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Langmuir 1997, 13, 4234-4238
Two Types of Salt Effects: For Aggregate Formation and for Electrostatically Stabilized Aggregate Formation Ellen Bo Tu, Guo-Zhen Ji, and Xi-Kui Jiang* Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, China Received December 6, 1996. In Final Form: March 6, 1997X The salt effects on aggregate (Agg) formation and the formation of electrostatically stabilized aggregate (ESAgg) have been investigated by means of kinetics and fluorescence spectroscopy. Two types of salt effects have been observed, one “conventional” and the other “unconventional”. For the formation of simple neutral Agg’s, the salt effect which facilitates the Agg formation is similar to the usual salting-out effect; however, for the formation of ESAgg’s, the salt effect is quite different from the usual salting-out effect. The results mentioned above also serve to show that there are structural differences between ESAgg’s and Agg’s.
Introduction Both inherent and external factors may affect the tendency of neutral organic molecules to form simple aggregates (Agg’s). Structure and shape are the main inherent factors.1 External factors of the medium may be epitomized by the term SAggP (solvent aggregating power).2 In order to explore the factors which affect the SAggP, the aggregating or coaggregating tendencies of organic molecules3-5 have been evaluated by means of kinetics or fluorescence spectroscopy in different aquiorgano solvents. Previously, SAggP has been studied from two perspectives, i.e., hydrophobicity of the organic component of an aquiorgano medium (Rekker’s Σf value) and Φ, volume fraction of the organic component of an aquiorgano mixture.3,4 For a particular binary system, the critical aggregate concentration (CAggC) of the aggregator (Agr), i.e., an organic compound that will form simple Agg’s in media with SAggP,5 will increase with increasing Φ (or decreasing SAggP).4c For aquiorgano binary mixtures with different organic components at the same Φ value, the SAggP will decrease with the increasing lipophilicities or deaggregating abilities of the organic components, which are estimable from the Σf values.4a For aquiorgano binary solvent systems, the medium with the highest SAggP is that one with Φ ) 0, i.e., pure H2O. The addition of an organic component can only reduce the SAggP of an aqueous medium. Obviously, it is of interest to investigate the aggregating tendencies of organic molecules when the SAggP of the system is larger than that of pure water. Addition of various amounts of various salts to the aqueous system may be a good way to raise or reduce the SAggP of the system; thus it is also a good way to study the salt effects on the aggregating tendencies of the Agr molecules. It is well-known that one can use solvent effects to alter either the development and course of a reaction (rate, yield, stereochemistry, regioselectivity) or the position of X
Abstract published in Advance ACS Abstracts, July 1, 1997.
(1) Jiang, X. K; Ji, G. Z.; Tu, B.; Zhang, X. Y.; Shi, J. L.; Chen, X. J. Am. Chem. Soc. 1995, 117, 12679. (2) (a) Jiang, X. K. Acc. Chem. Res. 1988, 21, 362 and references cited therein. (b) Tung, C. H.; Xu, C. B. Focus on photochemistry and photophysics; CRC Press Inc.: Boca Raton, FL, 1990; Vol. 4, Chapter 3, p 242. (c) Jiang, X. K. Pure Appl. Chem. 1994, 66, 1621. (3) Jiang, X. K.; Li, X. Y.; Huang, B. Z. Proc. Indian Acad. Sci. (Chem. Sci.) 1987, 98, 409. (4) (a) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. Acta Chim. Sin. 1984, 42, 1276; (Acta Chim. Sin. (Engl. Ed.) 1985, 111. (b) Fan, W. Q.; Jiang, X. K. J. Am. Chem. Soc. 1985, 107, 7680. (c) Jiang, X. K.; Ji, G. Z.; Luo, G. L. Chin. J. Chem. 1991, 9, 453. (5) Jiang, X. K.; Ji, G. Z.; Zhang, J. T. Langmuir 1994, 10, 122.
