Bull. Mater. Sci., Vol. 22, No. 3, May 1999, pp. 313-320. © Indian Academy of Sciences. Forces between colloidal droplets in the presence of a weak polyelectrolyte JOHN PHILIP*, O MONDAIN MONVAL t, F LEAL-CALDERON t and J BIBETTE t Metallurgy and Materials Group, lndira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India tCRPP-CNRS, Avenue Albert Schewitzer, 33600 Pessac, France Abstract. In this paper, we present the results on the forces between individual colloidal liquid droplets in the presence of a weak polyelectrolyte, poly(acrylic acid), using magnetic chaining technique. The effect of the repulsive forces have been investigated under different experimental conditions such as polyeleetrolyte concentration, adsorption time, salt concentration etc. At a PAA concentration of 0-01% (weight), a long range repulsive force proffie is observed due to the adsorption of polyelectrolyte on the droplet, without any irreversible aggregates even at very small inter-droplet spacing. Above a concentration of 0.01 wt% of PAA, formation of irreversible chaining of droplets is observed at short inter-droplet separations due to polyelectrolyte bridging. The onset of binding is also independently confirmed by microscopic observation. Compared to the slow adsorption on mica surfaces, the PAA adsorption on the colloidal droplets is found to be rapid. Up to 0.1 M NaCI, the range of repulsion and the hydrodynamic radius of the droplet is found to be increasing. Keywords. Colloidal droplets; polyelectrolyte. 1. Introduction of particles or for liquid particles coalescence may occur and the system will be unstable. If the net force is Colloid is a generic name for any dispersion in which repulsive, the system will remain stable. The height of the characteristic size of the dispersed domains is between the potential barrier decides the stability of the colloidal 10nm and 100m. This includes numerous systems system. Force measurement techniques have been widely such as foams (a gas dispersed in a liquid), smoke used to understand the interactions that give insight into (a solid dispersion in a gas), emulsions (one liquid the colloidal stability (Israelachvili 1985). dispersed in another) or paints (a solid dispersed in a Most of the time, electrostatically stabilized colloids liquid). Applications of colloidal science are in constant become unstable when the ionic strength of the medium progress in many aspects of life. Colloids are used in is increased sufficiently, due to the reduction in the a huge number of processes such as extracting oil from spatial extension of electrical double layers (e.g. clays, geological deposits, aerosols for dispensing domestic sols, lattices). Therefore, one major advantage of using products like shaving creams and deodorants, sprays macromolecules as stabilizers is that their double layers containing weedicide and insecticides, emulsions, gels, are less sensitive to electrolyte concentration. As a result, food processing, packaging industry etc. steric stabilization of colloidal dispersions has triggered Colloids generally require repulsive surface forces to a lot of interest in recent years due to its advantages become meta stable. The most important forces which over electrostatic counterpart and more importantly due determine the stability of colloidal dispersions are van to its numerous industrial applications (Napper 1983; der Waals dispersion forces, electrostatic and steric forces Patel and Tirrell 1989). due to polymers adsorbed at the interface. Other forces Stabilization of the colloidal dispersions with macro- that are important at short intermolecular distances are molecules has been explored in various technological solvation and hydrophobic forces. The classical applications for many years, however, the interactions Derjaguin-Landau-Verwey-Overbeck (DLVO) theory between polymer bearing surfaces could be studied combined the van der Waals forces with the electrostatic directly only after the introduction of the surface force double-layer forces to explain the stability of lyophobic measurement apparatus (Israelachvili and Tabor 1972; colloids. The net force acting between the colloidal Israelachvili and Adams 1978; Luckham and Klien 1984). particles determines the stability of the colloidal system. The elucidation of the structure of the adsorbed layer A net attractive force leads to the aggregation or clustering on the colloidal emulsion is important since it plays a major role in the stabilization of the colloids. Steric *Author for correspondence stabilization of the colloidal dispersions has triggered a 313 314 John Philip et al lot of interest due to its several advantages over its (Bibette 1991) was used to select emulsion droplets with electrostatic counterpart and more