Langmuir 1988,4, 3-12
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The Langmuir Lectures Self-Organization of Arnphiphiles D. Fennel1 Evans Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received October 21, 1987. I n Final Form: November 16, 1987 The self-assembly of amphiphiles to form micelles, vesicles, bilayers, and (with added oil) microemulsions is important in biological transformations and in many industrial processes. The hydrocarbon-solvent interactions that drive amphiphilic self-organization in water and other polar hydrogen bonding solvents are discussed. The interactions between aggregates that determine structure and reactivity are described with an emphasis on recent surface forces measurements. Amphiphilic structures determined by videoenhanced microscopy (VEM) and cryo-transmission electron microscopy (cryo-TEM) are discussed.
Introduction Amphiphiles self-assemble into a variety of microstructures that are physically, not chemically, associated. These structures, which include micelles, vesicles, liposomes, microtubules, and bilayers, constitute microphases possea~ingoil-like regions and large interfacial areas. Many biological and industrial processes utilize their properties. In biological processes, amphiphiles continuously transform from one microstructure to another in response to delicate changes in concentration, pH, ionic strength, and temperature. In conjunction with proteins, they form the super assemblies which mediate and control life processes. In industrial applications, such as the utilization of amphiphiles as drug delivery systems or microdomains for synthesis of small metal clusters, the goal is to set structure, maintain its integrity under adverse conditions, and then transform or disperse it at the end of the process. Successful realization of these goals depends on the ability to dictate the assembly, stability, and reactivity of the microstructures. These challenges are all encompassed in the question "What are the strategies for controlling amphiphilic assemblies?" During the past decade, remarkable advances toward answering this question have produced a renaissance in colloid and interface science. This paper will focus on three main issues: (1)What drives self-assembly? (2) What do we know about the structures that form? (3) What forces control the interactive properties of these systems? Solvent-Hydrocarbon Interactions That Drive Amphiphilic Self-Assembly Aggregation of amphiphiles is a physical process that has been documented in a variety of solvents, including water, hydrazine, ethylammonium nitrate, formamide, and the glycols. Because of its pivotal role in biological and industrial processes, water has been studied most extensively, yet even after 4 decades of intense work, a number of basic issues concerning the interaction between hydrocarbons and water remain unsolved. The story begins in 1945 with a seminal paper in which Frank and Evans1 analyzed the thermodynamics of (1)Frank, H. S.; Evans, M. W. J . Chem. Phys. 1946,13, 507.
Table I. Thermodynamics of Transferring Argon into Water at 25 OC Ar(CBHl2,X2= 1) Ar(H20, X, = 1) AGO = 2.48 kcal mol-' AHo = -2.68 kcal mol-' ASo = -17.3 cal mol-' K-'
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transferring rare gases from the vapor phase to water and other solvents. (For the purposes of this paper it is more convenient to consider the transfer of nonpolar groups from a reference solvent like cyclohexane, and we will focus on argon as a simple nonpolar molecule.) The pmitive free energy (Table I) simply reflects the truism that oil and water do not mix. The negative enthalpy and entropy were surprising, however, and differed from all other data then available. Frank and Evans concluded that nonpolar groups induced an increase in water structure in their immediate vicinity. Formation of solid stoichiometric clathyrates,2 in which each nonpolar group is completely caged inside a water cavity,is an extreme manifestation of this behavior. Although their lifetime is short ( T = s), remnants of these water structural cages persist even upon melting, and thermal fluctuations result in flickering clusters that continuously form and disperse around nonpolar group^.^ Unique water structural changes reflected by entropy changes thus appeared to control the insolubility of nonpolar moieties in aqueous solution. As more complex situations encountered in biological and industrial processes were considered, Frank and Evans' observations on the role of "hydrophobic interactions" stimulated others.4V6 Micellization of the surfactant tetradecyltrimethylammonium bromidee illustrates an extension of these ideals. The discussion below focuses on surfactants that form simple spherical micelles. These are the simplest aggregates in the hierarchy of amphiphilic microstructures and have well-defined cmc values that are amenable to (2) Davidson, D. W. Water-A Comprehensive Treatise, Vol. 2, Clathyrate Hydrates; Franks, F., Ed.; Plenum: New York, 1973, Chapter 3. (3) Frank, H.S.;Wen, W. Y. Discuss. Faraday SOC.1967,24, 133. (4)Kautzmann, W. Adu. Protein Chem. 1969, 14, 1. ( 5 ) Tanford, C. The Hydrophobic Effect,2nd ed.; Wiley: New York, 1980. (6) Evans, D. F.; Wightman, P. J. J. Colloid Interface Sci. 1982,86, 515.
