Subscriber access provided by Technical University of Munich University Library
Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Title: “Reduction in the Repulsive Forces between Two Charged Surfaces in Aqueous
2
Solutions containing Salts by a Liquid Flow”
3 4
Hayato Kawakami1 and Cathy E. McNamee1,*
5 6 7
1
8
Shinshu University, Tokida 3-15-1, Ueda, Nagano 386-8567, Japan.
Department of Chemistry and Materials, Faculty of Textile Science and Technology,
9 10 11
* Corresponding author:
email:
[email protected] Tel.: +81-(0)268-21-5585
12 13 14 15
Key-words: Atomic Force Microscopy; flow rate; water; NaCl; MgCl2.6H2O; silicon;
16
silica.
1 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Abstract
1 2
In spite of the fact that a flow is often present in the liquid in which charged
3
particles are dispersed, the effect of a flow on the forces controlling the dispersion is not
4
clear. Here, we used a combined Atomic Force Microscope-peristaltic pump system to
5
determine the effect of a flow in aqueous solutions between a negatively charged silica
6
particle and a negatively charged silicon wafer on the forces in the system. The effect
7
of a flow on the forces in water or aqueous solutions of NaCl or MgCl2.6H2O was
8
studied for salt concentrations lower than the concentrations needed to invert the charge
9
of the silica and silicon surfaces. This was done, in order to prevent the formation of a
10
reversed flow in the system due to a charge inversion of the silica surface. A flow was
11
seen to decrease the inter-surface repulsive forces, if the water contained salt (NaCl or
12
MgCl2.6H2O). An increased bulk salt concentration was also seen to decrease the
13
repulsive forces further in the presence of a liquid flow. The surface potentials and
14
effective ionic concentrations of the systems were determined by comparing the
15
experimental curves with the theoretically calculated ones. The surface potentials and
16
effective ionic concentrations were seen to decrease and increase, respectively, as the
17
flow rate and bulk salt concentrations were increased. This change was explained by the
18
shrinking of the diffuse layers by the liquid flow, due to part of the diffuse layer being
19
washed away by the flowing liquid.
20
2 ACS Paragon Plus Environment
Page 2 of 36
Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Introduction
2
The forces acting between particles in an aqueous solution determine whether
3
those particles will disperse or aggregate, where attractions will cause the particles to
4
aggregate and repulsions will cause the particles to disperse. The ability to control these
5
forces allows the physical properties of the systems to be controlled and therefore the
6
reliability of applications using particles to be improved. In real systems of particles in
7
aqueous solutions, the liquid often contains a flow. For example, flow would be present,
8
if the system is mixed or if there are vibrations. If heat is applied to the system, then
9
Marangoni flow may result.1 A difference in the concentration of ions or a pressure
10
difference in the system may also cause a flow.2 In spite of this, the forces acting
11
between two particles in solutions are usually experimentally investigated using the
12
Atomic Force Microscope (AFM) or Surface Force Apparatus3,4,5 in the absence of a
13
flow in the liquid. The effect of flow has indirectly been studied by determining the
14
effect of the flow brought about in the liquid by the movement of the two surfaces
15
during the force measurements or the depletion of material from the area between the
16
two surfaces on force-separation distance curves.6,7,8 The magnitude of this flow,
17
however, is smaller than that of the flow existing in real systems. The effect of a non-
18
negligible flow in the liquid on the inter-particle forces is therefore still unclear.
19 20
Electroosmotic studies have shown that if an external electric field is applied
21
tangentially to a charged surface in an aqueous solution containing electrolytes, then the
22
ions in the electrical double layer will move under the influence of the applied electric
23
field. Liquid will move with these ions, causing the liquid to flow due to a viscous force,
24
i.e. electroosmotic flow occurs.9 As the electrical double layer influences the magnitude
3 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
and range of the electrostatic force acting between two surfaces, a change in the
2
electrical double layer due to the presence of an electroosmotic flow may also affect the
3
forces acting between two charged surfaces in solution. The presence of a flow in the
4
liquid between two macroscopic charged surfaces would cause liquid to enter in and
5
flow through the space between the two surfaces. In a similar way as the application of
6
an electric field may create an electroosmotic flow that affects the electrical double
7
layer, the introduction of a flow in the electrolyte solution between two charged surfaces
8
may cause ions in the electrical double layer to move and the density of ions to change.
9
This change may affect the forces acting between the two surfaces in a liquid.
