Isooctane Microemulsions

Journal of Colloid and Interface Science 225, 259–264 (2000) doi:10.1006/jcis.2000.6771, available online at http://www.idealibrary.com on

Effects of Alkylamines on the Percolation Phenomena in Water/AOT/Isooctane Microemulsions L. Garc´ıa-R´ıo,∗ P. Herv´es,† J. C. Mejuto,†,1 J. P´erez-Juste,† and P. Rodr´ıguez-Dafonte† ∗ Departamento de Qu´ımica F´ısica, Facultad de Qu´ımica, Universidad de Santiago de Compostela, Santiago de Compostela, Spain; and †Departamento de Qu´ımica F´ısica y Qu´ımica Org´anica, Facultad de Ciencias, Universidad de Vigo, Vigo, Spain Received February 22, 1999; accepted February 10, 2000

We carried out an investigation on the influence of several alkylamines, frequently present in reactions carried out in microemulsions, on the properties of the water/AOT/isooctane system. The presence of alkylamines has an important effect on the electrical percolation phenomena. This effect of amines on the electrical percolation of microemulsions of AOT/isooctane/water can be explained by taking into account the ability of these substrates to associate with the AOT film in the microemulsion, the basicity of the amine, and the different solubility of the amine in the three pseudophases of the system. °C 2000 Academic Press Key Words: microemulsions; AOT; electrical percolation; amines.

INTRODUCTION

Research into the internal dynamics of water/oil microemulsions has largely concentrated on the phenomenon of electrical percolation (1), characterized by a sudden increase in electrical conductivity when either the temperature or the volume fraction of the dispersed phase reaches a critical value. Percolation therefore consists of a sharp change in electrical conductivity from very low values, typical of an array of disperse droplets in a nonconducting continuous medium, to values which are much higher by several powers of 10. Zana et al. have demonstrated a relationship between electrical percolation and the rate constants of mass transfer among droplets (2). The rate of the exchange process on the long time scale becomes larger with the percolation transition for microemulsions, but remains slower than the diffusion-controlled rate (3). Studies of the mechanism of charge transfer in AOT microemulsions near the percolation transition confirm this point (4). It is well known that a moderate concentration of some additives affects the percolation threshold (5). Matthew et al. (6) and Moulik et al. (7) found that those additives which make surfactant membranes more rigid hinder electrical percolation, whereas those that make the membranes more flexible favor it. These results do not suggest that percolation is associated with

1 To whom correspondence should be addressed. E-mail: xmejuto@setei. uvigo.es.

the formation of two-continuum structures in the microemulsion but rather that discrete droplets prevail. The percolation phenomenon can be explained in terms of the number of collisions between microdroplets. It is probable that in the vicinity of the percolation temperature the incidence of such collisions is much higher and leads to the formation of droplet clusters through which matter flows. In AOT microemulsions droplets are stable above the percolation threshold, while in the nonionic Igepal microemulsions the droplets collapse with the percolation transition (3). This behavior has been interpreted in terms of activation energy for the exchange process (3). The high activation energy for the exchange process in the nonionic microemulsion is due to this breakdown of structures. In this study we examined the effect of alkylamines on the conductivity of water/AOT/isooctane microemulsions. Our results provide us with a better understanding of the factors that influence mass transfer in microemulsions and reveal the significance of small amounts of additives [amines are frequently present in reactions carried out in microemulsions (8)] on the behavior and properties of water/oil microemulsions. In this respect, our results emphasize the need to exercise care in interpreting chemical reactivity data in microemulsions because the reactants themselves or their products may introduce significant changes into the system, particularly in the “fast” reactions, where mass transfer between droplets can determine the reaction rate. METHOD

AOT (Aldrich, 99%) was dried under vacuum and used without further purification. All other reagents were used as supplied by Aldrich and Merck without further purification. Microemulsions were prepared by direct mixing and the composition was kept constant and equal to [AOT] = 0.5 M (referred to total volume of microemulsion) and W = [H2 O]/[AOT] = 22.2. Electrical conductivity (κ) was measured with a Crison microCM 2202 conductivity meter with a 1.0 cm−1 cell; the frequency of the bridge was 3.8 kHz and the degree of precision was greater than 0.1%. The temperature was kept constant with a thermostat–cryostat within ±0.1◦ C. Viscosity was measured with a falling ball viscosity meter at 25◦ C.

