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Microstructure of Oil–Surfactant-Rich Phase in Relation to Liquid Scintillation Counting Efficiency
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
180, 22–26 (1996)
0269
Microstructure of Oil–Surfactant-Rich Phase in Relation to Liquid Scintillation Counting Efficiency GUN LUNDSTEN 1 Department of Physical Chemistry, A˚bo Akademi University, Porthansgatan 3-5, FIN-20500 A˚bo, Finland Received March 6, 1995; accepted November 13, 1995
of photons emitted from a solution containing the radioactive material of interest (11). The photons are emitted as a result of excitation of the solvent molecules by isotopic disintegration. They are not emitted by the excited solvent molecules directly but by a fluorescing compound which in turn becomes excited by the excited solvent molecules. For the scintillation efficiency to be high the solvent must have predominantly aromatic character (12). If the original sample is an organic solid or liquid with hydrophobic character, which directly dissolves into the aromatic solvent, there are usually no problems associated with sample preparation (13). The only consideration is that the sample should not contain compounds that decrease the scintillation efficiency too much. On the other hand, in most situations, the original sample has hydrophilic character and cannot be directly dissolved into an aromatic solvent. Suitable surfactants are needed to dissolve these compounds into aromatic solvents. In this case, the task is to find a mixture of an aromatic oil and surfactants that can dissolove large quantities of water at low contents of surfactant and high contents of aromatic oil. But as shown earlier, water is sparingly soluble at low surfactant contents (1–4, 9). However, if nonionic surfactants are mixed with sodium 1,4-bis(2-ethylhexyl)sulfosuccinate [AOT, which forms discrete water droplets with high stability over a wide composition range (14)], the mass fraction of solubilized water exceeds 0.5 at relatively low surfactant contents (9, 15). In this study, the phase diagram of the nonionic surfactant mixture alkyl polyoxyethylene type (Berol 227), mesitylene, and water has been determined. Conductivity and viscosity measurements were performed on the isotropic oil–surfactant-rich region (L2 phase) of the phase diagram. Earlier studies of microemulsions using these techniques (9, 16– 20) have clearly shown their value in identifying the structure and nature of microemulsions. Furthermore, the isotropic L2 phase was tested as a cocktail for liquid scintillation counting by determing the counting efficiencies of the mixtures. The properties of the L2 phases were compared with the properties of the commercially available scintillation cocktail Supermix.
The phase behavior at 298.2 K of a three-component system of 1,3,5-trimethylbenzene (mesitylene), water, and a nonionic surfactant of the nonyl phenyl polyoxyethylene type (Berol 227) was investigated. The microstructure of the isotropic oil–surfactantrich region in this system was examined by means of conductivity and viscosity measurements. At relatively low surfactant contents, open or bicontinuous structures are formed, but as the anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate is added, closed aggregates are formed. Some compositions of this pseudothree-component system were tested as cocktails for liquid scintillation counting. The counting efficiency seems to be independent of the microstructure in the isotropic oil–surfactant-rich phase, but as the one-phase border is crossed and birefringent phases are formed, the counting efficiency steeply decreases. When a twophase system of isotropic solutions is formed by crossing the onephase border, no abrupt change in the counting efficiency is recorded. At relatively low surfactant contents, the counting efficiency is approximately at the same level as for the commercially available scintillation cocktail Supermix. q 1996 Academic Press, Inc. Key Words: water-in-oil microemulsion; aromatic oil; alkyl phenyl polyoxyethylene; liquid scintillation counting; counting efficiency.
