Polymerizable Nonionic Microemulsions: Phase Behavior of H2O−n

4122

Langmuir 2000, 16, 4122-4130

Polymerizable Nonionic Microemulsions: Phase Behavior of H2O-n-Alkyl Methacrylate-n-Alkyl Poly(ethylene glycol) Ether (CiEj) O. Lade, K. Beizai, T. Sottmann, and R. Strey* Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburger Str. 116, D-50939 Ko¨ ln, Germany Received September 17, 1999. In Final Form: December 22, 1999

The phase behavior of ternary water-alkyl methacrylate-alkyl polyglycol ether (CiEj) systems has been examined. Specifically, using seven different alkyl methacrylates ranging from methyl to hexadecyl methacrylate and C10E6 as surfactant, vertical sections through the phase prism were determined, from which the phase inversion temperature, the upper and lower critical temperature of the three-phase body, and the efficiency of the surfactant and its monomeric solubility in the oil were obtained. Keeping hexyl methacrylate as oil-fixed, 18 different surfactants were applied including short- and long-chain surfactants such as C4E3 and C14E8. The microemulsion systems examined here show the same general patterns as the well-known nonionic microemulsions with alkanes as oil. Notably, the phase inversion temperature is highly dependent on the alkyl chain length of the oil, a fact that is often left out of consideration when choosing a surfactant in emulsion polymerization. For a given oil the phase inversion temperature can be adjusted by appropriate choice of the number of ethylene glycol units of the surfactant. The efficiency of the surfactant systematically depends on the alkyl chain length of both the surfactant and the oil. Interestingly, there is a striking parallel between efficiency of a surfactant and its monomeric solubility in the oil. Finally, in preparation for applying these systems to the synthesis of nanoscaled latexes in microemulsion polymerization the water-rich part of the phase prism was examined. Both the expected shape of the emulsification failure phase boundary and the near-critical phase boundary with its nonmonotonic decay characteristic of branched network structures are delineated. The results of some preliminary polymerizations are briefly discussed.

I. Introduction Microemulsions are being used for nanoparticle1,2 formation as well as in chemical synthesis,3-5 and increasingly also for polymer formation.6-8 However, in applications, often systematic investigations of the phase behavior are lacking, although the underlying microstructure in connection with the total area and elastic properties of the internal interface might be important parameters for the polymerization process. This insight led us to examine the phase behavior (and microstructure) of microemulsions first, before applying the systems to polymerization. In the past, many investigations concentrated on microemulsions with ionic surfactants. We explore here the adaptability of the nonionic alkyl poly(ethylene glycol) ethers (CiEj) for the polymerization of alkyl methacrylates. The possibility to adjust both the alkyl chain length i and the number of ethylene glycol units j of the surfactant enables one to tune the phase behavior more gradually than is feasible for ionic surfactants. (1) Eastoe, J.; Warne, B. Curr. Opin. Colloid In. 1996, 1, 800. (2) Sager, W. F. C. Curr. Opin. Colloid In. 1998, 3, 276. (3) Schwuger, M.-J.; Stickdorn, K.; Schoma¨cker, R. Chem. Rev. 1995, 95, 849. (4) Chhabra, V.; Free, M. L.; Kang, P. K.; Truesdail, S. E.; Shah, D. O. Tenside, Surfactants, Deterg. 1997, 34, 156. (5) Industrial Applications of Microemulsions; Solans, C., Kunieda, H., Eds.; Surfactants Science Series, Vol. 66; Marcel Dekker: New York, 1997. (6) Candau, F. In Polymerization in Organized Media; Paleos, C. M., Ed.; Gordon and Breach Science Publishers: Philadelphia, 1992. (7) Antonietti, M.; Basten, R.; Lohmann, S. Macromol. Chem. Phys. 1995, 196, 441. (8) Desai, S. D.; Gordon, R. D.; Gronda, A. M.; Cussler, E. L. Curr. Opin. Colloid In. 1996, 1, 519.

Microemulsions are macroscopically homogeneous, thermodynamically stable mixtures of at least two immiscible liquids and a surfactant. Microscopically, the amphiphilic surfactant molecules form a film separating the two incompatible solvents into two subphases. Depending on the surfactant strength, the temperature, and the ratio of the solvents, a wide variety of microemulsion microstructures, such as droplet, bicontinuous, and ordered structures, can be found. The characteristic length scales of these structures may range from 1 to 100 nm.9 Over the last 20 years, systems containing water, various n-alkanes (or other nonpolar liquids), and nonionic surfactantssfrequently of the CiEj type used in this studyswere investigated10 with regard to their phase behavior,11-14 microstructure,15-19 and interfacial properties.19,20 Although from an empirical point of view the phase (9) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (10) Kahlweit, M.; Strey, R.; Haase, D.; Kunieda, H.; Schmeling, T.; Faulhaber, B.; Borkovec, M.; Eicke, H.-F.; Busse, G.; Eggers, F.; Funck, Th.; Richmann, H.; Magid, L.; So¨derman, O.; Stilbs, P.; Winkler, J.; Dittrich, A.; Jahn W. J. Colloid Interface Sci. 1987, 118, 436. (11) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1973, 42, 381. (12) Shinoda, K.; Kunieda, H. J. Dispersion Sci. Technol. 1982, 3, 233. (13) Kahlweit, M.; Strey, R.; Firman, P.; Haase, D.; Jen, J.; Schoma¨cker, R. Langmuir 1988, 4, 499. (14) Schubert, K.-V.; Kaler, E. W. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 190. (15) Glatter, O.; Strey, R.; Schubert, K.-V.; Kaler, E. W. Ber. BunsenGes. Phys. Chem. 1996, 100, 323. (16) Langevin, D. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 323. (17) Lindman, B.; Olsson, U. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 344. (18) Talmon, Y. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 364. (19) Gradzielski, M.; Langevin, D.; Farago, B. Phys. Rev. E 1996, 53, 3900.

