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Reactive Surfactants in Heterophase Polymerization. 10. Characterization of the Surface Activity of New Polymerizable Surfactants Derived from Maleic Anhydride S. A ˆ bele,† M. Sjo¨berg,‡ T. Hamaide,§ A. Zicmanis,† and A. Guyot*,§ Faculty of Chemistry, University of Latvia, 19 Boulevard Rainis, LV-1586 Riga, Latvia; Institute for Surface Chemistry, Box 5607, S-11486 Stockholm, Sweden; and Laboratoire de Chimie et Proce´ de´ s de Polyme´ risation, C.N.R.S. ESCPE Lyon 43, Boulevard du 11 Novembre 1918, BP 2077, 69616 Villeurbanne Cedex, France Received June 12, 1996. In Final Form: October 21, 1996X Characterization of the surface activity of previously obtained polymerizable dialkyl maleates is performed to find out the relation between the structure of surfactants and their performances. The given polymerizable surfactants were synthesized for using in the emulsion polymerization. Three groups of dialkyl maleatessnonionics, cationics, and zwitterionicsswith different chain lengths of hydrophobic alkyl group are investigated. Critical micelle concentration (cmc) values are determined for water soluble surfactants. It is found that cmc decreases with increasing length of the hydrophobic alkyl group. For nonionic and cationic surfactants interfacial tension at the interface between water and dodecane is measured. Droplet size in oil-in-water (O/W) emulsions is determined for all given surfactants. Cationic and zwitterionic dialkyl maleates with the longest investigated alkyl chain (R ) C16H33, C17H35) provide good stability of O/W emulsions. In order to compare the obtained results, measurements with well-known surfactantss nonionic nonylphenol-poly(ethylene oxide) (NPEO10) and cationic hexadecyltrimethylammonium bromide (CTAB)sare performed.
Introduction A special class of polymerizable surfactants for emulsion polymerization derived from maleic anhydride, namely the dialkyl maleates, has been developed in the Laboratory of Chemistry and Processes of Polymerization of the CNRS (France). The different sets of surfactants were synthesized in order to use them in the emulsion polymerization of styrene, methyl metacrylate, butyl acrylate, and others, with the purpose of improving the surface characteristics of obtained latexes. The role of any surfactant during polymerization is to contribute emulsification of the monomer with limited solubility in water and maintain the colloidal stability of the final polymer dispersion. The investigated class of reactive surfactantsspolymerizable dialkyl maleatess contain an activated CdC double bond capable of copolymerization and not homopolymerization, providing an additional advantage for the final latex in this way. At the end of the polymerization process, surfactant can be covalently anchored onto the surface of the polymer and therefore cannot desorb from the latex during final application. It should give some improvements in the stability, chiefly in some possible events, including freezing and thawing, shear, addition of electrolytes, and longterm storage. A few preliminary experiments in styrene emulsion polymerization have been carried out, using such polymerizable surfactants.1,2 The first results are promising. So, maleates functionalized with quaternary ammonium salts give polystyrene particles with a diameter around 30 nm. Synthesized nonionic and, especially, * To whom correspondence should be addressed. † University of Latvia. ‡ Institute for Surface Chemistry. § Laboratorie de Chimie et Proce ´ de´s de Polyme´risation. X Abstract published in Advance ACS Abstracts, December 15, 1996. (1) Hamaide, T.; Zicmanis, A.; Monnet, C.; Guyot, A. Polym. Bull. 1994, 33, 133. (2) Zicmanis, A.; Hamaide, T.; Graillat, C.; Monnet, C.; A ˆ bele, S.; Guyot, A. Synthesis of new alkyl maleate ammonium derivatives and their uses in emulsion polymerization. Colloid and Polymer Sci., in press.
