The Journal of Physical Chemistry, Vol. 82,No. 7, 1978 81 1
Effect of Organic Additives on Micellar Systems two equations, we obtain the required result. (18) F. E. Harris and S. A. Rice, J. Chem. Phys., 25, 955 (1956); H. L. Friedman, "Ionic Solution Theory", Wiley-Interscience, New York, N.Y., 1962. (19) T. L. Hill, "Thermodynamics of Small Systems", Vols. 1 and 2, W. A. Benjamin, New York, N.Y., 1963-1964. (20) Note that since in equilibrium aG/ac, = (@/kT)M,n(section HI), we have ~(aGlac,)(ac,lap), = (,u/kT)(aN/ap),= 0 . (21) This term can be absorbed into redefined quantities S' = S - kand G' = G -t kT, which clearly does not change any of the micelle thermodynamic relations of ref 4, section 1 6 . 3 ~ . (22) I f the interaction terms for free amphiphiles in solution are kept in eq 20, so that c1 = q l v ; e p ' k r y l - l , then the first term of eq 44 becomes a(ln y l c , ) l a p for n = 1. (23) Equation 16.91 of ref 4 is in error by a factor of Tas is easily seen by dlmensional conslderatlons. (24) The communal free energy, which arises from the diffusion of molecules throughout the whole system volume, is less than kTper
(25) (26)
molecule for a fluid and can safely be neglected here. (For communal entropy, see ref 11, Chapter 8.) The function hl,(a,) measures the "free surface" available to the head group hard core of amphiphile a. (i) It takes energy kT( T 300 K) for an additional gauche rotation in an alkyl chain (P. J. Flory, "Statistical Mechanics of Chain Molecules", Interscience, New York, N.Y., 1969). (ii) The cohesive energy of organic fluids is roughly Ub -N( V,/ V)€b,N = number of molecules, V = volume, E, 20 kT(see, e.g., A. Wulf, J. Chem. -fblV, fhys., 64, 104 (1976)). Thus, U,'IN- -(VoIV)E,NIV v = VIN. The pair excluded volume for long rods (of length and diameter I , d) which make an angle y wKh one another is 212dsiny. The general problem for arbitrary rods was solved by Onsager (L. Onsager, Ann. N.Y . Acad. Sci., 51, 627 (1949)). Condition 70 for rods can be written McYY/ vo NPI Vo dll, and may be satisfied before the Onsager orientationaltransition takes place at NV/ V, 4 d l l (ref 27).
-
N
- -
(27)
(28)
-
-
-
Effect of Organic Additives on Micellar Systems Studied by Positron Annihilation Techniques' Yan-ching Jean and Hans J. Ache* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1 (Received October 26, 1977) Publication costs assisted by the Petroleum Research Fund
The rate constants for the reactions of positronium with nitrobenzene and cupric chloride in various aqueous micellar systems, such as sodium dodecylsulfate,hexadecyltrimethylammonium bromide, sodium octylsulfonate, hexadecylpyridinium chloride, and Tergitol-NPX, were measured in the presence of organic additives, such as benzene, benzyl alcohol, n-hexane, and 1-hexanol. The results show that the positronium reactivity toward nitrobenzene or cupric chloride substantially increases, when benzene or benzyl alcohol are added to sodium dodecylsulfate, hexadecyltrimethylammonium bromide, and hexadecylpyridinium chloride solutions, whereas only a slight increase was observed in Tergitol-NPX and no increase in sodium octylsulfonate solutions. In the former systems the rate constants are generally higher in the presence of benzene or benzyl alcohol than in aqueous solutions of nitrobenzene or cupric chloride and approach in the case of nitrobenzene values obtained for positronium reactions with nitrobenzene in benzene solution, whereas n-hexane or 1-hexanolgenerally exhibit only a small effect. A possible explanation for the observed behavior may be that the aromatic additives become incorporated into the micelle, possibly close to the micelle-water interface where they form clusters or aggregates in which the nitrobenzene probe molecule resides. In this benzene-like microenvironment the nitrobenzene molecule exhibits the same reactivity toward positronium as in benzene solutions. By the same token in the case of n-hexane or 1-hexanol additives, the corresponding rates slightly decrease with additive concentration and approach rate constants observed in a n-hexane-likeenvironment. It appears that the positron annihilation technique can provide a sensitive method of studying the microenvironment of probe molecules in micellar systems.