INTRODUCTION
The formation of surfactant aggregates and the solubility of water in mixtures of nonionic surfactants of the alkyl polyoxyethylene type and aromatic oils (1–4) or cyclohexane (5–8) have been extensively studied. Generally, at high surfactant contents, the water solubility is quite good due to surfactant aggregation. On the other hand, at low surfactant contents, aggregates solubilizing water are not formed and water is sparingly soluble (9). One application for these aromatic microemulsions is as cocktails for liquid scintillation counting (LSC) (10). LSC is a chemical spectroscopy method used to determine the amount of one or more substances labeled with a radioactive nuclide. The method of LSC is based on direct registration 1
To whom correspondence should be addressed. 22
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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MATERIALS AND METHODS
The nonionic surfactants Berol 227 (nonyl phenyl polyoxyethylene with a distribution in the number of ethoxy groups) and Berol 223 (surfactant mixture containing dipropylene glycol monomethyl ether at a mass fraction equal to 0.2) were supplied by Berol Nobel. Their chemical composition has not been determined. The anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT, ú98% purity) was obtained from Fluka. 1,3,5-Trimethylbenzene (mesitylene, ú98% purity) was supplied by Merck. OptiScint Hisafe based on diisopropylnaphthalene (DIN) containing fluorescing compounds (e.g., 2,5-diphenyloxazole, PPO) and Supermix scintillation cocktail were delivered by FSA Laboratory Supplies, England. All chemicals were used as supplied. The water was twice distilled. The nonionic surfactants and AOT were always mixed at equal masses. Starting with 20 stock solutions on the binary axis aromatic oil–surfactant, the approximate phase behavior was determined. In addition, about 50 stock solutions were prepared to the determine the exact phase boundaries. Water was added to these stock solutions and the samples were weighed into glass vials with screw caps and were then thermostated at 298.2 K in a water bath for at least 1 day. The different phases present were determined visually between crossed polarizers. The viscous samples were also examined after 2 months. All measurements were made at constant oil-to-surfactant mass ratios (9), r Å mo /ms Å wo /ws , as a function of the mass fraction of water, ww . The conductivities were measured using a Wayne–Kerr bridge with automatic recording of the resistances (21). The viscosities were determined at the shear rate (22) of 146 s 01 on a Bohlin VOR Rheometer from Bohlin Reologi AB, Sweden. For liquid scintillation counting, the samples containing radioactive sugar ( 14C, dpm Å 108,000) were prepared in glass vials. Samples were carefully shaken and allowed to stand for at least 2 days. Then, counting efficiencies were determined with a 1219 RackBeta liquid scintillation counter from Wallac Oy, Finland. RESULTS AND DISCUSSION
The phase diagram for the system Berol 227–1,3,5-trimethylbenzene (mesitylene) –water is shown in Fig. 1. Berol 227 is completely soluble in mesitylene and almost insoluble in water. At low Berol 227 contents, essentially no water could be dissolved into the mixture of Berol 227 and mesitylene. This indicates that a minimum amount of surfactant is necessary to dissolve or solubilize even small amounts of water (23). With increasing water content, an inverse hexagonal phase, F, appears. At intermediate Berol 227 contents, water can be solubilized up to a mass fraction of water, ww Å 0.5. At high Berol 227 contents, a lamellar liquid crystalline phase, D, is formed and further addition of water
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FIG. 1. Phase diagram for 1,3,5-trimethylbenzene (mesitylene), Berol 227, and water at 298.2 K. L2 denotes the isotropic solution phase, D the lamellar phase, E the hexagonal phase, and F the reversed hexagonal phase. wo , ws , and ww denote the mass fractions of oil, surfactant, and water, respectively. ( l — l ) Solubility limit of water with OptiScint Hisafe as oil.
transforms this phase to a hexagonal phase, E. The phase boundary of the isotropic L2 phase was determined within an accuracy of a mass fraction wi Å 0.01. The locations of the other one-phase boundaries are more approximate; the estimated errors may be as much as wi Å 0.05. The probable three-phase triangles are drawn on the basis of a few experimental points and by reference to other phase diagrams (24). Because of strong surfactant association even at low concentrations (9), sodium AOT in mesitylene solubilizes water at low AOT contents (15). Consequently, when AOT is mixed with Berol 227, the solubility of water is drastically increased, even in the mesitylene-rich corner of the phase diagram in Fig. 2. Furthermore, by addition of the anionic surfactant AOT, the lamellar phase melts due to decreased repulsive hydration forces between the lamellar bilayers (25), and more water is solubilized in the L2 phase. The commercially available scintillation cocktail Supermix dissolves a mass fraction of water ww Å 0.6. The variations of the relative conductivity as a function of the mass fraction of water for Berol 227 and the mixture Berol 227/AOT in mesitylene, respectively, are shown in Fig. 3. The relative conductivity was determined also for the commercially available scintillation cocktail Supermix. The general trend is an initial increase in the relative conductivity. At low mass fractions of water there is an excess of surfactant relative to the amount of solubilized water, and therefore this region is likely to consist of hydrated surfactant species (9, 16). As more water is added, the conductivity gradually increases due to increased hydration of the surfactants, thus giving a network of interacting species. With
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FIG. 2. Partial phase diagram for 1,3,5-trimethybenzene (mesitylene), Berol 227/AOT (equal masses), and water at 298.2 K. The symbols are the same as in Fig. 1.
