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Water solubilization capacity and conductance behaviors of AOT and NaDEHP systems in the presence of additives
Colloids and Surfaces A: Physicochemical and Engineering Aspects 197 (2002) 101– 109 www.elsevier.com/locate/colsurfa
Water solubilization capacity and conductance behaviors of AOT and NaDEHP systems in the presence of additives Quan Li, Tao Li *, Jinguang Wu College of Chemistry and Molecular Engineering, Peking Uni6ersity, Beijing 100871, People’s Republic of China Received 15 June 2000; accepted 25 June 2001
Abstract The influences of two typical additives, long-chain organic molecule (bis(2-ethylhexyl) phosphoric acid (HDEHP)) and inorganic electrolyte (sodium chloride), on the water solubilization capacity of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane solutions have been investigated. The presence of optimal content of HDEHP remarkably enhances the water solubilization capability of the NaDEHP system, while it decreases the solubilization capacity of the AOT system. The addition of low concentration of NaCl solution in the AOT microemulsions tends to enhance its solubilization capacity. The conductance behaviors of the AOT system have also been investigated, focusing on the influences of the HDEHP content, NaCl concentration, and temperature. For the water/AOT/n-heptane reverse micelles/microemulsions, with increasing water content, no percolation phenomena can be observed in the presence of HDEHP; whereas the percolation conductance occurs with the addition of NaCl solutions and the onset water content increases with an increase of the NaCl concentration. With an increase of temperature, the percolation conductance also occurs in the AOT system in the presence of HDEHP and the onset temperature for the percolation conductance decreases with increasing HDEHP content. The influences of the variables on the water solubilization capacity and conductance behaviors could be understood from their effects on the rigidity of the oil/water interface and the attractive interactions of the surfactant aggregates. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Water solubilization capacity; Conductivity; Additives; AOT; NaDEHP; Percolation
1. Introduction Surfactants, which contain both hydrophobic and hydrophilic components, are well known to be able to self-assemble in solutions and thus * Corresponding author. Present address: National Institutes of Health, Bldg. 10, Rm. 10D08, 10 Center Drive, Bethesda, MD 20892-1850, USA.
enhance the compatibility between immiscible solvents. Above a certain concentration, surfactant molecules self-assemble into aggregates in nonpolar solvents, with the hydrophilic portion of the molecule in the interior and the hydrophobic portion at the exterior of the aggregates. Such systems are referred to as a reverse micelles/microemulsions, which are isotropic and homogeneous on a macroscopic scale while heterogeneous
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on a microscopic scale. Investigations on these fluids have been stimulated by the specific phenomena they display, as well as by the possible applications in such diverse areas as pharmaceutical preparations, cosmetics, and enhanced oil recovery, etc. [1]. These applications depend crucially on the water solubilization capacity, which changes in response to the environmental variables, such as surfactant properties, composition, type of oil, temperature, valence of counterions, salt concentration, etc. [2– 4]. An important aspect of microemulsion design is its ability to solubilize a maximum amount of dispersed phase (i.e. water) into the continuous phase. Single surfactant does not necessarily produce the best microemulsions. Nonionic surfactant, acting as cosurfactant, shows dramatic influences on the water uptake capacity of the microemulsions formulated with ionic surfactant [2,3]. In this report, we examine the influences of a double-tailed cosurfactant, bis(2-ethylhexyl) phosphoric acid (HDEHP), on the water solubilization capacity of sodium bis(2-ethylhexyl) phosphate (NaDEHP) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) formulated systems. AOT has been widely used because of its ability to formulate reverse micelles containing large amounts of water without the addition of a cosurfactant. Recently, much attention has been focused on the NaDEHP due to its unique aggregation behaviors in nonpolar media [5]. These two surfactants are structurally related molecules, with identical hydrocarbon tails and counterions, while differ only in their headgroups. Our ongoing investigations have shown that despite the structural similarity, the spectroscopic properties [6,7] and the conductance behaviors [8,9] of the systems formulated with these two surfactants differ from each other significantly. The intention of this research is to examine the solubilization capacity of water into an oil phase and to improve the understanding of the synergism between a surfactant and a cosurfactant in nonpolar organic environment. We report the water solubilization capacity of the AOT and NaDEHP systems, focusing on the significant influences of the HDEHP content and the initial NaCl concentration. The roles played by the addi-
tives, temperature, and water content on the structural variation of the surfactant aggregates in the AOT formulated microemulsions were conveniently probed using electrical conductometry. The conductance behaviors were further interpreted by considering the influences of the variables on the rigidity and attractive interactions of the surfactant aggregates.
2. Experimental section
2.1. Materials and methods AOT, purity \ 98%, was purchased from Fluka and was purified following the procedure described elsewhere [10]. The other chemicals were purchased from Beijing Chemical Factory. n-Heptane (purity \ 99.5%) was used as received. Bis(2ethylhexyl) phosphoric acid (HDEHP) with purity 92% was purified by copper salt crystallization [11] and the purity of the resultant sample was 98%. The NaDEHP salt was prepared by mixing equivalent amounts of HDEHP and aqueous sodium hydroxide solutions. After the water was evaporated, the salt was dried in vacuum at 60 °C for 48 h and was stored over P2O5. Stock solutions of AOT or NaDEHP in n-heptane were prepared at room temperature, keeping the surfactant concentration at 0.25 mol l − 1. The necessary amount of distilled water or aqueous NaCl solution was gradually injected into the mixture of surfactant and n-heptane to examine the water solubilization capacity or to obtain the desired water content. For examining the water solubilization capacity, following each injection, the solutions became colorless or blue transparent after shaking. When the resulting solutions were homogeneous and transparent, more water or brine was added and the two phases were shaken vigorously. The development in the samples of persistently hazy or milky appearance after the addition of water or brines indicated that the maximum solubilization reached. In some cases, AOT was replaced with HDEHP to investigate the influences of the cosurfactant on the solubilization capacity and conductance behaviors of the microemulsions. The total concentration of AOT
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and HDEHP was kept constant. Water content (W0) is expressed as the molar ratio of the added water to the total content of surfactant(s) (surfactant+HDEHP, surfactant= AOT or NaDEHP). HDEHP was included when calculating the water solubilization capacity. The compositions were given in molar ratio.