10.1021/la991232i CCC: $19.00 © 2000 American Chemical Society Published on Web 04/06/2000

Polymerizable Nonionic Microemulsions

behavior, that is, the occurrence of different phases depending on composition and temperature of the system, can be considered to be well understood, a thermodynamic theory of predictive power is still lacking.13,14,21 Hence the investigation of a new class of hydrophobic liquids is not only important for application as polymerizable systems, but it also provides new experimental data for further theoretical developments. A. Polymerization of Ionic Microemulsions. Microemulsion polymerization aims to form potentially useful materials with structures as similar as possible to the microemulsions they stem from. Microstructures used in polymerization attempts have been mainly droplets and bicontinuous sponge phases. Typical results obtained so far in the literature indicate, however, that the microstructures of the microemulsion change during the reaction process. Polymerization of globular microemulsions, for instance, yields microlatexes with particles in the size range of 2 to 50 nm. The resulting particles are at least twice as large as the underlying microemulsion droplets. Typically the particle size increases by about a factor of 10.6-8 There are various strategies to obtain microporous materials from bicontinuous microemulsions. One strategy is to copolymerize both domains of the microemulsion with one copolymer being hydrophilic (e.g., acrylic acid) and the other being hydrophobic (e.g., methyl methacrylate).22 In most cases described a cross-linking agent was used. Another strategy is to polymerize only one domain of a bicontinuous microemulsion, which often leads to a destabilization of the systems, especially if the monomer itself is amphiphilic (e.g., charged monomers). By making use of vast amounts of surfactant and cross-linking agent it is possible to polymerize only one domain of a bicontinuous microemulsion, that is, the hydrophilic or the hydrophobic, respectively. It was found that during polymerization the original microemulsion structure is retained, but destroyed during the subsequent processing.23 Precipitation and polymerization of inorganic monomers (e.g., alkoxysilanes) in microemulsions show some similarities to the above-described methods.24,25 Except for a few reports of the application of technical-grade nonionic surfactants mainly by Candau and colleagues26-29susing water-soluble monomerssand Tadros and colleagues30,31susing methyl methacrylate and styrenesalmost all research was done using ionic surfactants. Here ternary microemulsions without cosurfactant are desirable, because short-chain alcohols might affect the polymerization process. N-Alkyltrimethylammonium halides, n-dialkyldimethylammonium halides, sodium dodecyl sulfate, bis(2ethylhexyl)sodium sulfosuccinate (AOT), and n-alkyldiethanolamines form ternary microemulsions with certain monomers. As an example the ionic microemulsion polymerization of the quasi-ternary system H2O-hexyl methacrylate-dodecyltrimethylammonium bromide/di(20) Leitao, H.; Somoza, A. M.; Telo da Gama, M. M.; Sottmann, T.; Strey, R. J. Chem. Phys. 1996, 105, 2875. (21) Sottmann, T.; Strey, R. J. Phys.: Condens. Matter 1996, 8, A39. (22) Palani Raj, W. R.; Cheung, H. M. In The Polymeric Materials Encyclopedia: Synthesis, Properties and Applications; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL 1996; Vol. 6. (23) Burban, J. H.; He, M.; Cussler, E. L. AIChE J. 1995, 41, 907. (24) Burban, J. H.; He, M.; Cussler, E. L. AIChE J. 1995, 41, 159. (25) Mukkamala, R.; Cheung, H. M. Langmuir 1997, 13, 617. (26) Holtzscherer, C.; Candau, F. Colloids Surf. 1988, 29, 411. (27) Buchert, P.; Candau, F. J. Colloid Interface Sci. 1990, 136, 527. (28) Candau, F.; Buchert, P. Colloids Surf. 1990, 48, 107. (29) Copart, J. M.; Candau, F. Colloid Polym. Sci. 1993, 271, 1055. (30) Lapent, C.; Tadros, T. F. Colloid Polym. Sci. 1991, 269, 114. (31) Girard, N.; Tadros, T. F.; Bailey, A. I. Colloid Polym. Sci. 1998, 276, 999.

Langmuir, Vol. 16, No. 9, 2000 4123

Figure 1. Schematic representation of the phase prism for a H2O-oil-CiEj system.

dodecyldimethylammonium bromide was studied extensively as to phase behavior, microstructure, kinetics, particle size, and monomer partitioning.32-35 Because only a few commercially available ionic surfactants are able to form ternary microemulsions with common monomers, recent work in this area was devoted to developing new surfactants by means of synthesis or exchange of the counterion;36 for example, gemini surfactants with either alkyl37 or oligo(oxyethylene)38 spacer chains were utilized for the microemulsion polymerization of styrene. For unknown reasons the phase behavior of nonionic microemulsions with polymerizable oils has not been investigated in the past. The only reference39 we were able to find reports the use of technical-grade fatty alcohol ethoxylates for the emulsion polymerization of alkyl methacrylates. B. Phase Behavior of Nonionic Microemulsions. At constant pressure the phase behavior of ternary microemulsions may be represented in a phase prism with the Gibbs triangle as base and temperature as vertical axis.13,14,21,40 Figure 1 shows a schematic phase prism. As is indicated by the test tubes and the slope of the tie lines, at low temperatures T the nonionic surfactant is more soluble in water, and at higher temperatures in the oil, respec(32) Lusvardi, K. M.; Schubert, K.-V.; Kaler, E. W. Langmuir, 1995, 11, 4728. (33) Morgan, J. D.; Lusvardi, K. M.; Kaler, E. W. Macromolecules 1997, 30, 1897. (34) Morgan, J. D.; Kaler, E. W. Macromolecules 1998, 31, 3197. (35) Co, C. C.; Kaler, E. W. Macromolecules 1998, 31, 3203. (36) Antonietti, M.; Hentze, H.-P. Adv. Mater. 1996, 8, 840. (37) Dreja, M.; Tieke, B. Langmuir 1998, 14, 800. (38) Dreja, M.; Pyckhout-Hintzen, W.; Mays, H.; Tieke, B. Langmuir 1999, 15, 391. (39) Mu¨ller, P. Ph.D. Thesis, Universita¨t zu Ko¨ln, 1995. (40) Kahlweit, M.; Strey, R. Angew. Chem., Int. Ed. Engl. 1985, 24, 654.

