J. Phys. Chem. 1992, 96,4653-4658 agreement with the average excess polarizability of 0.771 X cm3 determined in the present work. Based on this agreement, it is likely that the Q*values measured for LMM in the present work are due almost entirely to an intrinsic induced dipole moment. There is a problem with this analogy, because the theoretical treatment of OKonski and Krauseg should have included the intrinsic polarizability. If, however, the intrinsic polarizability is as much faster than the ion atmosphere polarizability as it seems from this work, it may contribute to Q* in a decoupled fashion.
Conclusions 1. Although the steady state specific Kerr constant versus conductivity data on monomeric L M M can be made to fit the O’Konski/Krauseg theoretical treatment, the individual variations of the P (the permanent dipole term) and Q (the induced dipole term) do not agree with the experimentally derived quantities P* and Q*which were determined using transient birefringence data. 2. However, the experimentally determined values of P* agree rather well with the same theoretically calculated values of Q that give agreement with the specific Kerr constants. In order to
4653
calculate these values of Q it was necessary to use an exceptionally low value for the mobility of the counterions. This implies that the calculated values of Q were actually estimates of a slow induced dipole moment that acted experimentally like a permanent dipole moment. 3. The experimentally determined average value of Q* is close to the value that can be calculated using the Peterlin-Stuartzz theory which determines only intrinsic induced polarization. 4. The electric polarization mechanism of L M M consists mainly of a slow induced dipole moment connected with ion atmosphere polarization with an added contribution from the intrinsic induced dipole moment. The molecule may also have a permanent dipole moment.
Acknowledgment. We thank the National Science Foundation, Structural Chemistry and Thermodynamics Program, for Grant No. CHE-8308089 in partial support of this work. We also thank W. F. Harrington and M. E. Rodgers for allowing J.F.C. to help in the preparation of myosin and L M M at Johns Hopkins University.
Characterization of a Four-Component Cationic Reversed Micellar System: Dodecyltrlmethylammonlum Chloride/Hexanol/n-Heptane and 0.1 M KCI Solution Christophe Petit; Andreas S. Bommarius,t Marie-Paule Pileni,*.+and T. Alan Hatton*,$ Laboratoire de Structure et Reactivite aux Interfaces, UniversitE de Paris VI, 75231 Paris Cedex 05, France, Department de Physico-Chimie, BP I21 Centre d’Etudes Nucleaires de Saclay, 91191 Gif-sur- Yvette Cedex, France, and Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received: July 18, 1991; In Final Form: January 2, 1992)
The reversed micellar region of the phase diagram for the DTAC/hexanol/n-heptane/buffer system has been characterized in terms of microemulsion droplet interfacial composition and size over a wide range of water-to-surfactant ratios, wo. Phase boundary titration, conductivity, SAXS, and QELS measurements are all consistent with each other, giving strong support for the existenceof a somewhat polydisperse dispersion of spherical droplets in the organic continuum stabilized by an interfacial cosurfactant-surfactant mixture of constant composition.
Introduction Reversed micellar systems have attracted considerable attention recently owing to their ability to host various hydrophilic components in organic solvents, thereby providing a versatile environment for carrying out a range of reactions and separati0ns.I The majority of investigations with reversed micelles have been carried out with three-component systems consisting of oil, surfactant, and an aqueous solution, which can be relatively easily characterized as to the limit of the reversed micellar region in the phase diagram, the micellar core, and hydrodynamic sizes as functions of water content, and the intermicellar interactions. The best known example of such a system is the anionic surfactant sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or AOT), a hydrocarbon as oil, and water. The behavior of four-component reversed micellar systems is more complex, as in addition to surfactant, oil, and an aqueous component, these systems also contain a cosurfactant, usually a medium-chain alcohol, which can act as a cosolvent as well. While there has been some work done on systems of this type,z in general they are less well characterized than the AOT system. Good characterization is a prerequisite for understanding changes in rate (i.e., kinetic and transport) and partitioning phenomena in reversed micellar systems ‘Authors to whom correspondence should be addressed. ‘Universitb de Paris VI and Centre $Etudes Nucleaires de Saclay. *Massachusetts Institute of Technology.
