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Interfacial Tension of the Water/n-Alkane Interface A. Goebel and K. Lunkenheimer* Max-Planck Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung Rudower Chaussee 5, D-12489 Berlin, Germany Received August 12, 1996. In Final Form: October 22, 1996X
Introduction The characterization of the water/oil interface is the subject of many investigations because of its relevance in various areas of chemistry, biology, and technology. Many groups are currently investigating certain aspects of the interfacial behavior of the water/oil interface. However, most experiments suffer from a lack of purity of the compounds used. To understand the underlying interfacial processes necessitates first of all to have available reliable data on the applied surfactants’ adsorption properties. These, however, can only be obtained from reliable experiments. In recent years there has been considerable progress in the characterization of the amphiphiles’ adsorption properties at the air/water interface which is mainly due to improvements in the experimental boundary conditions for research in surface chemistry. They refer to (i) modifications on the experimental conditions on the surfactant solutions’ surface tension measurement in order to get accurate values,1,2 (ii) the application of criteria to judge the necessary and sufficient grade of “surfacechemical purity”,3,4 and (iii) efficient methods to obtain this grade of purity.5,6 This together with a new approach to the surface equation of state has revealed a lot of novel adsorption features.7-11 Here we would like to report on some novel findings on the water/oil interfacial properties. According to the concept of “surface-chemical purity” any surfactant used “as received” does necessarily contain (at least) traces of its parent compounds. Since these parent compounds possess a much stonger surface activity than the amphiphile itself they will be preferentially enriched in the adsorption layer, thus falsifying the amphiphile’s surface properties. To get rid of such impurity effects necessitates the removal of these trace impurities efficiently from the system to be investigated. However, impurities may also originate from the solvent used, i.e. in most cases from water. If one investigates liquid/liquid interfaces such as oil/ water interfaces, the requirement of “surface-chemical X Abstract published in Advance ACS Abstracts, December 15, 1996.
(1) Lunkenheimer, K.; Wantke, K.-D. J. Colloid Interface Sci. 1978, 66, 579. (2) Lunkenheimer, K.; Wantke, K.-D. Colloid Polym. Sci. 1981, 259, 354. (3) Miller, R.; Lunkenheimer, K. Colloid Polym. Sci. 1986, 264, 273. (4) Lunkenheimer, K.; Miller, R. J. Colloid Interface Sci. 1987, 120, 176. (5) Lunkenheimer, K.; Pergande, H.-J.; Kru¨ger, H. Rev. Sci. Instrum. 1987, 58, 2313. (6) Lunkenheimer, K.; Miller, R.; Kretzschmar, G.; Lerche, K.-H.; Becht, J. Colloid Polym. Sci. 1984, 262, 662. (7) Lunkenheimer, K.; Haage, K.; Miller, R. Colloids Surf. 1987, 22, 215. (8) Lunkenheimer, K.; Hirte, R. J. Phys. Chem. 1992, 96, 8683. (9) Hirte, R.; Lunkenheimer, K. J. Phys. Chem., in press. (10) Lunkenheimer, K.; Laschewsky, A. Prog. Colloid Polym. Sci. 1992, 9, 239. (11) Lunkenheimer, K.; Czichocki, G.; Hirte, R.; Barzyk, W. Colloids and Surf., A 1995, 101, 187. (12) du Nou¨y, L. J. Gen. Physiol. 1919, 1, 521.
Figure 1. Water/n-decane interfacial tension.
