Langmuir 1989,5,66-70
66
cages, but in Cd(Cu)S-CdzoCul-X, as no Cd2+or Cu2+is present in the supercages, CdS and CuS may form separately and do not mix as in the case of Cd(Cu)SCd4lC~1-X. 3. CdS Formed in Zeolites with Channels. The CdS formed in zeolites with channels displays different absorption features from CdS formed in zeolites with cages. The intensity of the absorption spectra of CdS in zeolites with cages increases rapidly, and the spectra cover a rather narrow range of wavelengths, while the intensity of the absorption spectra of CdS in zeolites with channels increases slowly, and the spectra cover a wide range of wavelengths. These spectral features indicate a broad distribution of CdS particles in these systems compared to those in the zeolites with cages. Considering the specific structures of the zeolites with channels, it is not difficult to understand why the CdS in this kind of system displays broad particle size distribution. In zeolites with cages, the cages have rather narrow apertures for the entry and migration of molecules: a sodalite cage has a diameter 6.6 A, but the entry is only 2.1 A;an a-cage in zeolite A has a diameter 11 A with an entry of only -4 A; and the supercage in zeolite X has a diameter of 13 A,but the entry is only 7.4 A. Once CdS particles begin to form in such cages, further migration of particles through the narrow aperture aggregate to form long particles is impeded. However, in zeolites with channels, more freedom is offered to the CdS particles to aggregate along the channels, as the channels and the entry have similar dimensions and bigger particles may be formed. As a consequence (see Figure l),freedom of movement along the channels provides an opportunity for the CdS to form a wide distri-
bution of different particles, giving rise to the characteristic absorption spectra. The emission spectra of CdS in the zeolites with channels are similar to those observed in zeolites with cages. Figure 12B shows the changes of the maximum position of emission with increasing excitation wavelength.
Conclusion CdS formation in zeolites with cages and channels is affected by the sizes of the cages and channels and by the distribution of Cd2+cations among these cages. In some cases, the Cd2+ cations located in the small cages will contribute to the CdS formation, but this depends on how many Cd2+cations are located in the big cages. If the number Cd2+cations in the big cages is low, then Cd2+ cations located in the small cages will migrate out into the big cages. The number of CdS formed in the different cages is 1 or 2 in a sodalite cage, 4 in a a-cage, and 4-16 in a supercage, respectively. The CdS formed in zeolites with channels display a distribution of particles with different sizes due to the freedom CdS migration along the channels. Absorption spectra show different features for CdS formed in zeolites with cages and in zeolites with channels. The former covers a rather narrow range of wavelength, and the latter covers a wider range, as the particles of CdS are smaller and of a narrower size distribution in the former than in the latter.
Acknowledgment. We thank the National Science Foundation for support of this work. Registry No. CdS, 1306-23-6.
Metal/n-ZnO Interaction: Effect of the Surrounding Atmosphere on IR Transparency F. Boccuzzi,* A. Chiorino, G. Ghiotti, and E. Guglielminotti Dipartimento d i Chimica Inorganica, Chimica Fisica e Chimica dei Materiali, via P. Giuria 7, 10125 Torino, Italy Received March 2, 1988. I n Final Form: June 14, 1988 IR spectra of three different metaJ/ZnO samples (metal = Cu, Pt, Ru) under vacuum, in hydrogen, and in oxygen are discussed in comparison with those of pure ZnO. In all the reduced samples in vacuo, an electron transfer from the ZnO donor centers to the metal particles has been put in evidence. The growth of a broad absorption in the presence of H2 is interpreted as due to the repopulation of the ZnO donor levels as a consequence of the spillover of H atoms from the metal to ZnO, where they are adsorbed in a protonic form. The differences observed between the three examined samples depend on the different activity of the metals in dissociating H2. O2restores the IR transparency with production of water. The analysis of these phenomena leads to some information on the mechanism of H2sensihg on metal/ZnO systems.
Introduction Metal-semiconductor oxide systems are widely studied in the last years because of the applications that they have in catalysis and photocatalysis and as gas sensors. An important factor that may have a role in the properties of these systems is the electron transfer between metal particles and the support, induced by a difference in the Fermi energy levels. Moreover, the surrounding atmosphere (vacuum, hydrogen, oxygen, etc.) can be very important with respect to a change in the properties of the 0743-7463/89/2405-0066$01.50/0
metal semiconductor systems and in particular their electric and optical properties. In fact, on different systems of this kind an atmosphere of H2 induces a strong increase in the electric conductivity.' In this paper we illustrate the effect of the treatments and of the atmosphere on the IR transparency of three different M/ZnO systems (M = Cu, Pt, and Ru), in order to have spectroscopic information that can be related to (1) Aapnes, D. E.; Heller, A. J. Phys. Chem. 1983,87, 4919.