0743-7463/88/2404-003$01.50/0Q - 1988 American Chemical Societv
4 Langmuir, Vol. 4, No. 1, 1988
The Langmuir Lectures
Table 11. Thermodynamics of Micellization of C14H2$1J(CH,)3Br: Transfer of the Hydrocarbon Chain out of Water and into the Oil Interior of the Micelle at 25 O C AGO = -10.5 kcal mol-' AHo = -3.0 kcal mol-' ASo = 25 cal mol-' K-I
2
200180 160140
I20
IO0
60
80
40
\
theoretical analysis. These micelles are monodisperse, and interparticle interactions are at a minimum at the cmc. The processes that drive micellization are the transfer of hydrocarbon chains out of water and into the oil-like interior of the micelle and the opposing repulsions between the head groups as they move into close proximity at the micelle's surface. We can express the free energy of micellization in terms of the dressed micelle model7,*as RT In X,,, = Ag(HP) + Ag(HG) where RT In X,, is the total free energy of micellization, Ag(HP) is the free energy of transferring the hydrocarbon chain out of water and into the oil-like interior of the micelle, and Ag(HG) is the free energy associated with the head group interactions. We can calculate Ag(HG) from the nonlinear Poisson-Boltzmann equation that depends on the Debye length 1 / ~the , micellar aggregation number, and average head group area. As a check on the calculations, we can estimate &(HI') by summing -720 &/mol per methylene group and ---2300 cal/mol per methyl group.5 As shown in Table 11, the enthalpy and entropy changes parallel those for nonpolar groups in water. Arguments like these have been used to explain the interactions and folding of proteins. In all cases the focus has been on the entropic changes associated with changes in water structure. What is surprising in retrospect is the fact that enthalpic changes, which are as anomalous as the changes in entropy, receive so little attention. As Eddington observed, the answers we get to scientific questions are implicit in the way the questions are asked. In 1977 Shinodag suggested an alternative way of addressing the issue of hydrocarbon-water interaction. He plotted the solubility of alcohols and alkyl benzene as a function of the reciprocal of temperature (Figure 1). At high temperatures, the solubility in water decreases with temperature just as it does in virtually all other solvents. A t 90 "C, however, the solubility goes through a minimum and then actually increases as the temperature falls. An extrapolation of the high-temperature behavior is given by the straight lines. In other words, below 90 "C, as the structure of water increases, the solubility of hydrocarbon moieties and rare gases'O also increases. Every experimenter who works with water above room temperature is cognizant of this fact: in order to prevent bubble formation, aqueous solutions must be degassed. Thus, conclusions regarding the effect of water structure on the solubility of nonpolar groups apparently contradict one another. Interpretations based solely in terms of the temperature dependence of the free energy suggest that water structure increases the solubility. Interpretations in terms of the changes in AH and AS suggest the opposite. We can resolve this paradox by considering "hydrophobic processes" in water over an extended temperature range and in other polar solvents. We consider first the micellization of C14TABin water6 from 25 to 160 "C. Over this rather large temperature
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(7) Evans, D. F.; Mitchell, D. J.; Ninham, B. W. J. Phys. Chern. 1984, 88,6344. (8) Evans, D. F.; Ninham, B. W. J. Phys. Chern. 1983, 87, 5025. (9) Shinoda, K.J. Phys. Chern. 1977, 81, 1300. (10) Craretto, R.;Fernindez, R.; Japas, M. L. J. Chern. Phys. 1982,76, 1077.
