J. Phys. Chem. 1986, 90, 2521-2525 In order to relate the a exponent to other exponent already defined in the case of a fractal space, let us first notice that in the computer-simulated and the experimental patterns, there is generally only one path which connects two points (Le., there is no circle or closed lopp; also recall that in a fractal space the spectral dimension; d, governs the number of distinct sites N ( t ) that a random walker has visited from time 0 to time t:
-
N ( t ) tal2 (3) Under these circumstances and when 2 is smaller than 2, the exploration can be considered as compactlo which means that any point separated (Le., from the center) by a chemical distance L, smaller than L, has been reached by the random walker. L, is proportional to the square root of the elapsed time as in a classical one-dimensional diffusion problem. Combining the definitions of the fractal dimension (eq 1) and spectral dimension (eq 3) together with this consideration leads to N ( t ) = t’12 = (L,)’ RD (4)
-
Comparing this equation to the definition of the exponent a (eq 2) leads to a = D/J
(5) One way to compute the exponent a is then to determine 2 through a random walk problem (eq 3). This was the approach we used. We first began with the DLA model in order to check the accuracy of our program. We get d = 1.1 f 0.2 for the Witten-Sander model. This result is in agreement with previous determinations.” The large fluctuation we get are mostly due to the small number (1000) of sites which constitute=ouraggregate. In the case of th_e experimental pattern, we get d = 1.5 f 0.2. These values of d, from eq 5 with D = 1.7, lead to values of the = 1.1, which are very exponent a,CYDLA = 1.5 and aclcc,r~eposi,ion different from one another. This result confirms that the experimental pattern has more radial symmetry than that predicted by the DLA model calculated on a square lattice. There are several reasons that might accpunt for the difference between the calculated and the observed d . The DLA model is (9) Alexander, S.; Orbach, R. J . Phys. Lett. (Paris) 1982, 43, L625. (10) de Gennes, P. G. J . Chem. Phys. 1982, 76, 3316; C. R. Acad. Sei. (Paris) 1983, 296, Serie 11, 881. (1 1) (a) Meakin, P.; Stanley, H. E. Phys. Rev. Lett. 1983, 51, 1457. (b) Webman, I.; Grest, G. S. Phys. Rev. B 1985, 31, 1689.
2521
an ideal model for describing electrodeposition. It does not consider any transport properties which could be very important for the real physical systems. For example, in the electrodeposition process, the Cu aggregates have loose, porous structure instead of compact, crystalline structure. This might increase its resistance. Since the resistance within the aggregate is determined by the chemical length L, of the aggregate branches, for a certain radius from the center of the pattern, radial branches are expected to have lower resistance than branched ones. This leads to higher current density, and thus faster rate of deposition at the end of radial branches than at the end of more tortuous branches having the same distance from the center. Other possible causes for the more radial appearance of the observed pattern could simply be an inhomogeneous depletion of the Cu2+concentration during the deposition process. Due to the fragile nature of the two-dimensional pattern, stirring was not carried out. While we have blamed the disagreement between the observed and calculated spectral dimension on the deviation of the deposition process from the DLA model, some of the deviation might actually result from the use of a square lattice in the simulation calculation of the pattern according to the DLA model. It has already been pointed outI2 that the value of the fractal dimension, D, depends on the type of lattice used in these types of calculations. It might be that the value of the spectral dimension is also sensitive to the type of the lattice used. Studies are now in progress and aimed at examining the sensitivity of the observed fractal and spectral dimension to variation in the different experimental conditions. We are also examining the sensitivity of the spectral dimension of the calculated pattern on the type of lattice used in the calculation.
Acknowledgment. We thank Prof. S. Williams for letting us use his computer and digitizing system. Stimulating discussions with Dr. Z. Alexandrowicz, Dr. S. Alexander, Dr. H. Scher, and Dr. Sapoval are appreciated. We thank Mr. T. Corcoran for carefully reading the manuscript and acknowledge the support of the Office of Naval Research. P.E. thanks NATO for a travel grant. Registry No. Cu,7440-50-8. (12) Turkevich, L. A.; Scher, H.Phys. Reu. Lett. 1985, 55, 1026. (13) (a) Stanley, H.E. J . Stat. Phys. 1984,36, 843. (b) Vannimenus, J.; Nadal, J. P.; Martin, H. J . Phys. A: Math. Gen. 1984, 17,L351.