4124

Langmuir, Vol. 16, No. 9, 2000

Figure 2. (a) Schematic representation of the vertical section through the phase prism at constant oil/(water + oil) volume ratio, φ ) 0.5 [referred to as γ(T) section or fish plot in the text]. Characteristic parameters that can be extracted from such sections: Temperature T ˜ and surfactant mass fraction γ˜ at the fish tail point X ˜ . Tu and Tl are the upper and lower critical temperature of the three-phase body, respectively. γ0 represents the surfactant mass fraction monomerically dissolved mainly in the oil excess phase. (b) Schematic representation of the vertical section through the phase prism at a constant surfactant/(water + surfactant) mass ratio γa denoted as wB(T) sections.

tively. Therefore, at low temperatures (T < Tl) a surfactantrich water phase coexists with an excess oil phase, whereas at high temperatures (T > Tu) a surfactant-rich oil phase coexists with an excess water phase. These two situations are denoted by 2 and 2h , respectively. In the temperature region between these (Tl < T < Tu) three phases, that is, a water excess, an oil excess, and a surfactant-rich phase, often referred to as microemulsion, coexist. The most important features of the phase behavior are observed by performing characteristic sections through the phase prism. Keeping the oil/(oil + water) volume ratio at φ ) 0.5, and varying the surfactant mass fraction γ, Figure 2a is obtained. This section is indicated in Figure 1 as a gray highlighted plane. The coexistence curves show the well-known fish where at intermediate temperatures and low γ the threephase body occurs, whereas at high γ the one-phase region appears. From this section the following characteristic parameters can be extracted: from the point of contact of the one- and three-phase regions (the “fish tail point” X ˜) the minimum amount of surfactant mass fraction needed for complete solubilization of equal volumes of water and oil, denoted as γ˜ , can be obtained together with the corresponding temperature T ˜ . In addition, from the upper and lower temperatures of the three-phase body (Tu and Tl), the temperature extension ∆T ) (Tu - Tl) and the mean temperature Tm ) (Tu + Tl)/2 can be obtained, respectively. Tm, often referred to as phase inversion temperature,12 has the significance that at this temperature the system is in the balanced state. Here the

Lade et al.

surfactant film has zero mean curvature, resulting in a bicontinuous or lamellar microstructure.9 In general, Tm and T ˜ do not differ much because of the symmetry of the three-phase region. At γ0 the middle phase appears (fish head point). Its value represents the amount of surfactant monomerically dissolved in the excess water and oil phases. We will demonstrate its use in connection with the phase diagrams measured below. The phase behavior of ternary systems of the type water-n-alkane (Bk)-alkyl poly(ethylene glycol) ether (CiEj) has been studied systematically and general patterns were observed.13,41-44 An increase in the hydrocarbon chain length k of alkanes shifts T ˜ to higher temperatures, increases ∆T, and decreases the efficiency of the surfactant. Furthermore, for a given oil an increase of the surfactant alkyl chain i causes a remarkable increase in efficiency, that is, a decrease of γ˜ , but a moderate decrease of T ˜ . Differently, the efficiency decreases slightly when the headgroup size j is increased, whereas T ˜ increases significantly. It has been observed that the γ0 values show similar tendencies to the γ˜ values.45 For microemulsion polymerization of nanometer-scaled latexes, another section through the phase prism is of interest. Starting from the binary water-surfactant system the temperature extension of the one-phase region is determined as a function of the mass fraction of oil wB in the mixture at a constant surfactant/(surfactant + water) mass ratio γa. This section is also included in Figure 1. With increasing wB the phase transition curve between the 2 and the 1 phase region [the so-called emulsification failure (EF) boundary46] ascends. The phase transition curve (starting at the cloud point curve of the binary water-surfactant system) between the 1 and the 2h phase region descends steeply. Strictly speaking, this curve is not identical to the line of critical points, but runs for practical purposes indistinguishably close to it, so that it intersects the EF very near to the critical endpoint temperature Tl. By increasing wB further, the three-phase body appears at higher temperatures, indicating that the critical endpoint temperature and the lower temperature of the three-phase body are equal. Considering the shape of the upper phase transition curve, a significant difference in the behavior of weak and strong surfactants can be observed.44 Whereas for a weak surfactant the curve decreases monotonically down to Tl (see Figure 1), in the strong surfactant case the curve passes through a minimum to then increase monotonically up to Tl. The latter situation is depicted schematically in Figure 2b. The minimum is a consequence of a 2 h phase region in the form of a closed loop appearing at temperatures below Tl in the Gibbs triangle.43,44,47,48 In recent theoretical considerations these loops play an important role in explaining the origin of the three-phase body.49 C. Microemulsions with Polymerizable Oils. In the present study we examined the phase behavior for ternary nonionic systems using n-alkyl methacrylates (CkMA) as oil, where k is the alkyl chain length of the methacrylate. (41) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (42) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (43) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (44) Kahlweit, M.; Strey, R.; Busse, G. Phys. Rev. E 1993, 47, 4197. (45) Burauer, S.; Sachert, T.; Sottmann, T.; Strey, R. Phys. Chem. Chem. Phys. 1999, 1, 4299. (46) Safran, S.; Turkovich, L. Phys. Rev. Lett. 1983, 50, 1930. (47) Kilpatrick, P. K.; Gorman, C. A.; Davis, H. T.; Scriven, L. E.; Miller, W. G. J. Phys. Chem. 1986, 90, 5292. (48) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182. (49) Tlusty, T.; Safran, S. A.; Menes, R.; Strey, R. Phys. Rev. Lett. 1998, 78, 2616.

