J. Phys. Chem. 1992, 96, 5943-5948
5943
Effect of Wetting on the Reorientation of Acridine Orange at the Interface of Water and a Hydrophobic Surface J. D.Burbage and M.J. Wirth* Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716 (Received: December 5, 1991; In Final Form: March 10, 1992)
Acridine orange strongly adsorbs to the interface between water and the hydrophobic monolayer of n-octadecylsilanecovalently bonded to silica. The orientational dynamics of acridine orange were studied by frequency-domain fluorescence anisotropy for both in-plane and out-of-plane rotational motions. It was found that, at a coverage of approximately 60 A2 per alkyl chain, adsorbed acridine orange is virtually unable to rotate out of the surface plane but is able to rotate in an unhindered fashion in the plane of the surface. An out-of-plane orientational distribution having a standard deviation of 1S0 was attributed to roughness of the interface, which is significantly greater than the roughness of the underlying silica substrate. Perturbations on the orientational distribution by short-chain alcohols, which wet the interface, were investigated. It was found that the presence of adsorbed alcohol increases the out-of-plane interfacial roughness but has little effect on the in-plane orientational behavior of acridine orange.
Introduction The understanding of the behavior of surface-activemolecules at interfaces is important not only to physical chemistry but also to analytical chemistry (chromatography) and biochemistry and biology (membranes processes). The emergence of the field of molecular engineering has furthered interest in understanding molecular phenomena in chemical entities having molecular dimensions. The phenomena of wetting and adhaion are interfacial in nature, and advances in the understanding of these phenomena would assist in the design of composites and biocompatible materials. Surfactants are apt probes of interfaces by virtue of their tendency to concentrate at interfaces. Perhaps the most common properties of interfaces that are measured are the spreading and contact angle when two media are brought together. The contact angle allows calculation of the interfacial free energy, which is related to the amount of surface-active material that collects at the interface. From the free energy, one can infer more detailed molecular properties of the surfactant, such as orientation;' however, there is insufficient information for firm conclusions. New developments in experimentation have allowed more detailed properties of surfaces to be probed. Ellipsometry enables study of prewetting during the spreading Surface second harmonic generation and sum-frequency generation have been key spectroscopic techniques for probing surface-active molecule^^-^ because these measurements are intrinsically insensitive to bulk-phase contributions. These techniques provide information about molecular orientation, which is information not directly obtainable from free energy measurements. Recently, it was shown that surface second harmonic generation can also be used to sense molecular reorientation? Fluorescence depolarization experiments have the capability of probing molecular reorientation at surfaces, and in particular, these measurements provide separate information about in-plane reorientation and out-of-plane reorientation.I0 While the fluorescence technique does not distinguish surface from bulk molecules, many surfactants are sufficiently in excess at the interface that the bulk contribution to the signal is negligible. The attractive aspect of separately probing the two angular motions is that surfaces are inherently anisotropic: in-plane rotation can potentially provide information about the shear viscosity of the interface, while out-of-plane rotation provides information about how strongly oriented the surfactant is at the interface. The purpose of this work is to study the orientation and reorientation of a surfactant, acridine orange, at the interface of a hydrocarbon monolayer surface and an aqueous solution and to explore the effect of interfacial wetting on its reorientation To whom correspondence should be addressed.
behavior. The hydrocarbon monolayer is n-octadecyldimethylchlorosilane, which is covalently bonded to a flat silica gel substrate. This surface is heretofore referred to as the C I 8surface, as it is commonly called in chromatographic applications. The aqueous-phase composition has a varying amount of alcohol to control the amount of wetting at this interface.
