5020
J. Phys. Chem. 1983, 87,5020-5025
The Curious World of Hydroxide Surfactants. Spontaneous Vesicles and Anomalous Micelles B. W. Nlnham,t*5D. F. Evans,lt and G. J. Well' DepaHment of Chemical Engineering, Institute for Mathematics and Its Applications, and Department of 6iochemistry, University of Mlnnesota, Minneapolis, Minnesota 55455 (Received: April 2 1, 1983; In Final Form: Ju/y 1 1 , 1983)
Double-chained cationic surfactants typified by dodecyldimethylammonium bromide are insoluble in water, forming lamellar liquid crystal phases. They form vesicles only on prolonged sonication. If the halide ion is replaced by a hydroxide, the resulting surfactants are highly soluble and form spontaneously a clear solution which appears to comprise a mixture of small micelles and fairly monodisperse vesicles. The distribution of particle size changes with added base or with partial titration with acid (HBr, HC1, HF) which can sometimes yield vesicles with an initially unsymmetric distribution of anions. Evidence for these structures from quasi-elastic light scattering (QELS) and viscosity measurements and an account of their extraordinary properties are presented.
1. Statement of the Problem Practically all studies1V2on vesicles exhibit one common feature. The vesicles are not formed spontaneously and are unstable. This is because the starting compounds (in the biological domain typically phospholipids, in the chemical domain typically dialkyldimethylammonium halides and similar surfactants) are highly insoluble, forming liquid crystalline states. Vesicles are usually prepared by sonication, are not generally monodisperse, are almost certainly metastable, and gradually degrade over a period of days, weeks, or months, to the lamellar state from which they emerged. The equilibrium statistical mechanics of self-assembly of dilute surfactant solutions is by now well-established.3-6 Within this theoretical framework which includes metastable states single-walled vesicles are several possible allowed aggregates-spherical micelles, globular, oblate, prolate ellipsoids, cylindrical micelles, vesicles, liposomes, multilamellar bilayers, and inverse micellar structures. Which aggregates form is determined primarily by geometric packing of amphiphiles, hydrocarbon chain stiffness, and the hydrophobic-hydrophilic balance. Intramolecular interactions complicate the self-assembly process somewhat. There is no reason to believe that single-walled vesicles are-in general-in any way more unstable or disallowed than are normal micelles. Broadly pea king^,^ necessary (geometric) conditions for formation of aggregates are the following: (1)spherical micelles, u / a l < 1/3; (2) globular or cylindrical micelles, 1 / 3 < u / a l < lI2; (3) vesicles or bilayers, 1 / 2 < u / a l 51. Here u is the volume per hydrocarbon chain, or of the hydrophobic region of the surfactant, a is the head-group area, and 1 is an optimal hydrocarbon chain length related to the maximum extended length. (These conditions assume that these aggregates have a fluid-oil-like interior.) By changing chain length or chain stiffness (e.g., by varying temperature), hydrocarbon volume, or head-group area, one can in principle dictate which type of aggregate will form under prescribed conditions. Thus, e.g., for ionic surfactants, decrease in repulsive head-group interactions induced by added salt will decrease a, increase u l a l , and cause a 'Department of Chemical Engineering. *Institutefor Mathematics and Its Applications. Permanent address: Department of Applied Mathematics, Research School of Physical Sciences, Institute of Advanced Studies, Australian National University, Canberra ACT 2600, Australia. 11 Department of Biochemistry.
