5354
Langmuir 2002, 18, 5354-5359
Thermodynamic and Aggregation Properties of Gemini Surfactants with Hydroxyl Substituted Spacers in Aqueous Solution S. D. Wettig,† P. Nowak,‡ and R. E. Verrall*,§ Department of Biochemistry, University of Saskatchewan, 107 Wiggins Avenue, Saskatoon, Saskatchewan, Canada S7N 5E5, Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Box 97, Cambridge, Massachusetts 02138, and Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan, Canada S7N 5C9 Received December 10, 2001. In Final Form: April 8, 2002 Critical micelle concentration (cmc), degree of micelle ionization (R), headgroup area (a0), mean aggregation number, enthalpy of micellization (∆HM), and volume of micellization (∆Vφ,M) properties are reported for a series of gemini surfactants, 12-4(OH)n-12 (n ) 0, 1, or 2), with increasing hydroxyl substitution within the spacer group. The cmc and mean aggregation numbers are observed to decrease with increased hydroxyl substitution consistent with increasing hydrophilicity of the headgroup. Both a0 and R are unaffected by hydroxyl substitution. ∆HM° is observed to decrease (become more exothermic) with increased substitution of the spacer group, while ∆Vφ,M is observed to increase. These observations are rationalized in terms of changes in the solvation and conformation of the spacer group at the micelle-bulk solution interface. Free energies of micellization, ∆GM°, and entropies of micellization, ∆SM°, are calculated from the ∆HM°, cmc, and R values. The results indicate that the micellization process for all surfactants is entropy-driven.
Introduction Surfactants containing two hydrophilic and two hydrophobic groups have been the focus of considerable research interest since the early 1990s. Recently, a number of excellent reviews containing references to the bulk of the research carried out so far have described the solution behavior of these so-called “gemini” surfactants.1-3 They have a structure that can be thought of in terms of two typical surfactant molecules which are chemically linked at or near the headgroup. This chemical arrangement has been shown4 to provide a rather rich array of aggregate morphologies and solution properties that are dependent upon the nature and size of the linking group. The focus of this work was to examine, in greater detail, the effect of substitution in the alkyl spacer chain on the aggregation properties of the gemini surfactant. It has been well established that the variation in critical micelle concentration (cmc) and headgroup area (a0) of a gemini surfactant series having alkyl tails of fixed carbon atom length depends on the conformation of the spacer group at the micelle/water interface. Changes in the chemical structure of the spacer group have also been studied to determine the impact on the micellization properties of the gemini surfactants. For example, gemini surfactants with p-xylyl and acetylenic spacer groups were studied in an attempt to examine the effect of increased rigidity in the spacer on micellar properties.5-8 The results show that * To whom correspondence should be addressed. † Department of Biochemistry, University of Saskatchewan. ‡ Department of Chemistry and Chemical Biology, Harvard University. § Department of Chemistry, University of Saskatchewan. (1) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. Engl. 2000, 39, 1907-1920. (2) Rosen, M. J.; Tracy, D. J. J. Surfactants Deterg. 1998, 1, 547. (3) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (4) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (5) Menger, F. M.; Keiper, J. S.; Azov, V. Langmuir 2000, 16, 2062. (6) Song, L. D.; Rosen, M. J. Langmuir 1996, 12, 1149.
Chart 1. Structure of the Substituted Gemini Surfactants
any effect may only arise if the gemini compound has a spacer eight methylene units or more in length, i.e., greater than the equilibrium distance between the headgroups.5,8 The effect of substitution in the alkyl spacer group has been examined by surface tension methods for both the 12-3-12 with one hydroxyl substituted methylene group, and the 12-4-12 surfactant having two hydroxyl substituted methylene groups (Chart 1).6,7,9 The cmc decreased as a result of this substitution and was rationalized on the basis that the substituted spacer could form hydrogen bonds with water more readily, thus reducing the unfa(7) Rosen, M. J.; Song, L. D. J. Colloid Interface Sci. 1996, 179, 261. (8) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235, 310-316. (9) Rosen, M.; Liu, L. J. Am. Oil Chem. Soc. 1996, 73, 885.
