510
J. Phys. Chem. 1980, 84, 510-512
the decomposition of a nearly amorphous compound. The chemical composition, X-ray diffraction patterns, the dehydroxylation temperature of the “lithium containing A1(OH)3”precipitates as well as the p H decrease in the mother liquor, and the shape of the DTA curves indicate that lithium ions cause a crystallographic modification in precipitating pseudoboehmite. Such a definite effect may be well explained by intercalation of lithium ions. The is both and by taking into account the structural features of boehmite and pseud~boehmite’~ and the remarkable difficulty encountered in Li+ extraction from A1(0H)3 precipitates‘ The exact way of lithium incorporation should be further investigated. Acknowledgment. The authors are indebted to Mr. Aryeh Raz of the Industrial Research Administration of the Ministry of Commerce and Industry, and to Dr. J. A. Epstein of the Dead Sea Works for their initiative and interest. The financial support of the Ministry and of Israeli Chemicals Ltd. is appreciated.
References a n d Notes (1) M. E. Harris and K. S. W. Sing, J . Appl. Chem., 223 (1955). (2) J. W. Lucas, G. W. Newton, and K. S. W. Sing, J. Appl. Chem., 265 (1963). (3) E. Calvet, P. Boivinet, M. Noel, H. Thlbon, A. Malllard, and R. Tertian Bull. SOC. Chim. Fr., 99 (1953). (4) D. Pap&, R. Tertian, and R. Bais, Bull. Soc. Chim. Fr., 1301 (1958). (5) E. Matilevie, K. G. Mathai, R. H. Ottewill, and M. KerkeF, J . Phys. Chem., 65,826 (1961). (6) S. S. Singh, Can. J. Chem., 47,663 (1969). (7) P. H. Hsu and R. F. Bates, Miner. Mag., 33, 749 (1964). (8) J. Aveston, J . Chem. Soc., 4438 (1965). (9) C. Brosset, G. Blederman, and L. 0. Sillen, Acta Chim. Scad., 1917 (1954). (10) J. J. Fripiat, F. Van Cauwelaert, and H. Bosmans, J . fhys. Chem., 69, 2458 (1965). (11) T. Okura, K. Goto, and T. Yotuajanagi, Anal. Chem., 34,581(1962). (12) C. R. Frink and B. L. Sawhney, Soil Sci., 103, 144 (1967). (13) A. C. Vermeulen, J. W. Geus, R. J. Stol, and P. L. de Bruyn, J. ColW Interface Sci., 51, 449 (1975). (14) R. J. Stol, A. K. van Helden, and P. L. de Bruyn, J . ColloidInferface Sci., 57, 115 (1976). , sei., 103, 101 (1967). (15) p. H. H ~ U soil (16) R. D. Goodenough, U S . Patent 2964381 (1960). (17) N. P. Neipert and C. L. Bon, U.S. Patent 3306700 (1967). (18) 0.Lahodny-Sarc, L. Dragcevic, and D. Dosen-Sver, Ckys C/ayMiw., 26 153 (1978). (19) R. W. G. Wyckoff, “Crystal Structures”, Interscience, New York, 1964.
Surface Effects of Anisotropic London Dispersion Forces in n-Alkanest Frederlck M. Fowkes Depattment of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015 (Received Ju/y 30, 1979) Publication costs assisted by Lehigh University
Light-scattering and heats of mixing measurements by Bothorel, Patterson, and Tancrede have shown that in the higher n-alkanes adjacent chains are oriented parallel to one another, giving rise to an appreciable enhancement in adhesion; the effect has been termed correlated molecular orientation (CMO). Surface and interfacial tensions of alkanes vs. water show a specific CMO interaction between adjacent molecules of the higher n-alkanes which results in an anisotropic dispersion force component of the surface energy (ya) which is in excess of the normal isotropic dispersion force component of surface energy (yd). These findings suggest that the CMO contribution to cohesive energy and surface tension arises from an enhancement in the London dispersion forces between pairs of parallel molecules because of the much greater polarizability parallel to the chains. This interaction gives rise to local regions of optical anisotropy, to enhanced cohesive energy, and to enhanced surface tension, but does not contribute to the isotropic London dispersion farce field which governs the intermolecular interaction of alkanes with water. At 25 “C the CMO contribution to the surface energy of n-alkanes increases from zero for n-hexane to 2.9 mJ/m2 for n-hexadecane, but it is negligibly small for cyclic and highly branched alkanes. The dispersion force component of the surface energy of water is found to be 22.0 mJ/m2 at 25 “C, independent of what alkane is used as reference liquid.