transition from spherical micelles to cylindrical micelle^.^ If vesicles are to be an allowed structure, the require< u/al < 1 normally restricts one to doument that ble-chained surfactants. The design of stable vesicles is clearly a more delicate matter than the problem of micelles. Vesicles being much larger structures, one expects attractive interaggregate forces to play a correspondingly more important role-potential barriers to flocculation and subsequent fusion are lowered. And indeed, while not well documented, there is evidence that the lower the concentration of vesicles, the slower the flocculation rate. But inevitably for those systems studied so far a system of vesicles degrades to the lamellar state. The problem of designing either a stable dispersion of vesicles or a system which forms vesicles spontaneously can be tackled in three different ways. The first method is to impose a curvature on the system by using mixed surfactants. Thus, cholesterol mixed with phospholipid will reduce ( u / a l ) because of its smaller chain length, and tends to form vesicles over bilayem2 This route to a desired end product is analogous to the use of cosurfactants to form middle-phase microemulsions with ultralow surface tensions. This (easy) way suffers a corresponding disadvantage. It is difficult to unravel the fundamental process of aggregation except at the grossest level. In any event, such vesicles do not generally form spontaneously. A second method is to vary chain length and stiffness by working at a higher temperature. And indeed the size distribution of sonicated phospholipid vesicles appears' to remain constant above 55 "C. This is inconvenient. The third method is to vary head-group area, alter solubility, and simultaneously control the molecular forces responsible for degradation to the lamellar phase. That this may be possible is suggested by the work of Pashley,6 (1) J. Fendler, "Membrane Mimetic Chemistry", Wiley, New York, 1983. This book contains an extensive listing of the literature. (2) J. N. Israelachvili, S. Marcelja, and R. Horn, Q. Reu. Biophys., 13, 2, 121 (1980). (3) J. N. Israelachvili, D. J. Mitchell, and B. W. Ninham, J. Chern. Soc., Faraday Trans. 2, 72,1525 (1976);Biochern. Biophys. Acta, 470, 185 (1977). (4) D. J. Mitchell and B. W. Ninham, J. Chem. Soc., Faraday Trans. 2, 77, 601 (1981). (5) H. Wennerstrom and B. Lindman, Phys. Rep., 62, 1 (1979). (6) D. F. Evans and B. W. Ninham, J. Phys. Chern., in press. (7) R. Laughh, private communication. See also: A. L. Larrabee, Biochemistry, 18. 3321 (1979):D. Sornette, C. B. Hesse, and N. Ostrowsky, Biochimie, 63, 955 (1981). (8)R. M. Pashley, J. Colloid Interface Sci., 83, 531 (1981),and subsequent papers in this journal.
0022-3654/83/2087-5020$01.50/00 1983 American Chemical Society
Hydroxide Surfactants who showed that strong repulsive forces can be imposed on interactions between planar surfaces by changes in pH or salt. Thus, between mica surfaces separated by water, double-layer and van der Waals forces provide a good description8 of the forces operating when H+ is the counterion. With increasing salt concentration, specific cation adsorption induces very strong repulsive forces due to associated water structure. These forces provide a barrier to flocculation. Such phenomena are well-known in colloid science, e.g., in bubble interactions, anomalous behavior of thin soap films.g They account for stacking and unstacking behavior of thylakoid membranes.l0 Similar phenomena appear to be associated with the hydroxide ion, which behaves very differently from other anions. Thus, the cationic double-chained surfactants didecyldimethylammonium bromide and chloride and dioctadecyldimethylammoniumhalides are highly insoluble. The former forms a lamellar phase in water, and the latter is completely insoluble. These surfactants form vesicles on sonication."J2 By contrast, when ion exchanged so that the counterion is now OH-, these surfactants are soluble at very high concentrations (to at least 1M). No structure characteristic of liquid crystals is observed under a polarizing microscope. Since geometry normally dictates that these surfactants cannot pack into micelles, the only possible aggregates which can form would be cylindrical structures or vesicles. We shall show that this is not so here. Anomalous micelles and vesicles do form as a result of very strong repulsive forces associated with the hydroxide ion in water. These increase the area per head group in an aggregate, reducing v / a l from unity to a value at which such structures form spontaneously. On titration, the hydroxide ions are replaced by bromide ions causing a decrease in head-group area which allows such vesicles to be grown to any desired final state. We present here evidence for the existence of and nature of these unusual aggregates. 2. Experimental Section Solutions of didodecyldimethylammonium hydroxide (DDAOH) were prepared by ion exchange from the corresponding bromides (DDAB). The bromide surfactants were Eastman (recrystallized from acetone). (We have also worked with dioctadecyldimethylammonium hydroxide, whose properties are again sufficiently anomalous to deserve a report which will be given elsewhere.) The ionexchange resin (Fisher Rexyn R205) was used as received [capacity (1.3 mequiv/mL wet resin)]. Typical procedure was as follows: 50 mL of wet resin was placed into two 250-mL stoppered Erlenmeyer flasks. The resin was then soaked overnight in 200 mL of water and rinsed twice with 200 mL of double-distilled water, to remove resin decomposition products. Excess water was poured off into one flask, leaving a wet residue, and 2 g of DDAB was introduced. The flask was gently stirred and allowed to stand for 1h. At this point the surfactant is virtually completely ion exchanged to hydroxide form DDAOH which is in solution. Water (50 mL) was added to the contents of this flask and filtered through paper filters into the second (9) See references in B. W. Ninham, 'Proceedings of the UTAM-IUPAC Conference on Interactions in Dispersions of Colloidal Particles",
Adu. Colloid Interface Sci. (June 1982). (10) J. T. Duniec, J. N. Israelachvili, B. W. Ninham, R. M. Pashley, and S. W. Thorne, FEBS Lett., 129, 193 (1981). (11) T. Kunitake and Y. Okahata, J. Am. Chem. SOC.,99,3860 (1977). Reference 1 contains numerous references to subsequent work. (12) R. McNeil and J. K. Thomas, J. Colloid Interface Sci., 73, 522
(1980).
