1570
J . Phys. Chem. 1989, 93, 1570-1572
Electron Spin Resonance and Electron Spin Echo Modulation Studies of N,N,N',N'-Tetramethylbenzidlne Photoionization Adsorbed at the Interface of Polymeric Latices Piero Baglioni,*it Elisabeth Rivara-Minten, and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: July 1, 1988)
Electron spin resonance (ESR) and electron spin echo modulation (ESEM) of photoionized N,N,N',N'-tetramethylbenzidine (TMB) cation adsorbed at the interface of butadieneacrylonitrile-"thacrylic acid and butadienestyreneacrylic acid polymeric latices have been studied as a function of sodium dodecyl sulfate (SDS) concentration adsorbed at the latex interface. The photoionization yield of TMB in frozen latices mainly depends on the strength of TMB+-water interactions, which are enhanced by added SDS as measured by ESEM. An increase in the negative surface potential of the latex particles, due to the adsorption of SDS at the latex surface, does not affect the photoionization yield, showing that the particle surface potential has, for negatively charged systems, a secondary role in promoting the photoionization yield. Differences in the TMB' yield are found for the two polymeric latices and are attributed to the different latex compositions and/or different interfacial structures.
Introduction The photoionization process of N,N,N',N'-tetramethylbenzidine (TMB) in aqueous micellar solutions has been extensively studied. To understand the photochemical charge-separation process and the photocation stability, the effects of micellar counterions,'V2 micellar size and shape,3 micellar ~ h a r g e ,and ~ . ~the effects due addition have been investigated to alcohol^^*^ and crown ether2*6,7 in frozen micellar solutions. The main results show that the photoefficiency of the charge-separation process depends (1) upon the TMB+-water interaction at the micellar interface, (2) upon the micellar surface charge, and (3) upon the water organization or structure at the micellar interface. In a previous studya on the photoionization of TMB in mixed micelles of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium chloride (DTAC), we showed that the photoionization yield as well as the normalized deuterium modulation depth is almost constant over the mixed micellar composition range 1.0-0.4 mole ratio of SDS. The decrease of the net negative micellar charge, due to the increase of DTAC in the mixed micelle, was expected to produce an increase in the TMB+ yield due to the reduced repulsive barrier for electron escape from the m i ~ e l l e . We ~ attributed the invariance of the photoionization yield to the invariance of the TMB+-water interactions, and we suggested that the "charge effect" was only of secondary importance in the photoionization yield of TMB for neutral or negatively charged surfaces. However, in the SDS/ DTAC system the surface "neutralization" is associated with changes in the micellar system such as micellar growth and shape variation.I0 To overcome these limitations, we have studied in this work the photoionization of TMB at the surface of polymeric latices which change little in shape or size as the surface structure is varied. In such latices the surface charge is varied by the addition of SDS, which is adsorbed at the latex surface." Experimental Section The latices used in this study were a gift from Enichem Elastomeri, Milan, Italy. They were cleaned by ion exchange with mixed resins (Relite CES and Amberlite IRA458, both obtained from Rohm-Haas) and by the serum replacement method following the procedure of Vanderhoff et al.12313The cleaned latices were centrifuged. The supernatant (H20) was discarded and and the solid was dispersed by sonicating for replaced with D20, 30 min. This procedure was repeated three times. The latices were characterized by conductometric titration.I4 The main characteristics of the two latices are reported in Table I. SDS and TMB were purchased from Eastman Kodak. SDS was recrystallized three times from ethanol, washed with diethyl ether, Permanent address: Department of Chemistry, University of Udine, V. le Ungheria 35, Udine, Italy.
0022-3654/89/2093-1570$01.50/0
TABLE I: Characteristics of Latices A and B latex A composition butadiene, 65% acrylonitrile, 30% methacrylic acid, 5% concn, % (w/w) 3.8 6.6 PH 62.7 surf. tension,O dyn/cm av particle diam, nm 260 (monodispersed) surf. groups carboxyl 112/1000 A2 sulfate (from initiator) 2/1000 A* solvent composition (w/w) D20 = 96.1%
H20 = 3.9%
latex B butadiene, 36% styrene, 59% methacrylic acid, 5% 5.0 6.9 59.7 170 35/1000 A2 3/ 1000 A2 D20 96.7% H2O 3.3%
=
Air-water interface.
