Mechanical and thermodynamic properties of surfactant aggregates at the solid–liquid interface

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Abstract

Surfactants are widely used to stabilize colloidal systems in a variety of industrial applications through the formation of self-assembled aggregates at the solid–liquid interface. Previous studies have reported that the control of surfactant-mediated slurry stability can be achieved through the manipulation of surfactant chain length and concentration. However, a fundamental understanding of the mechanical and energetic properties of these aggregates, which may aid in the molecular-level design of these systems, is still lacking. In this study, experimentally measured force/distance curves between an atomic force microscope (AFM) tip and self-assembled surfactant aggregates on mica or silica substrates at concentrations higher than the bulk critical micelle concentration (CMC) were used to determine their mechanical and thermodynamic properties. The experimental curves were fitted to a model which describes the interaction between a hard sphere (tip) and a soft substrate (surfactant structures) based on a modified Hertz theory for the case of a thin elastic layer on a rigid substrate. The calculated mechanical properties were found to be in the same order of magnitude as those reported for rubber-like materials (e.g., polydimethylsiloxane (PDMS)). By integrating the force/distance curves, the energy required for breaking the surface aggregates was also calculated. These values are close to those reported for bulk–micelle formation.

Introduction

Self-assembled micellar surfactant aggregates are known to induce stability in particle systems under extremes in ionic strength and pH where polymeric and electrostatic dispersant schemes often fail [1], [2]. In a wide range of industrial particulate systems (e.g., pharmaceuticals, paints, coatings, and polishing slurries), surfactant structures are employed to ensure product quality and performance through inhibiting particle coagulation. The concentration regime at which these dispersants become effective largely depends on the surface properties of the adsorbent as well as the extent of intermolecular cohesion between the surfactant moieties [1], [2], [3], [4], [5], [6]. On hydrophilic substrates, there is a characteristic transition from attractive to repulsive surfactant structures, which occurs in the vicinity of critical micelle concentration (CMC) and corresponds to the evolution from hemimicellar to micellar aggregates at the solid–liquid interface.

The primary mechanism of surfactant-mediated stability on hydrophilic surfaces is attributed to the steric repulsion between the adsorbed micelle layer on the approaching surfaces [1], [2]. In many industrially significant processes, such as the mixing and flow of concentrated particulate suspensions and the extrusion of pastes, these layers interact with appreciable applied loads, leading to the deformation and possible destruction of the adsorbed surfactant aggregates. As processing environments become more extreme and performance tolerances become narrower, the mechanical and the thermodynamic properties of dispersant layers will become key parameters for the molecular-level design of advanced dispersion schemes. The yield strength and the elastic modulus of intervening steric layers influence the ensemble flow properties of contacting particulate systems by respectively controlling the mode and extent of deformation of the interacting surfaces. The energy required for the disintegration of surface micellar aggregates affects the upper limiting applied load for dispersant efficacy—above this value the micelles break and subsequent adhesion between the surfaces occurs.

Over the last decade, there have been a number of advances in using the atomic force microscope (AFM) for the measurement of the elastic response of deformable surfaces such as viscoelastic polymers [7], [8], [9], or biological samples [10], [11]. In these investigations the surfaces are compressed by indenting AFM tip into the sample and the resulting force curves are interpreted to investigate the substrate's elastic properties. A similar approach was adopted in this study to obtain the mechanical properties of the adsorbed surfactant aggregates. Since the deformable micellar layers in this investigation are only several nanometers thick, the indenting tip “feels” the rigid underlying substrate thereby augmenting the effective elastic modulus. To obtain the actual elastic modulus of the adsorbed self-assembled structures the finite layer effects are accounted for through the method proposed by Shull et al. [12], [13].

In the past, the majority of studies involving surfactants at the solid–liquid interface have focused on the self-assembly process [1], [2], [3], [4], [5], [6], adsorbed structure morphology [14], [15], [16], [17], [18], [19], or the interaction forces mediated by the surfactant structures [1], [20]. No experimental or theoretical efforts have been made to evaluate the mechanical and thermodynamic properties of these surface aggregates. The only exception is an article by Subramanian and Ducker [21], where specific (per unit area) energy of dodecyltrimethylammonium bromide aggregates on silica surfaces is evaluated from experimental force/distance curves using a glass particle (diameter = 8 μm) as the probe. This investigation provides a fundamental approach for estimating and experimentally deriving the yield strength and elastic modulus of adsorbed surfactant micelles as well as the energy required to destroy these structures. In a previous study, the maximum steric repulsion force has been shown to be directly proportional to the surfactant hydrocarbon chain length in a homologue [1]. In the present investigation, this finding is extended to the mechanical and thermodynamic properties of the surfactant aggregates. Additionally, surface-inspired morphological modifications of the adsorbate structure are shown to influence the effective mechanical and energetic properties of the dispersant layers elucidating the significance of micellar conformation on stability.

Section snippets

Experimental

Trimethylammonium bromide surfactants of varying chain length (C10−16TAB) and 99% purity were obtained from TCI America. The sodium dodecyl sulfate (SDS) was 99% pure as received from Aldrich Chemical Co. Surface tension versus log concentration curves for the surfactants used in this study did not exhibit a depression in the vicinity of the CMC, indicating that the reagents used were of high purity. The CMCs were also in good agreement with reported literature values [3]. All other reagents

Theory

Fig. 2 schematically illustrates the model employed to consider the interaction of the AFM tip and the adsorbed surfactant layer. To simplify the mathematical treatment of this phenomena the geometries of the AFM tip and the surfactant layer are idealized as a rigid sphere and a planar homogeneous elastic layer on top of a rigid substrate, respectively. R represents the radius of the tip and h0 is the undisturbed adsorbed layer thickness. As the AFM tip is pressed against the surfactant layer

Results and discussion

Fig. 3, Fig. 4 represent experimental force/distance curves (F vs h) for the interaction between the AFM tip and self-assembled surfactant aggregates on silica and on mica substrates, respectively. These curves were generated in solutions well above the bulk critical micelle concentration (CMC) in order to ensure that a saturated layer of micelle-like structures is present at the solid–liquid interface [1]. As mentioned earlier, 0.1 M NaCl was used as a background electrolyte to fully screen

Conclusions

In the current study, it is illustrated that experimental force–distance curves can be used to determine the fundamental mechanical and thermodynamic properties of micellar dispersant layers. The elastic modulus and yield strength of these intervening steric layers significantly impact particle–particle interactions during contact by modifying the mode and extent of deformation of the adsorbed surface layer under a given loading condition. Similarly, the energy required to destroy these surface

Acknowledgements

The authors acknowledge the financial support of the Engineering Research Center (ERC) for Particle Science and Technology at the University of Florida, the National Science Foundation (NSF) (Grant EEC-94-02989), and the Industrial Partners of the ERC for support of this research. The experimental help of Mr. Madhavan Esayanur is also acknowledged. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those

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