with respect to their properties. In this paper, we present a partial characterization of a cationic four-component reversed micellar system with the surfactant dodecyltrimethylammonium chloride (DTAC), hexanol as cosurfactant and cosolvent, n-heptane, and 0.1 M aqueous KCl solution at neutral pH. This system solubilizes water up to high molar water-surfactant ratios wo, a property lacking in many reversed micellar systems but important for investigations of rate and partitioning phenomena in microheterogeneous systems. The system investigated here and similar ones have been used by us in the investigation of an enzymatic reaction sequence and of reversed micellar coalescence and redispersion processes? and by Hilhorst et al. in a study of a coupled enzyme system involving cofactor regenerati~n.~The importance of having a well-characterized cationic reversed micellar system comparable to the well-known anionic AOT system for fundamental solute parti(1) Pileni, M. P., Ed. Structure and Reactivity in Reverse Micelles; Elsevier: Amsterdam, 1989. ( 2 ) (a) Abillon, 0.; Binks, B. P.; Otero, C.; Langevin, D.; Ober, R. J . Phys. Chem. 1988, 92, 4411. (b) Binks, B. P.; Meunier, J.; Langevin, D. Prog. Polym. Sci. 1989, 79, 208. (3) (a) Bommarius, A. S.;Hatton, T. A.; Wang, D. I. C. Manuscript in preparation. (b) Bommarius, A. S.;Holzwarth, J. F.; Wang, D. I. C.; Hatton, T. A. J. Phys. Chem. 1990, 94,1232. (4) (a) Hilhorst, R.; Spruijt, R.; Laane, C.; Veeger, C. Eur. J . Biochem. 1984, 144, 459. (b) Hilhorst, R. Ph.D. Thesis, Agricultural University; Wageningen, The Netherlands.
0022-3654/92/2096-4653$03.00/0 0 1992 American Chemical Societv
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The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
tioning studies has also recently been emphasized by Leodidis et al.5
Experimental Section Materials. DTAC was obtained from Tokyo Kasei Corp. in 99.6% purity and recrystallized once with ether/methanol before use. Reagent grade n-heptane from Mallinckrodt (Paris, KY) and purum grade hexanol from Fluka were used as received. Water was deionized and distilled. Cobaltous chloride, CoCl,, was obtained puriss. grade from Fluka and used as received. Potassium chloride was Suprapur grade from Merck (Darmstadt, F.R.G.). Phase Boundary Determination. The composition limits of the reversed micellar phase were measured by titrating either a mixture of oil, surfactant, and alcohol with buffer to the phase boundary (at which the w, ratio is at a maximum) or a mixture of oil, surfactant, and buffer with alcohol. The criterion for the phase boundary was a precipitous change in turbidity level from high to low at the lower or solubilization phase boundary (low alcohol content) or from low to high at the upper or haze boundary (high alcohol content). When the initial aqueous-surfactantsolvent mixture was to be titrated with hexanol, the mixture was formed by adding a 0.1 M KCl solution to a suspension of DTAC in n-heptane under vigorous mixing conditions in a vortex mixer to give the desired w, value. Hexanol was added to this turbid mixture in steps of 20 or 50 pL until the system became clear and nonviscous. A turbidity change at the phase boundary occurred within increments of only one or two 10-pL volumes and was easily detected. When the aqueous component was to be titrated to a mixture of DTAC in n-heptane and hexanol, it was added in 20-pL steps to a solution containing the desired initial amount of hexanol. After addition of any component, the samples were vortexed for at least 10 s. At points close to phase transitions, up to 20-30 min often elapsed between additions of hexanol. All phase boundary determinations were made at 25 f 0.5 O C . All DTAC samples were prepared by weighing in a specified amount of surfactant for each sample in a 20-mL test tube. The amount of DTAC was such that, at the desired surfactant concentration, the volume of oil needed was close to 5 mL. This procedure ensured accurate measurement of surfactant concentration. Interfacial Layer Composition. The procedure of Gerbacia et aL6 has been used in the past to determine the interfacial layer cosurfactant-to-surfactant ratio, obtained from the intercept of a plot of added hexanol versus oil required to reach the phase boundary. The slope of this curve gives the solubility of cosurfactant in the oil phase. This method was found to have insufficient precision for our purposes, however, and therefore a different procedure was adopted. At a fixed wo value, a DTAC suspension in oil of specified concentration, ranging from 0.025 to 0.4 M, was titrated with cosurfactant for turbidity change at the solubilization phase boundary. Following this, cosurfactant was added again until a rapid change in turbidity was observed at the haze phase boundary. In a plot of cosurfactant needed for turbidity change at this phase boundary versus surfactant concentration, the slope represented the cosurfactant/surfactant ratio at that wovalue and the intercept yielded the solubility of cusolvent in oil. The procedure was repeated at a number of different wo ratios. Conductivity. The conductivity measurements were made with a Tacussel CD810 conductivity meter using a GK2401C cell placed in the sample being studied. The data were taken in the middle of the reversed micellar region, at half distance between the solubilization and haze boundaries. The temperature was 25 f 0.5 OC, the DTAC concentration 0.1 M. The hexanol content of the continuum was 10%. The measurements were perfectly reproducible. Water Determination in Organic Phases. Water content was measured by the coulometric Karl Fischer titration method on ( 5 ) Leodidis, E. B.; Hatton, T. A. J . Phys. Chem. 1990, 94, 641 I . ( 6 ) Gerbacia, W.; Rosano, H. L.J . Colloid Interface Sci. 1973, 44, 242.
