J . Phys. Chem. 1986, 90, 1830-1834
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i n the metal-nitrogen bond in three directions would be accomin the pressure response of T , thus implicating AT‘,+. In the case L F process. The panied by AV, = 10 mL/mol for the C T of concentrated electrolyte solutions, molecular dynamic simu0.10-A geometrical factor is of the correct magnitude based on lations of ion hydration at 1 and 10000 bar suggest significant ’~ Franck-Condon analysis of L F states in related ~ o m p 1 e x e s . l ~ ~ ’ ~distortion of the H-bond network with increasing p r e s s ~ r e , in agreement with the loss in water structure observed in the difHence the sign and magnitude of A.V,’ for nonradiative electronic relaxation are explained by intrinsic volume changes of the two fraction patterns of LiCl solutions.20 If this decrease in water structure is the cause of the reduced pressure response of the k , states coupled in the transition. The data analysis reveals three additional effects in the pressure decay channel, then the opposite k,(P) results in urea solutions dependence of T . First, a solvent isotope effect in AV; is evident (Table IV) can be interpreted to suggest “structure-making’’ from Table 111. The influence of deuterium substitution on the abilities with pressure. This prediction has not been tested at high CT GS relaxation has been cited as evidence for the admixture pressures and further work is needed to mesh the interesting of charge-transfer-to-solvent states with effective vibronic coupling aspects of hydrogen-bonding and urea-water interactions2’ with between solvent molecules and the c ~ m p l e x . ~ ~The ~ .smaller ~ ~ ~ ’ ~ ~ its ’ ~ influence on excited-state solute processes. The inclusion of the temperature variable has been essential pressure increase of k2 in D 2 0 relative to H 2 0 and CH3CN (Table 111) is then attributed to poorer coupling with the OD environment in the interpretation of the high-pressure data on luminescent in the cybotactic cleft at high pressures. The presence of an isotope lifetimes of Ru(I1) complexes. Whenever thermally accessible effect on k2 and not k3 is entirely consistent with the weak coupling states are suspected to play a role in electronic relaxation, it will limit for the CT GS radiationless t r a n ~ i t i o n .Second, ~ the be imperative to include T in pressure studies of excited-state results in Table 111 demonstrate a small ligand effect in k , ( P ) , behavior. In summary, this project has demonstrated opposite with a greater pressure sensitivity indicated for phen ligands in pressure effects on the two nonradiative decay channels of ruthenium( 11) polypyridine complexes: a small and negative AT‘; H 2 0 and D20. The difference in AT‘,+ (Table V) can be assigned and a large positive AV,’. to the increased size and rigidity of phen vs. bpy ligands. Apparently, the ligand effect is only observable in the strongly coupled Acknowledgment. We acknowledge the donors of the Petrok3 decay channel. leum Research Fund, administered by the American Chemical Finally, significant differences in A V , - I are ~ observed with water Society, for partial support of this research. We are most grateful additives at a temperature where the thermally activated k3 process to Professor R. J. Watts for access to his lifetime instrumentation. is the dominant decay channel (Table IV). Electrolytes and urea in high concentrations evidently modify the solute-solvent inRegistry No. Ru(bpy),Clz, 14323-06-9;Ru(phen),CI,, 23570-43-6; HIO, 7732-18-5;DZO, 7789-20-0;CHlCN, 75-05-8. teractions in such a manner as to result in opposing effects on the net CT LF relaxation rate. The small effects noted in T” (Table IV) suggest that 0, is not responsible for the significant difference --+
-+
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(17) Hipps, K. W.; Merrell, G. A,; Crosby, G.A. J . Phys. Chem. 1976, 80, 2232. (18) Wilson, R. B.; Solomon, E. I . J . Am. chem. SOC.1980, 102, 4085.
(19) Jancso, G.; Heinzinger, K.; Kadnai, T. Chem. Phys. Lett. 1984, 110, 196. (20) Narten, A. H.; Vaslow, F.; Levy, H . A. J . Chem. Phys. 1973, 58, 5017. (21) Kuharski, R. A,: Rossky, P. J. J . A m . Chem. SOC.1984, 106, 5786.