importantly due to its a diameter of about 200 nm. The size distribution of the numerous industrial applications (Sato and Ruchs 1980; final emulsion was measured by using a Malvern Napper 1983; Th Tadors 1983, 1988). It is well known Instruments Master sizer (Ver. 2.14). that electrostatically stabilized colloids often coagulate when the ionic strength of the medium is increased 2.2 Polyelectrolyte sufficiently, due to the reduction in the spatial extension of the electrical double layers. Ionic strength higher than Poly(acrylic acid) (PAA) of molecular weight of 250,000 0.001 M, corresponds to a length of about 10 nm for a (Mw/MN=I.IO) was supplied by Rhone-Poulenc and 1 : 1 electrolyte and can lead to van der Waals attraction Aldrich Chemicals. PAA is a weak acid and its monomer in the dispersion. This is the range where electrostatic is shown below stabilization often fails. One major advantage of using macromolecules as stabilizers is that the double layers [-CH2-CH(CO2H)]. are less sensitive to electrolyte concentration. Measurement of forces acting between solid surfaces The unperturbed radius of gyration of this polymer in has been a topic of intense research over the last two good solvent (water) was found to be 35 nm (Brandrupp decades (Israelachvili 1985). Most of the force measure- and Immergutt 1975). PAA is a water soluble polyelec- ments have been carried out by using the novel surface trolyte and it can exhibit a large variety of behaviours force apparatus (SFA) introduced by Israelachvili and due to the amphiphilic nature of its constituents. Hy- Tabor (1972). Other relatively new force measuring tech- drophobic nature of the backbone makes the solubilization niques (Hansma et al 1988; Prieve and Frej 1990) such of the uncharged chains difficult. Theta temperature for as total internal reflection microscopy, atomic force PAA in water is about 20°C. The solubilization of PAA microscopy etc have also been used for such investi- in water at room temperature is due to electrostatic gations but occasionally compared to SFA. Recently, ,a repulsion between ionized carboxylic groups along the new technique (Leal et al 1994), called magnetic field chain. The non-ionic surfactants used for these investi- induced chaining technique (MCT) has been introduced gations was Nonyl phenol Ethoxylate (NP10). which allows the measurement of forces between two individual emulsion droplets. In contrast to other older C9H19-C6H4-O--(CH2CH20) lo-H. techniques, this method leads to the direct in situ mea- surement at the colloidal scale. In SFA, the force is measured between the semi macroscopic surfaces (mica) 3. Experimental whereas in MCT the forces between individual colloidal droplets are measured. Using this technique, one can Ferrofluid oil was used and the droplets were para- measure the colloidal forces in a wide variety of materials magnetic in nature (Rosensweig 1985). An applied field encountered in emulsions and dispersions. The present induces a magnetic dipole in each drop, causing them work is mainly aimed to understand the stability of to form chains. Without external field, these droplets emulsion droplets in the presence of polyelectrolyte. We have no permanent magnetic moments because of the present the force profiles-distance profiles in the absence random orientation of the magnetic grains within the and presence of polyelectrolytes (Philip 1995). droplets, due to thermal motion. An external magnetic field orients these magnetic grains slightly toward the field direction, which results in a dipole moment in each 2. Materials droplet. The magnitude of the magnetic dipole moment increases with the strength of the applied field until 2.1 Ferrofluid saturation is reached. The strength of this dipolar inter- action can be described by a coupling constant, which Ferrofluid oil consisted of a collection of ferromagnetic is the ratio of the contact dipole energy of two parallel domains of Fe203 dispersed in the octane and was dipoles to the thermal energy (Mou et al 1994). At low supplied by Rhone Poulenc Inc. The typical size of the concentration, one-droplet thick chains are well separated Fe203 particles was about 10nm. The inner surfactant and oriented along the field direction. Due to the presence used to stabilize the oxide particles against van der of the one-dimensional ordered structure, Bragg peak Waals attraction was oleic acid. The polar group of the can be observed, from which the inter-droplet separation adsorbed oleic acid associated with the oxide particle is estimated precisely. The condition for forming a linear surface and disguised the surface to look like part of chain is that the repulsive force between the droplets the continuous phase (octane). The technique used to must exactly balance