Polymerizable Nonionic Microemulsions

Langmuir, Vol. 16, No. 9, 2000 4125

Table 1. Characteristic Parameters of System H2O-Alkyl Methacrylate-C10E6 at O ) 0.5a

a

alkyl

T ˜ (°C)

Tu (°C)

Tl (°C)

∆T (°C)

γ˜

γ0

γi

γmon,b

methyl ethyl butyl hexyl octyl dodecyl hexadecyl

9.00 16.35 26.35 33.35 40.35 49.88 57.24

9.60 17.22 27.30 34.35 41.65 51.35 58.55

8.00 15.17 25.05 31.65 38.37 45.73 48.34

1.60 2.05 2.25 2.70 3.28 5.62 10.21

0.168 0.157 0.147 0.146 0.150 0.174 0.203

0.083 0.076 0.063 0.056 0.047 0.036 0.024

0.093 0.088 0.090 0.095 0.108 0.143 0.183

0.157 0.147 0.125 0.112 0.095 0.075 0.050

T and γ values carry a typical error of (0.10 °C and 0.010, respectively. Table 2. Characteristic Parameters of System H2O-Hexyl Methacrylate-CiEj at O ) 0.5a

a

CiEj

T ˜ (°C)

C6E3 C6E4 C6E5 C8E4 C8E5 C10E4 C10E5 C10E6 C10E7 C10E8 C12E5 C12E6 C12E7 C12E8 C12E9 C14E7 C14E8

13.00 32.13 46.30 14.55 30.55 4.76 20.57 33.35 43.14 51.53 14.38 26.87 37.27 46.13 53.10 32.60 41.35

Tu (°C)

Tl (°C)

∆T (°C)

32.33 46.61

31.14 45.43

1.19 1.18

31.45

28.58

2.87

21.70 34.35 44.95 52.83 15.30 27.65 38.15 47.25 54.50

19.05 31.65 41.90 50.05 13.45 25.65 36.23 45.15 52.00

2.65 2.70 3.05 2.78 1.85 2.00 1.92 2.10 2.50

42.65

40.30

2.35

γ˜ 0.248 0.251 0.260 0.190 0.191 0.128 0.139 0.146 0.152 0.155 0.092 0.102 0.105 0.113 0.126 0.076 0.081

γ0

γi

γmon,b

0.123 0.121 0.067 0.068

0.146 0.158 0.132 0.132

0.216 0.213 0.131 0.133

0.051 0.056 0.058 0.058 0.043 0.045 0.046 0.048 0.050 0.041 0.041

0.093 0.095 0.100 0.103 0.051 0.060 0.062 0.068 0.080 0.036 0.042

0.103 0.112 0.116 0.116 0.087 0.091 0.093 0.097 0.100 0.083 0.083

T and γ are accurate to within an error of (0.10 °C and (0.010, respectively.

The class of monomers chosen is not only of technical importance, but also is a model system for side-chainfunctionalized methacrylates. Making use of the detailed knowledge of the phase behavior, we expect to gain insight into the mechanisms of microemulsion polymerization and their dependence on temperature and composition, especially on the nature of the amphiphilic film. In particular, the above-defined two types of vertical sections through the phase prisms were performed. γ(T) sections (Figure 2a) provide us with the efficiency of the surfactant, for which γ˜ at the temperature T ˜ is a measure. The temperature extension of the three-phase body ∆T as well as the temperature at which the microemulsion is in the balanced state Tm are also obtained from the same section. From γ0 the monomeric solubility in the oil γmon,b will be obtained, as we show below. The wB(T) sections provide us with the EF boundary (lower phase boundary in Figure 2b), at which the microemulsion consists of rather monodisperse monomer droplets. The size of these droplets may be varied over certain ranges.9 The droplets may be formed by short-, medium-, and long-chained surfactants, and the temperature may be chosen as well. The synthesis of latexes in the nanometer scale from the droplets will be treated in a following publication.50 The potential use and the versatility of the present choice of components will become clear as the phase behavior is examined in the following sections. II. Experimental Section A. Materials. The water used was deionized and distilled twice. The CkMA have a purity >99% and were used as purchased from Polyscience (Eppelheim, Germany) or from Merck-Schuchardt (Hohenbrunn, Germany). To avoid polymerization during phase investigation the CkMA were used without removal of the inhibitors. The n-alkyl poly(ethylene glycol) ethers (CiEj) have a purity >98% and were purchased from Fluka (Neu Ulm, Germany), except C12E7 and C12E8, which were from Nikko (50) Lade, O. et al., in preparation.

