Probing the effect of superplasticizer adsorption on the surface forces using the colloidal probe AFM technique

https://doi.org/10.1016/j.cemconres.2004.07.008Get rights and content

Abstract

The interactions between adsorbed superplasticizer layers have been studied using direct force measurements, with an atomic force microscope. MgO was used as a nonreactive model system for cement, and a novel freeze granulation method for the production of spherical particles suitable for the colloidal probe atomic force microscope (AFM)-technique is reported. We found that the interactions induced by the adsorption of the anionic acrylic ester–ethylene oxide (AAE–EO) copolymers can be described as electrosteric, with an electrostatic and steric contribution. The range of the steric repulsion varied between 1.5 and 5 nm and could be related to the length of the grafted ethylene oxide chains. The lignosulfonate also induced a significant steric contribution that could be related to a dense coil conformation of the adsorbed superplasticizer.

Introduction

Superplasticizers are a type of dispersants that have been used to improve the flowability of cementitious systems for the last four decades [1]. The use of the first generations of superplasticizers, e.g., sulfonated naphthalene formaldehyde (SNF) and modified lignosulphonates (LS), resulted in significant improvements of the properties of fresh concrete and are still widely used. However, increasing demands on good flowability, extended working time and an increased need for a reduction in concrete porosity have created an interest in new types of superplasticizers, e.g., anionic acrylic ester–ethylene oxide copolymers (AAE–EO). Self-compacting concrete (SCC) is one application where it is especially important to have a good flowability because this concrete is not vibrated during placement [2].

Most of the different types of superplasticizers, both the early, first generation admixtures and the novel AAE–EO, are polyelectrolytes. Polyelectrolytes are a type of dispersant that is commonly used in, e.g., ceramics, paints, paper coatings and refractories [3]. Polyelectrolytes adsorb at the solid–liquid interface and infer a repulsive force, thus reducing or eliminating the adhesion between particles in close proximity. Polyelectrolyte adsorption is highly dependent on the electrostatic interactions between the polyelectrolyte and the surface; hence, the surface chemistry of the solid phase and the solution properties of the polyelectrolyte are important parameters, regulated by the pH and the ionic strength [4]. The pH controls the sign and density of charges on the surface and the degree of dissociation and the conformation in solution of a weak polyelectrolyte. The salt concentration also affects the conformation of the polyelectrolyte in solution and the screening of the electrostatic interaction between charged polymer segments and the surface charges.

The term electrosteric stabilization is often used to describe how polyelectrolytes act as dispersants. Electrosteric stabilization is a combination of an electrostatic double-layer repulsion and a steric repulsion, where the relative importance of the respective contributions is closely related to the polymer segment density profile at the interface. If the polyelectrolyte adsorbs in a flat conformation, the steric repulsion is short range and the stabilization mechanism is mainly electrostatic. This is usually the case when the polyelectrolyte is highly charged, having an extended conformation, and the particle surface is oppositely charged. With thicker adsorbed layers, having chains protruding into the solution, the steric contribution will become more important. However, there is always an electrostatic contribution because the adsorption of a charged polyelectrolyte usually induces a net surface charge. The relative importance of the steric and electrostatic contributions to the repulsion inferred by the adsorption of superplasticizers on cementitious particulate systems is still an unresolved issue, mainly because of the complex nature of the system. Superplasticizers that impose a significant change in the zeta-potential when adsorbed on the particles are usually interpreted to disperse cement through electrostatic stabilization, whereas steric stabilization is assumed to be the main mechanism when bulky polymers are adsorbed [5], [6]. Lewis et al. [7] also suggested that depletion forces are important for the stabilization of cementitious systems.