Introduction An important aspect in the evaluation of the physical and structural properties of micelles is the study of the location, disposition, and orientation of solubilized species in the micelle.2-6 Several techniques such as ESR, NMR, fluorescence probes, and other spectroscopic techniques have been employed to define the location of the solubilizate in the micelle. An important result of these investigations was that the solubilizate is relatively mobile, rather than being held a t a given position in a tight configuration. It appears that numerous parameters influence the solubilization process and the micellar interior can neither be considered completely hydrocarbon-like nor do the properties of the region near the micellar surface parallel those of bulk water. This situation seems to be even more complex in reacting systems, where the environment of initial, transition, and final states of the reactants in the micelle may differ. 0022-3654/78/2082-0811$01 .OO/O
In a previous paper7 we have reported on the results of a study in which the reactions of positronium with probe molecules such as nitrobenzene in micellar systems were investigated as a new tool for the determination of the location of the probe in the micelle. This new method is based on the fact that the rate constants for the reaction of the positroniums (Ps),which is the bound state of an electron and a positron, with a probe molecule is a function of the environment in which it O C C U ~ S .Thus ~ ~ ~ ~the nature of the microenvironment in which the probe molecule is located when it reacts with the Ps atom can be determined by comparing the observed rate constants in the micellar solutions with the rate constants obtained in standard solutions. As a result of these experiments we suggested that the nitrobenzene probe is in most of these micellar system located close to the surface or in the Stern layer.7 In the present study we have extended the application of the positron annihilation technique to the investigation
0 1978 American Chemical Society
812
The Journal of Physical Chemistry, Vo/. 82, No. 7, 1978
Y. Jean and
of the physical and structural changes which micelles undergo upon addition of solutes, in as much as they might affect the physical and chemical properties of solubilized species, such as the location of the probe molecules in the micelle, and the permeability of the micelle.2-6J1 For this purpose a series of experiments were carried out in which the rate constants for the reactions between Ps and nitrobenzene or Cu2+ions in various micellar systems, such as sodium dodecylsulfate, hexadecylpyridinium chloride, hexadecyltrimethylammonium bromide, sodium octylsulfonate, and Tergitol-NPX were determined in the presence of several organic additives, such as benzyl alcohol, benzene, n-hexane, and l-hexanol.
Experimental Section (A) Purity of Compounds. Sodium dodecylsulfate (NaLS), obtained from Aldrich Co., was recrystallized in 95% ethanol and dehydrated in a desiccator (with P20J under vacuum. The purification process was repeated until subsequent melting point measurements agreed with literature values within f1.0 “C. Hexadecyltrimethylammonium bromide (CTAB) and hexadecylpyridinium chloride (CPyC1) were obtained from Pfaltz-Bauer, Inc. and purified in a similar way by carrying out the recrystallization in methanol. Sodium octylsulfonate (NaOSO) was obtained from Pfaltz-Bauer Inc. and was purified in the same way as NaLS. The source of Tergitol-NPX was Union Carbide Co. All these latter compounds were of highest purity available (>99%) and were dehydrated by the addition of molecular sieves to the liquid compound. The water used in this investigation was demineralized and triple distilled with a purity better than 99.9%. (B) Positron Lifetime Measurements and Preparation of the Sample. Positron lifetime measurements were carried out by the usual delayed coincidence method as previously described.12 The resolution of the system, as measured by the fwhm of the prompt coincidence spectrum of a 6oCo source without changing the 1.27- and 0.511-MeV bias, was found to be less than 0.36 ns fwhm. Specially designed cylindrical sample vials (Pyrex glass 100 mm long and 10 mm i d . ) were filled with about 2 mL of the appropriate solution. The positron sources consisted of 3-5 mCi 22Nadiffused into a thin foil of soda lime glass. The relative amount