Langmuir 1988, 4, 1269-1273 molecules in the entire solution. The total dilution enthalpies shown in Figure 3 are summations of endothermic and exothermic peaks. The curve for OTG dilution in HzO can be characterized by three regions. In the premicellar region I, the injected micelles desintegrate completely and the enthalpy change for demicellization and loss of intermicellar interactions is recorded. Region I1 is the transition region around the cmc. In the postmicellar region 111, the injected micelles remain intact and only a very small enthalpy change for reduction of intermicellar interaction is measured. The enthalpy of micellization calculated as the difference in dilution enthalpy between regions I and I11 is +4.5 kJ-mol-l, a normal value for a nonionic surfactantaZ2 Comparison of the curve for the PPO solution with the curve for HzO reveals that PPO has only a small endothermic effect on the premicellar enthalpy of dilution. Furthermore, the transition region is located in the same concentration range, indicative of an unchanged cmc. However, a strong endothermic effect, +4.3 kJ-mol-l, is observed in the postmicellar region of the PPO solution. We contend that this value represents the enthalpy of interaction between PPO and the OTG micelles. Interestingly, Shirahama5 also found an endothermic enthalpy for interaction between PEO and SDS micelles (in a 0.1 M NaCl solution). Since the Gibbs free energy of micellization of OTG is unchanged by the presence of PPO, the endothermic interaction enthalpy is apparently compensated by a positive entropy change. This AHlAS compensatory behaviorz3probably originates largely from the release of water molecules from the hydrophobic hydration shells of the polymer disks upon interaction with the micelles. (22) (a) Olofsson, G. J. Phys. Chem. 1983,87,4000. (b) Corkill, J. M.; Goodman, J. F.; Harrold, S. P. Trans. Faraday SOC.1964, 60, 202. (23) Jolicoeur, C.; Philip, P. R. Can. J. Chem. 1974,52, 1834.
1269
We submit that PPO may well bind in more hydrophobic regions of the micellar core, as suggested by the fact that PPO is more soluble in apolar solvents, such as hydrocarbons, than in water. In the latter aspect, PPO is much different from the hydrophilic polymers PEO, PVP, and PVA-PVAc.16 If PPO penetrates more deeply into the micelle than generally assumed for polymer-micelle complexes,9steric hindrance between PPO and bulky sugar head groups of OTG will be minimized. Thus, the steric hindrance may well be compensated by the favorable free energy of transfer of PPO, and the overall free energy of formation of PPO-complexedmicelles is then quite similar to that for micellization in pure water.24 The much more hydrophilic PEO is insoluble in hydrocarbons, and PEOOTG interaction would force the polymer to reside at the surface of the micelle. This situation impedes PEO-OTG complexation as indicated by the microcalorimetric observation that PEO (0.5 gdL-l) does not affect the enthalpy of micellization mi, = +4.5 kJ-mol-') and exerts no effect on the enthalpy of dilution of OTG in the postmicellar region.
Acknowledgment. We are much indebted to G. Haandrikman for performing the microcalorimetric measurements. Registry No. PEO, 25322-68-3;PPO, 25322-69-4;PVP, 9003-39-8; (VA) (VAC) (copolymer),25213-24-5;HPC, 9004-64-2; OTG, 85618-21-9; CsE3,19327-38-9;2-(4-Decylpyridinium)ethyl sulfonate, 115757-12-5; (4-dodecy1pyridinium)methylcarboxylate, 115757-13-6; 2-(4-decylpyridinium)ethylsulfonate, 115796-70-8; (polyoxypropylene) (octyl 1-thio-P-D-glucopyranoside (complex), 115796-71-9. (24) Recent detailed calculations by N a g a r a j a ~ ~ (taking '~ into account a specified area, awl, per surfactant molecule that is shielded by polymer segments) also suggest that nonionic polymer-nonionic micelle association may occur while the cmc values are not significantly different from those of normal micelles.
