4237
MICELLAR EFFECTS ON ACIDITY FUNCTIONS
Micellar Effects on Acidity Functions' by C. A. Bunton and L. Robinson Department of Chemi8try, University of California at Santa Barbara, Santa Barbara, California 98106 (Received May 9, 1969)
An anionic detergent, sodium lauryl sulfate (NaLS), increases -HR, based on the ionization of tri-p-anisylmethanol, by up to 2.5 units, and -Ho', based on the protonation of p-nitroaniline or 1-amino-4-nitronaphthalene, by up t,o 1.0 unit. Salts decrease the micellar effecton these acidity functions in the sequence: nosalt > LiCl = NaBr = NaCl >> (CH&NCl. Added cetyltrimethylammonium bromide (CTABr) decreases both -HR and -Hot, with 10e3M CTABr decreasing -HR by 1.4 units and 10-2 M CTABr decreasing -Ho' by0.3 unit in 0.1 M' HC1. These results show that anionic micelles stabilize and cationic micelles destabilize a triaryl cation much more than an ammonium ion.
Introduction Long chain molecules or ions can aggregate in water to form micelles in which the polar or ionic end is exposed to the solvent. Ionic micelles are typically surrounded by a sheath of counterions, and therefore the ability of a micelle to incorporate organic substrate and to attract or repel ionic reagents at its surface changes the probability of many ion-molecule reaction~.~-' The acidity a t the surface of an ionic micelle should be different from that in the bulk of the solution, and it has been shown that the ionization of indicators is different in the micellar and aqueous phasesls and the apparent pK, of a Schiff's base is larger in an anionic and smaller in a cationic micelle than in water, based on the measured pH in the aqueous solution.S However, micellar effects upon the ionization of an indicator should depend not only upon the acidity, as measured by hydrogen ion activity or pH, but also upon the activity coefficients of the indicator and its conjugate acid or base in the micellar as compared with the aqueous phase (cf. ref 9). We can test this possibility by examining the protonation of two different bases, e.y., a primary amine and a tertiary alcohol, which have been used to establish the Ho' and the HRscales.10-12
+
Ho' ArNH2
HO'
= PKs
+ H + E ArNHs
- log(CBH+/CB)
HR ArsCOH
+ H+
-log(aH+fB/fBH+) (B = ArNH2)
+ Ar3C
+ HzO
(ROH = Ar3COH)
For a given solution the hydrogen ion activity will depend upon the presence of micelles, but not upon the indicator provided that it is in very low concentration,
and therefore by using different indicator systems we can find the extent to which interactions between the micelles and indicator bases and conjugate acids are important. We used tri-p-anisylmethanol as the HR indicator, and p-nitroaniline and 1-amino-bnitronaphthalene as Hammett indicators. The naphthylamine has not been used as a Hammett indicator before, but we used it in this work so that we could observe any effect of a bulky organic residue in the base. Our interest in this problem was stimulated by the fact that the relation between the various acidity scales depends on the strong acid, for example the difference between the HR and Ho' scales is greater for perchloric than sulfuric or hydrochloric acid. l 2 These differences have been shown to arise from the ability of a low charge density monoanion, such as perchlorate, to stabilize a carbonium relative to an anilinium ion, whereas a high charge density monoanion, such as chloride, does not have this ability.13 It therefore seemed probable that ionic micelles should have differential effects upon the equilibria between indicators and conjugate acids of different chemical structure. It is also (1) Support of this work by the National Science Foundation is grate fully acknowledged. (2) E. F. Duynstee and E. Grunwald, J . Amer. Chem. SOC.,81,4540, 4542 (1959). (3) M. T.A. Behme and E. H. Cordes, {bid., 87,260 (1965); M. T.A. Behme, J. G. Fullington, R. Noel, and E. H. Cordes, ibid., 87, 266 (1965). (4) F. M. Menger and C. E. Portnoy, ibid., 89,4698 (1967). (5) L.R. Romsted and E. H. Cordes, ibid., 90,4404 (1968). (6) C.A. Bunton, E. J. Fendler, L. Sepulveda, and K-U. Yang, ibid., 90,5612(1968). (7) C. A. Bunton and L. Robinson, ibid., 90,5972 (1968). (8) P. Mukerjee and K. Banerjee, J . Phys. Chem., 68, 3567 (1964). (9) C. A. Bunton, L. Robinson, and L. Sepulveda, J . Amer. Chem. Soc., 91,4813 (1969). (10) M. A. Paul and F. A. Long, Chem. Rev., 57,l (1957). (11) N.C. Deno, J. J. Jaruszelski, and A. Schreisheim, J . Amer. Chem. SOC.,77,3044 (1955). (12) E.M. Arnett and G. W.Mach, {bid., 88,1177 (1966). (13) C. A. Bunton, J. H. Crabtree, and L. Robinson, ibid, 90, 1258 (1968). Volume 78, Number 19 December 1969
4238
C. A. BUNTONAND L. ROBINSON
possible that ionic micelles, like simple electrolytes, could affect the rates of A1 and A2 reactions differently,13 but this aspect of the problem could not be studied until the indicator equilibria were understood.
