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Surface Properties, Micellization, and Premicellar Aggregation of Gemini Surfactants with Rigid and Flexible Spacers Li D. Song and Milton J. Rosen* Surfactant Research Institute, Brooklyn College of the City University of New York, Brooklyn, New York 11210 Received June 26, 1995. In Final Form: November 1, 1995X Micellization and premicellar behavior of the two series of cationic surfactants, each with two hydrophilic and two hydrophobic groups in the molecule (“gemini” surfactants), one series with a rigid, hydrophobic spacer, and second with a flexible, hydrophilic one, have been studied by use of surface tension measurements. The data show the expected regular increase in surface activity with an increase in alkyl chain length for the shorter chain homologs but show increased deviation from the regularity with an increase in chain length when the number of carbon atoms in the alkyl chain exceeds a certain number. This deviation in surface activity appears to be due to the formation of small, non-surface active aggregates. Equilibrium constants calculated for the aggregation reaction show that the conditions facilitating micelle formation also favor formation of these premicelles, such as lower temperature, stronger ionic strength of the solution, and increased alkyl chain length. Geminis with a flexible, hydrophilic spacer appear to aggregate more readily than geminis with a rigid, hydrophobic spacer. Their shorter homologs are also more surface active than those having a rigid, hydrophobic spacer.
Introduction A considerable number of investigations have recently been reported on the surface and micellar properties of surfactants containing two hydrophilic and two hydrophobic groups in the molecule, called gemini or dimeric surfactants. These include molecules that contain either a flexible hydrophilic,1-4 flexible hydrophobic,5,6 or rigid hydrophobic7,8 linkage between the two hydrophilic groups (“spacer”). The interest in these molecules appears to be due to their unusual surface and bulk properties. These include unusually high surface activity and low critical micelle concentration (cmc) values,9 unusual viscosity changes with an increase in surfactant concentration,10 and unusual micellar structures.11,12 Noteworthy among these unexpected properties is the observed increase in the value of the cmc of some of these compounds when the chain length of the alkyl hydrophobic group is increased beyond a critical length,7,8,13 contrary to the monotonic decrease generally observed with this change. The present study was undertaken to investigate this phenomenon. Materials and Methods A. Materials. Two series of bis quaternary ammonium surfactants were synthesized in our laboratory14 from the X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) Okahara, M.; Masuyama, A.; Sumida, Y.; Zhu, Y.-P. J. Jpn. Oil Chem. Soc. (Yukagaku) 1988, 37, 716. (2) Zhu, Y.-P.; Masuyama, A.; Okahara, M. J. Am. Oil Chem. Soc. 1991, 68, 268. (3) Rosen, M. J.; Zhu, Z. H.; Hua, X. Y. J. Am. Oil Chem. Soc. 1992, 69, 30. (4) Masuyama, A.; Hirono, T.; Zhu, Y.-P.; Okahara, M.; Rosen, M. J. J. Jpn. Oil Chem. Soc. (Yukagaku) 1992, 41, 301. (5) Devinsky, F.; Lacko, I.; Bittererova, F.; Tomeckova, L. J. Colloid Interface Sci. 1986, 114, 314. (6) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (7) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451. (8) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (9) Rosen, M. J. CHEM TECH 1993, 23, 30. (10) Kern, F.; Lequeux, F.; Zana, R.; Candau, S. J. Langmuir 1994, 10, 1714. (11) Zana, R.; Talmon, Y. Nature 1993, 362, 228. (12) Karaborni, S.; Esselink, K.; Hilbers, P. A. J.; Smit, B.; Karthauser, J.; Van Os, N. M.; Zana, R. Science 1994, 226, 254. (13) Zhu, Y.-P.; Masuyama, Y.; Nagata, T.; Okahara, M. J. Jpn. Oil Chem. Soc. (Yukagaku) 1991, 40, 473.