2.2. Measurements Electrical conductivity measurements were carried out using a hermetically sealed plate condenser-type glass cell with two rectangular Pt electrodes of 5×5 mm and a gap width of 5 mm. The variation of conductivity with water content was measured by dropwise addition of water into respective systems at 25 °C. Dependence of conductivity on temperature was investigated by changing the temperature in the range of 0–55 °C in a liquid paraffin bath under electromagnetic stirring to eliminate the temperature difference in the conductance cell.
3. Results and discussion
3.1. Influences of HDEHP on the water solubilization capacity of AOT and NaDEHP systems An important property of a microemulsion is its capability to solubilize water as microdroplets in nonpolar phase. The solubilization capacity of water in the mixture of surfactant and oil is strikingly influenced by the chemical nature of the cosurfactant used. We replaced part of the AOT and NaDEHP with HDEHP and examined the influences of HDEHP on the water solubilization capacity of the respective systems. The systems can take in certain amount of water and become turbid and eventually separate into two phases when excess water is added. The maximum amount of water that can be solubilized is determined from the appearance of permanent turbidity. Fig. 1 plots the experimental data of the water solubilization capacity of the NaDEHP and AOT systems as a function of the HDEHP content. For the NaDEHP system, with an increase of
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HDEHP content, the water solubilization capacity increases initially and reaches a maximum when the ratio of [NaDEHP]/([HDEHP] + [NaDEHP]) is 90%. Beyond that, the water uptake capacity decreases with a further increase of the HDEHP content, as shown in Fig. 1(a). The behavior of exhibiting a peak of solubilization capacity in the H2O/NaDEHP/n-heptane system in the presence of HDEHP at an optimal cosurfactant concentration is similar to the results reported earlier for the AOT formulated microemulsions with optimal chain-length of alcohols [12], Brij 52 [13], Span-20 [14], and acrylamide [15] as cosurfactants. In contrast, when the HDEHP was added into the H2O/AOT/n-heptane microemulsions, the water solubilization capacity decreases monotonically with increasing HDEHP content, as plotted in Fig. 1(b). For the AOT formulated microemulsions, the water solubilization capacity is geometrically related to the rigidity of the oil/water interface and the attractive interactions among the droplets. The significant differences in the influences of the cosurfactants on the water solubilization capacity of the AOT microemulsions are attributed to the different structures of the cosurfactants, which affect the fundamental properties of the surfac-
Fig. 1. The variation of water solubilization capacity as a function of surfactant composition in the NaDEHP (a) and AOT (b) formulated systems at 25 °C. The surfactant composition is defined as the ratio of [surfactant]/([surfactant]+ [HDEHP]), where surfactant refers to NaDEHP and AOT.
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tant aggregates at the oil/water interface (interfacial tension, preferred curvature, rigidity) and determine the ‘preferred’ size and shape of the aggregates. A possible explanation for the solubilization results with and without the presence of HDEHP can be made when the extent of HDEHP penetration into the surfactant layer is considered. HDEHP displays strong hydrophobic property and cannot partition equally between the oil and water phases. Its double-tailed structure and the large steric effects also hinder it from approaching the water pool as closely as AOT or NaDEHP does, which implies that most of the added HDEHP molecules distribute in the oil phase or penetrate into the interface of the surfactant aggregates. A dynamic equilibrium exists between the HDEHP present in the surfactant layer and in n-heptane. Experimentally, it has been shown that the increase of attractive interactions among the surfactant aggregates is associated with an increasing penetrable length of the interfacial region as evidenced from an increasing difference between the hard-sphere radius and the hydrodynamic radius [12,16]. The presence of HDEHP molecules in the oil/water interface increases the penetrable length, which would increase the attractive interactions among the droplets and thus decrease the water solubilization capacity accordingly. The earlier investigations demonstrated that an increase of the chain length of the oil used increased the strength of attraction among the droplets [12,13,16], which also support our arguments. Meanwhile, the partial replacement of the AOT with HDEHP decreases the number of the surfactant molecules that self-assemble into aggregates, which further decreases the water solubilization capacity. These two aspects account for the decrease of the water solubilization capacity of the H2O/AOT/n-heptane systems in the presence of HDEHP. One complication in the case of NaDEHP system is the potential formation of a network-like structure [5,17– 19]. Such scenario accounts for the low solubilization capacity of the H2O/ NaDEHP/n-heptane system without the presence of cosurfactants. The synergistic mixtures of HDEHP and NaDEHP show increased solubilization capacity of water at a certain ratio than the
Fig. 2. Solubilization capacity of aqueous NaCl solutions of different salinity in the water/AOT/n-heptane systems.
system made with NaDEHP only, which suggests that the addition of HDEHP destroys the liquid crystalline structure and favors the formation of microemulsion phase. However, the increase in the water solubilization capacity of the NaDEHP + HDEHP system is limited by the increased attractive interactions of the surfactant aggregates, similar to the influences of HDEHP on the AOT microemulsion. The balance between the two opposing effects accounts for the maximum value of the solubilization capacity. With a further addition of HDEHP in the system, after the maximum solubilization capacity, the effect of increasing interactions among surfactant aggregates is expected to become predominant and counteracts the effect of formulating microemulsions from the network-like structure, which leads to the decrease of solubilization capacity after the maximum, as shown in Fig. 1(a).
3.2. The influences of NaCl concentration on water solubilization capacity of AOT system It has been established that the addition of HDEHP in the AOT or NaDEHP formulated systems dramatically influences the water solubilization capacity. For the AOT system, the presence of HDEHP decreases the solubilization capacity, while the addition of optimal concentration of aqueous NaCl solutions can improve the solubilization capacity by influencing the structure
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Fig. 3. Variation of electrical conductivity against water content in H2O/AOT/n-heptane microemulsions when the ratios of [AOT]/([AOT]+ [HDEHP]) are 90% (a); 70% (b); and 60% (c). The curves with the symbols of (), ( ), and () represent the conductance behaviors of the systems with NaCl concentration of 10 − 4, 10 − 3, and 10 − 2 mol l − 1, respectively.
