Microemulsions with Alkyldimethyl Phosphine Oxides and Alkyldiethyl

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Microemulsions with Alkyldimethyl Phosphine Oxides and Alkyldiethyl Phosphine Oxides Renate Tessendorf,†,‡ Reinhard Strey,‡ and Cosima Stubenrauch*,† UniVersity College Dublin, School of Chemical and Bioprocess Engineering, Centre for Synthesis and Chemical Biology (CSCB), SFI-Strategic Research Cluster in Solar Energy ConVersion, Belfield, Dublin 4, Ireland, and Institut fu¨r Physikalische Chemie, UniVersita¨t zu Ko¨ln, Luxemburger Str. 116, 50939 Ko¨ln, Germany ReceiVed July 21, 2008. ReVised Manuscript ReceiVed August 12, 2008 Alkyldimethyl phosphine oxides (CnDMPO) as well as alkyldiethyl phosphine oxides (CnDEPO) with chain lengths of n ) 10 (decyl), 12 (dodecyl), and 14 (tetradecyl) were synthesized and purified to study how the formation of microemulsions depends on the size of the headgroup and on the length of the alkyl chain. For that purpose, equal amounts of water and n-octane were taken and surfactant was added to solubilize the two solvents. The resulting fish-shaped phase diagrams for C10DEPO, C12DEPO, and C14DEPO show that the longer the hydrophobic chain the more efficient the surfactant. Simultaneously, the extension of the lamellar phase (LR) shifts toward lower total mass fractions γ of the surfactant, i.e., the tendency to form lyotropic liquid crystals (LCs) increases. These trends are well-known for nonionic alkyl ethylene oxides and can thus be interpreted accordingly. What is astonishing, however, is the significant influence the size of the short side chains has. Replacing two methyl groups by two ethyl groups leads to a drastic drop of the three-phase region toward lower temperatures, while the efficiency remains nearly unchanged. Moreover, the tendency to form LCs decreases significantly.

1. Introduction Although amphiphilic phosphine oxides have been known for quite a while,1,2 they have not received the scientific attention they deserve. This is quite surprising as phosphine oxide surfactants are chemically quite resistant, pH- and temperaturestable.1,3 Because of these properties, applications can be considered for which the use of the two traditional classes of nonionic surfactants, namely, alkyl polyglycol ethers CiEj and alkyl polyglycosides CnGm, is impossible or at least difficult. For example, the properties of aqueous solutions of phosphine oxide3,4 and CiEj5 surfactants, respectively, are strongly temperature dependent. However, while CiEj surfactants degrade at high temperatures, the respective phosphine oxides are stable. In the case of CnGm surfactants, ring-opening reactions occur at extreme pH values, while phosphine oxide surfactants are pH-stable. In addition, the synthesis and purification of CiEj and CnGm surfactants in laboratory-scale quantities are much more timeconsuming than the synthesis of phosphine oxide surfactants. [Note: The situation is, of course, different for industrial production. Technical-grade CiEj and CnGm surfactants with a broad distribution of both the hydrophobic and the hydrophilic parts are easy to synthesize on large scales.6] * Corresponding author. E-mail: [email protected], Tel: +3531-7161923, Fax: +353-1-7161177. † University College Dublin. ‡ Universita¨t zu Ko¨ln.

(1) Kosolapoff, G. M.; Watson, R. M. J. Am. Chem. Soc. 1951, 73, 4101. (2) Herrmann, K. W.; Brushmiller, J. G.; Courchene, W. L. J. Phys. Chem. 1966, 70, 2909. (3) Laughlin, R. G. The aqueous phase behaVior of surfactants; Academic Press: London, 1994. (4) Blunk, D.; Tessendorf, R.; Buchavzov, N.; Strey, R.; Stubenrauch, C. J. Surfact. Deterg. 2007, 10, 155. (5) Sottmann, T.; Strey, R. Microemulsions, In Soft Colloids V-Fundamentals in Interface and Colloid Science, Lyklema, J., Ed.; Elsevier: Amsterdam, 2005; Chapter 5. (6) Eskuchen, R.; Nitsche, M. Technology and Production of Alkyl Polyglycosides, In Alkyl Polyglycosides: Technology, Properties and Applications, Hill, K., von Rybinski, W., Stoll, G., Eds.; Wiley-VCH: Weinheim, 1997; Chapter 2.