The Journal of Physical Chemistry, Vol. 87, No. 24, 1983
5021
flask, again stirred gently, and allowed to stand. This second exchange assured that all bromide was removed. Supernatant was then filtered into a third flask. In order to minimize contamination by COP,all manipulations involving the ion-exchange process and the preparation of solutions were carried out in a polyethylene glovebag inflated with COP-freeair. Standard AgN03 tests for bromide (surfactant solution acidified with HN03, pH 3) showed no trace of residual bromide. For the surfactant concentrations employed in this test, the surfactant is not precipitated by the nitrate. This procedure yields a concentrated solution of DDAOH which contains about 50% of the initial concentration of surfactant. The solution can be lyophilized to a fine white powder. However, if the lyophilization is taken too far, the dry residue sometimes takes on a yellow tinge which may indicate that the (fully) dried surfactant could be unstable. For this reason we have chosen to work with the solutions prepared as above, usually of the order of 0.05 mol/kg. The surfactant concentrations were determined by direct titration with standardized HCl or HBr using phenolphthalein. In the concentrated surfactant solutions (0.05 M) the titrations gave a well-defined stable end point. In more dilute solutions, titration to an end point gave solutions (protected from COz)which upon standing take on a pink color within 3 h and become a deeper pinkish hue within 12 h. This is remarkable. The phenomenon sugor gests immediately that some structures formed by M DDAOH must have an interior and exterior. If vesicles, leakage of indicator to the interior or exchange of exterior halide ions with interior OH- ions would account for the observation. Attempts to determine the pH with electrodes failed, because precipitation of the surfactant at the reference liquid junctions gave emf readings which changed with time. Use of agar-agar salt bridges to try to overcome this problem was unsuccessful. The samples used in the dynamic light scattering experiment~'~*'~ were either centrifuged (low fields to remove dust particles) or filtered through Millipore 0.22-llm filters. The viscosity measurements were carried out in suspended Cannon Ubbelohde viscometers. 3. Results Dynamic Light Scattering. We have obtained dynamic light scattering and viscosity data on DDAOH solutions. Typical results for didodecyldimethylammonium hydroxide are given in Tables I and 11. Table I is concerned with effects on measured diffusion constants at varying surfactant concentrations with added sodium hydroxide. Several comments are in order. The data are fitted to a form
'/z In ( c ( r ) )= a - b T + (c/2)? where ~ ( 7is) the correlation function. The quality factor (QF) defined as usual by QF = ((D - (D)2)z)/(0),2= c/b2, and ( D ) z= CIiDi/CIi,with Ii the intensity due to the ith species. For the B and C series, the observations taken over a range of sample times indicate either a mixed population, with varying proportions of small and large species, or a broad distribution. Hydrodynamic radii of the A series are clearly absurd and reflect electrostatic effects due to rapid exchange between species. These solutions appear to remain clear indefinitely. Flocculation (13) V. A. Bloomfield and T. K. Lim, Methods Enzymol., 48, 415 (1978). (14) C. H.Pletcher, R. M. Resnick, G. J. Wei, V. A. Bloomfield, and G.L.Nelsestuen, J. Biol. Chem., 255, 7433 (1980).
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Ninham et ai.