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phase, the oil/water interface, and the water pool. The addition of brine could be necessary to expel part of the AOT molecules from the aqueous phase into the organic phase to form reverse micelles, thus tending to increase the water solubilization capacity of the system [4]. In addition, the addition of NaCl solution decreases the attractive interactions among the droplets by making the interfacial layer more rigid [20], which also increases the water solubilization capacity. On the other hand, however, the increase in the salinity of the initial NaCl solution diminishes the effective polar area of the surfactant by decreasing the thickness of the electrical double layer around the polar group, which tends to increase the natural negative curvature of the surfactant monolayer [21,22]. The aggregation number would be reduced to be compatible with the decreased area of the headgroups. This effect leads to the decrease of the solubilization capacity of the AOT microemulsions. The addition of high concentration of NaCl solutions makes the latter effect play a dominant role and decreases the solubility capacity dramatically. The above effects are expected to counteract each other and lead to a maximum value of the water solubilization capacity, as shown in Fig. 2.
3.3. Influence of HDEHP on the conductance beha6iors of AOT system of surfactant aggregates. Fig. 2 plots the dependence of water uptake capacity on the concentration of aqueous NaCl solutions in the AOT microemulsions. With an increase of the initial concentration of the aqueous NaCl solutions, the water solubilization capacity increases initially and reaches a maximum. After that, it decreases dramatically with a further increase of salinity. The influences of the salinity on the water solubilization capacity of the AOT system can be elucidated by considering both the amount of surfactant molecules available for the establishment of aggregates and the rigidity of the surfactant membrane. With the hydrophilic sulfonate and the hydrophobic double chain, AOT molecules can be distributed in three different regions of the microemulsions— the organic
The influences of the additives on the interfacial structure of the surfactant aggregates can be easily detected by means of electrical conductometer. Fig. 3 presents the variation of electrical conductivity as a function of the content of aqueous NaCl solutions with different salinity in the AOT + HDEHP reverse micelles. Similar to that of the AOT system [13,23], the reverse micelles distinguish themselves with their very low conductivity and pronounced maximum conductivity within the solubilization capacity investigated. The conductivity-W0(s−W0) curves of the AOT systems appear bell-shaped with a maximum conductivity in each plot. What differentiates the AOT + HDEHP system from the AOT system is that, at room temperature, with increasing water content, no percolation phenomena can be ob-
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served in the AOT+HDEHP systems within the range of solubilization capacity, whereas percolation conductance occurs in the AOT reverse micelles [13]. In the AOT formulated reverse micelles, the disperse consists of water droplets surrounded by AOT molecules. The solubilized water molecules are preferably involved in ion hydration and associate tightly to Na+ and headgroups at low water content and thus tend to be immobilized. There is less bulk-like water to supply a diffusion environment for the hydrated Na+ counterions. Instead, the droplets carry excess charges through spontaneous charge fluctuations arised from the exchange of droplet core contents, i.e. water and counterions. The migration of the charged droplets in applied electric field leads to the low conductivity. As discussed in charge fluctuation model [23], the maximum conductivity is probably due to two antagonistic effects. As the water content exceeds the demand for hydrating surfactant headgroups and counterions, the excess water tends to form water pools in the cores of the surfactant aggregates. The hydrated counterions exchange and redistribute readily during the droplet collision and transient fusion process, which tends to increase the conductivity upon increasing W0 values. On the other hand, however, at high water content, an electrical double layer develops in the inner part of the droplets, with the negatively charged sulfonate anion in the inner wall and the positively charged Na+ counterions in the water pools, which leads to a size-dependent repulsion among the droplets [24]. According to Derjaguin, the repulsive energy of droplets containing such a double layer is proportional to first order to the radius of the droplet and the square of the potential droplet in the continuous oil phase [25]. The effect of repulsion is expected to counteract the effect of the increased mobility of ions, and become pronounced when more water is solubilized. The counteraction of these effects accounts for the maximum conductivity in the s −W0 curves. It can also be seen in Fig. 3 that the water content corresponding to the maximum conductivity in the s − W0 curves does not show much
differences with the addition of brine with different salinity, while changes with the AOT to HDEHP ratio. With an increase of HDEHP content (from Fig. 3(A–C)), the values of maximum conductivity shifts toward lower W0 values, which is in accordance with the results of the reverse micelles stabilized with AOT and Brij 30 [13], whereas it is opposite to the tendency of NaDEHP + HDEHP system [9]. Fig. 3 suggests that the aggregation state of the surfactant aggregates does not show significant variation when the NaCl solutions of different salinity were added into respective reverse micelles. The addition of high-salinity solutions in the system decreases the attractive interactions among the droplets by making the interfacial layer more rigid, which results in a slightly lower conductivity of the system. The difference in the conductivity of the systems with different salt concentrations decreases with an increase of the HDEHP content, as shown in Fig. 3(b and c). The variations in the maximum conductivity are attributed to the influences of HDEHP on the interfacial structure of the surfactant aggregates. As discussed above, the surfactant aggregates approach and fuse to form a short-lived dimmer and then separate to form two new isolated droplets. During this process, the counterions randomly redistribute and give rise to separately charged droplets, which migrate in the oil-rich medium under an electric field and result in the conductivity. The rigidity and the attractive interactions among the aggregates are probably the most important factors that determine the exchange rate of the ions and water molecules during the fusion process. The presence of HDEHP diminishes the rigidity of the membrane and increases the attractive interactions among the surfactant aggregates, which suggests that the opening of surfactant film to form transient dimers involves less activation energies. As a result, the conductivity of the reverse micelles with higher HDEHP content increases more readily with increasing water content and reaches a maximum at lower water content.
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Fig. 4. Dependence of logarithm conductivity on the content of aqueous NaCl solutions in the H2O/AOT/n-heptane microemulsions in the presence of NaCl solutions of various initial salinity.