cationic and zwitterionic maleates are efficient surfactants and should be useful for emulsion polymerization. Using investigated polymerizable surfactants in emulsion polymerization, it is important not to exceed 70 °C, because surfactants are sensitive to overheating, which can cause deamination. The double bond is sensitive to temperature as well; at temperatures around 100 °C the surfactant can form the trans-isomer after the breaking of the double bond. In this case the surfactant may be less useful for polymerization, because its reactivity has been changed. After the synthesis of a new class of surfactants, it is absolutely necessary to characterize their surface activity before using them in polymerization. Upon comparison of the obtained results, it is possible to estimate and to foresee their efficiency in emulsion polymerization. This paper gives some characteristics of the surface activity of dialkyl maleates synthesized in our laboratory. The synthesis and NMR characterization of the polymerizable maleates investigated in this paper, as well as some preliminary results in emulsion polymerization, have been described previously.1,2 Experimental Section All solutions were prepared with double-distilled water. Surface Tension Measurements. Surface tension measurements and interfacial tension measurements at the interface between water and dodecane were taken with a du Nou¨y ring tensiometer, KSV, Sigma 70. Droplet Size Measurements. Droplet size in oil-in-water (O/W) emulsions was measured by laser diffraction using a Mastersizer MS20, Malvern Instruments. Preparation of Emulsions. Surfactant (0.09 g), 3.00 g of oil (15.06 g of decane and 84.93 g of 1-bromodecane), and 96.91 g of water were mixed with an Ultra-Turrax T25, IKA-Labortechnic, for 8 min with a speed of 24000 turns/min at 25 °C. A 40.0 mM NaCl solution was used instead of water for evaluation of the effect of electrolyte. Emulsions for Interfacial Tension Measurements. Solutions of the nonionic surfactants 2-(N,N-diethylamino)ethyl alkyl maleates (1) in dodecane (25.0 mL, 0.5 mmol/L) were prepared. Water (25.0 mL) was added to the solutions.
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Table 1. List of Polymerizable Dialkyl Maleates and Their Abbreviations compound
alkyl chain (R)
structure
chemical title Nonionics 2-(N,N-diethylamino)ethyl alkyl maleate
1a 1b 1c 1d 1e
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35
2a 2b 2c 2d 2e
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35
3a 3b 3c 3d 3e
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35
2-(N,N,N-triethylammonio)ethyl alkyl maleate iodide
4d
n-C16H33
2-(N-allyl-N,N-diethylammonio)ethyl hexadecyl maleate bromide
5d
n-C16H33
6d 6f
n-C16H33 n-C18H37
Cationics 2-(N,N-diethylammonio)ethyl alkyl maleate chloride
Zwitterionics 2-[N,N-diethyl-N-(3-sulfopropyl)ammonio]ethylhexadecyl maleate
Solutions of the cationic surfactants 2-(N,N-diethylammonio)ethyl alkyl maleate chlorides (2), 2-(N,N,N-triethylammonio)ethylalkyl maleate iodides (3), and 2-(N-allyl-N,N-diethylammonio)ethylhexadecyl maleate bromide (4d) in water (25.0 mL, 0.5 mmol/L) were prepared. Dodecane (25.0 mL) was added to the solutions. All solutions were shaken by hand for approximately 1 min and left at room temperature in a separating funnel for 24 h. The layers were separated in order to get about 15 mL of each solvent. For 3d and 3e, it was necessary to use a centrifuge for separation of solvents. During the measurements, the separated layers of solvents were combined again and the values of interfacial tension at the interface between the two phases were measured.
Results and Discussion All investigated surfactants are listed in Table 1. Solubility. The nonionic surfactants 1 are not soluble in water at room temperature at 1% concentration. To improve the solubility of nonionics 1 in water, protonation of them (1) with hydrochloric acid was performed. As a result a new series of cationic surfactant 2 (hydrochlorides of 1) was obtained with better solubility in water in comparison with 1. Since nonionics 1 are not soluble in water at a reasonable temperature interval (5-80 °C), obviously they have no cloud point. The nonionics 1 are soluble in dodecane at room temperature. Zwitterionics are not soluble in dodecane or water at 1% concentration, except 2-[N,N-diethyl-N-(3-sulfopropyl)ammonio]ethyl hexadecyl maleate (5d), which has very low solubility in water. Cationics are not soluble in dodecane, and only three of them (3a, 3b, and 4d) are soluble in water at ∼1% concentration. The others have limited solubility in water, which depends on the length of hydrophobic alkyl chain.