Experimental Section Materials. The preparation and purification of most of the indicators and the detergents has been de~ c r i b e d . ~ Jl-Amino-4-nitronaphthalene ~ had mp 191” (ref 14, 191”), after recrystallization from aqueous ethanol. Measurement of Indicator Ratios. The indicator ratios, I = CB/CBH+or CROH/CR+ were measured spectrophotometrically,10-12 using a Gilford spectroThe wavelength! photometer as already de:cribed.la were approximately 3800 A for p-nitroaniline, 4420 A for 1-amino-4-nitronaphthalene and 4820 A for tri-panisylmethanol. For each measurement, a range of wavelengths was used to allow for spectral shifts caused by the detergent. For each acid concentration, the absorbances at A,, were measured for the acid-detergent mixture and for the detergent alone; the differences between these values were taken as the absorbance of the colored species. The indicator concentrations were: p-nitroaniline, 3 X 10-5 M ; 1-amino-4-nitronaphM ; tri-p-anisylmethanol, 3 X thalene, 2.5 X
M. One serious problem in these measurements is that it is virtually impossible to follow a greater than tenfold change in the value of I.’O This problem is not particularly serious for the measurements of Ho’ using the nitroamines because the detergents do not have very large effects upon the protonation of the indicators. However, the detergent effects upon the ionization of trianisylmethanol are so large that with 0.1-0.15 M acid, relatively low concentrations of NaLS are sufficient to lead to essentially complete ionization of the alcohol, and therefore we could not measure the maximum increase in -HR which could be brought about by the detergent. In order to avoid this problem, we studied the effect of NaLS upon --HE in both 0.01 and 0.1 M HC1, although in the absence of detergent, the value of I is inconveniently high in 0.01 M HC1. However, the experimental value of I for tri-p-anisylmethanol in 0.01 M HC1 is almost exactly 10 times that in 0.1 M HCl. With trianisylmethanol in 0.1 M HC1 the ionization is almost completely suppressed by M CTABr, and useful determinations of I could not be made at higher detergent concentrations. So far as we know the pK, value of 1-amino-4-nitronaphthalene has not been determined. We measured it spectrophotometrically1° using 0.1-1.2 M HC1. A plot of log(cHB+/CB’ CH+)against CHCIwas linear and exactly parallel to the corresponding plot for p-nitromilinelo and extrapolated to pK. = 0.36, as compared with 0.98-1.00 for p-nitroaniline.’O This difference in The Journal of Physical Chemistry
Cot M
Figure 1. Variation of Ho’with detergent at 25’. Solid points 0.1 M HC1; open points 0.01 M HCl: CTABr with p-nitroaniline; 0 , 0, NaLS with p-nitroaniline; W, 0, NaLS with 1-amino-4-nitronaphthalene.
+,
basicity is understandable in terms of the electronic effects of a phenyl as compared with a naphthyl group. Solubility Measurements. The solubilities of the indicator bases in aqueous detergents were measured by saturating the detergent solutions with the indicators by leaving the solutions for several days a t 25.0” with occasional shaking.’ The solutions were then filtered. For the experiments with the nitroamines, portions of the solution were diluted with water to bring the detergent concentrations below the cmc (critical micelle concentration), and the nitroamines were determined spectrophotometrically. For tri-p-anisylmethanol the solutions were diluted with aqueous acid, sufficient to ionize the alcohol completely. Sulfuric acid was used with CTABr and perchloric with NaLS. Control tests were carried out to make sure that Beer’s law was obeyed in these detergent solutions.
