Environ. Sci. Technol. 1992, 26, 2448-2453
Multiequilibria of 2-(2‘-Furany1)-1 H-benzimidazole Neutral and Protonated Forms in the Presence of Amphiphilic Aggregates Antdnio Lopes,t Antdnio L. MaGanita,t Fernando S. Plna,: Eurlco Melo,*,+ and Heinrich Wamhoff*
Centro de Tecnologia Qdmica e Bioldgica and IST, P-2780 Oeiras, Portugal, Faculdade de Cl6ncias e Tecnologia da Universidade Nova de Lisboa, P-2825 Monte da Caparica, Portugal, and Institut fur Organische Chemie und Biochemie der Universitat Bonn, D-5300 Bonn 1, Germany The solubility of 2-(2’-furanyl)-lH-benzimidazole(Fuberidazole) in water-surfactant systems is measured as a function of the pH. The partition constants between water and the micellar pseudophase of both neutral and protonated forms of Fuberidazole (pKal = 5.0) are determined in buffered solutions of an anionic surfactant, sodium dodecyl sulfate (SDS), a cationic surfactant, hexadecyltrimethylammonium chloride (HTAC), and a nonionic surfactant, polyoxyethylene(10) lauryl ether (C12E10)using the fluorescence anisotropy method, and the results compared with those obtained for the octanol-water system. Both forms of Fuberidazole are shown to dissolve readily in the polyoxyethylene region of the ClzElomicelles, with partition constants of 190 f 20 and 3.8 f 0.2 at 25 “C for the neutral and protonated forms, respectively. In SDS, much higher values of the partition constants are observed, respectively, 310 f 25 and 11600 f 1O00, presenting a very low temperature dependence. As expected, the cation is not soluble in HTAC micelles, but the neutral form presents a high partition constant of 850 f 80. Finally, we discuss the partition of Fuberidazole and protonated Fuberidazole between micelle and water as a function of the volume fraction of lipidic media, for different pH values.
partition constants between water and a pure organic medium, such as octanol, will not adequately model the partition between water and amphiphilic aggregates. In the pH range of natural aqueous media, both protonated and unprotonated Fuberidazoles, FubH+ and Fub, respectively, exist in solution [pKal = 5.0 and pK, = 11.7 (611. The equilibrium H
becomes more intricate when lipophilic aggregates, such as phospholipid bilayers or surfactant micelles, are present since we need to account for the solubility of Fub and FubH+ into the nonaqueous phase (7). A further complication arises when the molecular organizate possesses a charged interface where charged species can bind (8,9). In the presence of lipidic media the above equilibrium should be rewritten (10)
I
Introduction Fuberidazole [2-(2’-furanyl)-lH-benzimidazole] has been a fungicide of widespread use. We have already used it as a model compound to establish the techniques for studying the kinetics of the degradation pathways of a pesticide ( I , 2). Its photodegradation pattern and kinetics in acidic and neutral methanol is now well-established. We found that the degradation pattern in water is quite different from the one observed in organic media. Namely, while in acidic water a single photoproduct is formed, benzimidazole-2-carboxylic acid, in neutral or acidic methanol three photoproducts are obtained, l-methoxybenzimidazole (ca. 50%), and benzimidazole-2-carboxylic acid (ca.25%), and methyl 4-oxo-2-benzimidazolecrotonate (ca. 20%) (2). These findings induced us to investigate how this pesticide partitions between aqueous and microdispersed organic media, namely micelles, which may be present in natural waters, and to discuss the consequences of this complex solvent environment on its photodegradation pattern. Moreover, the partition coefficients of pollutants in octanol-water systems, Kow, have long been used as good predictors for the bioconcentration factors of such species (3-5). Many of the common pesticides, however, undergo acid-base equilibria under environmental pH conditions. The direct extrapolation of partition constants to the efficiency of uptake of these chemicals by an organism is, in this case, more complex. In fact, under particular ranges of pH, two or more species are present in water. Their +
Centro de Tecnologia Qu’mica e Biol6gica and IST.
