The adsorption at solution–air interface and volumetric properties of mixtures of cationic and nonionic surfactants

Abstract

Surface tension, density and conductivity measurements were carried out for systems containing mixtures of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Triton X-100 (TX100). The obtained results of the surface tension measurements were compared with those calculated from the relations derived by Joos, Miller and co-workers. From the comparison it appeared that using the modified adsorption isotherm derived by Joos the adsorption behaviour of CTAB and TX100 mixture can be predicted satisfactorily; however, the values of the surface tension of aqueous solutions of these mixtures calculated on the basis of the simple relationships of Miller et al. are a little higher than those measured in contrast to many two component systems of surfactants studied earlier. On the basis of the results obtained from the measurements and calculations it was found that there is a linear relationship between the surface tension and composition of CTAB and TX100 mixtures at their low concentration; however, in the concentration range corresponding to that of the saturated monolayer at the interface a negative deviation from the linear relationship is observed. This fact and the values of the parameters of molecular interactions in the mixed monolayer suggest that there is synergism in the reduction of the surface tension of aqueous solutions of CTAB and TX100 mixture when saturation of the monolayer is achieved. The negative parameters of intermolecular interaction in the mixed micelle and calculations based on MT theory of Blankschtein indicate that there is also synergism in the micelle formation for CTAB and TX100 mixture. It was also found that the values of the standard free energy of micellization for the mixture of CTAB and TX100 are somewhat lower than for individual components and they can be predicted on the basis of Maeda equation and the mole fraction of surfactant and free enthalpy of mixing CTAB and TX100 in the micelle. Our measurements and calculations indicate that the volume both of the individual surfactants and mixture studied decreases during the micellization process; however, at the concentration of the surfactants higher than CMC an increase of this volume is observed.

Introduction

The tendency of surfactants to adsorb at interfaces in an oriented fashion and micelle formation are their two fundamental properties [1], [2], [3].

The adsorption of surfactants at air–water interface controls the dynamic behavior of many important systems [4], [5]. The surfactants adsorption influence the stability of foams, the droplet size in jets and sprays, the spreading of drops on solid surfaces, and the smooth coating of multiple layers [6].

The micelle formation affects such interfacial phenomena as surface or interfacial tension reduction that do not directly involve micelles. Some of the micelles have the structure similar to that of the biological membranes and globular protein and catalytical properties [7], [8], [9].

Because of the specific surface and volumetric properties of the surfactants they are employed by organic chemists and biochemists in different industrial processes such as: ore flotation [10], coal transport [11], firefighting [12], emulsion polymerization [13], corrosion inhibition [14], oil recovery [15], cement hardening [16] and commercial laundering [17].

However, the systems employed in these applications almost always consist of mixtures of surfactants, because technical-grade surfactants are themselves mixtures, and the purification process may be difficult or excessively expensive, and the mixed systems often behave better than a single surfactant [18], [19], [20]. The widespread use of surfactant mixtures for industrial purposes has stimulated the interest of researchers, and in the last decade many papers have been published on the solution properties of mixed surfactant systems [21], [22], [23], [24], [25].

In these papers it is possible to find that the micelle and monolayer at water–air interface and the composition of two surfactant mixtures can be substantially different than the equilibrium composition in the bulk phase [2], [26]. Because of these differences a deviation from linear relationships between such parameters as surface or interfacial tension, wettability, CMC, standard free energy of adsorption and micellization and composition of surfactants is observed. Sometimes, the above mentioned parameters show a maximum or minimum at a given composition of the parameters. Our earlier studies showed that even for mixtures of two anionic surfactants, having different hydrophilic heads and a different length of hydrophobic alkyl tails, there is no linear relationship between the concentration excess at water–air and hydrophobic solid–water interfaces, the surface tension, critical micelle concentration and wettability of hydrophobic low energetic solids and the composition of the mixtures [3].

From the fundamental point of view mixtures of ionic–nonionic surfactants are more interesting because they often exhibit a highly nonideal behavior. Addition of a nonionic surfactant to an ionic surfactant micelle can reduce the electrostatic repulsions between the charged surfactant heads and greatly facilitate mixed micelle formation. Nonideal behavior of an ionic/nonionic surfactant mixture can also be influenced by other structural characteristics of the two surfactants, such as differences in the sizes of the surfactants heads or the lengths of the surfactants tails [27].

In the literature it is possible to find data concerning rather the anionic/nonionic mixtures of two surfactants than those of cationic/nonionic ones which are also used in many processes, for example as detergents for some materials.

Thus, the purpose of our studies was to determine the adsorption behavior of mixed layers basing on the equations of Gibbs, Joos, Miller and co-workers [2], [28], [29], [30], [31] as well as interactions between cationic and nonionic surfactants in the surface layers and micelles.

For this purpose the surface tension, density and conductivity of aqueous solutions of cetyltrimethylammonium bromide (CTAB) and p-(1,1,3,3-tetramethylbutyl)phenoxypoly(ethylene glycol), Triton X 100 (TX100) mixtures were measured.