J. Phys. Chem. 1985, 89, 2705-2708 offers a unique possibility of studying these parameters.
M,,, Mr,s
Acknowledgment. W e are greatly indebted to the late Prof. G. Aniansson for his stimulating encouragement and S U P P r t . His untimely death brought an end to a series of pioneering contributions in the field of micelle kinetics. Financial support by the Swedish Natural Science Research Council is gratefully acknowledged.
ml,m2
nl,n2
7,7 R
total concentration of A and B, respectively monomers of the components A and B, respectively monomer concentration of A and B, respectively ratio between the concentration of the material in the micellar form expressed as monomers and the monomer concentration of A and B, respectively Be =
M’ MO Mm
Btoi
- Aioi
total concentration of micelles parameters characterizing a Gaussian micelle distribution vectorial flow in the aggregation space components of the flow J(r,s); they are defined as the net number of aggregates that per unit time pass from r - 1 to r at constant s and from s - 1 to s at constant r , respectively rate constants of the inclusion of a monomer of component A and B into an aggregate Mrls and M,,, respectively disintegration rate constant of an aggregate M,,, into Mrl, and M,c-l, respectively average value of k,-(r,s) and k y ( r , s ) , respectively, in the region of proper micelles average value of kl-(r,s) and kT(r,s), respectively, in the minimum region of the micelle distribution equilibrium constant defined by M,, = K,JlrBls length of a parallelepipedal minimum in the micelle distribution concentration in a parallelepipedal minimum maximum concentration in the region of proper micelles concentration of micelles in a saddle-shaped “minimum”
a micelle containing r molecules of A and s molecules of B concentration of micelle M,, aggregation numbers of A and B, respectively, in a saddleshaped “minimum” number average of aggregation numbers of A and B, respectively, in the region of proper micelles quadratic number average of aggregation numbers of A and B, respectively, in the region of proper micelles resistance in the narrow passage relative amplitude of the fast and slow process in a relaxation experiment using conductivity detection width of a parallelepipedal minimum in the micelle distribution angle between the r and p axes (see Figure 1) central moments of the micelle distribution relative deviation from equilibrium of B, angle between the r axis and the largest main axis of the micelle distribution ul total conductivity of an ionic micellar solution molar conductivity of micelles M,,, and Mnl,n2,respectively derivative of A,, with respect to r and s, respectively total molar conductivity of the monomer species, i.e. A , = A,*
.
,“ I; I
E 6,
OXY
u, T
0
*
-
2705
+ X1B
sum of the molar conductivities of a micelle M,,, and r - s ions B, in the case u2 = 0 relative deviation from equilibrium of M,,s defined ,as .,,< u.. - r€ - sn. value of I,; in the micelle region during the slow process relative deviation from equilibrium of Al standard deviation characterizing the micelle distribution with respect to the “i” axis mixed second central moments of the micelle distribution with respect to the coordinate system (x,y) correlation between the two species with regard to their micelle-forming tendency a relaxation time; i l land T~~ refer to the fast process, and T~ refers to the slow process beginning of the fast process, i.e. at t = 0 end of the fast process an equilibrium value
-
Salt Effects on Photoionization Yields in Micellar Systems S. Hautecloque,* D. Grand, and A. Bemas L.A. 75 BBtiment 350, Universitd Paris-Sud, 91405 Orsay, France (Received: January 3, 1985)
The effect of added salts (LiCl, NaCI, CsC1, tetraethylammonium (Et4N) chloride, MgC1,) on the photoionization yields of perylene (Pe) incorporated in anionic NaLS micelles has been studied. The yields, measured from hydrated electron scavenging by NO3-, are found to decrease exponentially as the salt concentration is increased before reaching plateau values. The same regular trend is observed when extra Na+ counterions derive from an increase in surfactant concentration. A hydrophobic counterion such as Et4N+ appears much more efficient in reducing ae-and for high enough concentrations of hydrophobic salts, the NaLS interface electric potential ic/ seen by the photoejected electron can become positive, favoring geminate recombination reactions. When considering literature data pertaining to interface electric potential measurements obtained from the pK shift of various micellar probes, a linear law relating 9, to ic/ arises. The implications of such a correlation are examined.