* Universidade Nova de Lisboa. * Universitiit Bonn. 2448
Envlron. Sci. Technol., Vol. 26, No. 12, 1992
t
t
I
U
t
H’ where the subscript L stands for species dissolved into the lipid or somehow attached to the aggregate, and W for the ones in bulk water. The species HL+in the above scheme is not to be taken literally as a proton into the lipidic phase. In nonionic micelles, known to be largely penetrated by water ( l l ) ,the existence of charged species inside the micelle is quite acceptable. Conversely, in ionic micelles, it is expected that ions bind to the surface instead of entering into its hydrophobic core. At environmental pH both Fub and FubH+ coexist in solution, and the above scheme rules their distribution between water and lipid phases. In the present work, we study this multiequilibrium in model systems of watersurfactant using buffered solutions of an anionic surfactant, sodium dodecyl sulfate (SDS),a cationic surfactant, hexadecyltrimethylammonium chloride (HTAC), and a nonionic surfactant, polyoxyethylene(10) lauryl ether (C12Elo),The results are compared with those obtained for the partition of Fuberidazole in the 1-octanol-water system as a function of pH. Moreover, since Fuberidazole shows a systemic effect on plants (12), it should dissolve readily in water but still penetrate easily through biological membranes and other hydrophobic structures. We also
0013-936X/92/0926-2448$03.00/0
0 1992 American Chemical Society
discuss the mechanism of permeation of model phospholipid membranes to Fuberidazole, and its interconnection with the multiequilibrium (2). For the detection and localization of Fub and of FubH+ we used photophysical methods. Both species present a quite high fluorescence quantum yield, @f, of -0.9 (6) which allows us to work at very low concentrations, of the order of magnitude found in natural waters. The study of Fuberidazole, here presented, is representative of what we propose for a general preliminary approach to the study of the degradation of a pesticide in model natural media. In conjunction with the decomposition patterns obtained separately in pure organic media and in water, we can draw a more convincing picture of which photodecompositionand/or thermal decomposition products are to be expected in the natural environment. Formalism Partition Constants by Fluorescence Anisotropy. From the several different photophysical methods available to determine the partition constants of a molecular species between water and microdispersed lipophilic media (13-16),we have chosen the steady-state fluorescence anisotropy method (16). This is a generally suitable method for fluorescent solutes provided that (1) within the concentration range used, the intensity of the emission of the fraction present in one phase does not overimpose the other. Let us say that they should not differ more than about 5-10-fold. And that (2) there is a detectable difference of anisotropy in both media. This means that the ratio T/T (fluorescence lifetime over viscosity) is quite different for the two media. When a chromophore is excited with vertically polarized light, the intensity of the steady-state fluorescence collected with a vertically oriented polarizer, Iw, and the intensity of the horizontally polarized emitted light, IVH, relate to the anisotropy, r, according to the equation r = (IVV - IVH)/ (IVV + 2IVH) (3) Denoting rw and rL the emission anisotropy of the fluorophor in water and organic phase, respectively, we can use the property of additivity of anisotropy ( I 7)to write that
+
+
+
(4) r = fwrw fLrL fw+rw+ fL+rL+ where the coefficients fi represent the fractions of the total fluorescence emitted by the component i. Again, the subscripts W and L refer to the compound residing in each phase, water and lipid, respectively, and the superscript + indicates that the species is protonated. The relation between the fraction f and the molecular parameters, extinction coefficient a t the excitation wavelength, e, fluorescence quantum yield, @, and relative emission intensity at the analysis wavelength, g, and also with the concentration, c, of a given component, i, is easily derived fi = eicia?igi/Ce~ci@igi (5)