Chart 1. Structures of the Gemini Surfactants Being Studied
appropriate dihalide and N,N-(dimethylalkyl)amine: one series, p-[CnH2n+1N+(CH3)2CH2]2C6H4‚2Br-, with a rigid, hydrophobic spacer, and the other series, [CnH2n+1N+(CH3)2CH2]2CH(OH)‚2Cl-, with a flexible, hydrophilic spacer, referred to below as (CnN)2Ar and (CnN)2OH, respectively. Elemental analysis for (C18N)2Ar. Theory: C, 67.15; H, 11.08; N, 3.26. Found: C, 67.18; H, 11.42; N, 3.20. For (C12N)2OH, theory for the monohydrate: C, 65.52; H, 12.32; N, 4.83. Found: C, 64.89; H, 12.30; N, 4.88. Structures are shown in Chart 1. B. Surface Tension Measurements. These were made at 25 ( 0.05 °C and 50 ( 0.2 °C by use of the Wilhelmy plate technique, with a sand blasted platinum blade of ca. 5 cm perimeter. When measurements were taken at 50 °C, the sample container was covered with a large plastic box placed in the thermostat, through which air at 50 °C, saturated with moisture, was passed to decrease the evaporation of the sample solution. Instruments were calibrated against water that had been first deionized and then distilled twice, the last time from alkaline permanganate solution through a 3-ft Vigreaux column with a quartz condenser. All sample solutions were aged before taking measurements, the aging time varying from 10 to 40 min at 50 °C and from 2 to 24 h at 25 °C, depending on the surfactant molar concentration.
Results and Discussion Figures 1-3 show the surface tension (γ)-log surfactant molar concentration (log C) plots for the members of the (CnN)2Ar series at various temperatures and solution ionic strengths. Figures 4 and 5 show the γ-log C plots for (14) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci, in press.
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Figure 1. Effect of counterion on surface activity: surface tension vs log C of (CnN)2Ar at 25 °C: (9), C8 in 0.1 N NaBr; (b), C10 in 0.1 N NaCl; ([), C10 in 0.1 N NaBr; (2), C12 in 0.1 N NaCl.
Song and Rosen
Figure 4. Surface tension vs log C of (CnN)2OH in 0.1 N NaCl at 50 °C: (+), C8; (×), C10; (2), C12; (b), C14; ([), C16.
Figure 5. Surface tension vs log C of (CnN)2OH in 0.1 N NaCl at 25 °C: (+), C8; (×), C10; (2), C12; (b), C14; ([), C16. Figure 2. Surface tension vs log C of (CnN)2Ar in 0.01 N NaCl at 50 °C: (+), C8; (×), C10; (2), C12; (b), C14; ([), C16; (9), C18.
Figure 3. Surface tension vs log C of (CnN)2Ar in 0.1 N NaCl at 50 °C: (+), C8; (×), C10; (2), C12; (b), C14 experimental; ([), C14 expected; (9), C16.
members of the (CnN)2OH series at two different temperatures. Table 1 lists the surface properties of the geminis obtained from these plots. Included are values of the cmc, pC20 (negative log of the C20 value), γcmc (surface tension at the cmc), and Amin (minimum area per surfactant molecule of the aqueous solution/air interface).
Generally, measurements are best taken in solutions containing a swamping amount of electrolyte, to make unambiguous the value of the coefficient in the Gibbs absorption equation used to calculate the value of Amin. In a swamping amount of electrolyte, the value of the coefficient is one.15a In addition, there is no change in the ionic strength of the aqueous solution with a change in the concentration of the surfactant when the surfactant is ionic since a change in the ionic strength of the solution changes the effective charge of the hydrophilic group. This is particularly important when the surfactant, as in this case, has two ionic hydrophilic groups. Because of the low solubility of the members of the (CnN)2Ar series in water at 25 °C, only the C8 and C10 homologs could be measured at 25 °C in 0.1 N NaBr. The (C10N)2Ar and (C12N)2Ar homologs could be measured at 25 °C in 0.1 N NaCl, but not in 0.1 N NaBr, while the higher homologs could be measured, even in water, only at an elevated temperature (50 °C). As a result, the data obtained in 0.1 N NaCl are for the surfactants with Cl- counterions, rather than Br- counterions. Figure 1 shows the surface tension (γ)-log surfactant molar concentration (log C) plots for (C8N)2Ar and (C10N)2Ar in aqueous 0.1 N NaBr at 25 °C and of (C10N)2Ar and (C12N)2Ar in aqueous 0.1 N NaCl at 25 °C. It shows the expected increase in surface activityslower cmc and C20 (15) Rosen, M. J. Surfactants and Interfacial Phenomenas, 2nd ed.; Wiley: New York, 1989; (a) p 68 (b) pp 87, 136.