Formation of Adsorbed 02-and OH- on a Ag Electrode in Dilute CrOd2-Electrolytes Paul B. Dorain" and Jennifer L. Bates Chemistry Department, Amherst College, Amherst, Massachusetts 01002 Received February 29, 1988. I n Final Form: May 25, 1988 Surface-enhancedRaman scattering has been used to detect in situ the formation of 02-and OH-adsorbed M KC1. Both oxide and on a Ag electrode in an aqueous solution of 25 pM NazCr207and to hydroxide adsorbates are observed after completion of an oxidation-reduction cycle in a standard voltammetry experiment. As the Ag electrode voltage is swept cathodically, the Oad:- is protonated to form more OH,&-,but, unlike in MnO; solutions, no further protonation to form H20 occurs at any cell voltage. The addition of Ozgas to the electrolyte deprotonates the adsorbed OH- to form an oxide-coveredsurface. These results for Cr04z-solutions are contrasted with previous results for Mn04- electrolytes to obtain information about the nature of the metal-solution interface in chromate-passivatedmetal surfaces.
Introduction Understanding the interactions of oxide-covered metal surfaces with their environments is fundamental to understanding corrosion pr0cesses.l Only recently have in (1) (a) Stuve, E. M.; Madix, R. J.; Sexton, B. A. Surf. Sci. 1981, I l l , (b) Bange, K.; Madey, T. E.; Sass, J. K. Surf. Sci. 1987,138, 334. (c)
situ spectroscopic techniques been available to probe the detailed processes occurring in these systems as functions of concentration, pH, and electrochemical potential. Surface-enhanced Raman scattering (SERS), sensitive to less than monolayer coverage of adsorbates on selected metal surfaces,2 is one technique that has become widely
11.
Crowell, J. E.; Chen, J. G.; Hercules, D. M.; Yates, J. T., Jr. J. Chem. Phys. 1987,86, 5804.
(2) Moskovits, M. Reu. Mod. Phys. 1985, 57, 783.
0743-7463/88/2404-1269$01.50/00 1988 American Chemical Society
1270 Langmuir, Vol. 4, No. 6, 1988
used. Due to recent developments in detector instrumentation, SERS is a very effective tool for investigating electrode processes during cyclic ~oltammetry.~ We are interested in the nature of the protonation processes of the oxide layer on Ag electrode^.^ Electrolytic solutions containing dilute concentrations of alkali or alkaline earth halides and the tetrahedral oxyanions VOd3-, or Mn04- have been studied. Each solution has a different effect on the Ag metal-electrolyte interface. Both and Vo43- adsorb without decomposition and are protonated sequentially to form the adsorbed acids H,X04(3-n)-as the electrode voltage, VsCE, is swept cathodically (more negatively) relative to saturated calomel electrode. However, there are differences in the adsorption on Ag metal. properties of H,Po4(3-")- and H,vo4(3-n)H,V0,'3-")- remains contact adsorbed regardless of VScE and, therefore, interfacial pH.& At very negative VSCE,the vanadium ion is reduced but remains adsorbed even if H2 is produced. H,PO,(*")- forms H3P04,which then desorbs when the interfacial pH is brought below 4 by applying a negative VsCE to the Ag ele~trode.,~ Solutions with micromolar concentrations of MnO,behave quite differently.,"~~After the Ag electrode has gone through an oxidation-reduction cycle (ORC),a SERS peak, assigned to the Ag metal-oxide stretching mode, is observed at -600 cm-'. As V ~ is Eswept cathodically, the oxide layer is protonated to form adsorbed OH- (-500 cm-l) and, at very low interfacial pH, adsorbed H20 (-400 cm-'). The formation of adsorbed H20 is confirmed by the appearance of a strong peak due to the O-H stretching mode at -3500 cm-'. M The addition of O2 gas to an electrolyte with MnO,- and M KC1 has the remarkable effect of deprotonating the adsorbate layer stepwise to form the oxide layer. This reversible process is the result of the increased interfacial pH due to the reduction of dissolved 02. In this paper, we report the SERS spectrum obtained from a Ag electrode in a to M KC1 electrolyte containing 25 pM Na2Cr0,. The spectrum observed shows the formation of an oxide layer which is partially protonated to form adsorbed OH-. Unlike the spectra from MnO,- solutions, no evidence of additional protonation of the adsorbed OH- was seen. These results indicate that a significant factor in the passivation of metals by treatment with chromate solutions is the inability of the treated metal to form SERS-active sites with adsorbed H20
molecule^.^ Experimental Section A polycrystalline Ag electrode, polished with 0.5-pm alumina and washed with double-distilled H20, a Pt counter electrode, and a saturated calomel electrode (SCE) were placed in the electrolytic solution in a fused quartz optical cell. Standard methods of voltammetry were used to oxidize and reduce the Ag electrode surface.6 The voltage between the Ag electrode and the SCE was ramped linearly at 5 mV/s. Distilled H20 or commercial D20 and reagent grade chemicals were used to prepare all solutions. The pH of the electrolyte was monitored before and (3) Chang, R. K.; Laube, B. L. CRC Crit. Reu. Solid State Mater. Sci. 1984, 12, 1. (4) (a) Dorain, P. B. J.Phys. Chem. 1986,90,5808. (b) Dorain, P. B. J. Phys. Chem. 1986, 90, 5812. (c) Dinces, E. A. The Protonation of Vanadate Species Adsorbed onto Ag Electrodes; Amhent College Thesis, 1987. (d) Dorain, P. B.; Von Ftaben, K. U.; Chang, R. K. Surf. Sci. 1984, 148,439. ( 5 ) (a) Long, G. G.; Kruger, J., Black, D. R., Kuriyama, M. J . Electroanal. Chem. 1983,90,603. (b) O'Grady, W. E. J.Electrochem. Soc. 1980, 127,555. (6) Baird, A. J.; Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980.
