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Langmuir 1997, 13, 1368-1370
High-Frequency Surface Dynamics of Solutions of a Nonionic Surfactant J. C. Earnshaw* and C. P. Nugent The Department of Pure and Applied Physics, The Queen’s University of Belfast, Belfast BT7 1NN, Northern Ireland
K. Lunkenheimer Max-Planck Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Rudower Chausee 5, D-12489 Berlin-Adlerhof, Germany Received October 23, 1995. In Final Form: December 27, 1996X Surface light scattering has been used to study thermally excited capillary waves on solutions of a nonionic surfactant, n-decanoic acid in 0.005 M aqueous HCl. The capillary waves show no trace of the unexpected behavior previously found for ionic surfactants, suggesting that processes thought to act to reduce the stability of the dilatational surface waves in the ionic case are absent for the present system. The implications of this observation are discussed.
In certain recent studies of the surface dynamics of surfactant solutions, marked deviations from expectation have been observed.1-4 These observations appear to be consistent with predictions of a reduction in the stability of the dilatational modes of the adsorbed surface film due to the conversion of chemical energy into mechanical energy via the Marangoni effect,5 modified by the presence of an adsorption barrier.6 The details of this mechanism remain an open question: all surfactants for which such behavior has been observed have been ionic in character, and so some interest attaches to the question of whether it is observed for nonionic surfactants. The present Letter presents the results of a study of this point. The high-frequency dynamics of fluid surfaces is conveniently probed by light scattering from thermally excited capillary waves.7 The spectrum of light scattered by waves of a given wavenumber (q), which is measured in such experiments, reflects the complex propagation frequency (ω0 + iΓ) of that mode and also carries information on the several surface properties which affect the capillary waves. These include the surface tension (γ), which governs the capillary waves, and the dilatational elastic modulus () which affects the longitudinal waves.8 These two modes are coupled for the case of a liquid/air interface, as here. Dissipative effects in a surface film can be accounted for by expanding γ and as linear response functions:
γ ) γ0 + iωγ′
(1)
* To whom correspondence should be addressed. Telephone: +44-1232-245133. Fax: +44-1232-438918. E-mail: j.earnshaw@ qub.ac.uk. X Abstract published in Advance ACS Abstracts, February 1, 1997. (1) Earnshaw, J. C.; McLaughlin, A. C. Proc. R. Soc. London, Ser. A 1993, 440, 519. (2) Earnshaw, J. C.; McCoo, E. Phys. Rev. Lett. 1994, 72, 84. (3) Earnshaw, J. C.; McCoo, E. Langmuir 1995, 11, 1087. (4) Earnshaw, J. C.; Sharpe, D. J. J. Chem. Soc., Faraday Trans. 1996, 92, 611. (5) Chu, X.-L.; Velarde, M. G. Physicochem. Hydrodyn. 1988, 10, 727. (6) Hennenberg, M.; Chu, X.-L.; Sanfeld, A.; Velarde, M. G. J. Colloid Interface Sci. 1992, 150, 7. (7) Langevin, D. Light Scattering by Liquid Surfaces and Complementary Techniques; Dekker: New York, 1992. (8) Lucassen, J. Trans. Faraday Soc. 1968, 64, 2221, 2230.
S0743-7463(95)00923-1 CCC: $14.00
) 0 + iω′
(2)
It is relevant to note that γ′ and ′ primarily act to increase the damping of the capillary and dilatational waves, respectively. Experimentally, a laser beam incident upon the surface is scattered by thermally excited capillary waves. Light scattered by waves of a particular q is selected, and the spectrum of the scattered light (or, as in the present work, its Fourier transform, the photon correlation function of the scattered light) is determined (see ref 9 for fuller details). The data may then be analyzed by two independent methods. The first gives unbiased estimates of the frequency ω0 and damping constant Γ of capillary waves of the experimental q.10 The second analysis yields the four surface properties which affect the capillary waves (γ0, γ′, 0, ′) directly from the measured correlation function.11 We briefly note that the second, direct analysis involves fitting the data with the Fourier transform of the theoretical spectrum7 of thermally excited capillary waves on a surface supporting a molecular film, expressed as a function of the four properties just cited. As will emerge directly, this analysis has led to negative values of the dilatational surface viscosity ′ for solutions of ionic surfactants; these arise from the inadequacies of the model used, which apparently does not fully account for all processes occuring in these solutions. The deviations from expectation noted above for surfactant solutions were apparent in the results of both analyses. The q-dependence of the capillary wave propagation (i.e., ω0 and Γ) was perturbed for q J 1400 cm-1, the effects being greatest for Γ. The magnitude of these effects depended upon the concentration of surfactant, being greatest at low c.2,3 Secondly, the dilatational surface viscosity ′ was consistently found to be negative for all c and all q.3,4 Now, as noted above, ′ (when > 0) acts to increase the dilatational wave damping. These negative surface viscosities were interpreted as reflecting the action of some process (or processes) which reduced the damping of the dilatational waves from the value expected due to viscous damping in the aqueous subphase; they are thus effective properties only. Such a destabilizing (9) Earnshaw, J. C.; McGivern, R. C. J. Phys. D 1987, 20, 82. (10) Earnshaw, J. C.; McGivern, R. C. J. Colloid Interface Sci. 1988, 123, 36. (11) Earnshaw, J. C.; McGivern, R. C.; McLaughlin, A. C.; Winch, P. J. Langmuir 1990, 6, 649.
© 1997 American Chemical Society
Letters
Langmuir, Vol. 13, No. 6, 1997 1369
Figure 1. Light-scattering tension values (b) compared to the equilibrium isotherm (line). Here, and elsewhere, where not shown, the uncertainties are smaller than the plotted points.
process is the Marangoni effect,5 particularly in the presence of an adsorption barrier.6 Because such processes are not incorporated in the formulation of the spectrum of the thermally excited capillary waves7 used in the direct data analysis,11 the role of reducing the dilatational wave damping is played by ′, which perforce exhibits an effective value which is