There is a good deal of knowledge about the surface properties of dimethyl alkyl phosphine oxides with linear alkyl chains ranging from 9 to 14 carbon atoms. For example, extensive surface tension4,7-13 and surface rheology14-20 studies were carried out with a focus on the properties of C12DMPO. In addition, the binary water-surfactant phase diagrams of C8DMPO,2 C10DMPO,2 C12DMPO,2-4,21 and C14DMPO3,4,21 were studied, and the formation of microemulsions with C10DMPO, C12DMPO, and C14DMPO was investigated.21 Finally, there are studies on the interactions between C12DMPO surfactant layers adsorbed on hydrophobized mica22 and on the adsorption of C10DMPO and C12DMPO on silica.23,24 However, very few studies deal with the properties of the respective diethyl derivatives. Apparently, only the binary water-surfactant phase diagrams of C10DEPO and C12DEPO have been studied focusing on the occurrence of lyotropic liquid crystals (LCs) at high surfactant concentrations and on the miscibility gap.3 (7) Aksenenko, E. V.; Makievski, A. V.; Miller, R.; Fainerman, V. B. Colloids Surf., A 1998, 143, 311. (8) Dudnik, V.; Lunkenheimer, K. Langmuir 2000, 16, 2802. (9) Fainerman, V. B.; Miller, R.; Mo¨hwald, H. J. Phys. Chem. B 2002, 106, 809. (10) Fang, J. P.; Wantke, K.-D.; Lunkenheimer, K. J. Phys. Chem. 1995, 99, 4632. (11) Lunkenheimer, K.; Haage, K.; Hirte, R. Langmuir 1999, 15, 1052. (12) Lunkenheimer, K.; Haage, K.; Miller, R. Colloids Surf. 1987, 22, 215. (13) Makievski, A. V.; Grigoriev, D. O. Colloids Surf., A 1998, 143, 233. (14) Fainerman, V. B.; Miller, R.; Kovalchuk, V. I. J. Phys. Chem. B 2003, 107, 6119. (15) Fruhner, H.; Wantke, K.-D.; Lunkenheimer, K. Colloids Surf., A 2000, 162, 193. (16) Kovalchuk, V. I.; Kra¨gel, J.; Makievski, A. V.; Loglio, G.; Ravera, F.; Liggieri, L.; Miller, R. J. Colloid Interface Sci. 2002, 252, 433. (17) Noskov, B. A.; Alexandrov, D. A.; Miller, R. J. Colloid Interface Sci. 1999, 219, 250. (18) Noskov, B. A.; Grigoriev, D. O.; Miller, R. J. Colloid Interface Sci. 1997, 188, 9. (19) Wantke, K.-D.; Fruhner, H. J. Colloid Interface Sci. 2001, 237, 185. (20) Wantke, K.-D.; Fruhner, H.; Fang, J. P.; Lunkenheimer, K. J. Colloid Interface Sci. 1998, 208, 34. (21) Pospischil, K.-H. Langmuir 1986, 2, 170. (22) Herder, C. E. J. Colloid Interface Sci. 1991, 143, 1. (23) Pettersson, A.; Rosenholm, J. B. Langmuir 2002, 18, 8436. (24) Pettersson, A.; Rosenholm, J. B. Langmuir 2002, 18, 8447.

10.1021/la802333a CCC: $40.75  2008 American Chemical Society Published on Web 09/24/2008

Microemulsions with CnDMPO and CnDEPO

Figure 1. Molecular structures of (left) n-alkyldimethyl phosphine oxides (CiDMPO) and (right) n-alkyldiethyl phosphine oxides (CiDEPO).