S0743-7463(96)02077-X CCC: $14.00
chemical equilibrium. However, it is also possible to produce such changes by the addition to the reaction medium of chemically inert salts, and this is called a “salt effect”.6 Different ions of the salt can influence the water structure in different ways. Salt addition can alter interaction of organic molecules by either increasing or decreasing the solubility of the organic compound.7-9 Salting-out or salting-in is the variation of the solubility of organic compounds in water on addition of electrolytes.10 The salting-out coefficient kS is given by the Setschenow equation,11 i.e., log S0/S ) kSc, where S0 and S are the solubilities of the neutral molecule in the pure solvent and the salt solution, respectively, and c is the concentration in moles per liter. It was reported that for the univalent anions the salting-out effect decreases markedly in the order OH- > Cl- > Br- > NO3- > ClO4- > I-, and for the alkali metal cations the order of decreasing saltingout ability is Na+ > K+ > Li+, Rb+ > Cs+.12 The salting-in and salting-out effects can affect the kinetics of organic reactions carried out in water. Such is the case, for example, for Diels-Alder type reactions,13-16 and benzoin condensations17 described by Breslow et al. Thus, these workers observed a remarkable acceleration of reaction of cyclopentadiene with butenone when it is performed in water. This effect was attributed to hydrophobic interactions between the diene and dienophile. They reported that LiCl addition increases the hydrophobic effect through salting-out of the organic molecules from water, and thus accelerates the reaction. Conversely, as a result of salting-in, guanidinium perchlorate addition decreases hydrophobic effects and thus slows the reaction. It has been shown recently that in the concentration range of 10-7-10-5 (or 10-4 M), there is another structural niche for molecular assemblages between the simple Agg and the micelle, namely, the electrostatically stabilized aggregate (ESAgg), consisting of oppositely charged molecules with long flexible chains of roughly 8-16 (6) Loupy, A.; Tchoubar, B. Salt Effect in Organic and Organometallic Chemistry; VCH Verlagsgesellschaft: Weinheim, 1992. (7) Long, F. A.; McDevit, W. F. Chem. Rev. 1952, 52, 119. (8) Sergeeva, V. F. Russ. Chem. Rev. 1965, 34, 309. (9) Conway, B. E. Pure Appl. Chem. 1985, 57, 263. (10) Coetzee, J. F.; Ritchie, C. D. Solute-Solvent Interactions; Marcel Dekker, Inc: New York, 1976; Vol. 2, p 157. (11) Setschenow, M. Ann. Chim. Phys. 1892, 25, 226. (12) McDevit, W. F.; Long, F. A. J. Am. Chem. Soc. 1952, 74, 1773. (13) Rideout, R.; Breslow, D. C. J. Am. Chem. Soc. 1980, 102, 7816. (14) Breslow, R.; Maitra, U.; Rideout, D. Tetrahedron Lett. 1983, 24, 1901. (15) Breslow, R.; Maitra, U. Tetrahedron Lett. 1984, 25, 1239. (16) Breslow, R.; Guo, T. J. Am. Chem. Soc. 1988, 110, 5613. (17) Kool, E. T.; Breslow, R. J. Am. Chem. Soc. 1988, 110, 1596.