of positron annihilation occurring in the glass was found to be less than 270, for which corrections were made. The radioactive glass sources were suspended in the center of the ampoule and all solutions were carefully degassed by freeze-thaw techniques to remove oxygen. The vials were subbequently sealed off and the measurements carried out a t 22 “C. Results and Discussion (1) General Method of Data Analysis. The general method of the data analysis in micellar systems containing a probe molecule which is highly reactive toward Ps has been discussed in great detail in the previous paper7 to which reference is made. In analogy to the data analysis described in this reference it can be shown that X2, which is the slope of the long-lived component in the positron lifetime spectra, can be correlated to the pertinent rate constants for the reaction of the Ps with the various components of such a micellar system and their respective concentration by the following equation: A2
= Kmic(H,o)
KmicA{AmI + + K m i c { S m ) + Kobsdo{SO)
{Mm,O)I +
KH,OA{A(H,O)}
(1)
where Km,c(HzO) is the observed rate constants for the
H. J. Ache
reaction of Ps with the micelle in water, which was found (in separate experiments) to be less than lo7 M-l s-l. ITmi: and KH,OA are the observed rate constants for the reaction between Ps and the additive (A) in the micelle and in the aqueous phase, respectively, and are for the systems chosen in this study of the order of lo7 to lo8 M-l s-l. Kmicand Kobsdo are the observed rate constants for the reaction of Ps with the probe molecules (nitrobenzene or Cu2+)in the micellar or aqueous phase, respectively, they were found to be >lo9 M-l s-l . (M),(A),and (S)are the corresponding concentrations of the micelle, additive, and probe. Note that (S,) + (So]is the total concentration of probe molecules (ST). The subscript m refers to the micellar phase of the micellar solution, while the subscript zero refers to the aqueous phase. In the absence of additives eq 1 can be simplified to = K’mic(H,O){M’(H,O)
h’2
1 -k
K’micCS’M)
+ K’obsd’{S’O)
KmicACAmI
+
(2) The first term in eq 1 and 2 can be obtained from the measurement of h2 without probe molecules present, Le., {S,] + (So}= 0 or (S’,] + (So}= 0: = Kmic(H,O){M(H,O) 1 t = K’mic(H,O)
h’2°
K H , O CA(H,O)I
{M’(H,O)}
(3) (4)
By substituting eq 3 and 4 into eq 1 and 2, respectively one obtains
- 1 2 ’ = K m i c { s m l + Kobsd { S O 1 - A‘ 20 = K m i c { S ‘ m ) + KobsdCS’O)
(5) (6) At high surfactant concentration one can safely assume that a probe molecule such as nitrobenzene is completely located in the micelle, therefore {So)and {So} will be zero. Thus from the experimentally observed positronium reaction rates X2, Azo, and XfZo, the ratio K,ic/K’,,,ic can be directly determined:
A2
Kmic/K’mic
=
(A2
- X 2 O ) / ( h r 2 - h’2’)
(7)
It represents the relatives changes of the rate constant for the reaction between Ps and probe molecule due to the presence of the additive. In the case of Cu2+ ions which are insoluble in the hydrophobic core of the micelle but may be partially adsorbed on the surface of some of the micelles studied no such simplification can be made; thus
Or if one defines and then
( 2 ) The Effect of Additives on the Positron Annihilation Process in Micellar Systems. In the first series of experiments the effect of additives, such as benzyl alcohol, benzene, l-hexanol, and n-hexane on the positron lifetime spectra in aqueous micellar systems, such as NaLS, CTAB, CPyC1, NaOSO, and Tergitol-NPX was studied.
The Journal of Physical Chemistry, Vol. 82, No. 7, 1978 813
Effect of Organic Additives on Micellar Systems
1, vs ADDITIVE CONCENTRATION IN VARIOUS AQUEOUS M I C E L L A R
~
05
SYSTEMS 1 0 NoLS (311 m M ) A NoOSO ( 2 2 9 r n M ) 0 CTAB (231 m M ) +BENZYL A TERGITOL (232rnM) ALCOHOL D CPyCl (272mM) V CTAB ( 2 7 6 r n M ) + n HEXANOL
1
I
RELATiVE RATECONSTANTS K $ , ~ / K ; , ~ FOR THE REACTION OF Ps WITH NITROBENZENE ( 7 8 2 rnM) IN VARIOUS AOUEOUS MICELLAR SOLUTIONS 3 0 CONTAINING BENZYLALCOHOL L ADDlTTlVES
1 r
t 1.4
1
A TERGITOL
-
I
-
IO
I
03 100
O’
Id0
200
exptl error
200 300 m M ADDITIVE
( 2 3 2 rnM)
A
300
1
’
No O S 0 (229rnM
400
I
d0
mM B E N Z Y L A L C O H O L 400
500
Figure 3. Km,JKrmiC (relative rate constants for reaction of Ps with nitrobenzene in micellar solutions with and without additives present) vs. concentration (mM) of benzyl alcohol additives.