Dorain and Bates after the experiment and ranged from 4.6 to 6.3. The concentration of NazCr2O7was maintained at 25 pM, a value chosen to minimize the absorption of the laser beam and to assure a solution containing predominantly the HCr04- ion.7 To obtain approximate relative metal surface concentrations, ion-scatteringmass spectrometry (ISS) measurements were made on representative Ag electrodes that had undergone two oxidation-reduction cycles corresponding to a removal and replating of about 50 atomic layers of Ag. The ratio of Ag/Cr at 60 8,from the surface was 1011,and at 160 8, the ratio was 3511. Simultaneously measured secondary ion mass spectrometry (SIMS) yielded the same relative ratios at these two depths? It appears, therefore, that the fraction of Cr on the surface is not large but, as will be demonstrated, is important for the formation of these SERS-active sites. The Raman spectra were measured with an Ar ion laser operated at 514.5 or 488.0 nm at an incident power of 100 mW focused to 1 mm2 at the working electrode. The scattered light, collected with an f1.2 lens, was focused on the entrance slit of a triple spectrograph. The dispersed light was detected with an optical multichannel linear diode array with a resolution of 2.5 cm-' per diode. The data were collected and massaged by a Microvax computer.
Results Figure 1 shows the time and voltage evolution of the S E W spectra from a Ag electrode in a M KCl solution in D20containing 25 pM Na2Crz0,at a pD 6.3. The initial spectral frame is recorded at -0.1 VsCE as a second ORC linear voltage scan begins. As VsCE sweeps to a positive voltage, the Ag electrode is oxidized, forming an adherent layer of AgCl and Ag2Cr04and resulting in the loss of nearly all the SERS intensity. Upon reduction during the subsequent cathodic sweep, a roughened Ag surface is formed containing adsorbed Cr ions and C1-. A strong, broad, almost resolved SERS peak appears at 590 cm-' after the faradaic reduction has ceased at -0.05 VSCE. During the cathodic sweep, the peak becomes broader and more intense, reaching a maximum intensity at -0.8 VSCB Just prior to the onset of D2 gas formation, the peak becomes narrow. A distinct shoulder is observed at 642 cm-l on a strong peak at 567 cm-'. The intensity of both peaks diminishes as D2 gas is formed. Upon switching to an anodic sweep at -1.53 VSCE,the peaks become more resolved as the higher energy peak increases in intensity and the lower frequency peak decreases in intensity. A third ORC causes the reappearance of the original broad SERS peak. No other spectral feature is observed except a small peak at 240 cm-' that occurs at the end of each ORC and disappears during the cathodic sweep. This frequency shift and voltage behavior are characteristic of the stretching mode of Ag-C1. The absence of a strong SERS peak at -850 cm-l, due to the symmetric stretching mode of indicates that no adsorbed Cr02- or Cr20:- exists. Previous results have shown that Cr042-adsorbs on Ag colloids.'0 Thus the absence of any peak at 850 cm-* is an indication that Cr042-is not stable on a Ag electrode after an ORC. The results for H 2 0 solutions are the same except that the two peaks are barely distinguishable at any VSCE. At -0.45 VsCE during the most favorable conditions of an
-
-
(7) (a) Critical Stability Constants; Smith, R. M., Martell, A. E., Eds.; Plenum: New York, 1977. (b) Pourbaix, M. Atlas of Electrochemical Equilibria in Aqueous Solutions; Pergamon: Oxford, 1966. (8) Dorain, P. B.; Chen, T. T.; Chang, R. K. Proc. SPIE-Int. Soc. Opt. Eng. 1984,482,116. (9) The measurements were made by B. Laube of United Technology Co., Hartford, CT. (10) Feilchenfeld, H.; Siiman, 0. (a) J. Phys. Chem. 1986, 90,4590; (b) 1988, 92, 453.
