Langmuir 1994,10,2814-2816
2814
Methodology for Measuring the Critical Aggregate Concentration of Nonprobe Molecules Jin-Tao Zhang, Jin Nie, Guo-Zhen Ji, and Xi-Kui Jiang* Shanghai Institute of Organic Chemistry, Academia Sinica, 354 Feng-Lin Lu, Shanghai 200032, China Received November 19, 1993. I n Final Form: April 11, 1994@ A general methodologyhas been worked out for measuring the critical aggregate concentration (CAgC) of nonprobe molecules, Le., molecules which are not kinetic or spectral probes. The coaggregationoftarget molecules (A’s), i.e., p-nitrophenyl dodecanoate (C12) and hexadecanoate (C16), have been studied in 1 and n-heptyl pyrenyl ketone dioxane (DX)-H20 mixtures at 35 “C by using n-decyl pyrenyl ketone (e-1) (Py-8)as the fluorescent probes (B’s). The critical coaggregate concentrations of A with B (CoCAgC(A B)’s) have been measured. The CAgC’s oftarget molecules(C12 and C16)are then obtained by extrapolation of the plot of CoCAgC(A B) against probe concentration ([B]). All the CAgC values obtained are in good
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agreement with those measured kinetically. Critical aggregate concentration (CAgC) or critical coaggregate concentration (CoCAgC)is a n indispensible and reliable quantitative measure of the inherent aggregating tendency of a n aggregator (Agr),Le., an organic compound which tends to form simple aggregates (Ag‘s) or coaggregates (CoAg‘s) in a solvent with solvent aggregating power (SAgP).’ It is a basic and important concept which has led to interesting observations. For instance, the relative amounts of cholesteryl stearate, oleate, and linoleate in the arterial plaque have been found to be closely related to their relative coaggregating tendencies evaluated from their CoCAgC data, and systematic CoCAgC measurements of cholesteryl esters have led to the discovery of the important “chain-foldability effect”.1c,2However, most all previous CAgC measurements have been made on probe molecules which carry either hydrolyzable groups (kinetic probes) or fluorophores (fluorescent probes), and this state of affairs has seriously restricted systematic studies of structure-property relationships of the Agr’s because most organic molecules are “nonprobe” molecules. The development of a new methodology for measuring the CAgC of a nonprobe Agr, therefore, has become a ringing challenge that must be met. This paper reports our initial success in taking up this challenge. Our methodology makes use of the coaggregation of the target nonprobe molecule, designated as A, with a kinetic or fluorescent probe molecule, designated as B. Coaggregation of A and the kinetic-probe B will reduce the hydrolytic rate constant of B; coaggregation of A and the fluorescent-probe B will increase the IJImratio, where I , and I, are the fluorescence intensity of B monomer in the absence and presence of A, respectively; coaggregation of A and the fluorescent-probe B can also enhance the ratio Ze/lm, where Z, is the fluorescence intensity of the B-excimer. As Figure 1shows, either the k,dk,, vs [AI plot, where k,, and k,b are the hydrolytic rate constants of kinetic-probe B in the absence and presence of CoAgr A, or the IJIm vs [AI plot, or the IJZm vs [AI plot will give a CoCAgC value of A a t a certain Abstract published in Advance ACS Abstracts, J u n e 15,1994. (1)(a) Jiang, X. IC; Li, X. Y.; Huang, B. Z. Proc. Indian Acad. Sci. (Chem. Sci.) 1987,98,409.(b) Jiang, X. K. Acc. Chem. Res. 1988,21, 362. (c) Jiang, X. K. Plenary Lecture at the 34th IUPAC Congress, August 15-20, 1993,Beijing, and pertinent references cited therein. (2)(a1Jiang,X. K.; Ji, G. Z.; Nie, J.; Zhang, J. T. Chin.J. Chem. 1991, 9,559. (b) Jiang, X. K.; Zhang, J. T.; Tong, 2. H. Chin. Chem. Lett. 1992,3,673,675. (c) Jiang, X. IC;Zhang, J . T.; Tong, Z. H. Chin. J . Chem. 1993,11, 187. (d) Zhang, J.T.; Ji, G. Z.; Jiang, X. K. Chin. J . Chem. 1994,12,277. @
0743-7463/94/2410-2814$04.50/0
L
curve-b
[AI
Figure 1. Evalution of CoCAgC(A + B) from plots of k or I vs [A] at constant probe concentration [Bl. Curve-a: plot of k,d k , vs [A]; curve-b: plot of ZJIm or ZdZmvs [AI. concentration of B. With A always standing for the variable along the x-axis, we define CoCAgC(A B) as the critical coaggregate concentration obtained from the plot of kodkun, IdIm or IJIm on the y-axis against the concentration of the target nonprobe molecule ([AI) on the x-axis a t a constant concentration of the probe B (see Figure 1). Obviously, if a graded series of fixed concentrations of B, i.e., [Bl-a, [Bl-b, [BI-c, etc., are used, then a series of the corresponding CoCAgC(A B) values can be obtained. Yet another approach is to designate A = probe and B = target molecule, and plot k,dk,, against [A] a t a constant BIA molar ratio R. The CoCAgC thus obtained is designated as CoCAgC(A/B). Again, if a graded series of fixed R values are used, then a series of the corresponding CoCAgC(A43)’scan also obtained (cf. ref 2a). The present work, however, uses only the CoCAgC(A B) approach. In recent years we have observed quite a few linear correlations between CAgC’s or CoCAgC’s and some other variable, e.g., (1)chain length of the Agr,2d,3(2) SAgP in terms of 4 values, where 4 is the volume-fraction of the organic component of an aquiorgano binary m i ~ t u r e ; ’ ~ , ~ (3) t e m p e r a t ~ r e(4) ; ~ concentration of the salt additive.6
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(3) (a) Guthrie, J. P. J . Chem. Soc. Chem. Commun. 1972,897.(b) Guthrie, J. P. Can. J . Chem. 1973,51,3494. (4)(a)Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J . Chem. 1992,10,448. (b) Jiantz. X. K.: Shi. J . L.: Chen. X.. unuublished. (5)Zgang, J.’T.; Nie, J.: Sun, S . X.; Ji, G. Z.; Jiang, X. K. Chin. J . Chem. 1994,12,179.