0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 67
Effect of Atmosphere on Metalln-ZnO Interaction I
Scheme I. Energy Scheme of (a) Reduced ZnO and (b) Reduced ZnO in Contact with the Examined Metalsa
I
EA
3000
3800
2Ooo CR -1
700
Figure 1. IR transmission spectra, run at room temperature, of different samples reduced in a Hz-Nz mixture at 493 K: curve 1, pure ZnO; curve 2, Cu/ZnO; curve 3 Pt/ZnO; curve 4, Ru/ZnO. (-a+
the electrical properties of the same materials. Preliminary results on this subject, as far as Cu and Ru systems are concerned, have been presented in ref 2a and 2b.
Experimental Section The different samples examined were pressed in pellets and submitted to different thermal and chemical treatments in an infrared cell designed to treat the samples in situ. The IR spectra were recorded with a Perkin-Elmer580 B spectrometer interfaced with a 3600 Perkin-Elmerdata station. The reported spectra and the gas interactions were performed at room temperature (=300 K) . The ZnO was a commercial sample (Kadox 25, NJ). The Cu f ZnO system was obtained by reduction of an oxidic phase coming from the decomposition of a basic carbonate (C%JZno,&,(C03)2(0H)6; the Cu mean particle sizes are =40 A.h The Pt/ZnO sample (1%in atoms of Pt) was prepared (with ZnO Kadox as support) by photodeposition of Pt, starting from a chloroplatinic acid solution and following the method previously described for the photodeposition of Pt on Ti02.3 The Pt mean particle sizes, as determined by microgravimetricadsorption data and by TEM, are -40 A. The Ru/ZnO sample ( ~ 0 . 5 %in atoms of Ru) was prepared by impregnating the ZnO Kadox powder with a pentane solution of Ru3(CO)12and then evaporating the solvent at 323 KFb In this case the HRTEM micrographs do not show evidences of metallic particles, and the microgravimetric adsorption data confirm a cluster scale Ru dispersion (d < 10 A). The reduced samples were treated in a 3% Hzin Nz mixture up to 493 K and then outgassed at 623 K. After these treatments the metals are well reduced, while zinc oxide shows the same or a higher degree of nonstoichiometry than the pure phase reduced in the same conditions (as determined by the weight loss in microgravimetric experiments) according to literature data.4
Results and Discussion IR Spectra in Vacuo. Figure 1, curve 1, shows the effect of the reduction a t 493 K on the IR transparency of pure ZnO a complete loss of transparency occurs in all of the spectral range 3800-700 cm-'. The transparency is completely restored only after heating in O2a t 473 K. We have previouslysa shown in a study concerning a ZnO ~
~~~
~
(2) (a) Boccuzzi, F.; Ghiotti, G.; Chiorino, A. Swf. Sci. 1987,183, L285. (b) Guglielminotti, E.; Boccuzzi, F.; Ghiotti, G.; Chiorino, A. Surf. Sci. 1987,189f190, 331. (3) Krautler, B.; Bard,A. J. J. Am. Chem. SOC.1978,100, 5985. (4) Spencer, M. S. Surf. Sci. 1987, 192, 323, 329, 336. (5) (a) Boccuzzi, F.; Ghiotti, G.; Chiorino, A. J. Chem. SOC.,Faraday Trans. 2 1983, 79,1779. (b) Ghiotti, G.; Boccuzzi, F.; Scala, R. J . Catal. 1985, 92, 79.
