Electrochemistry of solutes in microemulsions: Electrostatically bound aqueous ions, surfactant-like ions and oil soluble molecules

Colloids and Surfaces, 56 (1991) 1-12 Elsevier Science Publishers B.V., Amsterdam

Electrochemistry of solutes in microemulsions: electrostatically bound aqueous ions, surfactant-like ions and oil soluble molecules* R.A. Mackay’ Department

of Chemistry,

Drexel University,

Philadelphia,

PA 19104 (USA)

(Received 26 July 1989; accepted 25 October 1990)

Abstract Electrochemical measurements of the diffusion coefficients (D) of ferricyanide and cadmium ions in ionic microemulsions have shown that electrostatically bound aqueous ions have the same value of D as do surfactant-like ions such as long alkyl-chain pyridinium ions. Cd (II) has been shown to be bound at a cetyltrimethylammonium bromide (CTAB ) interface, presumably as a cadmium bromide complex anion. Non-ionic oil-in-water microemulsions have been shown to exhibit percolation behavior as has been previously observed in water-in-oil systems. Ferrocene exhibits behavior similar to that of an oil molecule. Data obtained from NMR self-diffusion measurements of oil, water and surfactant as a function of composition in a CTAB microemulsion are in accord with those previously obtained for an anionic system by means of a salinity scan. These results suggest that the ferrocene is distributed between the oil and interface regions.

INTRODUCTION

Microemulsions are dispersions of oil and water containing relatively high proportions of a surfactant and, frequently, a cosurfactant (typically 580% ). These systems are clear or translucent, mechanically stable, and contain high phase volumes (e.g. large amounts of both oil and water). While current usage of the term microemulsion refers to thermodynamically stable systems, a number of reported “microemulsions” are demonstrably only kinetically stable. The colloidal aggregates existing in these media have been shown to be nanodroplets of oil-in-water, water-in-oil, or so-called bicontinuous structures. A recent review describes these salient features of microemulsions and provides many additional references [ 11. There have been a number of polarographic studies of electroactive probes in microemulsions [ 2-41 which have focused on probing their structure. Some consideration has also been given to studies of *Paper presented at the Congress on Applications of New Trends in Colloid and Surfactant Science, Torino, Italy, 5-8 June 1989. ‘Present address: Detection Directorate, U.S. Army Chemical Research, Development and Engineering Center, Aberdeen Proving Ground, MD 21010-5423, USA.

0166-6622/91/$03.50

0 1991-

Elsevier Science Publishers

B.V.

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solute distribution [ 51, measurement of aggregate and solute transport [ 3,5,6] and control of reactivity [ 71. These studies have established that the diffusion coefficients of relatively small ions, in non-ionic microemulsions with an aqueous continuous phase, are effectively those in free water decreased by the obstruction effect, since the ions are excluded from the “oil” disperse phase [ 81. The disperse phase in O/ W systems consists of the oil, emulsifier (surfactant plus cosurfactant ), and “bound” water which is motionally hindered [ 91. The effective viscosity of this bound water is about an order of magnitude higher than free water, and small aqueous ions obey the equation: D/D0 = (l-p)‘.5

(1)

where D and D, are the diffusion coefficients in microemulsion and water respectively, and p is the true phase volume, to be discussed below, which includes the bound water [ 91. The diffusion coefficients of surfactant-like electroactive species which are bound to the interface, such as long alkyl-chain pyridinium and viologen ions, have been shown to be essentially constant in ionic (both anionic and cationic ) microemulsions, and can be used to estimate the aggregate radii in systems containing nanodroplets [ $61. There have to date, however, been no reported studies of surfactant-like electroactive probes in non-ionic microemulsions, or of small aqueous ions which would be expected to be electrostatically bound to ionic microemulsions. We report here the first results of this type, as well as some preliminary results concerning a neutral, oil-soluble electroactive probe in a cationic microemulsion. EXPERIMENTAL