SURFACE SCIENCE, CLUSTERS, MICELLES, AND INTERFACES Cooling of Water/Oil Mlcroemulsions: The Cupric Probe Peter Bruggeller Institut Fur Anorganische und Analytische Chemie. Universitat Innsbruck, A-6020 Innsbruck, Austria (Received: April 15, 1985; I n Final Form: November 7 , 1985) Cupric chloride has been used as an EPR spin probe to test the possibility of ice formation in contrast to a vitrification behavior of the aqueous part of water/oil (w/o) microemulsions when they are cooled. The investigated w/o microemulsions contained cationic, anionic, and nonionic surfactants and the influence of a short-chain alcohol used as cosurfactant is examined. Furthermore, a surfactantless system is studied. In all cases vitrification of the water content is readily obtained at comparatively low cooling rates: ice formation is in one case observable when a w/o microemulsion is cooled very slowly. The others are capable of being cooled at any rate without any observable ice formation. The g,,splittings of the cupric EPR signals are very sensitive to environmental effects in the solid state, giving information about the participation of alcohols and/or surfactants in the coordination sphere of the cupric ion and concentration gradients within a more concentrated cupric chloride/ice phase. Introduction The appearance of microemulsions is possibly the last marked sign of amphiphile association into large aggregates before essentially complete breakdown of supramolecular organization. However, the subdivision on a microscopic level into hydrophilic 0022-3654/86/2090-1830$01.50/0
and hydrophobic domains alters the self-diffusion of the molecules 1-2 orders of magnitude.] In normal micelles NMR data clearly show that the chain termini within the micelles are “wet” on a ( I ) Fabre, H . et ai. J . Phys. Chem. 1981, 85, 3493.
0 1986 American Chemical Society
Cooling of Water/Oil Microemulsions time-averaged bash2 At the micelle surface the water molecules reorient only 2-3 times slower than in pure water3 and no longrange micelle water interaction has been observed. Angell and c o - w o r k e r ~have ~ ~ ~used microemulsions to study the “marginal” glass formation in molecular liquids. The main consequence of his study is that liquids which are not easily vitrifiable can be vitrified with low cooling rates in these disperse systems. The question whether the “droplets” contain real bulk liquids is difficult to answer since the droplets, if present at all, can collide and form “dimers” with transient fusion of the liquid pools.6 Both the formation and breakdown of these dimers are very rapid. However, since the modification of the bulk liquid structure does not exceed two to three surface layer^,^ there is certainly bulk liquid present even in the case of dynamic interfaces and no droplets present. Only when the cosurfactant, in most cases short-chain alcohols, is soluble in the dispersed liquid this liquid clearly becomes modified. EPR spectroscopy is a useful technique for probing the structure and dynamics of macromolecular systems.*-1° Cupric chloride as a spin probe has been used to study the freezing of water on Sephadex and in emulsions”*12and the water structure modification by glass surface^.^ Cupric chloride has three main advantages with respect to other spin probes: (1) cupric EPR-spectra are detectable even in low symmetry environments; (2) since two relaxation mechanisms govern its line width, it can be studied in a large field of solvent visco~ities;’~ and (3) the EPR spectra of cupric chloride show in the concentration range 0.10-4.3 M due to dipolar broadening and exchange narrowing a strong dependence of the signal from the concentration. When during cooling a concentration shift due to phase separation occurs this is reflected in a pronounced spectral ~ h a r g e . ~