further addition of water, the relative conductivity for Berol 227/AOT in mesitylene reaches a maximum at ww Å 0.05 and then decreases. At this maximum point closed inverse micelles are formed. The decrease in conductivity is due to the gradual replacement of hydrated surfactant species with microemulsion droplets and a decrease in the droplet concentration and mobility as the droplet size increases. This interpretation is also confirmed by the steep increase in the rela-
FIG. 3. Relative conductivity, k / k7, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( l ) mesitylene–Berol 227 at r Å 0.7/0.3, k7 Å 1.0 1 10 08 V 01 cm01 , ( . ) mesitylene– (Berol 227/AOT) at r Å 0.7/0.3, k7 Å 3.5 1 10 07 V 01 cm01 , and ( l ) Supermix scintillation cocktail, k7 Å 5.9 1 10 07 V 01 cm01 .
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FIG. 4. Relative viscosity, h / h7, at the shear rate 146 s 01 as a function of mass fraction of water, ww , at 298.2 K for the same systems as in Fig. 3. ( l ) Mesitylene–Berol 227 at r Å 0.7/0.3, h7 Å 2.3 1 10 03 Ns m02 ; ( . ) mesitylene– (Berol 227/AOT) at r Å 0.7/0.3, h7 Å 3.2 1 10 03 Ns m02 ; ( l ) Supermix scintillation cocktail, h7 Å 5.6 1 10 02 Ns m02 .
tive viscosity in Fig. 4. The relative conductivity for Supermix and Berol 227 in mesitylene increases by four decades as water is added, indicating formation of open aggregates or bicontinuous structures. The relative viscosity of Berol 227 in mesitylene increases steeply, indicating changes in the microstructure, whereas the relative viscosity of Supermix is unchanged by adding water. But h 0 is about
FIG. 5. Liquid scintillation counting efficiency, E, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( h ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.8/0.2, ( j ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.7/0.3, ( ú ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.5/0.5, and ( l ) Supermix scintillation cocktail.
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FIG. 6. Liquid scintillation counting efficiency, E, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( l ) OptiScint Hisafe–Berol 227 at r Å 0.7/0.3, ( L ) OptiScint Hisafe–Berol 227 at r Å 0.5/0.5, ( . ) OptiScint Hisafe– (Berol 227/AOT) at r Å 0.7/0.3, and ( l ) Supermix scintillation cocktail.
20 times higher for Supermix than for Berol 227 and Berol 227/AOT in mesitylene, respectively, indicating the presence of strongly interacting structures already in the waterfree Supermix. As the photons in the scintillation process are not emitted by the excited solvent molecules directly, a fluorescing compound has to be added. Therefore, the surfactants were dissolved in Optiscint Hisafe, which contains fluorescing compounds. In these surfactant–aromatic oil mixtures, the water solubility is now decreased compared with that in mesitylene, which is indicated in Figs. 1 and 2. In liquid scintillation counting the physical processes leading to light emmission are quite rapid, and thus the photons generated by disintegration can be considered to be emitted immediately and in one burst, or light pulse. The probability of detecting a disintegration is directly proportional to the mean number of photons in the light pulse. The overall counting efficiency, E, which is defined as the ratio of the rate of detected counts to the disintegration rate, is thus proportional to the number of photons in the light pulses. The count rate is usually given in counts per minute (cpm), and the disintegration rate is usually given in disintegrations per minute (dpm). CPM is often also used as a symbol denoting the count rate, and DPM is often used as a symbol denoting disintegration rate. The counting efficiency, E, is thus given by the relation E Å CPM/DPM (26). Figure 5 shows the counting efficiency for the mixture Berol 223/AOT in OptiScint Hisafe as a function of mass fraction of water (a partial phase diagram of the system mesitylene–Berol 223/AOT–water has been published in Ref. 9). The counting efficiency is evidently dependent on