10.1021/la011782s CCC: $22.00 © 2002 American Chemical Society Published on Web 06/05/2002
Gemini Surfactants with Hydroxyl Substituted Spacers
Langmuir, Vol. 18, No. 14, 2002 5355
vorable hydrocarbon-water contacts and making it easier for the spacer to locate at the micelle-water interface.9 In this work we present results of a comprehensive study of gemini surfactants having n-alkyl tails of 12 carbon atoms in length and a spacer group of 4 methylene units in length, with 0, 1, or 2 hydroxyl-substituted methylene groups (12-4(OH)n-12; n ) 0, 1, 2). The aggregation properties of these surfactants have been studied using specific conductance, surface tension, isothermal titration calorimetry, density, dynamic light scattering, and timeresolved fluorescence quenching methods. Experimental Section Materials. The gemini surfactants used in this study were synthesized according to procedures previously reported in the literature.9,10 Appropriate amounts of 1,4-dibromobutane, 1,4dibromo-2-butanol, or 1,4-dibromo-2,3-butanediol (for the 124-12, 12-4(OH)-12, and 12-4(OH)2-12 geminis, respectively) were reacted with 2 molar equiv (plus a 10% excess) of N,Ndimethyldodecylamine. The mixture was refluxed in HPLC grade 2-propanol (except for the 12-4-12 surfactant which was refluxed in acetonitrile) for 24 h. At the end of the reaction, the mixture was cooled and the solid material was recovered by filtration. It was recrystallized from acetonitrile (12-4-12) or a mixture of ethyl acetate/methanol (10:1 v/v), where appropriate, until a minimum in the surface tension vs log concentration profile in the postmicelle region was no longer observed. The composition and structure of the purified surfactants were confirmed by CH&N analysis and NMR spectroscopy, respectively. All surfactant solutions were prepared with water purified by using a Millipore Super-Q system. Methods. Critical micelle concentrations were determined by using both the electrical conductivity and surface tension methods. Specific conductivities were measured by using a Wayne-Kerr precision component analyzer (model 6425) operating at 1.5 kHz, with a Tacussel electrode having a cell constant of 1.15 cm-1. Experimental temperatures were maintained at 25 ( 0.1 °C, unless otherwise indicated, by means of a Haake (model F3) circulating water bath. Surface tension measurements were carried out by using a Kru¨ss (model K10T) tensiometer applying the du Nuoy ring technique. Surface tension values (γ) were corrected using the method of Harkins and Jordan.11 Experimental temperatures were maintained at 25 ( 0.1 °C using a Haake (model F3) circulating water bath. Enthalpies were measured by using a Calorimetry Sciences Corporation model 4200 isothermal titration calorimeter (ITC) with 1.3 mL cells. The sample and reference cells were filled with 1.0 mL and 1.1 mL of Millipore water, respectively. Concentrated surfactant solution was injected in 10 µL increments into the stirred sample cell using a 250 µL Hamilton syringe controlled by the injection apparatus of the instrument. A sample thermogram is shown in Figure 1. Enthalpies of micellization were obtained from the difference in the observed enthalpies (of dilution) above and below the cmc. The density values required for the calculation of the apparent molar volumes of the surfactants were obtained by using a Sodev model 02D vibrating tube flow densimeter12 following a technique previously described.13 The apparent molar volume (Vφ) of the aqueous surfactant was calculated from the relation
Vφ )
M 1000(d - d0) d mdd0
(1)
where M is the molar mass of the solute, d and d0 are the densities of the solution and solvent, respectively (in g cm-3), and m is the molality of the solution (in mol kg-1). (10) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (11) Harkins, W. D.; Jordan, H. F. J. Am. Chem. Soc. 1930, 52, 1751. (12) Picker, P.; Tremblay, E.; Jolcoeur, C. J. Solution Chem. 1974, 3, 377. (13) Huang, H.; Verrall, R. E. Can. J. Chem. 1997, 75, 1445.