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the Tp induced in the standard microemulsion by these additives. This table shows that n-alkylamines increase the percolation temperature. The effect on the percolation temperature increases with the length of the alkyl chain. Figure 2 shows the linear dependence of Tp on the number of carbon atoms. (b) Branched Primary Alkylamines The Tp of AOT/isooctane/water microemulsion ([AOT] = 0.5 M and W = 22.2) was measured in the presence of n-alkylamines with substituents in the hydrocarbon chain. Table 1 shows the percolation temperature induced in the AOT/ isooctane/water microemulsion by these amines. The presence

FIG. 1. Determination of percolation temperature of AOT/isooctane/water microemulsions, [AOT] = 0.5 M, W = 22.2, in the presence of [trimethylamine] = 0.01 M.

The temperature of the percolation threshold (Tp ) was determined as the location of the peak of a plot of (1κ/κ1T ) versus temperature T (see Fig. 1 for a typical example) (9). To determine the partition coefficients of amines (K ow ) between water and isooctane, mixtures of amines with fixed quantities of two solvents were prepared. These samples were stirred vigorously for at least 24 h and were subsequently kept at 25◦ C for 1 week. The concentration of amine in the organic phase was determined by spectroscopic measurements. When this was not possible, the concentration of amine in the aqueous phase was determined by titration with hydrochloric acid, using a mixture of methyl red and brom-cresol green as an indicator. The values listed in Table 1 were the average of at least 5–10 different measurements of amine concentration in the biphasic system. K ow was obtained using Eq. 1, K ow =

nw a /n w , n oa /n o

[1]

o where n w a is the number of moles of amine in water, n a is the number of moles of amine in isooctane, and n o and n w are the number of moles of isooctane and water, respectively.

RESULTS

The percolation temperature threshold Tp of a standard microemulsion with [AOT] = 0.5 M and W = 22.2, 33.6◦ C, was consistently increased by addition of moderate concentrations of alkylamines. The amines examined in this study can be classified in three groups: linear and branched primary alkylamines, and secondary and tertiary amines.

TABLE 1 Percolation Temperature Threshold Tp , Viscosity Ratios at 25◦ C of Water/AOT/Isooctane Microemulsions in the Presence of Various Additives ([AOT] = 0.5 M, W = 22.2), and Partition Coefficients of Amines between Oil and Water at 25◦ C Structure

Tp /◦ C

pK a

—

33.6 34.5 34.5 40.7 42.6 43.6 47.6 50.6 50.6 39.5 40.5

— 10.6421a 10.6421e 10.5321e 10.6421a 10.6121b 10.6521c 10.6421c 10.6321c 10.6321d 10.4321c

i-Pentylamine

41.7

10.6421d

t-Butylamine

40.6

10.6821b

neo-Pentylamine Methyl-n-octylamine Dimethyl-n-octylamine Dimethyl-ndodecylamine Dimethylamine Diethylamine

40.6 43.6 36.6 36.6

10.47 11.26 11.04 11.00

35.1 36.2

10.7721a 10.9421e

Dipropylamine Diethylmethylamine

37.1 35.0

11.0021b 10.2921f

Di-i-propylamine

35.4

11.0521d

Di-i-butylamine

34.4

10.5021g

Dibenzylamine

37.1

N -Methylbenzylamine

Amine Without additive n-Methylamine n-Ethylamine n-Propylamine n-Butylamine n-Pentylamine n-Octylamine n-Decylamine n-Dodecylamine i-Propylamine i-Butylamine

ηrelative 1

K ow —

0.55 0.34 0.10 0.04 0.02 0.02 0.16 0.07 0.20 0.70 0.84 0.83

0.03 0.01 0.04 0.01

0.94

1.30

0.87 0.79

0.74 0.22 0.05

0.98

0.03

8.84

0.80

0.11

34.0

9.3721h

0.99

0.04

Trimethylamine

34.8

9.8021a

Triethylamine

35.1

10.7121i

0.96

Piperazine

34.0

9.7321a

0.99

Piperidine

35.0

11.1221a

0.3

Pyrrolidine

43.0

11.3021a

6.4

0.04

>50

(a) Primary n-Alkylamines The percolation temperature, Tp , of a microemulsion with [AOT] = 0.5 M and W = 22.2 was measured in the presence of n-alkylamines. The amines studied are listed in Table 1 as well as