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the amount of aromatic oil. A decreasing mass fraction of aromatic oil, i.e., a decreasing oil-to-surfactant ratio, strongly decreases the counting efficiency. The same trends are observed for Berol 227 and Berol 227/AOT, respectively in mesitylene as shown in Fig. 6. Generally, for high oil-tosurfactant ratios, r, and low mass fractions of water, ww , the counting efficiency is approximately at the same level as for Supermix. Berol 227/AOT at r Å 0.7/0.3 forms closed water droplets on addition of water, and Supermix and Berol 227 at r Å 0.7/0.3 form bicontinuous or open structures. This indicates that the counting efficiency is independent of the microstructure. As the one-phase border in the phase diagram is crossed, the counting efficiency decreases dramatically and discontinuously, especially for Berol 223/AOT at r Å 0.2/0.8. After the water solubility limit is reached birefringent phases are formed and the counting efficiency decreases, probably due to absorption quenching (26). On the other hand, at the oil-to-surfactant ratio r Å 0.5/0.5, an equilibrium between two isotropic solution phases is formed when the one-phase border is crossed (Fig. 1). In this case no abrupt changes in the counting efficiency were recorded (Figs. 5 and 6) but because of the low content of aromatic oil, the initial counting efficiency is already low. CONCLUSIONS
In general, nonionic surfactants in aromatic oils do not form aggregates at low surfactant contents and consequently little or no water can be dissolved. At higher surfactant contents, open or bicontinuous structures are formed on addition of water. A mixture of nonionic surfactants and AOT already dissolves water at low surfactant contents due to the formation of closed surfactant aggregates, which solubilize the water. The different microstructures do not seem to have any effect on the liquid scintillation counting efficiency. At relatively low surfactant contents, the counting efficiency of the investigated systems is approximately at the same level as for the commercially available scintillation cocktail Supermix. As the one-phase border is crossed and birefringent phases are formed, the counting efficiency decreases discontinuously, probably due to absorption quenching. On the other hand, if an equilibrium between two isotropic phases is formed, no abrupt change in the counting efficiency is recorded when the one-phase boundary is crossed. ACKNOWLEDGMENTS Suomen Akatemia-Finlands Akademi is thanked for financial support. ˚ bo Akademi University, and Dr. Chris Brancewicz, Dr. Sune Backlund, A Clarkson University, are thanked for useful discussions and comments on ˚ bo Akademi the manuscript. Thanks are also due Ms. Patricia Snickars, A University, for help with experimental work, and Dr. Kenneth Rundt and Dr. Stefan Ja¨rnstro¨m, Wallac Oy, for help with the scintillation counting equipment.
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Marsden, S. S., Jr., and McBain, J. W., J. Phys. Chem. 52, 110 (1948). Mulley, B. A., and Metcalf, A. D., J. Colloid Sci. 19, 501 (1964). Zhu, D-M., Wu, X., and Schelly, Z. A., Langmuir 8, 1538 (1992). Almgren, M., van Stam, J., Swarup, S., and Lo¨froth, J-E., Langmuir 2, 432 (1986). Kumar, C., and Balasubramanian, D., J. Colloid Interface Sci. 74, 64 (1980). Zhu, D-M., Feng, K-I., and Schelly, Z. A., J. Phys. Chem. 96, 2382 (1992). Saito, H., and Shinoda, K., J. Colloid Interface Sci. 35, 359 (1971). Zhu, D-M., and Schelly, Z. A., Langmuir 8, 48 (1992). Lundsten, G., and Backlund, S., J. Colloid Interface Sci. 169, 408 (1995). Klein, R. C., and Gershey, E. L., Health Phys. 59, 461 (1990). Dayer, A., ‘‘An Introduction to Liquid Scintillation Counting.’’ Heyden & Son Ltd, London, 1974. Tschurlovits, M., Rajner, V., and Rank, D., Appl. Radiat. Isot. 42, 891 (1990). Peng, C. T., ‘‘Sample Preparation in Liquid Scintillation Counting’’, Amersham Corp., IL, 1976.