© 1997 American Chemical Society
Salt Effects on Aggregate Formation
methylenes.18 The average size of ESAgg has been found to be much smaller than that of a typical micelle.18d Evidently, the added structural feature, i.e., the existence of electrostatic “locks” between oppositely charged end groups, might render the ESAgg assemblages to be structurally different from simple Aggs. Therefore, a second objective of this work is to find out whether there are different salt effects for the formation of Agg and ESAgg assemblages. In the present study, the abovementioned question has been answered by measuring the salt effects on Agg and ESAgg formation processes using both kinetic and fluorescence spectral methods. Two fluorescence probes, namely, the cationic (ω-(2(R-naphthyl)ethoxyl)decyl)trimethylammonium bromide (FP+), and the anionic sodium ω-(2-(R-naphthyl)ethoxyl)undecanoate (FP-), together with the kinetic probe, n-octanoic acid p-nitrophenyl ester, were used in this study. It has been previously demonstrated that comparable amounts of FP+ and FP- will lead to ESAgg formation.18
Experimental Section 1H
Apparatus. NMR spectra were taken on a Varian EM 360 with TMS as the external standard. Mass spectra were obtained by using a Finnigan-4201 or Finnigan-8430 instrument. IR spectra were recorded on a Shimadzu IR-440. The fluorescence probes were prepared by methods reported elsewhere,18c and the kinetic probe was prepared according to ref 19. Salt additives (LiCl, NaCl, KCl, CsCl, LiBr, NaBr, KBr, NaI, KI, LiClO4, NaClO4, Li2SO4, Na2SO4, K2SO4, Na3PO4, Me4NCl, and guanidinium chloride) were analytical reagents and used without further purification. Kinetics. Water was twice distilled and dioxane (DX) was purified by standard procedures.20 NaOH and NaHCO3 (analytical reagents) were used for preparing the buffer solution. Kinetic measurements were made by using a Perkin-Elmer 559 UV-vis spectrophotometer equipped with a thermostat. A solution of specified concentration of the substrate (2.0 × 10-6 to 1.0 × 10-4 M) was prepared by first dissolving an accurately weighed sample in 10 mL of dioxane. This stock solution (ca. 10-2 M) was then diluted (from 100 to 1000 times) to the desired specified concentration ([Agr]i) (2.0 × 10-6 to 1.0 × 10-4 M). All kinetic experiments were performed at 35.0 ( 0.2 °C in aqueous NaOH-NaHCO3 (0.014 and 0.010 M, respectively) buffer solution containing various salt additives by the standard procedure described previously.5,18a,21,22 The increase in absorbance of p-nitrophenolate at 410 nm was then traced as a function of time. Pseudo-first-order rate constants (kob) were obtained in the usual manner. All the rate constants are accurate to within (10%. log kob was plotted against log[Agr]i, and the crossing point of the horizontal and the slanting line was taken as the CAggC value,1,22 as illustrated by Figure 1. Fluorescence Measurement. Emission spectra were recorded by a Perkin-Elmer LS-50 fluorescence spectrometer in (18) (a) Jiang, X. K.; Ji, G. Z.; Wang, J. S. Chin. Chem. Lett. 1991, 2, 813. (b) Jiang, X. K.; Ji, G. Z.; Wang, J. S. Chin. Chem. Lett. 1992, 3, 231. (c) Jiang, X. K.; Wang, J. S. Chin. J. Chem. 1993, 11, 472. (d) Zhang, J. T.; Tu, B.; Ji, G. Z.; Jiang, X. K. Chin. Chem. Lett. 1993, 4, 879. (e) Wang, J. S.; Zhang, H. Z.; Huang, W. Y.; Jiang, X. K. Tetrahedron 1994, 50, 10459. (19) Bodanszky, M.; et al. J. Am. Chem. Soc. 1959, 81, 5688. (20) Perrin, D. D.; et al. Purification of Laboratory Chemical, 2nd ed.; Pergamon, Oxford, 1980. (21) Jiang, X. K.; Hui, Y. Z.; Fan, W. Q. J. Am. Chem. Soc. 1984, 106, 3839. (22) Zhang, J. T.; Nie, J.; Ji, G. Z.; Jiang, X. K. Langmuir 1994, 10, 2814.