To fill this gap, we synthesized and purified alkyldimethyl phosphine oxides (CnDMPO) with chain lengths of n ) 8 (octyl), 10 (decyl), 12 (dodecyl), and 14 (tetradecyl), as well as alkyldiethyl phosphine oxides (CnDEPO) with chain lengths of n ) 10, 12, and 14.4 The respective molecular structures are shown in Figure 1. In our previous study, we investigated how the adsorption properties and the location of the miscibility gap of these surfactants depend on the size of the headgroup and on the length of the alkyl chain, respectively.4 We measured the surface tension isotherms from which the cmc, the minimum surface tension σcmc, the maximum surface concentration Γmax, and the minimum surface area Amin were obtained. As expected, for one homologous series a significant decrease of the cmc (∼factor of 10 per -CH2-CH2- unit), a slight decrease of σcmc, and an increase in Γmax, i.e., a decrease in Amin, were observed with increasing chain length. Comparing two surfactants of the same alkyl chain length, one finds that the cmc values of the CnDMPO surfactants are about two times higher than those of the corresponding CnDEPO surfactants. This observation can easily be explained by the higher water solubility and thus the lower surface activity of the CnDMPO surfactants. On the other hand, the Γmax values of the CnDMPO surfactants are larger than those of the CnDEPO surfactants, although the opposite trend would have been expected if the cmc played the main role (usually a lower cmc is mirrored in a higher Γmax). An explanation for this result is the larger headgroup of the diethyl derivatives which requires more space than the dimethyl headgroup. In addition to the adsorption properties, we studied the location of the miscibility gap as a function of both the alkyl chain length and the headgroup size. Again, for one homologous series one obtains the expected general trends with increasing alkyl chain lengths, namely, a shift of the miscibility gap toward much lower temperatures and concentrations. Comparing two surfactants of same alkyl chain length but different headgroup sizes, one also observes a significant shift toward lower temperatures if one replaces the two methyl side chains by two ethyl groups, while the concentration shift is not very pronounced. Both observations simply reflect the decreasing water solubility which can be tuned by increasing the length of either the main alkyl chain or of the shorter headgroup chains. Finally, the miscibility gaps of two surfactants with equal total number of carbon atoms are located at similar temperatures, while they are different regarding the concentration range. On the basis of the knowledge gained in our previous work, we wanted to go one step further and answer the questions: Are CnDEPO surfactants as efficient as CnDMPO surfactants regarding the formation of microemulsions? Is it also the total number of carbon atoms and not primarily the length of the main alkyl chain that determines the temperature range of microemulsion formation? As phase diagrams of ternary microemulsions depend on the miscibility gaps of the three binary systems water-surfactant, water-oil, and oil-surfactant,5 it was interesting to see to what extent the trends observed for the miscibility gaps of the binary water-surfactant systems4 are reflected in the respective phase diagrams of the microemulsions. To answer these questions, ternary phase diagrams of water-n-octane-CnDEPO and

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water-n-octane-CnDMPO were studied. Moreover a detailed analysis about the monomeric solubilities of phosphine oxide surfactants in both water and oil was carried out. We will compare the results and discuss differences and similarities in terms of the respective molecular structures.

2. Materials and Methods 2.1. Materials. n-Octane (purity of 99%) was purchased from Sigma-Aldrich. Water was purified by a Milli Q system or alternatively doubly distilled. The synthesis and purification of all surfactants is described in our previous study.4 2.2. Phase Behavior. The masses of the components water (A), oil (B), and surfactant (C) are denoted m(A), m(B), and m(C), respectively. The composition of the samples is given by the mass of oil in the water plus oil mixture

R)

m(B) m(A) + m(B)

(1)

and the overall mass fraction of the surfactant

γ)

m(C) m(total)

(2)

Various amounts of the components were weighed into test tubes containing a magnetic stirring bar and sealed with polyethylene stoppers. All samples were prepared at a 1:1 water-to-oil mass ratio, i.e., at R ) 0.5. The phase behavior of all microemulsions was studied as a function of the temperature T and of the surfactant mass fraction γ in a thermostatted water basin. Phase boundaries were determined with a precision of (0.5 K. The number of coexisting phases was determined by visual inspection in transmitted light. Crossed polarizers were used to detect the presence of anisotropic phases. Furthermore, the volumes of the coexisting phases in the three-phase region were measured.

