6220
J. Phys. Chem. 1995,99, 6220-6230
Salt Effects on Nonionic Microemulsions Are Driven by AdsorptiodDepletion at the Surfactant Monolayer Alexey Kabalnov,*J Ulf Olsson, and Hhkan Wennerstrgm Physical Chemistry I , Chemical Center, University of Lund,Box 124, S-22100, Lund, Sweden Received: October 18, 1994; In Final F o m : January 3, 1995@
The effects of inorganic salts on the phase equilibrium of the H ~ O - C ~ ? E ~ T Csystem I ~ H ~in~the Winsor I11 region have been studied. In agreement with the previous findings of Kahlweit et al., a regular Hofmeister trend has been observed: adding “hydrotropic” salts (NaI, NaSCN) makes water, in a phenomenological sense, a less polar solvent, and the bicontinuous microemulsion phase becomes water-rich, whereas with “lyotropic” salts (NaF, NaC1, NaBr), an opposite trend is observed. This behavior was previously attributed to a modification of water solution properties, and the brine was modeled as a pseudocomponent. In this paper, we argue instead that the salt effects on the phase equilibrium have an interfacial origin and are due to the salt adsorptioddepletion at the surfactant monolayer. A simple model relating the increment of the monolayer spontaneous curvature to the salt depletion at the monolayer has been proposed. Direct measurements of the salt adsorptioddepletion have been conducted. NaF, NaC1, and NaBr are shown to desorb, whereas NaI and NaSCN are shown to adsorb at the monolayer of the bicontinuous microemulsion phase, the Henry constant increasing in the above series from negative (NaF, NaC1, Nal3r) to positive values (NaI, NaSCN), in a good conelation with the microemulsion phase behavior. The relationship between the phase behavior and the adsorptioddepletion at the surfactant monolayer has been shown to be of a general nature and by no way limited to the case of aqueous solutions of inorganic salts: the same trends have been found with other water-soluble (dextran) and oil-soluble (perfluorohexane) additives.
1. Introduction The salt effects on the water solubility of nonionic compounds have been of interest since the pioneering study of Hofmeister2 and have been extensively reviewed3 recently. Let us outline the main ideas. (i) Solubility of organic compounds in water can be adjusted by adding inorganic salts. Most of the salts decrease water solubility of organic solutes (salting-out phenomenon), while some of them (NaI, NaC104, NaSCN) have an opposite action (salting-in). According to the salting-out strength at the given molar concentration, the anions can be classified into the so-called Hofmeister s e r i e ~ : ~
I- > C10,- > SCN- (1) The effect of the cation nature is usually smaller than that of the anion. (ii) The Hofmeister series is universal in the sense that the order of the sequence (1) does not depend on the nature of organic solute and applies, for instance, to alcohols, polymers, nonionic surfactants, proteins, etc. (iii) Two hypotheses have been proposed to explain the Hofmeister series behavior. According to one school of t h o ~ g h t , ~salts - ~ affect the “solvent quality” of water or, in other words, its x parameter. Further mechanistic interpretations along these lines invoke the salt effects on the structure of water: the salts on the left-hand side of the series (1) are believed to be “structure makers” while those on the right-hand side, “structure breaker^".^ Within this concept, the brine is modeled as a pseudocomponent, and the salt partitioning effects are believed to be negligible for the phase behavior. In an alternative interpretation of this the salting-in and salting-out phenomena have an interfacial origin. Salts adsorb or desorb at the water-organic solute @Abstractpublished in Advance ACS Absrructs, March 15, 1995.