3-[(N,N-dimethyl-N-(sulfopropyl)ammonio]propyl alkyl maleate
Table 2. Critical Micelle Concentration Values of Some Polymerizable Dialkyl Maleates conc of solutions used for cmc measurements comalkyl pound chain (R)
% conc
mmol/L
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C16H33 n-C16H33
0.73 0.78 0.84 0.12 0.49 0.97 1.02 0.27 0.42 0.08 1.12 0.001
20.0 20.0 20.0 2.5 10.0 20.0 20.0 5.0 7.0 1.25 20.0 0.02
2a 2b 2c 2d 2e 3a 3b 3c 3d 3e 4d 5d
surf cmc, tension at temp, mmol/L cmc, mN/m °C 6.46 0.93 0.28 0.02 0.01 9.12 2.63 0.18 0.05 0.02 0.07 0.004
28.6 26.7 32.8 34.1 34.2 29.4 28.9 32.5 34.0 35.7 38.5 31.9
26 26 26 26 26 20 20 26 24 20 25 26
It was possible to dissolve these surfactants only in concentrations lower than 1%. More precise concentrations of solutions used for cmc measurements are presented in Table 2. Critical Micelle Concentration. The critical micelle concentration (cmc) value, which is characteristic for a given surfactant, usually is determined by measuring the surface tension as a function of concentration. The cmc values described in this paper were obtained from surface tension measurements with the du Nou¨y ring method. The cmc values were measured for cationics 3 and 4d and for 2 (the last being obtained by protonation of 1) and for the zwitterionic surfactant 5d. The results are presented in Table 2. All surfactants investigated are obviously surface active substances, since the surface tension of water decreases when a surfactant is added. As a rule, the cmc values are lower the longer the hydrophobic alkyl chain length.
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A ˆ bele et al. Table 3. Interfacial Tension between Water and Dodecane of 0.5 mmol/L Solutions of Dialkyl Maleates interfacial tension σ, mN/m, 20-25 °C
Figure 1. 2-(N,N-Diethylammonio)ethyl alkyl maleate chlorides (2).
compound
alkyl chain (R)
nonionic 1
a b c d e 4d CTAB NPEO10
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C16H33 n-C16H33 n-C9H19
33.2 27.4 26.1 25.9 34.9
cationics 2
3
26.7 24.0 20.5 20.6 23.9 12.1 26.1 4.8 24.5 5.0 7.0-7.5 11.0
3.2
The cmc decreases with increasing length of the alkyl group, due to the increased hydrophobic character of the molecule.3 Surfactants with longer hydrophobic alkyl chains start to form micelles at lower concentrations because of lower monomer solubility in water. From our results a linear relation between log (cmc) and chain length for surfactants 2 and 3 was obtained (see Figures 1 and 2, respectively). With the exception of 3c, all points give a straight line on the plot. 3c shows a considerably lower cmc than expected (according to Figure 2, it should be 0.65 mmol/ L). The slope of the cmc curve (surface tension has a minimum, not constant value at the cmc) for 3c demands further investigation. From the most fitted lines to the points in Figures 1 and 2 it is possible to find the slope values of the straight lines. For Figure 1 (surfactants 2) the slope value is 0.30; for Figure 2 (surfactants 3) it is 0.29. It agrees well with theory, because usually for ionic surfactants the cmc decreases 4 times when the alkyl chain increases by two carbons, and the relation (log 4)/2 also gives the slope value 0.30.3 From the connection log (cmc)/chain length it is possible to foresee approximately the cmc values for this group of surfactants with different alkyl chain lengths. Interfacial Tension. The interfacial tension at the interface between water and dodecane was measured for three groupssfor nonionics 1 (soluble in dodecane), for cationics 2 and 3 (soluble in water), and for the cationic surfactant 4d (soluble in water). The values were compared with the interfacial tension between pure dodecane and pure water, i.e. 49.8 mN/m. The interfacial tension measurements of one group of surfactants show which chain length of the alkyl group R decreases the interfacial tension between oil and water in an emulsion most effectively for a given surfactant series. The lowest interfacial tension shows the optimal chain length.
