J. Phys. Chem. 1993,97, 7700-7705
7700
Reorientation of Acridine Orange in a Sodium Dodecyl Sulfate Monolayer at the Water/Hexadecane Interface D. A. Piasecki and M. J. Wirth' Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716 Received: February 3, 1993; In Final Form: May 17, 1993
A macroscopically oriented layer of sodium dodecyl sulfate (SDS)at the interface of water and n-hexadecane was investigated using acridine orange as a fluorescent probe. The range of angles through which the hindered out-of-plane reorientation occurs for the probe was found to correspond closely to the surface roughness predicted by the capillary wave model through the interfacial tension. The in-plane reorientation of the probe was found to be significantly slower than that in the absence of SDS,with little dependence on the SDS concentration. The in-plane and out-of-plane rotations of acridine orange appear to correspond to the two decay components measured for acridine orange in micellar solution, indicating that these macroscopically oriented surfactant monolayers can provide detailed insight into solute-micelle interactions. For SDS micelles, the behavior of the oriented interface suggests that the surfactants are aggregated with the probe in the micelle and that the SDS micellar surface is very rough. Introduction Due to their unique structures, micelles have enjoyed widespread use in a number of applications, ranging from industrial uses to analytical technique^.^-^^ In many of these applications, the solute-micelle interactions underlie the attractiveness of the technique. The differential interactions of solutes with micelles have resulted in a number of studies that provide information about micellization itself, as well as other micellar properties.l&ls Further fundamental studies are necessary to understand how solutes interact with micelles. The reorientation behavior of dyes attached to micelles provides information about micelle internal v i ~ c o s i t y ~ ~and J + ~the ~ geometry of dye atta~hment.~~,25.26 Several studies have been reported on the reorientation of cationic dyes attached to sodium dodecyl sulfate micelles. Phase fluorometry was used to characterize the reorientation of rhodamine 6G attached to sodium dodecyl sulfate micelles, and it was concluded that the reorientation was strongly hindered.25 In transient dichroism studies, several cationic dyes were studied, including acridine orange.26 The use of this dye is particularly informative because its high symmetry facilitates interpretation. A double-exponential anisotropy decay was observed, with the slower decay component attributed to acridine orange in the micelle and the faster decay component attributed to unbound acridine orange in water. The fast component was convoluted with the instrument response function, so it was not directly observable. Frequency domain spectroscopy was subsequently used to measure the reorientation of acridine orange attached to SDS mi~e1les.l~ The same doubleexponential decay was observed as in the transient dichroism experiments; however, there was sufficient time resolution to determine that both decay components were due to a single acridine orange species interacting with the micelle. This conclusion was confirmed by the lack of any concentration dependence in the data above the critical micelle concentration. The double-exponential decay was interpreted as due to the partially hindered, anisotropic reorientation of a single acridine orange species attached to the SDS micelle. The hindered anisotropic reorientation of acridine orange contains information about the geometry of dye attachment to the micelle. It was proposed that the long component arose primarily from the rotational diffusion about the axis of attachment and that the short component arose from wobbling
* To whom correspondence should be addressed.