“interface”, producing an increment in the solute free energy and, thereby, modifying the phase equilibrium. The controversy between these two interpretations is far from being resolved. This paper deals with the salt effects on the phase behavior of nonionic microemulsions. From a more general standpoint, salt effects on micellization and clouding of poly(ethy1ene oxide)-type nonionic surfactants have been studied both the~retically’~-’~ and experimentally.15-” In this study, however, we have been interested in the systems containing an oil as a third component, Le., microemulsions. In these systems, the surfactant molecules are located at the interface between the oil and water domains of various topology (spheres,lamellae, cylinders, bicontinuous structures). During the past few years, a better understanding of the phase behavior of nonionic microemulsions has been Figure 1 shows a typical phase diagram of a oil-water-ethylene oxide surfactant ternary system close to the balanced state. Let us move along the 1:l oil-to-water line in the direction of higher surfactant concentration. After a narrow two-phase region at very low surfactant concentrations, we cross a threephase triangle (so-called Winsor I11 region) where the bicontinuous microemulsion phase (m) (point A) coexists with a dilute solution of surfactant in water (1) (point B) and in oil (u) (point C).22 At higher surfactant concentrations, the bicontinuous microemulsion phase-(bicontinuous microemulsion phase lamellar phase)-lamellar phase sequence is observed. For our study, the Winsor 111three-phase region is of a crucial importance; let us consider it in more detail. The shape of the Winsor I11 three-phase triangle is strongly temperature dependent. An increase in temperature leads to a shift of point A to the oil comer until, at some temperature, the three-phase domain disappears. These temperature effects are attributed to the temperature dependence of the properties of the surfactant monolayer at the interface between the oil and water microdomains. Raising the temperature makes water a worse solvent
QQ22-3654/95/2099-6220~09.0Ql0 0 1995 American Chemical Society
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Salt Effects on Nonionic Microemulsions
oil
n
m-phase
water Figure 1. Schematic phase diagram of H ~ O - C I ~ E ~ - C I system O H ~ ~at 38.2 “C. Here u is the upper phase (dilute solution of C12E5 in oil), 1 is the lower phase (dilute solution of C12E5 in water), m is the middle phase (bicontinuous microemulsion), and La is the lamellar liquid crystalline phase. Triangle ABC is the Winsor 111 three-phase coexistence region of u-, m-, and l-phases. Only the phase regions relevant to our study are represented. The two-phase region at very low surfactant concentrations is not shown.
for poly(ethy1ene o ~ i d e ) ; as ~ ~a ,result, ~ ~ the surfactant polar heads contract, and the monolayer changes its spontaneous curvature (Le., bends toward water). For each microemulsion system, there is a specific temperature range of the three-phase body existence. Obviously, this range is dependent on the nature of the surfactant used. In this paper, we are however mainly interested in the effects of the nature of the “oil” and “water” phases24on the phase equilibrium and assume the nature of surfactant to be fixed. In general, using a more polar hydrocarbon as an oil component produces a shift of the three-phase body to lower temperatures. Even just a decrease of hydrocarbon chain length in the homologous series of aliphatic hydrocarbons produces a relatively large effect.25 Substituting water for a less polar solvent (e.g., formamide) has an opposite effect and the three-phase body moves to higher temperatures.26 The salt effects on the phase behavior of nonionic microemulsions formed by polyethoxylated surfactants have been extensively studied before in a series of papers by Kahlweit et al.25*27The authors found a regular Hofmeister trend: the “lyotropic” salts (NaCl) were shown to make water, in a phenomenological sense, a more polar solvent (the three-phase body moves to lower temperatures), whereas with “hydrotropic” salts (NaC104), an opposite trend is observed. The authors showed that hydrotropic salts are similar in their action to surface-active additives, e.g., sodium dodecyl sulfate, although the latter is effective at much lower concentrations. Despite a detailed and careful phenomenological description of the phase behavior, Kahlweit et al. made no attempts to interpret their results on a microscopic level. The objective of this paper is to show that the salt effects on the phase behavior of nonionic microemulsions have an
J. Phys. Chem., Vol. 99, No. 16, 1995 6221 interfacial origin and are due to the salt adsorptioddepletion at the microemulsion monolayer. Conceptually, this approach is similar to the one due to Aveyard? Schellman,8 and Piculell and Nilsson9-12 proposed for other systems. In the present paper we show experimentally that a brine in nonionic microemulsions is not a pseudocomponent: the salt-to-water ratio in the microemulsion phase is not the same as in the coexistent water phase. For lyotropic salts, the microemulsion phase is depleted in salt, wherem for hydrotropic salts, it is enriched in salt. A microemulsion phase has a definite internal structure: it consists of oil and water domains separated by the surfactant monolayer. Therefore, the (positive or negative) excess amount of salt in this phase has a clear-cut physical sense of an adsorption or depletion at the microemulsion monolayer. This effect, albeit small (for the salts studied, the salt-to-water ratio in the microemulsion and the water phase differs by a few percent), produces a shift in the monolayer spontaneous curvature that is sufficient to explain the microemulsion phase behavior. We argue that such a behavior is not an intrinsic feature of inorganic salts, or aqueous systems, but is related to adsorption or depletion at the surfactant monolayer. This paper has the following outline. First, we consider the binding isotherms of inorganic salts (NaF, NaCl, NaBr, NaI, NaSCN) and dextran to the H ~ O - C I ~ E ~ - C I O microemulsion H~~ monolayer. In the second part, we discuss the microemulsion phase equilibrium changes produced by these salts versus their partitioning between the microemulsion phases. In the third part, we present a simple model relating the increment of the monolayer spontaneous curvature, produced by a “salt”, to the value of depletion at the monolayer28 and compare the theory with experiment.
