Langmuir 1997, 13, 4933-4934
4933
Adsorption of Hexane at the Water/Vapor Interface Anjing Lou and Brian A. Pethica* Langmuir Center for Colloids and Interfaces, Columbia University, 500 West 120th Street, Room 911, New York, New York 10027 Received February 27, 1997X The interaction of hydrocarbon chains at aqueous interfaces is a major factor in defining the interfacial structures of monolayers and membranes formed by surfactants. The intermolecular pair potentials which can be expressed in the two-dimensional second virial coefficients (B2(T)) are basic to understanding these structures. In this report, measurements of the adsorption of hexane vapor on water as a function of the partial vapor pressure are reported for a static system in which the vapor pressure is controlled by mixing liquid hexane with either hexadecane or squalane. The B2(T) for hexane at the water/vapor interface is approximately estimated at one temperature. The results confirm that the extensive published data on the adsorption of alkanes on water obtained in a vapor flow system are incorrect, presumably due to inadequate vapor saturation.
The study of intermolecular forces between paraffin chains at interfaces is basic to the understanding of numerous surface and colloidal systems. Pair potentials may be estimated from measurements of two-dimensional second virial coefficients (B2(T)). A recent analysis of the available surface tension data for calculating B2(T) for normal paraffins at the water/vapor interface1 gives consistent estimates only for the series methane to butane.2,3 The interpretation of these estimates was shown to be consistent with the three-dimensional pair potentials in the alkane vapors without correction for interactions with the water molecules in the surface.1,4 Unfortunately the data for longer chain alkanes5-10 were found to be unsuitable for estimates of B2(T). One set of these results was obtained by a chromatographic method5-7 in which the water-wetted column area was known only to within a factor of 2. The second set8-10 obtained by measuring surface tension in an alkane vapor flow system proved thermodynamically inconsistent, and it was concluded that the alkane vapor pressures had been overestimated due to an inadequate vapor saturation technique.1 This Letter reports measurements in a static system of the adsorption of hexane vapor on water which appear to confirm this conclusion and which give an approximate estimate of B2(T) for hexane at the water/ vapor interface at one temperature. Methods The surface tension of water was measured by the drop-volume method with a micrometer-driven glass syringe using triply distilled water, the second stage being from alkaline permanganate. The air/water surface tension was routinely 72.0 ( 0.05 mN m-1 at 25 °C. The water/hexane vapor tension was measured by expressing the water drops into the vapor portion of a closed space partially filled with a liquid mixture of hexane and either hexadecane or squalane. The partial vapor pressure of the hexane X
Abstract published in Advance ACS Abstracts, August 15, 1997.
(1) Pethica, B. A. Langmuir 1996, 12, 5851. (2) Jho, C.; Nealon, S.; Shogbola, S.; King, A. D., Jr. J. Colloid Interface Sci. 1978, 65, 141. (3) Sachs, W.; Meyn, V. Colloids Surf. A 1995, 94, 291. (4) Pethica, B. A.; Glasser, M. L.; Mingins, J. J. Colloid Interface Sci., 1981, 81, 41. (5) Hartkopf, A.; Karger, B. L. Acc. Chem. Res. 1973, 6, 209. (6) Karger, B. L.; Castells, R. Y.; Sewell, P. A.; Hartkopf, A. J. Phys. Chem. 1971, 75, 3870. (7) King, J. W.; Chatterjee, A.; Karger, B. L. J. Phys. Chem. 1972, 76, 2769. (8) Hauxwell, F. Ph.D. Thesis, University of Bristol, England, 1969. (9) Hauxwell, F.; Ottewill, R. H. J. Colloid Interface Sci. 1968, 28, 514. (10) Hauxwell, F.; Pallas, N. R.; Pethica, B. A. Langmuir 1992, 8, 602.
S0743-7463(97)00220-5 CCC: $14.00
was controlled by varying the mole fraction of hexane in the mixtures. The hexane (spectroscopic grade) and hexadecane were more than 99% pure and used as received from Aldrich. The squalane (2,6,10,15,19,23-hexamethyltetracosane) was used as received from Sigma. Experiments were performed at room temperature, usually at 23 °C, with some variations between 22.5 and 23.5 °C. Drops were formed almost to the detachment volume and then left in contact with the vapor until no further decrease in detachment volume with time was observedstypically less than 10 min.