Experimental Section The water/oil (w/o) microemulsions were prepared by mixing the components and sonifying (a Branson sonifier B- 12 was used) the mixture in order to accelerate equilibration. The surfactants were obtained from Fluka, Switzerland; SDS (sodium dodecyl sulfate) was purissimum grade; CTACI (cetyltrimethylammonium chloride) and Triton X-1 00 (alkylphenylpoly(ethylene glycol)) were purified by conventional methods. Toluene, 1-butanol, 1hexanol, 1-octanol, 2-propanol, n-hexane, and cyclohexane were obtained from Merck, Germany, and used with purissimum grade purity. Distilled and deionized water was used. Cupric chloride was also obtained from Merck and was p.a. grade. The following five systems have been prepared: (1) A ternary system consisting of n-hexane, 2-propano1, and water (weight ratio 10.6:7.78:1.00; for all EPR experiments a 0.10 M CuCI, solution has been used instead of water) has been claimed to have w/o microemulsion properties.14J5 The composition used in this study is centered within the discussed microemulsion region. However, recent criticism against supramolecular organization in detergentless microemulsions exists.I6 (2) A w/o microemulsion containing 1-butanol, SDS, toluene, and water is described, e.g., in ref 17. The used composition of 4.88:2.44:6.56:1.00 is located in the middle of the clear w/o phase in the pseudoternary phase diagram. (3) A wjo microemulsion containing I-hexanol, Triton X-I 00, cyclohexane and water (weight ratio 8.20:2.00:6.16:1 .OO) is de(2) Menger, F. M.; Chow, J. F. J . Am. Chem. SOC.1983, 105, 5501. (3) Halle, B.; Carlstrom, Ci. J . P h j ~ s .Chem. 1981, 85, 2142. (4) MacFarlane, D. R.; Angell, C. A. J . Phys. Chem. 1982, 86, 1927. (5) Angell, C.A. et al. J . Phys. Chem. 1984, 88, 4593. (6) Eicke, H.F. et al. J . Colloid Interface Sci. 1976, 56, 168. (7) Briiggeller, P. J . Colloid Interface Sci. 1983, 94, 2. (8) Narayana, P. A. et al. J . Am. Chem. SOC.1981, 103, 3603. (9) Stilbs, P.;Lindman, B. J . Colloid Interface Sci. 1974, 46, 1. ( I O ) Narayana, P. A. et al. J . Phys. Chem. 1982, 86,3. (11) Briiggeller, P.; Mayer, E. J . Phys. Chem. 1981, 85, 4135. (12) Briiggeller, P. J . Disp. Sci. Techn. 1982, 3(4), 395. (13) Basetti, V. et al. J . Am. Chem. SOC.1979, 101, 19. (14) Smith, G . D.et al. J . Colloid Interface Sci. 1977, 60, 488. (15) Keiser, B. A. J . Phys. Chem. 1979, 83, 1276. (16) Stilbs, P.;Lindman, B. Prog. Colloid Polym. Sri. 1984, 69, 39. (17)Roux-Desgranges, G.et al. J . Colloid Interface Sci. 1981, 83, 569.
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1831
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I I I I 1’1 I H 2700 3000 3300G Figure 1. EPR spectra of a detergentless w/o microemulsion consisting of n-hexane/2-propanol/0.10M CuCI,-water solution (weight ratio 10.6:7.78:1) a t various temperatures: (A) 313 K, (B) 293 K,(C)253 K,(D)223 K,(E)173 K. I
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scribed, e.g., in ref 18. This system solubilizes up to 10% (vjv) water, existing as a clear, isotropic, spherical w/o microemulsion. (4, 5) Water/oil microemulsions containing 1-octanol have been studied, e.g., by Boutonnet et aI.l9 in the composition range which has been used in this study (weight ratios CTACI/ 1-octanol/H,O = 1.33 5.33: 1 .OO and SDS/ 1-octanol/H,O = 1.06:5.33 :1 .OO). All EPR spectra were measured on a Varian E-104 A spectrometer equipped with an E 257 variable-temperature unit at X-band frequencies.