Results The values of log I and AHo’ are given for p-nitroaniline and 1-amino-4-nitronaphthalene in 0.01 and 0.1 M HC1 in Table I and Figure 1. The values of log I are given to show the range of indicator ratios used in the work. The values of -A-Ho’ increase to a plateau (14) C. C. Price and S-T.Voong, “Organic Syntheses,” Coll. Vol. 111, John Wiley & Sons, Inc., New York, N.Y., 1955, p 664.
4239
MICELLAR EFFECTS ON ACIDITYFUNCTIONS Table I: Effect of NaLS upon H,'"
-
Aaid lo*cD,
M
0.01 M HC1
AH^'^
Log Ib
0.00 1.oo 1.50 2.00 2.50 3.00 5.00 7.50 10.0 20.0 30.0 40.0 50.0
0.97
-0.05
1.02
-AHo"
1.62d 1.58
0.04
1.50
0.78
0.19
0.35 0.14 0.12 0.13 0.16
0.62 0.83 0.85 0.84 0.81
' I n aqueous acid a t 25.0'.
Log IC
0.94 0.52 0.41 0.41 0.49 0.57
p-Nitroaniline.
Log Ib
-0.03 -0.03 -0.04
0.68 1.10 1.21 1.21 1.13 1.05
1-Amino-4-nitronaphthalene.
M
0.00 0.10 0.25 0.30 0.50 0.70 0.75 1.oo 2.00 3.00 4.50 5.00 6.00 7.50 9.00 a
2.5 M HClLogI - A H R
-0.01
-0.1 M HC1LogI -AHR
0.25 0.44 0.47
1.27
0.19 1.25 0.78 0.61 -0.43 -1.09 -1.29 -1.23 -1.34
0.02 0.49 0.66 1.70 2.36 2.56 2.50 2.61
-0.10 -0.67
-0.19 -0.22 0.06 0.35 0.92
0.00
-0.07
0.02 0.04
-0.14 -0.20 -0.26 -0.31 -0.45 -0.51 -0.56 -0.55
0.11 0.17 0.23 0.28 0.42 0.48 0.53 0.52
0.12
Table 11: Effect of NaLS upon HR' Acid 10'CD,
0.10 M HCl Log IC
AH^'^
M HClOdLogI AHR
-0.16
-0.17 -0.17
0.00
-0.23 -0.34 -0.64
0.06 0.17 0.47
-1.32
1.15
7
- AHo"
0.62 0.62 0.48
0.00 0.14
0.24
0.38
0.03 -0.06
0.59 0.68 0.72 0.80 0.80 0.80
-0.10
-0.18 -0.18 -0.18
Calculated value.
t
I n aqueous acid a t 25.0'.
and in some cases give a slight maximum. Relatively high concentrations of detergent could be used with these indicators, so that the indicator concentrations were always less than those of the detergents, and the detergent concentrations were generally considerably greater than the cmc. For NaLS the cmc = 4 X M in water,l5 and 2.5 X M in 0.05 M NaOH,' and the cmc should be considerably lower in 0.1 M than in 0.01 M acid because an increase in ionic strength decreases the cmc of an ionic detergent by stabilising the ionic micelle.16 The results given in Table I1 and Figure 2 show that in 0.1 M HC1 and 0.15 M HClOl - H R is increased sharply by relatively low concentrations of NaLS, but with 0.01 M HC1 where the cmc is higher than in 0.1 M HC1 the values of - A H g do not increase sharply until the detergent concentration becomes close to the cmc. However, the acidities in-
Cot M
Figure 2. Variation of H R with detergent a t 25' using tri-p-anisylmethanol: el CTABr in 0.1 M HC1; . , NaLS in 0.15 M HC10,; 0, NaLS in 0.1 M HC1; 0, NaLS in 0.01 M HC1.
crease at detergent concentrations below the cmc, possibly submicellar aggregates can stabilize the organic cations, just as they can be catalytically effective in reaction^.^^^^^^'^ In addition organic solutes can pro(16) M. L. Corrin and w. D. Harkins, J . Amer. Chew. SOC.,69, 679 (1947). (16) P. H. Elworthy, A. T. Florence, and C. E.Macfarlane, "Bolubilization by Surface-Active Agents," Chapman and Hall, London, 1968, Chapter I. (17) T. C. Bruice, J. Katzhendler, and L. R,Fedor, J . Amer. Chem. Soc., 90,1333(1968). Volume 73,Number 1.9 December 196.9
4240
C.A. BUNTON AND L. ROBINSON
mote micellization. lS One unexpected observation was that with 0.1 M HC1 NaLS in low concentration at first decreases - AHR. We do not understand this anomaly, but effects on the cmc of the detergent may be important. In all the systems which we have examined the plateau values of - AHO' or - AHR are greater for 0.01 than for 0.1 M HC1, simply because when there are sufficient protons in the solution to occupy the Stern layer of the detergent, additional protons have very little effect, and the increase in ionic strength should increase the aggregation number of the micelle and therefore decrease the coneentration of micelles for a given detergent concentration. Both -H I o and -H R are decreased by the cationic detergent CTABr (Table I11 and Figures 1 and 2). The cmc for this detergent is 7.8 X M in water and 3.2 X M in 0.05 M NaOH.' The effects of added salts are shown in Table IV.