of acridine orange about the attachment axis. One can isolate these two angular motions directly by using macroscopically oriented interfaces. The micellar interface that exists in solution between the water and the dodecylchains of the surfactant molecule can be mimicked by modifying the aqueous layer of a waterlhexadecane interface with sodium dodecyl sulfate, thus forming a partial monolayer. The density of head groups is expected to be comparable: chromatographic measurements of SDS at octadecylsiloxane/ water interfaces near the critical micelle concentration reveal a head group density of approximately 1.8 Nmol/m2, as estimated by desorbing SDS into methan01.~' This density is thus similar to that of the SDS micelle, which is estimated to be 1.3 pmollm2, based on an aggregation number of 66 surfactant molecules and a radius of 26 A, on the average. The purpose of this work is to investigate the anisotropic reorientations of acridine orange at the water/hexadecane interface in the presence of sodium dodecyl sulfate. Acridine orange, an amphiphilic molecule, is presumed to reside at the interface as shown in Figure 1, with the charged portion exposed to the aqueous phase and the uncharged portion residing strongly in excess at the interface. The in-planeand out-of-plane rotational motions of acridine orange at the interface can be measured separately for macroscopically oriented interfaces by appropriate choice of polarization conditions.2s In this work, frequencydomain fluorescence m e a s ~ r e m e n t sare ~ ~ used to obtain the subnanosecond time resolution necessary for studying molecular reorientation. "ry
Fluorescence anisotropy measurements provide information about molecular reorientation. Excitation of a sample with polarized light causes polarization of the emission intensity, which is related to the molecular reorientation. In an experiment involving surfaces or interfaces, the surface-bound solutes may have an orientation with respect to the laboratory coordinates. The anisotropy relations for probing in-plane and out-of-plane rotations of molecules at interfaces and surfaces have been described previously in detail.28 The coordinate system is shown in Figure 1. The x-y plane is the plane of the interface, and the z axis is the surface normal. Acridine orange reorients in the x-y plane through angles 4. To measure the reorientation of acridine orange through angles 4, the emission polarization is alternated between they and x
0022-3654/93/2097-1100~04.00/0 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7701
Acridine Orange in a SDS Monolayer
Aqueous Layer
Figure 1. Structure of acridine orange at neutral pH and its average orientation at a water/hexadecane interface.
axes, and the excitation polarization is fixed along they axis. The fluorescence light is collected along the surface normal. For inplane reorientation, r&), the fluorescence anisotropy, is defined as follows.
The subscripts denote the excitation and emission polarizations. Z,,b(t) is the time-dependent intensity of the emitted polarized light. Equation 1 is only strictly correct when the rotations inplane and out-of-plane are statisticallyindependent of one another. This assumption is satisfied when the out-of-plane orientational distribution is narrow. Acridine orange reorients out of the plane of the interface through angles 8. To study these out-of-plane rotations, the excitation polarization is varied between thez axis and they axis. The emission polarization is fixed at 45O. For out-of-plane reorientation, the fluorescence anisotropy is defined as follows.
The excitation and emission polarizations are again designated by the subscripts, and Z,,b(t) has its usual meaning. The subscript p is equal to the quantity (x+y)/2, indicating that no in-plane polarization discrimination exists for measuring ro(t). Ellipticityof thez-axis excitation intensity occurs when exciting via total internal reflection. The correction has been previously described.28 Also, the fluorescent probe must be in great excess compared to its solution concentration to avoid contribution to the signal from the bulk. This has been shown to be easily achievable with acridine orange.28v30J1 If the orientational distribution of adsorbates in the ground state is P(0) and the ground-state and equilibrium excited-state orientational distributionsare equal to one another, then the initial anisotropy, r(O), is the following:
r(0) = 1.5
J P ( e ) cos2(8 - e,) sin2(8 -),e
sin 0 dB
J P ( 0 ) sin2(0 - e,) sin 8 dB
-0.5
(3)
The anisotropy at time infinity is the following: r(m)
= 1.5
J P ( e ) cos2(8 - e,) sin 8 de
-0.5
(4)
J P ( 0 ) sin 0 dB The limits of integration are 0 to a ; 0, and 0, are the respective angles of the excitation and emission transition moments with respect to some reference axis in the molecule. Acridine orange
remains protonated at neutral pH in both the ground and excited states; thus, P(6) is the equilibrium orientational distribution for both. For acridine orange at water/monolayer interfaces, it has been suggested that out-of-plane reorientation is caused by interfacial roughness rather than intrinsic tilting of the molecule out of the interfacial plane.30 The capillary-wave modeP2 is an accepted description of interfacial roughness, where thermally-induced displacements of the interfacial boundary are opposed by the interfacial tension. The frequencies of the thermally-induced capillary waves are estimated to range from the molecular scale to the wavelength of light. The variance of the amplitude of surface displacements, ud2, arising from these capillary waves describes the interfacial roughness as a function of interfacial tension, y, as shown in eq 5.
k is the Boltzmann constant, 6 p is the density difference between the two liquids, and g is the gravitational constant. a / L is maximum frequency of the capillary wave, where L is the length of acridine orange (20 A).