The Journal of phvslcel Chemistry, Vol. 87, No. 24, 1983
TABLE I : Diffusion Constants and Viscosities for Didodecyldimethylamminium Hydroxide Surfactant Solutions as a Function of Added Sodium Hydroxide at 25 "C [surfactant], M
[NaOH], 108Dl, M cmz/s
0.0
0.050
A0
A1 A' A3
0.020 0.064 0.66
filtered
0.050
0
9.2 X
0 0.020 0.064 0.66
108D,, cm2/s
53 32 6.3 0.33 40
QF' 1
2
0.30 0.33 0.13 0.55 0.42
intensityg 1.2 3.2 2.2 220 3.2a,b
n,'
CP
R h ,e
9eff19d
1.10 1.07 1.32 70
A'
5 4 9
Rh
9'
A'
3.7 6.3 2.6 9.4 4.3
A BO Bl BZ B3 CO
o
0.92 x 10-3
(0.33) (0.33)
0.34 0.17 0.40
(0.30)
Cl C'
0.020 0.064 0.66
filtered
0
c,
(0.34) (0.40)
2.3 5.2 0.40 4.2
(1.1)
4.7
(0.49)
1.02 0.42 1.74
0.023
(0.28) (0.50) (0.45)
0.40 0.43 0.23 0.18 0.16
0.71 2.1 7.9 150 0.56 0.57 0.70 8.3 7.7a 1 6a
0.96 0.93 0.97 8.9 0.95 0.91 0.92 1.06
9 4 6
19 44 62
62 19 9 25
56 50 200 550 135
(660) (580) (780) (220) (480)
a Experiment done several months earlier; samples filtered through 2-pm Millipore filters; intensity difference reflects in Temperature = 20 "C. part new laser tube. q(0.02 M NaOH) = 0.894, q(0.064 M NaOH) = 0.916, n(0.066 M NaOH) = deff = (q - q0)/2.5q0;@ is actual volume fraction of surfactant computed by assuming surfactant density equals 1.03. unity. e R h calculated from Stokes Law with viscosities given in the table. f Where two entries are given in a column the data are best described by a two-parameter fit which indicates two populations. g lntensity in l o 4 counts/s.
TABLE 11: Diffusion Constants for Partially Neutralized Didodecyldimethylammonium Hydroxide Solutions
[surfactant core], M
B1 2 3 4 C1 2 3 4
0.92 X l o - '
0.92 X
lo-'
% neutralized HBra
108D, cm'/s
40 60
80
3.4 4.6 5.5
160 25 37 50 100
3.5 3.3 6.6 15.9
lO'Rh,
QF
I
A
0.5 0.45 0.20 0.27 0.28 0.5 0.4 0.3
0.80
7.1 4 4.9
0.89 0.44 0.44 0.98 0.47 0.65 5.2
9 7.3 7 1.5
% neutralized = 1OZ(molof HBr/mol of surfactant).
occurs only in excess of 1 M NaOH. We defer detailed interpretation to the Discussion section and remark only on the increase in size with decreasing surfactant concentration. Table I1 gives corresponding data with added HBr. Since, on complete neutralization, the system must eventually revert to liquid crystals, the meaning of hydrodynamic radii here is ambiguous. However, at concentrations B and C, the partially neutralized solutions remain clear at least for several months. Beyond about 60% neutralization, intensities increase dramatically for the B series, indicating growth of particles or increased particle size. With increased acid, the growth can be followed by eye, an initially clear solution exhibiting Tyndall scattering. At 0.05 M surfactant concentration, partially neutralized samples (>40%) go
TABLE 111: Comparison of Presumed Vesicle Size Deduced from Viscosity and Theoretical Predictions for Didodecyldimethylammonium Hydroxidea [surfactant], mol/kg
1 2 3 4 5 3Ab
[Br-1, M
11
@eff
9
0.0954
4.34 5.44 6.69 16.7 17.7
8.0
0.0551 0.0367 0.0275 0.0186 0.0159
0 0.0096 0.0143 0.0190 0.020
1.1048 1.0705 1.0562 1.1678 1.0734
0.074 0.124 0.0813
0.022 0.0147 0.011 0.0074 0.0064
0.0275
0.0143
1.071
0.080
0.011
0.080
@eff/@
radius, A
< 300 400 500-600 1250-1500 1250-1500 600-750
3BC 0.0275 0.0143 0.088 1.087 0.011 8.0 600-800 0.0222 0.017 1.162 0.121 0.0089 13.6 1000-1250 4B 0.0186 0.0190 1.259 0.166 0.0074 22.4 1500-2000 0.020 5B 0.0159 1.214 0.144 0.0064 22.5 1500-2000 6B 0.0139 0.0214 1.1849 0.131 0.0056 23 2750-2250 a q = q , ( l + 2.5@,ff),q o = 0.8906, T = 24.91 "C. Successive samples are made by increasing amounts of HBr solution. 9 is volume fraction of surfactant assuming density = 1. Sonicated for h. 20 h subsequent.