I 36
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28
24
32
I/T x 10'
Figure 1. Solubility of alkylbenzenes in water (expressed in mole fraction units) as a function of reciprocal temperature. Hypothetical curves (straight lines) give solubilities of hydrophobic
molecules in "structureless" water. Experimental curves demonstrate that water structure at lower temperatures increases the solubilities of hydrophobic moieties. From ref 9.
w
2z
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OO
50
100
TEMPERATURE,
150
"C
Figure 2. Micellization of tetradecyltrimethylammoniumbromide
(C,,TAB) in water as a function of temperature: the 10-fold increase in the critical micelle concentration (cmc) upon heating from 25 to 160 "C is shown (0); the decrease in micellar size (aggregation number) over the same temperature range is also shown (0).
range, in which water becomes a polar hydrogen-bonding solvent, the cmc increases by a factor of 10 while the aggregation number" decreases from 72 to 8 (Figure 2). Analysis of the data with the dressed micelle model gives the values of AGO, AH", and AS" for hydrocarbon transfer shown in Figure 3. The free energy is almost constant across the temperature range although it displays a small characteristic minimum at 90 "C. The "hydrophobic" driving force, therefore, is almost independent of temperature. The very large enthalpy and entropy changes compensate so as to leave the free energy almost invariant. At high temperatures, the large negative entropy reflects the ordering associated with transferring (11) Beealey, A. H. Ph.D. Thesis, Department of Chemical Engineering and Materials Science, University of Minnesota, 1987.
Langmuir, Vol. 4, No. 1, 1988 5
The Langmuir Lectures 1
l
l
l
I
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(C1,H2,)(Me),N Br
-
Table IV. Comparison of Thermodynamics of Micelle Formation and Hydrocarbon Transfer in Water and Hydrazine
HzO H4NZ
-16
-
A(AG)=0.3Kcal A(AH)=17.0 Kcal
-
- '
-30 I 25
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1
I
I
I
II
50
75
100
125
150
175
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Figure 3. Thermodynamics of the micellization of tetradecyltrimethylammonium bromide obtained by using the dressed micelle model and the data in Figure 2. Despite large changes in AH and TAS, the free energy of micellization (AG)remains almost constant across the entire temperature range from 25 to 160 O C . From ref 6. Table 111. Comparison of Hydrazine and property N~HI 1.69 mp, OC 113.5 bp, OC 380 T,,"C 145 P,, d m density at 25 "C, g 1.0036 7 at 25 "C, P 0.00905 y at 25 OC, deg cm-' 66.67 Lw, at mp, cal mol-' 3025 AS,,cal mol-' K-' 11 AHv at bp, cal mol-' 9760 A H v a t 25 OC, cal mol-' 10700 ASv at bp, gibbs mol-' 25.2 1.83-1.90 M(gas), D 51.7 e at 25 OC 1.4644 VD, p sp conductance at 25 OC, mho 3 X lo4 cm-' ion product, mol2 cmd 2 x lo-= CJliquid), cal mol-' K-' 23.62 (298 K) CJsolid), cal mol-' K-' 15.3 (274.7K)
Water' HL' 0.0 100.0 374.2 218.3 0.9971 0.008904 72.0 1440 5.3 9720 10500 26.1 1.85 78.3 1.3325 5 x 10-8 10-14 17.98 (298 K) 8.9 (273 K)
"Compiled by H. S. Frank and R. Lumry. Data for hydrazine taken from ref 14.
the hydrocarbon chain out of water and into the confines of a micelle and simultaneously orienting with the polar head group at the water interface. In other words, at high temperatures micellization is driven entirely by enthalpic changes. The same processes also must occur at 25 "C, but AH and AS are decorated by water structural changes that have only a small influence on water-nonpolar interactions. Further insight i n t ~ aggregation processes in general, and the special role of water in particular, comes from studies employing other polar solvents. Hydrazine provides particularly useful comparisons since it possesses many physical properties that are almost identical with those of water (Table 111). In fact hydrazine differs from water in just those properties (heat capacity, density maximum, dielectric constant, etc.) that have been considered unique physical properties of water. This observation led Frank that hydrazine might be viewed and L ~ m r y ' ~toJsuggest ~ (12)Lumry, R.; Frank, H. S. Proc. 6th Int. Biophys. Congr. 1978,7, 554.