0 1994 American Chemical Society
Critical Aggregate Concentration of Nonprobe Molecules
Langmuir, Vol. 10, No. 8, 1994 2815
t
1.40
1.30 Io/Im
1.20 1.10 1.00
I
I
[BI
Figure 2. Evaluation of CAgC of A from the CoCAgC(A
vs [B] plots.
CoCAgC(A+B)
+ B) c
0
1.0
0.5
1.5
[ C 1 6 ] (lo-%)
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Figure 4. Evaluation of the CoCAgC(A B)value of C16 (A) with Py-11 (B)at [Py-111 = 0.17 x M from the ZJZm vs
[C16] plot. Table 1. Effect of Py-n Concentration on the CoCAgC(A + B)Value@ M) for the Coaggregation of Py-11 with C16, or of Py-8 with C12, in DX-I310 at 35 "C [Py-n] Cn
M)
6 0.25 0.50 0.75 1.0 1.5 2.0 2.5 3.0 4.0 5.0 6.0 8.0
C12 0.25 0.42 0.35 0.27 0.21 0.15 0.09 0.30 1.64 1.46 1.10 0.84 0.34 0.08 C16 0.40b 0.71 0.59 0.50 0.38 0.28 0.24 0.10 0.04 0.44 2.6 2.3 2.1 1.9 1.8 1.5 a
Uncertainty: &5%. [F'y-ll]= 0.17,0.33,0.50,0.67,0.83,1.0, M, respectively.
2.0, 4.0 x
600
400
5 00
nm
Figure 3. The effect of C16 concentrationon the fluorescence spectra of Py-11 in the 6 = 0.40 DX-H20 at 35 "C. [Py-ll]= M): curve-a,0; b, 0.2; M, ,le =, 334 nm. [ClS] 0.17 x c, 0.4; d, 0.6; e, 0.8; f, 1.0; g, 1.2; h, 1.4; i, 1.7; j, 2.0.
Therefore, we naturally would expect that within a certain concentration range of B ([Bl) there would be a linear relationship between CoCAgC(A B) and [Bl. Ifthis line is extrapolated to [Bl = 0, then the value ofthe intersection on the y-axis should be a good approximation of the true CAgC of the target nonprobe molecule A, as illustrated by Figure 2. Obviously, the best way to establish the viability of our proposed methodology is to use, as our target molecule A, a probe molecule whose CAgC has already been reliably measured. If the CAgC value evaluated by the new methodology is quite close to the older known value, then the practicality and generality of our methodology might be considered to be demonstrated. In this work, the target molecules (A's) used are p-nitrophenyl dodecanoate ((312) and hexadecanoate (C16), and the fluorescence probes (B's) used are n-decyl pyrenyl ketone (Py-11) and n-heptyl pyrenyl ketone (Py8). The kinetically determined CAgC's of C12 in the 4 = 0.25 and 0.30 DX-H20 systems are 0.60 x M and 2.0 x M, respectively, and those of C16 in the 4 = 0.40 and 0.44 DX-H20 systems are 0.87 x M and 2.6 x M, respectively.laSb
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Experimental Section Material. Py-n (n = 8,111and C, (n= 12,161were prepared according to refs 7 and 8. All prepared compounds were indentified by 'H-NMR, IR, melting points, and elemental analysis. ( 6 )Jiang, X.
K.; Ji, G. Z.; Tu,B., unpublished.