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14. Jo´hannsson, R., and Almgren, M., Langmuir 9, 2879 (1993). 15. Lundsten, G., Backlund, S., and Kiwilsza, G., Prog. Colloid Polym. Sci. 97, 194 (1994). 16. Boned, C., Clausse, M., Lagourette, B., Peyrelasse, J., McClean, V. E. R., and Sheppard, R. J., J. Phys. Chem. 84, 1520 (1980). 17. Baker, R. C., Florence, A. T., Ottewill, R. H., and Tadros, Th.F., J. Colloid Interface Sci. 100, 332 (1984). 18. Chen, S. J., Evans, D. F., and Ninham, B. W., J. Phys. Chem. 88, 1631 (1984). 19. Peyrelasse, J., and Boned, C., Phys. Rev. A 41, 938 (1990). 20. Berg, R. F., Moldover, M. R., and Huang, J. S., J. Chem. Phys. 87, 3687 (1990). 21. Blokhus, A. M., Høiland, H., and Backlund, S., J. Colloid Interface Sci. 114, 9 (1986). 22. Chen, C-H., and Warr, G. G., J. Phys. Chem. 96, 9492 (1992). 23. Aveyard, R., Binks, B. P., Clark, S., and Fletcher, P. D. I., Prog. Colloid Polym. Sci. 79, 202 (1989). 24. Ekwall, P., in ‘‘Advances in Liquid Crystals’’ (G. H. Brown, Ed.), Vol. 1, p. 1. Academic Press, New York, 1975. 25. Jokela, P., and Jo¨nsson, B., J. Phys. Chem. 92, 1923 (1988). 26. Rundt, K., ‘‘On the Determination and Compensation of Quench in ˚ bo Akademi University, Liquid Scintillation Counting.’’ Ph.D. thesis, A ˚ bo, 1989. A
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Microstructure of Oil–Surfactant-Rich Phase in Relation to Liquid Scintillation Counting Efficiency GUN LUNDSTEN 1 Department of Physical Chemistry, A˚bo Akademi University, Porthansgatan 3-5, FIN-20500 A˚bo, Finland Received March 6, 1995; accepted November 13, 1995
of photons emitted from a solution containing the radioactive material of interest (11). The photons are emitted as a result of excitation of the solvent molecules by isotopic disintegration. They are not emitted by the excited solvent molecules directly but by a fluorescing compound which in turn becomes excited by the excited solvent molecules. For the scintillation efficiency to be high the solvent must have predominantly aromatic character (12). If the original sample is an organic solid or liquid with hydrophobic character, which directly dissolves into the aromatic solvent, there are usually no problems associated with sample preparation (13). The only consideration is that the sample should not contain compounds that decrease the scintillation efficiency too much. On the other hand, in most situations, the original sample has hydrophilic character and cannot be directly dissolved into an aromatic solvent. Suitable surfactants are needed to dissolve these compounds into aromatic solvents. In this case, the task is to find a mixture of an aromatic oil and surfactants that can dissolove large quantities of water at low contents of surfactant and high contents of aromatic oil. But as shown earlier, water is sparingly soluble at low surfactant contents (1–4, 9). However, if nonionic surfactants are mixed with sodium 1,4-bis(2-ethylhexyl)sulfosuccinate [AOT, which forms discrete water droplets with high stability over a wide composition range (14)], the mass fraction of solubilized water exceeds 0.5 at relatively low surfactant contents (9, 15). In this study, the phase diagram of the nonionic surfactant mixture alkyl polyoxyethylene type (Berol 227), mesitylene, and water has been determined. Conductivity and viscosity measurements were performed on the isotropic oil–surfactant-rich region (L2 phase) of the phase diagram. Earlier studies of microemulsions using these techniques (9, 16– 20) have clearly shown their value in identifying the structure and nature of microemulsions. Furthermore, the isotropic L2 phase was tested as a cocktail for liquid scintillation counting by determing the counting efficiencies of the mixtures. The properties of the L2 phases were compared with the properties of the commercially available scintillation cocktail Supermix.