Vacuum level
sample coming from the decomposition of a carbonate that the absorption in this spectral region is due to the photoionization of donor centers like ionized or neutral oxygen vacancies Vo+ and Voo formed during the reductive treatments; in fact, compared with the oxidized samples the reduced ones are less stoichiometric. However, in that case we observed some differences in respect with the present sample: (i) there was a residual transparency in the high-frequency range (P > 3000 cm-I); (ii) the contact with O2 a t 300 K partially restored the sample transparency. In our opinion the observed differences may be related to the different ratio between surface and bulk atoms in the two considered samples; in fact, the ZnO Kadox has a BET area of 10 m2/g whereas the ZnO ex-carbonate has a BET area of 21 m2/g. Moreover, differences in the exposed planes between the two samples, evidenced by SEM and a H2/C0 can explain the different reductivity of the two samples. On the contrary, the reduced Cu/ZnO sample and Pt/ZnO one (Figure 1, curves 2 and 3) exhibit, under vacuum, a similar and quite good transparency. The bands at =990 and 4370 cm-l, present in both the spectra, are due to bulk multiphononic modes of crystalline Zn0;6 differences between the two samples are observed in the hydroxyls (3600-3400 cm-') and carbonate-like (1600-1200 cm-l) regions as a consequence of the different preparations. The reduced Ru/ZnO has a transparency intermediate between reduced ZnO and the other two samples (Figure 1, curve 4). The reduced intensity in the M/ZnO samples of the band due to the Vo+ and Voo centers indicates that the metals modify the ZnO properties. Our interpretation is that an electron transfer occurs between the metals and the ZnO. In fact, either on the basis of an ideal Schottky behavior (typical of the ionic semiconductors) or taking into account the experimental M/ZnO Schottky barrier height,' the contact between n-ZnO and these metals must cause an electron transfer from the ZnO to the metals and the consequent depletion of the ZnO donor centers (Scheme I). The scheme is flat band because the ZnO crystallite sizes are quite small (500 A), and it is known that the Fermi energy of the semiconductor is controlled by the metal if the contact concerns more than 10l2atoms/cm2.8 Another criterium must be satisfied the metal (6) Thomas, D. G. J. Phys. Chem. Solids 1959,10,47. (7) (a) Brillson, L. J. Surf. Sci. Rep. 1982, 123. (b) Mead, C. A. Solid-state Electron. 1966, 9, 1023. (8) Morrison, S . R. In The Chemical Physics of Surfaces; Plenum:
New York, 1977.
Boccuzzi et al.
68 Langmuir, Vol. 5, No. 1, 1989
A
3ooo
I
I
I
I
I
I
2800
2Ooo
1500
800
ca -I
Figure 2. IR transmission spectra of room temperature H2 adsorption on M/ZnO samples reduced in a H2-N2 mixture. (a) Cu/ZnO: curve 1, background; curve 2, 20 Torr of Hz after 15 min; curve 3, in Hzafter 2 h. (b) Pt/ZnO: curve 1,background; curve 2,lO Torr of H2 (c) Ru/ZnO curve 1,background; curve 2, 10 Torr of Hz.
must be homogeneously dispersed on the whole semiconductor particle surface, to allow the space charge region to overlap. This condition is strongly dependent on the preparation method and seems fully satisfied for Cu and Pt samples, whereas this control seems to be not complete in the Ru/ZnO sample. IR Spectra in H2 and D2 Atmosphere. The IR spectrum of H2 adsorbed on pure oxidized ZnO a t room temperature shows bands a t 1710 and 3500 cm-' that have been assigned to the Zn-H and ZnO-H stretching vibrat i o n ~ small : ~ changes in the transparency are produced, only after a long contact time. Dz adsorption produces bands at 1230 and 2585 cm-', assigned respectively to Zn-D and ZnO-D stretching vibrations. On the reduced ZnO the H2 chemisorption cannot be studied spectroscopically as a consequence of the lack of transparency. In Figure 2a the transmission spectra of a Cu/ZnO reduced sample, in vacuo and in contact with 20 Torr of H2 a t different times, are shown. The transparency is gradually reduced in the whole spectral range, and exactly the same effect is produced with D2 interaction on a sample reduced in D2. Therefore, the absorption is clearly not of vibrational nature but of electronic nature. This behavior (9) Boccuzzi, F.; Borello, E.; Zecchina, A,; Bossi, A.; Camia, M. J. Catal. 1978, 51, 150.
