Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

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Abstract

Static and dynamic contact angles of aqueous solutions of three surfactants—anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C12E5)—were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid–vapor interface. Obviously, another process involving adsorption at the expanding solid–liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.

Introduction

Interest in dynamic wetting processes has immensely increased during the past 10 years [1], [2], [3], [4], [5], [6], [7], [8]. In many industrial and medical applications, some strategies to control drop spreading on solid surfaces are being developed. One possibility is that a surfactant is added to a liquid [3], [8], [9].

On the other hand, there are very few systematic investigations of quantitative relations between the chemical structure of surfactants and the spreading behavior of their aqueous solutions on solid polymer surfaces with different surface energies. Moreover, the main part of them is concerned with highly concentrated surfactant solutions (far above the corresponding critical micelle concentrations) [5], [10], [11], [12]. At the same time, in spite of their practical importance, there is a lack of literature data about the spreading dynamics of relatively dilute aqueous surfactant solutions on polymer surfaces.

The objective of this paper is to present some new results on surfactant solution dynamic behavior observed while measuring the contact angles at different surfactant concentrations both well below and close to the critical micelle concentration (cmc) on different well-prepared and -characterized polymer surfaces. Some problems associated with dynamic contact angle measurements such as evaporation and depletion effects will be also discussed.

Section snippets

Surfactants

The surfactants used in this study were anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C12E5), all with the same alkyl chain length. All surfactants were of high purity grade (⩾98%) from Fluka and were used without further purification. Aqueous solutions of these surfactants with desired concentrations in the cmc range of 0.0625 to 2 cmc were prepared by dilution from the corresponding stock solutions

Dynamic and quasi-equilibrium contact angles: evaporation and depletion effects

As an example, Fig. 1 demonstrates the averaged dynamic contact angle and drop base radius with their standard deviations for C12E5 solution at the cmc (31 mg/l) on the highly hydrophobic Teflon AF surface. As can be seen, the drop base radius regularly increases and then stays nearly constant, whereas the contact angle does not reach its equilibrium value.

In a typical experiment (Fig. 2), the drop volume decreases linearly with time beginning at approximately 5 s. We proceed from the

Summary

The wetting behavior of (relatively) dilute aqueous surfactant solutions over highly hydrophobic and moderately hydrophobic polymer surfaces was characterized by wetting dynamics measurements. In order to analyze the equilibrium state, an algorithm to correct the contact angle taking evaporation into account is presented.

Considering the dynamic wetting behavior, it was found that ionic surfactant solutions such as SDS and DTAB do not spread on low energy surfaces (⩽23 mJ/m2) at any

Acknowledgements

This research is supported by Sasol Germany GmbH. Two of the authors (K.S. and V.R.) acknowledge the Saxon Ministry of Science and Art (SMWK, Germany) for financial support. We are grateful to Zoltán Mészáros from Repallon GmbH (Switzerland) for giving much useful advice on the wetting measurements using the FibroDAT tester, as well as to Nicole Petong and to Mikhail Motornov, both from the Institute of Polymer Research Dresden, for performing the AFM experiments and ellipsometric measurements,

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