02and OH- Formation on a Ag Electrode
Langmuir, Vol. 4, No. 6, 1988 1271
VSWITC'H
Figure 1. SERS spectrum from a Ag electrode in a D20 solution of 25 pM Na2Cr207 and M KCl at a pD of 6.3. The excitation wavelength is 488 nm, the laser power is 100 mW, and the time interval between frames is 8.7 s (43.5 mVscE). A second ORC linear voltage scan begins at frame 2 (-0.10 VscE),sweeping anodically until frame 9 (0.21 VscE),cathodically until frame 49 (-1.53 VSCE), anodically again until frame 88 (0.21 VSCE),and cathodically again until the end of the experiment. anodic sweep, the peaks occurred at 573 and 642 cm-'. Several experiments to detect a peak at -3600 cm-' due to the symmetric stretch mode of adsorbed -OH showed no clear effect on the broad peak in the same region due to the normal Raman scattering of bulk water. This result is similar to that obtained with Mn04-. Presumably a surface selection rule is operative. Figure 2 shows the effect of periodic addition of O2 gas at 1 atm of pressure to the electrolyte at a distance of 1 cm from the Ag electrode. The cell voltage is maintained at -1.24 VsCEafter an initial ORC that removed and redeposited 10 layers of Ag. The first spectral frame shows that the 573-cm-' peak is the most intense under these conditions. The addition of O2 gas at frame 5 causes the 642-cm-' peak to appear. Stopping the O2 gas flow in frame 10 causes the 642-cm-l peak to decrease in intensity once again. The process is repeatable as shown in Figure 2. This result is similar to that obtained for cells where the Mn04- was substituted for CrO:-. It appears that the 642-cm-' peak is due to an adsorbed oxide layer formed by deprotonation of the adsorbed hydroxide appearing at 573 cm-'.
-
Discussion The SES peaks observed at 642 and 573 cm-' in H20 are assigned to the stretching modes of the adsorbed species (Ag,Cr)-0 and (Ag,Cr)-OH, where (Ag,Cr) indicates chromium species incorporated into the Ag surface. The assignments are based on the following facts: 1. The 642-cm-l peak occurs at the same frequency shift regardless of the isotopic composition of the solvent. On the other hand, the 573-cm-' peak in H20 is shifted by -7
cm-' to 566 cm-' in D20. Assuming a harmonic oscillator approximation for Ag-OH, the calculated shift is -14 cm-'. 2. The behavior of the two SERS peaks after an ORC in Cr0:- solution is reminiscent of the results previously obtained for MnO, solutions.4a During the cathodic sweep, the interfacial pH decreases due to electrostatic attraction of hydrated H+ ions.6 The (Ag,Cr)-0 species are protonated with a simultaneous electron transfer to form (Ag,Cr)-OH. This results in a decrease in the intensity of the 642-cm-' peak and an increase in the 573-cm-' peak. During an anodic sweep, the process is reversed as the hydroxide is deprotonated. 3. The addition of O2gas to the electrolyte demonstrates that the two peaks are related chemically. The 573-cm-' peak is generated by the addition of 02,while the cell voltage and bulk solution pH are such that adsorbed OHshould be the stable species. As in the case of the Mn04solutions, the oxygen reduction at the cathode according to the overall reaction O2 + 2H20 + 4e- = 40H- increases the interfacial pH and deprotonates the (Ag,Cr)-OH to form (Ag,Cr)-0. Stopping the flow of O2 decreases the interfacial pH, and the oxide is again protonated." The assignment of the SERS-active site is problematic. The only observed SERS spectrum for Ag20 has been observed on coldly deposited Ag films12at -200-500 cm-'. EELS measurements on Ag(ll0) give 315 and 630 cm-' for adsorbed oxygen species on Ag.13 In strongly basic NaOH (11) See ref 6. (12) Pettenkofer, C.;Pockrand, I.; Otto, A. Surf. Sci. 1983, 135, 52. (13) (a) Sexton, B. A.; Madix, R. J. Chem. Phys. Lett. 1980, 76, 294. (b) Barteau, M. A.; Madix, R. J. Surf. Sci. 1980, 97,101.