PERCOLATION IN WATER/AOT/ISOOCTANE MICROEMULSIONS

FIG. 2. Influence of the length of the carbon chain of primary n-alkylamines on the conductivity of AOT/isooctane/water microemulsions, [AOT] = 0.5 M, W = 22.2. (s) [n-propylamine] = 0.01 M, (d) [n-pentylamine] = 0.01 M, (n) [n-octylamine] = 0.01 M.

of substituents in the alkyl chain decreases the effect observed upon Tp as compared with primary n-alkylamines (Fig. 3). (c) Secondary and Tertiary Amines The percolation temperature threshold (Tp ) of a microemulsion with [AOT] = 0.5 M and W = 22.2, was measured in the presence of tertiary amines. The presence of different substituents on nitrogen atom decreases the effect on the percolation temperature as compared with the effect observed for n-alkylamines (Fig. 4). The amines studied and Tp are shown in Table 1. (d) Effects upon Viscosity

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FIG. 4. Influence of N substituents on n-alkylamines on the conductivity of AOT/isooctane/water microemulsions, [AOT] = 0.5 M, W = 22.2. (s) [noctylamine] = 0.01 M, (d) [methyl-n-octylamine] = 0.01 M, (n) [dimethyl-noctylamine] = 0.01 M.

are also manifested in the viscosity behavior. To confirm the behavior observed previously, the effect of amines on the viscosity of an AOT/isooctane/water microemulsion ([AOT] = 0.5 M and W = 22.2) at 25◦ C has been measured. Table 1 lists the ratio between the viscosity of microemulsions in both the presence and the absence of the additive. The viscosity ratio decreases for all amines studied. This ratio seems to correlate roughly with the effect of the amines on the percolation temperature threshold (Fig. 5). DISCUSSION AND CONCLUSIONS

It is well known that the percolation phenomena in microemulsions, which become apparent in electrical conductivity,

The effect of alkylamines on the percolation temperature of AOT/isooctane/water microemulsions apparently contradicts the behavior of small organic molecules. In fact, small organic

FIG. 3. Influence of the substituents on the carbon chain of branched primary n-alkylamines on the conductivity of AOT/isooctane/water microemulsions, [AOT] = 0.5 M, W = 22.2. (s) [n-butylamine] = 0.01 M, (d) [i-butylamine] = 0.01 M.

FIG. 5. Relationship between the viscosity ratio and the percolation temperature (δTp = Tpwith additive − Tpwithout additive ), [AOT] = 0.5 M, W = 22.2, [additive] = 0.01 M.

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GARC´IA-R´IO ET AL.

molecules, such as urea, thiourea, and amides, reduce the Tp value (5b). When small organic molecules associate with the AOT film, they decrease the curvature of the surfactant. This decrease would favor droplet fusion as a result of the decrease in the natural tendency of AOT to form inverted structures. In addition, the association of additives to the interfacial zone decreases the rigidity of the film and increases its deformability. In this respect, it is interesting to ascertain whether the role of small organic compounds in the interfacial region is to replace water molecules. This point is supported by studies on frozen vesicles in the presence of various organic molecules (10). These results suggest that the addition of moderate concentrations of such molecules “opens” the interface and facilitates water penetration into the vesicle structure. However, at high concentrations the additive replaces water molecules at the interface, thereby playing a direct role in the solvation of the head groups. However, the presence of amines implies an increase in the percolation temperature. Similar effects (an increase in Tp value) were found when long-chain alcohols were added to a microemulsion as cosurfactants. Shah et al. (11), using dynamic light scattering, studied droplet size and interactions, and concluded that the interactions decreased according to their structure. It is well known that changes in the interactions between droplets are correlated with changes in the value of the percolation temperature. An increase occurs in the value of Tp , and a decrease in the interaction between microdroplets correlates with this increase. Shah et al. made the distinction between short-chain and long-chain alcohols: the former increase interactions, while the latter decrease them. A decrease in interaction was also noticed for Aracel (a long-chain nonionic surfactant) (11). With regard to the longchain alcohols (12), when decanol is added to the system the resulting microemulsion has a higher percolation temperature, and the higher the concentration of alcohol, the greater the effect on the percolation temperature. If the cosurfactant added is a poly(oxyethylene)alkyl ether, the reverse is observed (12b); the percolation temperature decreases, and continues to fall as the concentration increases. The addition of oligo- and polyethylene glycols in AOT microemulsions strongly affects the phase boundaries and the percolation behavior of these systems. The influence of ethylene glycol and polyethylene glycols on systems stabilized by AOT and Igepal induces percolation in AOT microemulsions but stabilizes droplets in Igepal systems. This effect is contrary to almost all other solutes, which stabilize the same structure in ionic and nonionic systems. Schuebel and Ilgenfritz (13) assume that this behavior is due to the different interaction of ethylene glycols with the solvated polar headgroups of AOT and Igepal. With regard to the molecular weight dependence, Schuebel et al. studied three regimes in the AOT system and found that small oligoethylene glycols induce percolation, medium-size polymers increasingly stabilize droplets, and large polymers reach a regime where percolation is unaffected by changes of chain length. Effects of long-chain alcohols as an oil phase on percolation phenomena were also studied by