Introduction Interfaces or interphase surfaces play a prominent role in various chemical photochemical reactions occurring in heterogeneous media, in particular organized molecular assemblies. Especially, properties of charged interfaces in antact with electrolyte solutions are a subject of increasing interest, due partly to their possible implication in various biological systems. Addition of salts to ionic micelle solutions increases the degree of counterion-ion association with the charged headgroups, and hence reduces their mutual electrostatic repulsion. In turn, the cmc is reduced whereas the aggregation number and micellar diameter are increased. Such structural parameters have been 0022-3654/85/2089-2705$01.50/0
extensively examined as a function of added electrolyte concentration. On the other hand, micellar catalysis4 can be inhibited or practically suppressed by addition of salts to the reaction medium: (1) R. J. William, J. N. Phillips, and K. J. Mysels, Trans. Faraduy, Soc., 51, 728 (1955).
( 2 ) K. S.Birdi, S.Backlund, K. Sorensen, T. Krag, and S.U. Dalsager, J . Colloid Znrerface Sci., 66, 118 (1978). (3) R. Nagarajan and E. Ruckenstein, J . Colloid Interface Sci., 71, 580 (1979). (4) A. D. James and 74, 10 (1978).
B. H. Robinson, J. Chem. SOC.,Faraday Trans. I,
0 1985 American Chemical Society
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The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
a few chemical reactions (acid-base e q ~ i l i b r i aelectron-transfer ,~ reaction^,^ ...) have been shown to be very sensitive to the interfacial electric potential. The magnitude of the latter decreases upon addition of increasing concentration of organic or inorganic salts, according to a logarithmic law. Different models have attempted to rationalize counterion effects on charged interfaces. Interfacial potential values can be generally estimated from the Gouy-Chapman equation, the theory being based on the electrostatic and hydrophobic interaction in the Stern-Gouy ionic double layer. A detailed model of the interface was initially developed by Stigter,6 while a more sophisticated version,' which takes into account interface structure and counterion activity, is able to predict the dissociation degree of the surfactant headgroups. The proposed models are, however, difficult to test; experimental values depend on the method used and show considerable scatter. On the whole, pH indicators seem to be the most reliable probes of interfacial electric potential in micelles; even though different indicator dyes may lead to different absolute values, relative values appear m e a n i n g f ~ l . ~ * ~ ~ ~ In two recent studies,lO*"the photoionization yields (ae-)of hydrophobic solutes sequestered in either neutral (Mio) or anionic micelles (Mi-) were determined. In the latter case (NaLS micelles) a progressive decrease of @-, was recorded in the presence of increasing concentrations of NaCl. This observation was explained in terms of a concomitant decrease of the negative electric potential at the lipid-water interface. The purpose of the present work is to substantiate such an interpretation and to provide a more quantitative relationship between ionization yield and interfacial electric potential values. Organic and inorganic monovalent or divalent salts were thus added to NaLS micellar solutions and photoionization yields of perylene correlated with electric potential data reported in the literature.