is completely different from that of pure ZnO, on which, as shown, H2 produces only bands of vibrational nature and does not affect the overall transparency. On Pt/ZnO samples the effect of H2 admission is like that shown by Cu/ZnO, but it occurs more quickly (complete loss of transparency after 2 min, Figure 2b). The interpretation of the similar behavior of Cu and Pt/ZnO samples is that electrons released by H atoms, produced more or less quickly on the two metals, repopulate the donor levels of the reduced ZnO, emptied by the contact with the metal particles, as previously suggested. The contact of H2 on a reduced Ru/ZnO sample (Figure 2c) modifies the overall shape of the spectrum; in particular, the absorptions due to the ZnO multiphononic bands disappear. The same effect is produced by D2 interaction. A quite similar behavior was observed on pure ZnO heated in H2 at 433 KO ' or exposed to atomic hydrogen at room temperature." The explanation of these behaviors is that in these conditions the hydrogen, acting as a fully ionized donor, produces an accumulation layer and the population of the ZnO conduction band. The disappearance of the ZnO multiphononic bands, already observed in the electron energy loss spectra of monocristalline ZnO treated with atomic hydrogen,'l is ascribed to a phonon-plasmon coupling effect.lOJ1 In any case, in all the three M/ZnO samples examined an electronic effect of the H2 interaction has been observed: slow and limited on Cu/ZnO, rapid and stronger on Pt/ZnO, and dramatic, with loss of the ZnO multiphononic bands, on Ru/ZnO. In the first two cases a repopulation of the ZnO donor centers, empty as a consequence of the M/ZnO contact, occurs; in the last one there is also a population of the conduction band, as testified by the phonon-plasmon coupling effect observed. The H2-induced changes can be the consequence of different processes: change of the Fermi level of the metal upon hydrogen alloying, change in the surface dipole component of the metal work function a t the metal-semiconductor contact, or spillover of hydrogen atoms from the metal to the semiconductor, where they behave as ionized donors. However, the first two mechanisms may play an important role in metals, like Pd, that can absorb large quantity of H2. Cu, Pt, and Ru are poor absorbers of hydrogen; therefore, in these cases an effect of the spillover is more likely. The differences observed between the three examined samples reasonably depend on the different activity of the metals in dissociatively chemisorbing H2 and on the rate of surface diffusion. Actually, copper is quite inactive in the H2dissociation; only stepped planes have shown12some activity at 300 K. In fact, a 5 kcal/mol activation energy has been found13 for the dissociative adsorption of H2 on the stepped Cu(310) surface. A previous characterization with CO chemisorption of our Cu/ZnO reduced samples2 indicates that the copper particles expose stepped planes, like (211);therefore, the hydrogen dissociates, with a quite low rate, on these planes, spills over, and goes onto the semiconductor. Platinum is, instead, quite active in the dissociation of H2. In fact, Salmeron et showed that on Pt(ll1) there (10) Boccuzzi, F.; Morterra, C.; Scala, R.; Zecchina, A. J. Chem. Soc., Faraday Trans. 2 1981, 77, 2059. (11) Gersten, J. I.; Wagner, I.; Rosenthal, A.; Goldstein, Y.; Many, M.; Kirby, R. E. Phys. Reu. B 1984,29, 2458. (12) Pritchard, J.; Catterick, T.; Gupta,R. K. Surf. Sci. 1975, 53, 1. (13) Balooch, M.; Cardillo, M. J.; Miller, D. R.; Stickney, R. E. Surf. Sci. 1974, 46, 358. (14) Salmeron, M.; Garn, K. J.; Somorjaj, G. A. J. Chem. Phys. 1979, 70, 2807.
Effect of Atmosphere on Metalln-ZnO Interaction I
Langmuir, Vol. 5, No. 1, 1989 69
I
A
a
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6 4
1
r,
2
I 0
1
3cKK)
moo
2000
0
700
2Ooo -1
0 t/min
Figure 4. Inverse of IR absorbance versus the time of contact on Pt/ZnO samples: (a) H2 inlet; (b) O2 inlet.
Rate of Changes in the Absorbance of Pt/ZnO Sample in H2-0, Mixtures. The promptness in the
I
G I
3
t/mln
CI -1
m
2
1
700
Figure 3. IR spectra of room temperature H2-02 and D2-02 interactions on Pt/ZnO samples: (a) curve 1,background; curve 2,lO Torr of O2after H2 interaction; insert, difference spectrum in absorbance between curve 2 and curve 1. (b) curve 1,background; curve 2, 10 Torr of O2 after D2 interaction; insert, difference spectrum in absorbance between curve 2 and curve 1. is an energy barrier to the H2 dissociation of 0.5-1.5 kcal/mole and that the barrier is 0 on Pt(332). The ruthenium is the most active; in fact, large amounts of H2 are dissociated also on the flat Ru(001) plane a t low temperatures. Feulner and MenzeP have found that the saddle point between precursor and H chemisorbed species is 1 kJ/mol below the energy zero. On this basis we can explain the fact that on Ru/ZnO samples H2 chemisorption produces the greatest effect. IR Spectra in H2-0, Mixtures. After H2 adsorption O2 contact a t room temperature on the three M/ZnO samples restores the IR transparency. In the case of the Pt/ZnO and Ru/ZnO samples, bands due to water are also produced. In Figure 3 H2-02 and D2-02 interactions on Pt/ZnO samples are shown: the H2-02 mixture produces bands at 1620 and 920 cm-I that are depleted by evacuation a t room temperature, the D2-02 interaction produces bands a t 2600-2400, 1180, and ~ 7 0 cm-’ 0 (not reported in the insert). The first two bands are quite easily assigned to the “scissor” and “unallowed rotation” modes of molecular physisorbed water, while the last three are assigned to the “stretching”, “scissor”, and “unallowed rotation” modes of D20 physisorbed molecules. The transparency restoration as a consequence of the O2 adsorption indicates that a trapping of electrons on oxygen chemisorbed species occurs; also, the reaction of oxygen atoms with atomic hydrogen then occurs, leading to the water formation. (15) Feulner, P.; Menzel, D. Surf. Sci. 1985, 154, 465. (16)Mak, C. H.; Brand, J. L.; Deckert, A. A.; George, S. M. J. Chem. Phys. 1986,85, 1676.