Correlated Molecular Orientation In 1967 and 1968 Bothorel and co-workers published light-scattering studies1*2of n-alkanes which showed optically anisotropic regions attributed to parallel orientation of parts of adjacent molecules. The “correlated molecular orientation” (CMO) effect was strong in n-hexadecane and n-dodecane and negligible in n-hexane and in highly branched n-alkanes such as isohexadecane (2,2,4,4,6,8,8heptamethylnonane). In the following decade Patterson, Delmas, Tancrede, and others studied the calorimetry of mixing the higher Presented a t the Centennial Meeting of the American Chemical Society (New York, April, 1976) in the Kendall Award Symposium honoring R. J. Good. 0022-3654/80/2084-0510$0 1 .OO/O
n-alkanes with lower n-alkanes or with highly branched or cyclic alkane^.^-^ It was found that a positive term in
the heats of mixing always occurred when diluting the higher n-alkanes with branched or cyclic alkanes and this term was attributed to an enhanced interaction energy of the higher n-alkanes resulting from the parallel orientation of adjacent chains indicated by the light-scattering studies. This CMO contribution to the cohesive energy was observed at 25 “C only with n-alkanes above n-hexane and it increased with chain length. It was absent in highly branched or cyclic alkanes. The magnitude of this effect was determined by heats of mixing of many pairs of alkanes and over a wide range of compositions. An example is the heat of mixing of n-hexadecane with cy~lohexane,~ in which the CMO contribution to the partial molal heat 0 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84,
Dispersion Forces in n-Alkanes
TABLE I: Surface and Interfacial Tensions (dynlcm) vs. Water of n-Alkanes (20 ’C)“ n of y , d if Yzd = Y 2 YlZ Cfl H*fl +2 YZ WA -. ~6 7 8 9 10 11 12 14 16 a y 1=
18.41 20.28 21.78 22.96 23.89 24.78 25.48 26.69 27.64
50.80 51.23 51.68 51.96 52.26 52.51 52.86 53.32 53.77
40.41 41.85 42.90 43.80 44.43 45.07 45.42 46.17 46.67
22.17 21.59 21.12 20.89 20.66 20.49 20.24 19.97 19.70
72.80 dyn/cm; y 1 2values of Aveyard and
Hay don.
of dilution of n-hexadecane can be shown to be 1674 J/mol.
The Work of‘Adhesion of Alkanes to Water The work of adhesion between two liquids is actually the Helmholtz free energy change per unit area upon separating the two liquids: w‘4 = 71 + 7 2 - 712 in which y1 arid yzaxe the surface tensions and ylz is the interfacial tension. In 1957 Girifalco and Good proposed that the work of adhesion could be predicted by w”4=
2dJ12(Y1Y2)”2
in which dJ12 was a function of molecular volume^;^ the article pointed out that this equation did not apply to the hydrocarbon-water interface because of the dissimilarity of interfacial forces. Four years later the author proposed that, since the cohesion in “polar” liquids such as water results from dispersion forces (d), dipole interactions (p), hydrogen bonds (h), etc., the surface tension and work of adhesion could be treated as a series of independent terms:
+ YIP + Y l h WA = WAd + w A p + WAh 71 =
Yld
and that the dispersion forces between dissimilar materials could be accurately predicted by the geometric mean of the yld and yadvalues:lOJ1 WAd = 2(yldy2d)1/2 The work of adhesion between water and alkanes was assumed to result only from London dispersion forces ( WA = WAd) and SO yld for water was calculated from 71
+ Yz - 712 = 2(71dyz)1’z + 7 2 - yd2/4yz
71d = (71
From the datal available in 1961 on surface and interfacial tensions (vs. water) of alkanes, yd for water was estimated to be 21.8 f 0.7 mJ/rn2 at 20 “C. However, during the next few years more careful measurements by Aveyard and Haydon12and by Gillap, Weiner, and Gibaldi13showed that the value of yd for water appeared to be dependent on which n-alkane was used as reference liquid, with a steady decrease in yd as the chain length increased. On the other hand, yd values calculated from surface and interfacial tension of branched14 and cyclic alkanes15J6were always close to 22.0 mJ/m2 a t 20 “C. These results are illustrated in Tables I and 11. In a similar study by Johnson and Dettrel’ surface and interfacial tensions of n-alkanes vs. water were measured at 24.5 “C with results entirely consistent with those of Aveyard and Haydon. The interpretation is prophetic: “If we accept the basic concepts of Fowkes, this means that all the dispersion forces responsible for the surface tension