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Figure 1. log kob vs log[Agr]i plots for the hydrolysis of p-nitrophenyl octanoate in H2O at 35 °C in the absence of added salts. aqueous solution at 25 °C, using the excitation wavelength of 284 nm. Both FP+ and FP- have fluorescence λmax’s at 337 nm (monomer) and 420 nm (excimer); thus the latter was used to monitor excimer formation from FP+ or FP-. When the probes FP+ and FP- were used in the concentration range of 10-7-10-5 M in H2O, they only show 337 nm emission, and the emission intensity was in direct proportion to the concentration; thus both probes FP+ and FP- are in the monomeric state in this concentration range. In the presence of different salts at constant ionic strength (1.5), we measured the fluorescence spectra of FP+ and FP- at a fixed molar ratio ([FP+]:[FP-] ) 1:1). For illustration, Figure 2 shows the emission spectra of the FP+-FP- (1:1) system at different concentrations of the two probes, in the absence of salts. If we define R ) Ie/(Ie + Im), where Ie and Im are the fluorescence intensities at λmax of the excimer and monomer, respectively, and plot R against [FP+], then the crossing point of the horizontal and the slanting line can be taken as the critical concentration for coaggregate formation, i.e., CoCAggC, of FP+ and FP-. This is exemplified by Figure 3, which shows the plot of R vs [FP+] (or [FP-]) in the absence of salts.
Results and Discussion Salt Effect on the Formation of Simple Neutral Agg’s. Ionic strength (I, as calculated by I ) 1/2ΣCiZi2)23 represents a basic property of an aqueous solution of ionic salts; therefore, our CAggC measurements were performed at constant I. Owing to the limit of solubility of some salts, e.g., Na2SO4, our I value was fixed at 1.5 for all our measurements unless specified otherwise. CAggC values of p-nitrophenyl octanoate in H2O at 35 °C in the presence of different types of added salts at constant ionic strength (1.5) were measured as mentioned in the Experimental Section. We define ∆CAggC ) CAggC(0) - CAggC, where CAggC is the CAggC value of p-nitrophenyl octanoate in the presence of added salt and CAggC(0) is that in the absence of added salt. Therefore, a larger ∆CAggC value signifies a greater ability of the salt to facilitate Agg formation. The CAggC and ∆CAggC values are summarized in Table 1. At constant ionic strength (I ) 1.5), the order of the ability of the cations to increase the ∆CAggC value, i.e., to increase the SAggP and facilitate Agg formation is as follows: for Cl- salts, Na+, Li+ > K+ > Cs+; for Br- salts, Na+ > Li+ > K+; for I- salts, Na+ > K+*; for ClO4- salts, Na+ > Li+*; for SO42- salts, Na+ > Li+ > K+. Ability of anions to increase the ∆CAggC value is as follows: for Li+ salts, Cl-, SO42- > Br- > (ClO4-)*; for Na+ salts, Cl-, SO42- > Br- > PO43- > I- > ClO4-; for K+ salts, SO42-, Cl- > Br- > I-*, where the asterisk stands for a hindering effect for Agg formation. Quite similar orders of salt effects (23) Adamson, A. W. A Textbook of Physical Chemistry; Academic Press, Inc.: New York, 1973; p 518.
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Figure 2. Emission spectra of the FP+-FP- (1:1) system at different concentrations of the two probes in H2O at 25 °C.
Figure 3. R vs [FP] plots in H2O at 25 °C for the FP+-FP(1:1) system.
were reported by McDevit and Long;12 they measured the solubilities of benzene at 25 °C in water and in a number of electrolyte solutions and observed that for the univalent anions the salting-out effect decreases markedly in the order OH- > Cl- > Br- > NO3- > ClO4- > I- and for the alkali metal cations the order of decreasing salting-out effect is Na+ > K+ > Li+, Rb+ > Cs+. It was noted that salts of large ions and also perchlorate cause salting-in of benzene. In our experiment, quite similar results were observed, i.e., for salt additives with the same anions, Na+ ions show the largest facilitating effect for Agg formation while Li+ ions show a smaller effect, and the effect of K+ ion seems to be always smaller than that of the Na+ ion. Similar results were reported by Schott et al.24 and Maeda et al.25 Schott observed that Li+ salted-in the surfactants while Na+, K+, Cs+, and NH4+ salted-out the surfactants. Maeda reported that the transport rate of poly(vinylpyrrolidone) in aqueous dextran matrix was enhanced by the addition of CsCl or NaCl, but not by LiCl. (24) Schott, T.; Royce, A. E.; Han, S. K. J. Colloid Interface Sci. 1984, 98, 196. (25) Maeda, H.; Mashita, T.; Saaski, S. Chem. Lett. 1991, 635.