The relation between r, eq 4, and the partition constant KP = [ A L I L / [ A W I W (6) is given by
r= [H+lw YLr&p + Ywrwa + YL+rL+KP’Kaw
W+Iw + yw+rw+aKaw
where yi= ti@&!and a = (1 - xL)/xL with xL the fraction of volume occupied by the lipid aggregate. In our notation, the subscript that affects A refers to the environment and the subscript outside the brackets refers to the volume; e.g., [ALILstands for the concentration of A dissolved into the lipid phase referred to the volume occupied by the lipid. At pH values much larger than pKaw(but before any observable deprotonation of Fub), we expect that only the neutral species is present, and eq 7 reduces to r = (YLrSP + rwrw.)/(r&P
+Yw4
(8)
Conversely, if the pH is low enough, the fluorescence anisotropy, now for the emission of the protonated species, is given by the similar expression r = (YL+rL+Kp’+ rw+rw+a)/(rL’Kp’
+Y
W + ~
(9)
In the specific case of micelles, the value of XL may be expressed in terms of the concentration of surfactant, [SI, by means of the usual micelle parameters, the critical micellar concentration, cmc, the aggregation number, v, and the molar micelle volume, VM, accordingly to the equation
Therefore a will be given by v - VM([S]- cmc) a=
vM([s]- cmc)
(11)
By varying the concentration of surfactant we are able to change the fraction of the total volume of the solution constituted by the lipid pseudophase, xL (eq 10). Multiequilibria in the Presence of Charged Interfaces. In anionic micelles, a negatively charged interface is interposed between the water and the organic pseudophase. Positively charged species are expected to attach to the available binding sites of this interface, and for high FubH+ concentrations, a decrease in the ability of these micelles to solubilize molecular ions should be observed. In the scheme represented in eq 2, the equilibrium KpJ represents the binding step, and therefore, in the presence of binding this equilibrium constant should be dependent on the FubH+ concentration (18). Cationic micelles are known to correspond more closely to the hydrocarbon drop model. The solubility of a compound like the neutral Fuberidazole into this micelle is expected to be very high. However, there is no reason for the protonated cationic form to associate with such micelles. Materials and Methods All solutions were in Millipore water, buffered at either pH = 2.8 or 7.8 with phosphate salts. Both buffers are 50 mM in Na+. AU other solvents are spectroscopic or HPLC grade from Merck except for 1-octanol, which was Merck for synthesis >99% but presented adequate spectroscopic purity at X > 250 nm. Fuberidazole is 99% Riedel-deHaen, Pestanal, and its purity was checked by C, N, and H elemental analysis, TLC, and fluorescence. Poly(ethy1ene glycol) (( MW) = 200, PEG-200) and the surfactants SDS and CI2El0,purchased from Sigma Chemicals, were also used as received. Hexadecyltrimethylammonium chloride was extracted from Arquad 16-29 from Akzo Chemicals, Amersfoort,which was a generous gift from IrmBos Planas AlmasquB, Lda., Lisboa. The water present in Arquad Envlron. Sci. Technol., Vol. 26, No. 12, 1992 2440
16-29 was removed by azeotrope distillation with ethanol and the remaining solid recrystallized from diethyl ether. Solutions of the pesticide in water and surfactants were made with the help of a bath sonifier at concentrations lower than M. Fluorescence spectra, values of light scattering, and fluorescence anisotropy were obtained using a Spex Fluorolog 212 in the L conformation at a right-angle geometry. Care was taken to ensure that no observable photodecomposition took place during the measurements. In the case of the fluorescence anisotropy experiments, the instrumental anisotropy correction factor, G, was determined by the method of Azumi and McGlynn (19). Fluorescence decay times were obtained by time-correlated single-photon counting with an experimental setup described elsewhere (20). All the measurements have been performed at 25 f 1OC except when another temperature is explicitly stated. For each of the buffers, the values of critical micelle concentration for SDS and C12E10,in the presence of Fuberidazole, were determined by light scattering (21). The values