The role of adsorption of sodium bis(2-ethylhexyl) sulfosuccinate in wetting of glass and poly(methyl methacrylate) surface
Introduction
Surfactant adsorption is a transfer process of surfactant molecules from a bulk solution phase to the surface or interface. Adsorption of surfactants at solid–liquid interface systems plays a very important role in technological and industrial applications such as printing, detergency, cosmetics, mineral flotation, dispersion and others [1].
Adsorption of surfactants at polar solid–water interface is a complex process. A driving force of adsorption of surfactants is a combination of electrostatic and chemical interactions, lateral chain–chain associative interactions, hydrogen bonding and solvation of adsorbate species [2].
From the practical point of view the most interesting systems are those including water or different aqueous solutions. In three phase hydrophobic solid–water–air systems, surface active agents, for example surfactants, adsorb at water–air and solid–water interfaces reducing the water surface tension and solid–water interface tension to values which can cause contact angle decrease [3], [4]; however, in systems including hydrophilic solids the adsorption of surfactants at solid–water interface can decrease or increase or not change the solid–water interface tension [4]. Contact angle changes are sometimes difficult to predict. The solid–liquid interface tension can, among other things, be determined from the Young equation by measuring the contact angle [4], [5]:
This and Gibbs equation [4] were taken into account by Lucassen-Reynolds [6] to analyze the adsorption of a surfactant at solid–air, liquid–air and solid–liquid interfaces. Using the Lucassen-Reynders [6] equation it was found that for completely nonpolar surfaces, for example polytetrafluoroethylene (PTFE), the ΓSL and ΓLV were the same because the slope of the line representing the relationship between γLV cos θ and γLV was equal −1. It indicates that adsorption at solid–liquid and liquid–air interface is the same [4]. In the case of polar solids the slope of γLV cos θ versus γLV curve differs significantly from −1 [4], [7]. It results from the fact that adsorption of surfactants at solid–liquid interface is different than at liquid–air interface.
Because the wettability of solids by aqueous solution of surfactants and their adsorption at water–air and solid–water interface are strongly related, and adsorption of appositively charged surfactants and surfaces is not completely explained, we tried to determine the adsorption properties of aqueous AOT solutions at water–air, glass–water and PMMA–water interfaces in regard to glass and PMMA wettability by surfactant solutions.
For this purpose measurements of the contact angle of aqueous AOT solutions on glass and PMMA surface were made in the range of the surfactant concentration from 0 to 10−2 M/dm3, and the obtained results were analyzed by using the Lucassen-Reynders equation [6].
Section snippets
Materials
Sodium bis(2-ethylhexyl) sulfosuccinate (C20H37NaO7S) (AOT, Fig. 1) (Sigma–Aldrich), (purity ≥ 99%) was used for aqueous solution preparation. For preparation of AOT aqueous solutions doubly distilled and deionized water (Destamat Bi 18E) was used. The surface tension of water (γw = 72.8 mN/m) was always controlled at 293 K by Krüss K9 tensiometer under atmospheric pressure by the ring method before the solutions were prepared.
Poly(methyl methacrylate) plates (Z. Ch. Oświęcim, Poland) were cut from a
Wetting of glass and PMMA surfaces
In general wetting is a displacement of one fluid by another from the surface, but this term is commonly applied to the displacement of air from a solid surface by water or an aqueous solution. Three types of wetting have been distinguished: (a) spreading wetting, (b) adhesional wetting, (c) immersional wetting [4], [5]. For all the three types of wetting, reduction of the interfacial tension between solid and wetting liquid (γSL) is beneficial in contrast to reduction of the γLV which is not
Adsorption of AOT at solid–liquid–air system interfaces
Wetting process involves solid–liquid–air systems. Since wetting and adsorption in such systems are strongly related, analyzing the surfactant adsorption at the interfaces we referred our considerations to the Lucassen-Reynders equation [4], [5], [6]:where ΓSV, ΓSL and ΓLV are the surface excess of surfactant at solid–air, solid–liquid and liquid–air interface, respectively.
Bargeman and Van Voorst Vader [9] found that there is a linear relationship between the adhesion
Conclusions
In all we can state that:
- (a)
The wettability of glass and PMMA strongly depends on AOT concentration in solution, but the contact angle changes on glass and PMMA surface result probably more from decreasing surface tension of aqueous AOT solutions than adsorption of the surfactant at solid–liquid interface.
- (b)
There is a linear relationship between the adhesion tension and surface tension of aqueous AOT solutions both for glass and PMMA surface, but of quite different slopes, that indicates that in the
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