3. Results and Discussion 3.1. Phase Diagrams. At constant pressure, the phase behavior of ternary systems can be represented in an upright phase prism with an isothermal Gibbs triangle water(A)-oil(B)-nonionic surfactant(C) as the base and the temperature T as the ordinate. To investigate the phase behavior, sections are made through the phase prism, usually at constant water-to-oil ratio R or at constant surfactant mass fraction γ. In the first case, one studies the phase diagram as a function of the temperature T and the surfactant mass fraction γ, which leads to a fish-shaped phase diagram as schematically drawn in Figure 2. The surfactant mass fraction γ0 at the fish-head is a measure of the monomeric solubility of the surfactant in both solvents. It is only at γ > γ0 that the system has a microstructure and can thus be called a microemulsion. We will see in section 3.2 that the knowledge of γ0 allows us to determine the monomeric oil solubility of the respective surfactant. At γ > γ0 and at low temperatures, an oil-in-water microemulsion coexists with an excess oil phase (2), whereas at high temperatures, a water-in-oil microemulsion coexists with an excess water phase (2j). At intermediate temperatures and low surfactant mass fractions (γ0 < γ < γ˜ ) a bicontinuous microemulsion coexists with an excess water phase and an excess oil phase (3). By increasing γ, more water and oil can be solubilized by the microemulsion phase, as is indicated by the test tubes. At the characteristic surfactant mass fraction γ˜ , the system is able to completely solubilize water and oil; hence, a one-phase microemulsion appears (1). The point where the three- and one-phase regions meet is called the X˜ point or fish-tail, which is defined by the coordinates γ˜ and T˜. This point is of utmost importance, as it represents the minimum surfactant mass fraction needed to form a one-phase micro-

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Figure 2. Schematic phase diagram of a ternary system H2Oalkane-CiEj for equal volumes of water and oil as a function of the surfactant mass fraction γ and the temperature T. The surfactant mass fraction γ0 at the “fish-head” corresponds to the monomeric solubility of the surfactant in the solvents. The X˜-point (“fish-tail”) is defined by the surfactant mass fraction γ˜ for which at T˜ the one-phase region (1) is reached. 2, 3, and 2j denote water-, surfactant-, and oil-rich microemulsion in equilibrium with the corresponding excess phase(s). The test tubes show the number of coexisting phases at different T and γ (white ) excess phases, gray ) microemulsion). Three typical microstructures are also shown, namely, oil-in-water droplets (T < T˜), water-in-oil droplets (T > T˜), and bicontinuous microemulsions (T ∼ T˜).

emulsion and is thus a measure of the efficiency of a surfactant. See ref 5 and references therein for further details. 3.1.1. Alkyldimethyl Compared to Alkyldiethyl Phosphine Oxide Surfactants. 3.1.1.1. Influence of the Chain Length. Before presenting and discussing our results, it has to be mentioned that, strictly speaking, the symmetric fish-shaped phase diagram seen in Figure 2 is only obtained if equal volumes rather than equal masses of the two solvents are used. However, as we wanted to compare our results with the pioneering work of Pospischil,21 we decided to also use equal masses, i.e., R ) 0.5. Note that equal volumes of water and n-octane would correspond to R ) 0.41. Although the X˜-point of a phase diagram measured at R ) 0.41 would have been slightly shifted toward lower temperatures and surfactant mass fractions compared to that measured at R ) 0.5,25,26 the overall shape of the phase diagram is not affected significantly. Another reason for “asymmetric” fishshaped phase diagrams is the asymmetry of the miscibility gaps of the respective binary systems, which was discussed with one of the authors and the late Prof. H. Kunieda in 1985. While the lower critical point of the water-surfactant miscibility gap lies at very low surfactant concentrations, the corresponding upper critical point of the oil-surfactant miscibility gap lies at high surfactant concentrations, which, in turn, leads to a distortion of the phase boundaries. An example for this distortion can be seen below (see C10DEPO in Figure 4). The fish-shaped phase diagrams of the alkyldimethyl phosphine oxides C12DMPO and C14DMPO are shown in Figure 3, while those of the alkyldiethyl phosphine oxides C10DEPO, C12DEPO, and C14DEPO are shown in Figure 4. The corresponding coordinates of the fish-heads and the fish-tails are listed in Table 1. Note that a determination of γ0 via optical inspection is difficult and not very precise. Thus, this value was extracted from measurements of the volume of the middle phase as will be explained in section 3.2. The phase diagram of C10DMPO could not be determined, as the surfactant is too hydrophilic, i.e., the fish-shaped phase diagram would be located at temperatures above 100 °C if phase diagrams at these temperatures were measurable. (25) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. PhysChemChemPhys 1999, 1, 4299. (26) Burauer, S.; Belkoura, L.; Stubenrauch, C.; Strey, R. Colloids Surf., A 2003, 228, 159.