found in the buffers with pH = 2.8 and 7.8 are, respectively, (2.2 f 0.2) X and (1.4 f 0.1) X M for SDS, and (3.0 f 0.3) X loT4and (1.0 f 0.2) X M M for C12E10,in the same solutions [a value of 1.2 X is published for this surfactant in pure water (22)]. The aggregation number, Y,of SDS in our conditions of ionic strength is v(SDS) = 120 calculated by interpolation of literature values (23, 24). For the same surfactant the molar micelle volume, VM(SDS) = 40.4 M-', was also calculated by interpolation of the experimental values of the hydrodynamic radii given by Mazer et al. (25). The aggregation number and molar micelle volume for ClzElo were obtained from the equations given by Sato et al. (26), u(CIBE1O) = 62 and VM(C12Elo) = 78.1 M-'. Aggregation number and molar micelle volume of HTAC a t pH = 7.8 are 90 (21)and 32.1 M-' (27),respectively. Critical micelle concentration for the same surfactant is (8.0 f 0.4) X lo4 M determined as above. Fluorescence quantum yields in the presence and absence of O2were measured relative to naphthalene in degassed cyclohexane, aF = 0.23 f 0.02 (28). To obtain the parameters Kp, and Kp', and to refine the values of rL and rL+, for one given pH, eq 7, 8, or 9 is adjusted to the experimental values of the anisotropy, r, for different lipid fraction volumes, xL, (corresponding to different surfactant concentrations, [SI) by using the Marquardt algorithm for nonlinear least-squares minimization. In the least-squares computation, the errors in the values of r are assumed to be normally distributed. For the determination of KOw,we followed the method described by Hawker and Connell (29). Results and Discussion Fuberidazole in the Water-C12EloSystem. In Figure l a we present the absorption and fluorescence emission spectra of Fuberidazole in surfactant solution ([C12El,] = 0.2 M) at pH = 2.8 and 7.8. The shapes of the spectra are found to be independent of Fuberidazoleconcentration for values lower than 5.0 X lo4 M, but show a strong dependence on the concentration of surfactant. In particular, at pH = 2.8, the shoulder at 324.5 nm decreases markedly for lower ClzEloconcentrations. Comparison of these spectra with those obtained in buffered water solution (Figure lb) and in PEG-200 (not shown) leads to the following conclusions with respect to the environment of Fub and FubH" in these micelles: (1) The neutral form dissolves readily into the micelles, as can be inferred from the change in absorption and 2450
Environ. Scl. Technol., Vol. 26, No. 12, 1992
0.01
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emission maxima in the presence of micelles with respect to pH = 7.8 buffer. (2) The Fub molecule resides mainly in the polyethoxy external region of the micelle. Its spectrum in micelles is red-shifted with respect to the one obtained in water. In fact, it is similar to the spectrum obtained in PEG-200 and differs from the one in n-hexane (not shown). (3) At low pH, the amount of Fub in water is negligible, and the emission spectrum in C12Elois easily decomposed in two spectra: one that corresponds to Fub into the micelle and other similar to FubH+ in PEG-200; see Figure IC.Therefore, under these conditions, a fraction of nonprotonated Fuberidazole is present in the micelle. To obtain further evidence about the solubilization site of Fub into the micelle, we compared its emission maximum energy with that obtained in PEG-200-water mixtures; see Figure 2. The mixture that gives an equivalent shift has 70% (v/v) water. Therefore, the Fub molecules are exposed to a high amount of water and should be localized in the outer polyethoxy region of the C12E10 micelle. In fact, independent measurements of the amount of water in the polyethoxy region of the similar Triton X-100micelles suggested a volume fraction of -0.5 (30). In Table I, we present several photophysical constants for neutral and protonated Fuberidazole in water and in C12E10,SDS, and HTAC micelles. The low sensitivity of the probe to the polarity of the solvent, and the rationale for the variation of Xabs, hem,af,and rf with the solvent characteristics, have been presented elsewhere (6). Determination of these constants for FubH" into the micelles
>.