Results Figures 1-5 show the EPR spectra obtained when the corresponding w/o microemulsions 1-5 are cooled.20 All signals are interpretable in terms of the spin Hamiltonian for d9 ions in axial symmetryz1 7f= g,,PIfzSz + g,P(SxHx + SyH,) + A , , I J z + A,UxSx + and in fact do not appreciably differ from those of C U ( H ~ O ) ~ ~ + in water-glycerol solutions and glasses, respectively. The temperature dependence of the CU(H,O)~~’ EPR spectra is determined by spin rotational relaxation and modulation of the g and A anisotropies.22 Both mechanisms reflect the mobility Kurnar, C.; Balasubramanian, D. J . Phys. Chem. 1980, 84, 1895. Boutonnet, M. et al. Colloids Surf. 1982, 5 , 209. On quenching thin glass tubes of these microemulsions in liquid nitrogen only microemulsion 3 remains optically transparent, the others become turbid. Since pure l-octanol (at about 259 K), n-hexane, etc. also becomes turbid on cooling, because they crystallize, the different appearance of the cooled w/o microemulsions mainly reflects the different behavior of the hydrophobic parts of the w/o microemulsions. However, the EPR-sensitive probe cupric chloride is located in the hydrophilic part of the microemulsions and the EPR spectra give evidence whether this part vitrifies or crystallizes. (21) Hair, M.L.“Infrared Spectroscopy in Surface Chemistry”; Arnold: London, 1967. (22) Wilson, R.; Kivelson, D. J . Chem. Phj-s. 1966, 44, 169.
Briiggeller
1832 The Journal of Physical Chemistry, Vol, 90, No. 9, 1986
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2800 3100 3400G Figure 3. EPR spectra of a w/o microemulsion containing a nonionic surfactant: 1-hexanol/TritonX-lOO/cyclohexane/O.lO M CuCI,-water solution (weight ratio 8.20:2.00:6.16:1) at various temperatures: (A) 293 K, (B) 273 K , (C) 253 K, (D) 233 K, (E) 213 K. 2500
I I I I I I I I H 2400 2700 3000 3300G Figure 2. EPR spectra of a w/o microemulsion containing 1-butanolas cosurfactant. I-butanol/SDS/toluene/O. I O M CuCI2-water solution (weight ratio 4.88.2.44:6.56:1)at various temperature$: (A) 293 K, (B) 223 K, (C) 173 K, (D) I13 K
of the solvent molecules around the paramagnetic ion, the second being more important in the case of cooled microemulsions. If during cooling some of the cupric chloride is ejected from a solvent, the concentration of cupric chloride does not remain constant but increases." However, a concentration shift of cupric chloride in the range 0.10 M (concentration of an initial solution) to 4.3 M (eutectic composition) can easily be followed by EPR spectroscopy.' In the case of water as solvent the concentration dependence of the EPR spectra in this range is a very sensitive indication for ice formation. The EPR spectra of a cryoprotected (by at least 50% w/w glycerol) vitrified 0.10 M CuC12 solution shows a strong g, absorption at about 31 30 G and four weak g , absorptions between 2500 and 2980 G." During phase separation this spectrum irreversibly collapses giving a final single line at about 2980 G corresponding to CuCI, concentrations above 2.5 M in the solid state.' Figure 1 shows the EPR spectra of the cooled detergentless w/o system 1 consisting of n-hexane, 2-propano1, and a 0.10 M CuCI, solution (weight ratio 10.6:7.78:1.00). Figure 1A is the typical one-line spectrum at about 2970 G of CU(H,O),~+in an isotropic, liquid environment. The hyperfine structure (hfs) is only poorly resolved. At room temperature (Figure 1B) the hfs becomes more pronounced, indicating that a decrease in mobility leads to an enhancement of the Cu2+-2-propanol interaction. At 253 K (Figure 1C) the spectrum contains anisotropic contributions as a consequence of further reduced mobility and at 223 K (Figure 1D) the separation into gll (centered at about 2730 G) and g, absorption lines (at 3070 G) with A,, >> A , is complete. At about 173 K (Figure 1E) the spectrum consists of the typical strong g, absorption at 3 130 G and the weak gl,absorptions between 2500 and 2980 G (see above), indicating the limit of mobility. The splitting of the g absorption (more than four lines) is observable at thi\ temperature. It is due to a coordinative interaction of 2-propanol with Cu(H20)" All spectra are reversible with
