J. Phys. Chem. 1985,89, 2933-2937 phosphatidylserine or phosphatidic acid vesiclesz6 and is in the Caz+ concentration range used for inducing fusion.27 Since excimer formation is a bimolecular process, its concentration and relative emission intensity (E/M) are directly proportional to the microscopic PyPC concentration or inversely proportional to the microscopic volume confining the PyPC molecule^.^^-^^ For a mixture of PyPC-labeled and unlabeled vesicles, the occurrence of salt-induced fusion implies an increase of the available volume of amphiphile in which PyPC can dilute and a correspondent decrease of E/M. E / M decrease with time suggests that sonicated vesicles undergo fusion due to added salt (Figure 5A-C). However, one could also expect an analogous (45) A. K. Soutar, H. J. Pownall, A. S. Hu, and L. C. Smith, Biochemistry, 13, 2828 (1974). (46) H. J. Pownall and L. C. Smith, J . Am. Chem. Soc., 95,3136 (1973).
2933
E/M decrease if PyPC exchanged from labeled to unlabeled vesicles, as reported in cases of spontaneous phospholipid transfer!' As E / M for the mixture of labeled and unlabeled vesicles remains constant in absence of salt (see controls in Figure 5 ) , we consider our data as strong evidence of salt-induced fusion in sonicated amphiphile vesicles and absence of salt-induced fusion in large injected amphiphile vesicles. , Acknowledgment. The financial support from FAPESP, FINEP, and CNPQ is gratefully acknowledged. We are indebted to Drs. M. J. Politi and L. Sepulveda for careful reading of the manuscript. Registry No. DHP, 2197-63-9; DODAC, 107-64-2; N a , 7440-23-5; Ca, 7440-70-2. ~~
(47) M. A. Roseman and T. E. Thompson, Biochemistry, 19,439 (1980).
Micellar Mlcrofluldltles at Hlgh Pressures W. D. Turley and H. W. Offen* Department of Chemistry and The Marine Science Institute, University of California, Santa Barbara, California 93106 (Received: February 4, 1985)
The monomer/excimer intensity ratio and excimer lifetime of dipyrenylpropane (DPyP) dissolved in micellar media are used to infer the microfluidity of the solubilization site as a function of pressure to 300 MPa. The viscosity scale at 15.0 O C is calibrated with DPyP in isobutyl alcohol (5-90 cP) and used as a standard for the probe studies of SDS ( 1 9 cP), LDS (19 cP), CTAC ( 3 9 cP), and C&6 (57 cP). Their respective activation volumes of viscous flow, AV,*, are 13, 11, 24, and 25 cm3/mol. The marked difference in fluidity and its pressure coefficient of anionic surfactants as opposed to cationic and nonionic micelles is explained in terms of the relative strengths of headgroup hydration.
Introduction The utility of surfactant micelles lies in their ability to solubilize various types of molecules.'*z The use of fluorescent probes represents an important method for examining the probe microenvironment in micellar aggregates.14 The present work focuses on the pressure dependence of the effective micellar microviscosity q inferred from the intramolecular excimer kinetics of 1,3-di( 1pyreny1)propane (D P Y P ) . ~ Kinetic analysis of diarylpr~panes~' is based on the observation that electronic excitation of one aromatic chromophore in fluid media leads to fluorescence from two temporally and energetically distinct species-the monomer M* and excimer E*-as shown in Scheme I. The excited-state rate of association k, and dissociation kd are related to the fluorescence intensity ratio IM/IE, the excimer lifetime T E , and the radiative rate constants kMand kE of the respective M* and E* by zM/zE
= (kM/kE)(kd -k 7E-l)ka-l
(1)
This expression for two dynamically coupled species can be simplified in the low-temperature or high-viscosity ( q 1 5 cP) limit,5 (1) Fendler, J. H. "Membrane Mimetic Chemistry";Wiley: New York, 1982. (2) Mittal, K. L.; Lindman, B., Eds. "Surfactants in Solution"; Plenum Press: New York, 1984; Vol. I. (3) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (4) Singer, L. A. In "Solution Behavior of Surfactants", Vol. 1, Mittal, K. A., Fendler, E. J., Eds.; Plenum: New York, 1982. (5) Zachariasse, K. A. Chem. Phys. Lett. 1978, 57, 429. (6) Birks, J. B. "Photophysicsof Aromatic Molecules"; Wiley-Interscience: London, 1970. (7) Zachariasse, K. A.; Duveneck, G.; Busse, R. J . Am. Chem. Sm. 1984, 106, 1045.