Figure 1. Thermogram obtained for the 12-4-12 gemini surfactant. The micelle hydrodynamic radii and molecular weights were determined by using a Protein Solutions Inc. Dyna-Pro 99 dynamic light scattering instrument. The molecular weights could be calculated using the Dynamics software provided which is based upon a globular protein model. Aggregation numbers then could be determined from the ratio of the micelle and monomer surfactant molecular weights. However, verification of the suitability of application of this model to smaller sized aggregates was carried out by comparing the estimated aggregation numbers derived from this model with those obtained from fluorescence quenching methods for the series of alkyltrimethylammonium bromide (CnTAB, n ) 8, 10, 12, 14, and 16) surfactants. The globular protein model gave consistently lower aggregation numbers. As a result, aggregation numbers for the gemini surfactants were determined from dynamic light scattering measurements based on a molecular weight model derived from the aggregation numbers of the CnTAB surfactant series obtained from fluorescence quenching methods. The approach is described in more detail, below.
Results and Discussion The critical micelle concentration and degree of micelle ionization (R) determined from electrical conductivity measurements for the substituted gemini surfactants are given in Table 1. Estimates of values of R were obtained from the ratio of the slope of the specific conductance vs concentration curve (Figure 2) above and below the cmc.14,15 Also included in Table 1 are the cmc values and headgroup areas (a0) determined from plots of γ vs the logarithm of the surfactant concentration. The headgroup areas were determined from the surface excess concentration, Γ, according to eq 2
a0 ) (NAΓ)-1
(2)
where Γ is calculated from the Gibbs adsorption isotherm relation
Γ)-
1 dγ 2.30nRT d log C
(
)
T
(3)
The parameters R and T have their usual meaning and n is a constant dependent upon the number of individual ions comprising the surfactant. It can be seen that there is excellent agreement between the cmc values obtained from each method. Also, the values obtained for the cmc (14) Evans, H. C. J. Chem. Soc. 1956, 579. (15) Zana, R. J. Colloid Interface Sci. 1980, 78, 330.
5356
Langmuir, Vol. 18, No. 14, 2002
Wettig et al.
Table 1. Aggregation Properties of Hydroxyl Substituted (12-4(OH)n-12) Gemini Surfactants
a
n
cmca (mmol L-1)
R
0 1 2
1.17 ( 0.94 ( 0.02 0.87 ( 0.09
0.26 ( 0.25 ( 0.02 0.25 ( 0.02
0.04d
0.02d
cmcb (mmol L-1)
a0c (nm2/molecule)
RH (nm)
Nagg
1.1 ( 0.85 ( 0.18 0.75 ( 0.15
1.15 ( 1.19 ( 0.08 1.15 ( 0.04
1.712 1.656 1.550
36 33 30
0.1d
From specific conductance. b From surface tension. c Calculated using n ) 3 in eq 3.
Figure 2. Specific conductance versus surfactant concentration for the 12-4-12 (2), 12-4(OH)-12 (9), and 12-4(OH)2-12 (b) gemini surfactants.
agree well with those found in the literature, both for the 12-4-1216,8 and the 12-4(OH)2-129 surfactants. To our knowledge this work is the first to report the properties of the 12-4(OH)-12 surfactant. The cmc decreases with increased hydroxyl substitution of the surfactant spacer. There is no change observed (within experimental error) in the degree of micelle ionization, or in the headgroup area, with increased hydroxyl substitution. This is consistent with the idea that the spacer group is restricted to the micelle-water (or air-water) interface for short spacer groups.5,8 It is possible that substitution in a longer (i.e., >10 carbon atoms) spacer group may result in an increased value of a0 relative to the unsubstituted surfactant, due to the increased hydration of the substituted spacer and the likelihood of the spacer remaining at the interface rather than folding into the micelle core. The Gibbs energy change of micellization can be calculated from the following relation assuming a pseudophase-separation model for the aggregation process
∆GM° ) (3 - 2R)RT ln cmc
(4)
where R and T have their usual meaning, and the factor of (3 - 2R) accounts for the partial dissociation of the counterions from the micelle. It is important to note that values of ∆GM° for the gemini surfactants have been calculated more often by assuming a factor of (2 - R) to account for the degree of micelle ionization. This assumes the surfactant dissociates into two rather than three ions, and support for this assumption has been observed, experimentally, from neutron reflectivity studies of short spacer gemini salts. It was shown that headgroup areas determined from the Gibbs adsorption isotherm are in agreement with those determined from reflectivity measurements only when the surfactant is assumed to (16) Alami, E.; Levy, H.; Zana, R.; Skoulios, A. Langmuir 1993, 9, 940.
