Article Cite This: Langmuir XXXX, XXX, XXX−XXX
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Measurement of Forces between Supported Cationic Bilayers by Colloid Probe Atomic Force Microscopy: Electrolyte Concentration and Composition Matthew Leivers,†,‡ John M. Seddon,† Marc Declercq,¶ Eric Robles,§ and Paul Luckham*,‡ †
Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom ¶ The Procter & Gamble Company, Brussels Innovation Center, 1853 Strombeek Bever Temselaan 100, 1853 Grimbergen, Belgium § The Procter & Gamble Company, Newcastle Innovation Center, Whitley Road, Longbenton, Newcastle-Upon-Tyne NE12 9TS, United Kingdom
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ABSTRACT: The interactions between supported cationic surfactant bilayers were measured by colloidal probe atomic force spectroscopy, and the effect of different halide salts was investigated. Di(alkylisopropylester)dimethylammonium methylsulfate (DIPEDMAMS) bilayers were fabricated by the vesicle fusion technique on muscovite mica. The interactions between the bilayers were measured in increasing concentrations of NaCl, NaBr, NaI, and CaCl2. In NaCl, the bilayer interactions were repulsive at all concentrations investigated, and the Debye length and surface potential were observed to decrease with increasing concentration. The interactions were found to follow the electrical double layer (EDL) component of DLVO theory well. However, van der Waals forces were not detected; instead, a strong hydration repulsion was observed at short separations. CaCl2 had a similar effect on the interactions as NaCl. NaBr and NaI were observed to be more efficient at decreasing surface potential than the chloride salts, with the efficacy increasing with the ionic radius.
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the top of the aqueous phase or sinks to the bottom.1 With vesicle systems, as opposed to hard-sphere systems, there is also the possibility of vesicle fusion or the transformation into lamellar sheets.2 The stability of a colloidal vesicle dispersion is determined by the interactions between the vesicles within the dispersion. At small separations, assuming the vesicles are of identical composition, the interactions are generally dominated by hydration forces and attractive and short-range van der Waals forces.3 To counteract this, the majority of surfactant vesicle dispersions employ surfactants with ionic head groups forming charged vesicles. This creates an electrical double layer (EDL) at the surfactant−water interface, with its associated diffuse layer. The osmotic pressure generated when these diffuse layers overlap, e.g., when two charged vesicles approach each other, creates a repulsive interaction between the vesicles. This EDL interaction is comparatively long-range and tends to dominate the van der Waals interactions at large separations. The additive combination of the van der Waals and EDL interactions is known as the Derjaguin−Landau−Verway− Overbeek (DLVO) theory.4,5
INTRODUCTION Surfactant vesicle dispersions have many industrial applications and are widely used in several sectors, including healthcare, cosmetics, homecare, food, and other commercial products. The majority of these products are complex colloidal dispersions composed of multiple components and additives to fine-tune the physical and commercial properties of the dispersion. The formulation of these products requires precise control of the colloidal properties and an understanding of the stability of the colloidal system. The addition of perfumes, salts, polymers, dyes, and polyelectrolytes to these systems in commercial products makes the stability of these systems difficult to predict and to control. With the increasing awareness of the impact of plastic waste on our environment and recent initiatives to reduce its use, there have been movements within industry toward the use of more highly concentrated vesicle dispersions. This makes the understanding and prediction of the stability and phase behavior of these systems even more important. To be able to predict the stability of these systems, the interactions between the surfactant vesicles must be understood. Vesicle dispersions become unstable when the forces between them are not sufficiently repulsive. This can lead to flocculation, forming vesicle aggregates or flocs. These aggregates can lead to phase separation if the floc floats to © XXXX American Chemical Society
Received: October 22, 2018 Revised: December 7, 2018 Published: December 18, 2018 A
DOI: 10.1021/acs.langmuir.8b03555 Langmuir XXXX, XXX, XXX−XXX
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resulting in a decrease in the range of the repulsion measured between the bilayers. Halide salts further down the periodic table were observed to be more effective at reducing the repulsion between bilayers than lower atomic number halides. The results were compared to current DLVO models and suggested that the larger halide salts had a greater impact on the surface potential than the smaller halide salts. Counterionspecific effects are not accounted for by the standard DLVO theory; therefore, other models of the EDL are discussed in an attempt to explain this observation.
The DLVO interactions are generally good enough to describe the interactions within a simple colloidal system and to predict its stability.6 To optimize the desired properties of these dispersions for commercial applications, such as fabric conditioning, components such as salts, polymers, dyes, and perfumes are added to the vesicle system. These additives can interact with the vesicles and affect the interactions between vesicles and between individual surfactant molecules within the vesicles, perturbing the bilayer itself. As systems become more complex, the DLVO theory may not be sufficient to describe the interactions between the vesicles. Additional interactions, which are not accounted for by DLVO theory, such as depletion attraction, polymer bridging, hydration effects, and steric interactions, as well as any ion-specific effects play a role.7−10 With the push toward highly concentrated vesicle dispersions, the vesicle’s elasticity and deformation may also play a role in the intervesicle interactions.11,12 Additives acting on the bilayer rather than just the interaction between them may play a role, as well as any chemical degradation of the surfactant monomers. These also would not be accounted for in the DLVO forces. Supported bilayers have been used throughout the literature as models for lipid and surfactant bilayers and analogues for vesicle and liposome membranes.13−22 They enable the measurement of interactions between bilayers and thus the measurement of DLVO forces, depletion, steric, and other interactions between bilayers.23−26 However, due to the nature of supported bilayers, it is not possible to investigate affects arising from membrane bending and fluctuations. Supported bilayers have been used numerous times in the literature as model systems with surface force apparatus (SFA) and colloid probe atomic force spectroscopy to determine the interactions between surfactant vesicles in solution.27−34 In this work we directly measured the interactions between two cationic surfactant bilayers of di(alkylisopropylester)dimethylammonium methylsulfate (DIPEDMAMS) Figure 1
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EXPERIMENTAL SECTION
The cationic surfactant DIPEDMAMS was provided by Procter and Gamble, synthesized using an industrial fatty acid mixture of C18, C16, and C18:1 chains; therefore, the aliphatic tails are a statistical mixture of the three fatty acids; see Figure 1. NaCl, NaBr, NaI, and CaCl2 were obtained from Sigma-Aldrich. All solutions were prepared with deionized water,
