J. Phys. Chem. 1992, 96,9022-9025
9022
Reorientation of Acridlne Orange at Liquid Alkane/Water Interfaces M. J. Wirth* and J. D.Burbage Department of Chemistry di Biochemistry, University of Delaware, Newark, Delaware 19716 (Received: June 12, 1992; In Final Form: July 24, 1992)
The first study of a molecular reorientation at a liquid/liquid interface is described. The in-plane and out-of-plane orientational motions are investigated for acridine orange at the interface of water and several hydrocarbon liquids: n-hexadecane, isopropylcyclohexane,cis-decalin, and trans-decalin. The results reveal that the hydrocarbon viscosity has no direct influence on the in-plane reorientation of acridine orange. The results also reveal measurable differences in the out-of-plane orientational distribution, which is attributed to surface roughness. The roughness is unrelated to surface tension, suggesting that the surface roughness sensed by the probe is due to molecularity rather than to thermal capillary waves.
Introduction The behavior of molecules at interfaces is important to the areas of chromatography, biological interfaces, tribology, and molecular electronics. Molecular orientation at interfaces has been described by molecular dynamics and measured experimentally by surface second harmonic g e n e r a t i ~ n ~as- ~well as IR spectroscopy.8-'0 The intrinsic roughness of liquid/air" and liquid/liquid12J3interfaces, caused by thermal capillary waves, has been measured by light scattering. The amphiphilic nature of acridine orange makes it a suitable molecule for probing interfaces. Figure 1 depicts the structure of acridine orange at a water/hydrocarbon interface. While soluble in water, the uncharged portion of acridine orange is so strongly hydrophobic that the species exists appreciably in the form of dimers: at concentrations of 70 pM, equal concentrations of monomers and dimers exist.I4 Consequently, this solute resides strongly in excess at water/hydrocarbon interfaces. Spectrascopic studies of acridine orange at the interface of water and a covalently bonded hydrocarbon monolayer revealed considerable roughness of the interface.Is The substrate was estimated to have a negligible contribution to the roughness. The purpose of this work is to study acridine orange at a liquid/liquid interface, avoiding possible substrate contributions and thus probing the intrinsic roughness of the interface. This is the first study of the reorientation of a molecule at a liquid/liquid interface. The in-plane and out-of-plane rotational motions of acridine orange are probed as described previously.I6 Particular attention is paid to the effect of the nature of the hydrocarbon phase on the in-plane and out-of-plane rotational motions.
Theory The ability to measure separate anisotropy decays for in-plane and out-of-plane rotations is possible for molecules at interfaces by macroscopic orientation of the interface in laboratory coordinates. Figure 2 illustrates the coordinate system, where the x-y plane is the plane of the interface and the z axis is the surface normal. Rotations out of the x-y plane occur through angles 8, and rotations in the x-y plane occur through angles 4. The fluorescenceemission is collected along the surface normal. The anisotropy decay formulas for in-plane and out-of-plane reorientations have been derived previously.16 To study the reorientation through angles 6, the excitation polarization is varied between the z axis and the y axis, with the emission polarization fixed at 4 = 4 5 O . The anisotropy for these out-of-plane reorientations, re(?),is defined as follows: IzpW
re(t) = Z,(t)
- ZypW
+ 2ZYP(t)
(1)
The subscripts denote the excitation and emission polarizations. The subscript p represents the quantity (x + y)/2, which indicates that there is no polarization discrimination for 4. *To whom correspondence should be addressed.
The correction for the ellipticity of the z-axis excitation intensity, which occurs when exciting via total internal reflection, has been described previously.I6 It has been shown to be a correctable 8% contribution for the typical refractive indices used. The slight differences in refractive index of the alkanes used in this study yield no significant change in this correction factor. Upon exciting in-plane along the y axis and alternating the emission polarization between t h e y axis and the x axis, the reorientation through anglea # is probed. The anisotropy for in-plane reorientation, r6(t), is defined as follows:
The derivation of eq 2 assumes that 8 and # are statistically independent.
