M. Van Uffelen

Surface relaxation in spread insoluble monolayers of cholesterol and dipalmitoyl lecithin

Abstract Experimental evidence is given for time dependent surface tensions in spread insoluble monolayers of cholesterol and dipalmitoyl lecithin, indicating a relaxation mechanism in the surface. To understand this, it is necessary to account for the Principle of Braun-Le Châtelier. Two different techniques and subsequent analysis are presented: periodic compression and surface expansions, and stepwise surface disturbances. Most of the experimental results indicate a double consecutive reaction in the monolayer; the longest relaxation time for cholesterol is of the order of 102 s and for dipalmitoyl lecithin 103 s. Mixed monolayers of cholesterol and dipalmitoyl lecithin are also considered.

Adsorption kinetics with surface dilatation 4. Peaktensiometry—adsorption to a linearly expanded surface

Abstract Peaktensiometry refers to an experimental method in which the surface of a surfactant solution is expanded with a constant velocity. Since the dilatation rate for these types of experiments is not a constant, we attain no steady-state surface tension. Instead the curve Δσ vs. t (where σ is surface tension and t is time), shows a maximum (peak). The experiments in which the surface is expanded with a constant dilatation rate are also markedly affected by the speed of the surface expansion, as is observed in the peak height and in the position of the peak in the course of time. A theory is presented in order to explain this adsorption behaviour of surfactant solutions at the air/water interface. In fact, the theory can be generalized such that it applies for any type of surface dilatation rate.

Adsorption kinetics with surface dilatation 3. Adsorption to a surface expanded with a constant dilatation rate

Abstract A theory for the adsorption of a surfactant at the air/water interface, when the surface is expanded with a constant dilatation rate dln Ω/dt (Ω being the area), is presented. In order to verify the theory, two kinds of experiments were performed: (i) expanding the surface with a constant dilatation rate (area changes logarithmically), while measuring the surface tension as a function of time and (ii) expanding the surface by drawing a strip into the solution, while monitoring the surface tension with time. It is shown that not only the steady state surface tension (σ for t → ∞), contains valuable information, but also the transient surface tensions, which give additional information about the process that is going on, or at least confirm the results obtained from the steady state surface tension results only.

Study of the modulus of elasticity by linear compression of an adsorbed layer

Abstract It is shown that for an adsorbed monolayer, the modulus of elasticity can be obtained from linear compression of the surface. A theory has been developed using the diffusion penetration theory, in which convection due to the surface compression is neglected. The method, which is comparable with that of J. Lucassen and M. Van den Tempel (J. Colloid Interface Sci., 41 (1972) 491, and Chem. Eng. Sci., 17 (1972) 1283), is illustrated for adsorbed decanoic acid monolayers.

The relation between the surface elasticity modulus and the dynamic surface tension

When an air-liquid or liquid-liquid interface on which surfactant molecules are adsorbed is subjected to small periodic deformations, the surface tension also oscillates around its equilibrium value with the same frequency. If w is the radial frequency Q, is the area of the surface at rest, AQ is the amplitude of the surface oscillatory deformations and do is the corresponding amplitude of the surface tension oscillations, the modulus of elasticity c is given by

Adsorption kinetics with surface dilatation 5. Transient surface tensions of an adsorbed monolayer during compression at a constant dilatation rate

In the present article the integration of the convective diffusion equation for compression at a constant dilatation rate is presented, which enables us to describe the course of surface tension with time for the desorption rate of an adsorbed monolayer. The theory is evaluated by compression experiments performed with decanoic acid and Triton X-45.