-AGm, -A"P, ASm, surfactant T. "C kcal mol-' kcal mol cal mol-' K-' Cl4TAB 25 10.5 3.0 25 95 11.5 12.4 -2.5 166 10.6 15.6 -12 CIZTAB 35 7.1 11.0 -13 C120S03Na 35 8.6 14.0 -18
Table V. Thermodynamics of Transfer of Nonpolar Gases from Cyclohexane to EtNsNOs and H,O AHo ASo, AGO, kcal mol-' kcal mol-' cal mol-' K-' Kr FS" 1.6 -0.9 -9 HZ0 2.8 -2.9 -19 CH4 FS 1.6 -0.5 -4 HzO 2.9 -2.7 -18 CzHB FS 2.8 -1.0 -10 HZO 3.9 -2.1 -20 CIH10' FS (3.61) (-5.71) (-31.3) HzO (6.35) (-6.21) (-42.1) I
"The values for butane refer to the transfer from the gas phase to the fused salt and to water.
as a model for inhibited water. We have measured cmc's in hydrazine14 as a function of temperature and obtained AGO, AHo, and A S ' for the solvophobic contribution to micellization (Table IV). As the table shows, the thermodynamics are very similar to those observed in hightemperature water. Measurements in H4N2-H20mixtures gave values of AGO, AHo, and ASo that vary linearly with solvent composition.16 Ethylammonium nitrate (EAN) is a low-melting fused salt in which amphiphiles self-assemble.16 Table V compares the thermodynamics of transferring gases from cyclohexane to water and EAN.l' The trends in EAN are parallel to those in water but are only half the magnitude. This clearly demonstrates that the anomalous entropic and enthalpic changes are not unique to aqueous solutions. In fact, the basis for differentiating between water and other solvents rests not in the first derivatives of the free energy (AHand AS) but rather in the secondary derivatives like heat capacity, expansivity, etc.leJg Critical micelle concentrations are typically 10-15 times larger in EAN than in water,16 and aggregation numbers are considerably smaller.20 Phospholipid bilayers in water and EAN have the same bilayer spacing.21 Amphiphilic aggregation also has been documented in formamide22and ethylene While it is clear that cmc's are considerably higher in these solvents, no information on aggregate size or other related properties is (13)Lumry, R.; Battistel, E.; Jolicoeur, C. Faraday Symp. Chem. Soc. 1984,17,93.Lumry, R.; Frank, H. S. Proc. 6th Znt. Biophys.Congr. 1978, 7,544. (14)Ramadan, M.; Evans, D. F.; Lumry, R. J.Phys. Chem. 1983,87, 4538. (15)Ramadan, M.;Evans, D. F.; Lumry, R.; Philison, S. J. Phys. Chern. 1985,89,3405. (16)Evans, D.F.; Yamauchi, A.; Roman, R.; Casassa, E. Z. J.Colloid Interface Sci. 1982,86,89. (17)Evans, D.F.; Chen, S.-H.; Schriver, G. W.; Arnett, E. M. Arnett J. Am. Chern. SOC. 1981,103,481. (18)Lumry, R.; Gregory, R. B. In The Fluctuating Enzyme; Welch, G. R., Ed.; Wiley-Interscience: New York, 1986. (19)Mirejovsky, D.;Arnett, E. M. J. Am. Chem. Soc. 1983,105,112. (20)Evans, D.F.;Yamauchi, A.; Wei, G. J.;Bloomfield, V. A. J.Phys. Chem. 1983,87,3537. (21)Evans, D.F.; Kaler, E. W.; Benton, W. J. J. Phys. Chem. 1983, 87,533. (22)Ray, A. Nature (London) 1971,231,313;J . Am. Chern. SOC.1969, 91,6511.