(Tokyo, Japan). In contrast to technical-grade surfactants, the CiEj used here consist of the pure homologue. B. Phase Behavior. Every pseudobinary phase diagram at constant oil/(water + oil) volume ratio φ ) 0.5, and varying surfactant mass fraction γ was determined with at least two samples. Every ternary mixture was prepared by weighing sequentially water, oil, and surfactant up to a total mass of about 2 g into a test tube with a magnetic stirring bar. The occurring phases were determined visually in a water bath at constant composition by varying the temperature. One-phase microemulsions and lamellar LR phases are transparent and can be distinguished by birefringence of the LR phase. The first appearance of birefrigence is denoted in figures by LR. Typically, between these two phases exists a coexistence region, which has not been further investigated. Two- and three-phase regions are both turbid, and phase separation must be awaited. After every measurement consisting of the determination of at least two transition temperatures the surfactant concentration was decreased by adding equal volumes of water and oil. Considering that for γ0 the volume of the middle phase Vc vanishes and that for γ˜ the total volume is middle phase (Vtot ) Vc),

Φc )

γ - γ0 γ - γ0

(1)

Measuring Φc ) Vc/Vges as function of γ we obtain γ0 and γ˜ . γ0 is equal to the negative intersection of the curve with the Φc axis divided by the slope. Furthermore, γ˜ can be obtained as γ(Φc ) 1), which is independent of the determination from the γ(T) sections.

III. Results We determined γ(T) sections of 20 systems of the type water-n-alkyl methacrylate-n-alkyl poly(ethylene glycol) ether. The characteristic parameters are compiled in Tables 1 and 2 including T ˜ and γ˜ determinations for four further systems for which only the fish tail or the fish head yielding γ0 was determined. wB(T) sections in the water-rich part of the phase prism at a surfactant mass

4126

Langmuir, Vol. 16, No. 9, 2000

Lade et al.

Figure 3. Evolution of the fish plot in systems H2O-alkyl methacrylate-C10E6, where the alkyl chain length is as denoted by CkMA in the plot. O, 1, and 2 denote measured points, fish tail points, and fish head points, respectively. For guiding the eye we have connected the respective points by dotted lines. For dodecyl methacrylate the phase states 1, 2, 3, 2 h , for methyl methacrylate the characteristic parameters γ0, X ˜ (γ˜ , T ˜ ) are indicated.

fraction in the surfactant-water mixture of γa ) 0.04 were determined for nine systems. A. γ(T) Sections. To obtain an overview it has proven useful in the past to study γ(T) sections, so-called fishes. Here we systematically measured γ(T) sections as a function of k for C10E6 and as a function of i and j for hexyl methacrylate. Dependence on Alkyl Chain Length of Methacrylate k. For C10E6, γ(T) sections for seven systems including the three-phase body were recorded for methyl, ethyl, butyl, hexyl, octyl, dodecyl, and hexadecyl methacrylates (Figure 3). From Figure 3 one finds that γ˜ decreases for k ) 1 to 6 and then increases with further increasing k, whereas ˜ increases with the shift of T ˜ γ0 continuously decreases. T becoming smaller with increasing k, whereas ∆T always increases. One might note that the H2O-n-octane-C10E6 system has nearly the same ∆T and γ˜ as the hexadecyl methacrylate system. The characteristic parameters T ˜, γ˜ , γ0, and ∆T as a function of the methacrylate alkyl chain length k are compiled in Table 1. Next, C6MA is kept as the model oil for different CiEj, now varying the alkyl chain length i and the number of ethylene glycol units in the headgroup j, respectively. Dependence on Surfactant Alkyl Chain Length i. For investigating the effect of increasing the alkyl chain of the surfactant i, γ(T) sections were recorded for C6MA and the CiE5 and CiE8 surfactants (Figure 4a and 4b, respectively). Several features are readily observed with increasing i. γ˜ strongly decreases, indicating an increase in the efficiency of the surfactant. γ0 decreases, which means (as we shall see below) a decrease in the surfactant solubility in the oil. This might be surprising at first sight because a longer alkyl chain should lead to more favorable interactions with the oil, but apparently the temperature dependence of the solubility is important here. As can be seen, T ˜ decreases with increasing i. Considering the surfactants with j ) 5, the temperature extension of the

Figure 4. Evolution of the fish plot at increasing surfactant alkyl chain length i in systems H2O-hexyl methacrylate-CiEj, where j is either 5 or 8. Note that the fishes of C12E8 and C6E5 are located at almost identical temperatures and so are the mixtures between them, denoted by 4 (25% C6E5), 0 (50% C6E5), or 3 (75% C6E5), respectively.

three-phase body ∆T runs through a maximum. Starting from the long chains, first it increases from C12E5 to C10E5 and then decreases. The decrease of ∆T may be seen as a sign of an approach to a tricritical point13,51-54 for surfactants shorter than C6E5. Also, the shape of the C6E5 fish has already the form of near-tricritical three-phase bodies.51 For the long-chain surfactants with j ) 8, ∆T remains nearly constant. Interestingly, C6MA microemulsions with C12E8 and ˜ . This suggests making C6E5 have nearly the same T surfactant mixtures with gradual changing amphiphilic strength but similar T ˜ . We included in Figure 4a the onephase regions (fish tails) of three different surfactant mixtures with C6E5 contents of 25, 50, and 75%, together with that of pure C6E5 in the upper part of Figure 4. Upon closer inspection one finds that T ˜ does not change strictly linearly with the mixing ratio of the surfactants, but shows a slight maximum of about 0.2 °C higher than T ˜ of C6E5. Dependence on Surfactant Headgroup Size j. As can already be inferred from Figure 4, changing the headgroup size j has a strong effect on the mean temperature of the three-phase region. To see this more clearly, γ(T) sections were recorded for C10Ej and C12Ej surfactants, where j was varied from 5 to 8. Because we are interested (51) Kahlweit, M.; Strey, R.; Aratono, M.; Busse, G.; Jen, J.; Schubert, K.-V. J. Chem. Phys. 1991, 95, 2842. (52) Schubert, K.-V.; Strey, R. J. Chem. Phys. 1991, 95, 8532. (53) Wormuth, K. R.; Kaler, E. W. J. Phys. Chem. 1989, 93, 4855. (54) Ryan, L.; Kaler, E. W. Langmuir 1999, 15, 92.