Experimental Section Sodium dodecyl sulfate was obtained from Aldrich (98%) and was recrystallized once from ethanol before use. After this one recrystallization, solutionsof SDS in water exhibited no anomaly in the plot of air/water surface tension vs SDS concentration, and GC-MS confirmed the n-dodecanol impurity had been removed. A second recrystallization gave no difference in the data. Acridine orange (go+%), obtained from Eastman Kodak, was purified by column chromatography using a silica stationary phase and a methanol mobile phase. The water was purified by passage through Cole-Parmer Ion-X-Changer Research and Adsorber Cartridges, removing ion minerals and most organic compounds. At this stage, the water was equivalent to triplydistilled water. It was further purified by passage through a trifunctionally bonded C18 Sepak filter to further reduce any hydrophobic or surface-active impurities. The surface tension of the water agreed with literaturevalues. Then-hexadecane (99%) was obtained from Aldrich and was passed through a column of pure silica gel to remove any polar contaminants. The silica gel was purified by boiling in concentrated nitric acid, rinsing with pure water, and drying under nitrogen. Allsolvents and solutions were finally passed through 0.5-pm Millipore Universal solvent PTFE particle filters to assure that no dust particles were present in the sample cell. Measurements of interfacial tension were made using the CSCDuNouy Interfacial Tensiometer, CSC No. 70545, with a platinum-iridium ring. The ring was cleaned by flame before each measurement. Acridine orange and SDS were introduced to the interface by preparing an aqueous solution of the desired composition of SDS and an amount of acridine orange that would give a fluorescence signal of the same intensity as that for acridine orange at the interface of water and the octadecylsiloxanemonolayer on silica. This would provide for approximately the same amounts of SDS for each interface, assuming that the absorbance and quantum yields change negligibly. The octadecylsiloxaneinterface required a 12 nM acridine orange solution to provide an acridine orange coverage of 0.018 pmol/m2, as determined by quantitative desorptionmeasurements.28 The same amount of acridine orange gave the same signal size for the pure water/hexadecane interface, as one might expect. Larger amounts of acridine orange were required to give the same signal size when SDS was present: for example, the 8 mM SDS solution required an acridine orange concentration of 120 nM. Based on the adsorption isotherm of SDSat the water/alkane monolayer interface27 and the adsorption
Piasecki and Wirth
7702 The Journal of Physical Chemistry, Vol. 97,No. 29, 1993
pMTlgl FILTER
n -2
-
-4
c3
-6
W
3
n
-8
tI v,
POLAROID
-10 -12
LASER BEAM
W
2
L:::i
0.54mM’
-14
I
I 1
I
\
,
,
,
,
,
,
,
,
,
,
,
,
: PRISM PRISM -20
80
120
180
isotherm of acridine orange at this same interface, the SDS is expected to be in at least a 10-fold excess relative to acridine orange at the interface for each case. For the spectroscopicexperiment, the sample cell was identical to that used in previous experiments.” The optical arrangement is shown in Figure 2. The sample was excited with 1 mW of the 488-nm line of an argon ion laser. A Glan-Thompson prism was used to provide linearly polarized light, and a Pockels cell was used to electronicallycontrol the excitation polarization. A highextinction polaroid was used to select the polarization of the emission. A band-pass filter was used to isolate the fluorescence, which was then detected with a Hamamatsu 1635photomultiplier tube. The usual checks were made to ensure that negligible photodegradation occurred. The frequency-domain spectroscopicequipment used was also identical to that used previously, where the first, second, and fourth harmonics of the 82-MHz mode beat frequency of a modelocked Ar+ ion laser were used. The instrument was thoroughly tested to ensure linearity and accuracy under the conditions of the e ~ p e r i m e n t Fluorescence .~~ lifetimemeasurementswere made using the TENNELEC model TC 863 time-to-amplitude converter/singlechannel analyzer (TAC/SCA). Data were acquired using the Nucleus Series I1 personal computer analyzer (PCA11). For lifetime measurements, the sample was excited in-plane and the emission polarization was set at 45O. For the narrow out-of-plane orientational distributions, these polarization conditions provide as accurate of a measurement of the lifetime as do magic angle c0nditions.3~ The contribution from evanescent wave excitation of the blank was negligiblein all cases, and there was no detectable dependence of the data on acridine orange concentration at any of the acridine orange concentrations used. The presence of acridine orange was found not to affect the measured interfacial tension.