Solvent. Water was distilled twice and dioxane was purified by standard procedure. Fluorescence measurements were done in the mixture of dioxane (DX) and 0.36 M of NaCl (DX-H20). The 6 values used for C12 are 0.25 and 0.30, and for C16, 0.40 and 0.44. Fluorescence Measurement. Fluorescence spectraof Py-n in the absence and presence of C12 or C16 in DX-H20 mixtures at 35 "C were measured on a Shimadzu RF-51OLC fluorescence
spectrometer or on a Perkin-Elmer LS-50 Luminescence Spectrometer, by using the excitation wavelength of 334 nm.
Results and Discussion The CAgC values of Py-11 in the 4 = 0.40 DX-HzO mixtures were determined to be 4.4 x M and 6.4 x M, respectively,2dand those of Py-8 in the 4 = 0.25 and 0.30 DX-HzO were measured a t 35 "C to be 1.5 x M and 8.0 x M, respectively. Figure 3 illustrates the effect of C16 concentration on the fluorescence spectra of Py-11 ([Py-111 = 0.17 x M) in the 4 = 0.40 DX-HzO system at 35 "C. The fluorescence spectrum ofPy-11 in the absence of C16 gives only the monomer emission a t 420 nm, showing that Py11is in the monomeric state. The fluorescence intensity of monomeric Py-11 decreases with increasing C16 concentration, indicating that coaggregation has occurred between Py-11 and C16. The decrease in fluorescence intensity of monomeric Py-11 is most likely caused by the low polarity of the microenviroment of the coaggregates.2d Another possible cause might be the fluorescence quenching of Py-11 by C16 in the coaggregate~.~ The plot ofIJIm values against [C16]values from Figure 3 is shown in Figure 4, from which the CoCAgC (A B) M. Figure 4 shows that the is measured to be 0.71 x
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(7) Hui,Y. Z.; Fan,W. Q.; Wang, S. J.; Li, M. Z.Actu Chim. Sin. 1982,
40, 1148.
(8)Tong, Z. H.; Ouyang, X. X.; Liu, Y. Y. Photog?. Sci. Photochem. 1988,15. (9) Zhang, J. T.; Cui, X. L.; Ji, G. Z.; Jiang, X. K.; Tong, Z. H. Chin. Chem. Lett. l993,4,707.
Zhang et al.
2816 Langmuir, Vol.10,No.8,1994
Table 2. Comparison of CAgC Values0 M) of C12 and C16 Measured Spectroscopicallyby the Methodology Described in This Work with the Established Values Obtained Kinetically in DX-H20 at 36 "C Cn c12 c12 C16 C16 0.44 0.30 0.40 4 (DX-HzO) 0.25 2.7 1.9 0.79 fluorescence 0.49
4
kinetic& 0
1.0
% 2 . 0e
3.0
4.0
a
IFY-111 (10-5n)
Figure 6. Evaluation of CAgC of C16 (A) from the CoCAgC(A B) vs [Py-111([B]) plots. q5 = 0.40 DX-HzO system,35 "C.
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IdImratio is independent of [C161in the C16 concentration region of 0.2 x M to 0.6 x M, but in the concentration region of 0.8 x M to 1.7 x M, ZdZm is linearly related to [C161by the equation IJI,,, = 3.45 x lo4 [C16] 0.755, with r = 0.997 and n = 5. Dozens of these equations have been obtained from these ZdZmvs [C16] or [C121 plots, from which a series of CoCAgC(A B) values for Py-11 with C16, or for €37-8 with C12, a t graded of Py-n concentrations are obtained in the same way (Table 1). The plot of CoCAgC(A B) vs [Py-111in Figure 5 for the coaggregation of Py-11 with C16 in the q5 = 0.40 DXH 2 0 a t 35 "C shows that in the low Py-11 concentration M to 1.0 x M), the CoCAgC(A region (0.17 x B) is a linear function of [Py-111, i.e., CoCAgC(A B) = -0.583 [Py-111 0.79 x n = 6, r = 0.993. The extrapolation of the straight-line portion of the CoCAgC(A B) vs [Py-111plot until it intersects with the y-axis
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0.60
0.87
2.0
2.6
Uncertainty: f 5 % . Data from refs l a and 2a.
([e-11 =1 0) yields the desired CAgC value of C16, i.e., 0.79 x M. It is in good agreement with the CAgC measured by a completely M e r e n t (kinetic)method, which is 0.87 x M.1a,2a As expected,4b a t higher Py-11 concentrations (1.0 x M to 6.0 x M), the linear relationship between CoCAgC(A B) and [Bl no longer holds. Similarly, three other CAgC values evaluated by the methodology described in this paper for C12 and C16 in DX-H20 mixtures of different q5 values are listed together with the above-mentioned value in Table 2. It shows that the four newly evaluated values are in good or fair agreement with those obtained by the established kinetic method.'" Hopefully, application of this newly developed methodology to all kinds ofnonprobe moleculeswill greatly extend our understanding of the relationship between structure and aggregating tendency for many types of aggregators.
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Acknowledgment. We thank the National Natural Science Foundation of China for financial support.