"ry
A. Adsorption. The Gibbs surface excess, r, for a surfactant at an interface is proportional to the change in interfacial tension, y, with respect to bulk surfactant concentration, c:
r = -1/RT
dy/dc
(1)
A surface excess is exhibited by many types of amphiphilic molecules." The insight provided by the Gibbs equation is that the accumulation of surfactant at the surface is associated with a decrease in the interfacial tension, where the interfacial tension is equivalent to the free energy per area. From a spectroscopic point of view, the Gibbs equation is inconvenient because I' is not an absolute concentration; it the excess concentration. It is also difficult to predict the surface excess through dy/dc when there is more than one surfactant in the system, which is often the case in application. An alternative means of describing the accumulation of material at an interface is simply by the partition coefficient, K
K = as/ab (2) where a, and abare the activities of the surfactant at the surface and in the bulk. For sufficiently low concentrations, K can be expressed as the ratio of the surface concentration in mol/cm2 to the bulk concentration in mol/cm3. While this relation alone provides little insight in to the process of adsorption at the interface, K is amenable to direct spectroscopic measurement. To obtain further insight, one could relate asand abto the respective partition functions, Q, and Qb:l
K = QS/Qb
exP(-uo/kg
(3)
The density of states, as it affects Q,and the difference in zeropoint energy, Bo, determine the partition coefficient. In general, there is a lack of detailed information about the structure of the interface and the potential energy functions for a quantitative description. Experimental information about the partition coefficients and the orientational distributions of the surfactants will be valuable for advancing the understanding of the microscopic details of interfacial adsorption. B. Spectroscopic Measurement of Reorientation. The spectroscopic means of probing in-plane and out-of-plane reorientation has been described in detail previously,I0 and a brief outline of the technique is provided here for convenience. Figure 1 illustrates
0022-36S4/92/2096-5943%03.00/00 1992 American Chemical Society
5944
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992
%:
Z
Figure 1. Spherical polar and Cartesian coordinate systems for the experiment. The excitation beam is totally internally reflected at the interface. Its polarization is controlled to be along either the z axis or they axis. The fluorescence emission is collected by a detector positioned along the z axis. A Polaroid in front of the detector selects the emission
polarization. the coordinate system, where x and y are the in-plane axes and 4 represents in-plane angles; z is the surface normal, and 6 represents azimuthal angles. The photomultiplier is positioned along the z axis. Rotation through in-plane angles, which involves angles 4, are isolated by exciting with light polarized along t h e y axis and alternately measuring the fluorescence emission polarized along they and x axes. For each intensity, I,the first subscript represents the excitation axis and the second the emission axis: (4) Similarly, rotation through out-plane angles, which involves angles 6, are isolated by alternately rotating the excitation polarization between y and z , with no polarization discrimination for x and Y: zip
rB=
- IYP
I, + 2’yp
(5)
+
The term p is (x y)/2, which expresses the fact that no polarization discrimination is used in detecting the fluorescence emission. The normalizations were discussed previously.I0 Frequency-domain fluorescence measurements are used because they simultaneously allow the high sensitivity of fluorometry and picosecond time resolution by virtue of the ability to deconvolute the photomultiplier response without introducing drift. Upon excitation by a mode-locked laser, the time-domain intensity, I ( t ) , is the summation of the contributions from each mode beat frequency, v: I ( t ) = Ci(v)cos (27rvt + @(v)) (6) The phase shift @(v) and the amplitude i(v) are directly related to the functional form of I ( t ) . In frequency-domain anisotropy measurements, the difference in phase shifts for parallel and perpendicular emission is measured, as is the ratio of amplitudes for parallel and perpendicular emission. A tutorial review of this topic has been presented.I2 Through Fourier transform relations in nonlinear regression, the functionality of I(?) can be rec o ~ e r e d . ’ ~The ? ’ ~ relations between rs(t) and r,(t) to the frequency-domain parameters was described previously.I0 Experimental Section One difference from the system studied in previous work is that the surface has been prepared to have a higher coverage of alkyl group. The coverage was determined to be 60 f 10 A2 by infrared spectroscopy. This coverage corresponds approximately to the density associated with the liquid expanded phase of a Langmuir-Blodgett filmIs and is also close to the optimal density for liquid chromatography.I6 All reagents were highly purified. The water was distilled, deionized, and passed through activated charcoal and then through a Sep-pak n-octadecyltrichlorosilane column to remove any impurities that would have affinity for a water/hydrccarbon interface. The alcohols were distilled and passed through the same type of
