Elsevier

Applied Surface Science

Volume 254, Issue 9, 28 February 2008, Pages 2825-2830
Applied Surface Science

The role of adsorption of sodium bis(2-ethylhexyl) sulfosuccinate in wetting of glass and poly(methyl methacrylate) surface

https://doi.org/10.1016/j.apsusc.2007.10.024Get rights and content

Abstract

Advancing contact angles, θ, for aqueous solutions of the anionic surfactant, sodium bis(2-ethylhexyl) sulfosuccinate (AOT) were measured on glass and poly(methyl methacrylate) (PMMA) surface. Using the obtained results we determined the properties of aqueous AOT solutions in wetting of these surfaces. It occurs that the wettability of glass and PMMA by these solutions depends on the concentration of AOT in solution. There is almost linear dependence between the contact angle (θ) and concentration of AOT (log C) in the range from 5 × 10−4 to 2.5 × 10−3 M/dm3 (value of the critical micelle concentration of AOT—CMC) both for glass and PMMA surface. For calculations of AOT adsorption at solid (glass, PMMA)-solution drop–air system interfaces the relationship between the adhesion tension (γLV cos θ) and surface tension (γLV) and the Gibbs and Young equations were taken into account. From the measurement and calculation results the slope of the γLV cos θ  γLV curve was found to be constant and equal 0.7 for glass and −0.1 for PMMA over the whole range of AOT concentration in solution. From this fact it can be concluded that if ΓSV is equal zero then ΓSL > 0 for the PMMA–solution and ΓSL < 0 for glass–solution systems. It means that surfactant concentration excess at PMMA–solution interface is considerably lower than at solution-air interface, but this excess of AOT concentration at glass–solution interface is lower than in the bulk phase. By extrapolating the linear dependence between the adhesion and surface tension the value of the critical surface tension (γc) of wetting for glass and PMMA was also determined, that equaled 25.9 and 25.6 mN/m for glass and PMMA, respectively. Using the value of the glass and PMMA surface tension as well as the measured surface tension of aqueous AOT solutions in Young equation, the solid–liquid interface tension (γSL) was found. There was a linear dependence between the γSL and γLV both for glass and PMMA, but there were different slope values of the curves for glass and PMMA, i.e. −0.7 and 0.1, respectively. The dependence between the work of adhesion (WA) and surface tension (γLV) was also linear of different slopes for glass and for PMMA 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]:γLVcosθ=γSVγSL

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]:dγLVcos θdγLV=ΓSVΓSLΓLVwhere 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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