The phase behavior at 298.2 K of a three-component system of 1,3,5-trimethylbenzene (mesitylene), water, and a nonionic surfactant of the nonyl phenyl polyoxyethylene type (Berol 227) was investigated. The microstructure of the isotropic oil–surfactantrich region in this system was examined by means of conductivity and viscosity measurements. At relatively low surfactant contents, open or bicontinuous structures are formed, but as the anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate is added, closed aggregates are formed. Some compositions of this pseudothree-component system were tested as cocktails for liquid scintillation counting. The counting efficiency seems to be independent of the microstructure in the isotropic oil–surfactant-rich phase, but as the one-phase border is crossed and birefringent phases are formed, the counting efficiency steeply decreases. When a twophase system of isotropic solutions is formed by crossing the onephase border, no abrupt change in the counting efficiency is recorded. At relatively low surfactant contents, the counting efficiency is approximately at the same level as for the commercially available scintillation cocktail Supermix. q 1996 Academic Press, Inc. Key Words: water-in-oil microemulsion; aromatic oil; alkyl phenyl polyoxyethylene; liquid scintillation counting; counting efficiency.
INTRODUCTION
The formation of surfactant aggregates and the solubility of water in mixtures of nonionic surfactants of the alkyl polyoxyethylene type and aromatic oils (1–4) or cyclohexane (5–8) have been extensively studied. Generally, at high surfactant contents, the water solubility is quite good due to surfactant aggregation. On the other hand, at low surfactant contents, aggregates solubilizing water are not formed and water is sparingly soluble (9). One application for these aromatic microemulsions is as cocktails for liquid scintillation counting (LSC) (10). LSC is a chemical spectroscopy method used to determine the amount of one or more substances labeled with a radioactive nuclide. The method of LSC is based on direct registration 1
To whom correspondence should be addressed. 22
0021-9797/96 $18.00 Copyright q 1996 by Academic Press, Inc. All rights of reproduction in any form reserved.
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The nonionic surfactants Berol 227 (nonyl phenyl polyoxyethylene with a distribution in the number of ethoxy groups) and Berol 223 (surfactant mixture containing dipropylene glycol monomethyl ether at a mass fraction equal to 0.2) were supplied by Berol Nobel. Their chemical composition has not been determined. The anionic surfactant sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT, ú98% purity) was obtained from Fluka. 1,3,5-Trimethylbenzene (mesitylene, ú98% purity) was supplied by Merck. OptiScint Hisafe based on diisopropylnaphthalene (DIN) containing fluorescing compounds (e.g., 2,5-diphenyloxazole, PPO) and Supermix scintillation cocktail were delivered by FSA Laboratory Supplies, England. All chemicals were used as supplied. The water was twice distilled. The nonionic surfactants and AOT were always mixed at equal masses. Starting with 20 stock solutions on the binary axis aromatic oil–surfactant, the approximate phase behavior was determined. In addition, about 50 stock solutions were prepared to the determine the exact phase boundaries. Water was added to these stock solutions and the samples were weighed into glass vials with screw caps and were then thermostated at 298.2 K in a water bath for at least 1 day. The different phases present were determined visually between crossed polarizers. The viscous samples were also examined after 2 months. All measurements were made at constant oil-to-surfactant mass ratios (9), r Å mo /ms Å wo /ws , as a function of the mass fraction of water, ww . The conductivities were measured using a Wayne–Kerr bridge with automatic recording of the resistances (21). The viscosities were determined at the shear rate (22) of 146 s 01 on a Bohlin VOR Rheometer from Bohlin Reologi AB, Sweden. For liquid scintillation counting, the samples containing radioactive sugar ( 14C, dpm Å 108,000) were prepared in glass vials. Samples were carefully shaken and allowed to stand for at least 2 days. Then, counting efficiencies were determined with a 1219 RackBeta liquid scintillation counter from Wallac Oy, Finland. RESULTS AND DISCUSSION
The phase diagram for the system Berol 227–1,3,5-trimethylbenzene (mesitylene) –water is shown in Fig. 1. Berol 227 is completely soluble in mesitylene and almost insoluble in water. At low Berol 227 contents, essentially no water could be dissolved into the mixture of Berol 227 and mesitylene. This indicates that a minimum amount of surfactant is necessary to dissolve or solubilize even small amounts of water (23). With increasing water content, an inverse hexagonal phase, F, appears. At intermediate Berol 227 contents, water can be solubilized up to a mass fraction of water, ww Å 0.5. At high Berol 227 contents, a lamellar liquid crystalline phase, D, is formed and further addition of water
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FIG. 1. Phase diagram for 1,3,5-trimethylbenzene (mesitylene), Berol 227, and water at 298.2 K. L2 denotes the isotropic solution phase, D the lamellar phase, E the hexagonal phase, and F the reversed hexagonal phase. wo , ws , and ww denote the mass fractions of oil, surfactant, and water, respectively. ( l — l ) Solubility limit of water with OptiScint Hisafe as oil.