Table 1 shows that at constant ionic strength, the order of the ability of the univalent anions to facilitate Agg formation, or to increase the ∆CAggC value, is Cl- > Br> I- > ClO4-. In other words, the smaller the anion, the greater the ability of the anion to increase the SAggP of the medium. Under the same condition, the divalent anion SO42- shows a greater ability to increase the ∆CAggC values (almost the same as Cl-). It seems that sometimes multivalent ions may show a greater facilitating ability for Agg formation than do the univalent ions. In order to seek supporting evidence for this proposition, we examined the effects of univalent and divalent anions on the CAggC values of the kinetic probe at constant cation concentration. As shown in Table 2, the order of the ability of the anions to increase the ∆CAggC value at 1.0 M [Na+] is SO42- > Cl- and at 0.75 M [Na+] is SO42- > (PO43-) > Cl-. In agreement with the above-mentioned speculation, the divalent anion SO42- possesses a greater ability to increase the ∆CAggC value of the probe than the univalent anion Cl-. Owing to the fact that PO43- ion may hydrolyze to HPO42- and thus the true concentration of PO43- ion would be less than 0.25 M, we do not know for sure whether the PO43- ion is truly less effective than the SO42- ion for the facilitation of Agg formation. In order to examine the dependence of the CAggC on salt concentration, we measured the CAggC values of the kinetic probe in the presence of different concentrations of NaCl, as shown in Figure 4. Plotting the CAggC values against the concentrations of the NaCl yielded a straight line, as described by the equation: CAggC ) -0.77 × 10-5 [NaCl] + 1.57 × 10-5, r ) 0.986, n ) 5. The linear correlation also demonstrates the reliability of the CAggC measurements. All the above-mentioned results show that, in general, for the formation of simple neutral aggregates (Agg’s), the salt effect which facilitates the Agg formation is rather similar to the usual salting-out effect. Salt Effect on the ESAgg Formation. 1. Salt effect on the ESAgg formation of two fluorescence probes FP+ and FP-. In likeness to the previously mentioned approach, we define ∆CoCAggC ) CoCAggC(0) - Co-