3.4. Effect of NaCl concentration on the percolation water content Fig. 4 presents the variation of logarithm conductivity with the content of aqueous NaCl solutions in the H2O/AOT/n-heptane reverse micelles/ microemulsions. The logarithm conductivity increases with the addition of brine until a maximum. After that, it decreases with increasing content of NaCl solution and reaches a minimum. A further addition of the brine in the system leads to a dramatic increase of the electrical conductivity, indicative of percolation conductance. The occurrence of the percolation conductance reveals an increase of both the attractive interactions among the surfactant aggregates and the exchange rate of the dissolved materials between the microdroplets. The onset water content for the percolation conductance increases with the presence of NaCl solutions and continues to increase as the salinity increases, which can be understood from the influence of NaCl on the structure of the surfactant aggregates. It is generally considered that when percolation happens, the water droplets come in close contact and the charge carriers propagate by hopping between droplets [26] and/or by exchanging droplet contents during droplets coalesce and redisperse [27,28]. The solubilized water swells up the interfacial layer, which makes the overall structure less rigid and the exchange of droplet content
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is facilitated. The water globules in the oil continuum undergo ‘sticky collisions’ to form transient expanded clusters or conduits, through which sodium counterions migrate easily and hence increase the conductivity dramatically. The formation of the infinite percolating droplet network accounts for the percolation conductance. As noted earlier, the opening of surfactant film to form transient channels involves large activation energies related to the creation of local regions of positive curvature [29]. The addition of NaCl diminishes the effective polar area of the surfactants by screening the electrostatic repulsion, which makes surfactant membranes more rigid and decreases the attractive interactions among the droplets [30–32]. With the presence of high salinity NaCl solutions, large activation energies are needed to create positive curvature of local regions and thus hinder electrical percolation.
3.5. Influence of temperature on the conducti6ity of AOT + HDEHP system Fig. 5 presents the influence of temperature on the conductance behaviors of the water/AOT/nheptane microemulsions with varying HDEHP content at W0 = 50. The inset plots the variation of onset temperature as a function of AOT content.
Fig. 5. The logarithm conductivity as a function of temperature for the water/AOT/n-heptane microemulsions of varying HDEHP content at the W0 =50. ( ) 60%AOT+ 40%HDEHP, () 70%AOT+30%HDEHP, () 80%AOT+ 20%HDEHP, and (2) 90%AOT+10%HDEHP. The inset plots the dependence of the onset temperature for percolation conductance on AOT content, where the AOT content is the ratio of [AOT]/([AOT]+[HDEHP]).
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It can be seen in Fig. 5 that, below the onset temperature, the conductivity remains practically unchanged with the variation of temperature, which suggests that the local arrangement of the surfactant molecules is not measurably influenced by the temperature variation. The conductivity plot before the percolation conductance can be interpreted using the charge fluctuation model for spherical [23] and nonspherical [33] particles whose size is only determined by the amount of the solubilized water. In such a situation, the surfactant aggregates behave as the dispersion of conducting particles in non-conducting n-heptane solvent. The transport of the charged droplets, whose ionization is caused by the fluctuation of ion contents, conducts electricity. An increase in temperature would increase the ionization probability of the surfactant molecules, the frequency of droplet collisions, and the solubilizate exchange, thus increase the electrical conductivity [26–28]. With a further increase of temperature, after the onset temperature, the logarithm conductance exhibits a characteristic sigmoidal behavior of percolation in high temperature region, as presented in Fig. 5. A strong and sharp increase of conductivity, spanning four orders of magnitude over a narrow range of temperature can be observed in respective s – T curves. This behavior indicates that a different conducting process takes place in the high temperature region when the interaction between the individual droplets increases and the interfacial film is loosened. The electrical conductivity above the percolation threshold was due to the motion of surfactant counterions within transient water channels arised in droplet clusters upon opening of surfactant monolayers. The instability of the surfactant aggregates at high temperature is driven by the short-range attractive interactions between the droplets due to a decline in surfactant/oil compatibility with increasing temperature [12]. After the sharp increase, the conductivity remains practically unchanged upon increasing temperature, at considerably higher values than before the transition. As can be seen in the inset of Fig. 5, the onset temperature increases with an increase of the AOT content, which can also be understood from the structural change of the surfactant ag-
gregates. The changes in the value of the percolation temperature are correlated with the changes of the interactions between droplets. The presence of higher HDEHP concentration in the system increases the attractive interactions among the surfactant aggregates, and thus decreases the onset temperature for the percolating conductance.
4. Conclusions The influences of two typical additives, HDEHP and NaCl, on the water solubilization capacity of the H2O/AOT/n-heptane and H2O/ NaDEHP/n-heptane systems have been investigated. The effects of the variables (additives, water content, and temperature) on the structure of oil/water interface have also been examined using electrical conductometer. For the H2O/ AOT/n-heptane system, the presence of optimal content of HDEHP tends to increase the attractive interactions among the surfactant aggregates, thus reduces the water solubilization capacity and decreases the onset temperature for percolation conductance. However, the addition of NaCl solutions increases the rigidity of the membrane and, therefore, increases the threshold water content for percolation conductance. A maximum solubilization capacity could be obtained by maximizing the content of surfactant molecules available for forming aggregates and by increasing the rigidity of the surfactant aggregates. References [1] T.K. De, A. Maitra, Adv. Coll. Interf. Sci. 59 (1995) 95 References therein. [2] K. Kon-no, A. Kitahara, J. Coll. Interf. Sci. 41 (1972) 47. [3] D.J. Liu, J.M. Ma, H.M. Cheng, Z.G. Zhao, Coll. Surf. A 143 (1998) 59. [4] H.R. Rabie, D. Helou, M.E. Weber, J.H. Vera, J. Coll. Interf. Sci. 189 (1997) 208. [5] Z.-J. Yu, R.D. Neuman, Langmuir 11 (1995) 1081. [6] Q. Li, S.-F. Weng, J.-G. Wu, N.-F. Zhou, J. Phys. Chem. 102 (1998) 3168. [7] Q. Li, T. Li, J.-G. Wu, N.-F. Zhou, J. Coll. Interf. Sci. 229 (2000) 298. [8] Q. Li, T. Li, J.-G. Wu, J. Phys. Chem. 104 (2000) 9011. [9] Q. Li, T. Li, J.-G. Wu, J. Coll. Interf. Sci., 239 (2001) 522. [10] A.N. Maitra, H.-F. Eicke, J. Phys. Chem. 85 (1981) 2687.