Experimental Section Perylene (Pe) was incorporated in 0.1 M NaLS micelle (cmc M). Such N a L S concentration was chosen so as = 7.6 X to reach a high enough solute concentration. The NaLS solutions have been prepared by dissolving solid surfactant in the electrolyte solution. The solute (Pe) was dispersed in the micelle by magnetic stirring overnight at room temperature. The solutions were then filtered and the solubilizate concentration was determined by spectrophotometric measurement. The electrolytes were reagent grade (Prolabo or Merck) and used without further purification, except for the tetraethylammonium chloride (Et,NCl) which was recrystallized. In too concentrated salt solutions, the spherical micelles of NaLS tend to link with one another, forming microgel particles and interaction of charged groups is possible through cations: solutions are no more transparent or show a very high viscosity. The salt concentrations were thus kept below the following limits: [LiCl] I 1 M , [NaCl] I 0.6 M, [CsCl] I 0.15 M, [Et,NCl] 5 0.1 M, [MgC12] I5 X lo-* M. N a L S was purchased from Merck and used as supplied. Water was deionized and distilled in a quartz apparatus. The solutions were deaerated before UV illumination and the irradiation device has been described previously.1° It has been emphasized that the low light fluxes used prevent biphotonic processes. (5) M.Calvin, I. Willner, C. Laane, and J. W. Otvos, J. Phorochem., 17, 195 (1981). (6) (a)'D. Stigter, J. Phys. Chem., 68, 3603 (1964); (b) ibid., 78, 2480 (1974); (c) ibid., 79, 1008 (1975); (d) ibid., 79, 1015 (1975). (7) J. A. Beunen and E. Ruckenstein, J. Colloid Interface Sci., 96, 469 (1983). ' ( 8 j G . Charbit, F. Dorion, and R. Gaboriaud, J. Chim.Phys., 81, 187 (1984). (9) M.S. Fernandez and P. Fromhertz, J . Phys. Chem., 81, 1755 (1977). (10) A. Bemas, D. Grand, S. Hautecloque, and A. Chambaudet, J. Phys. Chem., 85, 3684 (1981). ( 1 1) D. Grand, S. Hautecloque, A. Bemas, and A. Petit, J. Phys. Chem., 87, 5236 (1983).
Hautecloque et al.
VI
NaLS CONCENTRATION
1t
I
1
,
I
/
1
,
NaLS M. * Figure 1. Perylene (Pe) photoionization yield (Awc = 250 nm) vs. NaLS concentration (Pe average concentration = M). 0.5
0.1
0.8
(a.u.1
3
I
0 .I
I
I
0.5
I
I
,
,
I
'
CXCll M.
Figure 2. Effect of monovalent salts on perylene photoionization yield in 0.1 M NaLS (A,,, = 250 nm). Pe average concentration = 2 X lo-' M: (0)Et,NCI; ( X ) CsC1; (A)NaCI; (0) LiC1.
The chemical scavenging of the hydrated photoelectron by NO< M) leads to NO2-, whose formation was ([N03K] = 2 X followed by the Schinn technique.12 All the experiments were run at least three times for each salt or surfactant concentration.
Results In the experiments reported below, counterions are issued either from the surfactant itself or from added salts. The results obtained illustrate at one and the same time the specificity of the counterions used and also a common behavior, in particular as regards to the interface electric potential. Surfactant Concentration E f f e c tFigure 1 displays the variation of the perylene photoionization quantum yield @e- measured at different surfactant concentration (0.02 M I (NaLS) 50.8 M), following steady-state irradiations at 250 nm. It appears that @edecreases up to a surfactant concentration of about 0.5 M, above which it remains constant. Such a @e- variation can be compared with that observed in the presence of added NaCl (see Figure 2); an increasing surfactant concentration leads to an increase of the free N a + which can "neutralize" or screen the micelle surface charge. Quantitatively, the same 3, value is obtained either from concentration c of added NaCl or from c X 5 of NaLS which accounts for the reported dissociation degree of NaLS, Le., =20%.13 On the other hand, Qe- vs. the log of the surfactant concentration gives a linear relationship, reminiscent of what was reported for interface potential vs. added salt c~ncentration.~ At higher NaLS concentration (>0.5 M) a logarithmic law is i10 longer obeyed and it can be remarked that, in such concen(12) M. B. Schinn, Ind. Eng. Chem. Anal. Ed., 13, 33 (1941). (13) J. K. Thomas, Acc. Chem. Res., 10, 133 (1977).
The Journal of Physical Chemistry. Vol. 89, No. 12, 1985 2707
Salt Effects on Photoionization Yields
1 **
1
+,.(a.u.)
(a.u.)
