J . Phys. Chem. 1992, 96, 950-961
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Figure 7. Number of octanoate molecules per micelle as a function of
the number of solubilized octane molecules. The sodium octanoate concentrationwas 20% before the solubilization started. molecule to the sodium octanoate micelle may, at high amphiphile concentrations, increase the micellar radius from 11 to 15 A. The increase in the micellar radius will give rise to a marked increase in the number of amphiphiles per micelle, as can be seen in Figure 7, where the number of amphiphiles per micelle is plotted as a function of the number of solubilized octane molecules per micelle. The amphiphile concentration in the system presented in Figure 7 is 20% at the start of the solubilization. The increase in the monomer number per micelle as a function of the number of solubilized molecules is not at all so pronounced for lower
7. Conclusions The results in this investigation may be summarized in the following way. If the micelle has a low charge density or if there is a high salt concentration in the system, hydrophobic, nonpolar molecules such as hydrocarbons are always solubilized in the interior of a micelle due to the large reduction in the interfacial energy thus produced. For systems with highly charged micelles, such as micelles of single-chained soaps, considerable amounts of hydrocarbons may also be solubilized in the palisade layer near the micellar surface. The reason is the favorable mixing with the amphiphiles and the reduction of the surface charge density. Polar molecules such as lona-chained alcohols are solubilized mainly in the palisade layer i n the case of micelles with a high surface charge density. An exception is the first solubilized molecule per micelle that is often solubilized in the interior of the micelle due to the considerable reduction of the micellar interfacial area per amphiphile attained by placing a molecule in the center of the micelle. A considerable amount of a polar solubilizate may also be solubilized in the interior of micelles with a low surface charge density.
Acknowledgment. Michael Aamodt thanks the Rogaland Research Institute, Stavanger, Norway for financial support through the Norwegian state project SPOR. Regi9t1-y No. Octane, 111-65-9;sodium octanoate, 1984-06-1; octanol, 11 1-87-5.
Solubillzatlon of Uncharged Molecules in Ionic Surfactant Aggregates. 2. Phase Equilibria Mikael Landgren, Michael Aamodt, and Bengt Jiinsson* Division of Physical Chemistry I , Chemical Center, University of Lund, P.O.Box 124, S-221 00 Lund, Sweden (Received: May 13, 1991; In Final Form: September 18, 1991)
This is the second of two papers dealing with the solubilization of nonionic solubilizates into ionic surfactant aggregates. In the first paper the solubilization into micellar aggregates was treated. Now we shall look at equilibria between the micellar phase and other phases, the lamellar phase and the reversed micellar phase. The model used is an extension of a model presented earlier dealing with three-component systems: ionic surfactant-water-long-chain4 alcohol (JBnsson, B.;WennerstrBm, H. J . Phys. Chem. 1987, 91, 338). The main new development is that the description of the micelle has been refined to also deal with solubilization of nonpolar solubilizates such as hydrocarbons. The micelle has been divided into two regions; one core region and one surface region. The polarity of a solubilizate is modeled by the difference in standard chemical potential of the solubilizate in the two regions, which is calculated from the surface tension between pure solubilizate and water. The phase boundary of the micellar phase against pure octane has been calculated. The effect of changing the volumeto-length ratio of the amphiphile is investigated. Phase diagrams for the system potassium decanoate-water-solubilizate are then calculated. The polarity of the solubilizateis gradually changed from that of octanol to that of octane. The general conclusions are as follows: (i) The solubility of octane in the micellar phase is increased as the ratio volumeto-length of the amphiphile is increased. (ii) The major reason for the much larger lamellar phase in the octanol compared with the octane phase diagram is that the polar head-groups of octanol are solvated by water, making it more favorable for the octanol to be in the lamellae than in a bulk solubilizate phase or in the core of the micelle. (iii) For the same reason there is a large reversed micellar phase in the octanol phase diagram where the head-groups of the octanol is in contact with the aqueous core of the micelle, while it is more favorable for the octane to form a pure solubilizate bulk phase.