Burbage and Wirth Seppak column. The acridine orange was obtained from Aldrich in the form of the hydrochloride hydrate and was purified by column chromatography using silica as a packing material. The spectroscopic method has been described previously.1° A power of 100 pW of 476-nm light from a mode-locked argon ion laser was used to excite acridine orange. A 72’ trapezoidal prism coupled the light into the silica substrate for total internal reflection, as depicted in Figure 1. The evanescent wave penetrates into the solution by a distance on the order of the wavelength of light. The resulting contribution to the fluorescence signal from acridine orange in the bulk was determined to be more than 4 orders of magnitude lower than the contribution from adsorbed acridine orange. The temperature of the flowing solution was contolled to be 23 f 1 “C. The housing for the silica plate and acridine orange solution is the same as previously described: all parts in contact with the solution are made of Teflon. Atomic force microscopy on the surface was performed by Imaging Services (Santa Barbara, CA) using a Nanoscope 11, and freshly cleaved mica was used for initial tip characterization. Resuits and Discussion A. Background Work. i. Surface Roughness. To ensure that it is realistic to separate in-plane and out-of-plane rotations quantitatively, the surface must be molecularly flat. The silica substrates are optically flat by virtue of polishing. For smaller scales, atomic force microscopy was used to examine the c18derivatized silica substrate. Since the coverage is low in density, and the force of the tip is large compared to van der Waals interactions, these profiles are expected to be characteristic of the silica substrate. Micrographs for the same type of substrate derivatized with (CH,),SiCl gave comparable results. Several different substrates with each type of derivatization were examined. Representative atomic force micrographs for a c18 surface are shown in Figure 2 for two scan sizes. In Figure 2a, the scan distance is almost as large as the wavelength of light, with each point separated in the scan be 1/200 of the full range. The surface exhibits remarkable flatness on the suboptical scale. In Figure 2b, it is shown that the surface remains flat all the way down to the molecular scale. Quantitatively, the standard deviation is 1.8 A for the 4000 X 4000 Az scan. Taking this worst possible case, if there were no noise in the measurement such that all of the standard deviation could be attributed to substrate roughness, and if the 1.8-A fluctuations occurred over the 16-A length of an acridine orange molecule, then this substrate roughness would contribute to the orientational distribution an amount of tan-’ (1.8/16), or 6.4O, out of the plane. Since the standard deviation decreases to 0.7 8, for the smaller scan range of 200 X 200 A2, the substrate roughness can contribute no more than tan-’ (0.7/16), or 2 S 0 , to the orientational distribution. In the subsequent spectroscopic studies of reorientation, any orientational distributions wider than a few degrees cannot reasonably be attributed to substrate roughness. ii. Nature of Acridine Orange Adsorption. Before interpretations about interfacial properties can be inferred, it is important to ensure that acridine orange resides a t the interface between the hydrophobic monolayer and the aqueous phase, as depicted in Figure 3. There are three other possible ways in which acridine orange could be adsorbed. (1) It is possible that acridine orange simply partitions into the hydrocarbon layer, rather than remaining at the interface. This possibility is rejected by the observation that acridine orange has an undetectably small solubility in alkanes (sub-nanomolar) while it has a very high solubility in water ( N 1 M). (2) Some of the residual silanols are sufficiently acidic to be in the form Wi-0 at neutral pH,”J8 which raises the possibility of the positively charged acridine orange being adsorbed at these silanols. This possibility was eliminated by the observation that the adsorption coefficient was negligibly affected upon “endcapping”, whereby the residual silanols are derivatized with chlorotrimethylsilane. Also, acridine orange is strongly in excess
01*ientationalDynamics of Acridine Orange
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5945
c
7
.y
. -
Figure 2. Atomic force micrographs for the CISsubstrate: (a, top) the 4000 X 4000 A* scan; (b, bottom) the 200
X 200
A* scan.
5946 The Journal of Physical Chemistry, Vol. 96, No. 1 4 , 1992
Burbage and Wirth 0
water
-4 h
m
U
QJ
y
-8-1
4
-12
V
I
D
,
l
&
o
i
0 --pure + 1 E X MeOH waiter
1o
-
2
80
t , 120
,
1
(nAlnM)b 14 13 1.7
solvent 3% PrOH 25% MeOH 5% PrOH
~~
----A
1 -
I
160
200
280
240
320
Frequency (MHz)
TABLE I: Effect of Solvent on the Accumulated Amount of Acridine Orange at the Interface'
accumulation
~
I
Figure 3. Orientation of acridine orange at interface.
solvent pure water 6% MeOH 18% MeOH
-5% PrOH - 2 5 X MeOH
L
-16
silica
L
~
accumulation (nA lnMlb 0.53 0.12 0.44
'The solvents are listed in order of decreasing interfacial tension, as per ref 21. bIndicates nanoamperes of current per nanomolar of acridine orange in the solvent.
