Langmuir 1992,8, 2347-2348
A Convenient Method for Measuring Pervaporation through Monolayers at the Gas-Water Interface'
2347
A
Mark Conner and Steven L. Regen' Department of Chemistry and Zettlemoyer Center for Surface Studies, Lehigh University, Bethlehem, Pennsylvania 18015 Received June 2,1992. I n Final Form: July 15, 1992
Introduction The amount of energy that is consumed each year by the chemical industry for separations is substantial. In the United States alone, it has been estimated that 4.2 quadrillion Btu's are spent on distillation, drying, and evaporation processes.2 These costs, together with the diminishing supply of natural energy resources, provide considerable incentive for creating alternate methods of separation, especially those that employ synthetic memb r a n e ~ .Recently, ~ Langmuir-Blodgett (LB) films have begun to attract interest as separation The fact that their structure is highly organized in three dimensions (existingas two-dimensional arrays of molecules that are layered onto a support), makes such material well-suited for adjustment and optimization of their transport properties. In principle, a systematicexamination of diffusionacross monolayers at the gas-water interface should assist the rational design of LB-based membranes for separations. In particular, the ability to precisely control the packing density of a monolayer affords an experimental means for sorting out barrier contributions due to molecular structure, composition, and packing. It thus provides an opportunity for definingthe relationshipsthat exist among molecular structure, composition, supramolecular structure, and permeation properties of LB films. Here, we describea convenient method that can be used for making such transport measurements. Specifically, we report a technique for measuring the efflux of volatile organics through surfactant monolayers, and demonstrate its utility by use of three permeants [tetrahydrofuran (T), cis-2,5dimethyltetrahydrofuran (CDT), and truns-2,5-dimethyltetrahydrofuran (TDT)], and films made from two different surfactants (cetyl alcohol and oleoyl alcohol).8 General Methods Unless stated otherwise, all reagents were obtained from commercial sourcesand used without further purification. Cetyl alcohol was recrystallized twice from n-hexane. Deionized water was purified by use of a Millipore Mill-Q filtering system containing one carbon and two ion-exchange stages. Surfactant solutions were prepared by direct weighing (1.00 mg/mL of (1)Supported by the Division of Basic Energy Sciences of the Departmentof Energy (DEFG02-85ER-13403) and by Air Products and Chemicals, Inc., Allentown, PA. (2) The United States Department of Energy Industrial Energy Program: Research and Development in Separation Technology, 1987. DOE Publication DOE/NBM-8002773. (3) Membrane Separation Systems: A Research Needs Assessment, 1990, DOE Publication DE90-011770. (4) Higashi, N.; Kunitake, T.; Kajiyama, T. Polym. J. 1987, 19, 289. I K ~ c + ~ - .D , ~. r d h , na A IT. n--- a . 1 -- r). n-I --_- r D Fiske, T. G.; Ringsdorf, H.; Schneider,J. Thin Solid Films 1989,180,241: (6) Albrecht, 0.; Laschowsky, A.; Ringsdorf, H. J. Membr. Sci. 1985, 22, 187. (7) Markowitz, M. A.; Janout, V.; Regen, S. L. J. Am. Chem. SOC.1989, 121,8192. @)An earlier report has described the permeation of radioactive hydrogensulfide from an aqueoussubphase through surfactantmonolayers and into air: Hawke, J. G.; Parts, A. G . J. Colloid Interface Sci. 1964, 19,448.
B
Flowmeter
Nitrqren In
Helium In
Detector
Figure 1. Schematicdrawing of the pervaporation chamber used in this study: (A) top view; (B) side view. n-hexane) and spread onto an aqueous surface area (74 cm2)by use of a 50-pLHamilton syringe. The total quantity of surfactant that was used in each experiment was approximately 6 X mol. Surface tensions were determined by use of tensiometer/ microbalance (NIMA, Model ST9000). The gas chromatograph that was employed was equipped with a capillary column and a flame ionization detector; separation of the ethers was baaed on differences in boiling point. Sampling was carried out at 5-min intervals via an automated sampler. Under the chromatographic conditions employed, the retention times of the permeants were T (1.96 min), CDT (2.59 min), and TDT (2.72 min); assignment of the cis/trans isomers was made on the basis of their relative retention times and known boiling points?