Anomalous Temperature-Dependent Reorientation of Cresyl Violet in 1-Dodecanol G. J. Blanchardt Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706
and M. J. Wirth* L-310, Lawrence Livermore National Laboratory, Livermore, California 94550 (Received: August 26, 1985; In Final Form: January 8, 1986) The temperature dependence of the dynamical behavior of cresyl violet in 1-dodecanol was studied by both microscopic and bulk physical techniques. Picosecond rotational diffusion measurements indicate that the solute experiences a very sharp temperature-dependent change in its environment. Steady-state absorption spectroscopy reveals that this effect is due to a change in the solvent-solute interaction of cresyl violet. I3C NMR measurement of the temperature dependence of the shows that the change in solvation is due to a previously unreported change in the solvent spin-lattice relaxation time (TI) state of the bulk solvent. Introduction Rotational diffusion of large molecules has been described successfully in the past by the Debye-Stokes-Einstein (DSE)
model.] In this model the rotational diffusion time of a spherical solute is given by rol = s V / k T (1)
Present address: Bell Communications Research Inc., 331 Newman Springs Road, Red Bank, NJ 07701.
p 84.
0022-3654/86/2090-2521$01.50/0
(1) Debye, P. Polar Molecules; Chemical Catalog Co.: New York, 1929,
0 1986 American Chemical Society
2522
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986
where q is the solvent viscosity, Vis the molecular hydrodynamic volume, and T is the temperature. Picosecond-resolved studies of polar dye molecules have shown that in low viscosity associative liquids the DSE model is obeyed closely.2 In higher viscosity solvents, however, the observed rotational diffusion times are faster than those predicted by the DSE model. It has been shown experimentally3that this model with a slipping boundary condition4 accounts for the deviation. Thus, in general, the DSE model is a good predictor of rotational diffusion for polar molecules. One polar dye molecule for which the DSE model, with either a stick or slip boundary condition, does not predict accurately rotational diffusion times is cresyl violet. In higher viscosity alcohol solvents the measured reorientation times are much longer than would be expected based on the calculated molecular v01ume.~ The data have been shown to be consistent with strong solvent attachment, presumably the result of hydrogen bonding. Thus, the larger effective volume of the reorienting species accounts for the observed behavior. Three alcohols were studied: ethanol, 2-propanol, and 1-decanol. It is expected, on the basis of the aforementioned data, that the reorientation of cresyl violet in 1-dodecanol, having a 12 carbon chain, would be even longer. With this in mind, the rotational diffusion behavior of cresyl violet in 1-dodecanol was examined in our laboratory. The result of this investigation was surprising in two ways: First, that cresyl violet reoriented inordinately fast even for the DSE model without solvent attachment. Second, and even more striking, is that the reorientation behavior slowed markedly upon increasing the temperature of the solution. Such information raises new questions about the nature of the solventsolute interaction. It is the purpose of this work to understand the anomalous reorientation behavior of cresyl violet in l-dodecanol.
Experimental Section All experiments were performed on a 15 p M solution of cresyl violet perchlorate (Kodak laser grade) in 1-dodecanol (Alfa 99+%). The cresyl violet steady-state fluorescence was quenched with KI (saturated in 1-dodecanol) to eliminate excited-state buildup due to the high repetition rate of the laser. Since the oxazine class of dyes are known to have negligible intersystem crossing: and all experiments were performed on the solute ground state, triplet-triplet absorption was assumed to be negligible. Ground-state rotational diffusion measurements were performed using picosecond pump-probe spectroscopy. The experimental apparatus and data acquisition system are similar to that used in a previous study.’ The detection electronics were modified slightly to increase both long-term signal stability and absolute sensitivity. This was accomplished through phase-locked generation of the modulation frequencies and single sideband sum frequency detection.8 In addition, the dye lasers have been modified to provide transform-limited p ~ l s e s . The ~ reduction in noise afforded by this improvement eliminates the coherent coupling artifact observed previously when the pump and probe wavelengths are different.’ The temperature of the flowed sample was controlled with a Neslab Model RTE-8 temperature controller and monitored at the sample cell by a Doric Trendicator Model 410A thermocouple thermometer. As a check on the reliability of the picosecond resolved measurements, we compared the rotational diffusion behavior of cresyl violet in ethylene glycol to the results obtained for the same (2) Millar, D. P.; Shah, R.; Zewail, A. H. Chem. Phys. Lett. 1979, 66, 435-440. (3) Moog, R. S.;Ediger, M. D.; Boxer, S. G.; Fayer, M. D. J. Phys. Chem. 1982.86, 4694-4700. (4) Hu, Chih-Ming; Zwanzig, Robert J . Chem. Phys. 1974, 60, 4354-4357. (5) Von Jena, A.; Lessing, H. E. Chem. Phys. 1979, 40, 245-256. (6) Drexhage, K. H. Top. Appl. Phys. 1973, I , 144-211. (7) Blanchard, G. J.; Wirth, M. J. J . Chem. Phys. 1985, 82, 39-44. (8) Andor, Laszlo; Lorincz, Andras; Siemion, Jeanne; Smith, Duane D.; Rice, Stuart A. Rev. Sci. Instrum. 1984, 55, 64-67. (9) Blanchard, G. J.; Wirth, M. J. Opt. Commun. 1985, 53, 394-400.