__
3
I
pure NILS
0.lM
0 0 0
N I L O + O . O l Y Ef,NCI
0 0 0
N a L S + O . l Y EI,NCI
+ ++
Erl] 35
\
I
1
240
250
270
260
280
Anm
Figure 3. Perylene photoionization yield vs. excitation wavelength h in (- -) 0.1 M NaLS; (0) 0.1 M NaLS 0.01 M Et4NCI;(0) 0.1 M NaLS + 0.1 M Et4NCI; (+) 3/1000 v/v Brij 35.
+
TABLE I: Hydrated Counterion Stoke's Radius" counterion hydrated Stoke's radius, A 2.30 Li+ 1.64 Na+ 1.14 cs+ Et4Nt
Mg2+
2.76 3.5
"References 16 and 17. tration range, the sphere-to-rod transition has been reported to take p l a ~ e . ' ~ ~ ' ~ Monovalent Salt Effect. The data obtained for various concentrations of monovalent salts are shown in Figures 2 and 3. Whatever the salt concentration, one observes the following sequence (Figure 2): 9e-(Et4N+) < 9.,-(Cs+) < 9,(Na+) < @,-(Li+). In the case of the inorganic salts, the effect of the counterions on 9,-appears more pronounced when the hydrated ion radius is smaller, as indicated in Table I. On the other hand, Et4N+ ion, although larger than Li+, has a drastic reducing effect on its hydrophobic character16J7 largely overcomes the influence of the counterion size. In the preceding paragraph, a logarithmic law relating 9,-and the surfactant concentration was noted. Here again 9,-vs. log c appears linear in the whole salt concentration range. Photoionization yields of Pe vs. the excitation wavelength are compared for pure N a L S micelles, for N a L S with increasing Et4NCl concentrations, and for neutral micelles (Brij 35) (Figure 3). For high enough Et,NCl concentrations, the photoionization yields found are lower than those corresponding to neutral micelles, suggesting an increased probability of geminate recombination Pe+ + electron. Divalent Salt Effect. Figure 4 displays the results obtained in the presence of MgC12 and for a low concentration range: 1-5 x M. Comparison of the 9., data - from Figures 2 and 4 shows that the same value of @.,- is obtained for a MgC12 concentration which is one order of magnitude lower than that of NaCl, a value which agrees with the electrostatic effect associated with the valence state. Discussion Previous experimentdo*" have indicated that the photoionization threshold energy of Pe in aqueous micellar aggregates was not significantly modified by the electric charge density at the micelle (14) (15) (16) (1967). (17)
K. J. Mysels, J . Colloid. Sci., 10, 507 (1955). K. J. Mysels and L. H. Princen, J . Phys. Chem., 63, 1696 (1959). P. Mukerjee, K. J. Mysels, and P. Kapauan, J. Phys. Chem., 71,4166 M. Almgren and S . Swarup, J . Phys. Chem., 87, 876 (1983).
I
I
I
I
5.10-'
Mg Cl* M.
.
Figure 4. Effect of a divalent salt (MgC12) on perylene photoionization yield (Aexc = 250 nm).