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Table 1. Surface Properties of the Geminis Investigated compound
media
temp (°C)
cmc (mol/L)
pC20
γcmc (mN/m)
Amin (nm2 × 102)
(C8N)2Ar (C10N)2Ar (C12N)2Ar (C14N)2Ar (C16N)2Ar (C18N)2Ar
0.01 N NaCl 0.01 N NaCl 0.01 N NaCl 0.01 N NaCl 0.01 N NaCl 0.01 N NaCl
50 50 50 50 50 50
3.16 × 10-2 6.31 × 10-3 4.57 × 10-4 1.38 × 10-5 8.71 × 10-6 3.80 × 10-6
1.90 2.66 3.93 5.44 5.98 5.62
41.2 40.6 40.2 40.0 38.4 43.8
50 58 78 72 98 44
(C8N)2Ar (C10N)2Ar (C12N)2Ar (C14N)2Ar (C16N)2Ar (C18N)2Ar
0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl
50 50 50 50 50 50
2.00 × 10-2 1.15 × 10-3 3.98 × 10-5 7.24 × 10-6 5.37 × 10-6 2.09 × 10-6
2.20 3.48 5.08 5.62 5.74 5.90
42.0 40.6 38.4 37.8 37.4 43.2
96 94 68 47 46 45
(C8N)2Ar (C10N)2Ar (C10N)2Ar (C12N)2Ar
0.1 N NaBr 0.1 N NaBr 0.1 N NaCl 0.1 N NaCl
25 25 25 25
1.20 × 10-2 6.31 × 10-4 1.10 × 10-3 3.31 × 10-5
2.80 4.72 4.10 5.60
39.4 37.0 40.6 39.6
68 98 93 95
(C8N)2OH (C10N)2OH (C12N)2OH (C14N)2OH (C16N)2OH
0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl
50 50 50 50 50
1.07 × 10-2 6.31 × 10-4 2.29 × 10-5 7.94 × 10-6 6.92 × 10-6
2.78 4.14 5.62 6.60 6.00
36.0 34.0 32.2 30.4 36.8
68 68 55 84 72
(C8N)2OH (C10N)2OH (C12N)2OH (C14N)2OH (C16N)2OH
0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl 0.1 N NaCl
25 25 25 25 25
9.55 × 10-3 3.98 × 10-4 9.55 × 10-6 1.00 × 10-5 3.16 × 10-5
3.10 4.60 6.04 6.22 5.10
38.2 35.8 33.0 32.6 41.4
74 73 51 55 47
Figure 6. log cmc vs alkyl chain carbon number (n) of (CnN)2Ar at 50 °C: (b), in 0.01 N NaCl; (9), in 0.1 N NaCl.
Figure 7. log cmc vs alkyl chain carbon number (n) of (CnN)2OH in 0.1 N NaCl: (b), at 25 °C; (9), at 50 °C.
(concentration to reduce the surface tension of the solvent by 20 mN/m) values (Table 1)swith an increase in the alkyl chain length and the expected decrease in surface activity with a less tightly bound counterion Cl- (due to the greater hydration of Cl-, compared to Br-). Figure 2 shows the γ-log C plots for members of the (CnN)2Ar series in 0.01 N NaCl aqueous solution at 50 °C. For (C8N)2Ar and (C10N)2Ar, the solutions near and above the cmc’s do not contain swamping amounts of NaCl. The values of the cmc, and possibly Amin may therefore be due to the surfactants with mixed counterions of Cl- and Br-. The plots of the C8, C10, C12, and C14 homologs show the expected increase in surface activity with an increase in the number of carbon atoms in the alkyl chain. However, the plots of the C16 and C18 homologs indicate anomalous behavior under these conditionsslarger γcmc and/or larger cmc and C20 values than expected. Figure 3 shows the γ-log C plots for the (CnN)2Ar series in 0.1 N NaCl at 50 °C. Here, the anomalous behavior is shown also by the C14 homolog. In Figures 4 and 5, showing the γ-log C plots for members of the (CnN)2OH
series in 0.1 N NaCl at 50 °C and 25 °C, respectively, anomalous behavior is shown by the C14 and C16 homologs. It is well-known that there is a linear relationship between the log of the C20 value or the log of the cmc and the number of carbon atoms (n) in the alkyl chain of the hydrophobic group of conventional surfactant5,15b. Figures 6 and 7 show plots of log cmc versus n for the (CnN)2Ar and (CnN)2OH series, respectively. Figures 8 and 9 show pC20 (-log C20) versus n for the (CnN)2Ar and (CnN)2OH series, respectively. When the value of n exceeds a certain value, which varies with the electrolyte content and the temperature of the solution, the log C20 and the log cmc values deviate from the expected linear relationship. This is the result of the unexpectedly low surface activity of the solutions. Menger and Littau,7,8 working with geminis having rigid hydrophobic spacers, including the (CnN)2Ar series investigated here, have observed an increase in the cmc of those geminis in water with an increase in the length of the alkyl chain to 16 carbons or more. They have suggested the formation of small, submicellar
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Song and Rosen Chart 2. Schematic Structure of Dimers
Figure 8. pC20 vs alkyl chain carbon number (n) of (CnN)2Ar at 50 °C: (b), in 0.01 N NaCl; (9), in 0.1 N NaCl.
