Influence of surfactants on the structure of the adsorption layer in the system: Carboxymethylcellulose/alumina

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Abstract

The influence of surfactants: anionic SDS, nonionic Triton-100 and their mixture (SDS/TX-100) on the structure of the adsorption layer in the system: carboxymethylcellulose (CMC)/alumina (Al2O3) was measured. The surface charge density of alumina and its zeta potential were determined in the presence of the CMC macromolecules and the surfactant particles. In order to gain more information about the structure of adsorption layer the amount of CMC adsorption in the presence of surfactants as well as the thickness of the CMC adsorption layer were determined. It was proved that in the presence of surfactants mixture (SDS/TX-100) the amount of adsorption of CMC is the highest and the obtained adsorption layer is the thickest. On the other hand, when Triton X-100 is added to the measured system the polysaccharide conformation is closely packed. The amount of adsorption is large but the thickness of the adsorption layer is relatively low. In the presence of SDS macromolecules of CMC form conformation expanded towards the bulk solution. Such a conformation is characterized by the smaller amount of adsorption of polymer but the larger thickness of the adsorption layer.

Research highlights

► The presence of SDS, Triton X-100 and their mixture causes the decrease of the surface charge density of Al2O3 and also the shift of pHpzc point to lower pH values. The main reason for that is the presence of the negatively charged groups not linked with the surface but present in the compact surface layer of Al2O3 ► The presence of CMC and surfactants causes the decrease of the zeta potential of Al2O3. This phenomenon comes from two effects a shift of the slipping plane towards the bulk solution by an adsorbing polymer or a polymer–surfactant complex and the presence of negatively charged groups in the diffused part of electric double layer. ► The addition of surfactants (SDS, Triton X-100 and their mixture) to the adsorption system strongly influences the amount of adsorption of carboxymethylcellulose on Al2O3 surface. The smallest values were observed in the presence of SDS, larger in the presence of Triton X-100 and the largest in the presence of SDS/Triton X-100. ► When Triton X-100 is added to the system: CMC/Al2O3 polymer conformation is full of closely packed macromolecules. The amount of adsorption is large but thickness of adsorption layer is small. ► In the presence of SDS macromolecules of CMC form conformation expanded towards the bulk solution. Such a conformation results in small amount of adsorption and large thickness of adsorption layer.

Introduction

Understanding the behaviour of polyelectrolyte/surfactant mixtures at the interface is very important because of their widespread industrial applications such as painting, detergency, water treatment, and oil recovery [1]. These mixtures are used to control parameters like colloidal stability, electrokinetic and rheological properties in different technological processes [2], [3], [4]. It was proved that polysaccharides strongly interact with surfactants [5], [6], [7], [8]. There are several types of interactions such as electrostatic attraction, covalent bonding, hydrogen bonding and non-polar interactions responsible for the adsorption process in the polymer/surfactant/solid system. Nevertheless, four main cases in the above-mentioned adsorption system can be distinguished:

  • only one of these two adsorbates has the affinity to the surface but they cannot bind to each other;

  • only one of these two adsorbates has the affinity to the surface but they also can bind to each other;

  • both the polymer and the surfactant have affinity to the surface but not to each other;

  • both the polymer and the surfactant have the affinity to the surface and to each other.