Figure 3. Phase diagrams of the ternary systems H2O-noctane-C12DMPO (top) and H2O-n-octane-C14DMPO (bottom) at a water-to-oil mass ratio of R ) 0.5. Anisotropic lamellar (LR) and hexagonal (H1) phases were observed by optical inspection through crossed polarizers.

It has to be mentioned that our results for C12DMPO and C14DMPO are in perfect agreement with those obtained by Pospischil,21 which further supports the purity of our surfactants.4 Looking at Figures 3 and 4, one clearly sees three effects that the increase of the alkyl chain length has on the phase behavior. First, the longer the hydrophobic alkyl chain, the lower the amount of surfactant required to solubilize equal amounts of water and oil in one phase (1). In other words, the longer the hydrophobic chain, the more efficient the surfactant, which is also reflected in the coordinates of the X˜ points (see Table 1). Second, all phase boundaries are shifted toward lower temperatures. Third, the extension of the lamellar phase (LR) shifts toward lower total mass fractions γ of phosphine oxide, i.e., the tendency of the system to form lyotropic liquid crystals increases. We did not determine the two-phase region H1 + 1 for C12DMPO, C14DMPO, and C14DEPO as the temperature ranges are very small and as the focus was on the microemulsion formation. To study the extension of the LC phases, the type of LC phase and especially the coexistence of microemulsion and LC phase 2H NMR measurements are very helpful as we have shown in previous studies on similar systems.27-29 One observation we want to mention is the fact that the maximum extension of the LR phase does not appear at T˜ but at lower temperatures. This has also been observed in ref 21 and indicates that the mean curvature H of the surfactant monolayer does not stay constant at T˜ but becomes negative with increasing γ. Thus H ) 0 is at T < T˜. This observation is certainly worth examining more closely. These three trends are well-known for nonionic alkyl ethylene oxide (27) Stubenrauch, C.; Frank, C.; Strey, R.; Burgemeister, D.; Schmidt, C. Langmuir 2002, 18, 5027. (28) Frank, C.; Sottmann, T.; Stubenrauch, C.; Allgaier, J.; Strey, R. Langmuir 2005, 21, 9058.

Microemulsions with CnDMPO and CnDEPO

Figure 4. Phase diagrams of the ternary systems H2O-n-octaneC10DEPO (top), H2O-n-octane-C12DEPO (middle), and H2O-noctane-C14DEPO (bottom) at a water-to-oil mass ratio of R ) 0.5. Anisotropic lamellar (LR) and hexagonal (H1) phases were observed by optical inspection through crossed polarizers.