4B'
TABLE IV : Viscosities o f Dodecyltrimethylammonium Hydroxide with Added NaOH and of Dodecyltrimethylammonium Bromide with Added NaBr a t 25 "C
0
0.1 0.3 0.9 1.7
0.20 0.20 0.198 0.185 0.17
1.294 1.234 1.228 1.325 1.508
3.58 3.10 3.11 4.27 6.63
0 0.1 0.3 1 2
0.20 0.20 0.20 0.20 0.20
1.294 1.152 2.371 gel-like solid gel
3.58 1.94 11.06 m
m
Hydroxide Surfactants
The Journal of Physlcal Chemistry, Vol. 87, No. 24, 1983 5023
cloudy within 1 week. With HCl or HF, such solutions remain clear. Viscosity. Changes in viscosity of DDAOH solutions upon successive addition of HBr are summarized in Table 111. As an aid to the strategy behind an interpretation of these measurements, we first describe parallel viscosity measurements on the single-chained surfactants dodecyltrimethylammonium bromide with added NaBr and the corresponding hydroxide with added NaOH (Table IV). Because of their small chain volume ( u / a l 5 1/3) (at low enough concentrations so that interaggregate interactions can be ignored) either surfactant can pack into spheres, or cylinders with added salt, but not vesicles or lamellar phase. Data listed in Tables I11 and IV are fitted to the Einstein formula q = qo(l + 2.54,ff)where is the effective volume fraction of surfactant (which depends on aggregate shape15). The viscosity of the background solvent qo can be taken to a good approximation as that of water qo = 0.890 at 25 “C. Actual volume fractions are computed from measured densities of the surfactant solutions. Note that 4eff= 4 for spherical micelles. Addition of a small amount (0.1 M) of electrolyte to either surfactant solution decreases the viscosity in a manner consistent with diminished electrostatic interactions between the charged micelles. A t higher electrolyte concentrations the results are very different. With increasing NaBr, the viscosity of ClzTAB increases rapidly as the aggregates undergo the typical transition from spheres to rods (deff m). At concentrations greater than 0.3 M NaBr the solution gradually becomes a gel. On the other hand, for DTAOH, with increasing NaOH, the viscosity increases only slowly up to a concentration of 1.7 M, and 4effincreases only by a factor of 2. Unlike the bromide case, large changes in micelle size and shape are not observed for the hydroxide. This is counterintuitive and we shall return to a rationalization in section 4. For the moment it suffices to note that head-group area per surfactant molecule in an aggregate must be larger for OH(and with addition of OH-) than for Br-. Consider now Table 111. The addition of HBr to the double-chained surfactant results in an effective volume fraction which increases continuously with replacement of the hydroxide by bromide counterions up to complete neutralization (sample 4 ) . No obvious transition to cylindrical shapes is observed. But agaig with increasing acid, Tyndall scattering becomes noticeable a t sample 3 and becomes more pronounced in samples 5 and 6. The aggregates are large. If they were single-walled vesicles, we can estimate their radii by the following simple argument. Let the radius of the vesicle be R. Its volume will be (4?r/3)R3. The vesicle comprises a curved bilayer of surfactant which encloses an inner water core. Let the thickness of the bilayer be E . Then the volume of surfactant is ( 4 a / 3 ) [ R 3- ( R - E)3]. Hence, the ratio of the effective volume fraction to actual volume fraction of surfactant is 4eff/4= R3/[R3 - (R - E)3] which determines R if 4eff/$and E are known. The outer surfactant molecules must be stretched5 close to a fully extended chain length = 16 A, the inner molecules cannot be fully extended, and bilayer studies indicate that in a fully compressed state these cannot take up a smaller length than 4A. Hence, 26 5 E 5 32 A. Since 4effis known, R is determined. (Note that were the thermodynamically favored state of the surfactant to be single-walled vesicles in dilute solution, the system would reach a prohibited random close packing configuration at a vesicle radius of
-
(15) C. Tanford, ‘The Physical Chemistry of Macromolecules”,Wiley, New York, 1961, Chapter 6.