The adsorption of polyelectrolyte or polyelectrolyte/surfactant mixture at the surface of the solid causes significant changes in the structure and properties of the electric double layer (edl). There are two major parameters characterizing the edl between the solid surface and the solution. The first one, the surface charge density, refers to the compact part of edl whereas the second one, the zeta potential, characterizes the diffused part of edl. There are two reasons responsible for changes in the surface charge density of the adsorbent in the presence of polyelectrolyte or surfactant. Firstly, an adsorbing polymer blocks a fraction of charged groups of the solid which leads to changes in the surface charge density of the solid. Secondly the adsorption of ionic polymer or polymer/surfactant mixture at the surface sometimes causes that the majority of its charged groups remains in the compact layer of the edl. Such a situation also leads to changes in the surface charge density. As far as the influence of polymer or polymer/surfactant mixtures on the zeta potential is concerned, there are three possible effects that influence changes in the zeta potential [9]. The first one is the shift of the slipping plane by a polymer adsorbing on the surface of the solid. The second one is changes in the position of counterions in the Stern layer. The last one is the presence of dissociated functional groups of polyelectrolyte or polyelectrolyte/surfactant mixtures in the diffused part of the electric double layer. When predominant effects are the shift of the slipping plane away from the solid surface and the presence of charge coming from dissociated carboxylic groups the zeta potential of the measured system will decrease. Whereas the predominant effect influencing the zeta potential is the change of position of counterions in the Stern layer the increase of the zeta potential is observed [10]. It should be stressed that these three effects may occur simultaneously. The value of the zeta potential depends on their quantitative contribution.

The amount of adsorbed polymer as well as the thickness of the polymer adsorption layer is very important parameters characterizing the structure of the adsorption layer in the system: polymer/metal oxide. The adsorption of polymer on the metal oxide surface is determined by many factors. Among them the most important ones are: (i) surface properties (the surface charge density, the surface energy), (ii) polymer properties (the charge density of the polyelectrolyte, molecular weight and its concentration), (iii) solution properties (pH, kind of background electrolyte and its concentration), and (iv) other properties (the addition of surfactant or polyvalent cation) [6]. The above-mentioned factors also influence the thickness of polymer adsorption layer. What is interesting, the increase of the adsorption amount does not mean that the thickness of polymer adsorption layer also increases. In some cases the polymers form conformation full of closely packed macromolecules (mostly as trains and short loops structures [11]) with a large amount of adsorption but small thickness of adsorption layer.

The aim of this paper was to investigate the influence of surfactants (SDS, Triton X-100 and their mixture) on the structure of the adsorption layer in the system: carboxymethylcellulose/alumina concerning the surface charge density of alumina, the zeta potential of this oxide, the adsorbed amount of CMC and the thickness of polysaccharide adsorption layer. The chosen systems attracted much attention owing to their applications in mineral processing [12].

Section snippets

Materials

Al2O3 produced by Merck was used as an adsorbent. The BET specific surface area for the sample was found to be 107 m2 g−1, whereas the average pores diameter of Al2O3 equalled 8.64 nm. The particle size distribution of the alumina sample determined with the use of a Malvern Mastersizer 2000, fell entirely in the range from 47.89 μm to 106.39 μm, with a volume average size of 71.57 μm. The adsorbent was washed with doubly distilled water until the conductivity of the supernatant was smaller than 2 μS cm

Results and discussion

Fig. 1 presents the influence of CMC and surfactants on the surface charge density of Al2O3. As one can see, the surface charge of alumina strongly depends on pH of the solution (Table 1). It is known that alumina point of zero charge pHpzc  7. At pH < 7 the solid surface is positively charged because of high concentration of AlOH2+ groups. At pH > pHpzc the oxide surface has negative charge coming from the increasing concentration of AlO groups. Another conclusion is that the presence of anionic

Conclusions

The presence of surfactants has a great influence on the structure the adsorption layer in the system: carboxymethylcellulose/alumina. It was proved that the presence of anionic SDS, nonionic TX-100 and their mixture causes the decrease of the surface charge density of Al2O3 and also the shift of pHpzc to lower pH values. The main reason for that is the presence of the negatively charged groups not bonded with the surface but present in the compact surface layer of Al2O3. Moreover, the presence

Acknowledgements

Author wishes to thank Prof. S. Chibowski for helpful discussions and stimulating critics, as well as Prof. W. Janusz and Dr. E. Skwarek for helping with the evaluation of changes in the concentration of various surface groups of alumina.

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