creased by steric hindrance to hydration of the hydroxyl group by the aryl groups, whereas there should be little steric hindrance to hydration of the amino group. In other systems we have used solubility measurements to determine the equilibrium constant for partitioning of the solute between the aqueous and micellar phases.l The treatment requires the assumption that the solute
/'
/ /
9 100
Log Ib
0.00 0.10 0.25 0.50 0.75 1.00 2.50 5.00 7.50 10.0 20.0 30.0 40.0 50.0
-0.03
AHP
/
/
/
/
I
80
/
?
,,l uv-
Table I11 : Effect of CTABr and Ha' and H R ~ 103CD,M
-
Log I Q
/
I
/
/ A .I / /-
/'
AHR %O
0.01
0.04
0.01 0.06 0.20 0.25 0.31 0.56 0.72 0.75 0.74
0.04 0.09 0.23 0.28 0.34 0.59 0.75 0.78 0.77
In 0.10 M HCl at 25.0'. methanol.
0.25 0.28 0.47 0.97 1.35 1.66
0.03 0.22 0.72 1.10 1.41
40
-
20
I
0.01
0.02
0.03
0.04
I
0.05
Cot M
For p-nitroardine.
'For trianisyl-
Figure 3. Solubilities of the indicators in detergent solutions at 250, Left hand scale is for tri-P-anisylmethano1 and 1-amino-4-nitronaphthalene; right hand scale is for p-nitroaniline: 0, 1-amino-4-nitronaphthalene in NaLS; 0, tri-p-anisylmethanol in CTABr, and 0 in NaLS; 0, p-nitroaniline in CTABr, and in NaLS.
+
Table IV : Effect of Salts upon AH@'and AHR"
--
Indicator--.
p-nitroanilineb
Salt
LogI
-AHQ'
tri-p-anisylmethano10 Log I -A H R
LiCl , NaCl (CH3)rNCl NaBr
0.12 0.21 0.23 0.40 0.21
0.85 0.76 0.74 0.57 0.76
-1.29 -0.87 -0.76 0.16 -0.80
With 0.03 M salt in 0.01 M HC1 and NaLS at 25.0". NaLS. e 0.006 M NaLS. a
2.56 2.14 2.03 1.11 2.07
* 0.03 M
Solubilities. The detergents solubilize both the nitromines and tri-p-anisylmethanol (Figure 3), but the effect is much greater with the tertiary alcohol, probbecause Of its higher molecular weight* In tion, the solubility of the alcohol in water will be deT h e Journal of Physical Chemistry
does not change the nature of the micelle significantly, and this assumption may not be valid for tri-p-anisylmethanol because the overall concentration of alcohol in 5 X M NaLS is 6 X M , and in 5 X M CTABr it is 9 X M , and we cannot assume that the solute will not affect the properties of the micelle.
Discussion Relation between Acidity and Detergent Concentration. At a given acid concentration, the values of -Hot and -H R reach plateaus with increasing detergent concentrations, and these limiting values should be reached when the base and its conjugate acid are wholly in the micellar At lower concentrations of deter(18) P.Mukerjee and K. J. Mysels, J . Amer. Chem. Soc., 77, 2937 (195s).
