12668
J. Phys. Chem. 1993,97, 12668-12669
Simple Apparatus for Solubilization of Volatile Substances into Surfactant Solution Yoshikiyo Moroi' and Tom0 Morisue Department of Chemistry, Faculty of Science, Kyushu University, Higashi-ku. Fukuoka 812, Japan Received: September 7, 1993; In Final Form: October 4, 1993'
A very simple glass vessel was devised as an apparatus to determine the solubilization of volatile or gaseous substances. The apparatus worked well for production of constant chemical potential of the solubilizate. With use of the apparatus, the first stepwise association constant of benzene and micelle of n-dodecylammonium perfluoroacetate was found to remain constant irrespective of the amount of benzene. This result indicates constancy of monomeric solubilizate concentration in aqueous surfactant solutions of various concentrations.
Introduction It is not easy to accurately determine the solubilizationamount of gaseous molecules into surfactant solutions at room temperature, because complex apparatus such as a vacuum line combined with an accurate pressure gauge is commonly employed for the purpose.192 This is also the case for solubility of gaseous molecules into aqueous solutions.3 As far as the maximum additive concentration is concerned, an excess of pure solid phase coexists with a surfactant solution phase, and the chemical potential of the solubilizate molecule in the excess phase can be kept constant at a definite temperature and pressure, because the degree of freedom is two for the solid phase by the Gibbs' phase rule. This condition is maintained in the case where the excess phase is in contact with surfactant solutions of various concentrations. This is the reason why the concentration of monomeric solubilizate is kept constant in thesurfactant solutions at a specifiedtemperature and pressure. In addition, this condition is quite useful to obtain the first stepwise association constant between a vacant micelle and a monomeric solubilizate m ~ l e c u l e . ~When . ~ solubilizates are organic liquid molecules, the excess liquid phase cannot be specified only by two degrees of freedom, temperature and pressure, because mutual dissolution of molecules between two phases takes place and another freedom, surfactant concentration for example, is necessary to specify the solubilization system. What is worse is that the liquid solubilizate is easily emulsified through itsdirect contact with surfactant solution. Consequently, keeping the chemical potential constant is quite important for the analysis of solubilization of liquid or gaseous solubilizate j n the surfactant solution. The chemical potential of any component is the same throughout phases in equilibriumfrom the restriction of thermodynamics. This condition can be effectively applied to produce identical chemical potential of gaseous solubilizate molecules against surfactant solutions of various concentrations. The apparatus shown in Figure 1 can meet this condition, where a gaseous phase is in contact with eight surfactant solutions of different concentrations. Inside the apparatus the chemical potential of the gaseous solubilizate molecule is kept constant throughout the phases, or the concentration of monomeric solubilizate can be set identical in the eight solutions. Whether this condition is held or not can be examined by the constancy of solubilizate concentrations in the surfactant solutions below the cmc. When this constancy is assured, the numerical analysis can be done to determine the first stepwise association constant as was made for solid solubilizates." To whom correspondence should be addressed. .Abstract published in Advance ACS Absrracrs, November 15, 1993.
0022-365419312097-12668$04.00/0
Figure 1. Solubilizationapparatus for volatile solubilizates: a = liquid solubilizate, b = surfactant solution, c = disk rotor, and d = magnetic
stirrer.
Experimental Section Solubilization. Eight aqueous solutionsof ndodecylammonium perfluoroacetate(DAPA) were separately injected into eight tubes of the apparatus; the first four below the cmc and the other four above the cmc. An aliquot of volatile liquidsolubilizate,benzene, was placed in the hollow place in the middle of the glass apparatus. Immediately after the placement, the cover was fixed, and the whole glass vessel was kept in a thermostat at 298.2 K controlled within fO.O1 K for 24 h, during which the surfactant solutions were agitated with rotors in the tubes. It took less than 1 min for the solubilizate to evaporate. After the equilibration, each surfactant solution was separately drawn into each injection tube through a small three-way stopcock connected with the injection tube as quickly as possible. The benzene concentration in the surfactant solutionswas determined spectrophotometricallyfrom the absorbance of 255 nm.
