The displacement of adsorbed polymer from silica surfaces by the addition of a nonionic surfactant

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Abstract

The adsorption of polyvinyl alcohol and Synperonic NP8 (nonyl phenol ethoxylate with an average of eight ethylene oxide groups per molecule) on fumed silica has been studied at various pH values. This was followed by an investigation of the competitive adsorption of NP8 and PVA. It was shown that NP8 can displace the polymer from the silica. This was attributed to a higher adsorption energy for the NP8 molecule compared with the value of the individual adsorbed PVA segments. Sediment volume experiments showed that the addition of NP8 to a colloidally stable silica dispersion with adsorbed PVA can induce flocculation as a result of displacement of some or all of the PVA chains from the surface. Initially the adsorption of the NP8 molecules caused an increase in the hydrophobic interaction (resulting in the flocculation) between the alkyl phenol groups which are oriented towards the bulk solution (since the PEO chains preferentially adsorb on the silica surface). Restabilisation at higher NP8 concentrations occur through the formation of bilayers with the PEO chains now dangling in solution.

Introduction

It is now fairly well established that the adsorption of polymers on most surfaces is ‘irreversible’ resulting in the so called high affinity adsorption isotherm [1], [2]. The adsorption is usually described in terms of tail–train–loop conformation. However, the segments in trains are not completely immobile and hence their adsorption may be described as reversible. For that reason, it has been postulated that the addition of small surface active molecules (usually referred to as displacers) may cause gradual desorption of the segments in trains with the ultimate displacement of the whole polymer molecule [3], [4], [5]. This may be understood if one considers the relative adsorption energy per segment of a segment and that of the displacer. The adsorption energy of a given polymer segment may not be high (of the order of a fraction of a kT unit). However, due to the large number of segments in trains (that can reach more than 10% of the total number of segments in a polymer) the total adsorption energy per molecule can be very large, reaching several kT units and hence the adsorption is virtually irreversible. If a small molecule, like a surfactant, i.e. the displacer, added during or after adsorption of the polymer, the higher adsorption energy per displacer molecule may exceed that of a single segment. This results in gradual displacement of the polymer segments from the surface and above a given concentration of the displacer complete desorption of the polymer molecule may take place,

To test the above hypothesis, we have investigated the competitive adsorption of poly(viny alcohol) and a small surfactant molecule, namely nonyl phenol with 8 mol of ethylene oxide on silica.

The above investigation is important both from a fundamental and applied points of view. In many formulations silica is used as a ‘thickener’ to reduce sedimentation of coarse suspensions. Polymers may be added as dispersants, whereas surfactants are included for wetting the powder into the liquid. In such complex formulations, it is important to know the role of competitive adsorption of the polymer and surfactant on the particle surface. In a previous study in our laboratory, we have found that the addition of a polymeric surfactant of the polyethylene oxide–polypropylene oxide type (Synperonic PE) resulted in the desorption of high molecular weight lignosulphonate [6] from the surface of coal particles. Kling and Ploehn [4] have recently reported the displacement of polyethylene oxide from polystyrene particles by a polyoxyethylene oxide alkyl phenol surfactant.

Section snippets

Materials

Silica was OX-50 supplied by Degussa (Germany) and was used as received. This is a fumed silica with a specific surface area of 50±15 m2/g with a mean primary particle size of 40 nm.

Poly(vinyl alcohol) (PVA) was GL-05 supplied by Nippon Gohsei (Japan). The polymer has an average molecular weight of 28 000 and is 88% hydrolysed poly(vinyl acetate).

Synperonic NP8 (nonyl phenol with 8 mol of ethylene oxide) was supplied by ICI surfactants (Wilton, UK) and was used as received.

Method

Stock dispersions of

Adsorption isotherms of PVA on silica at various pH values

Fig. 1 shows the adsorption isotherms of PVA on silica at various pH values. The results clearly show an increase in PVA adsorption with decrease in the pH. In addition, the isotherms become more and more of the high affinity type as the pH decreases. These results are similar to those previously obtained in our laboratory [8] on another silica sample, namely Cabosil M5 (a fumed silica with a surface area of 200 m2/g). Fig. 2 shows a plot of the plateau adsorption value versus pH. This figure

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