TABLE V : Viscosity of Sonicated Suspensions of Didodecylammonium Bromide [surfactant 1, M 0.015 0.010 0.005 0.003 0.002
q , cp
@eff
Oeifl@
1.7916 1.488 1.1677 1.1127 0.9903
0.374 0.243 0.1044 0.0451 0.0278
25 24.3 20.9 15.0 13.9
[Br-I, M 0.033 0.022 0.011 0.006 0.004
the order of 2000R a t 0.05 M. Such a system has several choices: collapse to a lamellar state, to microtubules (as is often observed), and/or to smaller vesicles and micelles.) These viscosity increases on replacement of hydroxide counterions with halides are indicative of increases in particle size. The second set of entries in Table I11 shows for 3A the effect on sample 3 of sonication, and 3B-6B effects of time and additiion of HBr to such a sonicated system. There is only slight change in sample 3 on gentle sonication for 0.5 h, which does not change over a 24-h period. Addition of HBr to this day-old sample results in an increase in #eff/4which, while slightly larger, parallels the values of solutions 4 and 5. Table V lists viscosity measurements on didodecyldimethylammonium bromide vesicles prepared by sonication ’0 ’ suspension of this surfactant. After prepof a 1.5 wt 3 aration, the suspension was diluted successively. Note that a t 0.15 M surfactant effective volume fractions are much the same as that for neutralized hydroxide surfactant solutions. A t lower concentrations apparent size decreases with concentration, reflecting increased electrostatic head-group repulsion. Sonication via a tip sonicator instead of an ultrasound bath will reduce the size of these vesicles. It appears that, even for the bromide surfactant, vesicle size depends on concentration also. 4. Discussion
The data presented in Tables 1-111 on didodecyldimethylammonium hydroxide are counterintuitive and puzzling. As such, and because of the nature of our experiments, they warrant scepticism. Nonetheless, we have been led inevitably to the conclusion that double-chained surfactants with hydroxide as counterions form a population comprising a mixture of vesicles and anomalous micelles. The goal of this discussion is to marshal1 and rehearse this and other evidence which justifies such a conclusion. Comprehensive reports via cold stage electron microscopy,16 which demonstrates the existence of reasonably monodisperse vesicles (300 A at M), via video-enhanced contrast polarization microscopy,” which shows up vesicles comparable in size with 1000-A latex spheres, with a considerable increase in number and size on addition of acid, and via aggregation number determinations18 by intramolecular excimer formation (with aggregation number = 50 in 0.05-10-3 M surfactant) will be given elsehwhere. All substantiate this picture. A second piece of evidence which provides some indication of how this situation might come about can be obtained from studies on the simpler single chemical surfactant systems dodecyltrimethylammonium hydroxide and dodecyltrimethylammonium bromide. We have determined the critical micelle concentrations (cmc’s) of (Cl2H,)(Me),N0H and (C14Hm)(Me)3NOH at 25 “C using (16) T. Talmon, D.F. Evans, and B. W. Ninham, Science, in press. (17) B.Kacher, D.F. Evans, and B. W. Ninham, to be submitted for publication. (18) S. Hashimoto, J. K. Thomas, D. F. Evans, S. Mukherjee, and B. W. Ninham, J. Colloid Interface Sci., in press.
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No. 24, 1983
conductance and fluorescence measurements and obtain M, respectively. These cmc’s of 3.3 X lo-* and 7.0 X values are twice as large as those for the corresponding and 3 X M). Since, for a given bromides (8 X hydrocarbon tail the hydrophobic free energies of transfer must be fairly similar, this variation in cmc must reflect increased head-group repulsion. We have also determined the change in cmc of the hydroxide surfactants on addition of NaOH. If these results are parameterized through the language of the familiar equilibrium dissociation model by plotting cmc vs. log (C(Na0H) + cmc), values 0.4-0.5 emerge. These values are half those of the corresponding bromides (0.8-0.86). Much more remarkable are determinationsl8 of aggregation number for tetradecyltrimethylammonium hydroxide micelles. These change only from 45 to 70 over the concentration range 0-1 M NaOH. The changes in viscosities for the single-chained surfactants (Table IV) on addition of electrolyte also reflect this difference between hydroxide and other anions. Thus, addition of NaBr to (spherical) micellar solutions containing tetradecyltrimethylammoniumbromides results in less curvature, smaller area, and growth to cylindrical micelles with their larger intrinsic viscosity. By contrast addition of NaOH results in almost no change in &ff consistent with the measured aggregation numbers. Transition to cylindrical shapes is not observed except at enormous concentrations; the viscosity varies only slightly. An interpretation of this micellar data6 which avoids the ion-binding concept leads to the conclusion that the hydroxide radical must be located on the average about 4 A further out from the charged surfactant head group than for the bromides and that the hydroxide micelles are smaller. This is a consequence of the extreme hydrophilic nature of the hydroxyl anion in water. The resulting increased Coulombic repulsion between the surfactant head groups gives increased head-group area, increased surface curvature, and decreased aggregation number. With these peculiarities of the hydroxide ion in mind, we consider the data on double-chained hydroxides in detail in an attempt to rationalize what must evidently be a very complex situation. Consider Table I, which lists measurements of diffusion constants in the presence of added NaOH. In the absence of added salt, the standard interpretation of QELS diffusion measurements breaks even for normal ionic micelles like SDS. Transport can occur via a complicated process which involves disassembly into monomers, coupled monomer and counterion transport, and reassembly. The absurd hydrodynamic radii (as also for SDS) inferred from application of Stokes law (cf. series A, 0.05 M) reinforce this conclusion. Fluorescence lifetime measurements give aggregation numbers N 50, so that only “radius” assigned to micelles here must be at least -20 A. What is surprising is that “hydrodynamic” radii are absurd even with added NaOH where electrostatic effects on transport would usually be screened.lg We infer that whatever small hydroxide surfactant aggregates are formed must be extremely labile at 0.05 M. This is consistent with low intensities observed. At lower surfactant concentrations (B and C), QELS data can be accommodated by postulating either a mixture of small micelles with larger particles in a bimodal distribution or a single broad distribution. The smaller component or tail of the distribution appears to increase in size with added hydroxide. Molecular weight averaged sizes are in the range 200-1000 A. Again such an interpretation must be tempered by the realization that the
-
(19)D.F.Evans,S.Mukherjee, D.J. Mitchell, and B.W.Ninham, J. Colloid Interface Sci., 93,184 (1983).