Figure 1. h, vs. additive concentration in various aqueous micellar systems at room temperature.
1 ,
’
3 4 r RELATIVE RATE CONSTANTS K,,,/K~,,
A, v s ADDITIVE CONCENTRATION IN VARIOUS AOUEOUS MICELLAR SYSTEMS
30
FOR THE REACTION OF Ps WITH NITROBENZENE ( 7 8 2 m M ) IN VARIOUS AQUEOUS MICELLAR SOLUTIONS CONTAINING BENZENE ADDITIVES
1
-I
I
E X P ERROR
0 NaLS (3llmM)
I
c
A TERGITOL(23ZrnM) ‘ I CPyCl (272rnM)
u22c
y 03
1
:
IO0
ZOO
mM
O6
exptl. error
r
100
200
3hO
460
500
1 600
mM BENZENE 300
400
500
600
ADDITIVE
Figure 2. h2 vs. additive concentration in various aqueous micellar systems at room temperature.
While the effect of additives on these micellar systems on the Ps formation probability as expressed by the intensity, 12,of the long-lived component in the positron lifetime is generally very small ( f 2 % absolute), over the whole range, i.e., up to 600 mM of additives, the corresponding positron decay rates, h2, show more drastic changes as seen in Figures 1 and 2, where h2 for the various systems is plotted as a function of additive concentration. However, no general trends emerge from these data; while in some systems benzyl alcohol additives reduce As, others enhance h2. With benzene a more consistent trend, Le., a slight enhancement of h2 with increasing additive concentration is observed. The cause for this behavior is probably due to changes in micellar shape or size upon addition of these compoundsll and will be discussed in context with the results obtained in the presence of probe molecules, see section 3 and 4.
Figure 4. Kmlc/K’mlc (relative rate constants for reaction of Ps with nitrobenzene in micellar solutions with and without additives present) vs. concentration (mM) of benzene additives.
(3) The Effect of Additives on the Positron Annihilation Process i n Micellar Systems in the Presence of Nitrobenzene as a Probe Molecule. As discussed in a previous paper7 a t high surfactant concentration one can assume that the relatively small amount of nitrobenzene molecules, less than 10 mM, is completely located in the micellar phase. Thus [So]and [S’,] will be equal to zero, and the ratio of Kmic/K&lc can be obtained from eq 7. It represents the relative changes of the Ps reaction rate constant caused ratios by the presence of the additives. These Kmic/K’mlc are plotted as a function of additive concentration in Figures 3-5 for the various micellar systems. The maximal concentrations of additives used in these studies and the absolute rate constants observed under these conditions are listed in Table I. Information about the environment of the probe molecules in the micellar systems can be obtained by comparing the observed rate constants for positron annihilation in the micellar systems with those obtained in the aqueous
814
Y. Jean and H. J. Ache
The Journal of Physical Chemistry, Vol, 82, No. 7, 1978
TABLE I: Rate Constants for Ps-Nitrobenzene Interactions in Various Aqueous Micellar Solutions with Different Additives Nitrobenzene, mM
a
Surfactants
Additive
0
7.82 7.82 7.82 7.82 7.82 7.82 7.82 7.82 7.82 9.33 9.33 9.33 7.82 7.82 7.82 7.82 7.82 7.82 7.82 7.82
1 1 5 mM 115 mM 3 1 1 mM 3 1 1 mM 3 1 1 mM 243 mM 243 mM 2 4 3 mM 276 mM 276 mM 276 mM 272 mM 2 7 2 mM 272 mM 229 mM 229 mM 2 3 2 mM 2 3 2 mM 2 3 2 mM
Experimental error is k0.04
X 1O1O M-' s-'.