1. Introduction
This is the second of two papers treating the solubilization of water-insoluble (or slightly soluble) molecules into ionic surfactant aggregates. The main aim of the first paper' was to clarify how different molecular properties of the amphiphile and the solubilizate affect the localization of the solubilizate in a micelle. The main aim of this paper is to clarify how different molecular (1) ,Aamodt, M.; Landgren, M.; JBnsson, B. J. Phys. Chem., preceding
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properties of the amphiphile and the solubilizate affect the extension Of the different phases in the three-component phase diagram: ionic surfactant-water-solubilizate. The ability to solubilize molecules that are normally slightly in an aqueous solution is a very important Property of Surfactant SOlUtiOnS, and this property is used in many different Processes, e&, in cleaning,' in RliCellar Catalysis? in paint man(2) BachtrBm, K. Thesis, University of Lund, Lund, Sweden, 1987. (3) OConnor, C. J.; Fendler, E. J.; Fendler, J. H. J. Am. Chem. Soc. 1973, 95, 600.
0022-3654/92/2096-950$03.00/00 1992 American Chemical Society
Uncharged Molecules in Ionic Surfactant Aggregates. 2
The Journal of Physical Chemistry, Vol. 96, No. 2, 1992 951 Octone
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Figure 1. Experimental phase diagrams of the ternary systems: (a) sodium octanoate (caprylate)-octanol-water; (b) sodium octanoate (caprylate)-octane-water at 20 O C , from ref 6.
ufacture, to mention only a few. It is not only important to understand the basic mechanism behind the solubilization phenomena when trying to construct a new detergent system for some specific technical application, for instance, but also more generally in biochemistry and medicine to understand the properties of different types of naturally occurring amphiphilic molecules. It has been observed experimentally that different liquidcrystalline phases may occur over large concentration domains in a three-component system consisting of an ionic amphiphile, water, and a long-chain alcohol$ see Figure 1. The reason for this rich phase behavior is fairly well understood today, and the equilibria between the different phases can be reproduced by the thermodynamic model presented in ref 5. The rich phase behavior is, however, drastically reduced when the long-chain alcohol is replaced by a hydrocarbon molecule, for instance, octane;4 see Figure 1. The reason for this reduction is less well understood. The previously developed thermodynamic model5 can describe only the solubilization of relatively polar molecules. The new thermodynamic model presented in the first paper in this series' is able to describe also the solubilization of nonpolar solubilizates. This is made possible by refining the description of the micelle. The micelle is divided into two regions, the core and the palisade layer. Depending on the polarity of the solubilizate it is preferentially solubilized in either the one or the other of these two regions. In the first paper we dealt only with the solubilization in micellar solutions. In this work the new model is combined with the thermodynamic model presented earlier for the liquid crystalline phases and the reversed micellar phase.5 It is not only the thermodynamicsof the micellar phase that must be treated when discussing the solubilization capacity of a micellar system, since the stability range of the micellar phase also depends on the thermodynamic properties of other types of phases that may be in equilibrium with the micellar phase (Ll).We will in this work calculate phase equilibria between the micellar phase and other phases. We will not treat all the different types of amphiphilic structures that may be in equilibrium with the micellar phase but instead concentrate on two typical structures, the reversed micellar phase (LJ and the lamellar phase (D). The lamellar phase is here chosen to represent a liquid-crystallinephase. In this paper, the different phases are first presented together with a discussion on how the systems have been modeled geometrically and which simplifying assumptions have been made. The different contributions to the free energy taken into account are then presented, emphasizing the contributions not discussed in paper 1. The model is then used in the last part of the paper to show how the stability range of the micellar phase depends on ~
~~~~
(4) Ekwall, P. M u . Liq. Cryst. 1974, 1, 1. (5) Jiinsson, B.; Wennerstriim, H. J . Phys. Chem. 1987, 91, 338. (6) Ekwall, P.; Mandell, L.; Fontell, K. Mol. Cryst. Liq. Cryst. 1969,8, 157.