FIG. 6. Relationship between the percolation temperature and the number of carbons in the hydrocarbon chain of the n-alkylamines (s) and the relationship between the percolation temperature and the partition coefficients of amines between oil and water at 25◦ C (d), [AOT] = 0.5 M, W = 22.2, [additive] = 0.01 M.

Ray and Moulik (12c), who observed the duality of the alcohols as a continuous phase and as cosurfactants. In view of the percolation temperatures, long-chain alcohols make the interface more rigid and hence make clustering aggregation, and consequently percolation, more difficult. Then we can assume that our n-alkylamines, as long-chain alcohols, are included in AOT film as cosurfactants. This implies that the interface is more rigid and the negative curvatures in the droplet structure are more favored. Hence it would be logical to assume that an increase in the number of carbon atoms in the alkyl chain would increase the effect on Tp . The sequence n-butylamine < n-pentylamine < n-octylamine < n-decylamine < n-dodecylamine is in accordance with this assumption (Fig. 6). This consideration can be supported when we observe the effect of K ow on the percolation for similar pK a values. In this case a relationship between Tp and log(K ow ) is observed (Fig. 6). This behavior is not surprising because both the number of carbon atoms and K ow will be associated with the substrate hydrophobicity and with their capacity for binding to the surfactant layer. Another point is the effect of short-chain amines. While small alcohols decrease the Tp (as normal organic substrates), in this study we observe that short-chain amines increase the Tp value (methylamine < ethylamine < n-propylamine < nbutylamine < n-pentylamine < n-octylamine < n-decylamine < n-dodecylamine). This behavior indicates that not only the substrate association with the surfactant layer, as in the case of alcohols, can explain the experimental results. Furthermore, it is significant that the effects of the amines on Tp seem to correlate with the pK a ’s of their conjugated acids. Tp values increase with increasing basicity: methylbenzylamine < trimethylamine < diethylmetilamine < triethylamine < dimethylamine < dipropylamine, whose conjugated acids have pK a ’s of 9.71, 9.80, 10.54, 10.71, 10.77, and

