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Reduction in the Repulsive Forces between Two Charged Surfaces in Aqueous Solutions containing Salts by a Liquid Flow Hayato Kawakami, and Cathy E. McNamee Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01336 • Publication Date (Web): 03 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018
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Title: “Reduction in the Repulsive Forces between Two Charged Surfaces in Aqueous
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Solutions containing Salts by a Liquid Flow”
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Hayato Kawakami1 and Cathy E. McNamee1,*
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Shinshu University, Tokida 3-15-1, Ueda, Nagano 386-8567, Japan.
Department of Chemistry and Materials, Faculty of Textile Science and Technology,
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* Corresponding author:
email:
[email protected] Tel.: +81-(0)268-21-5585
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Key-words: Atomic Force Microscopy; flow rate; water; NaCl; MgCl2.6H2O; silicon;
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silica.
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Abstract
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In spite of the fact that a flow is often present in the liquid in which charged
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particles are dispersed, the effect of a flow on the forces controlling the dispersion is not
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clear. Here, we used a combined Atomic Force Microscope-peristaltic pump system to
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determine the effect of a flow in aqueous solutions between a negatively charged silica
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particle and a negatively charged silicon wafer on the forces in the system. The effect
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of a flow on the forces in water or aqueous solutions of NaCl or MgCl2.6H2O was
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studied for salt concentrations lower than the concentrations needed to invert the charge
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of the silica and silicon surfaces. This was done, in order to prevent the formation of a
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reversed flow in the system due to a charge inversion of the silica surface. A flow was
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seen to decrease the inter-surface repulsive forces, if the water contained salt (NaCl or
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MgCl2.6H2O). An increased bulk salt concentration was also seen to decrease the
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repulsive forces further in the presence of a liquid flow. The surface potentials and
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effective ionic concentrations of the systems were determined by comparing the
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experimental curves with the theoretically calculated ones. The surface potentials and
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effective ionic concentrations were seen to decrease and increase, respectively, as the
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flow rate and bulk salt concentrations were increased. This change was explained by the
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shrinking of the diffuse layers by the liquid flow, due to part of the diffuse layer being
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washed away by the flowing liquid.
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Introduction
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The forces acting between particles in an aqueous solution determine whether
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those particles will disperse or aggregate, where attractions will cause the particles to
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aggregate and repulsions will cause the particles to disperse. The ability to control these
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forces allows the physical properties of the systems to be controlled and therefore the
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reliability of applications using particles to be improved. In real systems of particles in
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aqueous solutions, the liquid often contains a flow. For example, flow would be present,
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if the system is mixed or if there are vibrations. If heat is applied to the system, then
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Marangoni flow may result.１ A difference in the concentration of ions or a pressure
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difference in the system may also cause a flow.２ In spite of this, the forces acting
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between two particles in solutions are usually experimentally investigated using the
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Atomic Force Microscope (AFM) or Surface Force Apparatus３,４,５ in the absence of a
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flow in the liquid. The effect of flow has indirectly been studied by determining the
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effect of the flow brought about in the liquid by the movement of the two surfaces
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during the force measurements or the depletion of material from the area between the
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two surfaces on force-separation distance curves.６,７,８ The magnitude of this flow,
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however, is smaller than that of the flow existing in real systems. The effect of a non-
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negligible flow in the liquid on the inter-particle forces is therefore still unclear.
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Electroosmotic studies have shown that if an external electric field is applied
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tangentially to a charged surface in an aqueous solution containing electrolytes, then the
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ions in the electrical double layer will move under the influence of the applied electric
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field. Liquid will move with these ions, causing the liquid to flow due to a viscous force,
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i.e. electroosmotic flow occurs.９ As the electrical double layer influences the magnitude
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and range of the electrostatic force acting between two surfaces, a change in the
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electrical double layer due to the presence of an electroosmotic flow may also affect the
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forces acting between two charged surfaces in solution. The presence of a flow in the
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liquid between two macroscopic charged surfaces would cause liquid to enter in and
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flow through the space between the two surfaces. In a similar way as the application of
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an electric field may create an electroosmotic flow that affects the electrical double
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layer, the introduction of a flow in the electrolyte solution between two charged surfaces
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may cause ions in the electrical double layer to move and the density of ions to change.
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This change may affect the forces acting between the two surfaces in a liquid.
