T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered in U.S. Patent Ofice
@ Copyright, 1966, by the American Chemical Society
VOLUME 69, NUMBER 4 APRIL 15, 1965
Nuclear Magnetic Resonance, Dielectric, Near-Infrared, and
Cryoscopic Studies of Solubilized and Emulsified Water.
The
System Cyclohexane-Carbon Tetrachloride-Duomeen T Dioleate
by P. D. Cratin and B. K. Robertson’ Department of Chemistry, S p r i w Hill College, Mobile, Alabama
(Received February 19, 1964)
Nuclear magnetic resonance, dielectric, near-infrared, and cryoscopic methods have been used to demonstrate the existence of two distinct water species in water-in-oil emulsions formed from cyclohexane, carbon tetrachloride, and Duomeen T Dioleate (TDO). The solubilized species appears to be very strongly associated with the carboxyl groups of the surfactant; emulsified water appears only after the surfactant has been “saturated” with the solubilized moiety. The authors postulate that the surfactant-water complexes are a series of hydrates having the general formula TDO.nHzO. The marked increase in dielectric constant with decreasing frequency has been attributed to interfacial polarization. Dielectric studies suggest that micelles build up in cyclohexane (but are absent in carbon tetrachloride) before solubilization occurs; cryoscopic measurements indicate no such buildup.
Introduction It is not unreasonable to believe that water solubilized in an organic medium through the action of a surfactant should manifest properties which are substantially different from those exhibited by water emulsified in the same medium. Especially interesting, then, is the transition region where the solubilization point is slightly exceeded, and emulsion droplets first appear. Do two distinct water species exist in this region-each having its own set of properties due to environmental diff erences-or are all the water molecules equivalent and therefore indistinguishable? This problem-and related ones-have been subject to much theoretical
interest during the past few year^^-^; lacking, however, has been the experimental evidence needed to answer this question unequivocally, even for a single system. In an attempt to solve this problem, the authors have employed nuclear magnetic resonance, dielectric, spectral, and cryoscopic methods to study the properties of (1) Department of Chemistry, Texas A and M University, College Station, Texas. (2) M. E. L. McBain and E. Hutchinson, “Solubilization and Related Phenomena,” Academic Press, Inc., New York, N. Y.. 1955, pp. 188-190. (3) J. H. Schulman and T . S. McRoberts, Trans. Faraday SOC.,42B, 165 (1946). (4) A. V. Tobolsky and B. J. Ludwig, A m . Scientist, 51, 400 (1963).
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P. D. CRATINA N D B. K. ROBERTSON
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solubilized and emulsified water in the system cyclohexanecarbon tetrachloride-Duomeen T Dioleate.
Experimental Reagents. Duomeen T Dioleate (tallow 1,Bpropylenediamine dioleate) was obtained in the commercial grade, “ l O O ~ oactive,” from Armour Industrial Chemical Co. and used without further purification. The molecular weight determined from cryoscopic measurements agreed with that given by the manufacturer, about 1000, but was higher than the theoretical formula weight, 750. (The discrepancy is probably due to the presence of high molecular weight tertiary amines in the sample.) Cyclohexane and carbon tetrachloride were reagent grade chemicals purchased from Fisher Scientific Co. and Baker Chemical Co., respectively. Inasmuch as there were no detectable differences in the dielectric constants of the reagent grade chemicals and those of spectroscopic grade materials, further purification was deemed unnecessary. Distilled water was used in all experimental runs. Apparatus. Many of the practical difficulties