PERCOLATION IN WATER/AOT/ISOOCTANE MICROEMULSIONS

11.00, respectively, in bulk water. This suggests that the partial dissociation of the amine into ammonium and hydroxide ions, though negligible in comparison with the total concentration of amine, might be enough to affect the electrical percolation behavior of the microemulsion. It is well known that electrolytes can diminish the effective polar area of surfactants by screening electrostatic repulsion (5b, 14). This increases the curvature parameter of the surfactant, called the “critical packing parameter” (15). This parameter is defined as ν/al, where ν is the effective volume of a surfactant molecule, l the length of its hydrocarbon chain, and a the effective area of its polar head. Specifically, the presence of electrolytes will reduce a. The decrease in the surfactant polar head area induces a more markedly wedge-shaped structure in AOT and increases the natural negative curvature (i.e., the tendency to produce inverted structures) of the surfactant. This is consistent with the fact that salts diminish the maximum water solubilizing capacity of microemulsions since the solubilization process—through an increased droplet size—reduces the negative curvature of the interface, which counters the natural tendency of the surfactant. A tentative explanation based on the hypothesis of Rouviere et al. (16) could be that electrolytes might increase droplet sphericity. Although we assumed our droplets to be initially spherical, they may in fact be slightly distorted with local zones of decreased negative curvature. The addition of electrolytes may increase droplet sphericity in such a way that the same volume of the disperse phase could be accommodated in a more spherical structure, which would make a lower aggregation number compatible with a decreased area per polar head. Electrical conductivity of microemulsions is due to the passage of cations through transient channels formed between droplets that have collided. As noted by Robinson et al. (17), the opening of surfactant film to form transient channels involves large activation energies related to the creation of local regions of positive curvature. The less naturally prone the surfactant is to adopting positive curvature, the more difficult the mass-transfer process (and hence electrical conduction) will be. On the other hand the presence of “extra ions” in the microemulsion decreases attractive forces between droplets, which is seemingly related to droplet interpenetration. Theoretical calculations suggest (18) that the greater contributions to droplet–droplet attractive interactions occur in the overlapping region, a phenomenological parameter ξ being frequently used to characterize the depth of interpenetration between droplets. The value of ξ should decrease as the interface becomes stiffer and increase along with increasing salinity (19). The steric effect of the amine must be added to these two effects. Hence, the amines with a lower steric impediment reduce the percolation temperature; for example, propylamine (Tp = 40.7◦ C) and isopropylamine (Tp = 39.5◦ C); n-pentylamine (Tp = 43.6◦ C) and neo-pentylamine (Tp = 40.6◦ C). The combination of these three effects is probably the cause of the scattering observed in the correlations of Figs. 6 and 7. The quantification of the steric effects in an environment of reduced

263

FIG. 7. Relationship between percolation temperature and the pK a of the amine, [AOT] = 0.5 M, W = 22.2, [additive] = 0.01 M.

mobility, such as the interphase of the microemulsion, makes it impossible to carry out a multiparametric correlation using the experimental results. Interpretation of these results is hampered by the fact that all the amines used are more or less soluble in isooctane (Table 1 lists partition coefficients of amines), which apart from any other effects makes their real concentrations in the water droplets uncertain. This different solubility in the three pseudophases of the microemulsion can explain the behavior observed when substituents are introduced in the alkyl chain. Hence we can observe that the presence of substituents in the hydrocarbon chain decreases the effect of the amine on the Tp value: i-pentylamine < n-butylamine. Similar behavior was observed when the substituents were included in the nitrogen atom: n-octylamine >methyl-n-octylamine > dimethyl-n-octylamine or n-dodecylamine > methyl-n-dodecylamine. The percolation phenomena in microemulsions are also manifested in the viscosity behavior. The conductivity percolation curve is “paralleled” by a viscosity percolation curve. The observed decrease in viscosity correlated roughly with an increase in Tp and supported our assumptions. This reduction in viscosity caused by amines is attributable to a reduction in the attractive interactions among droplets, as occurs with the addition of longchain alcohols as cosurfactants (vide supra). The addition of amines apparently increases the rigidity of the interface, resulting in a decrease in the attractive interaction between droplets, and hence in viscosity. Previous studies have reported that the presence of electrolytes (NH4 Cl) decreases the exchange rate of material between microemulsions (20). This decrease in the exchange rate can be correlated with an increase in interfacial rigidity. To conclude, we can assume that the effect of amines on the electrical percolation of microemulsions of AOT/isooctane/ water can be explained by taking three different points into account, namely (a) the association of alkylamines with the AOT film as cosurfactants increasing the rigidity of the interphase as

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occurs when long-chain alcohols are added to the microemulsion as cosurfactants; (b) the different basicity of the amines which implies the presence of different amounts of hydroxyl and alkylammonium ions in the water phase for each amine; and (c) the fact that the presence of these ions will screen the head group, changing the effective area of the head group and increasing the natural negative curvature of the surfactant. Finally, we must add the steric effect of the amine in a restricted medium such as the AOT film. The different solubilities of the amine in the three pseudophases of the system, apart from any other effects, makes the real concentrations of the amine in the water droplets and in the AOT film uncertain. ACKNOWLEDGMENTS J.P.J. thanks the Ministerio de Educaci´on y Cultura for an FPU researchtraining grant. P.R.D. thanks the Universidad de Vigo for a grant. The authors are grateful to the Xunta de Galicia (project PGDIT99 PXI30104B) and Direcci´on General de Ense˜nanza Superior of Spain (project PB96–0954 and PB98–1089) for financial support. Also, the authors thank the reviewers for their useful comments.

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