and dried at 50 OC under moderate vacuum. TMB was dissolved in chloroform. A thin film of T M B was generated in vials by gently evaporating the chloroform under nitrogen atmosphere. The samples were prepared in nitrogen atmosphere when adding the latex and SDS. The samples were gently sonicated for 30 min and let stand overnight to reach thermodynamic equilibrium. All the samples were sealed in 2-mm i.d. X 3-mm 0.d. Suprasil quartz tubes and frozen rapidly in liquid nitrogen. Photoirradiation of TMB was carried out at 77 K for 6 min with a 150-W xenon lamp (Cermax) filtered with 10 cm of water (1) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J . Am. Chem. SOC.1984, 106, 4675; 1985, 107, 6467. (2) Baglioni, P.; Rivara-Minten, E.; Kevan, L. J . Phys. Chem. 1988, 92, 4726. (3) Narayana, P. A,; Li, A. S. W.; Kevan, L. J . Am. Chem. Soc. 1982,104, 6502. (4) Arce, R.; Kevan, L. J. Chem. SOC.,Faraday Trans I 1985,81, 1025, 1669. (5) Baglioni, P.; Kevan, L. J. Phys. Chem. 1987, 91, 2106. (6) Baglioni, P.; Kevan, L. Prog. Colloid Polym. Sci., in press. (7) Baglioni, P.; Kevan, L. J. Chem. Soc.,Faraday Trans. I 1988,84,467. ( 8 ) Rivara-Minten, E.; Baglioni, P.; Kevan, L. J . Phys. Chem. 1988, 92, 2613. (9) Maldonado, R.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J. Phys. Chem. 1984, 81, 3958. (10) Malliaris, A,; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (1 1) Baglioni, P.; Cocciaro, R.; Dei, L. In Surfactants in Solutions; Mittal, K. L., Chattoraj, D. K., Eds.; Plenum: New York, 1989. (12) Ahmed, S. M.; El-Aaser, M. S.; Pauli, G. H.; Poehlein, G. W.; Vanderhoff, J. W. J. Colloid Interface Sci. 1980, 73, 388. (13) Ahmed, S . M.; El-Aaser, M. S.; Micale, F. J.; Pochlein, G. W.; Vanderhoff, J. W. In Solution Chemistry of Surfacranrs; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 2, pp 853-864. (14) Buscall, R.; Ottewill, R. H. In Polymer Colloids; Buscall, R., Corner, T., Sageman, J. F., Eds.; Elsevier: New York, 1985; pp 141-217, and references therein.
0 1989 American Chemical Society
N,N,N',N'-Tetramethylbenzidine Photoionization
The Journal of Physical Chemistry, Vol. 93, No. 4, 1989 1571 z
0
I
TMB LATEX A = *
-1.0 0
25
50
LATEX B=A
75
100 SDS (mMi
Figure 3. Normalized deuterium modulation depth for TMB' in latices A and B as a function of SDS concentration.
I
-UJ5imV) 100
0
LATEX A=
-ATEX
B= A
TMB
s
L A T E X A=.
L A T E X B=A
-5
-4
-3
-2
-1
IOQ
SDS (Mi
Figure 4. Z potential for latices A and B as a function of SDS concentration.
E k?
0 02 5 0
525
5L0
75 - 100 2 0 0
SDS imM)
Figure 2. Relative photoionization yield of TMB' solubilized in latices A and B as a function of SDS concentration. The yield has been normalized to the TMB' photoionization yield in 0.1 SDS micellar solutions without latex taken as unity. and a Corning filter No. 7-51. This combination passes radiation centered at 370 nm with 80% transmittance. Electron spin resonance (ESR) spectra were recorded at 77 K on a Bruker ESP 300 ESR spectrometer. Two-pulse electron spin echo spectra were recorded at 4.2 K on a home-built spectrometer by using 40-ns exciting pulses. After the experiment each sample was thawed. The results obtained from samples that showed even partial coagulation were discarded. Z potential measurements were made by a Pen Kem Model 501 Z meter. The latex samples were diluted with a SDS solution to a concentration of 7.5 mg/mL of polymer. The solution was allowed to stand 1 day before the 2 potential measurements were taken.
Results Figure 1 reports two-pulse electron spin echo spectra at 4.2 K of photogenerated TMB+ adsorbed at the interface of type A or B latices, as described in Table I, in the presence of 2 X M SDS. The spectra exhibit modulation with a 0.5-ps period due to electron-deuterium dipolar interaction and an additional modulation of 0.08 ps due to TMB+-hydrogen interactions. Figure 2 shows the relative yield of TMB cation at 77 K as a function of the SDS concentration per liter of latex solution. The TMB' yield was obtained from doubly integrated ESR spectra at 77 K. The values reported are normalized with respect to those in 0.1 M S D S / D 2 0 micellar solution without latex particles for which solution the TMB+ yield has been taken as unity. Figure 3 shows the normalized deuterium modulation depth for the two latices as a function of SDS concentration. The normalized deuterium modulation depths were computed as previously d e ~ c r i b e d . ~ Finally, Figure 4 reports the trend of Z potential measurements for the two latices as a function of the total SDS concentration.