Blanchard and Wirth T (‘C) 31.6
33.7
36.2
30.4
27 0
29.5
24 7
I
I
35
40
45 (Io-:,
50
55
N-9
Figure 1. The experimentally determined decay constant of the anisotropy plotted vs. q / T (bottom ordinate) and 7‘ (top ordinate). The line drawn through the three high temperature points is the approximate best fit. T~~ = 629 ps for the best-fit line. TABLE I: Experimentally Determined Polarization Ratio of Cresyl Violet in 1-Dodecanol, Expressed as R ( t ) at 20-ps Delay, and the Decay Constant of the R ( t ) as a Function of Solution Temperature“ T , OC R(20)f95”/0 CL (max = 0.400) T,j f 95% CL,PS 24.7 0.344 f 0.020 636 f 32 27.0 29.5 29.8 30.1 30.4 31.6 33.7 36.2
0.338 f 0.014 0.332 f 0.014 0.338 f 0.012 0.336 f 0.018 0.332 f 0.012 0.336 f 0.012 0.330 f 0.014 0.344 f 0.014
630 f 20 655 f 17 136 f 22 932 f 46 1030 f 36 1181 f 37 1132 f 37 1102 f 31
“These data show that the cresyl violet diffusion constant is temperature dependent while the spectroscopic band polarization is temperature independent. Note: Xpump = 597 nm, Xprok = 580 nm. chemical system by Von Jena and L e ~ s i n g .We ~ found our results to be in agreement with theirs within the experimental error. Steady-state absorption measurements were made vs. air with a Perkin-Elmer-Hitachi Model 200 double-beam UV-visible spectrometer. The spectral resolution of this instrument was 1 nm for all measurements. The temperature of the sample was controlled and monitored with the same apparatus used for the rotational diffusion measurements. The absorption spectra were digitized and processed on a Harris/7 computer system. The temperature dependence of the solution viscosity was determined by measurement with a Cannon-Fenske viscometer in a home-built temperature bath. The set temperature of this bath is stable to fO.O1 K. The sample viscosities were compared with known standards for calibration purposes. The I3CNMR measurements of the spin-lattice relaxation time were done on a Bruker Model WP-270 270 MHz spectrometer using a commercial (Bruker) variable-temperature probe. The refractive index measurements were made with a Carl Zeiss differential refractometer. The sample temperature was regulated by a Haake Model FE temperature controller. The refractometer was calibrated with water, vs. air, and all sample readings were likewise made vs. air.
Results and Discussion The rotational diffusion time of cresyl violet as a function of temperature is shown in Figure 1. What is most unusual about these data is that at 30 OC there is a very rapid increase in the observed decay time of the anisotropy. Since the polarization ratio of the spectroscopic band examined is constant over the temperature range studied (see Table I), it must be the rotational diffusion constant of cresyl violet which is changing with tem-
The Journal of Physical Chemistry, Vol. 90, No. 11, 1986 2523
Reorientation of Cresyl Violet in 1-Dodecanol 39.0 I.
35.0
T ("C) 30.0
25.0
21 0
t
I
'
i
1
Figure 2. In q plotted vs. 1/T (bottom) and T (top). The temperature dependence of the solution viscosity is observed to obey the predicted behavior. D = neat l;dodecanol, r = 0.9998; S = the cresyl violet solution used for all other experiments, r = 0.9994. The origin of the different slopes of these two lines is discussed in the text.