0
)
D 3
.+ -
at the water/hexadecane interface, where there are negligible surface charges in the absence of acridine orange. The hexadecane had been passed through alumina before the experiment to remove polar impurities. Finally, the same fluorescence signal is observed when the pH of the solution is maintained a t 3, which fully neutralizes the surface.'* These observations indicate that the residual silanols on the surface are not accessible to the solute; presumably, this is because the surface has been derivatized to its steric limit by octadecylsiloxane groups. (3) It is possible that the presence of negatively charged surfactants at the interface would cause strong adsorption, as it is known that acridine orange strongly attaches to sodium dodecyl sulfate micelle^.'^ This possibility is eliminated by the use of ultrapure solvents. There is still a danger that a hydrolysis product such as CH3(CH2)17Si(CH3)20H, which could lose a proton, would be adsorbed to the interface; however, it is doubtful that this species is significantly ionized a t neutral pH. There is also very little of the hydrolysis product compared to the capacity of the surface for acridine orange. The signal intensities are quite reproducible, which further sheds doubt on the existence of a species of uncontrolled concentration giving rise to the adsorption. An important piece of evidence supporting the idea that acridine orange resides at the interface is that the amount adsorbed is decreased significantly upon wetting of the interface by alcohol. Methanol achieves more than half of saturated coverage at concentrations of 25% (v/v), while 1-propanol achieves saturated coverage at a concentration of 5% ( V / V ) .These ~ results have been confirmed by contact angle measurements for these same surfaces,21which allow calculation of the interfacial tension through the Young equation. Table I summarizes the amount of acridine orange accumulated at the interface as a function alcohol concentration. For low acridine orange concentrations at the interface, its partition coefficient is proportional to the fluorescence photocurrent normalized for the amount of acridine orange in solution. The data show that there is a large drop in amount of acridine orange adsorbed as the interface is wetted. For 5% 1-propanol, which provides saturated coverage of the interface, there is a factor of 30 decrease in the amount of acridine orange adsorbed. Surprisingly, the addition of 25% methanol decreased the amount of adsorbed acridine orange by a larger factor than did 5% propanol. The likely reason is that 25% methanol in water is a sufficiently high concentration that the hydrophobic effect is reduced in the bulk solvent. The hydrophobic effect is part of the driving force for adsorption. As an illustration of how significantly the hydrophobic effect is reduced, the solubility of pyrene
187. MeOH
'
I-
f
:
l ,
\\
pure water
.4
'' yroH
25X MeOH
\,
:i.? t
. I
0
L , , T
120
1
I
1
1
-
7
200 240 280 320 Frequency (MHz) Figure 4. Raw frequency-domain data for re(r) experiment. The solid curves are the theoretical curves for the analyzed decay parameters, which are listed in Table 11. 80
160
was measured spectroscopically to be 30 times higher in 25% methanol than in pure water. The interpretation that acridine orange resides at the interface is consistent with all observations, and it is concluded that acridine orange can be used to probe the properties of the water/CI8 interface. B. Reorientation of Acridine Orange at the Aqueous/CI8 Interface. Frequency-domain fluorescence anisotropy measurements were made for acridine orange adsorbed at the water/Cl8 surface. Very dilute submonolayers were used to ensure that energy transfer was not a problem: a 3 nM solution of acridine orange was used, which was previously found to correspond to approximately 3% of the surface concentration at which energy transfer just begins to