For all surfactants interfacial tension was determined using 0.5 mmol/L solutions of surfactants in dodecane (1) or in water (2, 3, 4d). To compare the results, 0.5 mmol/L solutions of two well-known surfactants (cationic CTAB; nonionic NPEO10) were prepared and interfacial tension was determined for them, too. The results of the interfacial tension measurements are given in Table 3. For the nonionics 1b, 1c, and 1d the interfacial tension values are approximately the same, but for 1a and 1e they are higher. An optimal alkyl chain length seems to be R ) C10H21 to C16H33. However there is a large difference between the obtained values for surfactant 1b (R ) C10H21) and for the well-known nonionic surfactant NPEO10 (R ) C9H19) (a branched hydrophobic group is equivalent to 10.5 carbon atoms in a straight chain3). The last works more effectively; it gives a very low interfacial tension (3.2 mN/m). The interfacial tension values for cationic group 2 are close to those for nonionics 1. There is no large difference in interfacial tension values depending on chain length for these surfactants. The minimal value (20.5 mN/m) is found for 2b. The interfacial tension for 2d is higher (26.1 mN/m) than the value of the cationic surfactant CTAB with the same alkyl chain length (11.0 mN/m). The cationics 2a-e appear to be not as efficient as the other cationic group 3. Cationics 3 show higher efficiency, especially those with longer chains. For this group (3), interfacial tension decreases with increasing alkyl chain length. From the obtained results one can get information about the optimal values of alkyl chain length for all three groups (1, 2, and 3) of surfactants. It is evident that the shortest chain (R ) C8H17) does not work well enough. For the individual cationic surfactant 4d the interfacial tension is found to be 7.0-7.5 mN/m, and it is lower than the value for CTAB. During the experiment it was very difficult to separate layers even after centrifugation of the solution several times. It looks like such a surfactant, containing an allyl group in the hydrophilic part of the molecule, could be very effective for the stabilization of emulsions. Interfacial tension is also related to initial droplet size in emulsions. From interfacial tension values one can foresee how the surfactant will work in an emulsionsthe lower the interfacial tension of a given surfactant, the smaller should be the initial droplets in the emulsion. Droplet Size. A key factor in achieving colloidally stable emulsions is to create small droplets in the emulsification process. A minimal amount of emulsifiers (surfactants) is desirable. To obtain small droplets at a low concentration of emulsifier, it is essential to optimize
(3) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; John Wiley & Sons, Inc.; New York, 1989; p 431.
(4) O ¨ stberg, G.; Bergentsta˚hl, B.; Hulde´n, M. J. Coating Technol. 1994, 66 (832), 37-46.
Figure 2. 2-(N,N,N-Triethylammonio)ethyl alkyl maleate iodides (3).
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Table 4. Droplet Size of Polymerizable Dialkyl Maleates in O/W Emulsion, µm compound
d (4;3)a d (4;3) + NaCl alkyl chain (R) 0 day 1 day 8 days 0 day 1 day 8 days
1a 1b 1c 1d 1e NPEO10
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C9H19
35.0 25.8 3.8 4.4 4.6 2.0
2a 2b 2c 2d 2e 3a 3b 3c 3d 3e 4d CTAB
n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C8H17 n-C10H21 n-C12H25 n-C16H33 n-C17H35 n-C16H33 n-C16H33
3.8 4.0 3.4 3.2 2.4 4.8 3.4 3.9 3.2 3.0 3.1 4.1
5d 6d 6f
n-C16H33 n-C16H33 n-C18H37
3.4 3.4 3.5
nonionics 45.6 >65b 41.0 41.0 6.2 5.2 7.2 5.2 5.3 5.0 1.9 2.0
39.2 19.2 4.3 4.2 4.2 2.1
50.1 48.1 38.1 19.2 15.0 2.2
>65b >65b 50.9 36.1 32.9 2.3
4.5 4.4 3.7 6.4 2.6 9.5 3.3 3.7 3.3 3.0 2.9 4.3
5.5 3.4 2.9 2.8 2.7 5.2 3.5 3.6 3.4 3.1 3.3 3.6
11.4 3.5 3.0 2.8 2.7 6.8 3.4 4.0 3.5 3.1 3.4 3.7
33.1 7.5 2.9 2.8 2.7 12.1 3.3 3.9 3.6 3.2 3.3 3.3
Zwitterionics 5.3 8.5 3.3 3.5 3.6 4.1
3.2 9.0 6.7
3.2 8.6 7.3
3.3 7.5 6.2
Cationics 4.0 4.6 3.7 4.5 2.5 4.5 3.7 4.2 3.7 3.1 3.1 5.1
Figure 3. Droplet size of 2-(N,N-diethylamino)ethyl alkyl maleates (1) in O/W emulsion.