Ninham et al.
transport process is probably anomalous and not understood. Nonetheless, we can be certain that there are large particles. Viscosity measurements (Table 111) are consistent with the supposition that these large aggregates are spontaneous vesicles, not large cylindrical structures. Fluorescence measurements appear to show the existence 50)-independent of surfactant of micelles ( N concentration-which increase to large aggregates with added NaOH. Cold stage electron microscopy, which presumably will not detect a micellar component (due to fast freezing), shows up relatively monodisperse vesicles (300-Adiameter) at M surfactant. Video-enhanced contrast polarization microscopy which can detect particles k 500 A shows no vesicles or larger structures for the A series, and an increasing population of apparent vesicles (identical in texture and size with latex spheres of radius 1000 A) with decreased surfactant concentration. For the B and C series, no sign of liquid crystallites has been observed with EM or VECPM, and the solutions seem to remain clear indefinitely. We can adduce more information from Tables I1 and IV which deal with the effects of added acid. Here we are on safer ground. Work on the single-chained surfactants already described shows that the head-group area associated with the OH- ion (and consequent curvature) is considerably larger than for the bromide or chloride anions which are in much closer proximity to the surface of an aggregate. Thus, titration with acid must result in decreased curvature, growth in aggregate size, and eventual reversion on neutrality to a liquid crystalline phase. The QELS data show a rapid increase in intensity around 50% neutralization-probably indicating the beginnings of such a change. The viscosity data (Table IV)are consistent with growth of vesicle-like structures and give values which compare nicely with sonicated vesicle suspensions of the bromide salts. Under VECPM, a t a fixed surfactant concentration, partial titration with acid shows a rapid increase of the population of vesicles of size 1000-2000 A, which grow out of the unobserved background which contains smaller micellar structures and vesicles. EM shows (at M surfactant) 25% neutralization of a mixture of vesicles (400-Adiameter) together with some liquid crystallites. With VECPM, very large, single-layered spherical aggregates, two- or three-layered structures (up to a few microns), and microtubules are observed at and beyond neutrality. The persistence time of these complicated dispersions (even when they are clearly unstable, at and beyond neutrality) is several months. Crude trapping experiments can and have been performed by titration with phenolphthalein (most standard techniques appear to fail, and hydroxide vesicle components cannot be sized by Sephadex columns). Titration of 0.05-0.02 M solutions yields nothing unexpected, consistent with the existence in such solutions of a predominantly micellar distribution. But after titration of M solutions pronounced leakage occurs after a few hours from sealed flasks. This is evidenced by a change from a colorless solution (apparently neutral) to pink (leakage of hydroxide). After repeated retitration an end point is reached where no further apparent leakage is observed. This is at leat consistent with the existence of a substantial vesicular component in the dilute solutions, a clue to structure which deserves further exploration.