occur.Io The liquid composition was varied among three different concentrations of methanol and two different concentrations of 1-propanol. The raw frequency-domain data are shown in Figure 4 for out-of-plane reorientation. The phase shifts are quite small and change little with frequency. This contrasts with the behavior we observe for a neutral probe, 1,4-bis(2-methylstyryI)benzene, where the phase shifts increase with frequency and the amplitude ratios are closer to unity.21 Since the angle of the transition moment is 12' from the long axis of acridine orange, as determined in the stretched film work of Matsuoka and Yamaoka,2z the low value of the amplitude ratio is indicative of strong in-plane alignment. The qualitative interpretation is that the rotation is very hindered: in the limit of perfect alignment in the plane of the surface, the phase shift would be zero and the amplitude ratio zero, a t all frequencies. The anisotropy data are quite different for in-plane reorientation. The raw frequency-domain data for in-plane reorientation are shown in Figure 5. The phase shifts and amplitude ratios are plotted on the same scale as the case for out-of-plane rotation in Figure 4 to allow convenient comparison between them. In contrast to the out-of-plane rotation, there is a large and fre-
The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 5947
Orientational Dynamics of Acridine Orange
TABLE II: Anisotrwy Decay Parametersa ~
mobile phase
18% U d H
1
o
+
80
,
120
#
l
~
200
160
l
a
240
t
280
~
~
320
Frequency (MHz)
2 * 0 7 - -
14
1.8
~
X2
r0
re(t)
pure water 6% MeOH
i
f
irl(ns)
0.05 0.06
'Tf >Tf
18% MeOH 3% PrOH 25% MeOH 5% PrOH
>TI
pure water 6% MeOH 18% MeOH 3% PrOH ' ' 25% MeOH 5% PrOH
410
0.08 0.09 0.12 0.12
>TI
>Tf >Tf
400 560 530 ~
540
-
590
r&)
1.o
1.o 1.o
1.o 0.99 0.89
1.5
-0.35 -0.34 -0.31 -0.31 -0.29 -0.30
1.6 1.1 1.5 1.1 1.3
0.36 0.36 0.32 0.34 0.30 0.31
6.8
10.8 3.2 3.1 6.0 1.2
"The value of T , fit ~ to in each case. The fluorescence lifetime of acridine orange at the interface, determined by independent measureis 1.9 ns.1 ment, upon Be As a self-consistency check the raw M,values also depend upon Bo.
X'"P(6) cos2 (e) sin2 ( e ( t ) ) sin 0 dB H
A
I
r(t) = 1.5
- 0.5
(9)
x*'2P((e) sin2 (O(t)) sin 0 dB The analysis of the data showed that T, > ~ i thus, , the expression for M,includes the correlation between the excitation and the emission angles. This simplification makes little difference to the results. 1 .o
80
120
160
200
240
280
X'/2P(B) cos2 (e) sin2 ( e ( t ) ) sin 0 dB
320
Frequency (MHz)
Figure 5. Raw frequency-domain data for the r&) experiment, in analogy to Figure 4.
quency-dependent phase shift, and the amplitude ratios are closer to unity. These features are indicative of significant rotation. The behavior of interfacial acridine orange in the presence of pure water is examined in part i, for both out-of-plane and in-plane reorientation. The effect of alcohol on these behaviors is amsidered in part ii. i. Pure Woter/CI8Interface. The results for the interfacial behavior in the presence of pure water are striking because they indicate that acridine orange rotates very little out-of-plane. Quantitatively,there must be some motion because the measured phase shifts are nonzero. Additionally, if acridine orange were aligned with its long axis perfectly in-plane, the amplitude ratio would be lower. The raw phase shift and amplitude data thus show that acridine orange is capable of rotation through a distribution of angles. To obtain an understanding of how acridine orange is reorienting about 8 at the interface, the data were analyzed through Fourier transform relations to fit to the general equation
4 2 ) = r(O)Lfexp(-t/r,i) + (1 -A
exp(-t/~,Jl
(7)
where T, is the orientational relaxation time constant. If T , ~= 05, then the anisotropy at infinite time, r(-), is r(0)(1 -8. In the fitting routines, a time constant of 1 s was uscd to approximate a.