0.09d
d
From ref 8.
dissociate into two rather than three ionic species.17 Nevertheless, the headgroup areas, a0, derived from eq 3 and reported in Table 1 are for n ) 3. Furthermore, irrespective of which relation is more valid, the values for ∆GM° calculated from these two methods will differ by a constant factor, provided that R does not vary significantly. Values of ∆GM° (Table 2) for the surfactants with substituted spacers have been calculated using both relations, and each data set shows minimal variation with increasing hydroxyl substitution. The experimental enthalpies obtained are shown as a function of surfactant concentration in Figure 3. The enthalpy of micellization (∆HM) was obtained from the difference in the trends in enthalpies above and below the cmc at a concentration equal to the cmc. Because of the low values of the cmc, ∆HM is assumed to be equivalent to ∆HM°. The values of ∆HM° obtained for the substituted gemini surfactants are given in Table 2. It is seen that the value obtained for the 12-4-12 surfactant is approximately double that reported previously by Bai et al.,18,19 while it is in excellent agreement with the value of -9.3 kJ mol-1 obtained by Grosmaire et al.20 The uncertainties in the experimental enthalpy values reported here were calculated on the basis of what is felt to be appropriate error propagation methods. The errors are larger than those reported by Bai et al.18,19 although the sensitivity of the instruments appears to be comparable. At this time we can offer no further explanation for the discrepancy between the values reported here and elsewhere for ∆HM° of the 12-4-12 surfactant. Small differences in ∆HM° are found between the mono- and disubstituted spacer systems in this work. There is a larger difference in ∆HM° between the surfactants having unsubstituted and monosubstituted spacers. The entropy of micellization, ∆SM°, has been determined in the usual manner from the free energy and enthalpy of micellization. Values of T∆SM° are also reported in Table 2, and in all cases T∆SM° > -∆HM° indicating an entropydriven micellization process, similar to that observed for the 12-s-12 series of surfactants.18,19 Bai et al.19 have analyzed thermodynamic results obtained for the 12-s-12 series of gemini surfactants in terms of four main energy contributions: van der Waals interactions between the alkyl tails, hydrophobic interactions, headgroup repulsions, and the configuration of the spacer chain. The processes occurring in solution which give rise to the various contributions involve (i) the transfer of the alkyl tails of the surfactant from an aqueous to a hydrocarbon environment, (ii) changes in the configuration of the alkyl tails, (iii) electrostatic and steric interactions between (and within) the surfactant headgroups, and (iv) changes in the solvation of the spacer group that may involve configurational changes provided the spacer is sufficiently long so that it can fold into the core of the micelle. Of these processes, in this work several can be (17) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 43924396. (18) Bai, G.; Wang, J.; Yan, H.; Li, Z.; Thomas, R. K. J. Phys. Chem. B 2001, 105, 3105. (19) Bai, G.; Yan, H.; Thomas, R. K. Langmuir 2001, 17, 4501-4504. (20) Grosmaire, L.; Chorro, M.; Chorro, C.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 2002, 246, 175-181.