temperature and in none of the spectra of Figure 1 any indication of ice formation is observable. The cooling behavior of the w/o microemulsion 2 with 1-butanol as cosurfactant (Figure 2) is very similar to that of the detergentless system 1. At room temperature (Figure 2A) the single line at 2970 G can be seen; no hfs indicates a weak or no Cu(H20),2+-l-butanol interaction. At 223 K (Figure 2B) the splitting into g, and gll absorption occurs and at about 173 K (Figure 2C) the glassy spectrum is reached. At 113 K (Figure 2D) the typical strong g, absorption at 3130 G and the split g,, absorption between 2500 and 2980 G can be seen. Also here all spectra are reversible with temperature and no indication of phase separation in the aqueous part of the microemulsion is observable. The w/o microemulsion 3 in Figure 3 containing a nonionic surfactant and 1-hexanol as cosurfactant behaves in an only slightly different manner. The single line at 2970 G at room temperature (Figure 3A) and the onset of hfs at 273 K (Figure 3B) are quite as before. Figure 3C represents a typical combination spectrum of an isotropic (centered at about 3020 G) and an anisotropic part (centered at about 3130 G) and at 233 K (Figure 3D) the separation into g, and gll is nearly complete. However, the rigid glass spectrum with its typical main absorption at 3130 G and only a weak g,lsplitting between 2500 and 2980 G is reached at only 213 K (Figure 3E) from where it remains constant to lower temperatures. All spectra are reversible with temperature and no indication of ice formation is observable, as would be indicated by an irreversible collapse o f one of the solid-state spectra. A comparison of the cooling behavior of the two w/o microemulsions 4 and 5 of Figures 4 and 5 gives very interesting results. Both w/o microemulsions contain 1-octanol as the hydrophobic component and nearly the same water content. CuClz is completely insoluble in I-octanol and even after addition of a surfactant to I-octanol (no matter if CTACl or SDS is used) and sonifying the mixture together with CuCl,(s), no EPR signal is observable i n the resulting solution, indicating that no CuCl,(s) has been
Cooling of Water/Oil Microemulsions
The Journal of Physical Chemistry, Vol. 90, No. 9, 1986 1833
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I 1 I I H 3000 33ooG Figure 4. EPR spectra of a w/o microemulsion containing a cationic surfactant: CTACI/ 1-octanol/O. IO M CuCI,-water solution (weight ratio 1.33:5.33:1) a t various temperatures: (A) 293 K, (B) 273 K, (C) 253 K, (D) 243 K, (E) 233 K, (F) 223 K, ( G ) 173 K .
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Figure 5. EPR spectra of a w/o microemulsion containing an anionic surfactant: SDS/l-octanol/O. 10 M CuCI2-water solution (weight ratio 1.06:5.33:1) at various temperatures: (A) 293 K, (B) 253 K, (C) 248 K, (D) 113 K, (E) quenched in liquid nitrogen and recorded a t 113 K.
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Figure 6. Splitting patterns of the first two peaks of the gl,absorption: (A) CTACl/l-octanol/O. IO M CuC12-water microemulsion cooled slowly to 113 K (quotient of the two peak heights = 6.05). (B) Microemulsion (A) cooled by immersion into liquid nitrogen and recorded at 1 1 3 K (quotient of the two peak heights = 7.6). (C) SDS/1-octanol/O.lO M CuC12-water microemulsion cooled by immersion into liquid nitrogen, annealed IO min at 228 K, recooled, and recorded at 113 K. (D) nHexane/2-propanol/O. 10 M CuCI,-water microemulsion cooled slowly to 113 K. (E) Microemulsion D cooled by immersion into liquid nitrogen and recorded at 113 K.