Petit et al. a Mettler DL 18 Automatic Titrator. The drift compensation for entering moisture from the surroundings was always less than 6 mg/min, and usually less than 1 mg/min of water. The organic sample volume was adjusted such that between 3 and 15 mg/mL water was titrated, wherever possible. Triplicate samples were analyzed. Water Structure in Reversed Micellar Pools. A 0.1 M solution of CoC1, in either distilled water or 0.1 M KC1 solution was injected into 5 mL of a 0.2 M DTAC solution in n-heptane and hexanol. The hexanol content was 10% based on n-heptane. The sample was vortexed for about 30 s and subsequently scanned for UV/vis absorption from 550 to 700 nm. The extinction at X = 694 nm, caused by the [ C O C ~ ~complex, ]~was plotted against the wo value to assess the availability of water molecules. Small-Angle X-ray Scattering (SAXS). The scattered X-ray intensity a t low angles was measured from 0.025 to 0.5 A-I, corresponding to a resolution in real space of 10 to 250 A, using a GDPA 30 goniometer and Cu K a radiation (1.54 A). The experimental arrangement has been described elsewhere.' Absolute scaling of the scattered intensity was achieved using water as a standard for apparatus calibration. The radius of gyration R, and the scattering intensity at zero angle Z(q = 0) were evaluated from a Guinier plot of In 1 vs q2, where q = 47r sin 8/X, and 20 is the scattering angle.* These plots showed good linearity at low q values, and a single slope was observed. Because the dialkyl chains and the solvent have, to a first approximation, the same electron density, the gyration radius is similar to the water pool radius, including the polar head group. A Porod plot of Z(q)dvs q provided both the characteristic radius R, and the Porod limit9 The characteristic radius is related to both the first maximum and the first minimum of this representation, given by 2.7/RC and 4.5/RC, respectively, in the case of homogeneous diffusing spheres. This provides a convenient measurement of the size of the droplets. The Porod limit at q = m is proportional to the total interfacial area per unit volume of the sample. A third estimate for the radius, Ri,,, can be obtained from the ratio I(q=O)/Q, where Q is the invariant defined by
Quasi-Elastic Light Scattering (QELS). A 136-channel Brookhaven 2130 A T correlator was used in the QELS experiments. The light source was an argon laser (5145 A) operating a t a power of 0.5 W. The measurements were made at 25 OC, at an angle of 45'. All samples were prefiltered and centrifuged to eliminate dust. The micellar diffusion coefficients were measured a t various volume fractions, obtained by diluting the initial 0.1 M DTAC solution with the continuum solvent at the appropriate composition established using the titration results. The hydrodynamic radii of the micellar aggregates were determined from QELS measurements of their diffusion coefficients using the Stokes-Einstein equation
Do= kT/67rqR
(2)
The apparent diffusion coefficient D measured at volume fraction @ greater than zero can differ from Do if there are interactions between the aggregates. At very low volume fractions, dependence of the apparent diffusion coefficient on the volume fraction is linear, and is given by D = Do(1 + cO)
(3)
where c is the virial coefficient. If the reversed micelles do not interact, c = 0, and the diffusion coefficient D does not change with aqueous volume fraction O. Do was obtained from the intercept of a plot of D vs 9. (7) Zemb, T.; Charpin, P. J . Phys. (Fr.) 1988, 49, 319. ( 8 ) Guinier, A.; Fournet, G. Small Angle Scattering of X-Rays; Wiley:
New York, 1955. (9) Porod, G. In Small Angle X-Ray Scattering Glatter, O., Kratky, 0.. Eds.; Academic Press: New York, 1982; Chapter 2.
Four-Component Cationic Reversed Micellar System 20
I
L
I
I
I
I
The Journal of Physical Chemistry, Vol. 96, No. 11, I992 4655
I
U p p e r ( h a z e ) phaaa boundary Turbid
\-
0 Upper p h a a a boundary
I
0 Lower p h a a a boundor
Clear,
2
0 " ~
region
4-
Turbid & Liquid c r y a t a l l i n s
0
4[
e
Lower (aolubilizotion) p h a a e boundary 1
I
l
I
I
4
I
0
10
20
30
40
50
Water-to-Surfactant
Figure 1. Phase boundaries for the reversed micellar region in the DTAC/hexanol/n-heptanelbuffer system (25 mM < [DTAC] < 400
mM).
wn
10 15 20 25 30 35 40 50 60 70
5.61 6.90 7.18 7.60 6.42 1.62 7.85 9.37 8.96 9.99
16.44 16.1 1 15.51 14.3 13.82 12.47 11.68
80
250 h
ln
(hexanol/DTAC ratio) lower phase upper phase boundary boundary 2.84 2.86 2.67 2.97 3.08 2.87 3.06 2.72 2.78 3.06
70
Ratio, wo
Figure 2. Hexanol-to-surfactantmolar ratio in the interface for the haze and solubilization boundaries of the reversed micellar region of the phase diagram.
