Langmuir 1987,3,234-239
234
in nucleation theory unless additional constraints are applied to the system which maintain the equilibrium between drop and vapor-even when the drop is not of nucleus size. Some consequences of these additional (virtual) constraints are discussed. One consequence is the validity of the Renninger-Wilemski observation. Finally we remark that if, in the future, a fully rigorous treatment of the relevant surface thermodynamics of the drop is undertaken, including a careful definition of the dividing surface and a consistent implementation of the
effects on radius, surface tension, and magnitudes of excess moles, there is some question as to whether this is the best route to follow, since it cannot be pursued without the molecular level information required, among other things, to locate the dividing surface. It might be better to utilize a nonthermodynamic molecular theory approach from the outset.
Acknowledgment. This work was supported by a grant from the CNRS-ATP Physique de L’Atmosphere.
Formation of Surfactant Double Layers on Laponite Clay Colloids’ T.Nakamura and J. K. Thomas* Department of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received M a y 16, 1986. I n Final Form: October 6, 1986 A very stable laponite (a synthetic hectorite) aqueous suspension was prepared, in which the clay has a double layer of hexadecyltrimethylammonium chloride (CTAC) on its surface. In this system the hydrophobic nature of the clay surface is dramatically improved; a typical CTAC/laponite system which contains 2 mmol of CTAC and 1 g of laponite in 1 L of aqueous suspension was able to dissolve 0.1 mmol of pyrene. The kinetics of pyrene fluorescence quenching and pyrene excimer formation reactions were examined in this system. The result of these analyses suggests that the CTAC molecules form a clusterlike double layer on the clay surface. Introduction During the last decade a marked interest has been focused on the photochemistry and photophysics of molecules in heterogeneous systems such as micelles, microemulsions, and vesicles.2 Recently, similar attention has been paid to the photochemistry and photophysics of materials adsorbed on clays, mainly because of their unique structure and catalytic n a t ~ r e . ~ - ~ In most cases luminescent cationic probe molecules such as tris(2,2’-bipyridine)ruthenium(II) ion and [4-(l-pyrenyl)butyl]trimethylammonium ion have been used to examine the nature of photophysical and photochemical processes which take place on clay surfaces, since these cationic molecules are strongly bound on the negatively charged clay surface. An attempt to alter the nature of clay surfaces has also been made by adding cationic surfactants such as hexadecyltrimethylammonium bromide (CTAB) into aqueous clay suspension^.^^*^ It has been (1) We thank the Army Research Office for support of this work via Grant DAG29-83-K-0129. We also thank Laporte Industries and Dr. R. Harrop for the laponite samples. (2) (a) Turro, N. J.; Braun, A. M.; Gratzel, M. Angew. Chem. 1980,19, 675. (b) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1983. Thomas, J. K. Chemistry of Excitation at Interfaces; ACS Monograph 181; American Chemical Society: Washington, DC, 1984. (3) (a) Krenske, D.; Abdo, S.; Van Damme, H.; Cruz, M.; Friplat, J. J . Phys. Chem. 1980, 84, 2447. (b) Abdo, S.; Canesson, P.; Cruz, M.; Friplat, J. J.; Van Damme, H. J. Phys. Chem. 1981, 85, 797. (4) (a) Dellaguardia, R.; Thomas, J. K. J . Phys. Chem. 1983, 93, 990; (b) 1983, 87, 3550; (c) 1984, 88, 960. (5) Kovar, L.; Dellaguardia, R.; Thomas, J. K. J . Phys. Chem. 1984, 88, 3595. (6) Schoonheydt, R. A,; Pauw, P. D.; Vliers, D.; De Schrijver, F. C. J . Phys. Chem. 1984,88, 5113. (7) Ghosh, P. K.; Bard, A. J. J . Phys. Chem. 1984, 88, 5519. (8) Nakamura, T.; Thomas, J. K. Langmuir 1985, I , 568. (9) Nakamura, T.; Thomas, J. K. J . Phys. Chem. 1986, 90, 641.
found that a clay suspension which contains CTAB concentration at twice the cation exchange capacity (CEC) of the clay is stable and that the clay surface is hydr~phobic.~~ Earlier studies reported on pyrene fluorescence quenching studies, but a quantitative analysis of the data obtained was not made because of the complex quenching behavior. The main difficulty arises from a relatively high Fe3+ content in the natural clay lattice structure which also efficiently quenches the pyrene fluorescence. The present study deals with the kinetic analysis of pyrene fluorescence quenching and pyrene excimer formation reactions in an aqueous laponite suspension containing hexadecyltrimethylammonium chloride (CTAC). Here, the quenching by Fe3+is avoided because laponite is a pure synthetic hectorite. Furthermore, CTAC is used in order to avoid quenching by Br-, thus essentially a single-exponential decay (i.e., simple kinetics) is obtained for the pyrene fluorescence. The kinetic data obtained are explained in terms of known kinetic models which have already been established. The studies comment on the surfactant-clay structures that are formed.
Experimental Section Chemicals. Laponite RD, which is a synthetic hectorite, was obtained from Laporte Industries and was used without further purification. The CEC of laponite was determined as 0.8 mequiv/g of laponite by the methylene blue adsorption method.l0 Pyrene from Sigma Chemical Co. was recrystallized 3 times from ethanol. Nfl-Dimethylaniline (DMA)from Eastman Kodak Co. was distilled over zinc powder. Dodecylpyridinium chloride (DPC)from Matheson Coleman & Bell was recrystallized 2 times (10) The method recommended is by Georgia Kaolin Co. Tech Data Sheet 47.
0743-7463/87/2403-0234$01.50/00 1987 American Chemical Society
Langmuir, Vol. 3, No. 2, 1987 235
Formation of Surfactant Double Layers
d
1 t
6 I
-
'I 0
0
\
9
5
10
2
[CTACI / mM
Figure 1. III/I ratio of the third &d f i t vibronic band of pyrene fluorescence as a function of CTAC concentration in laponite suspension (1 g/L): [pyrene] = 1 X lo* M (O), 5 X lo4 M ( 0 ) ; * indicates the III/I ratio in water.
0' 0
"
"
I
0.5
'
"
"
1
'
CQladded/mM
from ethanol. Hexadecyltrimethylammonium chloride (CTAC) and didodecyldimethylammonium bromide (DDAB) from Eastman Kodak Co, and tridodecylmethylammonium chloride (TDAC) from Polyscience Inc. were used as received. Dimethyldioctodecylammonium bromide (DOAB) from Eastman Kodak Co. was recrystallized 2 times from an acetone and ethyl acetate mixture. Hexadecylpyridinium chloride (CPC) from Sigma Chemical Co. and nitromethane (NM) from Eastman Kodak Co. were used as received. Sodium iodide, sodium bromide, and sodium sulfate were obtained from Fisher Scientific Co. and were used without further purification. Equipment. A PRA LN 100 pulsed N2 laser with pulse duration 500 ps, energy 2.5 mJ/pulse, and wavelength 337.1 nm was used to excite pyrene, and fast (