transforms this phase to a hexagonal phase, E. The phase boundary of the isotropic L2 phase was determined within an accuracy of a mass fraction wi Å 0.01. The locations of the other one-phase boundaries are more approximate; the estimated errors may be as much as wi Å 0.05. The probable three-phase triangles are drawn on the basis of a few experimental points and by reference to other phase diagrams (24). Because of strong surfactant association even at low concentrations (9), sodium AOT in mesitylene solubilizes water at low AOT contents (15). Consequently, when AOT is mixed with Berol 227, the solubility of water is drastically increased, even in the mesitylene-rich corner of the phase diagram in Fig. 2. Furthermore, by addition of the anionic surfactant AOT, the lamellar phase melts due to decreased repulsive hydration forces between the lamellar bilayers (25), and more water is solubilized in the L2 phase. The commercially available scintillation cocktail Supermix dissolves a mass fraction of water ww Å 0.6. The variations of the relative conductivity as a function of the mass fraction of water for Berol 227 and the mixture Berol 227/AOT in mesitylene, respectively, are shown in Fig. 3. The relative conductivity was determined also for the commercially available scintillation cocktail Supermix. The general trend is an initial increase in the relative conductivity. At low mass fractions of water there is an excess of surfactant relative to the amount of solubilized water, and therefore this region is likely to consist of hydrated surfactant species (9, 16). As more water is added, the conductivity gradually increases due to increased hydration of the surfactants, thus giving a network of interacting species. With
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FIG. 2. Partial phase diagram for 1,3,5-trimethybenzene (mesitylene), Berol 227/AOT (equal masses), and water at 298.2 K. The symbols are the same as in Fig. 1.
further addition of water, the relative conductivity for Berol 227/AOT in mesitylene reaches a maximum at ww Å 0.05 and then decreases. At this maximum point closed inverse micelles are formed. The decrease in conductivity is due to the gradual replacement of hydrated surfactant species with microemulsion droplets and a decrease in the droplet concentration and mobility as the droplet size increases. This interpretation is also confirmed by the steep increase in the rela-
FIG. 3. Relative conductivity, k / k7, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( l ) mesitylene–Berol 227 at r Å 0.7/0.3, k7 Å 1.0 1 10 08 V 01 cm01 , ( . ) mesitylene– (Berol 227/AOT) at r Å 0.7/0.3, k7 Å 3.5 1 10 07 V 01 cm01 , and ( l ) Supermix scintillation cocktail, k7 Å 5.9 1 10 07 V 01 cm01 .
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FIG. 4. Relative viscosity, h / h7, at the shear rate 146 s 01 as a function of mass fraction of water, ww , at 298.2 K for the same systems as in Fig. 3. ( l ) Mesitylene–Berol 227 at r Å 0.7/0.3, h7 Å 2.3 1 10 03 Ns m02 ; ( . ) mesitylene– (Berol 227/AOT) at r Å 0.7/0.3, h7 Å 3.2 1 10 03 Ns m02 ; ( l ) Supermix scintillation cocktail, h7 Å 5.6 1 10 02 Ns m02 .
tive viscosity in Fig. 4. The relative conductivity for Supermix and Berol 227 in mesitylene increases by four decades as water is added, indicating formation of open aggregates or bicontinuous structures. The relative viscosity of Berol 227 in mesitylene increases steeply, indicating changes in the microstructure, whereas the relative viscosity of Supermix is unchanged by adding water. But h 0 is about
FIG. 5. Liquid scintillation counting efficiency, E, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( h ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.8/0.2, ( j ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.7/0.3, ( ú ) OptiScint Hisafe– (Berol 223/AOT) at r Å 0.5/0.5, and ( l ) Supermix scintillation cocktail.
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FIG. 6. Liquid scintillation counting efficiency, E, at 298.2 K as a function of mass fraction of water, ww , at constant oil-to-surfactant mass ratios, r. The systems are ( l ) OptiScint Hisafe–Berol 227 at r Å 0.7/0.3, ( L ) OptiScint Hisafe–Berol 227 at r Å 0.5/0.5, ( . ) OptiScint Hisafe– (Berol 227/AOT) at r Å 0.7/0.3, and ( l ) Supermix scintillation cocktail.