6 Langmuir, Vol. 4, No. 1, 1988 I
3 - Methylsydnone
The Langmuir Lectures Table VI. Thermodynamic Function for Transfer of 1 mol of Argon from Cyclohexane to Water and to Hydrazine kcal mol-’
AS, gibbs mol-’
(1) Ar(X2 = 1 in C6HlZ)= Ar(XZ= 1 in H4Nz) 2.84 (2) Ar(XZ= 1 in H4Nz) = Ar(X, = 1 in HzO) -0.36 (3) Ar(X, = 1 in CBHIP)= Ar(Xz = 1 in H20) 2.48
2.27 -4.95 -2.68
-1.9 -15.4 -17.3
step
__
(P)”3
Figure 4. Free energy of transfer of argon, methane, ethane, and n-butane from several liquids to the gas hase. The liquids are plotted according to their value of yluliFwhere y is the surface tension and u is the molar volume. This (Gordon) parameter is a measure of a liquid’s “cohesiveness”,and liquids with Gordon parameters above about 13 tend to promote aggregation of amphiphilic molecules One exception is the aprotic 3-methylsydnone where ylv’I3 z 15 but where no aggregationphenomena have been
detected (see text).
available. Investigation has not confirmed reports of micellization in other solvents such as dimethyl sulfoxide.23 As one might expect, cmc’s correlate with solvent polarity. This can be placed on a quantitative basis (Figure 4) by plotting the transfer of free energy vs y/V I 3where y is the surface tension and V is the molar v~lume.’~ This latter quantity, which was proposed by Gordon,24 is analogous to the Hidelbrand solubility parameter and permits us to consider fused salts with no detectable vapor pressure. The experimental observation is that as y/V I 3 decreases from water to ethylene glycol, cmc’s increase and become progressively broader. Aggregation numbers diminish. Note that in Figure 4 the nonaqueous data points for argon approximate a straight line. The value for water lies considerably below the extrapolated value and suggests (as in Figure 1) that water structural effects increase the solubility of nonpolar moieties. Whether amphiphilic aggregation can ever occur in an aprotic solvent has been an unresolved issue. All aprotic solvents investigated previously have Gordon parameters that fall below those of ethylene glycols. 3-Methylsydnone is one exception.% It possesses a high dielectric constant ( E = 144), a very large dipolar moment (p = 7.3 D), and a cohesive energy density12 that lies between the cohesive energy density of hydrazine and EAN. Conductance and surface tension measurements, flooding polarizing microscopy, and diffusion interfacial transport experiments% on a variety of single- and double-chain amphiphiles give absolutely no indication of amphiphilic aggregation at any concentration or under any circumstances. Thus, it appears that multiple solvent hydrogen bonding is a necessary prerequisite for amphiphilic self-assembly. In summary, amphiphilic self-assembly is a physical process that occurs in polar, multiple-hydrogen-bonding solvents. The driving force for aggregation correlates with solvophobicity except in aqueous solution. In this most important and exotic of all solvents, this situation is more complex, and the key issues have remained unresolved for (23) Singh, A. N.;Saleem, S. M.; Singh, R. P. J.Phys. Chem. 1980,84, 2191. (24) Gordon, J. E. The Organic Chemistry of Electrolyte Solution; Wiley: New York, 1975; pp 158-162. (25) Beesley, A. H.;Evans,D. F.; Laughlin, R. G., submitted for publication in J . Phys. Chem. (26) Laughlin, R. G.; Munyon, R. L. J . Phys. Chem. 1987, 91,3299.