Results and Discussion PervaporationChamber. A schematic drawing of the chamber that has been designed for measuring pervaporation is shown in Figure 1. A miniature trough was fabricated from a solid Teflon block (117 X 117 X 26 mm), having well-dimensions of 86 X 86 X 7 mm. One central channel (6 mm diameter), extending across the inlet side of the trough, generates a constant flow of purge gas (nitrogen) over the aqueous subphase through seven evenly-spaced ports. A similar channel/port assembly, located on the outlet side of the trough, directs the effluent toward a Valco six-port sampling valve (100-pL sampling loop) which is connected to a gas chromatograph. The entire trough is encased in a gas-tight, Plexiglas enclosure equipped with a removable lid. Pervaporation Measurments. In a typical experiment, 50 mL of an aqueous solution containing T (1.67 mM), CDT (0.84mM), and TDT (0.84 mM), is added to tho
'l'eflnlr
tn\iiah fif thn
nnrxranArat;r\-
fihamhnm
A
n-hexane solution of a given surfactant (ca. 10pL) is then injected onto the aqueous surface, and the hexane allowed to evaporate for 60 s. The surface tension of the film is then measured (Wilhemy plate, 2 X 1.0 cm filter paper), and the chamber sealed by reattaching the Plexiglas lid. (9) Gagnairo, D.; Monzeglio, P. Bull. SOC.Chim Fr. 1965,474.
0743-7463/92/2408-2347$03.00/00 1992 American Chemical Society
2348 Langmuir, Vol. 8,No. 9, 1992
Notes
Table I. Permeation of Tetrahydrofurans through Surfactant Monolayers surfactant
film pressure (dvn/cm)
none cetyl alcohol cetyl alcohol oleoyl alcohol
0.0 12.6 36.7 32.4
T
flux (mol/(cm2.s))a CDT TDT
1.70 f 0.05 3.47 f 0.10 1.37 f 0.01 2.33 f 0.01 0.76 f 0.07 1.28 & 0.13 1.69 f 0.07 2.64 f 0.12
1.80 f 0.05 1.27 f 0.01 0.66 f 0.08 1.48 0.05
*
Average values i one standard deviation from the mean (three independent experiments). These initial flux rates were obtained after 15 min of sampling and were unchanged after an additional 15-min period. 0
The flow of nitrogen that is fed into the chamber during these steps, and throughout the entire experiment, is maintained a t 20 f 1 mL. The effluent is analyzed at 5-min intervals. At the end of each experiment the lid is removed and the surface tension analyzed; in all cases, it was foundtobeunchanged (k2.ldyn/cm). Surface tension measurmenta that were made on the etheral subphase, in the absence of added surfactant, gave values that were identical (within experimental error) to permeant-free water (typically 74.2 f 0.2 dyn/cm). In Table I we report initial flux values for each permeant in the absence and in the presence of surfactant mono-
layers, expressed in units of mol/(cm2.s). What is immediately obvious from these data is that the release of the ethers into the vapor phase is significantly retarded by the presence of each of the monolayers, and that the film barrier is a function of ita surface pressure and the specific structure of the surfactant that is employed. Specifically,flux of T, CDT, and TDT through monolayers of cetyl alcohol is reduced, considerably, on going from 12.6 to 36.7 dyn/cm. Moreover, monolayer films made from oleoyl alcohol (having a cis double bond in the middle of the alkyl chain) exhibit less of a barrier toward these ethers at 32.4 dyn/cm, than does cetyl alcohol a t 12.6 dyn/ cm. Finally, these data clearly demonstrate that the flux rates are strongly dependent on the composition and structure of the permeant. The experimental technique described herein should be applicable to a wide range of permeanta such as amines, alcohols, ethers, etc., as well as a variety of monolayerforming surfactants. Such application would afford a data base from which a fundamental understanding of transport through LB films may be developed.1° (10) For fundamental investigations of water permeation through spread monolayers, see: Retardation of Evaporation By MonolayerTransport Processes; LaMer, V. K., Ed.;Academic Press: New York, 1962.