Polymerizable Nonionic Microemulsions

Figure 5. Evolution of the fishes with increasing number of headgroup units j for H2O-hexyl methacrylate-C12Ej. The dotted lines are to guide the eye.

in efficient systems, we report and analyze the γ(T) sections for the C12Ej systems. With increasing j one observes in Figure 5 that γ˜ increases, that is, the efficiency of the surfactant decreases moderately, whereas γ0, that is, the monomeric solubility of the surfactant, increases only slightly, but monotonically. Not unexpectedly, T ˜ increases strongly, whereas the temperature extension ∆T of the three-phase body is nearly constant (for exact numbers see Table 2). From Strong to Weak Surfactants. As noted already in connection with Figure 4, by decreasing the hydrophobic chain length a tricritical point might be approached. There is considerable interest in identifying tricritical points, as it has both theoretical as well as practical importance. The evolution of the phase behavior,13 the microstructure,52,53,55 and the interfacial properties51 undergoes dramatic changes in the vicinity of tricritical points. Previous research44,56 seems to indicate that to reach tricritical points the strength of the surfactant has to be weakened. Therefore, the polarity of either the hydrophilic or the hydrophobic solvent has to be reduced52 or raised,53 respectively. The versatility of the CiEj surfactants in conjunction with the methacrylates studied here permits us to select a series of surfactants so that the transition beyond the tricritical point can be realized. To this end we started with C14E8 and decreased i by 2 and j by 1 to stay temperature-wise in the experimental window (Figure 6). Starting with the γ(T) section of the long-chain C14E8, one finds a low γ˜ . Near X ˜ a lamellar liquid crystalline phase is shown in the fishtail, which had been omitted in Figure 4 for clarity. By decreasing the surfactant chain length from C12E7 to C8E5, the efficiency of the surfactant decreases, whereas the monomeric solubility γ0 increases. Furthermore, T ˜ shifts to lower temperatures, but the shift from C10E6 to C8E5 is smaller than expected from the systematic trend of the longer-chain surfactants. This trend change becomes absolutely indisputable when one (55) Gradzielski, M.; Langevin, D.; Sottmann, T.; Strey, R J. Chem. Phys. 1996, 104, 3782. (56) Leitao, H.; Telo da Gama, M. M.; Strey, R. J. Chem. Phys. 1998, 108, 4189.

Langmuir, Vol. 16, No. 9, 2000 4127

Figure 6. Fish plots for the H2O-hexyl methacrylate-CiEj systems. With decreasing surfactant chain length the systems become more weakly structured and a trend reversal is observed. For C4E3 the three-phase has disappeared.

proceeds to C6E4, where T ˜ even moves up, and the height of the three-phase body shrinks dramatically. This evolution of the phase behavior is typical when approaching the vicinity of a tricritical point. Decreasing the surfactant chain length further, that is, proceeding to C4E3, the threephase body has gone altogether, which means a tricritical point is passed. B. wB(T) Sections. To synthesize monodisperse latexes, a detailed knowledge of the location of the EF boundary, at which the microstructure of the microemulsion is nearly globular, is required. As we will discuss in detail in the next paper of this series, it is possible to calculate the radii of droplets near the EF boundary from phase behavior measurements, provided the area of the surfactant molecule at the internal interface is known. For the present systems the radii are in a range of 2-20 nm.57 Therefore we measured wB(T) sections (γa ) 0.04) as a function of the surfactant headgroup size j and of the amphiphilic strength of the surfactant using C6MA as monomer. Dependence on Surfactant Headgroup Size. In Figure 7 the wB(T) sections are shown for the C12Ej surfactants varying j from 5 to 9. Considering first the C12E5 system, the typical shape of these sections is notable. Starting at the cloud point of the binary water-C12E5 system the upper boundary resembles near-critical phase transitions and decreases when C6MA is added (full points). As is typical for strong surfactants, this phase boundary undergoes a minimum and finally merges with the EF boundary (hollow points).43,44,48 An increase of j from 5 to 9 reduces the depth of the upper phase boundary minimum and also reduces the oil solubility, that is, the surfactant becomes less efficient. From Weak to Strong Surfactants. For the surfactants introduced in Figure 6, wB(T) sections were measured (Figure 8). The phase diagrams demonstrate the different shapes of the upper near-critical phase transition curves and the (57) Sottmann, T.; Strey, R., in preparation.

4128

Langmuir, Vol. 16, No. 9, 2000

Figure 7. Vertical section through the phase prism at a constant surfactant/(water + surfactant) mass ratio of γa ) 0.04 for H2O-hexyl methacrylate-C12Ej systems. The onephase and the three-phase regions meet at Tl (marked by 1), where also the maximum oil solubility in a one-phase microemulsion is reached. At higher and lower temperatures the 2h and 2 regions are found, respectively.

maximum amount of C6MA that can be solubilized in a one-phase microemulsion. The former exhibits a minimum for the two strongest surfactants C12E7 and C14E8, whereas for the other weaker surfactants it decays monotonically. With increasing amphiphilic strength the C6MA solubility is enhanced by one order of magnitude from wB ) 0.013 (C6E4) to wB ) 0.13 (C14E8). IV. Analysis and Discussion In this section we further analyze the data and discuss systematic trends that can be seen from the synopsis of the results for 20 different systems. A. Fish Tail Points X ˜ . For many applications the knowledge of the X ˜ point suffices to choose the appropriate surfactant and to estimate its properties.21 Therefore we

Lade et al.