purity” must not only cover the amphiphile and water but also the oil phase. With respect to liquid/liquid interfaces’ purity this matter has so far been taken into consideration too little. This contribution suggests a simple test to evaluate whether the necessary grade of the oil’s purity is achieved and presents novel results on the interfacial tension of water/n-alkane systems in dependence on the hydrocarbon chain length. Experimental Section Interfacial Tension of an interface. Its measurement can easily be carried out and gives reproducible and reliable results provided that errors due to frictional and straining effects in the adsorption layer are avoided.1,2 In our study the water/oil interfacial tension was determined by the ring method by means of an automatic Lauda tensiometer taking into consideration necessary modifications,1,2,13-15 and by the Lasda pendant drop technique.16 The accuracies of these methods at the measuring temperature of 22 °C are (0.1 and (0.2 mN/m, respectively. Generally, doubtful results are observed when the substances are used “as received”. This is demonstrated in Figure 1 using a sample of n-decane (Baker, 99%). The low value of γ together with its decay with time indicates the presence of unwanted compounds of amphiphilic character at the interface. These impurities might originate from the crude oil’s native residues but also from oxidation processes taking place during the alkanes’ separation and fractionation. Several purification techniques are known to remove the contaminants from the oil phase.17 Here we suggest a simple test to evaluate the oil’s purity. Since the oil’s amphiphilic impurities have a high affinity towards the water/oil interface it seems appropriate to remove them at this location of their enrichment. This is analogous to the purification procedure used for air/water interfaces.5 After shaking a sample of n-decane with water in a separating funnel and removing the formed interface, the interfacial tension of the remaining oil against a new sample of water was again measured (Figure 2a). By repeating this procedure the grade of the (13) Huh, C.; Mason, S. G. Colloid Polym. Sci. 1975, 253, 566. (14) Lunkenheimer, K.; Miller, R. Tenside Deterg. 1979, 16, 312. (15) Lunkenheimer, K. J. Colloid Interface Sci. 1989, 131, 580. (16) Semmler, A.; Ferstl, R.; Kohler, H.-H. Langmuir, in press. (17) Bunge, W. Methoden der Organischen Chemie (Houben-Weyl); Georg Thieme Verlag: Stuttgart, Germany, 1959.
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Figure 2. Water/n-decane interfacial tension in dependence on time: (b) adsorption state γa; (2) after compression γd; (a) after removing the interface by means of a separating funnel; (b-d) after several times passing a column of alumina. hydrocabon’s purity increased, but not significantly, as the area of the water/oil interface where the contaminants adsorb is rather small. Our experiences reveal that solid adsorption with basic alumina is the method of choice in order to efficiently remove the contained impurities (Figure 2b-d).
Results and Discussion A suitable check of the purity of a fluid interface is to compress it and monitor the changes of the measured quantity with time. No difference between the equilibrium values before (ya) and after (yd) compression should appear.4 The compression of the water/oil interface can easily be carried out by reducing the volume of a pendant drop of water immersed in the oil phase. In Figure 2 the purification process of n-decane is monitored by the measurement of the interfacial tension of the water/n-decane interface before (γa) and after (γd) compression in dependence on time and on the the number of “purification cycles”. The compression of the adsorption layer was performed after the system’s adsorption equilibrium had been established. On the basis of the procedure of surface-chemical purification at the air/water interface performed automatically in the high-performance purification apparatus,5 the term “purification cycles” refers now to the number of runs of the oil’s passing through the alumina column (Figure 2b-d). The dynamic interfacial tension behavior shown in Figure 2 for the water/n-decane interface resembles very much the effect of strongly surface active trace impurities at air/water interfaces.3-6 By analogy with the criteria of “surface-chemical” purity put forward by us earlier in refs 3 and 4, the necessary and sufficient state of “interfacechemical” purity will be reached if either the difference between the two apparent equilibrium interfacial tension values in the case of adsorption (γa) and desorption (γd) does vanish or the dependence of the apparent interfacial tension value γa on the “purification coordinate” j does remain constant, i.e. (dγa)/dj ) 0. Evidence is provided by Figure 3. Consequently, the state of “interface-chemical” purity of the water/n-decane interface is guaranteed for the system’s state 2d only.
Figure 3. Water/n-decane equilibrim interfacial tension in dependence on the number of purification cycles.