surface. On the other hand, addition of NaCl and LiCl, progressively decreasing the magnitude of interfacial negative potential of NaLS, was found to affect drastically the shape of the solute photoionization efficiency curves 9,-=AX). Present results complement and corroborate the initially proposed interpretation. Addition to aqueous N a L S of increasing concentrations c of monovalent or divalent counterions induces in all cases studied a continuous exponential decrease of the photoionization yield @%-. Similarly, ae-decreases regularly when the extra Na+ counterions originate from an increase in the surfactant concentration. Such findings are in contrast with recent results pertaining to tetramethylbenzidine photoionized at 77 K in frozen NaLS micelles,'* in which case 9,-was found to increase for [NaCl] I0.15 M, and then to decrease. The important conclusion was then drawn that salt addition can be used to optimize charge separation. It should be underlined, however, that the two sets of experiments differ in at least three respects: solute, temperature, and analytical method for measuring ionization yields. The present results display not only a general decrease of aewhen the salt concentration is increased but also an ion selectivity. To account for interfacial potential modification by added counterions, screening and binding effects are generally invoked, but ion specificity has not been satisfactorily explained. In particular, adsorption at the interface, exchange reactions with surfactant counterions, and interion reactions are difficult to evaluate (or to treat theoretically). Electrostatic effects of counterions are obvious; the binding degree of counterion at the interface seems to depend on the size and on their valence state. of hydrated The present results concerning the effect of monovalent counterions agree with such considerations. Smaller hydrated ion radii favor a closer approach of the interface and decrease the interfacial electric potential Photoionization efficiency curves of Pe obtained from Li+, Na+, Cs+ aqueous micellar solutions illustrate such an influence. The role of the counterion hydrophobicity has also been underlined previously, especially in the case of organic i o n ~ . ' ~In, ~ ~ the present study, Et4N+ evidences the effect of the ion hydrophobicity. As shown in Figure 3, ionization yields obtained in N a L S micellar solutions in the presence of Et4N+ can be even lower than those relative to neutral micelle solutions. A significant adsorption or intercalation of Et4N+ is thus assumed to occur, in addition to the exchange reaction with the surfactant Na+ ion. Above a certain Et,N+ concentration, NaLS micelles would behave like positively charged entities and an inversion of the electric potential sign can be expected to occur more readily for longer chain hydrophobic additives. The above considerations have emphasized the counterion specificity. On the other hand, when interfacial electric potential
+.
~
(18) R.Maldonado, L. Kevan, and E. Szajdzinska-Pietek, J . Chem. Phys.,
in press. (19) C. C a b s , J . Phys. Left., 44, 997 (1983).
2708
Hautecloque et al.
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985
t
I
Smv
90-h u . ) )i:
Na' (NaLS)
0 :
Na* (NaCl)
A : LI
3
2
I
(LICI)
e:
c** (CSCI)
0:
EI,N*(Et,NCI)
A?
r '/
1\
0
5
-
0
0.05
0.1
CEt4NCLI M.
Figure 5. (a) (0) Variation of interfacial electric potential $ as a function of added Et4NCI concentration (from ref 4). (b) (X) Perylene photoionization yield (this work) after normalization at [Et,NCl] = 0.
data are considered, a general quantitative correlation between ionization yield and electric potential # arises. In the particular case of Et4NCl, our results are in good agreement with those obtained from pK shift measurements of various dyes used as probes in NaLS micelles in the presence of Et4NCL4 The surface electric potential # calculated from the equation due to Hartley20 and expressed in millivolts shows a decrease from -124 mV (in the absence of added salt) to -6 mV when 0.1 M of Et4NC1 is added. Figure 5 gives the variation # = f(c) taken from ref 4. When normalized at c = 0 our 0,- yields (circles) and Robinson's potential values (crosses) fall exactly on the same curve, which is a strong indication that the variation of ae-with c indeed reflects the variation of the interface electric potential. More generally, to obtain an estimation of the electric potential variation upon any salt addition, we have used an interesting model recently proposed by Gaboriand et a1.8*2'which takes into account the specificity of added counterions. Applied to acid-base equilibria in micellar systems, the model allows us to deduce, for any type of surfactant, two parameters characterizing the various counterions, one of the two expressing the interaction of counterion with the interface. From the knowledge of these parameters, the variation # =f(c) can be derived and the experimental ionization yields correlated with #. This @e- vs. # plot (Figure 6) shows that whatever the size and hydrophobicity of the counterions, they all give rise to the same curve, and the photoionization yields depend linearly on the interphase electric potential. It may be added that the # values obtained by Robinson4 and Stigter22 for NaLS-NaCl solutions also fall on the same straight line. In short, the present results suggest the following concluding comments: (a) Acid-base equilibria and electron photoejection processes show the same electric potential dependence. (20) G.S. Hartley and J. W. Roe, Trans. Faraday Soc., 36, 107 (1940). (21) F. Dorion, Thesis, Paris, 1982. (22)D. Stigter, J . Colloid. Interface Sci., 23, 319 (1967).
(23)S. C.Wallace, G. E. Hall, and G. A. Kenney-Wallace, Chem. Phys., 49,279 (1980). (24)P. Cordier, unpublished results.
