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Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces Haim Diamant and David Andelman*

Summary: Kinetics of Surfactant Adsorption at Fluid-Fluid Interfaces
Haim Diamant and David Andelman*
School of Physics and Astronomy, Raymond and BeVerly Sackler Faculty of Exact Sciences,
Tel AViV UniVersity, Ramat AViV, Tel AViV 69978, Israel
ReceiVed: February 7, 1996; In Final Form: May 20, 1996X
We present a theory for the kinetics of surfactant adsorption at the interface between an aqueous solution and
another fluid (air, oil) phase. The model relies on a free-energy formulation. It describes both the diffusive
transport of surfactant molecules from the bulk solution to the interface and the kinetics taking place at the
interface itself. When applied to nonionic surfactant systems, the theory recovers results of previous models
and justifies their assumptions. Common nonionic surfactants are predicted to undergo a diffusion-limited
adsorption, in accord with experiments. For salt-free ionic surfactant solutions, electrostatic interactions are
shown to drastically affect the kinetics. The adsorption in this case is predicted to be kinetically limited, and
the theory accounts for unusual experimental results obtained recently for the dynamic surface tension of
such systems. Addition of salt to an ionic surfactant solution leads to screening of the electrostatic interactions
and to a diffusion-limited adsorption. In addition, the free-energy formulation offers a general method for
relating the dynamic surface tension to surface coverage. Unlike previous models, it does not rely on
equilibrium relations which are shown in some cases to be invalid out of equilibrium.
1. Introduction
Aqueous solutions of surface-active agents (surfactants) play
a major role in various fields and applications, such as biological


Source: Andelman, David - School of Physics and Astronomy, Tel Aviv University


Collections: Materials Science; Physics