TABLE I: Phase Boundaries and Interfacial Compositions hexanol in interfacial composition
continuum, vol % lower phase upper phase boundary boundary
80
5
200
.-h> +
150
$
100
.-+ U C
6.34 4.87 3.48 3.05 2.85 2.54 2.77 3.10
Results and Discussion Phase Diagram. No surfactant precipitation occurred upon addition of the buffer at low surfactant concentrations, between 25 and 75 mM. For DTAC concentrations in heptane beyond 0.2 M, a white, highly viscous liquid was formed initially, but as the phase boundary was approached during the titration, the viscosity decreased substantially. At each wovalue investigated, titration curves of hexanol addition needed for phase transition versus surfactant concentration gave linear plots with DTAC concentration (10 data points, 25 mM < [DTAC] < 400 mM), with regression coefficients of greater than 0.98. The continuum contents of hexanol were determined from the intercepts at [DTAC] = 0 mM, and the interfacial cosurfactant-surfactant ratios from the slopes. Figure 1 shows the minimumand maximum amounts of hexanol in the continuum, in volume percent, required to form a clear, nonviscous phase a t various water-surfactant ratios wo,an ionic strength of I = 0.1 M, and a temperature of 25 f 0.5 OC; the data are given in Table I. The solubilization phase boundary at all watersurfactant ratios, as well as the haze boundary above wo = 25, can be represented by linear equations for the limiting amount of hexanol in the continuum volume as a function of wo: vhcx,lpb = 5.49 f 0.56 + 0,064 f 0.010wO (0 < w o 5 79) (4a) Vhcx,upb= 19.21 f 0.27 - 0.110 f 0.008WO (25 IW O I79) (4b) The intercept of the solubilization phase boundary at 5.5 vol 7% hexanol represents the minimum amount of hexanol in the continuum required for a clear, nonviscous phase. The equation for the haze phase boundary is valid only at wovalues larger than 25; at lower w o values, the clear, nonviscous phase extended to much higher hexanol levels with a limiting wovalue between 15 and 25. The maximum capacity of the microemulsion for the aqueous solution, corresponding to the highest possible w o value for the formation of reversed micelles, is at the intersection of the solubilization and haze phase boundary lines, which occurs at wo = 79 and a hexanol continuum content of 10.5 vol %, or 0.84 M.
8
50
0
4
I
I
20
40
80
Water- t o - Su rf a c t a n t Rat io,
80
wo
Figure 3. Conductivity of the reversed micellar solutions as a function of microemulsion water content. No percolation phenomena are indi-
cated. This point has been verified experimentally. Interfacial Composition. The experimentally-determined interfacial cosurfactantsurfactant ratios, given in Table I, are shown for the two phase boundaries in Figure 2. The average ratios for the two boundaries did not differ significantly: the values were 2.89 f 0.14 for the solubilization phase boundary for all wo,and 2.86 f 0.20 for the haze boundary for wo> 25; the combined value was 2.88 f 0.17. From this ratio, and with the assumption that the interface consisted only of surfactant and cosurfactant, the mole fraction of the cosurfactant in the interface was calculated to be 0.742. Since the composition of the interfacial layer was the same over a significant portion of the phase boundary, it is plausible to assume that the interfacial composition was also the same at compositions away from the phase boundaries in the interior of the reversed micellar phase region. Conductivity. The conductivity of the reversed micellar solutions was measured at several wovalues from 0 to 70. The results shown in Figure 3 indicate that the conductivity values were very low and therefore that the aggregates were unconnected, and no percolation phenomena were evident. The conductivity pattern as a function of wo,in which there is a maximum in the conductivity at some low wo,resembles that reported for AOT reversed micellar solutions.I0 Beyond this peak, the decreasing conductivity with increasing wohas been successfully interpreted by Eicke et a1.l0 in terms of a charge fluctuation model. Small-Angle X-ray Scattering. The good agreement between the two estimates R, and I$ for the water pool radii obtained from the SAXS experiments and tabulated in Table I implies that the scattering patterns observed were due to spherical droplets. This is further confirmed by a simulation of the scattering patterns carried out using the known electronic contrast and polar volume and the estimated micellar radius for a wo value of 20. The scattered X-ray intensity Z(q) is given by where P ( q ) is the structure factor and S(q) the form factor de(10) Eicke, H. F.; Borkovec, M.; Das-Gupta, B. J . Phys. Chem. 1989, 93, 314.