Gemini Surfactants with Hydroxyl Substituted Spacers
Langmuir, Vol. 18, No. 14, 2002 5357
Table 2. Thermodynamic Properties of Micellization for the 12-4(OH)n-12 Series of Gemini Surfactants n
∆GM° a (kJ mol-1)
∆HM° (kJ mol-1)
T∆SM° a (kJ mol-1)
∆GM° b (kJ mol-1)
∆HM° (kJ mol-1)
T∆SM° b (kJ mol-1)
0 1 2
-41.5 -43.2 -43.7
-9.2 ( 1.4 -12.1 ( 1.4 -11.5 ( 1.2
34.3 31.1 32.2
-29.1 -30.2 -30.6
-9.2 ( 1.4 -12.1 ( 1.4 -11.5 ( 1.2
21.9 18.1 19.1
a
Calculated assuming ∆G ) (3 - 2R)RT ln cmc. b Calculated assuming ∆G ) (2 - R)RT ln cmc.
Figure 3. Observed enthalpies versus surfactant concentration for the 12-4-12 (2), 12-4(OH)-12 (9), and 12-4(OH)2-12 (b) gemini surfactants. Lines are for visualization purposes only.
eliminated as possible explanations for the observed decrease in the absolute magnitude of ∆HM° between the compounds having a singly and doubly substituted spacer. Since the length and composition of the alkyl tails is kept constant, the contribution to ∆HM° arising from the transfer of the alkyl tails from the bulk to the micelle should also remain constant. Similarly, since the length of the spacer group is also held constant, contributions arising from headgroup steric repulsions and repulsions between the charge centers should also remain fixed. Support for this assumption can be seen in the near constant values obtained for R and a0. This leaves changes associated with the transfer of the spacer group from the bulk to the micellar phase, namely, changes in the solvation of the spacer group, as a possible explanation for the observed difference in ∆HM° for the hydroxylsubstituted surfactants. The enthalpy change associated with the transfer of both the alkyl tails and an alkyl spacer group of a gemini surfactant from an aqueous to a micellar environment is exothermic in nature.21,22 Short spacers in gemini compounds are known to reside at the surface of micelles.8,5 As a consequence, they can prevent contact between alkyl chains and the water. Previous work19 has shown that ∆HM° changes relatively little for gemini surfactants having unsubstituted spacer lengths of s ) 3-6. Substitution of hydroxyl groups in the spacer should diminish the hydrophobic nature of this segment and further enhance the “anchoring” of the spacer at the interface. Whether the disubstituted would be twice as effective as the monosubstituted spacer is unlikely for the following reasons. The difference between the ∆HM° values for the (21) Bashford, M. T.; Woolley, E. M. J. Phys. Chem. 1985, 89, 3173. (22) Evans, D. F.; Wennerstom, H. The Colloidal Domain: Where Physics, Chemistry, Biology, and Technology Meet, 1st ed.; VCH Publishers: New York, 1994.
Figure 4. Apparent molar volume versus surfactant concentration for the 12-4-12 ((), 12-4(OH)-12 (2), and 124(OH)2-12 (9) gemini surfactants. Solid lines are fits to the pseudo-phase-separation model.
substituted spacers may arise from impeded rotation of alkyl chains about the C2-C3 carbon bonds of the spacer when the species are in the micelle. While this effect is likely to be less of a factor for a four-carbon spacer compared to two- and three-carbon spacers, nevertheless, it may still contribute to the observed differences. The preferred conformation of the spacer in the micelle would be to expose the OH groups to the hydrophilic side of the micelle-water interface. To achieve this, the OH groups in the disubstituted spacer may have to occupy a gauche or eclipsed conformation and these conformations may enhance steric hindrance between the alkyl chains when they occupy the cis position in a micelle. This could lead to a relatively less negative change in ∆HM° between the gemini surfactants having mono- and disubstituted spacers. Substitution of the first hydroxyl group would not impose such a steric constraint, and consequently, the change in ∆HM° would be more negative with respect to the unsubstituted spacer. The apparent molar volumes obtained for the substituted gemini surfactants (Figure 4) were analyzed assuming a pseudo-phase-separation model for micelle formation.23 Above the cmc the experimental data can be fit to the relation
Vφ ) Vφ,M -
cmc ∆Vφ,M m
(5)
where Vφ,M is the value of the apparent molar volume of the surfactant in the micellar phase and ∆Vφ,M is the volume change due to micelle formation.24 Experimental data for all surfactants were fit to eq 5, using values for the cmc obtained from conductivity measurements, and keeping Vφ,M and ∆Vφ,M as adjustable parameters. Results (23) Desnoyers, J. E.; DeLisi, R.; Perron, G. Pure Appl. Chem. 1980, 52, 433. (24) Frindi, M.; Michels, B.; Zana, R. J. Phys. Chem. 1992, 96, 6095.