SCHEME I
+ i DPyP
where the dissociation rate becomes negligible (7E-l >> kd) and excimer formation is viscosity controlled. If diffusion of the aromatic chromophores to achieve the excimer conformational state is rate limiting, the applicable equation is kL1 = cq, where c is a constant that depends only on temperature for a given solute/solvent system.6 These two approximations lead to a simple relation between observables and q of the solubilization sites: 7EzM / I E
= (kM / kE)Clf
(2)
Application of eq 2 to pressure studies requires the reasonable assumption that the ratio of radiative transition rates are pressure independenkg Then the pressure coefficient of the microviscosity is d In q/dP = d In (TEZM/ZE)/dP
(3)
and the activation volume of viscous flow is Av,* = RT[d In ( ~ E I ~ / I ~ ) / ~ P ] ~ (4) Equations 2 , 3, and 4 can be used to evaluate q(0) and q(P)of micelle-probe aggregates provided that the bulk viscosity and its (8) Johnson, P. C.; Offen, H. W. J . Chem. Phys. 1972, 56, 1638. Offen, H. W.; Phillips, D. T. J . Chem. Phys. 1968, 49, 3995.
0022-365418512089-2933$01.50/00 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 13, 1985
TABLE I:
Turley and Offen
Surfactant/Probe Data
aggre[surfactant], gation mM no.
surfactant (CH,)(CH2)IIOS03-Na* (CH,)(CH,) I ,OSO 10 ns is absent. The first option is eliminated because probe microcrystals quench monomer emission. The observed DPyP fluorescence pattern and the absence of a pressure effect in the crystalline surfactant phase of SDS is consistent with the trapping of DPyP molecules into two types of conformations: a syn isomer which can achieve the excimer geometry in less than 10 31s' and the constrained anti isomer which gives only monomer emission. In summary this work has demonstrated a complex fluidity pattern in surfactant-probe aggregates. Interfacial forces attributed to headgroup hydration add significant constraints to hydrocarbon chain packing within the micellar core. Interpretation of the pressure response of dipyrenylpropane fluorescence does not require water to penetrate into the micellar interior. Registry No. DPyP,79480-81-2; SDS,151-21-3; LDS,2044-56-6; CTAC, 112-02-7; C12E6, 3055-96-7.
(39) Tanaka, M.; Kaneshina, S.;Shin-No, K.; Okajima, T.; Tomida, T. J . Colloid Interface Sci. 1974, 46, 132. (40) Cabane, B.; Duplessix, R.; Zemb, T. in ref 2, p 373.
(41) Harada, S.;Nakagawa, T. J . Solution Chem. 1979, 8, 267. (42) Vikingstad, E.; Hoiland, H. J . Colloid Interface Sci. 1978, 64, 510.
Simultaneous Calorimetric Determination of Equilibrium Constant and Enthalpy Change of Hydrogen-Bond Complexes in Dilute Solutions of Phenol with Pyridine in Carbon Tetrachloride J. Mullens,* J. Yperman, J. P. Franqois, and L. C. Van Poucke Limburgs Universitair Centrum, Department SBM, Universitaire Campus, B- 361 0 Diepenbeek, Belgium (Received: October 23, 1984; In Final Form: February 18, 1985)
A calorimetric study in CC14has been camed out on the 1-1 complex between phenol and pyridine at 25 O C at low concentrations. The values of the equilibrium constant (44.61 0.18) and the enthalpy change (-31 797 55 J mol-') have been determined simultaneously. A method is proposed to calculate the stability of the results obtained in minimization programs by mapping the area around the minimum.
*
Introduction Most thermodynamic data of hydrogen-bond complexes have been obtained by spectroscopic techniques. The enthalpy changes are usually determined by temperature variation of the equilibrium constant. Hydrogen-bond complexes have for the greater part enthalpy changes between l o a n d 5 0 kJ mol-'. In the literature differences up to 15 kJ mol-' are reported for the same system in the same condition. There is no doubt that spectroscopic methods are irreplaceable in the detection and in the study of the hydrogen bond, but there is also a great need for direct and accurate calorimetric measurements. In view of the recent technical developments and 0022-3654/85/2089-2937$01.50/0
*
refinements in calorimetric equipment, together with the facilities of the computer, everything is available to work off arrears. Besides several graphical methods there are also computational methods for the simultaneous calorimetric determination of equilibrium constants and enthalpy changes: a general review with many references can be found in papers by Fenby and Hepler and by Lamberts.' We have used a least-squares method for the investigation of the complexation between pyridine and phenol in carbon tetrachloride at 25 OC. The stability of the result (1) Fenby, D. V.; Hepler, L. G. Chem. SOC.Rev. 1974,3, 193. Lamberts, L. bnd. Chim.Belge 1971, 36, 347.
0 1985 American Chemical Society