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Water solubilization capacity and conductance behaviors of AOT and NaDEHP systems in the presence of additives Quan Li, Tao Li *, Jinguang Wu College of Chemistry and Molecular Engineering, Peking Uni6ersity, Beijing 100871, People’s Republic of China Received 15 June 2000; accepted 25 June 2001
Abstract The influences of two typical additives, long-chain organic molecule (bis(2-ethylhexyl) phosphoric acid (HDEHP)) and inorganic electrolyte (sodium chloride), on the water solubilization capacity of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) and sodium bis(2-ethylhexyl) phosphate (NaDEHP) in n-heptane solutions have been investigated. The presence of optimal content of HDEHP remarkably enhances the water solubilization capability of the NaDEHP system, while it decreases the solubilization capacity of the AOT system. The addition of low concentration of NaCl solution in the AOT microemulsions tends to enhance its solubilization capacity. The conductance behaviors of the AOT system have also been investigated, focusing on the influences of the HDEHP content, NaCl concentration, and temperature. For the water/AOT/n-heptane reverse micelles/microemulsions, with increasing water content, no percolation phenomena can be observed in the presence of HDEHP; whereas the percolation conductance occurs with the addition of NaCl solutions and the onset water content increases with an increase of the NaCl concentration. With an increase of temperature, the percolation conductance also occurs in the AOT system in the presence of HDEHP and the onset temperature for the percolation conductance decreases with increasing HDEHP content. The influences of the variables on the water solubilization capacity and conductance behaviors could be understood from their effects on the rigidity of the oil/water interface and the attractive interactions of the surfactant aggregates. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Water solubilization capacity; Conductivity; Additives; AOT; NaDEHP; Percolation
1. Introduction Surfactants, which contain both hydrophobic and hydrophilic components, are well known to be able to self-assemble in solutions and thus * Corresponding author. Present address: National Institutes of Health, Bldg. 10, Rm. 10D08, 10 Center Drive, Bethesda, MD 20892-1850, USA.
enhance the compatibility between immiscible solvents. Above a certain concentration, surfactant molecules self-assemble into aggregates in nonpolar solvents, with the hydrophilic portion of the molecule in the interior and the hydrophobic portion at the exterior of the aggregates. Such systems are referred to as a reverse micelles/microemulsions, which are isotropic and homogeneous on a macroscopic scale while heterogeneous
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on a microscopic scale. Investigations on these fluids have been stimulated by the specific phenomena they display, as well as by the possible applications in such diverse areas as pharmaceutical preparations, cosmetics, and enhanced oil recovery, etc. [1]. These applications depend crucially on the water solubilization capacity, which changes in response to the environmental variables, such as surfactant properties, composition, type of oil, temperature, valence of counterions, salt concentration, etc. [2– 4]. An important aspect of microemulsion design is its ability to solubilize a maximum amount of dispersed phase (i.e. water) into the continuous phase. Single surfactant does not necessarily produce the best microemulsions. Nonionic surfactant, acting as cosurfactant, shows dramatic influences on the water uptake capacity of the microemulsions formulated with ionic surfactant [2,3]. In this report, we examine the influences of a double-tailed cosurfactant, bis(2-ethylhexyl) phosphoric acid (HDEHP), on the water solubilization capacity of sodium bis(2-ethylhexyl) phosphate (NaDEHP) and sodium bis(2-ethylhexyl) sulfosuccinate (AOT) formulated systems. AOT has been widely used because of its ability to formulate reverse micelles containing large amounts of water without the addition of a cosurfactant. Recently, much attention has been focused on the NaDEHP due to its unique aggregation behaviors in nonpolar media [5]. These two surfactants are structurally related molecules, with identical hydrocarbon tails and counterions, while differ only in their headgroups. Our ongoing investigations have shown that despite the structural similarity, the spectroscopic properties [6,7] and the conductance behaviors [8,9] of the systems formulated with these two surfactants differ from each other significantly. The intention of this research is to examine the solubilization capacity of water into an oil phase and to improve the understanding of the synergism between a surfactant and a cosurfactant in nonpolar organic environment. We report the water solubilization capacity of the AOT and NaDEHP systems, focusing on the significant influences of the HDEHP content and the initial NaCl concentration. The roles played by the addi-
tives, temperature, and water content on the structural variation of the surfactant aggregates in the AOT formulated microemulsions were conveniently probed using electrical conductometry. The conductance behaviors were further interpreted by considering the influences of the variables on the rigidity and attractive interactions of the surfactant aggregates.
2. Experimental section
2.1. Materials and methods AOT, purity \ 98%, was purchased from Fluka and was purified following the procedure described elsewhere [10]. The other chemicals were purchased from Beijing Chemical Factory. n-Heptane (purity \ 99.5%) was used as received. Bis(2ethylhexyl) phosphoric acid (HDEHP) with purity 92% was purified by copper salt crystallization [11] and the purity of the resultant sample was 98%. The NaDEHP salt was prepared by mixing equivalent amounts of HDEHP and aqueous sodium hydroxide solutions. After the water was evaporated, the salt was dried in vacuum at 60 °C for 48 h and was stored over P2O5. Stock solutions of AOT or NaDEHP in n-heptane were prepared at room temperature, keeping the surfactant concentration at 0.25 mol l − 1. The necessary amount of distilled water or aqueous NaCl solution was gradually injected into the mixture of surfactant and n-heptane to examine the water solubilization capacity or to obtain the desired water content. For examining the water solubilization capacity, following each injection, the solutions became colorless or blue transparent after shaking. When the resulting solutions were homogeneous and transparent, more water or brine was added and the two phases were shaken vigorously. The development in the samples of persistently hazy or milky appearance after the addition of water or brines indicated that the maximum solubilization reached. In some cases, AOT was replaced with HDEHP to investigate the influences of the cosurfactant on the solubilization capacity and conductance behaviors of the microemulsions. The total concentration of AOT
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and HDEHP was kept constant. Water content (W0) is expressed as the molar ratio of the added water to the total content of surfactant(s) (surfactant+HDEHP, surfactant= AOT or NaDEHP). HDEHP was included when calculating the water solubilization capacity. The compositions were given in molar ratio.