AH,
AG, kcal mol-’
many decades. The story that is emerging is that below 90 “C hydrocarbons are more soluble in aqueous solution than one would predict solely on the basis of water’s polarity. The result of water’s unique structural effects is to increase solubility and thereby lower cmc’s. We can illustrate these ideas by comparing the solubility of argon in water and in hydrazine (Table VI).12J3 The ideal is to divide the dissolution process in water into two imaginary steps. In the first step, we create a hole in water and insert the argon with the proviso that water is inhibited in the sense that we do not permit it to structurally respond to the pressure of the nonpolar group. Because of the physicochemical similarities between hydrazine and water (particularly enthalpy of vaporization and surface tension), hydrazine is proposed as a model for inhibited water. In the second step, we relax the inhibitions, which corresponds to the transfer of argon from hydrazine to water. Large negative changes in AH and A S take place, accompanying the formation of flickering clusters, but they are compensated so that the change in free energy is very small. While this is a heuristic argument, it illustrates the point in a concrete way. Relative to other liquids, water possesses an enormous diversity of fluctuating states that display comparable free energy but vastly different entropy and enthalpy. A nonpolar molecule imposes on water a succession of these states in a way that actually increases the molecule’s solubility at low temperatures. In such a situation, the compensated contributions to the enthalpy and entropy dominate and the measured values provide no useful insight as to why processes happen. Table VI illustrates this. Whenever a large number of molecular configurations of comparable free energy exist, we find enthalpy-entropy compensati~n.~~,~~
Structure Most colloid scientists carry around in their mind’s eye a collection of amphiphilic structures. They are the idealized abstractions and summaries of knowledge gained from light, X-ray, and neutron-scattering measurements. Once we move beyond single spherical micelles at the cmc, however, polydispersity and interactions become important issues, and we must incorporate them into the models employed for the interpolation of scattering data. Because these two factors severely limit the amount of structural information scattering measurements can give us about more complex systems, direct visualization can teach us a great deal in such situations. Before we pursue this point, it will be useful to describe a guide that relates amphiphilic molecular structures to the assemblies they form. This is the surfactant number defined as ulal where u is the volume of the hydrocarbon chain, 1 is its length, and a is the head group area. The surfactant number is a measure of local curvature at the aggregate-solvent interface that ignores particle interactions. This concept, which originated with Tartar,29was (27) Benizinger, T. H.Nature (London)1971, 229, 100. (28)Benizinger, T. H.; Hammer, C. Curr. Top. Cell. Regul. 1981,18, 475.
The Langmuir Lectures developed by Tanford! and waa refined by Mitchell and Ninham” and by Gelbart.3’ predicts spherical micelles for u l l a < ItQ, cylindrical micelles for < u/la < bilayers and vesicles for ‘I2< v/la < 1, and inverted structures for u / l a > 1. A useful characterization of amphiphilic systems requirea more than an knowledge of local curvature, however. In many situations one needs to know the relationship between microscopic structure and macroscopic properties such as viscosity, visual appearance, etc. In this regard the surfactant number is limited. When the interface is highly curved, i.e., u l l a < ‘I3or ulla >> 1, aggregate size is small and bulk properties can usually be related directly to structure. In between these extremes, which include cylindrical micelles, vesicles, and bilayers, the relationship between microscopic and macroscopic properties becomes less well defined (aswe will see below) and often depends on preparative methods and previous history. Recent developments in light (video-enhanced microscopy)” and electron m i c m p y (cryotransmission electron have expanded our ability to directly -see“ amphiphilic structures. Sinre these techniques were discussed in detail recently,” we will summarize them briefly here. In light microscopy, mntraat limitations restrict the size of aggregates we can select for study, while resolution limitations restrict the amount of structural information we can extract from microscopic observation. Many surfactant aggregates like vesicles and microtubules are of inherently low contrast, and the eye cannot distinguish them from the background solution even with differential interference