Out-of-PlaneReorientation. The raw frequency-domain data for the reorientation of acridine orange out of the interfacial plane, Le., through angles 8, are plotted in Figure 3. The points represent the experimental data while the solid lines represent the theoretical curves calculated from the anisotropy decay parameters, as discussed later. In the case of the pure water/ hexadecane system, the reorientation of acridine orange appears to be strongly hindered because the phase shifts approach zero and the amplitude ratios are far from unity. These data agree with those reported in a study of several water/alkane interfaces,2*930931where it was shown that these data are explained by the orientation shown in Figure 1. Figure 3 also shows that, as the concentration of SDS increases, acridine orange becomes increasingly free to rotate, as indicated by the larger phase shifts and the increase in amplitude ratios toward unity. However, the reorientation is still quite hindered in all cases because the
240
280
320
0.8
0.6
W
n 3
!=
d
0.4
3 0.2
o m d
TABLE I: Anisotropy Decay Parameters for the Out-of-Plane Reorientdon of Acridine Orpage at the Interface of Water and BHexadecane in the Preseace of Varying SDS Concentratio~P [SDSI (mM) 0 0.54 2.0
4.0 6.0 8.0 a
Results and Discussion
200
FREQUENCY (MHz)
Figure 2. Optical schematic of the liquid/liquid experiment.
70
f
(ns)
* 0.5 0.7 * 0.5 0.2
0.8 0.4 0.7 0.2 0.8 i 0.3 0.9i 0.5
0.03 i 0.02 0.040i 0.008 0.038 i 0.004 0.07 0.01
** 0.02 0.14 i 0.03 0.12
-0.42 i 0.01 -0,402 i 0.004 -0.393 i 0.006 -0.380 i 0.006 -0.366 i 0.008 -0.365 0.008
XZ -
1.o 1.o 1.o 1.a 1.3 1.8
Error bars represent the ranges over which xz increases by 25%.
amplitude ratios remain far from unity. Quantitative interpretation of the behavior of acridine orange in the presence of SDS requires recovery of the decay parameters from the frequencydomain data. The data were found to fit well to a double-exponentialdecay having an infinite component. The anisotropy decay parameters of eq 6,recovered from the out-of-plane reorientation data of Figure 3, are listed in Table I. The curves obtained from the fitting procedure are the solid curves in Figure 3. Theoretically, the most negative possible value of r6(0)is-Q.45, based on the transition moment of acridine orange being 12O from its long axis. This value of re(0) would be observed for perfect alignment of acridine orange in the plane of the interface, as illustrated in Figure 1. By contrast, a
Acridine Orange in a SDS Monolayer
The Journal of Physical Chemistry, Vol. 97, No. 29, 1993 7703
TABLE Ik Measured Surface Tension, y, and the Corresponding Orientational Distribution, u& Calculated from the Data in Table I and the Distribution of Vertical Displacements, Ud' lSDSl (mM) Y (dyn/cm) ue (deg) Ud (A) 0