Phase maps Compositions, materials, and methods of determination of the phase maps have been previously described. The systems employed for the electrochemical measurements are: cetyltrimethylammonium bromide (CTAB ) /l-butanol/ hexadecane/water [lo]; sodium cetyl sulfate (SCS)/l-pentanol/heavy mineral oil (Nujol)/water [3,11]; Tween 60/l-pentanol/hexadecane/water [ 121; and Brij 96/l-butanol/hexadecane/water [ 131. Tween 60 is polyoxyethylene (20)sorbitan monostearate and Brij 96 is polyoxyethylene (10)oleyl ether. The phase maps for these microemulsions are shown in Fig. 1. All of the electrochemical studies were carried out by varying the phase volume along a water dilution line. This line connects a point on the emulsifier-oil (E-O) axis with the water (W) apex. The oil concentration in the fixed E/O ratio is specified as percent (by weight) initial oil. The CTAB/2_butanol/benzene/water system [ 141 was employed for the NMR self-diffusion measurements.

Fig. 1. Phase maps of microemulsion systems. The axes are weight percent of water (W), oil (O), and emulsifier (E) which is composed of a fixed ratio of surfactant to alcohol cosurfactant. The emulsifieris (a) Tween60 (66%)/l_pentanol(34%), (b) Brij96(65.7%)/1-butano1(34.3%), (c) SCS (39.6%)/l-pentanol(60.4%), (d) CTAB(50%)/1-butano1(50%). The oil is hexadecane in (a), (b) and (d), and heavy mineral oil in (c).

Electrochemical measurements The polarographic measurements were made with a three compartment cell as described previously [ 151. The working electrode was a dropping mercury electrode except for ferricyanide which employed a rotating platinum electrode, and ferrocene for which glassy carbon was used [ 161. All measurements were at 25’ C with 0.1 M KC1 as supporting electrolyte. NMR self-diffusion coefficients The self-diffusion coefficient of all of the components in the CTAB/2-butanol/benzene/water microemulsion were measured by means of an NMRpulsed field gradient technique [ 171. Corrections were made for the exchange of protons between water and the alcohol [ 91. RESULTS

AND DISCUSSION

Aqueous ions in ionic systems It might be expected that highly charged aqueous ions would be electrostatically bound at the interphase region of an ionic microemulsion, just as ions are observed to bind to simple aqueous micelles. As a result of the lower surface potentials in microemulsions, the binding would be expected to be weaker than in the corresponding micelles [3,18]. This expectation is realized for the [Fe (CN )6] 3- ion in cationic CTAB microemulsion as shown in Fig. 2.

4

1

0

0.2

0.4

0.6 Pcomp

1

0.8

Fig. 2. Polarographic diffusion coefficient of ferricyanide ion in microemulsion normalized to the value in water (D/D") vs. compositional phase volume (p co,,,P) for the CTAB/l-butanol/hexadecane (10% initial) (0) and SCS/l-pentanol/mineral oil (21% initial) (0).

The essentially constant value of the diffusion coefficient (D) is precisely the expected behavior for droplet diffusion, and the absolute value (6.7. 10m7 cm* s-l) is in agreement with that obtained from alkyl pyridinium and viologen probes [5,6]. The ferricyanide ion in an anionic SCS microemulsion is shown for comparison, and the value of D increases with increasing water content (decreasing p) as is expected for an ion in the aqueous phase being affected by the obstruction effect, Eqn (1). The physical basis for this effect is the exclusion of the species undergoing transport from some fraction of the entire volume. In this case, the ferricyanide is effectively confined to the aqueous regions of the microemulsion. The compositional phase volume, pcomp,is the volume of oil and emulsifier as given by

Pcomp=l--w

(2)

Here, w is the weight fraction of water and g is the specific gravity of the microemulsion (close to unity for all the systems studied here). The true phase volume p in Eqn ( 1) is given by P =Pcomp+PbLu