2. Experimental Section 2.1. Materials. C 12E5 (pentaethyleneglycol mono-n-dodecyl ether, Nikko Chemicals Co., GC pure, water content 0.33%, ignition residue 0.003%), n-decane (Sigma, 99%), 1-decene (nCsH19=CH2, Aldrich, 94%, main impurities branched decenes and decane), and n-perfluorohexane (n-C&4, 95%, Aldrich, main impurities perfluoropentanes) were used without further purification. The salts NaF (J. T. Baker, 99.3%), NaCl (Sigma, ACS Reagent grade, >99.9%), NaBr (J. T. Baker, 99.3%), and NaI (Merck, “Suprapur” >99.9%), NaSCN (Mallinckrodt, analytical grade, >99.9%) where dried in oven at 110 “C for 1 day before use. For the colorimetric determination of thiocyanate, copper(II) sulfate pentahydrate (CuS045H20, J. T. Baker, 99.9%), pyridine (Mallinckrodt, analytical grade), and chloroform (Merck, 99%) were used as received. Dextran PL 1s (Heifer & Langren Dormagen, Adn = 918, M , = 1092), which will be referred to below as dextrose hexamer (DXg), was used as received. According to a chromatogram supplied by the manufacturer, the sample contained 90% of tetramer-decamer mixture, the rest of the sample being lower oligomers. In all the experiments, Millipore-filtered water was used. 2.2. Methods. Phase Diagrams. As a reference microemulsion system, we used the H ~ O - C I ~ E ~ - C Imicroemul~H~~ sion system close to the balanced state (38.2 “C); see Figure 1. At the surfactant concentration used (3.4% v/v), and in absence of salts, this system is in a Winsor 111three-phase equilibrium. The upper phase represents a relatively weak (1.5% v/v) surfactant solution in oil; the lower phase is essentially pure water, with C12E5 concentration being below 0.01% V / V .The ~~ middle bicontinuous phase contains equal volumes of oil and water and ca. 7% v/v of C12E5. The samples with added salts (or dextran) were prepared in a following manner. A 555 mg sample of a 8.64% solution of Cl2E5 in decane and 750 mg of
Kabalnov et al.
6222 J. Phys. Chem., Vol. 99, No. 16, 1995
n U
+
0 0
0.02 0.04 0.06 0.08 0.1 0.12 C*,, 4000
8000
12000
h rS/m
Figure 3. Increment of lower phase conductivity with respect to that of stock solution, dA (,uS/cm), vs NaCl concentrationin stock solution, csall(M). Conductivity of NaCl stock solution, A, is shown as second scale on the X-axis. Note the nonzero conductivity increment in the blank experiment due to ionic impurities in C I ~ E S .
Csalt = 1% Acsalt = 0.01% Figure 2. Scheme of salt partitioning experiment.
a brine (or a dextran solution) were placed into 5 mm NMR tubes which were immediately flame-sealed. The samples with added 1-decene or perfluorohexane were prepared in a similar manner, with the difference that 750 mg of a 6.45% C12E5 solution in water and 555 mg of a CloH20 or c6F14 solution in decane were mixed. The samples prepared were ca. 1:1 in oilto-water volume ratio. After sealing, the samples were placed into a thermostated bath where they were slowly rotated for several hours to ensure mixing the components at the given temperature (38.2 f 0.1 "C). After this procedure, the samples were rapidly transferred into a double-walled glass beaker filled with water and thermostated by water flow of an MGW thermostat (Lauda) at the same temperature. The equilibrium was usually established within a few days. Relative volumes of the coexisting phases were measured by using a ruler. Partitioning Measurements. ( i ) The Basic Procedure. The amount of a salt adsorbeddepleted by the microemulsion phase was measured by controlling the salt concentration in the coexistent lower (water) phase. We measured the difference in the salt concentrations of the stock salt solution (prior to addition of the oil and surfactant) and that of the corresponding lower phase (Figure 2). This procedure is based on the fact that the lower phase is essentially free of oil and surfactant. The surfactant concentration is low ((0.01 %) and remains so after the salt has been added. Decane cannot be detected at all (