The success of the purification scheme was additionally checked in the following manner using the fact that the interfacial tension has a constant and definite value whenever all materials are surface-chemically pure. The interfacial tension of the purified oil was measured, and subsequently an amount of the very same oil was again dispersed in a new sample of water to dissolve eventually interface-active materials which were not yet removed by the solid adsorbent (Figure 3, j ) 6). An interfacechemically pure state is indicated by constant readings of the interfacial tension of both batches then. Various n-alkanes were purified in this manner. The specifications of the hydrocarbons are listed in Table 1. The results of the measured interfacial tension values in dependence on the carbon number NC of the n-alkane at 295 K are presented in Figure 4 and Table 2. Evaluating these results, there are two striking features. Firstly, the interfacial tension values determined for the n-alkane/water interfaces for which the sufficient grade of purity is in fact guaranteed are generally found to be
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Figure 4. Water/n-alkane interfacial tension. Table 1. Specifications of the Oils Used substance
specifications
manufacturer
n-pentane n-hexane n-heptane n-octane n-nonane n-decane n-undecane n-dodecane n-tridecane n-tetradecane n-hexadecane
>99.5% >99.5% >99.5% >99.5% >99% >99% >99% >99% >99% >99% >98%
Fluka Fluka Baker Fluka Fluka Baker, Merck, Aldrich Merck, Aldrich Merck, Aldrich Merck Merck, Fluka Fluka
Table 2. Interfacial Tension Water/n-Alkane and Surface Tension n-Alkane/Air (T ) 22 °C) n-alkane
γ(water/n-alkane) (mN/m)
σ(n-alkane/air) (mN/m)
pentane hexane heptane octane nonane decane undecane dodecane tridecane tetradecane hexadecane
50.9 51.4 51.9 52.5 52.4 53.2 53.1 53.7 54.0 54.5 55.2
15.9 18.3 20.05 21.55 22.7 23.7 24.6 25.3 25.95 26.4 27.2
somewhat higher than those given in the literature.18,19 This is in line with common findings on the effect of surfaceactive trace impurities on the surfactant solutions’ surface tension. As a rule, removing these trace impurities from the surfactant solutions will result in a certain increase in the corresponding solution’s surface tension value. In addition, a clear hint to the amphiphilic character of surface-active compounds in commercially available oils is given by this behavior. Secondly, a pronounced evenodd effect is found. This represents a remarkable finding. Phenomena of alternation are of common occurence in bulk properties of homologous series of organic compounds. (18) Aveyard, R.; Haydon, D. A. Trans. Faraday Soc. 1965, 61, 2255. (19) CRC Handbook of Chemistry and Physics, 75th ed.; CRC Press: Boca Raton, FL, 1994.
They were observed for melting points, molecular volumes, solubilities in water, viscosities, dipol and magnetic moments, reaction rates, constants of complex formation, hydrogenation enthalpies of ethylenes, etc.20-22 Even-odd effects are also known for bulk properties of aqueous solutions of surfactants such as micellar properties and Krafft points.23-25 We have detected alternation effects in the adsorption properties of amphiphiles of different chemical structures like standard free energy of adsorption, saturation adsorption (limiting surface area demand per molecule adsorbed), and surface interaction parameter only recently.9-11,26,27 So far a real theoretical explanation of the effects of alternation in the adsorption parameters is still missing. There are two hypotheses worth considering. One is due to Gutman.28 It relies on the donor-acceptor approach. It can be used for qualitatively explaining alternation phenomena in adsorption properties of amphiphilic compounds. However, generally the alternation effect is related to the molecules’ different packing constraints for even- and/ or odd-numbered homologues. This was shown quantitatively by Marcelja for the chain ordering in liquid crystals.29 As donor-acceptor effects ought not to be encountered at the pure water/oil interface, the occurrence of the alternating behavior in their interfacial tension values is a hint for orientation at the interface.29,30 In a recent study of the adsorption of n-alkane vapors on water the measured adsorption energies for the