0.54 2.0 4.0 6.0 8 .O
56.2 30.4 23.9 15.8 10.9 9.4
7*2 11k1 13* 1 15* 1 18+ 1 18* 1
12
1.2 h 0.3 2.0 0.2 2.2 0.2 2.6 0.2 3.0 f 0.2 3.1 0.2
**
0 The interfacial tension was measured with a typical 95%confidence interval of 0.2 dyn/cm.
completely isotropic orientational distribution of acridine orange molecules would give rg(0) = -0.20. Since the values of rg(0) approach the case of extreme alignment in the plane, but are somewhat less than this extreme value, there must be a narrow distribution of orientations about the interfacial plane in each case. The distributionwidens with increasingSDSconcentrations, as indicated by the less negative values of re(0). Further, if the values off were zero, the orientational distributions would be static. The small, nonzero values off, which increase as rg(0) decrease, show that acridine orange reorients through its orientational distribution, in each case. This is expected for a liquid system. The orientational distribution can be calculated quantitatively to compare the behaviors of different SDS concentrations. The out-of-plane orientational distribution, P(8), is characterized from the experimental values of rg(0) andf, where re(-) = rg(O)(l -A, using eqs 3 and 4. For conservatism's sake, P(8) is assumed to be Gaussian. It is also assumed that the mean of P(8) is exactly in the plane of the interface, based on the fact that rg(0) approaches -0.45 and that the symmetric molecule would be expected to have a symmetric distribution with respect to 8. To calculate P(8), the directions of the transition moments must be known: OLr has been previou~ly~~ measured to be 12O, but Om is not known. The value of et,, has only a small effect on the computed width of P(8). We used both re(0) and f to arrive at consistentvalues for the standard deviation of P(8),which reveals a e,,,, of 1 8 O . This amounts to a 6O angle between the excitation and emission transition moments, which is not surprising. The computed widths change only about 10%by this change in et,,. The experimentally determined widths of P(8), expressed as standard deviations, ug, are summarized in Table 11. These show quantitativelyhow much the width of the orientationaldistribution increases as the amount of SDS is increased. There are two possible explanationsfor an increasein ug with SDSconcentration. First, the surfactant layer might not orient acridine orange as strongly as the bare water/hexadecane interface. Second, the orientational distribution might be attributable to interfacial roughness, which increases with decreasing surface tension. The first possibility cannot be directly tested because there is insufficient prior knowledge of what to expect for the range of angles over which the molecule can reorient. On the other hand, the second possibility can be investigated by using the capillary wave model for interfacial roughness. The standard deviation, Ud, of the random vertical displacements of the rough surface can be estimated from the standard deviation of the angular distributions of acridine orange through Ud = 10 sin(ug), where 10 A is half the length of the acridine orange molecule. This formula simply converts angle into vertical distance from the mean surface plane, as it is valid for small ug. A molecular-scale probe only senses the very high-frequency capillary waves, as low-frequency waves do not significantly tilt the angle of the molecule; therefore, the values of ud sensed by acridine orange would be smaller than those measured by light scattering. According to eq 5, a plot of Udz vs [ k T / ( 4 r y ) ]ln[y?r2/(6pgL2)1, where y is given in Table 11, would be linear with a zero intercept
0
2
4
6
8
10
12
Figure 4. Dependence of the experimentally determined variance of vertical displacements, u,j2,on the predicted value of Ud* by the capillary wave equation, Udz = kT/(4ry) ln[rzr/(6~gt2)1. if interfacial roughness were the cause of the out-of-plane orientational distributions. A plot of Ud2 vs [kT/(4ry)] ln[rr2/(6pgL2)],shown in Figure 4, establishes that there is clearly a linear relation over this very wide range of interfacial tensions (9.4-56.2 dyn/cm) and that the intercept is zero within experimental error. The first point on the line is from the interface with no SDS. The slope of the plot is less than unity: acridine orange apparently senses an interfacial roughness, Ud, that is about 3 times less than that predicted by the capillary wave model. This is consistent with the notion that acridine orange senses a smaller ud due to its small length. The results are quite insensitive to