4656 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992
Petit et al.
2
/-
0 EO
V
Rc
0
-2 -4
-a
-6
P
I
I
4 0 1
I
i
-I -10 -4
-2
-3
-1
10
0
In q
20
30
40
50
(10
70
Ratio, wo
Water-to-Surfactant
Figure 5. Variation of the micellar radius with water content, w,,, as determined from Guinier (R,) and Porod (R,) representations of small-angle X-ray scattering results. TABLE II: Radii and Porod's Limit WO
0 10 0.00
0.05
0.10
0.15
0.20
9
Figure 4. Different representations of small-angle X-ray scattering results for a w, of 20, compared with model predictions based on spherical droplets having radius 30 A. (a, top) In I(q) vs I n q; the higher observed intensity at low q could be due to some intermicellar interactions, or to polydispersity in the sample. (b, bottom) Porod plot; the positions of the
minimum and maximum are consistent with spherical micelles having radius 30 A, but are not as pronounced as the model predictions. This is probably due to polydispersity in size or form. scribing intra- and intermicellar interactions, respectively. The structure factor is given by
P(q) = @P,IvFAP%Z(qR)
(6)
where V, F,and Ap are the polar volume fraction, singleparticle volume, Thompson's factor, and the variation of the average electron density, respectively, and f,(x) = 3(sin x
- x cos x)/x3
(7)
The form factor was obtained using the Hayter-Penfold method." In this model, we assumed a hard-sphere system with no interactions. The good agreement between the simulated and experimental data obtained by plotting In I ( q ) vs In q (Figure 4a) shows the validity of the structural hypothesis. The underestimation of the scattered intensity at low q could be due either to intermicellar interactions or to some polydispersity in the sample, as has been noted for AOT.12313The P o d representation (Figure 4b) gives the correct value of the radius, but the absence of a pronounced minimum in the experimental patterns is generally attributed to a relatively high degree of polydispersity of the microemulsion dr0p1ets.l~ The situation is similar to that of AOT reversed micelles,13although in our case the polydispersity is higher. Such effects can be caused by fluctuations about a mean spherical shape in the micelles, even in a monodisperse system. It is difficult to discriminate between these shape fluctuations and size polydispersity. Indeed, as is well-described by Zemb et al.,15 any scattering intensity measurement can be explained in terms of a microstructure made up from either intracting monodisperse (11) Hayter, J. B.; Penfold, J. Mol. Phys. 1981, 42, 109. (12) Brochette, P. Thesis, University P et M Curie, Paris, 1987. Steeb, C.; Hofmeier, U.; (13) Hiificker, R.; Eicke, H. F.; Sager, W.; Gehrke, R. Eer. Eunsenges. Phys. Chem. 1990, 94, 677. (14) Cabane, B. In Surfactants in Solution; Zana, R., Ed.; Dekker: New York, 1986. (15) Eamb, T. N.; Hyde, S . T.; Derian, P. J.; Barnes, I . S.; Ninham, B. J . Phys. Chem. 1987, 91, 3814.