response to the changes in the surrounding atmosphere depends on different factors, such as temperature, gas concentration, metal loading, and presence of other active gases in the ambient. In the same experimental conditions differently pretreated samples of the same material have shown quite different rates in the response to the different atmospheres. Some experiments were performed where the sample absorbance was recorded versus the time of contact with different atmospheres, in a continuous way, at fixed frequency. The analysis of the rate of absorbance changes with H2 and O2 contact on differently treated Pt/ZnO samples indicates that the order of the production and depletion of electrons is 2 in fact, a linear relationship is observed between 1/A (inverse of absorbance) at fixed frequency ( u = 1230 cm-’) and the time for the two kinds of adsorption (Figure 4). This kind of relationship indicates that the diffusion process from the metal to the semiconductor and on the semiconductor is not the rate-determining step. In fact, in the Pt/A120, system, where the dissociative adsorption of H2 on platinum and the transfer over the phase boundary are accomplished instantaneously (the diffusion of atomic hydrogen on the oxide surface is the rate-determining step), a linear relationship is observed between the amount of hydrogen diffused and the square root of the time.’? Therefore, the observed behavior indicates that in our case the rate-determining steps of the two reactions are reasonably the dissociations of the two molecules
+ 2Pt(s) 02 + 2Pt(s)
Hz
-
-
2Pt-H
(1)
2Pt-0
(2)
followed by a rapid spillover of the atomic species and by production (1) or trapping (2) of electrons on the ZnO. Considerable experimental evidence has been collected for the spillover of hydrogen species. Hydrogen is not, however, the only species which can migrate from an activator to a support. Although the amount of data concerning the spillover of oxygen is much smaller, it seems, at the present time, well established.18 We observe, in conclusion, that the presence of metal particles on the ZnO modifies the hydrogen chemisorption on this oxide at room temperature, i.e., from heterolytical and neutral species as Zn-H and ZnO-H on pure ZnO to a protonic form when metal particles are present. This phenomenon probably occurs because, in the presence of (17) Kramer, R.; Andre, M. J. Catal. 1979.58, 287. (18)Curtis Conner, W., Jr.; Pajonk, G. M.; Teichner, S. J. Adu. Catal. 1986, 34, 1.
Langmuir 1989,5,70-77
70
metal particles, the hydrogen attains the ZnO surface in an atomic form.
Acknowledgment. The financial support of the Italian CNR "Progetto Finalizzato Materiali e Dispositivi per 1'
Elettronica a Stato Solido" is acknowledged. Registry No. ZnO, 1314-13-2;Cu, 7440-50-8;Pt, 7440-06-4; Ru, 7440-18-8;H,, 1333-74-0;O,, 7782-44-7;H, 12385-13-6;0,
17778-80-2;H ' , 12408-02-5;HZO, 7732-18-5;D,,7782-39-0.
Hardness Tolerance of Anionic Surfactant Solutions. 1. Anionic Surfactant with Added Monovalent Electrolyte Kevin L. Stellner and John F. Scamehorn* Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019 Received April 21, 1988. In Final Form: July 17, 1988 Precipitation of sodium dodecyl sulfate by calcium has been reported over a wide range of concentrations with and without added sodium chloride. Below the cmc, when no micelles are present, surfactant precipitation can be described by an activity-based solubility product written between the surfactant and the calcium. Above the cmc, where micelles exist, the solubility product must be written between the unbound (unassociated)divalent counterion and the monomeric (unassociated)anionic surfactant activities. A model has been developed that can predict the precipitation boundary by using the solubility product combined with material balances for each species and information about binding of counterions on the charged micelles. The hardness tolerance (minimum calcium concentration required to cause precipitation) of this system has been shown to increase by as much as a factor of 25 upon addition of 0.1 M NaC1, indicating the value of monovalent electrolyte in enhancing the hardness tolerance of anionic surfactant solutions.