20 times higher for Supermix than for Berol 227 and Berol 227/AOT in mesitylene, respectively, indicating the presence of strongly interacting structures already in the waterfree Supermix. As the photons in the scintillation process are not emitted by the excited solvent molecules directly, a fluorescing compound has to be added. Therefore, the surfactants were dissolved in Optiscint Hisafe, which contains fluorescing compounds. In these surfactant–aromatic oil mixtures, the water solubility is now decreased compared with that in mesitylene, which is indicated in Figs. 1 and 2. In liquid scintillation counting the physical processes leading to light emmission are quite rapid, and thus the photons generated by disintegration can be considered to be emitted immediately and in one burst, or light pulse. The probability of detecting a disintegration is directly proportional to the mean number of photons in the light pulse. The overall counting efficiency, E, which is defined as the ratio of the rate of detected counts to the disintegration rate, is thus proportional to the number of photons in the light pulses. The count rate is usually given in counts per minute (cpm), and the disintegration rate is usually given in disintegrations per minute (dpm). CPM is often also used as a symbol denoting the count rate, and DPM is often used as a symbol denoting disintegration rate. The counting efficiency, E, is thus given by the relation E Å CPM/DPM (26). Figure 5 shows the counting efficiency for the mixture Berol 223/AOT in OptiScint Hisafe as a function of mass fraction of water (a partial phase diagram of the system mesitylene–Berol 223/AOT–water has been published in Ref. 9). The counting efficiency is evidently dependent on
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the amount of aromatic oil. A decreasing mass fraction of aromatic oil, i.e., a decreasing oil-to-surfactant ratio, strongly decreases the counting efficiency. The same trends are observed for Berol 227 and Berol 227/AOT, respectively in mesitylene as shown in Fig. 6. Generally, for high oil-tosurfactant ratios, r, and low mass fractions of water, ww , the counting efficiency is approximately at the same level as for Supermix. Berol 227/AOT at r Å 0.7/0.3 forms closed water droplets on addition of water, and Supermix and Berol 227 at r Å 0.7/0.3 form bicontinuous or open structures. This indicates that the counting efficiency is independent of the microstructure. As the one-phase border in the phase diagram is crossed, the counting efficiency decreases dramatically and discontinuously, especially for Berol 223/AOT at r Å 0.2/0.8. After the water solubility limit is reached birefringent phases are formed and the counting efficiency decreases, probably due to absorption quenching (26). On the other hand, at the oil-to-surfactant ratio r Å 0.5/0.5, an equilibrium between two isotropic solution phases is formed when the one-phase border is crossed (Fig. 1). In this case no abrupt changes in the counting efficiency were recorded (Figs. 5 and 6) but because of the low content of aromatic oil, the initial counting efficiency is already low. CONCLUSIONS
In general, nonionic surfactants in aromatic oils do not form aggregates at low surfactant contents and consequently little or no water can be dissolved. At higher surfactant contents, open or bicontinuous structures are formed on addition of water. A mixture of nonionic surfactants and AOT already dissolves water at low surfactant contents due to the formation of closed surfactant aggregates, which solubilize the water. The different microstructures do not seem to have any effect on the liquid scintillation counting efficiency. At relatively low surfactant contents, the counting efficiency of the investigated systems is approximately at the same level as for the commercially available scintillation cocktail Supermix. As the one-phase border is crossed and birefringent phases are formed, the counting efficiency decreases discontinuously, probably due to absorption quenching. On the other hand, if an equilibrium between two isotropic phases is formed, no abrupt change in the counting efficiency is recorded when the one-phase boundary is crossed. ACKNOWLEDGMENTS Suomen Akatemia-Finlands Akademi is thanked for financial support. ˚ bo Akademi University, and Dr. Chris Brancewicz, Dr. Sune Backlund, A Clarkson University, are thanked for useful discussions and comments on ˚ bo Akademi the manuscript. Thanks are also due Ms. Patricia Snickars, A University, for help with experimental work, and Dr. Kenneth Rundt and Dr. Stefan Ja¨rnstro¨m, Wallac Oy, for help with the scintillation counting equipment.
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
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