surfactants5,25,30,31 and can thus be explained in the same way. Briefly, the longer the alkyl chain, the more densely packed and thus the more rigid the monolayer is at the water/oil interface, which, in turn, leads to both a higher efficiency and a higher tendency to form lyotropic liquid crystals. The temperature shift can be explained by the increasing hydrophobicity of the surfactant. The higher the hydrophobicity, the lower the degree of dehydration of the headgroup required to change the curvature of the monolayer from positive to negative. In other words, the higher the hydrophobicity, the lower the temperature at which the dehydration is sufficient to change the curvature. A more detailed comparison between phosphine oxide and ethylene oxide surfactants will be given in section 3.1.2. 3.1.1.2. Influence of the Head Group Size. After having studied the influence of the chain length, we will now focus on the (29) Frank, C.; Strey, R.; Schmidt, C.; Stubenrauch, C. J. Colloid Interface Sci. 2007, 312, 76. (30) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (31) Lade, O.; Beizai, K.; Sottmann, T.; Strey, R. Langmuir 2000, 16, 4122. (32) Sottmann, T.; Strey, R.; Chen, H.-S. J. Chem. Phys. 1997, 106, 6483. (33) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (34) Burauer, S.; Sottmann, T.; Strey, R. Tenside, Surfactants, Deterg. 2000, 37, 8.

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influence the headgroup has on the phase behavior. With regard to the classical CiEj surfactants, an increasing headgroup renders the surfactant more hydrophilic, which, in turn, shifts the X˜ points to higher temperatures and surfactant mass fractions. Looking at the structure of the phosphine oxide surfactants, one sees that an increase in the headgroup can only happen if one exchanges the two methyl groups. In the present study, we exchanged the two methyl by two ethyl groups, which, in turn, renders the surfactant more hydrophobic, as already indicated by the cmc values and the miscibility gaps of the binary water-surfactant systems.4 If adding two methyl groups to the side chains had the same effect as adding them to the main alkyl chain, the phase diagrams of CnDEPO and Cn+2DMPO would be the same. Another consequence would be that the X˜-point shifts to lower temperatures and surfactant mass fractions. Finally, the formation of LC phases would be more pronounced for CnDEPO compared to CnDMPO. A look at the phase diagrams, however, tells us a different story. C12DEPO Compared to C14DMPO. Looking at Figures 3 and 4, one clearly sees that the same total number of C-atoms does not lead to the same phase diagrams. C14DMPO is by far more efficient than C12DEPO (much lower γ˜ ), and the LC phase formation is much more pronounced. However, the increased efficiency is not accompanied by a decrease but by an increase of T˜. (The same trends are seen if one compares C10DEPO to C12DMPO.) As the increase of T˜ cannot be explained by the miscibility gap of the binary water-surfactant systems, it can only be caused by differences in the binary oil-surfactant systems, which have not been investigated. However, even without knowing the binary oil-surfactant phase diagrams a reasonable explanation can be given. [Note: The miscibility gap of the binary system water-C14DMPO (water-C12DMPO) is at lower temperatures compared to that of the system water-C12DEPO (water-C10DEPO).] From surface tension measurements, the minimum surface areas were derived, namely, Amin ) 0.50 nm2 for C12DEPO and Amin ) 0.31 nm2 for C14DMPO.4 The corresponding Amin-values at the water/oil interface of the microemulsion are expected to be similar. An extensive SANS study revealed that the area per surfactant molecule at the water/oil-interface is independent of the oil and the length of the alkyl chain, while it increases linearly with the headgroup size.32 On the other hand, the area per surfactant molecule at the water/air-interface depends on both the length of the alkyl chain and the size of the headgroup. If the Amin-values were the tuning parameter, one would expect T˜(C12DEPO) > T˜(C14DMPO). As the opposite is observed, it can only be the actual surface area Aact that counts, which is the hydrated headgroup. As T˜(C12DEPO) < T˜(C14DMPO), the headgroup of DMPO is obviously much larger and much more energy for curvature changes is required. This can only be the case if the hydration shell of a DMPO headgroup is larger (more water molecules) than that of a DEPO headgroup. The reason for a higher hydration degree is most likely the higher polarity of the DMPO headgroup, which would also automatically lead to stronger surfactant-water interactions. It would be very interesting to quantify both the number of water molecules per headgroup and the hydration energy per water molecule. C12DEPO Compared to C12DMPO. For the sake of clarity, the phase diagrams of water-n-octane-C12DMPO (see Figure 3) and of water-n-octane-C12DEPO (see Figure 4) are replotted in Figure 5. Looking at Figure 5 and Table 1, one sees that the size of the short side chains has a significant influence on the phase behavior. Three points are important to mention. First, replacing two methyl groups by two ethyl groups leads to a