a d(4;3): volume weighted mean diameter, µm. b DS: data saturationsdroplets are too large for the chosen lens (interval 0.180 µm).
the emulsification conditions with regard to type of emulsifier and emulsification temperature.4 O/W emulsions for droplet size measurements at room temperature were prepared using a definite amount (0.09% for all solutions) of surfactant as stabilizer. A mixture of decane and 1-bromodecane has been used in the emulsions, and not pure decane, because the mixture of decane (20.6 volume %) and 1-bromodecane (79.4 volume %) has a density close to that of water and it was used in order to minimize the effect of gravity on the stability of the emulsion. The initial droplet size, the droplet size after 1 day, and that after 8 days time were measured. From the results after 1 day and 8 days, it is possible to make conclusions about the stability of the emulsion. In order to check the effect of electrolyte, the same measurements were performed in the presence of NaCl. Electrolyte should influence the stability of emulsions where an ionic surfactant is used. In this case the electrostatic repulsion between the droplets in the emulsion hinders the coalescence of droplets. When NaCl is added, the electrostatic repulsion is screened and the stability of the emulsion decreases. Electrolyte should not influence the stability of emulsions in the case of nonionic surfactant because there are only steric repulsions between droplets. All the obtained results from droplet size measurements are presented in Table 4. For nonionic surfactants 1 droplet size decreases with increasing alkyl chain length (see Figure 3). The emulsion that is most stable in time is the one stabilized with the longest examined chain (1e). It’s droplet size changes in the interval 4.6-5.3 µm. In comparison with NPEO10, whose droplet size changes in the interval 1.9-2.0 µm, 1b has a big initial droplet size (25.8 µm), and there is a large difference in stability as well. With the addition of 40.0 mM NaCl droplet size grows considerably for all five chain lengths. There is no difference in initial droplet size (with electrolyte or not), but after 8 days with NaCl droplet size is bigger than 30 µm for all surfactants (1a-1e).
Figure 4. Droplet size of 2-(N,N-diethylammonio)ethyl alkyl maleate chlorides (2) in O/W emulsion.
Addition of electrolyte does not influence NPEO10. A decrease in stability is observed for emulsions of 1 with electrolyte. One can suppose that nonionic 1 does not work efficiently enough for stabilization of the emulsion, since the droplets are rather big even without electrolyte and addition of electrolyte decreases the stability even more. The cationic surfactants 2 and 3 generally show the same tendency of nonionics 1sdroplet size becomes smaller and emulsions more stable with increasing alkyl chain length. Really small droplet size (2.9-3.1 µm) and excellent stability in time and in the presence of electrolyte were only shown by the individual cationic surfactant 4d (see Table 4). Within the cationic group 2 surfactant 2e is the most stable in time, and it has a smaller droplet size than 3e and 4d (the best surfactants from other cationic groups). Surfactant 2d gives a smaller initial droplet size than CTAB (3.2 and 4.1 µm, respectively). It is difficult to see any tendency with addition of electrolyte for this group (see Figure 4). In group 3, the quaternary ammonium surfactant with the longest alkyl chain (3e) works most efficientlysthe droplet size with it reaches 3.0-3.1 µm. Surfactant 3d has a lower initial droplet size than CTAB (3.2 and 4.1 µm, respectively). With the exception of 3a, other emulsions in this group show good stability in time, and droplet size is not influenced by the addition of electrolyte (see Figure 5). Measurements with zwitterionics show the same initial droplet size for all three investigated surfactantssfor 2-[N,N-diethyl-N-(3-sulfopropyl)ammonio]ethyl hexadecyl maleate (5d) and for 3-[N,N-dimethyl-N-(sulfopropyl)ammonio]propyl alkyl maleates (6d, 6f). Emulsions with surfactants 6d and 6f are stable in time. These emulsions are stable in time as well in the presence of NaCl, only
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Figure 5. Droplet size of 2-(N,N,N-triethylammonio)ethyl alkyl maleate iodides (3) in O/W emulsion.