No. 5, 7980 511
TABLE 11: Surfaces and Interfacial Tensions (dyn/cm) vs. Water for Branched or Cyclic Alkanes (20 C )
trans-decalin cis-decalin 3-ethylhexane
29.89 32.18 21.54
51.40 51.74 50.8
51.29 53.24 43.54
22.00 22.02 22.00
15, 1 6 15, 16 14
of hexadecane are not operative across the interfaces studied”. Discussion The Correlated molecular orientation indicated by optical anisotropy suggested parallel orientation of the higher liquid n-alkanes. The enhanced cohesive energy observed by calorimetry for the same liquids could be explained by closer intermolecular distances, as suggested much earlier by Moore, Gibbs, and Eyring.18 Another explanation appears more relevant. The attractive energy between CH2 or CH, groups in adjacent molecules is due to the London dispersion forces: U = -3/4a2hvo/r6 in which U is the potential energy of two interacting groups of polarizability a and of London frequency YO a t a separation distance r. Adjacent parallel alkane chains can have a much stronger attraction than randomly oriented chains because of the anisotropy of the polarizability of the C-C bond.lg Denbighm estimated that parallel to the C-C bond cm3, but that in the perthe polarizability is 18.8 X pendicular direction it is only 0.2 X cm3, Using Denbigh’s method of accounting for bond angles one can calculate that the polarizability per CH2 parallel to the length of an extended n-alkane chain is 30% greater than the average of all directions. Thus two adjacent parallel chains can interact far more strongly than when randomly oriented. At the interface the water molecules cannot sense the polarized dispersion forces, for water is essentially isotropic (nearly equally polarizable in all directions). The result is that the anisotropic dispersion forces in n-alkanes give rise to a cohesive energy contribution not sensed by an adjacent water phase. This contribution to surface tension we shall label ya, and indicate the isotropic dispersion force contribution to surface tension by yd. Thus for n-alkanes y = yd ya
+
As cyclic and highly branched alkanes are observed not to have correlated molecular orientation, yzafor these liquids should be negligibly small, and so yld for water calculated by assuming yzd = y2 should be correct. As shown in Table 11, these data predict that yld for water a t 20 “C is 22.00 mJ/m2. If then 7: for water is really always 22.00 mJ/m2 at 20 “C, we can calculate yt and yzafor all n-alkanes from the measured values of y 2 and y12,as shown in Table 111. The yzavalues for the higher n-alkanes are not inconsequential, reaching a t least 10% of the total surface energy for nhexadecane. Since the calorimetric studies have provided heats of correlated molecular orientation in various liquid n-alkanes, it is instructive to see how these values relate to the yza values. In the-case of n-hexadecane, the partial molal heat of CMO, AH2a= 1674 J/mol. The contribution of this energy to surface tension is 7za = ARZa/NA2 where N is Avogadro’s number and A 2 is the area per molecule. For the observed y2aof 2.89 mJ/m2, Az is 96 A2
512
J. Phys. Chem. 1980, 84, 512-517
TABLE 111: Effect of Correlated Molecular Orientation on Surface Tension (dynlcm) of n-Alkanes (20 C)" n of CtlHZ,,, 6 7 8 9 10 11 12 14 16 a
Yz 18.41 20.28 21.78 22.96 23.89 24.78 25.48 26.69 27.64
Yz 18.41 19.90 20.91 21.80 22.43 23.08 23.44 24.22 24.75
y z a (CMO)
0 0.38 0.87 1.16 1.46 1.70 2.04 2.47 2.89
Based on Aveyard and Haydon data."
per molecule, very close to the best estimates for n-hexadecane molecules oriented parallel to the surface.
Conclusions 1. The surface energy of alkanes involves two independent terms, one involving isotropic London dispersion forces (yzd)and one involving anisotropic London dispersion forces (yza). 2. The anisotropic London dispersion forces result from the anisotropy of polarizability parallel to n-alkane chains, which gives rise to the optical anisotropies previously observed by light scattering and which provides the enhanced cohesion observed by both calorimetry and surface tension. 3. The anisotropic dispersion forces can be sensed only by molecules which are anisotropic in polarizability. Water molecules a t the interface are isotropic and do not sense the CMO contribution to cohesion in the n-alkanes. Therefore the work of adhesion to water is not related to the yzaof the n-alkanes.