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Table 1. Coordinates of the “Fish-Head” (γ0,T˜) and the “Fish-Tail” (γ˜ ,T˜), respectively, for Various Water-oil-non-Ionic Surfactant Systemsa surfactant oil T˜/°C γ˜ γ0 γ˜ calc γ0,calc C10DMPO C12DMPO

n-octane n-octane

C14DMPO

n-octane

C10DEPO C12DEPO C14DEPO C14DMPO C14DMPO C8E5 C10E5 C12E5

n-octane n-octane n-octane n-decane n-dodecane n-octane n-octane n-octane

-b 81.7 ∼82.5 50.4 50.7c 62.4 41.0 25.2 61.7c 73.8c 61.8d 45.1d 32.6d

-b 0.145 ∼0.15c 0.033 0.036c 0.313 0.173 0.075 0.055c 0.085c 0.299d 0.149d 0.047d

-b 0.146

0.006

0.029

0.003

0.312 0.176 0.070

0.018 0.011 0.006

0.017d 0.008d 0.007d

a The index “calc” indicates that these values were determined via the relative volume fractions (see section 3.2 for details). All measurements with phosphine oxide surfactants were carried out at a water-to-oil mass ratio of R ) 0.5, while those with the CiEj surfactants (gray-shaded) were carried out at a water-to-oil volume ratio of φ ) 0.5. b Temperature >100 °C. c ref 21. d ref 25.

Figure 5. Same phase diagrams as in Figures 3 and 4 of the ternary systems H2O-n-octane-C12DMPO (top) and H2O-n-octane-C12DEPO (bottom).

drastic drop of the three-phase region and thus of T˜ to lower temperatures. Second, the efficiency of the system is not affected significantlysthe γ˜-values of the ethyl derivatives are only slightly higher than those of the corresponding methyl derivatives, which means that the CnDEPOs are a little bit less efficient. Third, the tendency of C12DEPO to form lyotropic liquid crystals is lower than that of C12DMPO, i.e., the LC regions are less extended. In other words, changing the size of the short side chains allows one to influence the extension of the LC phases without significantly affecting the efficiency. Thus, the modification of the short side chains is an additional parameter via which one can tune the phase behavior. The same trends are seen if one compares C14DEPO to C14DMPO. How can these observations be explained?

Temperature Shift. The shift to lower temperatures (over 40 °C) can be explained by the higher hydrophobicity of the ethyl derivatives and simply reflects the shift of the respective miscibility gaps.4 Note that the concentration shift of the miscibility gap is not very pronounced compared to the significant temperature shift (over 20 °C). This trend is also observed for the phase behavior of the respective ternary systems, which illustrates the direct correlation between binary base systems and the location of the three-phase region.5 If the explanation given above for the higher T˜-value of C14DMPO compared to C12DEPO was right, the smaller and weaker hydration shell of C12DEPO compared to C12DMPO would also contribute to the considerable shift to lower temperatures. Efficiency and Extension of LCs. Extensive measurements at the X˜ points of water-oil CiEj microemulsions illustrated that the lower the rigidity of the monolayer the less efficient the microemulsion.32 Moreover, the efficiency of a microemulsion and LC formation are directly correlated (see ref 5 and references therein). Thus. the observed shift to lower efficiencies and the lower tendency to form LC phases could be explained by a lower rigidity of a C12DEPO compared to a C12DMPO monolayer (see section 3.1.2). Anticipating that this is the case, one has to answer the question: Why is a DEPO monolayer less rigid than a DMPO monolayer? At the X˜ points, the average curvature is zero and the monolayer is packed as densely as possible. Due to the larger headgroup, it is impossible to pack a DEPO layer as densely as a DMPO layer, which, in turn, leads to a lower rigidity. The ethyl side chains of the DEPO headgroup can be considered as “branches” which are usually attached to the main alkyl chain in order to suppress LC formation. Another point that influences the rigidity is the interaction between the adsorbed surfactant molecules. The higher the interaction, the more densely packed and thus the more rigid is the monolayer. In the present case, however, it is most likely the steric constraint (ethyl versus methyl group) that dominates the packing and thus the rigidity. 3.1.2. Phosphine Oxide Compared to Ethylene Oxide Surfactants. Pospischil concluded his study with the following statement:21 “The results of these studies show that the phase behaviour of the ternary system H2O-oil-CiDMPO is quite similar to that with CiEj as nonionic amphiphile.” However, as Pospischil did not specify the “CiEj” with which he compared the CiDMPO, it is hard to tell what he was referring to. Since the publication of Pospischil’s study, an enormous amount of data have been generated that allow us to compare the properties of these two classes of nonionic surfactants in more detail. In order to do so, we checked all available data for the ternary systems water-n-octane-CiEj, which have been summarized in a convenient way by plotting the γ˜ -values versus the corresponding T˜-values.25 The result is a net with straight lines that connect surfactants of the same headgroup and alkyl chain, respectively. Looking at this net, one can immediately extract the CiEj surfactants that have the same X˜-point as the phosphine oxide surfactants (see also data in Table 1). It holds: CnDEPO: C10DEPO ∼ C8E5 C12DEPO ∼ C10E5 C14DEPO ∼ C12E5 CnDMPO : C12DMPO ∼ C11E8 C14DMPO ∼ C13E7