measured fish tails of several additional systems, each of which provides us with T ˜ and γ˜ (see Table 2). All coordinates of the X ˜ points measured are plotted in Figure 9. X ˜ points of systems with i ) const, j ) const., and ∆i/∆j ) 2/1 are connected by a line. Lines belonging to the same condition are nearly parallel. Together with the k dependence (Figure 3) this will enable other workers to estimate the X ˜ point for microemulsions containing different oils. As mentioned above, hexyl methacrylate may be seen as a short-chain alkane as to its phase behavior in CiEj systems exhibiting qualitatively similar general patterns. A quantitative comparison with H2O-n-octane-CiEj systems reveals a weaker dependence of T ˜ and γ˜ on i and j.45 Drawing back the attention to Figure 4 one realizes that there is an unexpected difference between pure surfactants and surfactant mixtures. One would expect mixtures of C6E5 and C12E8 at mixing ratios 2:1 or 1:2 to behave like systems with pure C8E6 or C10E7, respectively. From Figure 9 we would estimate T ˜ to be considerably lower than those of C6E5 and C12E8, whereas the real mixtures have, according to Figure 4, a T ˜ very close to those of C6E5 and C12E8. B. Monomeric Solubility (γmon,b) of Surfactant in Hexyl Methacrylate. Sometimes γmon,b is in the literature referred to as the critical micelle concentration cµc or cmcb 43,58-61 As can be seen from the γ(T) sections in Figure 3, a relatively high monomeric solubility of the surfactant is found for the methacrylates, decreasing monotonically with increasing k. Considering that the baseline of the three-phase triangle at Tm (cf. Figure 1) connects the waterrich and oil-rich excess phases with γmon,a and γmon,b at R ) 0 and R ) 1, respectively, one finds45 for the monomeric solubility of the surfactant in the oil

γmon,b )

γ0 + γmon,a [R(1 - γ0) - 1] γ0 + R(1 - γ0) - γmon,a

≈

γ0 γ0 + R(1 - γ0) (2)

where R ) mb/(ma + mb) is the oil/(water + oil) mass ratio. In general γmon,a, the critical micelle concentration (cmc) of long-chain surfactants (i g 8), is at least a factor of 10 less than γmon,b and can be neglected, justifying the

Figure 8. Vertical section through the phase prism at a constant surfactant/(water + surfactant) mass ratio of γa ) 0.04 for H2O-hexyl methacrylate-C12Ej systems.

Polymerizable Nonionic Microemulsions

Langmuir, Vol. 16, No. 9, 2000 4129

Figure 9. Coordinates of the X ˜ points for H2O-hexyl methacrylate-CiEj systems denoting the minimum surfactant mass fraction γ˜ needed to form a one-phase region and the corresponding temperature T ˜ . Lines are to guide the eye.

approximation in eq 2. For the C6Ej surfactants it is sufficient to take γmon,a as the cmca ) 0.02.44,62 For practical purposes γ˜ is important because it indicates the amount of surfactant actually needed to form a one-phase bicontinuous microemulsion. However, for the assessment of the amphiphilic strength of a surfactant it is more important how much surfactant resides in the internal interface at the fish tail point

γi ) γ˜ - γ0

1 - γ˜ 1 - γ0

(3)

As shown in Figure 10, for surfactants with i ) const. only a weak dependence of γmon,b on j is observed, whereas for surfactants with j ) const. the dependence on i is strong. Comparing Figures 9 and 10 it becomes obvious that these trends are strikingly similar to those of γ˜ . At closer inspection there are, however, some differences. In Figure 9 the distance between nearly parallel lines connecting surfactants with i ) const. is only slightly changing with i. Contrariwise, in Figure 10 the distance between two of these lines increases strongly with decreasing i. All in all, comparatively large values of γmon,b ranging from γmon,b ) 0.083 (C14E8) to γmon,b ) 0.213 (C6Ej) are obtained. For comparison, in n-alkane systems γmon,b is about one order of magnitude lower for a given surfactant, but the dependence of γmon,b on i and j is stronger.45 C. Determination of Tl. As pointed out above, there are two simple ways to determine the temperature of the lower critical endpoint Tl, namely the two different sections through the phase prism shown schematically in Figure 2. Of course the upper critical endpoint temperature Tu can be obtained from a section analogous to that shown (58) Kunieda, H.; Shinoda, K. J. Colloid Interface Sci. 1985, 107, 107. (59) Binks, B. P.; Fletcher, P. D. I.; Horsup, D. I. Colloids Surf. 1991, 61, 291. (60) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I.; Ye, X. J. Chem. Technol. Biotechnol. 1992, 54, 231. (61) Schott, H. J. Pharm. Sci. 1995, 84, 1215. (62) Pakusch, A. Ph.D. Thesis, Universita¨t Go¨ttingen, 1983.

Figure 10. Monomeric solubility γmon,b of CiEj surfactants in hexyl methacrylate at T ˜ . Lines are to guide the eye.

in Figure 2b, starting with an oil-surfactant mixture and adding in that case water. The temperature of the intersection of the two phase boundaries then depicts Tu. To quantify the accuracy of the two independent determination methods we compare the two values for the nine systems for which wB(T) sections were measured (Figures 5, 6 and 7, 8). The typical difference between the two values is 0.15 K, which can be considered to be within experimental error. Larger differences occur for the C6E4 and C8E5 where the wB(T) sections give up to 0.50 K higher values, and for the C12E9 and C14E8 where the wB(T) sections give up to 0.55 K lower values. Impurities of the surfactants result in slightly different Tl values depending on the amount of oil in the mixture (which might be a solvent for the impurities).63 These may be reasons for these (small) differences observed. D. Further Discussion. The increase of efficiency with increasing k in Figure 3 for the short-chain methacrylates is somewhat unexpected because usually a shorter alkyl chain of the oil means a higher efficiency for a given surfactant. Considering the monomeric solubility of the surfactant (eq 2), though, the amount of surfactant residing at the internal interface γi, an inverse measure of the amphiphilic strength of the surfactant, increases as expected from C2MA to C16MA (Table 1). There is, however, still an anomalous increase in γi from methyl to ethyl methacrylate. A reason for this might be an unfavorable interaction between the relatively polar methyl methacrylate and the alkyl chain of the surfactant. It is noteworthy that γ˜ , T ˜ , ∆T for long-chain methacrylates, and the alkanes starting from octane all increase in the same fashion as k is increased. From this we conclude that the phase behavior of alkyl methacrylates may be conceived as a continuation of the alkane series toward low chain lengths. These alkanes, though existing, cannot be investigated at ambient pressure because of their high vapor pressure. Additionally, the three phase bodies of the long-chain methacrylates become more and (63) Schubert, K.-V.; Strey, R.; Kahlweit, M. J. Colloid Interface Sci. 1991, 21, 141.