Figure 2. Schematic representationof a cell in the normal micellar phase.
different molecular properties of the amphiphile and of the solubilizate and how the extension of the different phases in the ternary phase diagram is affected by changing the polarity of the solubilizate gradually from a pure hydrocarbon to a long-chained alcohol. 2. TheMicelle
A micelle is a dynamic, fluctuating entity which changes in both size and shape?V8 Also the mean properties vary with parameters such as the total concentration and the temperature. Close to the cmc micelles are often spherical, but at higher concentrations, especially in the presence of a third component, they may grow, forming nonspherical aggregates. To describe such a system in molecular detail is very difficult, and simplifications must be introduced to obtain a model that is both computationally feasible and susceptible to qualitative analysis of model effects. The main simplifications introduced in the model used here are as follows: (1) The amphiphilic aggregates contain sufficiently many molecules so that the properties of the aggregates approach those of a macroscopic system, i.e., the effects of the complex molecular interactions can be described with a few parameters due to averaging effects. (2) The interior of a micelle is assumed to have a fully fluid character. No counterions, charged head-groups, or water molecules are allowed inside the micelle. (3) The micelle is assumed to consist of two regions, the inner region, here called the core, with only solubilizate molecules and the outer region, here called the palisade layer, with both amphiphile and solubilizate molecules (Figure 2). In the palisade layer all molecules are anchored at the micellar surface. This sets an upper limit for the thickness of the palisade layer equal to the length of an extended surfactant chain. (7) Wennerstriim, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (8) Halle, B.; Landgren, M.; Jiinsson, B. J. Phys. (Paris) 1988,49, 1235.
Landgren et al.
952 The Journal of Physical Chemistry, Vol. 96, No. 2, 1992
cell border
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Figure 3. Schematic representation of a lamellar system.
(4) All micelles in a system have the same radius (aggregation number) and are always assumed to be spherical. Fluctuations in shape and size are ignored. The most severe limitation in this model is probably the restriction that we assume that the micelle is always spherical. This restriction prevents the pure surfactant micelle that has no core region from growing. In fact micelles want to grow in many systems, especially at high amphiphile concentrations.
3. Lamellar Phase The lamellar liquid-crystalline phase consists of alternating water layers and bilayers of ionized surfactant molecules with solubilized molecules oriented with their hydrophilic groups facing outward towards the surrounding water layers (Figure 3). The interior of the amphiphile layers has in various investigations been characterized as a hydrocarbon region of fully fluid character. The interfacial area per amphiphile/solubilizate with the surrounding water is often at a constant minimum value of 25 A2 determined by the packing of the fluid chains and independent of the thickness of the water layer. For highly charged systems a significantvariation of the area per amphiphile can be observed.4 This effect is discussed in section 5.9 and in refs 5 and 9. The assumptions made in our model to describe a lamellar phase are as follows: (1) The lamellar phase consists of alternating layers of water and bilayers of amphiphile with solubilizate. The layers have an infinite extension in two dimensions. An amphiphile layer is assumed to consist of only one region, a surface region much like the palisade layer in the micellar phase, where the amphiphiles and solubilized molecules are mixed. It is assumed that no region . with only solubilizate is present. It can be shown that with the assumptions made in the model and with the energy contributions taken into account, such a region is unstable compared to the pure solubilizate.l o (2) The interior of the layers is assumed to have a fully fluid character. No counterions, charged head-groups, or water molecules are present inside the layers. (3) All amphiphilicand solubilized molecules in a bilayer are anchored at the interface toward the water.
4. Reversed Micellar Phase Less is known about the aggregation behavior in the isotropic L2 phase. In our treatment the reversed micelle is assumed to consist of an aqueous core with water and counterions. The core is surrounded by a palisade layer of amphiphilic molecules and molecules penetrating from an outer continuous bulk solution. The bulk solution consists of solubilizate with dissolved water. When the solubilizate is a hydrocarbon the amount of dissolved water (9) Jiinsson, B.; Wennerstriim, H. J . Colloid Interface Sci. 1981,80,48. (10) Jiinsson, B. Thesis, University of Lund, Lund, Sweden, 1981.
Figure 4. Schematic representation of a cell in the reversed micellar phase.
is very small,