(3)

where pbWis the volume fraction of interfacially bound water [ 191. The situation for Cd(I1) in the same cationic CTAB microemulsion yields some unexpected results. As shown in Fig. 3, this ion behaves as though it were bound. The most probable explanation for this behavior is that the cadmium is bound as anionic cadmium bromide complex ions. Given the formation constants for these complexes in water [20], and realizing that the effective bromide ion concentration in the interphase is of the order of a few molar, it is readily apparent that effectively all the cadmium ion is bound as [CdBr4]*-. Support for this explanation has been provided by studies with copper (II) in a waterin-oil microemulsion containing added CTAB where the results indicated that

0.2 0 4 ,000 v

0.1 u

_

n

00 0

1

I

I

0.2

0.3

0.4

0.5 Pcomp

0.6

0.7

Fig. 3. Polarographic diffusion coefficient of cadmium ion in CTAB/l-butanol/hexadecane (10% initial) microemulsion normalized to the value in water (D/D”) vs. compositional phase volume (Pcomp 1.

4-

PI 0

x

3-

L

i

” 02-

l-

01

0

1

0.2

I

I

0.6

0.4

,

0.8

Pcomp

Fig. 4. Polarographic diffusion coefficient (D) of l-dodecyL4cyanopyridinium ion vs. composi(10% initial) microemulsion. tional phase volume (pcump) for the Brij 96/l-butanol/hexadecane The solid line is a fit of the data for 0.3
the copper was bound to the interface as anionic copper bromide complex ions [211. Alkylpyridinium

ions in nonionic systems

Electroactive long alkyl-chain organic ions, as discussed above, can be used to obtain droplet diffusion coefficients in ionic microemulsions. When these measurements were extended to non-ionic microemulsions, surprising results were obtained. Figures 4 and 5 show data for the diffusion coefficient of 1-dodecyl-4-cyanopyridinium ion in the Brij microemulsion and both

6

09 0

0.2

0.6

0.4

0.8

Pcomp

Fig. 5. Polarographic diffusion coefficient (D) of 1-dodecyl- (0 ) and 1-cetyl (0 )-4-cyanopyridinium ions vs. compositional phase volume (p,,,, ) for the Tween 60/l-pentanol/hexadecane (7% initial) microemulsion. The solid line is a fit of the data for 0.3
1-dodecyl- and 1-cetyl-4-cyanopyridinium ions in the Tween microemulsion respectively. It may be noted that both systems exhibit identical behavior, namely, a decreasing value of D with increasing water content (decreasing pcomp), This behavior parallels that observed for the conductivity in some water-in-oil microemulsions, and was explained on the basis of the percolation and effective medium theories [22,23]. In this case, there would be percolative behavior exhibited at a high water content by the oil nanodroplets in the oil-in-water region. At phase volumes beyond the percolation threshold, the Effective Medium Theory (EMT) has been shown to fit the W/O conductivity data. These systems can be considered to be conductor-insulator mixtures where the “oil” pseudo-phase is the conductor for the diffusing water-insoluble alkyl pyridinium ion and the water pseudo-phase is the insulator. The operative EMT expression is given by [ 23 ] o/o0

= 3/2 (P, --P)

(4)

where pc has the value l/3. It may be noted that there are effects due to the interacting and dynamic properties of microemulsions which can change both the value of pc and the critical element for p 0.5. The microemulsion viscosity is significantly

Fig. 6. Polarographic diffusion coefficient of 1-cetyl-4-cyanopyridinium ion (D5’*) vs. corrected phase volume (p) for the Tween 60/l-pentanol/hexadecane (10% initial) microemulsion, according to Eqn (5 ) .