20 30 40 50 60
lim(q-m) I(q)q4, io5 cm-'
R,,A 10 f 1
1.2 1.5 1.2 1 .o 1.1
1.1
18f2 30 f 2 43f2 54f2 65f5 85f5
Rp A 12 f 0.5
Ri,,, A
18f0.5
13f1 20 f 2 32f3
29 f 0.5
40f1 51f2 60f2 78f5
40f5 45f5 55f5
ellipsoids or polydisperse spheres, provided one admits a free choice of the mass distribution function and the interaction potential. Note that the radius Rhv determined from the invariant of the scattering spectrum given by eq 1 is consistently lower than the other two estimates by about 25-30%. This could again reflect the high degree of polydispersity in the sample, as the weighting given to the small particles may be greater in this case than for the other two methods for estimation of the radius. Figure 5 shows that the water core radius increased linearly with the water content, w,. Geometric argumentsi6 predict that the radius R is given by
R = 3V/S
(8) where Vis the polar volume and the S the total surface area of the spheres per unit total volume of reversed micellar solution. Assuming the polar volume is simply that associated with the water pools themselves, we can write V = v,N[H~O]
(9)
where u, is the volume of a water molecule, taken to be 30 A3, and N is Avogadro's number. The total surface area is due to contributions from both the surfactant (DTAC) and the cosurfactant (hexanol) at the interface, and is given by S = N(u,[DTAC] + Uh[heX]i) = uN[DTAC] (10) where usand o h are the molecular areas occupied by the surfactant and the cosurfactant, respectively, at the interface, and u = (us + Xuh) is the effective interfacial area per surfactant molecule, X denoting the molar cosurfactant-to-surfactant ratio in the interface. The concentration of hexanol which is associated with the interface is [hexlie The size of the micelle is then given by
R =~v,w~/o (11) The linear relationship observed in Figure 5 suggests that u (or S) is constant over the wo range studied. This is corroborated I,"), which was found to be constant by the Porod limit (lim(q--) at all water contents, also indicating an invariant total (or system) surface area (Table 11). The slope obtained from Figure 5 is equal to 1.1, from which we can estimate o to be 99 AZ. (The finite intercept in this figure indicates that part of the surfactant headgroup region is included in the polar volume of the micellar core estimated by the SAXS experiments; this is simply a constant (16) Pileni, M . P.; Zemb, T.; Petit, C. Chem. Phys. Lett. 1985, 118. 414.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4651
Four-Component Cationic Reversed Micellar System N
.-0
a
v
; 1 - 1 z
Y
150
n L
125
n
0
G
-
4
........ ......f.-.....f .......f
100
I
20
10
30
40
Water-to-Surfactant
50
Ratio, we
0
2.0
0 1.s
1.o
Y
.-L
0.0 I 0.00
0.02
0.06
0.04
0.08
I
0.10
Polar Volume Fraction
Figure 7. Variation of the apparent diffusion coefficient of DTAC reversed micelles with volume fraction for two different water contents. No significant interdroplet interactions are indicated by these results.
that should be added to the expression (9) for R.) A second estimate can be obtained using the Porod limit lim Z(q)& = 27r(A~)~S ,?--
where Ap is the difference in electronic density between the solvent 0.244 e/A3) and the polar core (0.4 e/A3). A value for u of 90 is found. A limiting value for the area of the alcohol molecule can be estimated from a CPK model and is found to be approximately 18 AZfor hexanol. Assuming that the surface area per polar head group of DTAC can be approximated by that determined for tetramethylammonium bromide (‘ITAB) in normal micelles,” we take u, to be 40 A2. With these estimates, and noting that the interfacial composition obtained from the titration experiments gives a A-value of 2.85, a lower limit for u of 90 A2can be obtained, consistent with the earlier estimates. The effective surfactant head coverage can also be deduced from the Z(q=O) value obtained from the Guinier plot, using the equation
B
Z(q-0) = O(Ap)*F(4r/3)R3
(13)
where CP is the polar volume fraction and F is the Thompson factor, equal to 7.9 X cm2. Figure 6 shows that, for wo > 10, u was approximately constant in value between 85 and 100 Az.An invariant surfactant head coverage implies a constant cosurfactant-surfactant ratio, so again the SAXS data support the findings of the titration experiments. QELS Experiments. Figure 7 shows the variation of the diffusion coefficient with the volume fraction for the system studied at wo of 20 and 50. From these plots, it can be deduced that the virial coefficient is small, indicating very small interdroplet interactions. The hydrodynamic radius determined from the dif~~~~~~
~
~
~
~
~~~
(17) Seno, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H . J . Colloid Interface Sci. 1980, 78,57.
I
0.0
.
-
01-0.3M 0.4 M
0 I
1
1
‘.
\\- .
Water-to-Surfactant
SO
UI
X
,
I
Y
Figure 6. Dependence of the total surface area per surfactant molecule on water content of the reversed micellar solution, as deduced from I(q = 0) at DTAC = 0.1 M.
-
I
1
-
0.1
0.3
0
0
,
-
0.4
-+ 0.2
OO
0.5
N
I
f
‘
a
0.6
--
-
-
Ratio, w o
Figure 8. [CoCI4]*-absorption profiles as determined by micellar water content.