choice of L,which is the only adjustable parameter. A slope less than unity is not surprising because the orientational distribution of the molecularscale probe has preferential sensitivity for the high-frequency part of the capillary wave spectrum. Much of the contribution to the predicted value of Ud is from the lower-frequency capillary waves. The out-of-plane orientational distributions of acridine orange for these varying amounts of SDS are thus attributable to interfacialroughnessinduced by lowering the interfacial tension. In-PlaneReorientation. The orientationaldistributionsthrough out-of-plane angles, 8, are sufficiently narrow to attempt a study of the reorientation with respect to in-plane angles, 4. To test the assumption of statistical independence of rotational diffusion in 4 and 8 requires knowing how much the rotational diffusion through 4 depends on angle 8. This cannot be known a priori: the distributions are assumed to be sufficiently small that comparisons can be made. The raw frequency-domain data for the in-plane fluorescence anisotropy, r4(t),are shown in Figure 5. The phase shifts and the amplitude ratios are plotted as a function of mode-beat frequency for acridine orange residing at the pure water/ hexadecane interface, as well as in the presence of SDS modifier. Again, the points represent the experimental data while the solid lines represent the theoretical curvecalculated from the anisotropy decay parameters. In all cases, both the phase shifts and the amplitude ratios increase as a function of frequency, as expected. In contrast to the out-of-plane reorientation, these data show no obvious sign of hindered rotation, in that the phase shifts are large and the amplitudes ratios are close to unity. The raw data reveal that SDS,in any of the concentrations used, causes a large change in the in-plane rotational diffusion compared to that of the pure water/hexadecane interface. This change cannot be attributed to differences in P(8) because the behavior is similar for all of the SDS concentrations, and the only jump in behavior occurs from the pure interface to the lowest SDS concentration, where ug changes only by 4'.
7704 The Journal of Physical Chemistry, Vol. 97, No. 29, 1993
18
/-. L? 16
w
a
w
14 12
t-
4
10
I ' " 8
W
I
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200
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FREQUENCY (M Hz)
(7) The anisotropydecay parameters recovered from the fit are listed in Table 111. In the caseof acridineorange at a water/hexadecane interface, the results are identical to those previously determined, wherer4(0) = 0.34 f 0.08 and T # = 0.3 f 0.1 ns. In no case was a better fit obtained when either a double-exponentialor hindered rotor equation was used. While the raw data show that the behavior has little dependence on SDS concentration, the fit is poor at low SDS concentration, preventing quantitative interpretation for the cases of 0.54 mM and 2.0mM SDS. A possible cause of a poor fit is heterogeneity. Forthe high concentrations, the fit is good in each case, revealing that acridine orange is able to completely reorient in the plane of the interface. The time constant for reorientation is significantly increased with the addition of SDS to the pure interface: T# is 0.3 ns for the pure interface, and T~ increases to nearly 1 ns in the presence of SDS. It is surprising that an approximately 10-fold change
Piasecki and Wirth in the amount of SDS at the interface causes little change in the raw data for in-plane reorientation. A possible explanation is that SDS forms aggregates with the oppositely charged acridine orange even at low concentrations of SDS. This would also be consistent with heterogeneity at low concentrations. As further evidence of charge playing a role, acridine orange was found not to adsorb in any detectable amount to a waterln-hexadecane interface modified by cetyltrimethylammonium chloride (which has the same charge as acridine orange) even at minute amounts of the surfactant. Comparison with Micellar Behavior. As detailed earlier, acridine orange was found to reorient as a double-exponential decay in micellar solution.14The slow decay constant, attributed to rotation of acridine orange about the axis of attachment, is thus analogous to the rotation of the probe in the plane of the SDS modified liquid/liquid interface. However, T # 1 ns for the interfacial surfactant layer, while = 1.7ns for the micellar system. A plausible explanationfor the differencein reorientation times is a difference in viscosity. Assuming