2.2. Measurements Electrical conductivity measurements were carried out using a hermetically sealed plate condenser-type glass cell with two rectangular Pt electrodes of 5×5 mm and a gap width of 5 mm. The variation of conductivity with water content was measured by dropwise addition of water into respective systems at 25 °C. Dependence of conductivity on temperature was investigated by changing the temperature in the range of 0–55 °C in a liquid paraffin bath under electromagnetic stirring to eliminate the temperature difference in the conductance cell.
3. Results and discussion
3.1. Influences of HDEHP on the water solubilization capacity of AOT and NaDEHP systems An important property of a microemulsion is its capability to solubilize water as microdroplets in nonpolar phase. The solubilization capacity of water in the mixture of surfactant and oil is strikingly influenced by the chemical nature of the cosurfactant used. We replaced part of the AOT and NaDEHP with HDEHP and examined the influences of HDEHP on the water solubilization capacity of the respective systems. The systems can take in certain amount of water and become turbid and eventually separate into two phases when excess water is added. The maximum amount of water that can be solubilized is determined from the appearance of permanent turbidity. Fig. 1 plots the experimental data of the water solubilization capacity of the NaDEHP and AOT systems as a function of the HDEHP content. For the NaDEHP system, with an increase of
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HDEHP content, the water solubilization capacity increases initially and reaches a maximum when the ratio of [NaDEHP]/([HDEHP] + [NaDEHP]) is 90%. Beyond that, the water uptake capacity decreases with a further increase of the HDEHP content, as shown in Fig. 1(a). The behavior of exhibiting a peak of solubilization capacity in the H2O/NaDEHP/n-heptane system in the presence of HDEHP at an optimal cosurfactant concentration is similar to the results reported earlier for the AOT formulated microemulsions with optimal chain-length of alcohols [12], Brij 52 [13], Span-20 [14], and acrylamide [15] as cosurfactants. In contrast, when the HDEHP was added into the H2O/AOT/n-heptane microemulsions, the water solubilization capacity decreases monotonically with increasing HDEHP content, as plotted in Fig. 1(b). For the AOT formulated microemulsions, the water solubilization capacity is geometrically related to the rigidity of the oil/water interface and the attractive interactions among the droplets. The significant differences in the influences of the cosurfactants on the water solubilization capacity of the AOT microemulsions are attributed to the different structures of the cosurfactants, which affect the fundamental properties of the surfac-
Fig. 1. The variation of water solubilization capacity as a function of surfactant composition in the NaDEHP (a) and AOT (b) formulated systems at 25 °C. The surfactant composition is defined as the ratio of [surfactant]/([surfactant]+ [HDEHP]), where surfactant refers to NaDEHP and AOT.
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tant aggregates at the oil/water interface (interfacial tension, preferred curvature, rigidity) and determine the ‘preferred’ size and shape of the aggregates. A possible explanation for the solubilization results with and without the presence of HDEHP can be made when the extent of HDEHP penetration into the surfactant layer is considered. HDEHP displays strong hydrophobic property and cannot partition equally between the oil and water phases. Its double-tailed structure and the large steric effects also hinder it from approaching the water pool as closely as AOT or NaDEHP does, which implies that most of the added HDEHP molecules distribute in the oil phase or penetrate into the interface of the surfactant aggregates. A dynamic equilibrium exists between the HDEHP present in the surfactant layer and in n-heptane. Experimentally, it has been shown that the increase of attractive interactions among the surfactant aggregates is associated with an increasing penetrable length of the interfacial region as evidenced from an increasing difference between the hard-sphere radius and the hydrodynamic radius [12,16]. The presence of HDEHP molecules in the oil/water interface increases the penetrable length, which would increase the attractive interactions among the droplets and thus decrease the water solubilization capacity accordingly. The earlier investigations demonstrated that an increase of the chain length of the oil used increased the strength of attraction among the droplets [12,13,16], which also support our arguments. Meanwhile, the partial replacement of the AOT with HDEHP decreases the number of the surfactant molecules that self-assemble into aggregates, which further decreases the water solubilization capacity. These two aspects account for the decrease of the water solubilization capacity of the H2O/AOT/n-heptane systems in the presence of HDEHP. One complication in the case of NaDEHP system is the potential formation of a network-like structure [5,17– 19]. Such scenario accounts for the low solubilization capacity of the H2O/ NaDEHP/n-heptane system without the presence of cosurfactants. The synergistic mixtures of HDEHP and NaDEHP show increased solubilization capacity of water at a certain ratio than the
Fig. 2. Solubilization capacity of aqueous NaCl solutions of different salinity in the water/AOT/n-heptane systems.
system made with NaDEHP only, which suggests that the addition of HDEHP destroys the liquid crystalline structure and favors the formation of microemulsion phase. However, the increase in the water solubilization capacity of the NaDEHP + HDEHP system is limited by the increased attractive interactions of the surfactant aggregates, similar to the influences of HDEHP on the AOT microemulsion. The balance between the two opposing effects accounts for the maximum value of the solubilization capacity. With a further addition of HDEHP in the system, after the maximum solubilization capacity, the effect of increasing interactions among surfactant aggregates is expected to become predominant and counteracts the effect of formulating microemulsions from the network-like structure, which leads to the decrease of solubilization capacity after the maximum, as shown in Fig. 1(a).
3.2. The influences of NaCl concentration on water solubilization capacity of AOT system It has been established that the addition of HDEHP in the AOT or NaDEHP formulated systems dramatically influences the water solubilization capacity. For the AOT system, the presence of HDEHP decreases the solubilization capacity, while the addition of optimal concentration of aqueous NaCl solutions can improve the solubilization capacity by influencing the structure
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Fig. 3. Variation of electrical conductivity against water content in H2O/AOT/n-heptane microemulsions when the ratios of [AOT]/([AOT]+ [HDEHP]) are 90% (a); 70% (b); and 60% (c). The curves with the symbols of (), ( ), and () represent the conductance behaviors of the systems with NaCl concentration of 10 − 4, 10 − 3, and 10 − 2 mol l − 1, respectively.