-
-
5. Summary
This paper began with a remarkable observation, that double-chained hydroxide surfactants dissolve spontaneously, a qualitatively different behavior from that of the corresponding halides. The origins of this phenomenon
J. Pbys. Chem. 1983, 8 7 , 5025-5032
have been traced via a study of corresponding singlechained surfactants to the much larger head-group area associated with the OH- ion, which, with ita strong affinity for water, sits much further from a charged micellar surface than the other anions. In the language of a current theo $ ~ of surfactant aggregation, for the dialkyl halides ulal, = 1 (bilayers favored), and for the dialkyl hydroxides ulal, i= lI2. Theoretically one expects peculiar behavior when such a high degree of curvature, opposed by chain packing, is imposed. It has been shown: although only within the pseudophase approximation for dilute solutions, that depending on chain stiffness, as head-group area is lowered by, e.g., increasing salt or counterion concentration, it is possible for the favored aggregates to be bilayers a t u / a l = lI2, revert to vesicles as u l a l increases, and eventually revert again back to bilayers. The analysis of a situation which allows a distribution of aggregate sizes instead of the pseudophase approximation is more complicated. Clearly interaggregate forces, and entropy, and the physical limitations of close packing of large aggregates a t high concentrations are all involved. It seems fairly clear that dialkyldimethylammonium hydroxides do form spontaneous stable vesicles whose size
5025
and distribution depend on concentration and salt type. Even microtubules can be constructed from these syst e m ~ . ~There ’ is some evidence that vesicles formed by partial titration have an asymmetric distribution of anions and with differing interior and exterior pH. This is important. For mooted pragmatic reasons and because of their evident biological interest the subject of what might be termed vesicle chemistry is an expanding phase. If the phenomena that we have reported hold up to intensive scrutiny, the possibility exists that some design features necessary for the making of vesicles for a particular task might be revealed. Acknowledgment. We thank Dr. Y. Talmon, Dr. B. Kacher, and Dr. J. K. Thomas for collaborative work on electron microscopy, VECPM and intramicellar excimer fluorescence studies, Dr. V. A. Parsegian for assistance, criticism, and advice, and R. G. Laughlin for much helpful criticism. D.F.E. acknowledges support of U.S. Army Contract DAA G29-81-K-0099. G.J.W. acknowledges support of Dr. V. Bloomfield through NSF grant PCM 81-18107. Registry No. DDAOH, 23381-53-5.
Ion Binding and the Hydrophobic Effect D. Fennel1 Evans*t and B. W. Nlnhamt*§ Department of Chemlcal Engineering and Institute for Mathematics and Its Applications, University of Minnesota, Minneapolis, Minnesota 55455 (Received: Aprll25, 1983; In Flnal Form: July 28, 1983)
Two apparently opposing interpretations of ionic surfactant aggregation are reconciled. Phenomenological constants ascribed to ion association at micellar surfaces are reinterpreted and shown to emerge naturally from a treatment of electrostatic head-group interactions via an explicit approximate solution of the nonlinear Poisson-Boltzmann equation. Given critical micelle concentrations (cmc’s) and aggregation numbers, one can calculate the free energies, enthalpies, and entropies of micellization. Enthalpy-entropy compensation is studied with water as solvent, and earlier conclusions based on the phenomenological model are confirmed. Hydrazine exhibits very different behavior. By removing one parameter from consideration,the theory shows considerable light on the nature and importance of chain interactions in determining micellar structure, and on the validity of fluid-interior models for micelles.
Introduction In ionic micellar chemistry, no concept is more firmly entrenched than a belief in ion binding.’ Such a concept is not ab initio unreasonable. All observations can indeed be subsumed into a single phenomenological “constant” which characterizes a presumed degree of dissociation of counterions a t the surface of a micelle. And this “constantn has been seen in the past as the one firm piece of information on which to build an understanding. On the other hand, extant theories2” make no appeal to ion binding and usually assume that micelles are fully dissociated. There is an apparent dichotomy here which it is our purpose to resolve. While the ion-binding model appears to account for many features of micellar chemistry quite satisfactorily, Department of Chemical Engineering. *Institute for Mathematics and Its Applications. $Permanent address: Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra ACT 2600, Australia.
its phenomenological base masks and obscures all information on the molecular interactions which determine the shape, size, and energetics of micelle formation. The estimated degree of counterion binding as determined from experiment varies widely (*20%) depending on the experimental technique employed6 and is thus not even an operationally well-defined quantity. Since the determination of micellar thermodynamic quantities and the interpretation of almost all transport and equilibrium measurements is necessarily model dependent, the use of (1) Mukerjee, P. J. J. Phys. Chem. 1962, 66, 1375. (The thermodynamics of micelle formation was first described by an equation of this form in 1935 by G. S. Hartley in his well-known book. Given the then wide acceptance of the Bjerrum theory of ion pairing in electrolytes, it seemed sensible). (2) Tanford, C.“The Hydrophobic Effect”; Wiley: New York, 1973. (3) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. SOC., Faraday Trans. 2 1976, 72, 1525. (4) Mitchell, D. J.; Ninham, B. W. J. Chem. SOC.,Faraday Trans. 2 1981, 77, 601. (5) Wennerstrom, H.; Lindman, B. Phys. Rep. 1979, 52, 1. (6) Kresheck, G. C. In “Water-A Comprehensive Treatise”; Franks, F., Ed.; Plenum Press: New York, 1975; Vol. 4,Chapter 2.
0022-3654/83/2087-5025$01.50/00 1983 American Chemical Society