fusion coefficient a t zero volume fraction is equal to 38 A at wo = 20, and 115 A at wo = 50. The value obtained at wo = 20 is in good agreement with that estimated from SAXS measurements when the chain length of the alkyl group is accounted for. The good agreement between the SAXS and the QELS measurements, the absence of interactions, and the good fit in the simulation of the SAXS scattering intensity profiles all point to the presence of spherical reversed micelles, and indicate that the microemulsion dilution procedure required for the QELS measurements was valid at this wo value. In practice, the dilution of the microemulsion system could not be continued indefinitely; at DTAC concentrations lower than about 7 to 8 mM no correlation function could be found for the reversed micellar diffusion coefficient, i.e., no aggregates were formed; this concentration can be identified as a critical micelle concentration (cmc) for this particular set of solution conditions. At high water content, wo = 50, the interactions were also not significant, but the radius obtained with QELS was much higher than that obtained with SAXS (for example RsMs = 60 A and RQEU= 1 1 5 A). This could be due to a high degree of polydispersity of these reversed micelles, consistent with the observation of the SAXS experiments, since in QELS the larger aggregates are given greater weight in the detection scheme. Character of Water in DTAC Reversed Micellar Pools. Solutions of 0.2 M DTAC with a water content of 1 < wo < 20 were probed by UV/vis spectrophotometry to study the equilibrium between CO(H,O),~+and [ C O C ~ ~ ] ~ As - . ~illustrated * in Figure 8, the microemulsion system displayed a peak at 694 nm in the range 1 < wo < 7, indicating preferential formation of the tetrachlorocobalto complex. The absorption maximum shifted slightly from wo = 2.4 a t Z = 0.3 M (no KCl added) to wo = 3.25 at Z = 0.4 M. No peak was found at 694 nm for wo > 7. These results for Co2+ absorption suggest that, a t wo < 5-7, water molecules are bound tightly to the cationic DTAC headgroups whereas at wo> 5-7, where no absorption at 694 nm was detected, the properties of micellar water approached those of bulk water. All structural, partitioning, or rate process investigations at low wo values, therefore, should take into account the reduced availability of water molecules at those conditions.
Conclusions A knowledge of the phase behavior, reversed micellar sizes, and distribution of components in reversed micellar systems is often indispensable for the correct interpretation of kinetic, transport, and partitioning results. The partial characterization of the four-component cationic reversed micellar system, dodecyltrimethylammonium chloride (DTAC), hexanol, n-heptane, and aqueous buffer, presented in this paper was undertaken to support some of our studies on enzymatic reactions in reversed micellar systems and to confii some of the assumptions regarding reversed micellar characteristics made in an investigation of micellar coalescence exchange rates. The composition of the interfacial layer, expressed as the molar ratio of cosurfactant (hexanol) to surfactant (DTAC), was found to be constant over a significant portion of the reversed micellar (18) McNeil, R.; Thomas, J. K. J . Colloid Interface Sci. 1981, 83, 57.
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J. Phys. Chem. 1992, 96, 4658-4666
region of the phase diagram, implying also an invariant total surfactant headgroup area in this region. These results were supported by QELS and SAXS measurements, which demonstrated that the micellar radius changes linearly with watersurfactant ratio, w,,. The measured total surfactant headgroup area agreed well with values estimated from phase boundary titration results. Conductivity measurements indicated no onset of percolation over the composition range studied. Above wo values
of 5-7, the water in the reversed micellar cores had the properties of bulk water.
Acknowledgment. This work was supported by the National Science ~oundationthrough a Presidential Young Investigator's Award to T.A.H., with matching funds from Dow Chemical Co. Registry No. DTAC, 112-00-5;KC1, 7447-40-7; [COC~.,]~-, 1433708-7; hexanol, 11 1-27-3; heptane, 142-82-5.
Energy-Transfer and Exciton-State Relaxation Processes in Allophycocyanin Warren F. Beck7 and Kenneth Sauer* Department of Chemistry and Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: July 22, 1991; In Final Form: January 27, 1992)
We have employed picosecond spectroscopictechniques to characterize the photophysics of the phycocyanobilin chromophores in linker-free allophycocyanin isolated from the cyanobacterium Synechococcus PCC 6301 (AN1 12 mutant). In analogy with the known structure of the related phycobiliprotein C-phycocyanin, allophycocyanin is probably organized as a ringlike homotrimer; the monomeric units are composed of an a and a ,9 subunit, each of which binds a phycocyanobilinchromophore via a thioether linkage to a cysteine residue at amino acid position 84. We observe bidirectional excitation transfer in the a,9 monomer between the a84 and ,984 chromophores with a 140-ps time constant. Absorption anisotropy measurements show that the transition dipole moments of the a84 and ,984 chromophores are tilted 13 9' apart in a@ monomers. In trimers, however, the a84 and 884 chromophores in adjacent a@ monomers are brought close together, forming strong chromophore-chromophore interactions across the intermonomer interface. We interpret the observed photophysics using a model consisting of exciton levels formed by mixing of the monomeric singlet-state levels of the a84 and ,984 chromophores in the trimers; a system of three symmetry-equivalent but well separated chromophore dimers is formed, which produces a pair of triply degenerate singlet exciton states. We assign an ultrafast ( q 2 - p ~time constant) anisotropy and photobleaching transient observed only in trimers to an interexciton level transition; the transient occurs with a polarization change that is consistent with a transition between the orthogonal upper and lower exciton states. The upper exciton state also relaxes directly to the ground state through a decay process with a 45-ps time constant. We attribute the heterogeneous relaxation of the upper exciton state through these two paths to an inhomogeneous broadening due to site heterogeneity, which was previously observed in C-phycocyanin in hole-burning experiments at low temperature (Kohler et al. Chem. Phys. Lett. 1988, 143, 169). Excited-state absorption, originating from the lower exciton state, is assigned to a transition yielding a doubly-excited exciton state (van Amerongen; Struve, J . Phys. Chem. 1991, 95, 9020). Excitation transfer among the degenerate lower exciton states is detected in terms of a 70-ps anisotropy decay observed in the photobleaching and stimulated emission. The interexciton level transition rapidly concentrates excitation in the lower exciton state of allophycocyanin trimers; this kind of spectral relaxation process may be important in facilitating directional excitation transfer in reaction center/ light-harvesting protein assemblies.