associated with employing conventional permittivity cells (i.e., those having immersed electrodes) were overcome by using the Wayne Kerr (D121) cell, which has noncontacting electrodes. Since the electrodes contact neither the solution nor the sample container, surface adsorption and catalysis, poisoning, boundary effects, etc , are not encountered. Such a cell is limited, however, to use with solutions whose dielectric constants do not exceed 5. In this study, the dielectric constants were calculated using capacitance values obtained with a Wayne Kerr universal bridge (B221) and the D121 permittivity cell a t 159, 1592, and 15,000 C.P.S. Because the frequency of the bridge oscillator was fixed at 1592 c.P.s., a General Radio oscillator (1210 C) and power supply (1203 B) were used to achieve the remaining frequencies. A Ballantine vacuum tube voltmeter (300-H) served as the null detector. Sm1.r. spectra were obtained with a Varian 60-Mc. proton resonance spectrometer. Near-infrared spectra were obtained with a Beckman DK-1 recording spectrophotometer using 1-cm. Corex cells. A Beckmann freezing point apparatus was used for all cryoscopic measurements. The temperature sensor was a Sargent thermistor bridge equipped with a lowrange thermometric element. The bridge unbalance was fed into a Hewlett-Packard (425A) d.c. microvoltammeter and amplified, and the output was displayed on a Varian (G-11A) recorder. The Journal of Physical Chemistrp
Technique and Reproducibility of Emulsions. One of the difficulties in any emulsion study is that of reproducibly formulating the systems with which one is to work. In this investigation we feel that we have overcome this problem-within limits-by using sonic energy to generate our emulsions. (The criterion of reproducibility is the dielectric constant.) A Sonifier 75-H sonic generator proved most effective in bringing about rapid solution and emulsification of water in the organic phase, which contained weighed amounts of surfactant. The resulting mixture was subjected to sonic energy until solution (or emulsification, as the case may be) was complete, usually in about 10 sec. The liquid was rapidly transferred to the previously calibrated dielectric cell, and the capacitance was determined as quickly as possible, in most cases within 30 sec. Table I illustrates the typical precision with which emulsions were reproduced in this study. Table I : Precision of Emulsion Formulation@ % H20
-Dielectric Run A
constant. e-Run B
0.00 0.90 1.88 2.97 4.14 5.45 6.88 8.53 10.3 12.4
2.08 2.15 2.22 2.31 2.43 2.565 2.74 2.93 3.15 3.48
2.08 2.155 2.23 2.33 2.44 2.58 2.74 2.93 3.15 3.48
Ratio r d r g
1,000 1.002 1.005 1.009 1.004 1 ,006 1,000 1.000 1,000 1,000
Organic phase consisted of 72 ml. of cyclohexane, 28 ml. of carbon tetrachloride, and 0.30 g. of TDO. 2’ = 25’.
Results and Discussion 1 . Nuclear Magnetic Resonance Studies. Conclusive evidence for the existence of two distinct water species in an emulsion was gathered from studies of the n.m.r. spectra, whose tracings are shown in Figure 1. The first (partial) spectrum gives the chemical shifts assigned5*to the carboxyl protons at 7.22 p.p.mesband to the olefinic hydrogens at 3.88 p.p.m. When 0.15% H 2 0 is added to the solution, the chemical shift of the carboxyl protons moves to higher magnetic field strengths, and the peak area ascribed to these protons increases. Once the solubilization point is reached-
( 5 ) J. D. Roberts, “ Nuclear Magnetic Resonance,” McGraw-Hill Book Co., Inc., New York, N. Y., 1959, p . 23. (b) All measurements
were made relative to the methylene protons of cyclohexane set a t zero p.p.m.
STUDIES OF SOLUBILIZED AND EMULSIFIED WATER
V=1.20 ?o‘
Hp
V a 0.60 ‘loD20 7.22
PPM
3.88 3.38
Figure 1. Nsclear magnetic resonance spectra of mixtures containing 4.0 g. of TDO in 100 ml. of cyclohexane and varying quantities of water.