The effect of a dispersant is often investigated with methods providing macroscopic (sedimentation and rheology), as well as microscopic information (electrokinetics). These methods, however, contribute only with indirect information on the actual interaction between the particles constituting the colloidal dispersion. Direct measurements using the Surface Forces Apparatus (SFA) or the Atomic Force Microscope (AFM) offer a possibility to directly probe the interactions between surfaces. The AFM method for measuring surface forces, often referred to as the “colloidal probe method”, was first used by Ducker et al. in 1991 [8] and has, since then, been applied to a number of different systems [9], [10], [11], [12], [13]. The AFM method has also been applied to cement and cement-like systems immersed in a liquid with [14], [15] and without the addition of polymer [16], [17]. However, the use of reactive surfaces with a poorly defined geometry and surface roughness makes a thorough evaluation of the details of the inferred surface forces difficult.

The aim of the present work is to use the AFM colloidal probe technique to directly measure the interactions inferred by adsorption of different superplasticizers and elucidate the characteristics of the electrosteric repulsion. We have used MgO as a nonreactive model system for cement. Previous work by Flatt et al. [18] showed that the surface chemistry and adsorption properties of MgO are quite similar to cement when the powder is immersed in an electrolyte that mimics the ionic composition of cement water. We have developed a technique to produce spherical MgO probes suitable for accurate direct force measurements using the AFM. The direct force measurements suggest that the adsorbed layer thickness is relatively thin (1–5 nm) and that the electrostatic interactions are of importance, also for the novel AAE–EO superplasticizers.

Section snippets

Polymers

Two modified anionic acrylic ester type polymers with grafted ethylene oxide chains are used in this study. The two polymers mainly differ with respect to the length of the grafted ethylene oxide chains. The ethylene oxide (EO) branches of Conpac 30 (Perstorp Speciality Chemicals, Sweden) are about 60–80 units long and the overall molecular weight around 100,000 g/mol. The length of the EO chains for Ultra 1 (Cementa, Sweden) are shorter; FTIR and GC-MS analysis indicates a length between 25

Results and discussion

The adsorption of superplasticizers at the solid–liquid interface induces interparticle forces that are of both steric and electrostatic origins. The range and magnitude of each contribution depends on several parameters, e.g., adsorbed amount, degree of dissociation of the polyelectrolyte backbone, net surface charge density and ionic strength [19]. Having a reactive system, i.e., cement, where the surface chemistry of the solid phase and the electrolyte composition changes with time creates a

Summary and conclusions

A novel technique based on freeze granulation of a dense suspension of Mg(OH)2, followed by sintering at 1700 °C, was developed to synthesize spherical MgO probes for atomic force microscopy. Direct measurements of the forces between such a spherical probe and a flat MgO substrate showed that the surfaces attract each other in an electrolyte that mimics cement water at pH 10. However, the adsorption of superplasticizers eliminates the attraction and induces a repulsive force between the

Acknowledgements

The results presented here are part of a joint project that was run as a collaboration among six companies and two research institutes, and partly funded by NUTEK. The industries were Borregaard Lignotech, Perstorp Speciality Chemicals, Elotex, Cementa, Nobel Biocare and Doxa Certex, and the institutes were YKI, Institute for Surface Chemistry and the Swedish Ceramics Institute. All partners are gratefully acknowledged. Ozark Technical Ceramics are acknowledged for providing the flat MgO

References (22)

  • W.A. Ducker et al.

    Direct measurement of colloidal forces using an atomic force microscope

    Nature

    (1991)
  • Cited by (121)

    • Advances in surface properties characterization and modification for lignin

      2023, International Journal of Biological Macromolecules
    • Fresh and hardened properties of GGBS-contained cementitious composites using graphene and graphene oxide

      2021, Construction and Building Materials
      Citation Excerpt :

      ASTM C1875 [61] was followed in the present study for ICP-OES method. In the current investigation, the values provided by the literature [54,62] were used as a starting composition for calibration. Further, the composition of the calibration solution was modified with the results of the initial series to ensure coverage of the entire range of concentrations measured.

    View all citing articles on Scopus
    View full text