Experimental Section The water was purified by distillation and passage through an ion exchanger, a charcoal filter, and a t-C 18 Sepak filter. Acridine orange (go+%) was obtained from Eastman Kodak and purified by column chromatography using silica as a stationary phase and methanol as a mobile phase. The alkanes, n-hexadecane (99%), isopropylcyclohexane (99%), cis-decalin (99%). and transdecalin (99%), were obtained from Aldrich and purified by passage through silica to remove polar contaminants. The structures of the alkanes are shown in Figure 3. The silica itself had been purified by boiling in concentrated nitric acid, rinsing with purified water, and drying under nitrogen. All solvents were passed through 0.5-pm Millipore universal solvent PTFE particle filters as a final step. The viscosity measurement for isopropylcyclohexane was camed out using a Cannon-Fenske viscometer and a Neslab temperature bath. The viscosities of the other n-alkanes were obtained from the 1iterat~re.l~ The interfacial tensions were determined using a DuNouy tensiometer with a platinum-iridium ring. An optical schematic diagram is shown in Figure 4. The experiment was performed on a floated Newport optical table. The sample cell was constructed by sealing Pyrex windows in square glass tubing. To obtain the 72" angle of incidence desired for total internal reflection, BK 7 glass wedge prisms were attached to the cell windows. Glycerin was used as an index matching fluid. The interface between water and the alkanes formed a meniscus in the cell; however, the curvature of the meniscus was calculated to be negligible and was confirmed experimentallyby the fact that the same data were obtained upon translation of the cell. One milliwatt of the mode-locked 488-nm line of the argon ion laser was focused to a 1-mm spot for excitation. The polarization of the excitation beam was achieved with a Glan-Thompson prism and controlled with a Pockels cell. The emission was collected along the surface normal. The emission polarization was selected with a high-extinction polaroid, and the fluorescence was isolated with a bandpass filter and detected with a Hamamatsu 1635 photomultiplier tube.
0022-365419212096-9022%03 .OO/O 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 22, 1992 9023
Reorientation of Acridine Orange
TABLE I: Tiw-Domrin Decay Panmetera for the Out-of-Ptun Reorientation of Acridine Orange at the Interface of Water rad A b n e SolventO solvent r(O) 70, ns f X2 Ywa u, deg T W I PS 0.01 0.02 0.9 54 4.4 1 n-hexadecane -0.428 -0.406 0.6 0.01 0.9 50 isopropylcyclohexane 8.0 3 4.363 0.6 0.02 0.9 55 12 7 cis-decalin 4.366 2.5 0.02 1.o 55 12 7 rrans-decalin "The interfacial tension between the alkane and water is indicated as ywa,in dyn/cm.
Water
LASE
Y
PRISM
PRISM
Figure 4. Optical schematic diagram of liquid/liquid experiment. 0
Figure 1. Structureof acridine orange and its average orientation at a water/alkane interface.
i
w -12 Ln
6
I
IA
f
-
trans-Decalin
~
i o - cis-Decalin
a . ,
t
-16
~
ltopropylcyclohexone
I
- 2 0 L T - 80
120
7---4
- 1 7 7 - 7 7 7
160
240
200
320
280
FREQUENCY (MHz)
0.8
i -
s
-
6 E 0.6
-
c
.....
__
Vcrins-Decolin o - cis-Decolin + - Isopropylcyclohexane
,
n:!exadec.ana
......
'! 1
J
W
0 3
-
5a 0.4 2 ot-r--77-----r 80
120
I
160
200
I
I
240
l -
280
320
FREQUENCY (MHz)
Figure 2. Coordinate system used in the experiment. The x-y plane is the plane of the interface. (a) In-plane rotations occur through angles 6. (b) Out-of-plane rotations occur through angles 8.
to obtain the decay information. The data were analyzed as described previously.I8
n-hexadecane
r-7
isopropylcyclohexane
cisdecalin
Figure 5. Raw frequency-domain data for r&). Points are the experimental data and solid curves are the theoretical values calculated from the decay parameters given in Table I. Typical error bars (95% confidence intervals) are shown.
uans-decalin
Figure 3. Structure of alkanes used in forming the liquid/liquid inter-
face.
The output of the photomultiplier was directed to frequencydomain electronics, which have been described previously.'s The mode beats of the argon ion laser constitute modulations at 82 MHz and its harmonics. Phase shifts and amplitude ratios for the two polarizations were obtained as a function of mode beat
Results and Discussion I. Out-of-Plme Reorientation. The raw frequency-domain fluorescence anisotropy data for out-of-plane reorientation are shown in Figure 5. The data are indicative of strongly hindered rotation in all cases: the phase shifts are small and the amplitude ratios are far from unity and vary little with frequency. The data were found to fit to an exponential decay of the form The values of the time-domain parameters are listed in Table I. The timc-domain parameters show that the rotation is strongly hindered; i.e., f