4130

Langmuir, Vol. 16, No. 9, 2000

more asymmetrical, that is, Tm * T ˜ . The latter behavior is uncommon for ternary systems at φ ) 0.5 and might stem from partitioning of the methacrylate between the bulk and the amphiphilic film. Therefore the methacrylate acts partly as a cosurfactant rendering the amphiphilic film more hydrophobic as the surfactant-to-methacrylate ratio is decreased, that is, at lower γ. As known from other cosurfactants, for instance alcohols, this effect increases with increasing chain length of the cosurfactant.64

Lade et al.

transfer rates. Accordingly, the monomerically dissolved surfactant does not play the role of a strong transfer agent. The preliminary results show already that microemulsion polymerization of ternary nonionic microemulsions is feasible and promises interesting new insights. These results on the polymerization as well as dynamic light scattering and small-angle neutron scattering will be presented in due course. VI. Conclusions

V. Some Preliminary Nonionic Microemulsion Polymerizations The phase behavior presented above enables us to choose the compositions and temperatures for microemulsion polymerization. For instance, the polymerization of H2OC6MA-C12E7 (γa ) 0.04 and wB ) 0.02) was performed at T ) 25.5 °C using a 60Co-γ-radiation source (∼7 Gy/h). (For details we have to refer to the forthcoming paper50.) Strong scattering of light is observed, whereas in transmitted light the sample appears orange-colored, but otherwise transparent. The one-phase mixture becomes two-phase above 56 °C. The particle radii were determined by a standard dynamic light-scattering device (ALV 5000) at several different angles at T ) 25 °C. The hydrodynamic radii of microemulsion droplets were measured without dilution; the latex particles were diluted 20 times to avoid particle interaction. The evaluation of the intensity autocorrelation function was performed with CONTIN.65 The (effective) hydrodynamic radius was calculated using the Stokes-Einstein equation assuming the viscosity of pure water. The molecular weight distribution was measured by a gel permeation chromatographer (Water 501) with refractive index detector (Water 401) after precipitation of the polymer with methanol and dissolving it in tetrahydrofuran. According to the well-known mechanism of microemulsion polymerization,33,34 the hydrodynamic radius increases during the polymerization process. In our case from 5.6-nm microemulsion droplets, latex particles of 21.8-nm radius form with a polydispersity index of 0.03. In nondiluted latex solutions CONTIN detects a second radius fraction presumably representing empty micelles with a radius of about 4 nm. This may be understood if one considers the number density of objects to be stabilized. The total number density of monomer droplets and polymer particles decreases at nearly constant total volume. The excess surfactant may thus be forced to form empty micelles. The molecular weight average of the polymer was found as Mw ) 2.1 × 106 g/mol with a polydispersity of Mw/Mn ) 1.6. This comparatively high molecular weightscompared to emulsion poylmerizationspresumably arises from low termination and (64) Penders, M. H. G. M.; Strey, R. J. Phys. Chem. 1995, 99, 10313. (65) Provencher, S. W. Comput. Phys. Commun. 1982, 27, 213.

The phase behavior of ternary nonionic systems with methacrylates as oil examined here shows similar general patterns as water-alkane-CiEj microemulsions. These are specifically (a) the temperature-driven appearance of a three-phase region and, thus, a phase inversion; (b) the minimum in the upper phase boundary in diagrams such as present for strong surfactants in Figures 7 and 8; (c) the formation of a lamellar liquid crystalline phase for very strong surfactants such as C14E8; (d) the existence of a tricritical point for weak surfactants. The class of alkyl methacrylates enlarges the experimental window of basic microemulsion research toward more polar oils than the well-studied alkanes. Compared with the latter, the homologue series of methacrylates examined here for the first time exhibits a phase behavior characteristic for very short-chain alkanes. Although the model of an effective carbon number of the oil requires careful application, it seems to describe the observed trends. The ternary microemulsions presented offer themselves for microemulsion polymerization. Not only do the monomers differ considerably in their monomeric solubility in water, but also are the glass temperatures of their polymers expected to be grossly different, and the polymerization rate constants may vary significantly. It will therefore be of particular interest to determine the polymer properties and the polymerization mechanism as a function of the alkyl chain length of the monomer and the amphiphilic strength of the surfactant. These results will be published in the near future. The phase behavior of microemulsions containing polymerizable monomers should be directly applicable to technical formulations. Here two aspects are important. First, compared with the well-established cationic surfactants usually applied to microemulsion polymerization of styrene, nonionic CiEjbased surfactants are more efficient and nontoxic. Second, the possible alkyl chain length variation of the alkyl methacrylates and liberty in choosing the reaction temperature are potentially useful. Acknowledgment. Part of the material presented is based upon activities supported by Deutsche Forschungsgemeinschaft under grand STR 311/2-1. LA991232I