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.-VI8
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z
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.-t VI
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Table I. Photophysical Parameters of Neutral and Protonated Fuberidazole in Water and in Micelles of CIZEIO, SDS, and HTAC at 25 O C
Wm
(nm) af (ns)
q (ns)
r
Fub
water ClZElO SDS HTAC
306.0 308.0 307.5 308.0
32150 32100 31700 32300
324.5 325.0 325.0 326.0
0.87 0.92 0.86 0.85
l.60 1.63 3.56 1.70
0.015 0.105 . 0.058 0.105
313.0 314.5 316.0
32300 30900 31700
336.0 338.0 338.5
0.93 0.93 0.94
1.68 0.014 1.70 0.103 3.85 0.063
FubH+
water C12ElO SDS
0.3
XL
(v/v)
Figure 2. Variation of the fluorescence anisotropy, r , (m) and of the energy of the emission maximum (0) of Fuberidazole in mixtures of PEG-200-water as a function of the volume fraction of PEG-200. The lines overimposed on the experimental points do not derive from any model and are merely guides for readability. Also shown are the values obtained in CI2El0 micelles.
species ,A, ab emax and solvent (nm) (M-'cm-')
0.2
0.1
has been performed at high concentration of surfactant (0.2 M) and low enough pH to ensure that all the Fuberidazole is, for practical purposes, in the protonated form. Fluorescence anisotropy values in micelles are obtained, from the plots of r as a function of xL, by extrapolation for xL = 1. It is to be noted that the fluorescence anisotropy in pure poly(ethy1ene glycol) (PEG-200)is r(Fub) = 0.280 and r(FubH+) = 0.274, values that are much larger than the ones found in ClzElomicelles. From the Perrin equation (31),the difference in anisotropy reveals a difference in microviscosity of -3.5-fold. Despite the errors underlying the extrapolation from fluorescence anisotropy to viscosity (32),the observed difference can only originate in a large increase in the fluidity of the polyethoxy chains in the micelles due to the penetration of water. As already pointed out, a large amount of water is mixed with the outer chains in these micelles. In Figure 2, the fluorescence anisotropy in micelles is compared with the one observed for a series of mixtures of PEG-200-water. Again, a high concentration of water (ca. 45% v/v), although smaller than our previous result, is obtained. The lower value is a consequence of the reduced mobility of the polyethoxy chains when anchored to the core of the micelle, leading to a larger microviscosity. A considerable difference in fluorescence anisotropy between water and micellar media is, as previously stated, an important feature to obtain accurate partition constanta out of the anisotropy method. In our case we observe an increase of ---fold in r from water to micelles (only 4X
Figure 3. Experimental values and best fk to eq 7 (solid h e ) of the fluorescence anisotropy of Fuberidazole In CI2El0 micellar medla at pH = 7.8 (0)and 2.8 (0).
Table 11. Equilibrium Constants for Fub and FubH+ in ClzElo-,SDS-, and HTAC-Water Systems at 25 O C
KP Kp' KaW
C12Elo-water
SDS-water
HTAC-water
190 f 20 3.8 0.2 1.0 X
308 f 25 11600 k lo00 1.0 X
850 f 80
*
1.0 x 10-5
in SDS and HTAC). This, in conjunction with a quite high fluorescence quantum yield of Fuberidazole in both aqueous and organic environment,is an excellent condition for application of the method. The equilibrium constants of the reactional scheme 2 are obtained from the evolution of the fluorescence anisotropy, r, with the concentration of surfactant, [SI (see eqs 7 and ll),or with the volume fraction of lipidic phase, xL (see eq 10). At pH = 7.8, we may neglect the contribution of the protonated form to the intensity of fluorescence. Therefore, r as a function of xL is described by eq 8. The experimental data fit this equation quite well with Kp = 190, as shown in Figure 3. At pH = 2.8, both protonated and nonprotonated forms coexist, as was already concluded from spectroscopic evidence; see Figure IC. Under these conditions our data are modeled by eq 7, and the resulting fit is also presented in Figure 3. The parameters leading to the above plots are summarized in Table 11. The spectral decomposition presented in Figure ICallows us to calculate, for the acidic solution, the total amount of nonprotonated Fuberidazole present in both water and organic phase. In this way, we can directly measure the concentrations of Fub and FubH+ in each medium and compare the result with that predicted by our fluorescence anisotropy method. Since the electronic absorption spectra in water and micellar media of either Fub and FubH+ are not exactly coincident, the method is only approximate. For the solution with xL = 0.252, shown in the Figure IC,the spectral decomposition method estimates that 17% of all Fuberidazole is in the neutral form. For the same solution, the anisotropy method predicts 15.6%. The agreement between both values is quite convincing in regard to the accuracy of the results obtained. To further validate the method, we have verified that no change in anisotropy is observed when, for the same concentration of surfactant, we vary the concentration of solute from to M. Therefore, the same relative amount of fluorophores in each phase is maintained.