4. The isotropic London dispersion force contribution to the surface energy of water is 22.0 mJ/m2, no matter what alkane is used as reference liquid. 5. At 20 " C yzaincreases from zero for n-hexane to 2.89 mJ/m2 for n-hexadecane. 6. From the heat of dilution of n-hexadecane (in cyclohexane) and the y$ of 2.89 mJ/m2 the surface area per molecule of n-hexadecane is estimated to be 96 A2, corresponding to an orientation parallel to the surface.
References and Notes P. Bothorel, C. Clement, and P. Maravol, C . R. Acad. Sci., 264, 568 (1967).
P. Bothorel, J. Colloid Sci., 27, 529 (1968). V. T. Lam. P. Plcker. D. Patterson. and P. Tancrede. J. Chem. Soc.. Faradav Trans. 2. '70. 1465 (19741. M. D. Goucher and D.'PattersOn, J.' Chem. Soc., Faraday Trans. 2, 70, 1479 (1974). P. Tancrede, D. Patterson, and V. T. Lam, J . Chem. Soc., Faraday Trans. 2, 71, 985 (1975). G. Delmas and S. Turrell, J . Chem. Soc., Faraday Trans. 7 , 70, 572 (1974). P. Tancrede, P. Bothorel, P. de St. Romain, and D. Patterson, J. Chem. SOC.,Faraday Trans. 2 , 73, 15 (1977). P. Tancrede, D. Patterson, and P. Bothorel, J. Chem. Soc., Faraday Trans. 2, 73, 29 (1977). L. A. Girlfalco and R. J. Good, J. Phys. Chem., 61, 904 (1957). F. M. Fowkes, J. Phys. Chem., 66, 382 (1962). F. M. Fowkes, Ind. Eng. Chem., 56, no. 12, 40 (1964). R. Aveyard and D. A. Haydon, Trans. Faraday Soc., 61, 2255 (1965). W. R. Gillap, N. D. Weiner, and M. Gibaldi, J . Am. OilChem. Soc., 44, 71 (1967). D. K. Owen, J . Phys. Chem., 74, 3305 (1970). W. F. Seyer and C. H. Davenport, J . Am. Chem. SOC.,63, 2425 (194 1). W. E. Rose and W. F. Seyer, J. Phys. C o l l o ~ Chem.,55, 439 (1951). R. E. Johnson, Jr., and R. H. Dettre, J . Colloid Interface Sci., 21, 610 (1966). R. J. Moore, P. Gibbs, and H. Eyring, J. Phys. Chem., 57, 172 (1953). F. M. Fowkes, J . Colloid Interface Sci., 28, 493 (1968). K. G. Denbigh, Trans. Faraday Soc., 36, 936 (1940).
Influence of Ligand N-H Oscillators vs. Water 0-H Oscillators on the Luminescence Decay Constants of Terbium( 111) Complexes in Aqueous Solution Simon Salama and F. S. Richardson" Department of Chemisty, University of Virginia, Charlottesville, Virginia 2290 7 (Received August 22, 7979)
Potentiometric titration data, absorption spectra, and luminescence decay measurements are used to investigate the formation and structure of the complexes formed by iminodiacetic acid (IDA) and N-methyliminodiacetic acid (MIDA) with trivalent terbium ions (Tb3+)in aqueous solution under variable pH conditions. It is shown that under high pH conditions (pH >8 for the IDA complexes and pH >9 for the MIDA complexes) ninecoordinate tris-terdentate chelate structures are the dominant species. Furthermore, it is shown that Tb3+-ligand chelation begins and proceeds over the pH region corresponding to terbium-induced early deprotonation of the ligand ammonium groups. Luminescence decay measurements carried out in HzO/DzOsolvent mixtures are used to determine the number of water molecules bound directly to Tb3+in the Tb(1DA) and Tb(M1DA) systems under variable pH conditions. From these results the progressive binding of ligand donor groups (and the displacement of coordinated water molecules) is followed as a function of solution pH. A detailed comparison of the luminescence decay data for the Tb(1DA) and Tb(M1DA) systems under high pH conditions yields the relative efficiencies of ligand N-H oscillators vs. water 0-H oscillators as nonradiative deactivators of the Tb3+ emitting state. The ratio of these efficiencies (N-H/O-H) is found to be -2/3.