Results and Discussion The absorbances of the solutions are plotted against the surfactant concentrations in Figure 2 for three different amounts of benzene placed in the hollow. From the relation between these concentrations, it is possible to calculate the first stepwise 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 49, 1993 12669
Letters I
I
0 0
10
30
20
40
Surfactant Concentration lmmol-dm"
Fig1ue2. Concentration change ofbenzenewithsurfactantconcentrations.
n " -
0
10 C
. cmc I
20
30
mmol,dm"
Figure 3, Plots of ([R,] - [R])/[R] against micellar concentration (C - cmc) of DAPA for determination of the K I / N value.
association constant K1 by the following e q ~ a t i o n : ~ where [R,] is the total solubilizate concentration, [R] is the monomeric solubilizate concentration, C is the total surfactant concentration, and N is the micellar aggregation number. Here constancy of monomeric surfactant concentration is assumed due to a relatively small total surfactant concentration. The plots obeying eq 1 are shown in Figure 3. The three relationships from the three different benzene amounts agree within experimental error. This agreement strongly suggests the rationality of the
present solubilization theory. From the slope the KIIN value can be obtained to be 16.7 mol-' dm3. In any case, the KI value is difficult to compare with reported values due to the different theoretical treatment, although the solubilization of benzene has been a matter of interest.gJ0 As mentioned in the Introduction,theconstancy of thechemical potential of solubilizate was confirmed by the constant solubilizate concentrations below the cmc. Volatile solubilizate can easily escape from surfactant solution. This was also the case for the present solubilizate, benzene. After the struggle to stop the benzene escape from the surfactant solutions in preliminary experiments, the authors used the stopcock as mentioned above. Then, it became possible to keep the benzene in the injection tube without loss for more than 1 h. Another problem was the transport of solvent water from tube to tube. Fortunately, however, the solvent transport was so small that the surfactant concentration change could be neglected over a period of 24 h. A systemin which the chemical potential of a certain component can be controlled is quite useful for thermodynamic analysis of the component solution. The above glass vessel can be used for this purpose. When it is necessary to vary the chemical potential of solubilizate, nonvolatile-solvent solution of volatile solubilizate can be placed in the hollow instead of pure liquid solubilizate: n-dodecane solutions of benzene in with different benzene concentrations are examples. This vessel also can be used for determination of dissociation constants, or acidity constants of volatile acids and bases, because the chemical potential of undissociated chemical species can be kept constant.11J2 The authors propose this vessel for thermodynamic analysis of volatile or gaseous chemical species and for understanding of their physicochemical properties in solutions. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research No. 03453013 from the Ministry of Education, Science, and Culture, which is greatly acknowledged. References and Notes (1) Tucker, E. E.; Christian, S.D. J . Chem. Thermodyn. 1979,11,1137. (2) Mathcson, I. B. C.; King, Jr., A. D. J . Colloid Inrerface Sci. 1979, 66, 464. (3) Wishnia, A.; Pinter, Jr., T. W. Biochemistry 1966, 5, 1534. (4) Moroi, Y. J . Phys. Chem. 1980,84, 2186. ( 5 ) Moroi, Y. Micelles; Theorericaland Applied Aspecrs; Plenum: New York, 1992; Chapter 9. (6) Moroi, Y.; Sato, K.; Matuura, R. J . Phys. Chem. 1982.86, 2463. (7) Moroi. Y.: Noma. H.: Matuura, R. J. Phys. Chem. 1983.87, 872. (8) Moroi, Y.; Matuura, R. J. Colloid Inte$ace Sci. 1988, 125, 463. ( 9 ) Simon, S.A.; McDaniel, R.V.; McIntosh, T. J. J . Phys. Chem. 1982, 86, 1449. (IO) Tucker, S. A.; Christian, S.D. J . Colloid Inrerface Sci. 1985, 104, 562. (11) Muckerjee, P.; Moroi, Y. Anal. Chem. 1978,50, 1589. (12) Moroi, Y.; Matuura, R. Anal. Chim. Acta. 1983, 152, 239.