increased at this point, although the system is still quite fluid. At a value of P camp= 0.5, p is equal to 0.64 which corresponds to the value for random closepacked spheres (0.63). It is possible that this anomalous post-percolative behavior corresponds to the transition to a bicontinuous structure [ 1,251. In the vicinity of the percolation threshold, the scaling law is given by [ 231 D(P) x (P-Po)s’5

(5)

where the value of p,, is expected to be 0.29 for spheres that can overlap with centers distributed randomly. A plot of D 5/8vs. p for the Tween system is shown in Fig. 6, yielding a value of po= 0.22. Similar results are obtained for the Brij microemulsion. On the basis of these results, we conclude that in non-ionic oil-in-water-type microemulsions the nanodroplets undergo “sticky” collisions which result in percolative behavior for species which diffuse with the surfactant, such as the surfactant-like long alkyl-chain pyridinium ions. Ferrocene in a cationic system The electroactive neutral, water-insoluble molecule ferrocene has been examined in the same CTAB/l-butanol/hexadecane/water microemulsions employed for the above studies [ 161. The value of D is seen in Fig. 7 to increase with increasing pcomp.Although it may appear qualitatively to be the same as for the pyridinium ions in the non-ionic systems discussed above, the behavior is quantitatively quite different. At the high water end, the value of D for ferrocene is about an order of magnitude higher than in the non-ionic systems, and corresponds to the expected droplet diffusion coefficient (broken line in Fig. 7 ). None of the data obtained to date on these ionic O/W systems indicate that the interactions between droplets, particularly at lower values of pcomp,

8

0

0.2

0.6

0.4

0.8

Pcomp

Fig. 7. Diffusion coefficient of ferrocene (D) vs. compositional phase volume (p,,,) in the CTAB/ 1-butanol/hexadecane (10% initial) microemulsion. The broken line is the value obtained from bound or long alkyl-chain electroactive ions over the entire range of available phase volumes. The arrows labelled a and b represent the corrected phase volumes corresponding to random and hexagonal close packed spheres respectively. Ferrocene data are from Ref. 16.

are other than repulsive. A possible explanation for this behavior comes from previous studies of NMR self-diffusion of the components of an anionic microemulsion composed of sodium dodecyl sulfate, butanol, toluene and brine [26]. In this case, brine salinity was varied to scan O/W, through bicontinuous, to W/O regions. The water and oil diffusion coefficient varied by two orders of magnitude. The values of D for water and surfactant were about the same at the W/O end, and similarly for oil and surfactant at the O/W end. We have obtained NMR self-diffusion data in a CTAB/2_butanol/benzene/ water system by varying only composition, not salinity, similar to the electrochemical studies. A phase map of this system [ 141 has been reproduced in Fig. 8, showing the composition employed for the NMR measurements. Quasi-elastic light scattering (QLS ) measurements have shown that aggregates of some type exist at all of the compositions (points 1-5in Fig. 8) along the demixing line, although the QLS data alone cannot distinguish between droplet or bicontinuous structures. The results of the NMR measurements are given in Table 1. As has been shown [ 271, the apparent self-diffusion coefficient measured by NMR in an O/W system must be corrected by a factor ( 1-pcomp)as discussed in a recent study of water self-diffusion in ionic and non-ionic microemulsions -p w,the volume fraction of water, the corrected value (DC) [ 91. Since l-pPcompfor the self-diffusion coefficient for water is given by D,/P,,where D, is the

W

0

Fig. 8. Phase map of a CTAB/l-butanol/benzene microemulsion system. The emulsifier (E) is 40 wt.% CTAB 60 wt.% 2butanol, the oil (0) is benzene, and W is the water apex. The compositions labelled 1-5 correspond to those in Table 1. TABLE 1 Component diffusion coefficients determined by means of NMR in the CTAB/2_butanol/benzene microemulsion at 25’ C Composition

1 2 3 4 5

% H,O”