-
Environ. Scl. Technol., Vol. 26, No. 12, 1992
2451
I
/ /
Table 111. Partition Constants, K p , and Fluorescence Anisotropy in SDS Micelles, rL,as a Function of Temperature for p H = 7.8 and 2.8
0.06 2.
a
p H = 7.8
0 + L 0
temp('C) 20 25 27 35 40
.-
v1
:
0.04
al 0 8
0.00 0.000
0.001
o.co2
0.02
0.04
x, Figure 4. Experimental values and best fit to eq 7 (solld line) of the fluorescence anisotropy of Fuberidazole in SDS mlcellar media at pH = 7.8 (0)and 2.8 (0).
It is also interesting to compare the results of Kp obtained for these nonionic micelles with those of Kow for the same species. Due to its relatively high hydrophobicity, the neutral form is more soluble in 1-octanol, Ko,(Fub) = 380 f 20, than in the highly hydrated external region of these micelles, Kp = 190. As expected, the opposite is observed for the protonated form, which is quite soluble in CI2El0micelles, Kp' = 3.8, and practically insoluble in l-octanol, Kow(FubH+) < 0.2. Fuberidazole in the Water-SDS System. The photophysical parameters of Fub and FubH+ in SDS micelles are presented in Table I. The values obtained are in agreement with the high degree of exposure of the molecule to water, as expected from independent studies (33). In the case where, instead of a true solubilization, a binding takes place, a similar set of values for the above properties would be obtained. Therefore, to elucidate the solubilization mechanism, a careful check of the dependence of the value of the constanta Kp and Kp' on the concentration
p H = 2.8
KP
RL
KP
rL
335 f 30 308 f 25 300 f 30 290 f 25 280 f 30
0.062 0.058 0.055 0.050 0.048
12600 f 1000 11600 f 1000 11800 f 1300 10500 f 1000 11700 f 2000
0.067 0.063 0.063 0.060 0.056
of Fub and FubH+, respectively, has to be performed. The fib to the experimental r obtained for [FubIbM = 5 X lo* M with eq 8 for pH = 7.8, and with eq 7 for pH = 2.8, are presented in Figure 4. The resulting partition constants of neutral and protonated forms of Fuberidazole between water and SDS micellar pseudophase are presented in Table 11. An extensive search of saturation of the binding sites of the SDS micelles by either Fub or FubH', until concentrations of ca.0.5 molecule per micelle, was carried out. Since we did not observe any variation in Kp or Kp', there is no experimental evidence of binding, at least within this concentration range. In Table 111,the results obtained for five temperatures are presented for both pH = 7.8 and 2.8. As expected, the anisotropy values decrease with the increase in temperature. However, the decrease in the partition constants is relatively small, less than 20%. Therefore, we conclude that, within the environmental temperature range, no significant changes in partition are to be expected. Fuberidazole in the Water-HTAC System. In this water-surfactant system, the anisotropy results given in Table I for Fub in the micelles are in agreement with a quite viscous environment,as expected from literature data (34). The fluorescence lifetime, again similar to the ones obtained in homogeneous media, does not present the abnormally high value observed in SDS. This value can be attributed to the intense electrical field at the solubilization site near the SDS-water interface. The strong 1 .o 0.6 0.6
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4.0
6.0
6.0
10.0
0.0
0.5
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Figure 5. Partition of Fuberldazole between water and micelles as a function of the volume fractlon occupied by the organic pseudophase, xL: Species FubL(-), F u h (--), FubH,' (---), and FubHw+(-) In (a) C12E,0at pH = 4.0, (b) CI2El0 at pH = 6.0, (c) SDS at pH = 4.0, and (d) SDS at pH = 6.0. 2452
Envlron. Scl. Technol., Vol. 26, No. 12, 1992
affinity of Fub for this micelle (see Table 11) results from the large polarizability of its core. As expected, there is no detectable solubility of the cation into the micelles of HTAC.