MICELLAR EFFECTS ON ACIDITY FUNCTIONS
(Dn = micellized detergent) gent, the values of -Ho’and -HRwill depend in part upon the solubilities of the bases and conjugate acids in the aqueous and micellar phases, and on the way in which the micelles affect the properties of water and hence, the equilibria in the aqueous phase, and the relations between -Ho’and -HRand detergent concentrations will be very complicated. For these reasons it is the values of -Hot and -HR in the plateau region which are most informative. We could not use one concentration of acid for all our experiments, but NaLS increases -HRmuch more than -Ho’ in the plateau region (Figures 1 and 2). The difference must depend upon the relative stabilities of the bases and their conjugate acids in the micellar phase, and it is probable that the triarylmethyl cation gains more stability in the micellar phase than does the anilinium ion. An ammonium ion gains much of its stability in water by hydrogen bonding, which would be lost, at least in part, if the positive charge were to be close to the surface of the anionic micelle. On the other hand, the large carbonium ion, in which the charge is delocalized over aryl groups, should not have a large solvation energy, and should gain considerable stability by incorporation into the micelle.l9 Anionic micellar effects upon go’and HR are therefore similar to, but much larger than, the effects of a large, low charge density anion, such as perchlorate, which increases -HR much more than -Ho.13 This similarity does not necessarily mean that there is direct association between carbonium and perchlorate ions in water, but in a 1 M lithium or sodium perchlorate solution, a large amount of water will be closely bound to the inorganic cation, and average interionic distances will be relatively small, even for free ions,21and the perchlorate and carbonium ions will be surrounded by loosely held layers of water which will be released if the ions come close enough. The free energy gained by the increasing entropy of the water, plus the electrostatic interaction of the ions, could offset the loss of hydration energy of the ions or ionic micelles. (This association is therefore analogous to hydrophobic bonding between nonpolar solutes.)22 The increase of -H0’ (1.05 as compared with 0.85 for 0.01 M HC1) in NaLS is only slightly greater when 1-amino-4-nitronaphthalene rather than p-nitroaniline is used as the indicator, showing that the size of the organic residue is not particularly important ; in agreement with our assumption that we are seeing a difference between the two indicator systems which depends on the difference between a carbonium and an anilinium ion.
4241 The decrease of acidity brought about by the cationic detergent, CTABr, is easily understood, because incorporation of the base into the micellar phase will protect it from protonation and the ammonium or carbonium ions will be less stable in the micellar as compared with the aqueous Our experiments show that we cannot use indicator measurements to determine micellar effects upon the “inherent” acidity or basicity of the medium i.e., the ability o f the medium to donate or accept hydrogen ions, because, as in any system, the measured effect depends not only upon the hydrogen ion activity, but upon the indicator base and its conjugate acid. We cannot estimate how much of the micellar effects upon Ho‘ and HRare due to changes in hydrogen ion activity and how much to changes in the activity coefficient ratio of the base and its conjugate acid, but the large differences between - AHo‘and - AHRshow that this second effect must a t least be large for the alcohol and its carbonium ion. One problem in evaluating the effects is that -HR includes the water activity, but all the evidence suggests that the solutes will be in the water rich outer layer of the micelle,23so that the bulk water activity should not be an important factor (cf. ref 12). Salt E$ects. Counterions inhibit catalysis by ionic rnicelle~,~ and - ~ their ~ ~ ~ efficiency ~~ increases with decreasing charge density of the ion. The same pattern is exhibited in the present systems (Table IV), for example tetramethyl-ammonium suppresses the acidity much more than does lithium, (based on either Ho’ or HRvalues in the plateau region) and in both cases it is the bulky low charge-density ions which are the most effective, because they interact most strongly with the micelle and prevent it interacting with the reactive ions. The effect on HRis much greater than on Ho‘. A small contributory factor could be that it is the small ions which increase acidity most in aqueous a ~ i d ~ ,in~part ~ , by ~ ~decreasing l ~ ~ water activity and salting out the indicator base. However the effect of 0.03 M salt upon acidity is very small and should be negligible as compared with that caused by the micellesalt interaction. (19) The importance of hydrogen bonding in determining the differences between the various acidity scales has been noted by many workers.% 20 (20) R. W. Taft, J . Amer. Chem. SOC.,82,2965 (1960). (21) E. M. Kosower, “An Introduction to Physical Organic Chemistry,” John Wiley & Sons, New York, N. Y., 1968, p 345. (22) G. Nemethy and H. A. Scheraga, J . Phys. Chem., 66, 1173 (1962). (23) J. C. Eriksson and G. Gilberg, Acta Chem. Scand., 20, 2019 (1966); cf., M. Muller and R. H. Birkhahn, J . Phys. Chem., 71,957 (1967). (24) C. A. Bunton and L. Robinson, J . Org. Chem., 34, 773, 780 (1969). (25) C. Perrin, J . Amer. Chem. Soc., 86, 256 (1964); B. J. Huckings and M. D. Johnson, J . Chem. SOC.,5371 (1964).
Volume 73, Number 12 December 1969