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In previous studies, the presence of a flow has been shown to cause a negatively
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charged latex particle moving parallel to a negatively charged glass wall in the presence
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of a liquid to lift off from the wall. １０ This phenomenon has been called “electrokinetic
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lift”. The addition of salt to an aqueous solution or a change in the liquid viscosity was
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shown to affect the magnitude of the electrokinetic lift. １０ This lift was explained by a
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hydrodynamic stress arising from an electroviscous flow along the surface of the
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particle.１１,１２ The forces responsible for these changes by the liquid flow are, however,
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still not clear.
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Flow has also been reported to alter the interfacial chemistry of silica surfaces in
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water.１３ The presence of a flow in the water was seen to modify the surface charge of
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silica by alterations in the near surface ionic distributions. This change was explained
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by the perturbation of the water molecules at the silica-water interface by the flow. As a
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flow in the liquid can affect the interfacial chemistry of silica surfaces in water, a flow
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in the liquid between two charged surfaces is expected to affect the inter-surface forces
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of the system.
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The properties of interfacial water have also been reported to be different than
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those of bulk water.１４ Additionally, the properties of water confined between two
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surfaces have been shown to possess different structural and dynamical properties than
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the bulk, where the type of the solid surfaces influences these properties.１５ The charge
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density near the surface of a silica pore containing salt has been reported to change in
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the presence of an electrodynamic flow.１６ The conductance of silica nano-channels
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containing aqueous solutions of potassium chloride has also been shown to increase
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relative to the bulk properties, when the ionic concentration is low.１７ This result was
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explained by electrostatic effects of the channel surface charge on the fluid. High
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concentrations of NaCl (concentrations ≥4 M１８) or MgCl2 (concentrations ≥0.4 M １９)
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have been shown to invert the charge of a silica surface in aqueous solution, due to the
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surface charge overcompensation by the cations, i.e. overscreening. Such a charge
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inversion has been reported to reverse the direction of the electro-osmotic flow in a
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channel.２０,２１ As the forces acting between two surfaces separated by a liquid are
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known to be affected by the charge density of the surface and the properties of the
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solution, the forces between two charged surfaces are thought to change in the presence
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of a liquid flow and in the presence of salts.
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The change in the forces acting between a charged particle and a charged plate
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in liquid due to the introduction of a flow in the liquid can be better understood, if we
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measure the forces between a charged particle and a charged substrate separated by
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liquid in the absence and presence of a liquid flow. The effect of the ionic concentration
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on the changes in the forces due to the flow can be studied by using pure water and
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aqueous solutions containing varying amounts of salts in the water. The effect of flow
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on the forces between charged surfaces in aqueous salt solutions can be understood
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more easily, if salt concentrations lower than the concentrations needed to invert the
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charge of the surface are chosen. This will prevent the formation of a reversed flow in
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the system due to the charge inversion of the silica surfaces, allowing the effect of flow
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on the forces to be more easily understood.
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In this study, we investigated how the presence of a flow in a liquid affected the
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forces and physical properties of a system of charged particles in aqueous solutions.
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This was demonstrated by using a combined Atomic Force Microscope (AFM)-
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peristaltic pump system to experimentally determine the effect of a flow between a
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charged silica particle attached to an AFM cantilever (“silica probe”) and a silicon
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wafer in water and aqueous solutions containing salts on the forces in the system. NaCl
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and MgCl2.6H2O were chosen as the salts, in order to determine whether the type and
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valency of the ions in the solution influences how a flow affects the forces.
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Concentrations of NaCl and MgCl2.6H2O (concentration≤10 mM) lower than the
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concentrations reported to invert the charge of a silica surface were chosen, １８,１９ in
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order to see the effect of flow on the forces more clearly. We determined the effect of
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the NaCl or MgCl2.6H2O concentration and the liquid flow strength on the force-
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separation distance curves. This study will give more information as to how a flow can
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affect the interactions between charged particles dispersed in an aqueous solution.
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Experimental: Materials
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Sodium chloride (99.5% purity, Wako Pure Chemical Industries, Japan),
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Magnesium chloride.hexahydrate (MgCl2.6H2O, JIS Special Grade, Wako Pure
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Chemical Industries, Japan), ethanol (JIS Special Grade, Wako Pure Chemical
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Industries, Japan) and acetone (JIS Special Grade, Wako Pure Chemical Industries,
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Japan) were used. The water was distilled and de-ionised (Milli-Q Direct, USA) to give
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a resistivity of 18.2 MΩ cm and a total organic content