that is, when the solution becomes saturated with solubilized water-the upfield movement of the chemical shift and the increase in peak area cease. (In this example, the solubilization point occurs at 0.62% H20.) Water added after the solubilization point is reached causes the sample to become turbid ; the protons of this “excess” water-actually , emulsified water-are seen in the n.m.r. spectrum as a distinct signal occurring a t 3.38 p.p.m. (0,85y0spectrum). Increasing the water concentration to 1.20y0 increases the peak area of the emulsified species but does not affect that of the solubilized moiety. The last spectrum of Figure 1 shows how heavy water, D20, affects the magnitude and change of the carboxyl proton chemical shift. Inasmuch as deuterium does not respond to n.m.r., the use of D2O removes the contribution of the water protons. The existence of a single n.m.r. signal up to the solubilization point indicates a very rapid exchange of protons between the solubilized water and the carboxyl groups. Once the solubilization point is exceeded, a new n.m.r. signal characteristic of the protons of free ( i e . , emulsified) water appears. That two distinct signals are noted in the n.m.r. spectrum shows that (1) two distinct water species coexist in these emulsions and
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(2) the rate of proton exchange between the solubilized water and the emulsified species is indeed a very slow one. 2. Dielectric Constants. A . Frequency Dependence of Dielectric Constants. The dependence of the dielectric constant on the frequency a t which measurement is made is depicted in Figure 2. In the range 1592-15,000 c.P.s., the values of the dielectric constants are identical (within experimental error) and appear to be independent of the frequency. Somewhere below 1592 c.P.s., the dielectric constant begins to increase as the frequency decreases. The cause of this phenomenon is interfacial polarization.6 As pointed out by Smyth,’ interfacial polarization arises only when the bulk phases differ appreciably in dielectric constant (e) and electrical conductivity (k) so that elk2 # t2kl. Under these conditions, a charge accumulates at the oil-water interface. This accumulation of charge requires current to flow through the dielectric phase; the current flows so slowly that it is observed only in the low frequency range of measurement. B. Dielectric Constants and Water Concentration. According to Callinan and co-workers,* there is a linear relationship between the logarithm of the dielectric constant and the volume yo of water in an oil-continuous phase emulsion. Our findings agreed with theirs, provided that the water content was high and the surfactant concentration was low. When the reverse was true, the addition of water effected little or no change in the dielectric constant.
MOLE FRACTION HZO Figure 2. Dependence of the dielectric constant upon frequency of a mixture containing 72 ml. of cyclohexane, 28 ml. of carbon tetrachloride, and 0.30 g. of TDO aa a function of the water concentration.
Hanai, ICoZloid Z . , 177, 57 (1961). (7) C. P. Smyth, “Dielectric Behavior and Structure,” McGraw-Hill Book Co., Inc., New York, N. Y., 1955, pp. 52-54. (8) T. D. Callinan, R M. Roe, and 3. B. Romans, Anal. Chem., 28, (6) T.
1911 (1956).
Volume 69,Number 4
April 1966
P. D. CRATINAND B. K. ROBERTSON
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0.2
VOLUME % HzO
Figure 3. Dependence of the logarithm of the dielectric constant upon the water concentration of a mixture containing 4.0 g. of TDO in 100 ml. of cyclohexane.
A typical example of how low water and high surfactant concentrations affect the dielectric constant is given in Figure 3. With the addition of small quantities of water, up to 0.250/, HzO in this case, there is no perceptible change in log e. Between 0.25 and 0.75% HzO,the change in log a is much more pronounced. At approximately 0.i'5Y0 HzO, the relationship between log E and yo HzO becomes linear and remains so until demulsification begins. The solubilization point, as determined by the first appearance of turbidity, occurred a t 0.62Y0 HzO. A graph of A log e / A V vs. V showed that the maximum in the first-derivative plot corresponded to the solubilization point. Once again, it is quite evident that solubilized water exhibits polar properties that are substantially different from those manifested by the emulsified species. It was not surprising to find that the quantity of water solubilized was directly proportional to the amount of surfactant present; it was unusual, we thought, that the composition of the organic phase should play such a dominant role in determining the solubilizing power of the surfactant. These phenomena are discussed in detail in section 5 of this paper. 3. Near-Infrared Spectral Studies. Near-infrared spectra were obtained on the cyclohexane-surfactant system containing varying quantities of water, using the former components as a blank. In all cases, an absorption band occurred at 1.935 I.L. As shown in Figure 4, when the peak height (which is directly proportional to the absorbance) is plotted us. the water concentration, two intersecting straight lines result. The first line, passing through the origin, gives only the response due to solubilized water. The second line The Journal of Phyaical Chemistry
0
' 0
0.2
0.4
0.6
0.8
1.0
Volume T' Hz0.