1212 Langmuir, Vol. 4 , No. 6,1988
Dorain and Bates
800
600
400
Raman Shift (cm-l) Figure 2. SERS spectrum from a Ag electrode in a D 2 0 solution of 25 WMNa2Cr207and lo-’ M KCl at a pD of 6.3. The excitation wavelength is 488 nm, the laser power is 100 mW, and the time between frames is 2.7 s. The cell voltage is held constant at -1.24 VSCEafter an initial ORC. O2 gas is bubbled through the solution from frames 5 and 10, 22 to 32, 42 to 50, 62 to 72. solutions, SERS peaks a t 490 and 430 cm-l are observed which have been assigned to “Ago”, a mixed valence compound of Ag,O and Ag,03.14 Attempts by several groups to measure ordinary Raman scattering from AgzO powders have failed. However, infrared measurements show a fundamental absorption a t 545 cm-’. It is clear from these results that the vibrational frequency of atomic oxygen adsorbed on Ag metal lies in the spectral region from -300 to 600 cm-’. Several of the vibrational modes of chromic oxide and chromic hydroxide derivatives are also expected to occur in this region. The major IR absorptions of Crz03due to K = 0 phonons occur at 550 and 618 cm-’.I5 IR measurements on various preparations of hydrated CrzO3 powders show a very broad peak centered at 550 cm-’. Anhydrous Cr,03 exhibited two peaks a t -565 and 625 cm-’.le From these results it is clear that the SERS-active site could be a pure Ag site, a localized Cr species large enough to approximate bulk Cr, or a site involving both Cr and Ag. The pure Ag site can be eliminated because these SERS effects are never seen in the absence of Cr0:-. Because Cr metal itself is not expected to give rise to enhanced Raman scattering,l’ we suggest that the active
(14)(a) Kolz, R.; Yeager, E. J. Electroanal. Chem. 1980,111, 105. (b) Temperini, M. L. A.; Lacconi, G. I.; Sala, 0. J. Electroanal. Chem. 1987, 227, 21. (15) Marshall, R.; Mitra, S. S.; Gielisse, P. J.; Plendl, J. N.; Mansur, L. C. J . Chem. Phys. 1965, 43, 2893. (16)Burwell, R. L., Jr.; Haller, G. L.; Taylor, K. C.; Read, J. F. Adu. Catal. 1969, 20, 1. (17) DiLella, D. P.; Gohin, A.; Lipson, R. H.; McBreen, P.; Moskovits, M. J . Chem. Phys. 1980, 73, 4282.
site involves both metals, symbolized by (Ag,Cr). The (Ag,Cr)-0 site differs from the (Ag,Mn)-0 site in that the hydroxide-covered surface cannot be protonated further as a function of VSCE. In contrast, the (Ag,Mn)-0 site may be protonated to form adsorbed water and a hydrogen-bonded water dimer. It is known that impurity metals in some catalysts drastically modify the chemical product distribution.18 In electrochemical cells, similar effects are observed when SERS is used. For instance, the substitution of MgCl, for KCl in electrolytes containing Mn04- causes the voltage-dependent appearance of adsorbed OH- rather than adsorbed Hz0.19 The incorporation of Cr into Ag during an ORC seems to modify the surface potential such that the interfacial pH is never sufficiently low to cause appreciable protonation of the hydroxide to form adsorbed HzO. Chromate solutions are commonly used to passivate metals, particularly steels.20 The rate of corrosion of Ag metal is also decreased by treatment with chromate solutions,21yet Mn04- solutions increase the rate of corrosion. The anticorrosion properties of chromate-treated metals apparently stem form the inability of the surface to form adsorbed interfacial HzO regardless of the potentials present. A surface containing adsorbed H 2 0 such as that obtained in Mn04- solutions may effect a less random transition to the bulk solution than a surface covered with (18) (a) March, N. H. Chemical Bonds Outside Metal Surfaces; Plenum: New York, 1986. (b) Alvey, M. D.; Lanzillotto, A.; Yates, J. Surf. Sci. 1986, 177, 278. (19) (a) Dorain, P. B. J. Phys. Chem. 1988,92,2546-2549. (b) Chen, T. T.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1984,108, 39. (20) Weber, W. J. Physicochemical Processes; Wiley-Interscience: New York, 1972. (21) Dettner, H. W. Plating 1961, 48, 285.