Introduction The hardness tolerance of an anionic surfactant is defined as the minimum concentration of multivalent cation necessary to cause precipitation of the surfactant. Precipitation is an important phenomena because it is wellknown to potentially limit the usefulness of anionic surfactants in detergency applications in hard water (water containing a high concentration of calcium and/or magnesium). It can also restrict the ability to apply surfactants in enhanced oil recovery of reservoirs with high hardness levels. Conversely, the use of multivalent cations to precipitate anionic surfactant can be useful, as illustrated by a proposed process for the recovery of surfactant from surfactant-based separation pr0cesses.l In this study, the precipitation phase boundary (boundary between the region where precipitate forms and where solutions remain isotropic) is reported for mixtures of sodium dodecyl sulfate (NaDS) and calcium chloride (CaC1,) over a wide range of concentrations. A model is developed which can be used to predict the precipitation phase boundary for this system when NaCl is also added. In the following paper in this issue (part 2 of this series), the model is expanded to include the addition of nonionic surfactant to the system to enhance the hardness tolerance and is tested against experimental precipitation phase boundaries for that system. In previous related work, we have studied salinity tolerance of anionic surfactant solutions in the presence of nonionic surfactant2and precipitation of anionic/cationic surfactant mixture^.^
Experimental Section Materials. Sodium dodecyl sulfate (NaDS) obtained from
Fisher Scientific had a purity greater than 95%. This was recrystallized twice from a 50150 mixture of water and ethanol and then dried under vacuum with low heat.
* Author to whom
correspondence should be addressed.
0743-7463/89/2405-0070$01.50/0
The NaCl, CaC12,and MgClzwere Fisher reagent grade and were used as received. The water was distilled and deionized. Methods. Precipitation Boundaries. A series of solutions, each with the same concentration of NaDS and NaCl (when present) but with varying CaClzconcentrations,were prepared in 100-mL volumetric flasks. All experiments in this study were performed at 30 i 0.05 "C, since precise temperature control is essential to obtain accurate precipitation phase boundaries. Surfactant solutions can remain supersaturatedfor long periods of time before precipitation is complet&6 therefore, all solutions were cooled to near freezing temperatures to force precipitation to occur. The solutions were then placed in the 30 "C water bath, shaken periodically, and allowed to equilibrate for at least 4 days. Whether or not crystals were present after equilibration determined if the initial solution composition was inside the precipitation phase boundary. Using simple visual detection, we found that the concentration of CaClzthat determined a point on the precipitation boundary was accurate to within *lo% at all NaDS concentrations. Other worker^^^^ have used laser scattering to obtain more accurate phase boundaries in similar systems. However, the observed precipitation phase boundaries were not significantly different when laser techniques were used for detection in this work. We have found that the laser technique can give improved detection results in precipitating anionic/ cationic surfactant systems3 cmc Determination. Surface tension measurements were used to determine the critical micelle concentration (cmc) for each system of interest by a break in the surface tension versus logarithm of surfactant concentrationcurve. A DuNuoy ring ten(1) Brant, L. W.; Stellner,K.L.; Scamehorn, J. F. In Surfactant-Based Separation Processes; Scamehorn, J. F., Harwell, J. H., Eds.; Marcel Dekker: New York, Chapter 12, in press. (2) Stellner, K.L.; Scamehorn, J. F. J. Am. Oil Chem. SOC.1986,63, 566. (3)Stellner, K.L.;Amante, J. C.; Scamehorn, J. F.; Harwell, J. H. J. Colloid Interface Sci. 1988, 123, 186. (4) Peacock, J. M.; Matijevic, E. J.Colloid Interface Sci. 1980,77,548. (5) Fan, X.-3.;Stenius, P.; Kallay, N.; Matijevic, E. J.Colloid Interface Sci. 1988, 121, 571. (6)Matheson, K.L.;Cox, M. F.; Smith, D. L. J. Am. Oil Chem. SOC. 1985,62, 1391. (7) Smith, D. L.; Matheson, K. L.; Cox, M. F. J. Am. Oil Chem. SOC. 1985, 62, 1399.
0 1989 American Chemical Society