dissolved. Water dissolves up to about 0.05% (w/w) 1-octanol and 1-octanol dissolves up to about 1% (w/w) water.23 However, the weight ratio of 1-octano1:water (Le., a 0.10 M CuCl, solution) is 5.33:l.OO for both the CTACI (weight factor 1.33)/1-octanol/O.lO M CuCI2 microemulsion and the SDS (weight factor 1.06)/1-octano1/0.10 M CuCI2 microemulsion. Since as mentioned earlier water structuring from the present dynamic interfaces has its limitations, there is certainly bulk aqueous phase present with CuClz being located within the mobile water cores in these two microemulsions. The two microemulsions contain a cationic and an anionic surfactant, respectively. (i) In the cationic case the single EPR line at about 3000 G at room temperature (Figure 4A) nearly remains constant down to 243 K (Figure 4B-D). Figure 4E,F shows the transition region from isotropic to anisotropic behavior due to the reduced mobility of the cupric ion and at 173 K the glassy stat: is reached (Figure 4G), with the typical 3130-G absorption and a strong gll splitting. All spectra are reversible with temperature and there IS no indication of ice formation. (ii) In the anionic case there is also the single line at about 3000 G at room temperature (Figure 5A) which shows hfs at 253 K (Figure 5B). Then at 248 K (Figure 5C) a new type of signal appears, centered at 2980 G, and corresponding to cupric chloride concentrations above 2.5 M (see above) caused by extended ice formation, This spectrum nearly remains the same down to 113 K (Figure 5D)except that the low concentration part of Figure 5C at about 3150 G intensifies. The spectra 5A-D reflect an irreversible change and the typical last spectrum of Figures 1-4 with the strong gL absorption at 3 130 G and the weak g,:peaks between 2500 and 2980 G cannot be reached with the low cooling velocity used here. If the microemulsion of Figure 5A is quenched in liquid nitrogen then the glassy spectrum (Figure 5E) with the strong absorption at 3130 G and the gl,absorption between 2500 (23) Apelblat, A. Ber. Bunsenges. Phys. Chem. 1983, 87 ( l ) , 2
J . Phys. Chem. 1986, YO, 1834 -1838
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and 2980 G reflecting the initial concentration of 0.10 M CuCl, without phase separation is obtained. It has no indication of g!, splitting. Figure 6C: shows that this quenched microemulsion is metastable and ice formation occurs when it is warmed. Annealing it I O min at 228 K leads to the beginning of a gllsplitting caused in that case by a CuClz concentration gradient7 Longer annealing times and/or higher temperatures lead to further phase separation. The formed ice melts at about 273 K indicated by EPR spectral changes. Figure 6A,B,T),E shows that the gllsplitting observed in Figures I , 2, and 4 is kinetically controlled (only the splitting of the first g,,absorption peak between 2450 and 2590 G is shown). In Figure 6A,B the CTACl/ 1-octanol/O.lO M CuCI, microemulsion 4 is cooled at different rates. The two peak quotients of Figure 6A,B show, that the g,lsplitting depends on the cooling velocity. This effect can very clearly be observed in the detergentless n-hexane/2-propanol/O. I O M CuCI, system (Figure 6D,E), where the gll splitting very strongly depends on the cooling velocity.
Discussion It is a common feature of this study that w/o microemulsions which contain a short-chain alcohol like the detergentless nhexane/2-propanol/water system and the 1-butanol/SDS/ toluene/water microemulsion show no indication of ice formation during cooling. The short-chain alcohols are soluble in water, thus acting as cryoprotecting agents. There is no pure bulk water present in these systems. The microemulsion 3 of Figure 3 containing 1-hexanol, Triton X- 100, cyclohexane, and water has ambiguous properties with
respect to the bulk structure modification problem. Water dissolves up to about 7% (w/w) l - h e ~ a n o lwhich , ~ ~ is very low compared to the 50% (w/w) glycerol needed to vitrify an aqueous solution completely by quenching it in liquid nitrogen. However, a slight influence of the dissolved 1-hexanol cannot be ruled out. The above problem does not exist for the w/o microemulsions consisting of CTACl/ 1-octanol/water and SDS/ 1 -octanol/water. As mentioned earlier neither is water modified by 1-octanol nor call 1 -octanol take up the water present in these systems. Nevertheless the two microemulsions 4 and 5 behave very differently. The difference has no simple explanation since the water content of both microemulsions is nearly the same. As to the freezing behavior of the aqueous parts the two microemulsions could be distinguished by saying one contains "bound" water---it vitrifies, the other "free" water- -ice is formed. However, since the amount of bound water is not strongly affected by the chemical nature of a ~ u r f a c e the , ~ observed different behavior is probably better explained in terms of different kinetics toward ice formation than of some differences in the free energies, which the terms free and bound water automatically imply. As stated earlier' for o/w systems the same is true as for these w/o microemulsions: if the glass transition temperature or the melting point of the hydrophobic part lies higher than Tgfor pure water, highly dispersed subunits of some thousands of mobile water molecules could be reached-subunits with possibly nonergodic behavior. Registry No. YOS, 151-21-.3; CuCI,, 7447-39-4; n-hexane, I 10-53-3; 2-propano1, 67-63-0; I-butanol, 71-36-3; toluene, 108-88-3; 1-hexanol, I 1 1-27-3; Triton X-100, 9002-93-1;cyclohexane, 110.82-7; I-octanol, I 1 1-87-5.