10 11
In previous studies, the presence of a flow has been shown to cause a negatively
12
charged latex particle moving parallel to a negatively charged glass wall in the presence
13
of a liquid to lift off from the wall. 10 This phenomenon has been called “electrokinetic
14
lift”. The addition of salt to an aqueous solution or a change in the liquid viscosity was
15
shown to affect the magnitude of the electrokinetic lift. 10 This lift was explained by a
16
hydrodynamic stress arising from an electroviscous flow along the surface of the
17
particle.11,12 The forces responsible for these changes by the liquid flow are, however,
18
still not clear.
19 20
Flow has also been reported to alter the interfacial chemistry of silica surfaces in
21
water.13 The presence of a flow in the water was seen to modify the surface charge of
22
silica by alterations in the near surface ionic distributions. This change was explained
23
by the perturbation of the water molecules at the silica-water interface by the flow. As a
24
flow in the liquid can affect the interfacial chemistry of silica surfaces in water, a flow
4 ACS Paragon Plus Environment
Page 4 of 36
Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
in the liquid between two charged surfaces is expected to affect the inter-surface forces
2
of the system.
3 4
The properties of interfacial water have also been reported to be different than
5
those of bulk water.14 Additionally, the properties of water confined between two
6
surfaces have been shown to possess different structural and dynamical properties than
7
the bulk, where the type of the solid surfaces influences these properties.15 The charge
8
density near the surface of a silica pore containing salt has been reported to change in
9
the presence of an electrodynamic flow.16 The conductance of silica nano-channels
10
containing aqueous solutions of potassium chloride has also been shown to increase
11
relative to the bulk properties, when the ionic concentration is low.17 This result was
12
explained by electrostatic effects of the channel surface charge on the fluid. High
13
concentrations of NaCl (concentrations ≥4 M18) or MgCl2 (concentrations ≥0.4 M 19)
14
have been shown to invert the charge of a silica surface in aqueous solution, due to the
15
surface charge overcompensation by the cations, i.e. overscreening. Such a charge
16
inversion has been reported to reverse the direction of the electro-osmotic flow in a
17
channel.20,21 As the forces acting between two surfaces separated by a liquid are
18
known to be affected by the charge density of the surface and the properties of the
19
solution, the forces between two charged surfaces are thought to change in the presence
20
of a liquid flow and in the presence of salts.
21 22
The change in the forces acting between a charged particle and a charged plate
23
in liquid due to the introduction of a flow in the liquid can be better understood, if we
24
measure the forces between a charged particle and a charged substrate separated by
5 ACS Paragon Plus Environment
Langmuir 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
liquid in the absence and presence of a liquid flow. The effect of the ionic concentration
2
on the changes in the forces due to the flow can be studied by using pure water and
3
aqueous solutions containing varying amounts of salts in the water. The effect of flow
4
on the forces between charged surfaces in aqueous salt solutions can be understood
5
more easily, if salt concentrations lower than the concentrations needed to invert the
6
charge of the surface are chosen. This will prevent the formation of a reversed flow in
7
the system due to the charge inversion of the silica surfaces, allowing the effect of flow
8
on the forces to be more easily understood.
9 10
In this study, we investigated how the presence of a flow in a liquid affected the
11
forces and physical properties of a system of charged particles in aqueous solutions.
12
This was demonstrated by using a combined Atomic Force Microscope (AFM)-
13
peristaltic pump system to experimentally determine the effect of a flow between a
14
charged silica particle attached to an AFM cantilever (“silica probe”) and a silicon
15
wafer in water and aqueous solutions containing salts on the forces in the system. NaCl
16
and MgCl2.6H2O were chosen as the salts, in order to determine whether the type and
17
valency of the ions in the solution influences how a flow affects the forces.
18
Concentrations of NaCl and MgCl2.6H2O (concentration≤10 mM) lower than the
19
concentrations reported to invert the charge of a silica surface were chosen, 18,19 in
20
order to see the effect of flow on the forces more clearly. We determined the effect of
21
the NaCl or MgCl2.6H2O concentration and the liquid flow strength on the force-
22
separation distance curves. This study will give more information as to how a flow can
23
affect the interactions between charged particles dispersed in an aqueous solution.
24
6 ACS Paragon Plus Environment
Page 6 of 36
Page 7 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Langmuir
1
Experimental: Materials
2
Sodium chloride (99.5% purity, Wako Pure Chemical Industries, Japan),
3
Magnesium chloride.hexahydrate (MgCl2.6H2O, JIS Special Grade, Wako Pure
4
Chemical Industries, Japan), ethanol (JIS Special Grade, Wako Pure Chemical
5
Industries, Japan) and acetone (JIS Special Grade, Wako Pure Chemical Industries,
6
Japan) were used. The water was distilled and de-ionised (Milli-Q Direct, USA) to give
7
a resistivity of 18.2 MΩ cm and a total organic content