perature.' In the interpretation of the data in Figure 1, it is necessary to determine the solution viscosity experimentally. The temperature dependence of viscosity is shown in Figure 2 for both the sample and the neat solvent. The viscosity displays no discontinuity or change of slope over the temperature range of interest, indicating that the observed rotational diffusion behavior is not due to a sudden change in the solvent bulk viscosity at 30 O C . The slope of the temperature dependence of the sample viscosity is slightly different from that of the neat solvent due to the addition of the KI quencher to the sample. At temperatures above 30 OC the reorientation time is temperature dependent, and below 30 OC the reorientation time is temperature independent within the experimental error. The reorientation times for cresyl violet in ethanol, 2-propanol, and 1-decanol are all observed to be longer than expected based on the DSE model with a sticking boundary c o n d i t i ~ n .In ~ contrast, its reorientation time in 1-dodecanol is shorter at all temperatures than that predicted by stick dynamics. This result indicates that the mechanism of reorientation for cresyl violet in 1-dodecanol is different than that for the other above-mentioned alcohols. The hydrogen-bonding interactions known to be responsible for the behavior in short-chain alcohols are not observed for 1-dodecanol. Reorientation times shorter than those predicted by the sticking boundary condition are typically interpretable in terms of slip dynamic^.^ The temperature dependence of reorientation in the slip limit is describable by the following equation: where 6 is the slip coefficient, T , is ~the value ~ of the reorientation time calculated from eq 1, and T,: is the zero viscosity intercept. To determine whether or not the slip boundary condition describes the cresyl violet reorientation behavior above 30 "C, we compared the slip coefficient observed experimentally to a theoretical slip coefficient calculated from the solute molecular geometry. The theoretical value of the slip coefficient can be obtained as follows. The anisotropy decay curve is fit to the effective geometry of a symmetric rotor. This gives a quite reliable ratio of the minimum-to-maximum molecular axis lengths because the directions of the pumped and probed transition dipoles are well characterized.' From the experimental anisotropy decay, cresyl violet has a minimum/maximum ratio of 0.54, which agrees well with the estimate of 0.5 by Millar et alezbased on the molecular structure. From this ratio the theoretical slip coefficient is determined to be 0.21 .4 The experimental value of the slip coefficient is determined from the slope of the T~~ temperature dependence. T~~ for each T is calculated from the experimental anisotropy decay curves through the Chuang and Eisenthal equations.I0 T ~ , Dr ~) is ~ calculated (
WAVELENGTH (nm) Figure 3. Absorption spectra of cresyl violet in 1-dodecanolas a function of temperature. The three spectra with maxima at 613.4 nm were taken at solution temperatures of 25.9, 28.4, and 29.8 " C . The three spectra with maxima at 607.7 nm were taken at 31.6, 34.3, and 36.5 "C. based on the solute molecular volume2 and the experimentally determined values of q. The resulting value for the slip coefficient is 0.22, in close agreement with the theoretical slip coefficient of 0.21. The rotational diffusion behavior is thus described well by slip dynamics. In the stick limit, the adjacent solvent molecules move with the reorienting molecule, while in the slip limit the solute merely displaces the relatively immobile solvent molecules. It is possible for cresyl violet to be hydrogen bonded to the solvent and yet reorient according to slip behavior if the lifetime of the hydrogen bonds is short compared to the solute reorientation time. While the reorientation data do not reveal the specific nature of the solvent-solute interaction, they do show that the degree of interaction is increased abruptly at 30 O C . r0:, the zero viscosity intercept, determined from our data is 157 ps, which corresponds closely to the reorientation time observed below 30 O C (155 ps). Thus the apparent temperature independence below 30 "C can be explained as originating from cresyl violet reorienting in a very low viscosity environment. The data are consistent with cresyl violet residing in a polar (possibly H-bonded) environment above 30 "C, while it is solvated primarily by the alkyl chains below 30 O C . An experimental method which can be used to corroborate the chemical identity of the average solvation environment is the steady-state absorbance measurement of the spectroscopic band position. It is well-known that the cresyl violet electronic absorption spectrum is solvent dependent. Its spectrum is blue shifted in hydrogen-bonding solvents due to solvent attachment to the ring nitrogen in the chromophore." The idea that cresyl violet (10) Chuang, T. J.; Eisenthal, K.B. J . Chem. Phys. 1972,57,5094-5097. (1 1) Hammond, Peter, Lawrence Livermore National Laboratory, private communication, 1985.