Salt Effects on Aggregate Formation
Langmuir, Vol. 13, No. 16, 1997 4237
Table 1. CAggC (10-5 M) and ∆CAggC (10-5 M) Values of p-Nitrophenyl Octanoate in H2O at 35 °C, in the Presence of Different Added Salts at Constant Ionic Strength (1.5) salt
CAggC
∆CAggC
salt
CAggC
∆CAggC
none 1.5 M LiCl 1.5 M NaCl 1.5 M KCl 1.5 M CsCl 1.5 M LiBr 1.5 M NaBr 1.5 M KBr
1.66 ( 0.05 0.50 ( 0.02 0.49 ( 0.02 0.68 ( 0.03 0.91 ( 0.05 0.87 ( 0.03 0.71 ( 0.03 1.02 ( 0.03
0.00 1.16 1.17 0.98 0.73 0.79 0.95 0.64
1.5 M NaI 1.5 M KI 1.5 M LiClO4 1.5 M NaClO4 0.5 M Li2SO4 0.5 M Na2SO4 0.5 M K2SO4 0.25 M Na3PO4
1.41 ( 0.03 1.86 ( 0.07 1.95 ( 0.05 1.58 ( 0.04 0.56 ( 0.02 0.49 ( 0.02 0.62 ( 0.04 0.81 ( 0.02
0.25 -0.20 -0.29 0.08 1.10 1.17 1.04 0.85
Table 2. CAggC (10-5 M) and ∆CAggC (10-5 M) Values of p-Nitrophenyl Octanoate in H2O at 35 °C, in the Presence of Sodium Salts of Cl-, SO42-, and PO43- at Constant Cation Concentration [Na+] ) 1.0 M
[Na+] ) 0.75 M
salt
CAggC
∆CAggC
salt
CAggC
∆CAggC
0.5 M Na2SO4 1.0 M NaCl
0.49 ( 0.02 0.76 ( 0.03
1.17 0.90
0.25 M Na3PO4 0.375 Na2SO4 0.75 M NaCl
0.81 ( 0.02 0.56 ( 0.04 0.91 ( 0.02
0.85 1.10 0.75
Table 3. CoCAggC (10-5 M) and ∆CoCAggC (10-5 M) Values of the FP+-FP- (1:1) System in H2O, 25 °C in the Presence of Different Added Salts at Constant Ionic Strength (1.5) salt
CoCAggC
∆CoCAggC
salt
CoCAggC
∆CoCAggC
none 1.5 M LiCl 1.5 M NaCl 1.5 M KCl 1.5 M LiBr 1.5 M NaBr 1.5 M KBr
1.54 ( 0.05 1.42 ( 0.04 0.42 ( 0.02 0.40 ( 0.02 0.72 ( 0.02 0.70 ( 0.02 0.66 ( 0.02
0.0 0.12 1.12 1.14 0.82 0.84 0.88
0.5 M Li2SO4 0.5 M Na2SO4 0.5 M K2SO4 1.5 M LiClO4 1.5 M GnCl 1.5 M N+Me4Cl-
0.40 ( 0.04 0.58 ( 0.02 0.20 ( 0.04 0.82 ( 0.10 0.80 ( 0.08 2.60 ( 0.10
1.14 0.96 1.34 0.72 0.74 -1.06
Table 4. CoCAggC (10-6 M) and ∆CoCAggC (10-6 M) Values of the FP--S 16+ System in H2O, 25 °C in the Presence of Different Added Salts at Constant Ionic Strength (1.5) and Constant [FP-] (1.0 × 10-5 M) salt
CoCAggC
∆CoCAggC
salt
CoCAggC
∆CoCAggC
none 1.5 M LiCl 1.5 M NaCl 1.5 M KCl
4.0 ( 0.7 6.0 ( 0.6 1.0 ( 0.05 0.87 ( 0.05
0.0 -2.0 3.0 3.1
1.5 M LiBr 1.5 M NaBr 1.5 M KBr 1.5 M Me4NCl
1.7 ( 0.2 0.91 ( 0.05 0.60 ( 0.09 9.1 ( 1.0
2.3 3.1 3.4 -5.1
Figure 4. Linear dependence of CAggC on the concentration of NaCl.