Discussion The SDS adsorption at the latex interface occurs with a gradual coverage of the latex particle until the total surface area available is Occupied by SDS monomer^.^^^^^ A hydrophobic probe molecule
such as a nitroxide, adsorbed at the latex surface, can monitor the adsorption process. It has been f o ~ n d ' ~that ~ ' ~a ~nitroxide '~ molecule located at the surface of the latex environment can follow variations of the surface environment until the surface is completely covered by SDS monomers. At higher SDS concentrations no changes of the latex particle surface environment can be detected by a nitroxide probe, and the latex surface or interface seems to be similar in local polarity and viscosity to an SDS micellar interface. Considering the above, since TMB is a hydrophobic molecule we expect the TMB' photoionization yield and the deuterium modulation depth to be dependent on the amount of SDS adsorbed at the latex interface only up to monolayer coverage. For SDS concentrations greater than the amount necessary to saturate the latex interface, the TMB' photoionization yield and the deuterium modulation depth are expected to be almost constant and similar to the values found for TMB+ in micellar solutions. Figure 2 reports the TMB' photoionization yield for both latices as a function of SDS concentration. It is seen that (a) the photoionization yield increases with increasing SDS concentration and plateaus for a SDS concentration of about 30-40 mM depending on the latex and (b) the plateau value of the relative TMB+ yield is unity for latex A (the same as for TMB' in SDS micellar solutions) and about 20% lower for latex B. Thus TMB in latex A behaves as expected, while the lower plateau yield in latex B suggests different interfacial structures for latices A and B. Previous ~ o r k ~has - ~shown * ~ a~ direct correspondence between the efficiency of charge separation by TMB' photoionization and the strength of TMB'-water interactions as measured by ESEM. The normalized deuterium modulation depth, which is a measure of the strength of TMB+-water interactions, increases upon addition of SDS as shown in Figure 3. This shows the same trend as the TMB' yield in Figure 2, which indicates a correlation between the TMB' yield and the strength of TMB+-water interactions. However, the deuterium modulation depth is greater for latex B than for latex A, while the reverse is seen for the photoionization yield. This difference indicates that although the trend of the photoionization yield is correlated to the trend to stronger TMB+-water interactions, other factors related to the (15) Baglioni, P.; Cocciaro, R.; Dei, L. J . Phys. Chem. 1987, 91, 4020. (16) Kevan, L. In Photoinduced Electron Transfer; Fox, M. A., Chanon,
M., Eds.; Elsevier: Amsterdam, in press. (17) Baglioni, P.; Kevan, L., manuscript in preparation.
1572
J . Phys. Chem. 1989, 93, 1572-1575
latex particle surface structure must also be involved. Such factors could include the interface structure, the water distribution at the interface, the chemical composition, and the surface potential. Of these factors the surface potential can be directly investigated experimentally. The surface potential is a function of the latex particle charge and can be directly measured by a Z meter. Figure 4 shows that the Z potential becomes more negative for both latices as the SDS concentration increases. This is consistent with the adsorption of SDS leading to an increase in the negative surface charge. Quantitative values of the surface charge or charge variation are not directly determinable since the Z potential is the overall charge of the colloidal particle along with its electrical double layer.I8 From a qualitative point of view an increase in the negative surface charge between the TMB location and the bulk water should lead to a decrease in the photoionization yield.8J6 This is opposite to what is observed in Figure 2, so the trend in photoionization yield is not correlated to the Z potential trend. This supports, at least for negatively charged particles, that the photoionization yield does not correlate with interfacial particle charge. Therefore, the most important contribution to the TMB photoionization yield, in negatively charged particle systems, is the TMB+-water interaction. This result confirms deductions from the mixed micellar system SDS/DTAC8 The difference of about 20% in the TMB+ yield between latices A and B seems to be (18) Hunter, R. J. Zeta Potential in Colloid Science: Principles and Applications; Academic Press: New York, 1981.
related to different interfacial structure perhaps associated with the different chemical compositions of the two latices since the TMB’ yield in latex A versus latex B is not correlated to the deuterium modulation depth or to the Z potential.