15.0 60.0 34.0 9.6 6.3

Diffusion coefficient (NMR) (m2 s-l* log) D H,O

D CsHs

D CTAB

D 2-Butanolb

1.62 1.39 0.94 0.52 0.54

0.24 0.32 0.60 0.95 0.13

0.10 0.13 0.14 0.14 0.14

0.47 0.44 0.42 0.42 0.54

“Compositions 1 and 2 lie along the line connecting the W-apex with the 70-E/30-0 composition on the E-O axis (Fig. 8). Similarly, composition 3 lies on the 35-E line parallel to the W-O axis, and compositions 4 and 5 lie on the line connecting the O-apex with the 25-W/75-E composition on the W-E axis. bCorrected for proton exchange [ 91.

value of D measured by NMR. If the oil is assumed to be confined mainly to the oil region (although some benzene does penetrate the interface), and the surfactant confined to the interface, which is assumed to be composed of the emulsifier although some cosurfactant is distributed in the oil and aqueous pseudo-phases, then the corrected values shown in Table 2 are obtained. These corrected values for water, oil and surfactant are plotted in Fig. 9. It may be noted that D’s varies by only a factor of two over the entire range of (p0,,<0.4) and 0: P camp examined, and coincides with the values of 0:

10 TABLE Corrected

2 diffusion

coefficients

determined

by NMR

for the CTAB/2-butanol/benzene/water

microemulsion Composition”

% H,O

DwPwb,C

1 2

75.0 60.0

0.83

0.02 0.05

0.02 0.04

3 4

34.0 9.6

0.05 0.03

0.23 1.06

0.05 0.03

“The compositions

1.22

DsPE~

correspond

to those in Fig. 8. coefficient (m’s_‘* log), pi is the compositional volume fraction for 0 (oil) and E (emulsifier). The emulsifier consists of the surfactant (S), plus the

bD is the NMR self-diffusion i= W (water),

alcohol cosurfactant. “The values of D for pure water and benzene are 2.5 and 2.3 (m’s_‘*

log) respectively.

“0 x

7 *

"E ti-

dO.lO-

0.001 0

0.2

0.4

0.6

0.6

Fig. 9. Corrected diffusion coefficient (Dip;) vs. compositional phase volume (p,,,) for the CTAB/ 2_butanol/benzene/water microemulsion (points 1-5, Fig. 8). IJ, water (D,p,); 0, benzene

(D,p,);

+, (CTAB

(DSpE). The lines are provided only as guides for the eye.

(P comp>0.9). The absolute values of Ds (2*10W1’ -5*10W1’) are in accord with those expected for aggregate diffusion coefficients. The essential implication to be derived from these results with regard to the ferrocene diffusion coefficients is that they may be qualitatively explained by a distribution of the electroactive probe between the oil and emulsifier pseudophases. Studies are currently in progress to examine the concentration and

11

compositional vide sufficient

dependence information

of the ferrocene diffusion coefficient in order to proto develop and test a quantitative model.

SUMMARY

It is concluded that electrochemical measurements can distinguish between free and interface-bound aqueous ions in ionic microemulsions. Free ion data provide the true phase volume, while bound ion data give the aggregate diffusion coefficient. Under appropriate circumstances, cations can be bound to a cationic interface as an anionic surfactant counterion complex as a result of the high effective counterion in the Stern layer. Surfactant-like ions provide a measure of the droplet and interfacial diffusion in ionic systems, and information on percolative behavior in non-ionic systems. The oil soluble neutral molecule, ferrocene, appears to partition between the oil core and interfacial layer, and may provide information on the transition from an O/W to a bicontinuous region. ACKNOWLEDGMENT

The author thanks R. Agarwal and N.S. Dixit for performing some of the electrochemical measurements, Dr. A. Brajter-Toth for the ferrocene measurements, and F.D. Blum and E. Cheever for the NMR measurements. The support of the U.S. Army Research Office and the National Science Foundation is gratefully acknowledged.

REFERENCES

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5 6 7 8

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