Langmuir 1988, 4, 1273-1277 adsorbed 0 or OH-. Further SERS experiments using TcOC, an excellent passivating agent, should be a test of this proposal.
Conclusions In this paper evidence is presented to support the following conclusions: 1. (Ag,Cr)-0 and (Ag,Cr)-OH are formed on a Ag electrode after an ORC in a solution of KC1 containing micromolar quantities of Na2Cr207.The (Ag,Cr) adsorption site is similar to the (Ag,Mn) site discussed previously.48JJ 3. The (Ag,Cr)-0 is protonated to form adsorbed hydroxide, (Ag,Cr)-OH, as V~CE is swept cathodically. The addition of O2gas to the electrolyte deprotonates the hydroxide to form the (Ag,Cr)-0 in a reversible process that
1273
involves increasing the interfacial pH. 3. The (Ag,Cr)-OH is not further protonated a t any obtainable VscE, indicating that the surface potential for Ag metal with a -1% Cr impurity level is strongly modified compared to the case when Cr is replace with Mn. This difference in the ability of the surface to form SERS-active HzO suggests that the passivation characteristics of CrOd2--treatedmetals are related to the H20metal interface.
Acknowledgment. The partial support of this research by the Office of Naval Research is gratefully acknowledged. Dr. Bruce Laube and United Technology Corporation made the ISS and SIMS measurements, for which we are appreciative. Registry NO.Ag, 7440-22-4;0'-, 16833-27-5;OH-, 14280-30-9; 0 2 , 7782-44-7.
Relationship of Structure to Properties of Surfactants. 14. Some N-Alkyl-2-pyrrolidones at Various Interfaces Milton J. Rosen,* Zhen Huo Zhu, Ben Gu, and Dennis S. Murphy Surfactant Research Institute, Brooklyn College, City University of New York, Brooklyn, New York 11210 Received March 4, 1988. I n Final Form: June 9, 1988 The interfacial properties (maximum interfacial excess concentration, rmax; minimum area/molecule at the interface, A h ; efficiency of interfacial tension reduction,p(C,,); effectivenessof interfacial tension reduction, 7rmax; standard free energy of adsorption, AGO,& of some well-purified N-alkyl-2-pyrrolidones, a new class of commercially available surfactants, with C8 to Clz alkyl chains, have been investigated at the aqueous solution-air, -Parafilm, -Teflon, and -hexadecane interfaces. The small hydrophilic group permits close packing of the molecules at the interface when the alkyl chain is not branched. The compounds show high surface activity in aqueous solution, the C12compound reaching a minimum surface tension of about 26 mN m-l (dyn/cm). Because of their limited water solubility,the compounds show no micelle formation in water at 25 "C. For the C8 and Clo compounds, properties at the air and Parafilm interfaces are almost identical. A t the Teflon interface, there is a 98% homologue purity and purified by reduced pressure distillation.' The fol-
lowing data were obtained on these compounds:
R 4 - l
P 0
which are surface-active molecules with small hydrophilic heads, have recently become commercially availab1e.l This investigation explores the properties of some molecules of this type a t various interfaces. The only previous information on the interfacial properties of these materials is (1) Personal communications, GAF
the surface tension data of Nakagaki2on aqueous solutions of N-dodecyl-2-pyrrolidone.
Corp., Wayne, NJ.
% purity
compd C2,6P C8P ClOP C12P
byGC 99.5 99.5 99.8 99.0
calcd
%C %H 73.0
11.8
74.6
12.1 12.3
73.0 75.8
11.8
found
%N % C 7.1 72.4 7.1 73.0 6.2 73.8 5.5 75.5
%H
% N
12.0 11.8 12.2 12.4
6.6 6.6 6.1 5.4
Before being used for surface tension measurements, aqueous solutions of the surfactants (in water that had been first deionized (2) Nakagaki, M.; Shimabayashi, S. Nippon Kagaku Kaishi 1973, No. 11, 2056.
0743-7463/88/2404-1273$01.50/0 0 1988 American Chemical Society