Conclusions The photoionization yield of TMB+ in frozen latices depends on the photocation-water interaction and is a function of the amount of SDS adsorbed at the latex interface. An increase in the negative charge of the latex particle does not affect the photocation yield, showing that the surface charge plays a secondary role for the photocation yield in negatively charged dispersed systems. The TMB+ yield difference of about 20% found for the two latices is not related to a corresponding trend in the TMB+-water interactions or to the particle surface charge. This result suggests that the interfacial structure of the latices have a role in the photocation generation, even if secondary to that of the TMB+-water interactions. Acknowledgment. This work has been supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U S . Department of Energy. P.B. thanks the MPI for partial financial support. Thanks are also due to Drs. R. Cocciaro and L. Dibuo of Enichem Elastomeri, Ravenna, Italy, for synthesis and purification of the latices. Registry No. TMB,366-29-0;TMB+,21296-82-2; SDS,151-21-3; butadiene-acrylonitrile-methacrylic acid polymer, 9010-81-5; butadiene-styrene-acrylic acid polymer, 25085-39-6.
Electron Spin Echo Modulation Studies of Doxylstearic Acid Spin Probes in Frozen Vesicles: Interaction of the Spin Probe with D,O and Effects of Cholesterol Addition Thomas Hiff and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 (Received: August 15, 1988)
Electron spin echo studies have been carried out for a series of x-doxylstearic acid (x = 5 , 7, 10, 12, and 16) spin probes in frozen deuteriated aqueous solutionsof phospholipid vesicles and cationic dioctadecyldimethylammonium chloride (DODAC) vesicles. Modulation effects due to interactions of the nitroxide group of the spin probes with D20give information about the conformations of the probes and the degree of hydration of the surfactant headgroups as well as about the degree of packing of the alkyl chain. We show that DODAC headgroups are more hydrated than choline headgroups and that the doxylstearic acid probes show a larger tendency for bending in DODAC vesicles than in phospholipid vesicles. Upon addition of cholesterol into phospholipid vesicles, the headgroups are separated and their degree of hydration increases.
Introduction Photoinduced charge separation of photosensitive solutes and the subsequent charge transport in organized molecular assemblies are typical models for artificial photosynthetic rea~tions.l-~ Among other colloidal systems, vesicles have been widely used to mimic natural membranes?+ Photoionization and net charge separation may be markedly affected by the structural parameters of the vesicle, such as counter ion^,'*^ head group^,^^'^ and hydrocarbon tail length"^'* of the amphiphiles, or by adding salt^^^,'^ J. K. Ace. Chem. Res. 1977, 10, 133. (2) Calvin, M. Ace. Chem. Res. 1978, 11, 369. (3) Gratzei, M.; Thomas, .I. K. J . Phys. Chem. 1974, 78, 2248. (4) Hurley, J. K.; Tollin, G. Sol. Energy 1982, 28, 187. (5) Fendler, J. H. Ace. Chem. Res. 1980, 13, 7. (6) Colaneri, M. J.; Kevan, L.; Thompson, D. H. P.; Hurst, J. K. J . Phys. Chem. 1987, 91, 4072. (7) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J . A m . Chem. Soc. 1985, 107, 184. (8) Jones, R. R. M.; Maldonado, R.; Szajdzinska-Pietek, E.; Kevan, L. J . Phys. Chem. 1986, 90, 1126. (9) Plonka, A,; Kevan, L. J . Phys. Chem. 1985, 89, 2087. (IO) Fang, Y.; Tollin, G. Photochem. Photobiol. 1984, 39, 685 (1) Thomas,
0022-365418912093-1572$01.50/0
or slightly water soluble alcohols or ch01esterol.l~ Stable nitroxide radicals have been widely used as spin probes in studies of biological membranes and membrane mimetic systems.I6-l8 Among others, x-doxylstearic acids of the formula HOOC (CH2 )x-2-C-(CH2)
’0
I ~ - ~ C H ~
‘NO.
L+
have turned out to be particularly useful, for they are sparingly soluble in water but can be readily incorporated into heterogeneous (11) Narayana, P. A.; Li, A. S. W.; Kevan, 104, 6502.
L. J . A m . Chem. Soc. 1982,
(12) Hiff, T.; Kevan, L.J . Phys. Chem., in press. (13) Plonka, A.; Kevan, L. J . Chem. Phys. 1985, 82, 4322. (14) Hiromitsu, I.; Kevan, L. J. Phys. Chem. 1986, 90, 3088. (15) Hiromitsu, I.; Kevan, L. J . A m . Chem. Soc. 1987, 109, 4501. (16) Spin Labeling Theory and Application; Berliner, L. J., Ed.; Academic Press: New York, 1976. (17) Fendler, J. H.; Fendler, E. J. Catalysis in Micelles and Macromolecular Systems; Academic Press: New York, 1975. (18) Marsh, D. In Membrane Spectroscopy; Grell, E., Ed.; SpringerVerlag: Berlin, 1981; Chapter 2, p 51.
0 1989 American Chemical Society