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phase, the oil/water interface, and the water pool. The addition of brine could be necessary to expel part of the AOT molecules from the aqueous phase into the organic phase to form reverse micelles, thus tending to increase the water solubilization capacity of the system [4]. In addition, the addition of NaCl solution decreases the attractive interactions among the droplets by making the interfacial layer more rigid [20], which also increases the water solubilization capacity. On the other hand, however, the increase in the salinity of the initial NaCl solution diminishes the effective polar area of the surfactant by decreasing the thickness of the electrical double layer around the polar group, which tends to increase the natural negative curvature of the surfactant monolayer [21,22]. The aggregation number would be reduced to be compatible with the decreased area of the headgroups. This effect leads to the decrease of the solubilization capacity of the AOT microemulsions. The addition of high concentration of NaCl solutions makes the latter effect play a dominant role and decreases the solubility capacity dramatically. The above effects are expected to counteract each other and lead to a maximum value of the water solubilization capacity, as shown in Fig. 2.
3.3. Influence of HDEHP on the conductance beha6iors of AOT system of surfactant aggregates. Fig. 2 plots the dependence of water uptake capacity on the concentration of aqueous NaCl solutions in the AOT microemulsions. With an increase of the initial concentration of the aqueous NaCl solutions, the water solubilization capacity increases initially and reaches a maximum. After that, it decreases dramatically with a further increase of salinity. The influences of the salinity on the water solubilization capacity of the AOT system can be elucidated by considering both the amount of surfactant molecules available for the establishment of aggregates and the rigidity of the surfactant membrane. With the hydrophilic sulfonate and the hydrophobic double chain, AOT molecules can be distributed in three different regions of the microemulsions— the organic
The influences of the additives on the interfacial structure of the surfactant aggregates can be easily detected by means of electrical conductometer. Fig. 3 presents the variation of electrical conductivity as a function of the content of aqueous NaCl solutions with different salinity in the AOT + HDEHP reverse micelles. Similar to that of the AOT system [13,23], the reverse micelles distinguish themselves with their very low conductivity and pronounced maximum conductivity within the solubilization capacity investigated. The conductivity-W0(s−W0) curves of the AOT systems appear bell-shaped with a maximum conductivity in each plot. What differentiates the AOT + HDEHP system from the AOT system is that, at room temperature, with increasing water content, no percolation phenomena can be ob-
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served in the AOT+HDEHP systems within the range of solubilization capacity, whereas percolation conductance occurs in the AOT reverse micelles [13]. In the AOT formulated reverse micelles, the disperse consists of water droplets surrounded by AOT molecules. The solubilized water molecules are preferably involved in ion hydration and associate tightly to Na+ and headgroups at low water content and thus tend to be immobilized. There is less bulk-like water to supply a diffusion environment for the hydrated Na+ counterions. Instead, the droplets carry excess charges through spontaneous charge fluctuations arised from the exchange of droplet core contents, i.e. water and counterions. The migration of the charged droplets in applied electric field leads to the low conductivity. As discussed in charge fluctuation model [23], the maximum conductivity is probably due to two antagonistic effects. As the water content exceeds the demand for hydrating surfactant headgroups and counterions, the excess water tends to form water pools in the cores of the surfactant aggregates. The hydrated counterions exchange and redistribute readily during the droplet collision and transient fusion process, which tends to increase the conductivity upon increasing W0 values. On the other hand, however, at high water content, an electrical double layer develops in the inner part of the droplets, with the negatively charged sulfonate anion in the inner wall and the positively charged Na+ counterions in the water pools, which leads to a size-dependent repulsion among the droplets [24]. According to Derjaguin, the repulsive energy of droplets containing such a double layer is proportional to first order to the radius of the droplet and the square of the potential droplet in the continuous oil phase [25]. The effect of repulsion is expected to counteract the effect of the increased mobility of ions, and become pronounced when more water is solubilized. The counteraction of these effects accounts for the maximum conductivity in the s −W0 curves. It can also be seen in Fig. 3 that the water content corresponding to the maximum conductivity in the s − W0 curves does not show much
differences with the addition of brine with different salinity, while changes with the AOT to HDEHP ratio. With an increase of HDEHP content (from Fig. 3(A–C)), the values of maximum conductivity shifts toward lower W0 values, which is in accordance with the results of the reverse micelles stabilized with AOT and Brij 30 [13], whereas it is opposite to the tendency of NaDEHP + HDEHP system [9]. Fig. 3 suggests that the aggregation state of the surfactant aggregates does not show significant variation when the NaCl solutions of different salinity were added into respective reverse micelles. The addition of high-salinity solutions in the system decreases the attractive interactions among the droplets by making the interfacial layer more rigid, which results in a slightly lower conductivity of the system. The difference in the conductivity of the systems with different salt concentrations decreases with an increase of the HDEHP content, as shown in Fig. 3(b and c). The variations in the maximum conductivity are attributed to the influences of HDEHP on the interfacial structure of the surfactant aggregates. As discussed above, the surfactant aggregates approach and fuse to form a short-lived dimmer and then separate to form two new isolated droplets. During this process, the counterions randomly redistribute and give rise to separately charged droplets, which migrate in the oil-rich medium under an electric field and result in the conductivity. The rigidity and the attractive interactions among the aggregates are probably the most important factors that determine the exchange rate of the ions and water molecules during the fusion process. The presence of HDEHP diminishes the rigidity of the membrane and increases the attractive interactions among the surfactant aggregates, which suggests that the opening of surfactant film to form transient dimers involves less activation energies. As a result, the conductivity of the reverse micelles with higher HDEHP content increases more readily with increasing water content and reaches a maximum at lower water content.
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Fig. 4. Dependence of logarithm conductivity on the content of aqueous NaCl solutions in the H2O/AOT/n-heptane microemulsions in the presence of NaCl solutions of various initial salinity.