5358
Langmuir, Vol. 18, No. 14, 2002
Wettig et al.
Table 3. Additivity and Experimental Volume Data for a Series of 12-4(OH)n-12 Gemini Surfactants n
V° (cm3 mol-1)a
Vφ,cmc (cm3 mol-1)b
Vφ,M (cm3 mol-1)b
∆Vφ,M (cm3 mol-1)
0c 1 2
583.0 586.3 589.6
584 ( 1 587 ( 1 587 ( 1
597.7 ( 0.4 603.6 ( 0.6 605.7 ( 0.9
13.5 ( 0.9 17 ( 1 19 ( 1
a From additivity method. b From a fit of experimental data to the pseudo-phase-separation model. c From ref 8.
of the fits are presented in Table 3. The apparent molar volume of the surfactant at the cmc, Vφ,cmc, was obtained from Vφ,M and ∆Vφ,M. The values of Vφ,cmc are close to the values of V° obtained from a group additivity method previously described.8 Given the low values of the cmc properties of these salts (Table 1), these results are not unexpected. Also, the small differences may be due, in part, to the fact that the group additivity approach is not completely satisfactory because it assumes each group in the molecule contributes to the total volume as if it were independent of the group position and the nature of its surrounding. The structural organization of water molecules in the region of the monomer headgroups and spacer is difficult to describe quantitatively. However, it is known that quaternary ammonium headgroups are not well hydrated.25 Consequently, hydroxyl group substitution into the spacer should transform its hydration cosphere from one that is hydrophobic to one that is hydrophilic in nature. The increase in Vφ,cmc between the nonsubstituted and monosubstituted spacer in the 12-4-12 series is ca. 3 cm3 mol-1 (Table 3) and, presumably, occurs because the hydration water in the monosubstituted spacer is less structured than that in the nonsubstituted spacer. In Table 3 it is seen that the derived values of Vφ,cmc for the 2- and 2,3-substituted spacers of the 12-4-12 series are approximately equal. Of interest is the fact that the infinite dilution apparent molar volumes of 2-butanol26 and 2,3-butandiol27 in water at 298 K differ by ca. 0.7 cm3 mol-1, the diol being smaller. The results reported, here, are somewhat consistent with those of the substituted butanols. Further, they suggest that, at least in the monomer (premicelle) concentration range, the quaternary segments of the gemini compounds in this 12-4-12 series contribute a nearly constant amount to the cationic volume. The substitution of a single hydroxyl group in the 124-12 spacer increases both Vφ,M and ∆Vφ,M (Table 3). However, the increase in these properties upon inserting a second hydroxyl group in the methylene group R to the initial substitution is much less. As was argued in the discussion of the ∆HM° results, above, the cis configuration of the alkyl groups of the gemini cations in the micelle state must place the hydration cospheres of the hydroxyl groups of the 2,3-disubstituted spacer in an unfavorable intramolecular interaction. The volume change would be negative when water is released to the bulk as the more structured cospheres of the monomer are forced to assume a less structured configuration when the spacer is confined to the micelle interface. Thus the increase in the volume of micellization is attenuated. The use of the globular protein model to analyze the dynamic light scattering (DLS) data for the CnTAB (25) Huang, H. Systematic Study of Ionic Surfactant and Ethoxylated Alcohol Mixed Micelles. Ph.D. Thesis, University of Saskatchewan, 1997. (26) Origlia, M. L.; Woolley, E. M. J. Chem. Thermodyn. 2001, 33, 451-468. (27) Hawrylak, B.; Gracie, K.; Palepu, R. J. Solution Chem. 1998, 27, 17-31.