2524
cc
Blanchard and Wirth
The Journal of Physical Chemistry, Vol, 90, No. 11, 1986
T ("C)
~
L 240
260
28 0
300
TEMPERATURE
32 0
340
29,.0 27.4
24.7
46.9 50.4
57.0
36 0
I'C)
Figure 4. The refractive index plotted vs. temperature. The line is drawn through the data points as a visual aid only. The refractive index of the neat solvent and the cresyl violet solution was observed to be identical a t all temperatures examined.
is solvated by alkyl chains below 30 O C and by hydrogen-bonding groups above 30 "C can be tested by examining the temperature dependence of its absorption band. Figure 3 shows the absorbance data as a function of temperature. Three absorbance spectra were obtained at different temperatures below 30 O C and all have their maxima a t 613.4 nm. The three absorbance spectra obtained above 30 OC all have maxima at 607.7 nm. The distinct and uniform band shifts on either side of 30 OC indicate an abrupt change in the nature of the solvation environment. The same transition temperature as seen for the rotational diffusion behavior is observed. Further, the shift above 30 O C is to the blue, indicative of increased hydrogen bonding to cresyl violet above the transition temperature. The spectral data thus reinforces the physical interpretation of the rotational diffusion data. It is noted that the temperature-dependent solvation effect of cresyl violet is reversible. It is therefore not due to a chemical decomposition of the cresyl violet molecule. In addition, the absorption spectrum of cresyl violet in ethylene glycol has no such temperature-dependent shift in this temperature range. Thus, the observed effect is not an intramolecular property of cresyl violet. The possibility that the KI quencher or any extraneous water might be responsible for the temperature-dependent equilibria was eliminated by reproducing the spectral shift behavior with unquenched and dried solutions. The temperature-dependent solvation is thus a property of the cresyl violet-1-dodecanol system. Normally hydrogen bonding to cresyl violet would be expected to decrease with increasing temperature. Thus, it is of interest to determine why the opposite behavior is observed experimentally. A shift in the equilibrium of solvation sites would be expected to vary slowly with temperature. The sharpness of the temperature dependence suggests that a phase change occurs at 30 O C . Since 1-dodecanol is a liquid below this temperature, such a phase change would involve a mesophase to isotropic liquid transition. 1-Dodecanol is not known to form a liquid crystal, although its structure does allow for this possibility. N o visual evidence of liquid crystal formation is detectable for 1-dodecanol between crossed polarizers, but the ordered domains may be smaller than the wavelength of visible light. A simple means of detecting a phase transition is through the temperature dependence of the refractive index. In the absence of a phase transition, it is expected to be a smoothly varying function.' The experimentally observed temperature dependence of the I-dodecanol refractive index is shown in Figure 4. The data reveal a deviation from the smooth temperature dependence. This deviation occurs a t 30 OC, the same temperature at which the abrupt changes were observed for the rotational diffusion and spectral shift data. The refractive index behavior is identical for both the cresyl violet solution and the neat solvent, indicating that the effect is due to a change in the solvent and the cresyl violet is merely a spectroscopic probe for this change. The cause of the
i
-
c,-c4
33.8
38.2
41.0
7 /T
(id3~PK')
Figure 5. The temperature dependence of the T I nuclear spin-lattice relaxation time of different carbons in the alkyl chain of I-dodecanol. Only C,, and C,, deviate from the behavior predicted theoretically.
anomalous reorientation behavior must therefore be sought through a study of the neat solvent 1-dodecanol. The possible changes in the interactions of 1-dodecanol involve either the hydroxyl group or the alkyl chain. Calculations based upon proton N M R datal2 suggest that the solvent exists primarily (>90%) as hydrogen bound trimers over the temperature range studied, with no sharp changes. As an independent check on this conclusion, the temperature dependence of the infrared spectrum of the 0-H stretching vibration was measured. There was no change detectable over the temperature range of 25-35 OC, within the experimental uncertainty. Thus, the change in solvation properties does not appear to involve a change in the hydrogen bonding among solvent molecules. The conformation of alkyl chains is often studied by Raman measurement of the C-C stretching region.I3 Investigation by this method revealed no discernable change in the trans and gauche conformation populations, within the experimental uncertainty. The Raman bands corresponding to different chain conformations are, however, strongly overlapping. Also, unlike the IR measurement, which probes only the hydroxyl environment, the Raman spectrum probes all parts of the chain. Thus, the Raman measurement does not disqualify the possibility of a conformational change in the alkyl chain at 30 OC. A method better suited to the study of long alkyl chains is I3C N M R because individual types of carbon atoms can be distinguished. The spin-lattice relaxation time ( T , )of a given nucleus is expected to increase in proportion to q/T,I4 with the assumption that the correlation times of the carbon motions are fast compared to the Larmour frequency. Previous experiments have established that this relationship is valid for a given solvent; however, deviations are observed when comparing different types of solvent^.'^ It is thus expected that a change in the structure of the liquid would also give rise to a deviation in the simple proportionality between T I and q / T . The relation between T I and q / T for 1-dodecanol is presented in Figure 5 . The data reveal breaks in the linear dependence for (12) Makarov, M.G.; Gus'kov, A. K.; Shvets, V. F. Zh. Fiz. Khim. 1982, 56,
71-75.