Second Harmonlc Generation at a Silver Electrode in the Presence of Phthalazine D. F. Voss,*' M. Nagumo,* L. S. Goldberg,+and K. A. Bundingt Naval Research Laboratory, Washington, DC 20375-5000 (Receiced: April 19, 1985; In Final Form: November 13, 198.5)
Second harmonic generation (SHG) and surface-enhancedKaman scattering (SERS) are used to examine the phthalazinesilver electrode interface over a wide potential range. The SHG signal is found to exhibit hysteresis negative of the potential of zero charge, in contrast to SERS, which shows the expected irreversible degradation of signal. Hysteresis in the second harmonic signal is likely the result of anion-mediated reorientation of phthalazine.
Introduction Because it is sensitive to the boundary between isotropic media, second harmonic generation (SHG) has recently attracted interest as a probe of processes occurring at surfaces, particularly electrodes. S H G is well suited to study the electrochemical environment because it is an optical probe and may complement more established techniques such as surface-enhanced Raman spectroscopy (see ref 1 for a recent review and complete bibliography). The first SHG studies of electrodes were those of Lee and coworkers, who investigated S H G at electrically charged surfaces of silver and silicon and related the observed signal to the surface electric field.2 Shen and co-workers reported SHG and SERS from silver electrodes in the presence of pyridine and pyrazine, and carried out a number of studies of electrochemical cycling and ad~orption.~"Richmond' studied SHG from silver electrodes in several electrolytes and correlated features in the observed signal with fundamental electrode properties. For silver in several electrolytes the potential at which the intensity began to increase
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'Laser Physics Branch. Bio/Mol&lar Engineering Branch.
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coincided with the potential of zero charge (pzc), an important electrochemical parameter that is otherwise difficult to measure on solid electrodes. (However, as our studies of SHG from gold electrodes indicate, this result does not hold for all electrode materials*.) Corn et examined SHG from thiocyanate and urea (both specifically adsorbed species) on a smooth silver electrode by exciting thin film surface plasmon modes for signal (1) Chang, R. K.; Laube, B. I.. CRC Crif. Reo. .Solid State Mater. Sci. 1985, 12, 1-73. (2) l e e , C . €1.; Chang, K.K.; Bloembergen, N. Phys. Reo. Lett. 1967, 18,
167--168.
( 3 ) Chen, C. K.; Heinz, "IF.; . Ricard, D.; Shen, Y. R . Phys. Reti. l e u . 1981, 46, 1010-1012. (4) Heinz, T. F.; Chen, C.K.; Ricard, 11.;Shen, Y. R. Chem. Phys. Lett. 1981, 83, 180-182. (5) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y. R. Chem. Phys. Lett. 1981,83, 455-458. ( 6 ) Chen, C. K.; Heinz, T. F.; Ricard, D.; Shen, Y . R. Phys. Reti. B 1983, 27, 1965-1979. (7) Richmond, G. L. Chem. Phys. Lett. 1984, I J O , 571-575. (8) Nagumo, M.; Bunding, K. A.; Voss, D. F., manuscript in preparation. (9) Corn, R. M.; Romagnoli, M.; Levinson, M. D.; Philpott, M . R. Chem. Phys. Lett. 1984, 106, 30-35.
This article not subject to U S . Copyright. Published 1986 by the American Chemical Society