CAggC, where CoCAggC is the CoCAggC value of FP+ and FP- in the presence of added salts, and CoCAggC(0) is that in the absence of added salt. The CoCAggC and ∆CoCAggC values are listed in Table 3. From Table 3 it can be seen that the orders of the ability of the cations to increase the ∆CoCAggC values are quite different from those for the Agg formation. The most conspicuous difference between the two sets of data is the effect of K+, i.e., for Agg formation the ability of K+ to facilitate the formation process is smaller or much smaller than that of Na+ (and Li+), but for ESAgg formation, the ability of K+ to facilitate the formation process becomes comparable to, or even slightly larger than that for Na+ (and Li+) (actually it occupies the top effectiveness position in all the five sets of our data summarized in Tables 3 and 4). Apparently, since ESAgg is made up of oppositely
charged long-chain molecules, while Agg is made up of simple neutral molecules, differences in structural features between the two types of molecular assemblages could be the main cause of the differences in salt effects. In order to obtain additional evidence for the aforesaid argument, we further examined the salt effects of three special salts, i.e., LiClO4, Me4NCl, and GnCl (guanidinium chloride), because they were found to be salting-in agents.26 Interestingly, it was found that only Me4NCl hinders ESAgg formation, but both ClO4- and Gn+ ions facilitate ESAgg formation, a behavior usually not expected for salting-in additives. Apparently, the net effect of an added salt on ESAgg formation is the result of subtle and complicated interactions among several contributing factors, e.g., size and polarizability of the added cations and anions, and differences in their binding interactions with the organic probe molecules for the different salt ions, as well as the hydrophobicity, polarizability, and shape of the organic probe molecules. However, although we are not able to rationalize our results in more detailed or theoretical terms, we are still able to conclude that two types of salt effects have been observed: one for Agg formation and one for ESAgg formation. 2. Salt Effect on ESAgg Formation from the FP-S-16+ Pair. It has been previously established that fluorescence probe molecules (FP- or FP+) can pair up with surfactant molecules (S-n- or S-n+) of opposite charge to form ESAgg’s.18c Therefore, with an objective to demonstrate that our observed salt-effects on the FP-FP+ pair are not trivial or fortuitous, we measured the (26) Breslow, R.; Guo, T. Proc. Natl. Acad. Sci. U.S.A. 1990, 87, 167.
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Figure 5. Effect of the concentration of S-16+ on the emission spectra of FP- in H2O at 25 °C, [FP-] ) 1.0 × 10-5 M.
fluorescence spectra of FP- with increasing concentrations of S-16+, i.e., cetyltrimethylammonium bromide, in the presence of various added salts at constant ionic strength (1.5). Emission spectra of FP- with increasing concentrations of S-16+ in the absence of salts are shown in Figure 5. Again, as described before, by plotting R against log [S-16+] ([S-16+], concentration of S-16+), at constant concentration of FP- ([FP-] ) 1.0 × 10-5 M), ∆CoCAggC values at constant [FP-] have been evaluated and summarized in Table 4. Our data show, for Cl- salts, the order of the ability of the cations to facilitate the FP--S-16+ ESAgg formation is K+, Na+ > Li+*, where the asterisk sign again stands for a hindering effect. This order is rather similar to that observed for the FP--FP+ pair. Particularly noteworthy is the fact that while Li+ is usually a salting-out agent, here it becomes a retarding agent for ESAgg formation. Perhaps Li+ and Cl- ions are too small to fit in the FP-S-16+ ESAgg assemblage, although its size suits Agg’s pretty well. This notion is in harmony with our data for the bromides. Although the LiBr salt is no longer a retarding agent for the FP--S-16+ ESAgg formation, the order of facilitating abilities remains K+ > Na+ > Li+. Certainly, if we suspect that some ions are too small, we should also suspect that some ions would be too large; in other words, we should expect ions (positive and negative)
of proper (or optimum) sizes (e.g., K+ and Br- for the FP-S-16+ pair) would be the most effective ions in promoting a certain ESAgg species. This expectation seems to have been materialized again by the exceptionally large hindering effect of the Me4N+ ion (∆CoCAggC ) -5.1) for the FP--S-16+ ESAgg formation. In conclusion, in studying salt effects on Agg and ESAgg formation, we have found two types of salt effects, i.e., one conventional and the other nonconventional and rather novel. For Agg formation, the effect bears resemblance to the classical salting-out effect; for ESAgg formation, the effect is quite different from the conventional. The existence of two types of salt effects also indicates that the ESAgg concept is well-founded. Quite likely, ESAgg’s possess a higher degree of structural complexity. In short, among other factors, e.g., charge, polarizability, flexibility, shape, etc., of the probe molecules and salt ions, proper sizes for the added salt ions are also important factors in the facilitation of the formation of molecular assemblages in solvents with SAggP. Acknowledgment. We thank the National Natural Science Foundation of China for financial support. LA9620770