*
Introduction In recent years, the structures of several photosynthetic energyand electron-transfer proteins have been determined by crystallographic methods. These include the photosynthetic reaction centers of two species of purple non-sulfur bacteria'" and four examples of light-harvesting proteins: the bacteriochlorophyll a protein from the green bacterium Prosthecochloris a e ~ t u a r i i , ~ - ~ the phycobiliproteins C-phy~ocyanin'*~~ and C-phycoerythrocyanin,14 and most recently the LHCII light-harvesting complex associated with photosystem II.I5J6 Given these structures, an unmecedented omortunitv now exists for studv of the elements of 'protein/chromophore irchitecture that co&rol energy and electron transfer in chromoproteins. One feature that the known protein structures referred to above have in "man is a clustering of the bound tetrapyrrole or porphyrin chromophores. Rather than containing chromophores bound nonspecifically and with random orientational order, the known structures covalently bind several (6-15) chromophores in close proximity and with apparently functionally important mutual orientations between donor-acceptor electronic transition dipole moments. Because the electronic interactions formed between the chromophores mediate the rate of energy and/or electron transfer, the clustered arrangement of chromophores found in the known protein Current address: Department of Chemistry, Vanderbilt University, P.O. Box 1822 Station B, Nashville, TN 37235.
structures probably represents an important structural/functional motif* (1) Deisenhofer, J.; Epp, 0.; Miki, K.; Huber, R.; Michel, H. Nature 1985, 318, 618-624. (2) Deisenhofer,J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385-398, (3) Deisenhofer, J.; Michel, H. Science 1989, 245, 1463-1473. (4) Allen, J. P.; Feher, G.; Yeates, T. 0.;Rees, D. C.; Deisenhofer, J.; Michel, H.; Huber, R. Proc. Narl. Acad. Sci. U.S.A. 1986,83, 8589-8593. ( 5 ) Allen, J. P.; Feher, G.; Yeates, T. 0.; Komiya, H.; Rees, D. C. Proc. Not/. Acad, sei, (I.s.A. 1987, 84, 5730-5734. (6) Tronrud. D. E.; Schmid. M. F.: Matthews. B. W. J . Mol. Biol. 1986. I88, 443-454. (7) Fenna, R. E.; Matthews, B. W. Nature 1975, 258, 573-577. (8) Fenna, R. E.; Ten Eyck, L. F.; Matthews, B. W. Biochem. Biophys. Res. Commun. 1977. 75.751-756. (9) Matthews, B.'W.i Fenna, R. E.; Bolognesi, M. C.; Schmid, M. F.; Olsen, J. M. J . Mol. Biol. 1979, 131, 259-285. (10) Schirmer, T.; Bode, W.; Huber, R.; Sidler, W.; Zuber, H. J . Mol. Biol. 1985, 184, 257-277. (11) Schirmer, T.; Huber, R.; Schneider, M.; Bode, W.; Miller, M.; Hackert, M. L. J . Mol. Biol. 1986, 188, 651-676. (12) Schirmer, T.; Bode, W.; Huber, R. J . Mol. Biol. 1987, 196,671495. (13) Duerring, M.; Schmidt, G. B.; Huber, R. J . Mol. Biol. 1991, 217, 577-592. (14) Duerring, M.; Huber, R.; Bode, W.; Ruembeli, R. Zuber, H. J . Mol. Biol. 1990, 211, 633-644. (15 ) Kiihlbrandt, W. In Current Research in Photosynthesis; Baltscheffsky, M., Ed.; Kluwer Academic Publishers: Dordrecht, 1990; Vol, 11, pp ..
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(16) Kiihlbrandt, W.; Wang, D. N. Nature 1991, 350, 130-134.
0022-3654/92/2096-4658%03.00/0 0 1992 American Chemical Society