Figure 9. pC20 vs alkyl chain carbon number (n) of (CnN)2OH in 0.1 N NaCl: (b), at 25 °C; (9), at 50 °C.
aggregates in solution when the alkyl chain is 16 carbons or more in those geminis and have presented some evidence for their existence. Premicellar Equilibrium Constants and Aggregation Numbers Since, as we have seen in Figures 2 and 3, an increase in the alkyl chain length to a certain value results in a smaller increase in surface activity than expected for both the (CnN)2Ar and (CnN)2OH series, it can be assumed that any premicellar aggregates formed have little or no significant surface activity. With this assumption, it is possible to calculate an equilibrium constant, Keq, for the formation of these small aggregates and also to determine their aggregation number. The interaction of surfactant (monomeric) molecules to form small aggregates (Sx) can be described by the equation
xS h Sx
(1)
Keq ) [Sx]/[S]x
(2)
from which
Values of [Sx] and [S] for use in eq 2 are obtained in the following manner. We can estimate the values of C20 and γcmc, for the higher homologs showing deviation from pC20 versus n and log cmc versus n linearity, in the absence of premicellar aggregation by assuming that without premicellar aggregation they would fall on linear plots of pC20 or log cmc versus n and estimating the value of γcmc from
the values of the lower homologs. We can now construct the γ-log C plots for the higher homologs, expected in the absence of premicellar aggregation, from the values of C20 and γcmc, since the plot is linear between these two points. One such plot is shown in Figure 3. Since the premicellar aggregates are assumed to have no surface activity, the observed γ values of the solutions at different surfactant concentrations must be due only to the monomeric S. The value of [S] can then be obtained at any value of γ from the γ-log C plot expected for that homolog in the absence of premicellar aggregation. The value of [Sx] is the difference between the total surfactant concentration in the solution phase at any value (from the experimental γ-log C plot) minus [S] at the same γ value, divided by x. Different integral values of x can be taken and Keq values calculated at different concentrations. Results are shown in Table 2. Since the units for K2 are (mol/L)-1, for K3 (mol/L)-2, and for K4 (mol/L)-3, to compare these values for constancy we have used (K3)1/2 and (K4)1/3. These are also listed in Table 2. It is apparent that the values, compared in this manner, have the same degree of constancy, indicating that a series of oligomers are formed. It is to be expected that, in these premicellar aggregates, the molecules of geminis will be arranged with their (similarly charged) hydrophilic groups at opposite ends of the structure and with their hydrophobic groups oriented toward each other in a manner somewhat similar to that in a very small bilayer or lamellar micelle (Chart 2). Since aggregation of any type involves the close approach of two similarly charged monomeric molecules to each other, it is to be expected that aggregation will be facilitated by the same factors that facilitate micellization, e.g., an increase in the length of the alkyl hydrophobic groups, a decrease in the charge on the hydrophilic