that the SDS head group densities are similar, the dynamics of the SDS head groups can differ due to the viscosities of the tail group regions, since these two groups are attached. The viscosity in the interior of the micelle, as measured p r e v i o ~ s l yis, ~on ~ the order of 8 CPat 25 "C. The viscosity of the "interior" of the interface, Le., the hexadecane side of the interface, cannot be predicted straightforwardly, but it is probably significantly less than 8 CPbecause the bulk viscosity of n-hexadecane is 3.3 CPat 25 OC. This idea that the faster reorientation is simply a consequence of viscosity is further supported by fluorescencelifetimes. The fluorescence lifetime of acridine orange increases from 1.8 ns in water to 3.0 ns in the micelle, while the fluorescence lifetime remains 1.8 ns for acridine orange in the SDS interfacial layer. In the absence of quenching, lifetime increases with the rigidity of the environment; therefore, the shorter lifetime is consistent with the interfacial layer having a lower viscosity than the micelle. The interpretation of the fast decay component in the original micellestudies14isthat it is analogous to out-of-plane reorientation in the liquid/liquid studies. For the micellar system, the range of angles over which acridine orange is able to reorient was found to be u = 23". For the oriented interfacial SDS layer, the range of angles was found to be u = 18'. The similarity suggests that the hindered rotation of acridine orange in the micelle is based on the same phenomenon as that for the interfacial layer, which is roughness. This implies that, for the micelle, the hindered rotation of acridine orange occurs in concert with motions of SDS molecules that create a rough micellar surface. Equation 5 showed that interfacial roughness is primarily controlled by the ratio ( k T / y ) the , ratio of the interfacial tension, which keeps the interface smooth, and thermal energy, which randomizes the positions of the molecules. One can think about the SDS micelle as having an interfacial tension, despite the macroscopic nature of this term. Analogous to macroscopic interfaces, interfacial tension would be the energy required to increase the area of the micellar surface by a given amount. From the measured roughness of the micellar interface, Le., ug = 2 3 O , one can calculate Ud = 3.9 A and extrapolate from Figure 4 an interfacial tension of 6 dyn/cm for the SDS micelle. This is an interesting exercise because it provides a unique way of thinking about micelles: their surfaces are rough because their interfacial tensions are small. Another parameter that is obtained from the experiment is the decay time for out-of-plane reorientation, which is related to the average distance between roughness features and the speed at which these roughness features pass acridine orange. Neither of these parameters is known for either the micelle or the interfacial SDS layer, but comparisons can be made. The decay constant is much slower in the interfacial case. The experiment indicates that, while the amplitudes of vertical displacements are com-
Acridine Orange in a SDS Monolayer parable for the two systems, the roughness features at the surface of the micelle pass through acridine orange at a greater rate than do those of the liquid/liquid interface. This is perhaps a consequence of the small size of the micelle, which can have only high-frequency waves on its surface.
Conclusions These experiments reveal parallels between the behavior of acridine orange attached to SDS micelles and to SDS monolayers at water/hexadecane interfaces. For in-plane reorientation, the slow reorientation apparently is due to aggregation of SDS molecules with the oppositely charged acridine orange molecule. Differences in reorientation behavior for acridine orange in the interfacial layer and the micelle appear to owe to viscosity differences. The viscous interior of the SDS micelle might be better mimicked in future experiments using n-alkylsiloxane monolayers instead of hexadecane as the organic phase. Choice of the appropriate chain length could provide the right match. For out-of-plane reorientation, the similarity in surface roughness apparently owes to a similarity in interfacial tensions for the surfactant-modified interface and the SDS micellar interface. Acknowledgment. This work was supported by the National Science Foundation under Grant CHE-911354. References and Notes (1) Nomura, M.; Ikoma, J.; Fujita, K. Polym. Mater. Sci. Eng. 1991, 64, 310.
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