Figure 6. Initial droplet size in an O/W emulsion of nonionic surfactant 1e.
Figure 7. Initial droplet size in an O/W emulsion of cationic surfactant 3e.
A ˆ bele et al.
Figure 9. Droplet size of dialkyl maleate iodide (3a) in O/W emulsion at different concentrations of surfactant.
One can see from droplet size and interfacial tension measurements that generally the lower the interfacial tension between water and dodecane (with a surfactant in the system) the smaller the initial droplet size in emulsion. The amount of substance used in the emulsions was close to their cmc value for all cationic surfactants. The cmc is higher only for 3a; the concentration of surfactant in the emulsion was 1.86 mmol/L, but the cmc is 9.12 mmol/L. Therefore it was necessary to check if increasing the concentration of this surfactant gives a more stable emulsion. The smallest droplet size is obtained with a 10.00 mmol/L concentration of surfactant (see Figure 9). It is reasonable, because the cmc of 3a is 9.12 mmol/L. But at all concentrations (interval 1.86-10.00 mmol/L) the stability of the emulsion remains good enough only for 1-2 days. After 8 days the droplet size increased considerably, from 2.8 up to 10.2 µm, and particles of surfactant were seen on the bottom. Addition of an electrolyte increases the droplet size even more. It reaches 37.7 µm for a 10.00 mmol/L concentration. The smallest influence of electrolyte is observed for the smallest (1.86 mmol/L) surfactant concentration; the droplet size from 4.8 µm with electrolyte grows up to 12.1 µm after 8 days. For all concentrations with NaCl there were oil-droplets observed on the bottom. One can conclude from these measurements that there is no use in working at high concentrations of surfactants (higher than 0.09% for all solutions), even if the cmc differs from this value. A higher concentration gives a smaller initial droplet size (1.86 mmol/L, 4.8 µm; 10.00 mmol/L, 2.8 µm) but, unfortunately, does not provide good stability of the emulsion. Conclusions
Figure 8. Initial droplet size in an O/W emulsion of zwitterionic surfactant 6f.
the initial droplet size with electrolyte becomes approximately two times bigger than that without. It is difficult to make conclusions about 5d, because it shows higher stability and smaller droplet size with electrolyte. In Figures 6-8 three diagrams of initial droplet size distribution for nonionic 1e, cationic 3e, and zwitterionic 6f are presented.
Decreasing droplet size and increasing emulsion stability with increasing alkyl chain length of the examined polymerizable surfactants are observed in all cases. The influence of electrolyte is bigger for unstable emulsions and smaller for stable emulsions in both the nonionic and ionic surfactant cases. Nonionics 1 do not provide stability of the emulsion in the presence of NaCl. Both cationic groups 2 and 3 are quite suitable for the stabilization of emulsions. From examined surfactants the optimal alkyl chain length seems to be R ) C17H35. Amine salt 2e gives a smaller initial droplet size, but quaternary ammonium salt 3e provides a higher stability of the emulsion. The presence of electrolyte does not influence emulsion stability. Cationic surfactant 4d gives a small droplet size and quite good stability, and it certainly looks promising for stabilization of emulsions.
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Higher concentrations (>0.09% for all solutions) of surfactant give a smaller initial droplet size for cationic 3a but does not provide the necessary stability of emulsions. The zwitterionics 6d and 6f give stable emulsions, only the droplet size is two times bigger in the presence of electrolyte. Surfactants with improved characteristics of surface activitysthe cationics 2e, 3e, and 4d and the zwitterionics 6d and 6f scan be selected from the results above. It would be useful to try them in polymerization reactions to see if they really work efficiently in emulsion polymerization. Final conclusions could be made only after quality analysis of obtained latexes. Experiments of emulsion polymerization with the most promising of the
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investigated surfactants have been started, and the results will be published later. Regardless of promising results, one should remember that surface characterization experiments are not adequate for those in emulsion polymerization. It is possible that other ingredients (monomer, initiator system, buffers, and others) and conditions (temperature, pressure, rate of mixing) during the process of polymerization might significantly alter the results presented in this paper. Acknowledgment. The research work performed at the Institute for Surface Chemistry was possible only due to a scholarship presented by the Nordic Council of Ministers to one of the authors (S.A ˆ .). LA960577N