Comparing the two different types of surfactant, one clearly sees a relation between CnDEPO and CiE5. Note that the same efficiencies can be reached even if the alkyl chain length of the CiE5 surfactants is two methyl groups shorter than that of the corresponding CnDEPO [Note: The situation is different for systems with oils that penetrate the surfactant layer. In this case, high monomeric solubilities were found for very efficient systems.34 Thus, all considerations in the paper at hand refer to systems in which the oil is a linear n-alkane]. As already mentioned

Microemulsions with CnDMPO and CnDEPO

Langmuir, Vol. 24, No. 20, 2008 11395

above, it has been shown that, the higher the rigidity of the monolayer, the more efficient the microemulsion. According to SANS measurements in water-n-octane microemulsions, it holds for the rigidity constants of the CiE5 surfactants κ(C8E5) ) 0.47 kT < κ(C10E5) ) 0.65 kT < κ(C12E5) ) 0.92 kT,32 while the κ-values for systems of similar efficiency do not differ very much. Thus, SANS measurements of the CnDEPO systems are planned to show that the rigidity is indeed the major player and to better understand the difference between the DEPO and the DMPO surfactants. If the rigidity was indeed the major tuning parameter, one would obtain similar κ-values for C10DEPO and C8E5, C12DEPO and C10E5, and C14DEPO and C12E5. Regarding the CnDMPOs, no homologous CiEj series could be found. However, a comparison of C12DEPO (C10E5) with C12DMPO (C11E8) clearly illustrates and supports what was mentioned before: C12DMPO (C11E8) is slightly more efficient than C12DEPO (C10E5), while it is much more difficult to dehydrate the headgroup and thus induce a change in curvature. 3.2. Monomeric Solubilities. In one-phase microemulsions, the surfactant molecules partition between the microscopic water/ oil interface and the subphases in which they are dissolved monomerically. In the case of two- or three-phase systems, they also dissolve monomerically in the coexisting excess phases and adsorb at the macroscopic interfaces between the phases. Neglecting the amount that is adsorbed at the macroscopic interfaces in multiphase systems, we still have to tackle the problem with the monomerically dissolved surfactant, as this part is not available for creating the microscopic water/oil interface required for microemulsion formation. The monomeric solubility of surfactants in water γmon,a equals the cmc and can thus easily be determined from surface tension measurements.33 Quite an elegant way to obtain the monomeric solubility of surfacants in oil γmon,b is to determine the surfactant mass fraction γ0 at the fish-head (see Figure 2) as it holds25,34

γmon,b )

γ0 + γmon,a(R(1 - γ0) - 1) γ0 ∼ (3) γ0 + R(1 - γ0) - γmon,a γ0 + R(1 - γ0)

Three comments have to be made in relation to eq 3. First, the approximation can be used if γmon,a