3.4. Effect of NaCl concentration on the percolation water content Fig. 4 presents the variation of logarithm conductivity with the content of aqueous NaCl solutions in the H2O/AOT/n-heptane reverse micelles/ microemulsions. The logarithm conductivity increases with the addition of brine until a maximum. After that, it decreases with increasing content of NaCl solution and reaches a minimum. A further addition of the brine in the system leads to a dramatic increase of the electrical conductivity, indicative of percolation conductance. The occurrence of the percolation conductance reveals an increase of both the attractive interactions among the surfactant aggregates and the exchange rate of the dissolved materials between the microdroplets. The onset water content for the percolation conductance increases with the presence of NaCl solutions and continues to increase as the salinity increases, which can be understood from the influence of NaCl on the structure of the surfactant aggregates. It is generally considered that when percolation happens, the water droplets come in close contact and the charge carriers propagate by hopping between droplets [26] and/or by exchanging droplet contents during droplets coalesce and redisperse [27,28]. The solubilized water swells up the interfacial layer, which makes the overall structure less rigid and the exchange of droplet content
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is facilitated. The water globules in the oil continuum undergo ‘sticky collisions’ to form transient expanded clusters or conduits, through which sodium counterions migrate easily and hence increase the conductivity dramatically. The formation of the infinite percolating droplet network accounts for the percolation conductance. As noted earlier, the opening of surfactant film to form transient channels involves large activation energies related to the creation of local regions of positive curvature [29]. The addition of NaCl diminishes the effective polar area of the surfactants by screening the electrostatic repulsion, which makes surfactant membranes more rigid and decreases the attractive interactions among the droplets [30–32]. With the presence of high salinity NaCl solutions, large activation energies are needed to create positive curvature of local regions and thus hinder electrical percolation.
3.5. Influence of temperature on the conducti6ity of AOT + HDEHP system Fig. 5 presents the influence of temperature on the conductance behaviors of the water/AOT/nheptane microemulsions with varying HDEHP content at W0 = 50. The inset plots the variation of onset temperature as a function of AOT content.
Fig. 5. The logarithm conductivity as a function of temperature for the water/AOT/n-heptane microemulsions of varying HDEHP content at the W0 =50. ( ) 60%AOT+ 40%HDEHP, () 70%AOT+30%HDEHP, () 80%AOT+ 20%HDEHP, and (2) 90%AOT+10%HDEHP. The inset plots the dependence of the onset temperature for percolation conductance on AOT content, where the AOT content is the ratio of [AOT]/([AOT]+[HDEHP]).
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It can be seen in Fig. 5 that, below the onset temperature, the conductivity remains practically unchanged with the variation of temperature, which suggests that the local arrangement of the surfactant molecules is not measurably influenced by the temperature variation. The conductivity plot before the percolation conductance can be interpreted using the charge fluctuation model for spherical [23] and nonspherical [33] particles whose size is only determined by the amount of the solubilized water. In such a situation, the surfactant aggregates behave as the dispersion of conducting particles in non-conducting n-heptane solvent. The transport of the charged droplets, whose ionization is caused by the fluctuation of ion contents, conducts electricity. An increase in temperature would increase the ionization probability of the surfactant molecules, the frequency of droplet collisions, and the solubilizate exchange, thus increase the electrical conductivity [26–28]. With a further increase of temperature, after the onset temperature, the logarithm conductance exhibits a characteristic sigmoidal behavior of percolation in high temperature region, as presented in Fig. 5. A strong and sharp increase of conductivity, spanning four orders of magnitude over a narrow range of temperature can be observed in respective s – T curves. This behavior indicates that a different conducting process takes place in the high temperature region when the interaction between the individual droplets increases and the interfacial film is loosened. The electrical conductivity above the percolation threshold was due to the motion of surfactant counterions within transient water channels arised in droplet clusters upon opening of surfactant monolayers. The instability of the surfactant aggregates at high temperature is driven by the short-range attractive interactions between the droplets due to a decline in surfactant/oil compatibility with increasing temperature [12]. After the sharp increase, the conductivity remains practically unchanged upon increasing temperature, at considerably higher values than before the transition. As can be seen in the inset of Fig. 5, the onset temperature increases with an increase of the AOT content, which can also be understood from the structural change of the surfactant ag-
gregates. The changes in the value of the percolation temperature are correlated with the changes of the interactions between droplets. The presence of higher HDEHP concentration in the system increases the attractive interactions among the surfactant aggregates, and thus decreases the onset temperature for the percolating conductance.
4. Conclusions The influences of two typical additives, HDEHP and NaCl, on the water solubilization capacity of the H2O/AOT/n-heptane and H2O/ NaDEHP/n-heptane systems have been investigated. The effects of the variables (additives, water content, and temperature) on the structure of oil/water interface have also been examined using electrical conductometer. For the H2O/ AOT/n-heptane system, the presence of optimal content of HDEHP tends to increase the attractive interactions among the surfactant aggregates, thus reduces the water solubilization capacity and decreases the onset temperature for percolation conductance. However, the addition of NaCl solutions increases the rigidity of the membrane and, therefore, increases the threshold water content for percolation conductance. A maximum solubilization capacity could be obtained by maximizing the content of surfactant molecules available for forming aggregates and by increasing the rigidity of the surfactant aggregates. References [1] T.K. De, A. Maitra, Adv. Coll. Interf. Sci. 59 (1995) 95 References therein. [2] K. Kon-no, A. Kitahara, J. Coll. Interf. Sci. 41 (1972) 47. [3] D.J. Liu, J.M. Ma, H.M. Cheng, Z.G. Zhao, Coll. Surf. A 143 (1998) 59. [4] H.R. Rabie, D. Helou, M.E. Weber, J.H. Vera, J. Coll. Interf. Sci. 189 (1997) 208. [5] Z.-J. Yu, R.D. Neuman, Langmuir 11 (1995) 1081. [6] Q. Li, S.-F. Weng, J.-G. Wu, N.-F. Zhou, J. Phys. Chem. 102 (1998) 3168. [7] Q. Li, T. Li, J.-G. Wu, N.-F. Zhou, J. Coll. Interf. Sci. 229 (2000) 298. [8] Q. Li, T. Li, J.-G. Wu, J. Phys. Chem. 104 (2000) 9011. [9] Q. Li, T. Li, J.-G. Wu, J. Coll. Interf. Sci., 239 (2001) 522. [10] A.N. Maitra, H.-F. Eicke, J. Phys. Chem. 85 (1981) 2687.
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