(13) Gaber, B.P.;Peticolas, W. L. Biochim. Eiophys. Actu 1977,465, 260. (14) Bloembergen, N.;Purceil, E. M.; Pound, R.V.Phys. Reu. 1948, 73, 619-7 12.
Reorientation of Cresyl Violet in 1-Dodecanol the last two carbons on the chain. Both breaks occur at or near 30 OC, the same temperature at which the rotational diffusion change, absorption band shift, and refractive index dip occur. In the N M R data, the inordinate increase in T I above the transition temperature is indicative of a sudden decrease in the motional correlation time. The data thus reveal that the change occurring in the bulk solvent above 30 O C involves an increase in the motional freedom of the end carbons of the chain. The N M R and refractive index measurements, and calorimetric evidence that the solvent melts at 24 OC, combine to point to a structural change in the liquid solvent. Further, the increased solvation of cresyl violet by the alkyl chains below the transition temperature is consistent with the N M R evidence that the chains are more ordered at low temperature. The evidence from these independent measurements points clearly to an abrupt change in the average conformation of the alkyl chains: Such behavior is consistent with a phase transition a t 30 O . C . The phase existing below 30 OC shows motional hinderance characteristic of mesophases, and the transition to the isotropic liquid is characteristically sharp. However, no birefringence is observed under a polarizing microscope nor is there a detectable change in the temperature dependence of the bulk viscosity. The mesophase of 1-dodecanol must therefore have a domain size smaller than the wavelength of light. Differential scanning calorimetry (DSC) shows no thermal activity in the region of 30 OC. However, no conclusion can be drawn from this result because the instrument sensitivity is 0.2 J / g while the enthalpy of the mesophase transition can quite conceivably be less than this. Further experiments need to be performed to obtain an understanding of the 1-dodecanol mesophase. It may seem surprising that a phase transition in 1-dodecanol has not been observed previously. However, the phase change was observed by rotational diffusion measurements rather than by the conventional methods of DSC and birefringence. Additionally, observation of the phase change was dependent upon the fortuitous choice of the solute cresyl violet becuase its solvation environment is sensitive to the solvent structure. Phenoxazone 9, which is similar in structure to cresyl violet but nonionic, detects no transition in 1-dodecanol at 30 OC, from spectral band position measurements. Since 1-dodecanol is a frequent solvent choice in reorientation experiments, the phase transition apparently does
The Journal of Physical Chemistry, Vol, 90, No. 11, 1986 2525 not affect the behavior of many dyes. A transition in the cresyl violet solvation environment is reasonable to expect, based upon the parameters controlling solubility. Solubility is controlled by the chemical potential, p. If it is assumed for simplicity that there are two statistically independent solvation environments, one being the hydrogen-bonding environment and the other being characterized by interactions with the alkyl chains, the chemical potential is CL
= POH
+ PCC
(3)
With these assumptions, the ratio of the cresyl violet concentration in each solvated form is related to the values of the standard chemical potentials. For 1-dodecanol, the solubility of cresyl violet is about 4 orders of magnitude lower than that for methanol, indicating a much less favorable solvation free energy for hydrogen bonding. The value of pOH thus approaches the value of pcc. Further, pcc > k T due to the unfavorability of alkyl solvation of cresyl violet. Under these circumstances, eq 4 shows that moderate changes in pcc arising from the conformational transition can markedly affect the concentration ratio. Thus for this case of low solubility, it is reasonable to expect that a conformational change in the alkyl chain can cause a large change in the solvation environment of .cresyl violet.
Acknowledgment. This work was supported by the National Science Foundation Grant CHE-8306697 and performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-ENG-48. We are grateful to Professor J. L. Schrag for the loan of the Neslab temperature controller and to Professor J. D. Ferry and Dr. C. J. Carriere for the assistance and use of their viscosity and refractive index measurement systems. We are indebted to Dr. Bruce Adams for his kind help with the N M R data acquisition and for several stimulating discussions. We are also grateful to T. J. Weight for his assistance in construction of the phase-locked detection system. Registry No. KI, 7681-1 1-0; cresyl violet perchlorate, 52659-20-8; 1-dodecanol, 112-53-8.