Figure 4. Near-infrared absorbance (1.935 p ) of a mixture containing 4.0 g. of T D O in 100 ml. of cyclohexane as a function of the water concentration.
probably results from a combination of several factors: (1) the absorption of light by the solubilized species, (2) the absorption of light by the emulsified moiety, and (3) the scattering of light by the emulsion droplets themselves. We believe that the effect of scattering contributes very little to the over-all effect, for, although the particle size and wave length of light used in the measurement are of the same order of magnitude, there is only one point where absorption occurs, i.e., 1.935 p . Were the scattering significant, a more or less broad absorption would be noted over a relatively large range of wave lengths. The intersection of these lines corresponds to the solubilization point found by other methods, 0.62% HzO. 4. Cryoscopic Measurements. The large freezingpoint depression constant of cyclohexane (Kr = 20) made this solvent attractive for use in determining the molecular weight of TDO by the cryoscopic m e t h ~ d . ~ The molecular weight lo was found to be about 1000 by this method; thus, the number of micelles in cyclohexane at 6.5' (the freezing point) is indeed negligible. It should be emphasized, however, that micellization and solubilization are temperature-dependent properties of this system" so that the conclusions drawn from measurements made at freezing temperatures may not be valid for the system at room temperature. (9) F. Daniels, et al., "Experimental Physical Chemistry," McGrewHill Book Co., Inc., New York, N. Y., 1956, p. 65 ff. (10) Theoretical formula weight = 750. (11) See ref. 2, pp. 113-122,149 ff.
STUDIESOF SOLUBILIZED AND EMULSIFIED WATER
To determine the degree of dissociation of the TDOHzO complex, additional cryoscopic studies were made using water as the solute in cyclohexane-surfactant mixtures. Water concentrations as high as 1 m caused no depression of the freezing points of these mixtures. We interpret this to mean that, in the solubilization range, each water molecule is bound to the carboxyl groups of the surfactant so tightly that no detectable dissociation takes place. Once emulsification occurs, the water droplets are too large (1 p in diameter) to manifest colligative properties. Thus, it appears that at this temperature water is tied up with the surfactant, not in micelles, but as hydrated species. This disagrees with Pink’s findings12 but lends support to some early work carried out by Weichherz. l 3 , I 4 5 . Solubilization Point and Surfactant Concentration. The dependence of the solubilization point on the composition of the organic phase was determined as a function of the surfactant concentration in mixtures containing varying percentages of cyclohexane and carbon tetrachloride. As one would predict, the quantity of water solubilized is directly proportional to the amount of surfactant present for a fixed composition of the organic phase. There is, however, wide variance in the solubilizing power of a fixed quantity of surfactant from one composition to another. Figure 5 graphically depicts the total amount of water solubilized as a function of the amount of surfactant present in cyclohexane, carbon tetrachloride, and two of their mixtures. The slopes of the lines are interpreted to be average, albeit approximate, formulas of the water-surfactant complexes in each system. These are in cyclohexane, TDO. 10-llH20; in 72-28, TDO.8-9HzO; in 50-50, TDO * 7-8H20 ; and in carbon tetrachloride , TDO . 5-6Hz0. We can envision two interpretations of these data: (1) if we assume that the complexes are built up from a surfactant molecule and n molecules of water, i e . , a series of hydrates having the general formula T D 0 . n H2O , then the solubilization point merely represents the place where one (or more) of the hydrates precipitates from solution, and all higher hydrates are similarly insoluble; (2) if we extrapolate the curves in Figure 5 to zero water concentration, we find that those representing 100 and 50% CC1, pass through the origin, whereas those of the 100 and 72% cyclohexane do not. According to R o s s , ~ the~ intercept ~~~ gives a measure of the c.m.c. of the surfactant since this is the concentration of material needed before any water can be solubilized. If this is true, then micelles are present in the 100 and 72y0 cyclohexane samples and are absent in the 100 and 50% CCl, solutions.
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700
600
0 CYCLOHEXANE
P
MOLES TOO x lo4
Figure 5 . Quantitative solubilization of water as functions of amount of surfactant and composition of organic phase.
