorescence

spectrum

of pyrene.

form the excimer. The increase in fluorescence intensity (F2/F1) becomes larger abruptly.

of FJF,

12 x102

vs C,,As

(mol

16

24

l-9

concentration.

determine the CMC and the second CMC of these mixed cationic/anionic surfactants in aqueous solutions. Acknowledgement

Conclusions (1) The aggregation

number

of alkyl

sulfonate

micelles increases with increase in NaCl concentration. (2) The aggregation number of the mixed anionic/cationic micelles does not change with the addition of NaCl when the molar ratio of anionic to cationic surfactant is I : 1. Both the addition of anionic surfactant to cationic surfactant, and of cationic to anionic surfactant can cause a larger increase in N. With the molar ratio of cationic to anionic surfactant far from 1 : 1, the N value of

We wish to thank to the Natural Sciences Fund Foundation of China for financial support and the aid of Mr. Yu Chun and Mrs. Ye Jian-Ping (Institute of Photographic Chemistry, Academia Sinica, Beijing 100 10 I, People’s Republic of China). References J.M. Corkill, J.F. Goodman, SD. Harrold and J.R. Tate, Trans. Faraday Sot., 63 (1967) 247. J.H. Buckingham, J. Lucassen and F. Holloway, J. Colloid Interface Sci., 67 (1978) 423. E.H. Lucassen-Reynders, J. Lucassen and D. Giles, J. Colloid Interface Sci., 81 (1981) 150. B.Y. Zhu and M.J. Rosen, J. Colloid Interface Sci., 99 (1984) 435. A. Nakamura and M. Muramastsu, J. Colloid Interface Sci., 62 (1977) 165. D. Groalozyk, J. Colloid Interface Sci., 77 (1980) 68.

these combined systems increases with the increase in concentration of the added NaCl and decreases with the rise in temperature. (3) The fluorescence method can be used to TABLE

2

The second

CMC values

System

C,zAs

C,Asj’CTAB

C,,As,‘CTAB

C,,As/DTAB

Temperature (‘C) Second CMC (M)

40 8.0. IO-*

40 1.5*10-4

40 5.5*10-4

50 2.0.10-3

G.Z. Li et al./Colloids 7 8 9 IO 11 12

Surfaces

A: Physicochem.

Eng. Aspects 76 (1993) 2.57-265

A. Malliaris, W. Binana-Linbele and R. Zana, J. Colloid Interface Sci., 110 (1986) 114. T. Kato, M. Iwal and T. Seimiya, J. Colloid Interface Sci., 130 (1990) 253. T. Kato, H. Takeuchi and T. Seimiya, J. Colloid Interface Sci., 140 (1990) 253. R. Zana, Surfactant Sciences, Vol. 22, Marcel Dekker, New York, 1987, Chapter 5. P. Lianos, J. Lang, C. Strazlelle and R. Zana, J. Phys. Chem., 86 (1982) 1019. M. Aoudia, M.A.J. Rodgers and W.H. Wade, J. Colloid Interface Sci., 101 (1984) 472.

13

14 15 16 17 18

265

M. Almgren and S. Swarup, in K.L. Mittal and B. Lindman (Eds), Surfactant in Solution (I), Plenum Press, New York, 1984, p. 613. N.J. Turro and A. Yekta, J. Am. Chem. Sot., 100 (1978) 595. M. Almgren and J.E. Lofroth, J. Colloid Interface Sci., 81 (1981) 486. P. Lianos and R. Zana, J. Phys. Chem., 84 (1980) 3339. P. Lianos and R. Zana, J. Colloid Interface Sci., 84, (1981) 100. J.K. Thomas, Chem. Rev., 80 (1980) 283.