NaLS NaLS NaLS NaLS NaLS CTAB CTAB CTAB CTAB CTAB CTAB CPyCl CPyCl CPyCl NaOSO NaOSO Tergitol Tergitol Tergitol
0 0
1.02 0.83 1.52 0.45 1.42 0.86 0.57 1.37 1.76 0.53 0.51 0.39 0.35 1.20 0.76 0.59 0.51 0.50 0.70 0.88
4 8 3 mM benzyl alcohol
0 4 8 3 mM benzyl alcohol 5 6 3 mM benzene
0 483 mM benzyl alcohol 5 6 3 mM benzene
0 239 mM 1-hexanol 383 mM n-hexane
0 483 mM benzyl alcohol 563 mM benzene
0 4 8 3 mM benzyl alcohol
0 483 mM benzyl alcohol 563 mM benzene
TABLE 11: Rate Constants for the Ps-Cu*+ Interaction in Various Aqueous Micellar Solutions with Different Additives Present CuCl,, mM 25.07 25.07 25.07 25.07 25.07 25.07 25.07 25.07 25.07 20.05 20.05 20.05 a
Surfactant
Additive
0 295 295 295 272 272 272 229 229 232 232 232
mM CTAB mM CTAB mM CTAB mM CPyCl mM CPyCl mM CPyCl mM NaOSO mM NaOSO mM Tergitol mM Tergitol mM Tergitol
0 0
0.23 0.08 0.22 0.18 0.05 0.15
483 mM benzyl alcohol 5 6 3 mM benzene
0 483 mM benzyl alcohol 563 mM benzene
0.13
0
0.15 0.15 0.22 0.19 0.20
483 mM benzyl alcohol
0 483 mM benzyl alcohol 563 mM benzene
Experimental error is r 0 . 0 2 x loLoM-I s-l.
phase or in a hydrocarbon system having a hydrocarbon chain length similar to those of the surfactants forming the micelles. On the basis of this comparison we previously reached the conclusion7 that the nitrobenzene probe molecule is located in most of the micellar systems under investigation in the Stern layer, with the exception of the NaOSO solutions, where the nitrobenzene may be located in a hydrocarbon-like environment. From Figure 3, where the relative changes of the rate are plotted as a function of benzyl constants, Kmic/Kkic, alcohol concentration, it is obvious that the addition of benzyl alcohol causes an enhancement of the Ps reactivity with nitrobenzene in NaLS, CTAB, CPyC1, and to a lesser degree in Tergitol while no effect or a slight decrease is seen in the case of NaOSO. Similar trends, although definitely less pronounced in the case of NaLS, can be recognized if benzene is the additive as shown in Figure 4. A completely different behavior is observed for 1hexanol and n-hexane additives, as demonstrated for the CTAB system in Figure 5, where the changes of Kmic/KLlc are displayed as a function of the nature and concentration of the various additives. Since, as discussed a b o ~ e , ~the - l ~rate constants for Ps reactions with probe molecules such as nitrobenzene are sensitive to the nature of the environment in which they occur it seems logical to relate these trends to changes of
I
- RELATIVE
I
I
I
I
R A T E C O N S T A N T S K,,,lc/K~lc 3 0 - F O R T H E REACTION O F PS W I T H N I T R O B E N Z E N E IN A N AQUEOUS M I C E L L A R
-
u
a Y
\
2
a
Y
0'
Id0
260
300
460
m M ADDiTiVE
do
do
Figure 5. Relative rate constants for the reaction of Ps with nitrobenzene in CTAB solutions (243-276 mM) as a function of additive concentration.
the environment of the nitrobenzene probe in the micelle. By comparing the absolute rate constants Kobsd a t high additive concentrations as listed in Table I with those
Effect of Organic Additives on Micellar Systems
obtained for the nitrobenzene probe molecule in pure water, benzyl alcohol, benzene, and other hydrocarbon solutions or mixtures of hydrocarbons, alcohols, and water, as listed in Table 11, it becomes rather obvious that, e.g., the addition of benzene to CTAB results in absolute rate constants, Kobsd, which are definitely greater than those observed in the aqueous phase, or in homogeneous mixtures of the corresponding hydrocarbons with the same amount of benzene, and approach those observed in benzene solutions. In other micellar systems such as NaLS, CPyC1, and Tergitol, the effect of benzene additives is less pronounced. The implication of these results seems to be that the addition of benzene to the micellar systems, most obvious in the case of CTAB, provides an environment for the nitrobenzene probe molecules which is benzene-like. One can visualize that this is accomplished by benzene cluster formation and benzene gel formation within the micelle.llj Evidence for such a benzene microenvironment in which solubilized species may reside has recently been obtained from optical spectra and the measurement of dielectric constants in aqueous NaLS systems to which benzene was added.13 The authors postulate that the solubilized benzene is concentrated primarily a t the micelle-water interface rather than in the hydrocarbon core. A very similar behavior is observed for the Ps rate constants in micellar systems containing benzyl alcohol additives. In this case NaLS and CTAB show the most pronounced increase, with Kobsdvalues exceeding even those obtained for Ps nitrobenzene interaction in pure benzyl alcohol solutions, whereas on the other hand a slight decrease is observed in NaOSO solutiofis. If the above assumption, namely, that additives such as benzene or benzyl alcohol are forming clusters a t the micelle- water interface in which the probe molecule can be located is correct, then one would expect that the greatest effect on the Ps-nitrobenzene rate constants is seen in those cases where the nitrobenzene is initially, Le., without