Results and Discussion The change of the air/water surface tension with the small temperature variations was readily followed. The corresponding temperature variation of the surface pressure (π) in the presence of hexane vapor (measurable to a lower limit of (0.1 mN m-1) could not be observed, and all the results are given as for 23 °C. The choice of hexadecane and squalane as diluents to control the hexane vapor pressure was based on their low vapor pressures, as estimated by extrapolation of their high-temperature boiling points at several pressures.11,12 For hexadecane at 23 °C the vapor pressure is estimated as 5.2 × 10-4 mmHg. For squalane at the same temperature the vapor pressure is estimated as 1.7 × 10-8 mmHg. Hexane vapor pressures (p) have been measured for mixtures with both hexadecane11 and squalane13 and activity coefficients calculated. Furthermore, it has been shown that placing drops of liquid hexadecane or squalane on water does not lower the surface tension.14,15 In agreement with this observation, we find no change in the surface tension of water exposed over pure liquid hexadecane or squalane. However, even if the pure higher paraffins show no adsorption to the air/water interface via the vapor phase, it is possible that either may show adsorption at an interface partially occupied by the hexane. Aveyard et al.14,15 in fact show that liquid hexadecane in contact with water containing surface active solutes can spread into the surface layer, whereas squalane shows very small or zero spreading on surfactant solutions. It may therefore be anticipated that if neither higher paraffin adsorbs into a water surface partially covered with hexane, the surface (11) McGlashan, M. L.; Williamson, A. G. Trans. Faraday Soc. 1961, 57, 588. (12) Beilstein Handbook of Organic Chemistry. Tabulated boiling Points for Squalane (Registry Number 776019). (13) Ashworth, A. J.; Everett, D. H. Trans. Faraday Soc. 1960, 56, 1609. (14) Aveyard, R.; Cooper, P.; Fletcher, P. D. I. J. Chem. Soc., Faraday Trans. 1990, 86, 3623. (15) Aveyard, R.; Binks, B. P.; Fletcher, P. D. I. Langmuir 1995, 11, 2516.
© 1997 American Chemical Society
4934 Langmuir, Vol. 13, No. 19, 1997
Figure 1. Surface pressure of hexane at the water/vapor interface as a function of mole fraction of hexane in hexadecane or squalane at 23 °C.
pressures observed for a given hexane pressure will be the same for both diluents. If there is coadsorption, it is unlikely that both diluents would coadsorb equally, and mixtures of hexane with hexadecane would show a higher surface pressure than mixtures with squalane. Squalane is unlikely to show any effect, as judged from the liquid drop experiments, and the very low vapor pressure of squalane would in any case require an equilibrium adsorption time much in excess of 10 min. Figure 1 shows the results for the hexane surface pressure as a function of mole fraction of hexane. Hexane vapor pressures were obtained by interpolation from the published data for mixtures with hexadecane11 and squalane.13 For mixtures with hexadecane11 both the experimental hexane vapor pressures and the activity coefficients are recorded at several temperatures from 20 to 60 °C down to hexane mole fractions of 0.03. For mixtures with squalane13 only the activity coefficient are recorded, on the basis of experiments down to mole fraction 0.2 at 20 and 30 °C. The activity-coefficients show little variation with temperature. The conversion to vapor pressure for hexane/squalane mixtures involves use of a form of the Bertholet equation of state for hexane vapor (a minor correction). Vapor pressures for mixtures below mole fraction 0.2 in squalane were taken from the line for a theoretical equation, which fits well above 0.2. The corresponding variation of the surface pressure with hexane vapor pressure is given in Figure 2. At low p, the results from mixtures with hexadecane or squalane cannot be distinguished. At higher p, π for hexane vapor over hexadecane mixtures appears to be somewhat larger than that for squalane mixtures, suggesting some coadsorption
Letters
Figure 2. Surface pressure of hexane at the water/vapor interface as a function of hexane partial vapor pressure at 23 °C. Pressure units are as used in ref 9, 11, and 13 (760 mmHg ) 1 atm ) 1.013 × 105 Pa). The lower broken line gives the (interpolated) results of Hauxwell.9
of hexadecane as the surface area of hexane at the water surface falls below about 4 nm2/molecule. Also shown in Figure 2 is the surface pressure for hexane as obtained by interpolation of the data from the vapor flow experiments.8 It does appear that those results give too low a surface pressure, probably due to overestimating the hexane vapor pressure, as discussed previously.1 Correspondingly the conclusion1 that the heats and entropies of adsorption of the higher alkanes calculated from these measurements10 are in error is confirmed. The data shown in Figure 2 are not sufficiently precise for a good estimate of B2(T) for hexane by the methods described earlier.1 Estimates of the initial slope (R) of the π-p results for hexane/squalane mixtures by graphical plotting or from least squares fitting of a polynomial quadratic in p give values between 15 and 18 mN m-1 atm-1, in the units used previously.1-3 This is in reasonable agreement with an estimate of 15.5 mN m-1 atm-1 by extrapolating the values of R obtained for the range methane to butane (Figure 6 of ref 1). Estimates of the curvature of the π-p plot are less reliable due to the large error in π/p at low p values. Estimates of B2(T) for hexane on water give only an order of magnitude of (negative) 1 nm2/molecule. Better estimates for hexane and higher homologs will require direct control and measurement of the vapor pressure without diluent liquids and more accurate low surface pressures with a different method such as the Wilhelmy plate technique originally used by Hauxwell and Ottewill.9 LA970220Z