The analysis of the data according to eq 7 fit best to an infinite ~ a small value off for the anisotropy decay. Such value of T , and behavior is interpreted as hindmd rotation through a limited range of angles between 90° and 90° 80.'0*23-27 The ground-state orientational distribution can be described by a Gaussian, P(0)
-
P(e) = expl-(8
- 78°)2/(28,2)]
(8)
The 78O derives from the fact that the transition moment of acridine orange is 12O from its long axis and therefore, 78O from the surface normal. The parameters r(0) and r ( = ) both depend
M, =
A''*P(d) sin2 (e) sin2 ( e ( t ) ) sin 8 de
(10)
The regression provided a very narrow range of values for 40) but a wider range of values off and 7,. Using the well-defined values of r(0) from the regression and the experimentally determined values of M,,the range of values off were narrowed to less than +O.Ol through eqs 9 and 10. The results of Table 11, first entry, show that, for pure water, f = 0.05. Given the well-characterized value off, one can now proceed with the interpretation from the hindered rotation model. Using eq 8, 15O is the range of angles through which the transition moment of interfacial acridine orange is distributed with respect to 8, where pure water is the solvent. Furthermore, the acridine orange is redistributing within this distribution over time; i.e., it is not a static distribution. This must be the case because the phase shifts are nonzero. A surprising results is that the time, T,,, required to redistribute over this narrow range of angles is rather long: at least several nanoseconds. If one were to predict from the dimensions of acridine orange the time scale of reorientation, which increases with a viscosity of a few centipoise would correspond to a reorientation time of a few picoseconds. This constitutes a 3 order of magnitude discrepancy between the prediction and the experimental result. It is doubtful that the effective viscosity of the interface is 1000 CP or greater. A more satisfying explanation would be that there is actually little rotational diffusion about 8; instead, there is a static orientational distribution across a rough interface, and acridine orange translationally diffuses across the rough interface. Bareman et al. have shown through molecular dynamics simulations that the exterior surface of C,8chains, which are attached to a planar surface, becomes increasingly rough as the density drops from 21 to 35 A2/chain.28 For the surface used here, the coverage is 60 A2f chain, which would presumably be rougher yet. Experiments are under way to characterize the translational diffusion properties. The in-plane reorientation of acridine orange is more straightforward. In Table 11, the first entry under r J f ) shows
5948 The Journal of Physical Chemistry, Vol. 96, No. 14, 1992 TABLE 111: Orientational Distributions for Hindered Rotation‘ alcohol conc. 8, (deg) y.,b alcohol conc. 8, (deg) 0% 6% M e O H 18%MeOH
15 16 18
29 22 11
3% P r O H 25% M e O H
5%PrOH
21 23 23
11
8 4
“ T h e estimated values of the interfacial tension between the solution and the CI8surface, indicated a s ysl,are from ref 21. *Expressed in dyn/cm.
that the anisotropy decay is a single-exponential function with no infinite component, and the reorientation time is 0.5 ns. The strong alignment of acridine orange in the plane of the surface assists in the interpretation of r+. With reference to Figure 3, the anisotropy decay is attributed to rotational diffusion about the symmetry axis of acridine orange. The high alignment permits estimation of an effective in-plane component of the viscosity tensor of the interface, which from the dimensions of acridine orange and the Perrin relations29is estimated to be 2 cP. The low value of the apparent viscosity is surprising considering the bulkiness of the chains. The single-exponential decay for r,(t) indicates that acridine orange is free to reorient over all angles 9, with no obvious evidence of hindered rotation. The fact that the fit is not of high quality (x2 = 7) suggests either nonexponential decay or some inhomogeneity of the acridine orange ensemble on the time scale of rotational diffusion. Inhomogeneity is the likely origin because the rs(t)results showed that the orientational distribution about 0 is slow on the time scale of reorientation in 4. ii. Effect of Wetting of the Interface. As outlined earlier, alcohols adsorb to CISsurfaces, and small amounts of alcohol in water lower the surface tension of the water/CI8 interface significantly. Since partial wetting by alcohol is generally correlated with faster transfer dynamics between water and it is interesting to probe its effect upon the rotational dynamics of acridine orange at the interface. Figures 4 and 5 show that there is an experimentally measurable effect of alcohol on the raw frequency-domain data. Alcohol causes a small but experimentally significant perturbation of the orientational behavior for both the in-plane and outsf-plane cases. To reveal the nature of these perturbations, the analyzed data summarized in Table I1 are examined. The results of Table I1 shows that the primary effect of wetting by alcohol is that the fractional contribution, f, of the orientational distribution becomes larger as more alcohol covers the surface. In each case a selfconsistent set of values of r(O), M, and f were obtained, for which low x2 values indicate excellent fit to the experimental data. The increasingf translates into a larger eo as alcohol covers the surface. The calculated values of eo for each solvent composition are summarized in Table 111. The values of 8, gradually increase with increasing surface coverage of the alcohol. As with the case for pure water, the apparent reorientation time, T , ~ is, longer than the fluorescence lifetime. Adsorption of alcohol thus increases the roughness of the interface. For r+(t),the raw data show a miniscule change in the orientational behavior upon wetting, except for the case of 5% 1propanol. The analyzed data show that the reorientational behavior is more complicated than simple single decay, as indicated by the values of xz exceeding unity significantly. Again, 5% 1-propanol is the exception. The analyzed data indicate that the
Burbage and Wirth reorientation time is approximately 0.5 ns. For the case of 5% 1-propanol, the analyzed data reveal the same fast decay component but also a slow decay component, which was not evident for any of the other cases. The 5% 1-propanol solution has the alcohol concentration that was found to provide the highest surface coverage of alcohol and the most interfacial roughness. The roughened interface thus perturbs the in-plane reorientation in that acridine orange is no longer able to reorient over all in-plane angles on the time scale of the fluorescence emission. A possible interpretation is that the roughness features approach a distance in the xy plane on the order of the size of acridine orange; thus, acridine orange encounters roughness before it can completely reorient in 4. These studies show that the effects of wetting are (1) to reduce the partition coefficient of the surfactant and (2) to cause a wider out-of-plane orientational distribution of the surfactant. It thus appears that wetting makes the water/C18 interface rougher. These experimental results show that wetting by alcohol does not cause faster orientational dynamics of acridine orange at the interface.
Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-8814602). Registry No. M e O H , 67-56-1; P r O H , 71-23-8; acridine orange, 6561-2.
References and Notes (1) 1990. (2) 1286. (3) (4) (5)
MacRitchie, F. Chemistry at Interfaces; Academic Press: New York, Heslot, F.; Cazabat, A. M.; Levinson, P. Phys. Reu. Len. 1989, 62,
Novotny, V. J.; Marmur, A. J. Colloid InterfaceSci. 1991, 145, 355. Shen, Y. R. Annu. Rev. Mater. Sci. 1986, 16, 69. Bhattacharyya, K.; Castro, A.; Sitzmann, E. V.; Eisenthal, K. B. J. Chem. Phvs. 1988.89. 3376. (6) Goh, M. C.; Hicks, J. M.; Kemnitz, K.; Pinto, G. R.; Heinz, T. F.; Eisenthal, K. B. J. Phys. Chem. 1988, 92, 5074. (7) Zhao, X.; Goh, M. C.; Eisenthal, K. B. J . Phys. Chem. 1990,94,2222. (8) Huana, J. Y.; Suwrfine. R.; Shen. Y. R. Phvs. Rev. A 1990.42.3660. (9) Castro, A.; Sitzmann, E. V.; Zhang, D.; Eisenthal, K. B. J. Phys. Chem. 1991, 95, 6152. (10) Wirth, M. J.; Burbage, J. D. Anal. Chem. 1991, 63, 1312. (1 1) Chattoraj, D. K.; Birdie, K. S. Adsorption and the Gibbs Surface Excess; Plenum Press: New York, 1984. (12) Wirth, M. J. Prog. Anal. Specrrosc. 1988, 1 1 , 361. (13) Klein, U. K. A.; Haar, H.-P. Chem. Phys. Lett. 1978, 58, 531. (14) Lakowicz, J. R.; Cherek, H.; Maliwal, B. P.; Gratton, E. Biochemistry 1985, 24, 316. (15) Knobler, C. M. Science 1990, 249, 870. (16) Sentell, K. B.; Dorsey, J. G. Anal. Chem. 1989, 61, 930. (17) Nawrwki, J. Chromatographia 1991, 31, 177. (18) Lambert, W. J.; Middleton, D. L. AMI. Chem. 1990, 62, 1585. (19) Chou, S.-H.; Wirth, M. J. J. Phys. Chem. 1989, 93, 7694. (20) Scott, R. P. W.; Simpson, C. F. Faraday Symp. Chem. SOC.1980, 15, 69. (21) Montgomery, M. E.; Green, A. M.; Wirth, M. J. Anal. Chem. 1992, 64, 1170. (22) Matsuoka, Y.; Yamaoka, K. Bull. Chem. SOC.Jpn. 1979,52, 3163. (23) Kinosita, K.; Kawato, S.;Ikegami, A. Eiophys. J. 1977, 20, 289. (24) Lipari, G.;Szabo, A. Eiophys. J. 1980, 30, 489. (25) Szabo, A. J . Chem. Phys. 1984,81, 150. (26) Kinosita, K.; Ikegami, A.; Kawato, S. Eiophys. J. 1982, 37, 461. (27) Lipari, G.; Szabo,A. J. Chem. Phys. 1981, 75, 2971, (28) Bareman, J. P.; Cardini, G.;Klein, M. L. Phys. Rev. Letr. 1988,60, 2 152. (29) Perrin, F. J. Phys. Radium 1936, 7, 1. (30) Cole, L. A,: Dorsey, J. G.Anal. Chem. 1990, 62, 16.