additives present, near the micelle-water interface or in the Stern layer. In our previous paper7 we have postulated this to be the case in the NaLS, CTAB, CPyC1, and Tergitol systems, whereas the indication was that nitrobenzene is located in the hydrocarbon-like layer of the NaOSO micelles. The results shown in Figures 3-5 and Tables I and I1 are consistent with the expected trend. The fact that the rate constants, Kobsd, in the presence of benzyl alcohol exceed those observed in pure benzyl alcohol solution requires some additional discussion. The fact that Kobsd approaches values typically found in aromatic solvents might suggest that the benzyl alcohol molecules form aggregates in the micelles which show a particular arrangement, e.g., with the aromatic ring directed to the center of the cluster and the -OH group pointing outward. In such a case one could rationalize the similarity of the rate constants observed in micellar systems with benzyl alcohol additives and in (pure) benzene solutions. Further substance to the postulate that these additives form clusters in or a t the micelle surface is added by the fact than 1-hexanol and n-hexane additive in the CTAB system slightly decrease the Ps rate constants, again consistent with the hypothesis that in this case nitrobenzene is located in a 1-hexanol or n-hexane microenvironment, where the rate constants for Ps-nitrobenzene interactions, as shown in Table 11, are considerably lower. The above explanation postulates that the effect of the additive is a change of the environment of the probe
The Journal of Physical Chemistry, Vol. 82, No. 7, 1978 015
molecule, which subsequently affects the rate of the reaction between Ps and the nitrobenzene probe. This is in contrast to the previous interpretation of the effects of additives preferred by several authors where the increase of the reactivity of a probe molecule in a micelle in the presence of additives such as benzyl alcohol was interpreted in terms of an enhanced permeability of the micellellhiallowing, e.g., a quencher molecule to approach the probe more freely. An unambiguous answer to this question is difficult to obtain. Studies of the microviscosity, e.g., of pyrene probes in NaLS with or without additives seemed to indicate very little change in the environment.llh However, pyrene is supposedly located deeper in the core and its environment may therefore not be subjected to changes in the presence of such additives. It could very well be that the positron annihilation technique does not pick up differences in the permeability of micelles because of the intrinsic properties of the Ps atom, and recognizes only variations in the local environment of the probe molecule, while fluorescence techniques are affected by both factors. (4) The Effect of Additives on the Positron Annihilation Process in Micellar Systems in the Presence of Cu2+Ions. As discussed in the previous paper,I Cu2+ions are adsorbed on the surface of ionic micelles due to an electrostatic attraction between surface charge and Cu2+ ions or counterion charge and Cu2+ions. As a result of this adsorption process the rate constants between Cu2+ions and Ps dropped considerably below the values observed in aqueous solutions of Cu2+. The reason for the reduced reactivity of Ps toward adsorbed Cu2+may be seen in the fact that the Cu2+when adsorbed on the surface loses the character of the hydrated Cu2+by partial bond formation, complexation, etc. with the surface molecules, and thus reacts a t a different rate with Ps. An indication how the surface of the micelles is changed in the presence of additives, such as benzyl alcohol or benzene, may therefore be obtained by comparing the rate constants for Ps interactions in micellar solution containing Cu2" ions with or without the presence of these additives. Since Cu2' ions can be considered insoluble in the hydrophobic micellar phase, one can assume S[], and S [],' in eq 8 equal to zero, and one obtains for the ratio of the Ps reaction rate constants with or without additives present the following equation:
(Kobsdo and Kbbsdo are the Ps reaction rate constants for Cu2+in the presence or absence of additives, respectively). The ratio Kobsdo/Kbbsdo which represents the change caused by the additives is plotted for various aqueous CuC12-micellar solutions in Figures 6 and 7, with the absolute values of Kobsd under the conditions listed in Table 111. The results show that both benzene and benzyl alcohol significantly increase the Ps reaction rates in micellar systems, such as CPyCl and CTAB, where previous results have shown a drastic drop in the Cu2+-Ps reaction rates when micelles are formed, while no such effect is observed in the case of Tergitol, which as a neutral micelle has shown no attraction for Cu2+ions. It seems that these results suggest that the addition of benzene or benzyl alcohol to the CpyCl or CTAB leads to drastic changes of the micelle structure in or close to the micellar-water interface, caused by the formation of benzene clusters etc. (see above) resulting in a release of CuZc otherwise adsorbed on the micellar surface.