The problem is quite perplexing. From cryoscopic studies, there appear to be few, if any, micelles in cyclohexane, and results can be interpreted on the basis of hydrate formation; as noted before, however, the fact that micelles are absent a t 6.5” does not necessarily mean they are absent at 25”. On the other hand, the solubilization plots mentioned in this section lead one to believe that a c.m.c. does exist in 100 and 72% cyclohexane mixtures. Unfortunately, the n.ni.r. spectra give no additional information as to the aggregation of the “complex.” The extreme solubility of the surfactant in cyclohexane prohibited our using surface tension measurements to determine a c.m.c. by that method. (It may be well to mention that TDO is practically insoluble in water; it does, however, show some surface activity here.) 6. Temperature and Salt Efects. Although no quantitative data were obtained on our systems, it was noted qualitatively that the solubilization point was dependent upon the temperature and salt content. An increase in temperature lowered the amount of water that could be solubilized. This indicates that the formation of the surfactant-water complex is an exothermic process. Sodium chloride decreased the quantity of water solubilized by the surfactant. This is probably due to
(12) (13) (14) (15) (16)
R. C . Pink, J. Chem. Soc., 53 (1939). J. Weichherz, Kolloid-Z., 47, 133 (1929). J. Weichherz, ibid., 49, 158 (1929). S. Ross and J. B. Hudson, J . Colloid Sci., 12, 523 (1957). S. Ross, private communication.
Volume 69,Number 6 April 1966
W. F. SAGER, N. FILIPESCU, AND F. A. SERAFIN
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the competition of the salt and the TDO for the water molecules. Acknowledgment. We wish to express our appreciation to the Jersey Production Research Co. for permis-
sion to publish this work and to Dr. Walt Naegele of Esso Research and Engineering for his help in obtaining and interpreting the n.m.r. spectra. We also extend thanks to Mr. J. W. Coryell, who aided us in the cryoscopic and surface tension measurements.
Substituent Effects on Intramolecular Energy Transfer. I.
Absorption and
Phosphorescence Spectra of Rare Earth p-Diketone Chelates
by W. F. Sager, N. Filipescu, and F. A. Serafin Department of Chemistry, The George Washington University, Washington 6,D . C.
(Received June 1 , 1964)
The absorption and phosphorescence spectra of differently substituted rare earth @-diketonates have been investigated. The influence of the substituents on the electronic states of the organic ligand is discussed in relation to the over-all intramolecular energy migration,
Introduction Rare earth ions incorporated in organic chelates by coordination through a donor atom such as oxygen or nitrogen when excited in the region of light absorption associated primarily with the organic ligand exhibit characteristic intra-4f parity-forbidden fluorescence similar to the inorganic single crystal system. Direct excitation of the metal ion is not responsible for the line emission which, instead, is the result of an intramolecular energy transfer from the excited electronic states of the organic complex to the localized 4f energy levels of the chelated ion. The efficiency of excitation varies greatly with the nature of the ligand, temperature, and so1vent.l The over-all absorption-line emission under near-ultraviolet excitation involves (1) ground singlet -+ excited singlet absorption, (2) radiationless intersystem crossing from the excited singlet to the lowest lying triplet state, (3) transfer of energy to the chelated ion, and (4) characteristic ionic fluorescent emission. Recent studies of the emission spectra of rare earth P-diketonates have been concerned with the path of energy migration within these complex molecules.2 In these investigations only a The Journal of Physical Chemistry
restricted number of chelating agents have been studied (dibenzoylmethane, benzoylacetone, and acetylacetone). The present work is concerned with the influence of substituents attached to a 8-diketone rare earth chelate structure on the emission spectra under excitation with near-ultraviolet radiation, the intramolecular energy transfer, and the quantum states involved. The first part covers the investigation of the absorption spectra and phosphorescence spectra of a number of substituted gadolinium p-diketone chelates, indicative of the excited electronic states of the organic ligand.
Experimental Preparation of 6-Diketones. The following diketones, reagent grade, were purchased : dibenzoylmethane, benzoylacetone, theonyltrifluoroacetone, acetylacetone (1) s. I. Weissman, J . Chem. Phycr., 10,214 (1942). (2) (a) G.A. Crosby, R. E. Whan, and R. IM.Alire, ibid., 34, 743 (1961); (b) R. E. Whan and G. A. Crosby, J . Mol. Specfry.,8 , 315 (1962); (e) G.A. Crosby, R. E. Whan, and J. J. Freeman, J . Phys. Chem., 66, 2493 (1962); (d) J. J. Freeman and G. A. Crosby, ibid., 67, 2717 (1963); (e) G. A. Crosby and R. E. Whan, ibid., 36, 863 (1962).