contrast optics. We can overcome this limitation by using a video camera that responds linearly to intensity and a comwter capable of real-time digital image . . processing. Two imDortant imaee-orocessine stem are (1) background subtraction aid 12) gray-&Je*transformation. Background subtraction removes optical noise arising from inaccessible dirt and lens imperfections. This noise appears as a mottled background pattern that obscures the objects of interest. We can remove it by storing the mottled image in the computer and subtracting it frame by frame from the actual pattern. Gray-scale transformation increases the contrast between image and background. With a very low contrast image (such as small unilamellar vesicles) all the information may be contained in only 20 of the 256 gray levels detected by the video camera. The transformation proceas changes all the levels above and below these 20 gray levels to black and white, respectively, and expands the original 20 shades of gray into 256 shades. Other more complex mathematical transformations, including edge enhancement, random noise elimination, and automatic particle sizing, can be carried out at the same time. As a result of this contrast enhancement, we can detect isolated colloidal particles like u n i l a m e h vesicles or latex spheres as small as 600 A. Given sufficient contrast to distinguish a particle from its background, there is theo(29) Tartar, H.V.J. Phys. Chem. 1965,59,1195. (30) Mitehell, D. J.; Ninham, B.W. J. Chem. Soc., Famday l h m . 2 1981,77,609. (31) Ben-Shad, A,; Szleifer, J.;Gelbart,W.M.h. Natl. A d . Sei. U.S.A. 1904,81, m1. (32) InouB, S. Video Miemcopy; Plenum: New York, 1986. (33) Adrian, M.; Duboehet, J.; McDoweU, J.; MeDonell, A. w.Nature (London) 1984,308,32. (34) Miller, D. D.; Bellare, J. R; Evans, D. F.;Talmon, Y.;Ninham, B. W.J. Phys. Chem. 1987,91,674. (35) Kachar. B.;Evans, D.F.;Ninham, B.W. J. Colloid Interface Sci. 1984, IW,287.
Langmuir, Vol. 4, No. I, I988 I
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:F.?d Figure 5. Uncontrolled neutralization of M didodecyldimethylammonium hydroxide with HBr results in a violent transformation of micelles to vesicles and liposomes, as visualized by this video-enhanced micrograph. Bar = 10 pm. From ref 34. retically no limit to the size of the isolated particles that microscopes can detect. However, with such small particles, the video image is enlarged due to diffractions. As a result, we cannot obtain any useful information about particle size. Because of the contrast-enhancing nature of VEM, its resolution limit appears to be a factor of 2 smaller than that of normal microscopy. We can detect extended structures as small as l(t60 nm, and this enables us to visualize the bilayer structure of large vesicles and liposomes. The main advantages of VEM are that it gives us the ability to (1)observe the dynamics of amphiphilicstructure in real time without perturbing the system, (2) follow flow and/or chemically induced transformations, (3) record the video information and subsequently reanalyze the data a t a later time, and (4) use time-lapse recording to follow transformations that occur over a period of hours or days. It is useful to remember that photographs of the video images convey only a small fraction of the information we can obtain in a video film. Neutralization of didodecyldimethylammonium hydroxide micelles by hydrogen bromide involves all these factors.” The change of counterion results in the transformation of a clear micellar solution (aggregation number 40)= to liposomes and bilayers. Simple mixing of the two solutions between the coverslip and slide results in uncontrolled convection and an almost explosive formation of large liposomes like those shown in Figure 5. However, we can obtain more useful information about such transformations by using a microscopic flow cell.” In our cell, the two solutions are pumped into two entrance ports so that a sharp interface forms down the length of the rectangular channel. When the flow is stopped, neutralization takes place across the interface. The micelle-liposome transformation (Figure 6) occurs via “worms” or microtubules that are oriented Dreferentiallv in the direction of the acid-base concentrkion gradient. These worms disappear as liposomes form. We note that many transformations between various microstructures appear to involve worms as intermediates. Figure 7 (a sample of a 6-month-old, 1.7 wt % sodium 8’-hexadecylbenzenesulfonate (SHBS)) shows a typical polydisperse liquid crystalline aggregate with a wide div(36)Brady, J.; Evans,D. F.;W a n , G. G.; Greiasr, F.;Ninham, B.W. J. Phys. Chem. 1986,90,1853.
8 Langrnuir, Vol. 4, No. 1, 1988
The Langmuir Lectures
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