group, and a reduction in thermal motion. All these factors should increase the value of Keq. Inspection of the values of Keq in Table 2 shows that this is indeed the case. In 0.01 N NaCl at 50 °C, where the charge on the hydrophilic groups is greater than in 0.1 N NaCl, and where thermal agitation is greater than at 25 °C, we would expect that the Keq values for a particular chain length would be smallest and that only for the longest chain homologs would the γ-log C plots deviate from their expected positions (Figure 2), while in 0.1 N NaCl at 25 °C the Keq values for a particular chain length would be greatest. This accounts for the positions of the γ-log C plots of the higher homologs, relative to the lower homologs in Figures 2 and 3, and the chain length at which deviation of the log C20 and log cmc-n plots from linearity first appears (Figures 6-9). Inspection of the Keq values in Table 2 also reveals that there is a stronger tendency to form premicellar aggregates in geminis with a flexible, hydrophilic spacer
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Langmuir, Vol. 12, No. 5, 1996 1153
Table 2. Keq of Investigated Geminis at Four Different Surface Tensions K4
(K4)1/3
1.80 × 2.61 × 106 3.61 × 106 4.63 × 106 1.8-4.6 × 106
1018
3.20 × 1.11 × 1018 3.24 × 1019 7.67 × 1019 3.2-76.6 × 1018
1.47 × 106 1.03 × 106 3.19 × 106 4.25 × 106 1.5-4.2 × 106
1.39 × 1018 2.41 × 1018 4.81 × 1018 1.05 × 1019 1.4-10.5 × 1018
1.18 × 109 1.55 × 109 2.19 × 109 3.24 × 109 1.2-3.2 × 109
9.34 × 1025 2.28 × 1026 7.20 × 1026 2.50 × 1027 0.93-25.0 × 1026
4.54 × 108 6.11 × 108 8.96 × 108 1.36 × 109 4.5-13.5 × 108
2.02 × 108 3.36 × 108 4.20 × 108 4.81 × 108 2.0-4.8 × 108
1.94 × 1015 5.37 × 1015 8.87 × 1015 1.28 × 1016 1.9-12.8 × 1015
4.40 × 107 7.33 × 107 9.42 × 107 1.13 × 108 4.3-11.3 × 107
2.10 × 1022 9.26 × 1022 2.10 × 1023 3.82 × 1023 2.1-38.2 × 1022
2.76 × 107 4.52 × 107 5.94 × 107 7.26 × 107 2.8-7.2 × 107
3.02 × 1012 5.71 × 1012 7.93 × 1012 9.97 × 1012 3.0-9.8 × 1012
1.11 × 1021 3.63 × 1021 6.66 × 1021 1.05 × 1022 1.1-10.5 × 1021
3.33 × 1010 6.02 × 1010 8.16 × 1010 1.02 × 1011 3.3-10.2 × 1010
4.56 × 1029 2.59 × 1030 6.29 × 1030 1.25 × 1031 0.46-12.5 × 1030
7.70 × 109 1.37 × 1010 1.85 × 1010 2.32 × 1010 7.7-23.2 × 109
2.44 × 106 3.21 × 106 4.01 × 106 4.67 × 106 2.0-4.8 × 108
1.63 × 1012 1.79 × 1013 2.12 × 1013 5.29 × 1013 1.6-52.9 × 1012
1.28 × 106 4.23 × 106 4.60 × 106 5.29 × 106 1.3-5.3 × 106
1.22 × 1018 8.89 × 1018 1.28 × 1019 3.82 × 1019 1.2-38.2 × 1018
1.67 × 106 2.08 × 106 2.34 × 106 3.37 × 106 1.7-3.4 × 106
9.21 × 108 1.80 × 109 2.20 × 109 3.64 × 109 9.2-36.4 × 108
1.61 × 1016 5.15 × 1016 7.33 × 1016 1.71 × 1017 1.6-17.1 × 1016
1.27 × 108 2.27 × 108 2.71 × 108 4.14 × 108 1.3-4.1 × 108
3.18 × 1023 1.65 × 1024 2.75 × 1024 9.06 × 1024 3.2-90.6 × 1023
6.83 × 107 1.18 × 108 1.40 × 108 2.08 × 108 6.8-20.8 × 107
2.50 × 1011 2.73 × 1011 2.81 × 1011 5.21 × 1011 2.5-5.2 × 1011
8.29 × 1019 1.07 × 1020 1.33 × 1020 3.16 × 1020 8.3-31.6 × 1019
9.10 × 109 1.03 × 1010 1.15 × 1010 1.78 × 1010 9.1-17.8 × 109
3.11 × 1028 4.73 × 1028 7.08 × 1028 2.15 × 1029 3.1-21.5 × 1028
3.14 × 109 3.62 × 109 4.14 × 109 5.99 × 109 3.1-6.0 × 109
7.34 × 106 1.03 × 107 1.16 × 107 1.35 × 107 7.3-13.5 × 106
9.32 × 1012 2.39 × 1013 3.23 × 1013 5.70 × 1013 9.3-57.0 × 1012
