pubs.acs.org/Langmuir © 2009 American Chemical Society
Influence of Wetting and Dispersing Agents on the Interaction between Talc and Hydrophobic Particles )
Viveca Wallqvist,*,† Per M. Claesson,†,‡ Agne Swerin,† Joachim Schoelkopf,§ and Patrick A. C. Gane§, † YKI, Ytkemiska Institutet AB/Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, § Omya Development AG, CH-4665 Oftringen, Switzerland, and Department of Forest Products Technology, Faculty of Chemistry and Materials Science, Helsinki University of Technology, FI-02015 Helsinki, Finland )
‡
Received January 16, 2009. Revised Manuscript Received March 5, 2009 The interactions between a natural talc surface and a model hydrophobic particle have been investigated in aqueous solutions by employing the atomic force microscopy (AFM) colloidal probe technique. The results demonstrate the presence of long-range attractive forces due to bridging via preadsorbed or induced bubbles/cavities. Due to the natural heterogeneity of talc, and the stochastic nature of the bubble bridging process, the variability in the range and magnitude of the attraction is larger than that for cases when other interactions predominate or than that when only model surfaces are used. Addition of poly(acrylic acid), a common dispersing agent, did not affect the measured forces. Thus, we conclude that poly(acrylic acid) does not adsorb to the basal plane of talc. In sharp contrast, addition of Pluronic PE6400, a nonionic triblock polymer used as wetting agent, resulted in complete removal of the bubble-induced attractive force. Instead, a short-range steric repulsion is the dominating feature. Clearly, Pluronic PE6400 is able to displace air bubbles from the surface and prevent their formation when the particles come into contact. These are suggested to be important features of efficient wetting agents.
1. Introduction Talc, with the chemical formula Mg3Si4O10(OH)2, is the softest mineral on earth. The reason for its softness stems from its chemical structure (Figure 1), which consists of a magnesium based octahedral layer sandwiched between silica rings through shared oxygen atoms. Because both sides of this structure expose an oxide surface, individual talc platelets are held together only by weak van der Waals forces. Compared to other silicates, talc is relatively hydrophobic due to the oxide surfaces. The oxide surfaces hydrate to a limited extent on exposure to ambient moisture, and some silanol groups are formed,1 which improves the compatibility of talc with aqueous media somewhat. The magnesia edges become hydrolyzed in water and are not hydrophobic but exhibit some hydrophilicity accounting for the mineral’s alkaline pH. In nature, what is called “talc” is often a blend containing predominately (>90%) talc mineral mixed with other minerals such as tremolite (CaMg3(SiO3)4), serpentine (3MgO 3 2SiO2 3 2H2O), and anthophyllite (Mg7 3 (OH)2 3 (Si4O11)2.). The properties of talc (platyness, softness, hydrophobicity, organophilicity, inertness, and mineralogical composition) provide specific functions in many applications. In the paper industry, talc is used as a filler and for coating purposes.2 It improves printability,2 reduces surface friction,2 and is active in pitch control.3 In the agriculture and foods industry, talc is used as an inert carrier, as an antistick agent (e.g., in chewing gums), or as *Corresponding author. Telephone: +46 8 50106091. Fax: +46 8 208998. E-mail:
[email protected]. (1) Ciullo, P. A.; Robinson, S. Paint and coating industry magazine, January 2003. (2) Likitalo, M. Talc; Papermaker Science and Technology: Helsinki, 2000; Vol. 11, pp 106-119. (3) Allen, L. H.; Cavanagh, W. A; Holton, J. E.; Williams, G. R. Pulp Pap. 1993, 67, (13), 89–91. (4) The Industrial Minerals Association, www.ima-na.org (accessed online 01/ 12/2008).
Langmuir 2009, 25(12), 6909–6915
an anticaking agent.4 In the ceramics and coating industries, talc is also widely used, for example, as filler in primers and top-coats, powder coatings, joint cements, and crack fillers.4 Furthermore, talc is used as a structural stabilizer in plastics, as a viscosity reducer in rubber, and as a bacteria ballast to increase sedimentation in wastewater treatment.4 Talc powder can be manufactured into a suspension without any additives, but it requires a lot of energy and results in low solids content. In 1956, Lamar proposed a method for dispersing talc using nonionic surfactants.5 He found that wetting agents with the hydrophilic portion consisting of ethylene oxide, for example, “Pluronics” in which a poly(propylene glycol) base has various proportions of ethylene oxide condensed on it, are effective for use together with talc. He produced nonviscous slurries of