816
The Journal of Physical Chemistry, Vol. 82,No. 7, 1978
Y. Jean and
I
I
H.J.
Ache
34s
RELATIVE
RELATIVE RATE C O N ~ T A N T S
RATE CONSTANTS K,o,,/K~~, FOR THE REACTION OF Ps WITH CuClp ( 2 0 - 2 5 mM)
38 THE REACTION OF Ps WITH CuCI2 ( 2 0 - 2 5 mM) IN VARIOUS AQUEOUS MICELLAR SOLUTIONS CONTAINING BENZYLALCOHOL ADDITIVES
3 0 - IN VARIOUS AQUEOUS MICELLAR SOLUTIONS CONTAINING BENZENE ADDITIVES -
t
/
O/CTAB(295rnM)i
VI
0:
1 t
11
I
/
.
O6
200
100
300
400
22
CUCI, CTAB (25 mM)
c
Y
\ VI
0Y8
-
IS;
/
t
/ /
1
i
/
i -1 (2,32 mM)
06'
IO0
200
300 400 m M BENZENE
500
600
Figure 7. Kobsd/K'obsd (relative rate constants for reaction of Ps with CuCI, in micellar solutions vs. concentration (mM) of benzene additives.
500
rnM BENZYLALCOHOL
Figure 6. Kobsd/K'obsd (relative rate constants for reaction of Ps with CuCI, in micellar solutions) vs. concentration (mM) of benzyl alcohol additives.
TABLE 111: R a t e Constants for Ps-Nitrobenzene Interactions in Various Solutions
Water 1-Pentanol Benzyl alcohol n-Hexane n-Octane n-Dodecane n-Hexadecane Benzene Water t 3 8 6 mM b e n z y l a l c o h o l n-Dodecane t 242 mM b e n z y l alcohLO1 n-Dodecane t 5 6 3 mM benzene n-Hexadecane t 5 6 3 mM benzene 239 mM h e x a n o l n-Hexadecane n-Hexadecane t 3 8 3 mM n-hexane
gations of changes introduced by the presence of additives in micellar systems. It appears that additives such as benzyl alcohol, benzene, 1-hexanol, and n-hexane form clusters in most of the micellar systems studied in this research a t or near the micellar-water interface also leading to changes in the micellar surface charge. References and Notes
Solutions
+
(25mM)
A (20 mM)
NaOSO ( 2 2 9 m TERGl TOL ( 2 4 2 rnM)
IO
26 -
1.00 0.86 1.16 0.30 0.69 1.28
1.15 2-56 0.82 1.02 1.06 1-00 0.96 0.97
An interesting case is the NaOSO system. Prior experiments7 with this system containing Cu2+ions have shown a clear drop of the rate constants for the reaction between P s and Cu2+upon micelle formation, which indicated extensive Cu2+adsorption on the surface of this micelle. The present results as shown in Figure 6 provide no evidence for a release of Cu2+from the surface in the presence of benzyl alcohol additivies. We would like to interpret these results by assuming that if this micelle solubilizes benzyl alcohol a t all, that it becomes incorporated not in form of clusters near the surface but more likely in a homogeneous form in the hydrocarbon-like core of the micelle consistent with the fact that the rate constant for the Ps-nitrobenzene interaction is noqaffected by the presence of additives either, as discussed in section 3.
Summary The results of this study have shown that the positron annihilation technique is a sensitive tool for the investi-
Work supported by the US. Energy Research and Development Administration. Acknowledgment is made to the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for the partial support of this research. J. K. Thomas, Acc. Chem. Res., 10, 133 (1977). J. H. Fendler and E. J. Fendler, "Catalysis in Micellar and Macromolecular Systems", Academic Press, New York, N.Y., 1975. E. J. Fendler and J. H. Fendler, Adv. fhys. Org. Chem., 8, 1472
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