Table 4. Mean Aggregation Numbers for a Homologous Series of CnTAB Surfactants Determined from Fluorescence Quenching (FQ) and Dynamic Light Scattering (DLS) Methods n
Nagg (FQ)
RHa (nm)
Nagg (DLS)b
Nagg (DLS)c
8 10 12 14 16
23d 39d 57d 69e 83f
1.015 1.168 1.389 1.786 2.451
14 17 24 39 75
25 39 53 72 82
a Determined experimentally in this work. b Calculated using the globular protein model. c Calculated using the model developed in this work. d From ref 25. e From: Lianos, P.; Zana, R. Chem. Phys. Lett. 1980, 76, 62. f From: Jobe, D.; Verrall, R.; Skalski, B. Langmuir 1993, 9, 2814.
surfactant series produced mean aggregation number data that were consistently lower than those determined from fluorescence quenching methods (Table 4). This suggested that the globular protein model for the calculation of molecular weights may not be appropriate for the determination of micellar molecular weights, at least in the case of quaternary ammonium surfactants. Consequently, a calibration curve for the determination of the molecular weight of the micelle from the experimentally determined hydrodynamic radius (RH) (DLS method) was developed using molecular weights (MW) calculated from aggregation numbers determined using fluorescence quenching
MW ) -9.855(RH)2 + 50.79(RH) - 30.04
(6)
where the molecular weight is in kilodaltons and RH is in nanometers. Mean aggregation numbers were calculated on the basis of this model (DLS method) for the CnTAB surfactants (Table 4) and show good agreement with the values obtained from fluorescence quenching. While DLS measurements should normally give higher estimates of mean aggregation numbers due to the fact the scattering entity is a hydrated micelle, the derived relation, eq 6, implicitly, does not account for this difference. The aggregation numbers determined for the 12-4-12 and 12-4(OH)2-12 surfactants using the DLS method (see Table 1) agree well with previously reported values of 3028 and 324 for the 12-4-12 surfactant and 3429 for the 12-4(OH)2-12 surfactant. The aggregation numbers for the substituted geminis are observed to decrease upon increased hydroxyl substitution. This result is consistent with an increase in the hydrophilicity of the surfactant headgroup and results, generally, in a decrease in the mean aggregation number of the resulting micelle.30 Conclusions The results obtained for the cmc and mean aggregation numbers for the series of hydroxyl-substituted gemini surfactants decrease with increased substitution, consistent with those expected for increased hydrophilicity of the surfactant headgroup. The headgroup areas obtained remain approximately constant, indicating that substitution has no significant effect on the headgroup area of the surfactant. It is possible that hydroxyl substitution may have a more substantial effect on the headgroup areas of gemini surfactants having longer spacer groups, which are known to become incorporated into the micellar core. (28) Wettig, S. Studies of the interaction of gemini surfactants with polymers and triblock copolymers. Ph.D. Thesis, University of Saskatchewan, 2000. (29) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148-6154. (30) Myers, D. Surfactant Science and Technology; VCH, Inc.: New York, 1988.
Gemini Surfactants with Hydroxyl Substituted Spacers
Values of ∆Vφ,M increase upon substitution in the spacer group, consistent with a disruption of the hydration of the surfactant headgroup upon increased hydroxyl substitution. Values of ∆HM° are observed to decrease, i.e., become more exothermic upon hydroxyl substitution. It is likely that substitution of the spacer group not only causes a disruption of the interaction of the spacer with water but also results in steric factors that must also be accounted for. The values obtained for T∆SM° indicate that, as for the 12-s-12 series of gemini surfactants, the micellization process for the substituted surfactants is entropy-driven. It is observed from both volume and enthalpy measurements that the addition of a second hydroxyl substitutent
Langmuir, Vol. 18, No. 14, 2002 5359
has less of an effect on the thermodynamic properties of micellization as compared to the addition of a single substitutent. Acknowledgment. The authors thank the Saskatchewan Structural Sciences Center (SSSC) for the use of the isothermal titration calorimeter and the dynamic light scattering instrument. Financial assistance provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged. LA011782S