evennumbered alkanes were found to be higher than those for the odd-numbered ones.31 Another group investigated the orientation of some water/n-alkane interfaces by applying total internal reflection second-harmonic generation. The measured surface nonlinear susceptibilities suggest a comparatively high degree of interfacial order for the evennumbered alkanes while the investigated water/heptane and water/nonane interfaces exhibit a higher degree of disorder.32 Even-odd effects of solid n-alkanes can be related to the different packing of the alkane molecules in their solid state33-35 and their rotator phases.36,37 Further evidence with direct structural information is needed to prove if and why molecules with odd-numbered (20) Ansell, M.; Gigg, R. H. In Rodd’s Chemistry of Carbon Compounds Geoffey, S., Ed.; Elsevier Publishing Company: Amsterdam, London, New York, 1965; Vol. I, Part C, p 124. (21) Salukaev, L. P. Homologisacija Organitscheskich Molekul Izdatelstvo Woroneskogo Universiteta: Woronesh, 1968. (22) Salukaev, L. P. Z. Org. Chim. 1976, 12, 1600. (23) Mukerjee, P. Kolloid Z. Z. Polym. 1969, 236, 76. (24) Oss, N. van; Haak, J. R.; Rupert, L. A. M. Physico-chemical properties of selected anionic, cationic and nonionic surfactants; Elsevier: Amsterdam, 1993. (25) Ueno, M.; Takasawa, Y.; Miyashige, H.; Tabata, Y.; Meguro, K. Colloid Polym. Sci. 1981, 259, 761. (26) Lunkenheimer, K.; Burczyck, B.; Piasecki, A.; Hirte, R. Langmuir 1991, 7, 1765. (27) Lunkenheimer, K.; Holzbauer, R.; Hirte, R. Prog. Colloid Polym. Sci. 1994, 97, 116. (28) Gutman, V. The Donor Acceptor Approach to Molecular Interactions; Plenum Press: New York and London, 1978. (29) Marcelja, S. J. Chem. Phys. 1974, 60, 3599. (30) Tamaki, K. Bull. Chem. Soc. Jpn. 1965, 38, 1987. (31) Hauxwell, F.; Pallas, N. R.; Pethica, B. A. Langmuir 1992, 8, 602. (32) Conboy, J. C.; Daschbach, J. L.; Richmond, G. L. Appl. Phys. A 1994, 59, 623. (33) Mathisen, H.; Norman, N.; Pedersen, B. F. Acta Chem. Scand. 1967, 21, 127. (34) Norman, N.; Mathisen, H. Acta Chem. Scand. 1972, 26, 3913. (35) Small, D. M. The Physical Chemistry of Lipids. Handbook of Lipid Research; Plenum Press: New York, 1986. (36) Ungar, G. J. Phys. Chem. 1983, 87, 689. (37) Ungar, G.; Masic, N. J. Phys. Chem. 1985, 89, 1036.
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hydrocarbon chains orient differently from those with even-numbered chains. Conclusions Investigating interfacial properties of liquid/liquid interfaces needs carefully to check whether surface-active trace impurities are present in both solvents. As was shown in this study using water/n-alkane interfaces, such trace impurities may especially be introduced by the oil phases. There was no alkane “as received” which indeed did fulfill the requirements of “interface-chemical” purity. Hence, it is required not only to judge the interfaces’ purity but also to apply efficient methods to remove falsifying amphiphilic impurities from the oils used “as received”. The criteria derived for “surface-chemical” purity of air/water interfaces can be applied analogously to water/oil interfaces to evaluate the necessary and sufficient grade of the interface’s purity. Therefore, it is only necessary to follow the interfacial tension in dependence on increasing grade of purity.
Notes
Interface-active trace impurities contained in the oils can be removed efficiently by using solid adsorbent columns. Thus, any study on adsorption properties of amphiphiles at water/oil interfaces has to take care that amphiphilic trace impurities have to be removed from both the surfactant, which contains the impurity as a parent component, and the oils. Since the latter was hardly taken into account so far, it is not surprising that the distinct even-odd behavior in the water/n-alkanes interfacial tension data was detected only now when the necessary purity of the concerning interfacial phases was guaranteed. Unfortunately, so far there is not yet a satisfactory theory which can explain the pronounced even-odd effects either of the pure interface or of the adsorbed amphiphiles’ properties. In addition, to understand the even-odd phenomenon of the neat interface, further investigations on the structure of the boundary layer between the two liquid phases are necessary. LA960800G