Conclusions The first striking conclusion that we draw from Table I1 is that not only is the neutral form of Fuberidazole highly soluble in neutral micellar media but also the protonated form will solubilize preferentially into these molecular organizates. In anionic micelles of SDS, especially for the case of the cation, the solubility is still higher. The equilibrium constants herein determined allow us to represent the concentrations of the various species as a function of the volume fraction of lipidic media. Such plots for the two amphiphiles, SDS and C12E10,at pH = 4.0 and 6.0, give a more precise view of what is to be expected when Fuberidazole is in a natural environment; see Figure 5. In this figure, the results for the nonionic surfactant C12E10 are depicted in plots a and b, and those for the anionic surfactant SDS in plots c and d. While for C12Elomore than 75% of the total Fub is protonated and dissolved in the bulk water at pH = 4.0and x L < 0.01, in SDS micelles nearly all of the compound is present as FubHL+, at the same pH and lipid volume fraction; see Figure 5a and c. The high affinity of FubH+ for the SDS micelles is again evident at pH = 6.0. In ClzElo the total amount of the protonated form, in both water and micelle, is always below lo%, whereas in SDS the protonated form bound to the For HTAC micelles becomes predominant for XL > the picture is much simpler but similar to CI2El0.The only difference is that no protonated form is found inside the micelle. Also, the neutral form dissolves more efficiently into the lipid. We can conclude from the above results that the photodegradation pattern of this pesticide should be strongly dependent on the presence of microdispersed organic matter in water. This change in the photoproduct distribution is variable with pH and the type of headgroups in the amphiphilic substances. The permeation of Fuberidazole through biological membranes is a consequence of this partitioning of either Fub or FubH+ between lipidic and aqueous media. In fact, at physiological pH some of the Fuberidazole is solubilized in the aqueous phase as FubH', while its neutral form is mainly soluble in the lipidic phase. In this way, by a successive deprotonation-protonation mechanism, the Fuberidazole molecules can freely cross biological barriers. This explains its systemic action in spite of its hydrophobicity (12). We believe that, with the present work, we went a step further in the techniques for modeling of the natural environment in the laboratory. The methods herein developed allow a better understanding of the retention of the pesticides in the soil and in biological lipidic reservoirs (35, 36). Moreover, the knowledge of the characteristics of the adjacent surroundings of the pesticide makes the prediction of the final decomposition products more realistic. Registry No. Fuberidazole, 3878-19-1. Literature Cited (1) Mhvak, M. R.; Sidky, M. M.; Wamhoff, H. Chemosphere 1983,12, 1611.
(2) Melo, M. J.; Pina, F.; Maqanita, A. L.; Melo, E, C.; Herrmann, C.; Farater, R.; Koch, H.; Wamhoff, H. Photcchemistry of 242-Furyl) Benzimidazole(Fuberidazole). 2. Naturforsch., in press. (3) Jafvert, C.; Heath, J. Environ. Sci. Technol. 1991,%, 1031. (4) Jafvert, C. Enuiron. Sci. Technol. 1991, 25, 1039. (5) Kile, D.; Chiou, C. Enuiron. Sci. Technol. 1989, 23, 832. (6) Melo, M. J.; Pina, F. S.; M a w t a , A. L.; Melo, E.; Wamhoff,
H. Excited States of 1-H-Benzimidazole,2-(2-Furanyl) (Fuberidazole)and its Cationic and Anionic Forms. Submitted for publication in J . Photochem. (7) Westman, J.; Boulanger,Y.; Ehrenberg, A.; Smith, I. Bio-
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