3.05 × 106 4.86 × 106 5.68 × 106 7.55 × 106 3.1-7.6 × 106
1.33 × 1019 6.22 × 1019 1.01 × 1020 2.70 × 1020 1.3-27.0 × 1019
2.37 × 106 3.96 × 106 4.66 × 106 6.46 × 106 2.3-6.5 × 106
1.19 × 1010 1.20 × 1010 1.31 × 1010 1.43 × 1010 1.2-1.4 × 1010
6.90 × 1017 7.97 × 1017 1.05 × 1018 1.55 × 1018 6.9-15.5 × 1017
8.31 × 108 8.93 × 108 1.02 × 109 1.24 × 109 8.3-12.4 × 108
6.46 × 1025 5.98 × 1025 9.43 × 1025 1.89 × 1026 6.5-18.9 × 1025
4.01 × 108 3.91 × 108 4.55 × 108 5.74 × 108 4.0-5.7 × 108
compounds
K2
K3
(C14)2OH (0.1 N Cl-) (50 °C)
3.69 × 4.65 × 106 5.92 × 106 6.70 × 106 3.7-6.7 × 106
3.24 × 6.79 × 1012 1.30 × 1013 2.14 × 1013 3.2-21.4 × 1012
2.34 × 1010 2.87 × 1010 3.62 × 1010 4.99 × 1010 2.3-5.0 × 1010
range: (C16)2OH (0.1 N Cl-) (50 °C) range: (C14)2OH (0.1 N Cl-) (25 °C) range: (C16)2OH (0.1 N Cl-) (25 °C) range: (C14)2Ar (0.1 N Cl-) (50 °C) range: (C16)2Ar (0.1 N Cl-) (50 °C) range: (C18)2Ar (0.1 N Cl-) (50 °C) range: (C16)2Ar (0.01 N Cl-) (50 °C) range: (C18)2Ar (0.01 N Cl-) (50 °C) range:
106
1012
than in those with a rigid, hydrophobic one. This may be because the hydrophobic groups in the former molecules can pack together more closely than in the latter. This can be seen from the Amin values in Table 1 for the lower homologs of the two different gemini types under similar conditions. In 0.1 N NaCl at 50 °C, (C8N)2Ar has a Amin value of 0.96 nm2, while for (C8N)2OH the value is 0.68 nm2; (C10N)2Ar has a Amin value of 0.94 nm2, while for (C10N)2OH the value is 0.68 nm2; for the C12 homologs, the values are 0.68 and 0.55 nm2, respectively. In 0.1 N NaCl at 25 °C, (C10N)2Ar has a Amin value of 0.93 nm2, and the (C10N)2OH value is 0.73 nm2; for the C12 homologs, the values are 0.95 and 0.51 nm2, respectively. The energy for this closer packing (required to overcome the repulsion involved in bringing the two similarly charged quaternary ammonium groups close together) may come from the energy released upon hydrogen bond formation between the hydroxyl group in the spacer and water molecules. This energy release would not be present for geminis with a hydrophobic spacer. Conclusions The surface activity of short alkyl chain (e.g., C8-C12) quaternary ammonium geminis with either rigid, hydro-
(K3)1/2 106
phobic or flexible, hydrophilic spacers increases regularly with an increase in the alkyl chain length of the hydrophobic group. When the carbon atom number of the chains is above a certain value, which depends upon the molecular environment (temperature and solution ionic strength) there is a deviation, which increases with chain length, from this behavior. This appears to be due to the formation of premicellar aggregates, with little or no surface activity. Aggregation is facilitated by the same factors that favor micellization: larger solution ionic strength, lower temperature, and longer alkyl chain lengths of the hydrophobic groups. Geminis with a flexible hydrophilic spacer appear to aggregate more readily than those with a rigid hydrophobic one.
Acknowledgment. This material is based upon work supported by grants from the National Science Foundation (Grant CTS-9312715), the Colgate-Palmolive Company, Reckitt and Colman Co., Rhoˆne-Poulenc Surfactants and Specialties, and Witco Corp. LA950508T
