Measurement of forces between supported cationic bilayers by colloid probe atomic force microscopy: Electrolyte concentration and composition Matthew Leivers,y,z John M. Seddon,y Marc Declercq,{ Eric Robles,x and Paul Luckham∗,z yDepartment of Chemistry, Imperial College London, London, UK zDepartment of Chemical Engineering, Imperial College London, London, UK {The Procter & Gamble Company, Brussels Innovation Center, 1853 Strombeek Bever Temselaan 100, Belgium xThe Procter & Gamble Company, Newcastle Innovation Center, Whitley Road, Longbenton, Newcastle-Upon-Tyne NE12 9TS, UK E-mail: [email protected] Phone: +44 (0)2075 945583 Abstract The interactions between supported cationic surfactant bilayers were measured by colloidal probe atomic force spectroscopy and the eect of dierent halide salts was investigated. Di(alkyl iso-propyl ester) dimethyl ammonium methylsulfate (DIPED- MAMS) bilayers were fabricated by the vesicle fusion technique on muscovite mica. The interactions between the bilayers were measured in increasing concentrations of 1 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 eect on the interactions as NaCl. NaBr and NaI were observed to be more ecient at decreasing the surface potential than the chloride salts, with the ecacy increasing with the ionic radius. Introduction Surfactant vesicle dispersions have many industrial applications and are widely used in sev- eral sectors, including healthcare, cosmetics, homecare, food and other commercial prod- ucts. The majority of these products are complex colloidal dispersions composed of multiple components, and additives to ne tune the physical and the commercial properties of the dispersion. The formulation of these products requires precise control of the colloidal proper- ties 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 dicult 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 towards the use of more highly concentrated vesicle dispersions. This makes the understanding and prediction of the stability and phase behaviour of these systems evermore 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 suciently repulsive. This can lead to occulation forming vesicle aggregates or ocs. These aggregates can lead to phase separation if the oc oats to 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 2 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 the 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 at the surfactant water interface, with its associated diuse layer. The osmotic pressure generated when these diuse layers overlap, e.g. when two charged vesicles approach each other, creates a repulsive interaction between the vesicles. This elec- trical double layer (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 Figure 1: Structure of the DIPEDMAMS surfactant, where R is a fatty alkyl chain originating from a fatty acid mixture of C16, C18 and C18:1 The DLVO interactions are generally good enough to describe the interactions within a simple colloidal system and to predict its stability.6 To optimise 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, aect the interactions between vesicles, and between individual 3 surfactant molecules within the vesicles, perturbing the bilayer itself. As systems become more complex the DLVO theory may not be sucient to describe the interactions between the vesicles. Additional interactions, which are not accounted for by DLVO theory, such as depletion attraction, polymer bridging, hydration eects, and steric interactions, as well as any ion specic eects play a role.710 With the push towards 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.1322 They enable the measurement of interactions between bilayers and thus the measurement of DLVO forces, depletion, stearic and other interactions between bilayers.2326 However, due to the nature of supported bilayers, it is not possible to investigate aects arising from membrane bending and uctuations Supported bilayers have been used numerous times in the literature as model systems with Surface Force Apparatus (SFA) and colloid probe atomic force spectroscopy for determining the interactions between surfactant vesicles in solution.2734 In this work we directly measured the interactions between two cationic surfactant bilay- ers of di(alkyl iso-propyl ester) dimethyl ammonium methylsulfate (DIPEDMAMS) gure 1 using colloid probe atomic force spectrometry. We investigated the eect of increasing NaCl concentrations on the interactions and how dierent sodium halide salts aect the vesicle vesicle interactions. Atomic force microscopy was also used to assess the impact of the halide ions and their concentration on the topography of the surfactant bilayers formed on the solid substrate. All halide salts had the eect of screening the vesicle's electrical charges from each other, 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 eective at reducing the repulsion between bilayers than lower atomic number halides. The results were compared 4 to current DLVO models, and suggested that the larger halide salts had a greater impact on the surface potential than the smaller halide salts. Counter ion specic eects are not accounted for by the standard DLVO theory, therefore other models of the electrical double layer are discussed in attempt to explain this observation. Experimental The cationic surfactant, di(alkyl iso-propyl ester) dimethyl ammonium methylsulfate (DIPED- MAMS) was provided by Procter and Gamble. Synthesised using an industrial fatty acid mixture of C18, C16 and C18:1 chains, therefore the aliphatic tails are a statistical mix- ture of the three fatty acids see gure 1. NaCl, NaBr, NaI and CaCl2 were obtained from Sigma-Aldrich. All solutions were prepared with deionised water, <18mS, ltered through an activated carbon lter to remove organic impurities. Preparation of supported lipid bilayers Supported surfactant bilayers were formed by the vesicle deposition technique.35 The vesicle dispersion was fabricated by melting together 0.9mg of the surfactant and 0.1mg of perillyl alcohol then suspending in 1ml of deionised water and sonicating the sample in an ultrasonic bath for 20 minutes, until the turbidity had dissipated indicating the formation of vesicles. Perillyl alcohol was added to lower the uid to gel transition temperature of the surfactant system to facilitate the bilayer formation. Bilayers were formed by pipetting 100µl of the vesicle dispersion onto freshly cleaved mica and allowing the system to equilibrate at room temperature for 40 minutes. The system was then ushed with 10 times the volume of deionised water to remove any excess undeposited vesicles from the mica. For the colloid probe atomic force spectroscopy, the probe was brought in contact with the freshly cleaved mica and withdrawn by 500µm before the vesicle dispersion was added allowing the formation of surfactant bilayers on both the probe and 5 Cantilever Glass Surfactant Bilayer Mica Figure 2: Schematic of the colloidal probe experimental set up 6 the at substrate. Atomic Force Microscopy All measurements were made using a JPK Nano wizard 4 AFM and analysed using the JPK data processing software. All probes were cleaned prior to use by UV/ozone plasma for 15 minutes. Colloid Probe