Colloid Stability: The Role of Surface Forces - Part I: 1 (Colloids and Interface Science) - Hardcover

Hardcover

ISBN 10: 3527314628 ISBN 13: 9783527314621

Publisher: Wiley-VCH, 2006

Synopsis

The first modern approach to relate fundamental research to the applied science of colloids, this series bridges academic research and practical applications, thus providing the information vital to both. Written by the very top scientists in their respective disciplines, this volume discusses the nature of various forces, as well as the influence of surface forces on the stability of dispersions, their measurement and role in adsorbed polymers and liquid films.
For surface, polymer and physicochemists, materials scientists, and chemical engineers.

About this title

About the Author

After finishing his PhD at Alexandria University, Tharwat Tadros was appointed lecturer in Physical Chemistry (1962-1966) at the same University. Between 1966 and 1969, he spent a sabbatical at the Agricultural University of Wageningen and T.N.O in Delft, the Netherlands. Thereafter he worked at I.C.I. and ZENECA until 1994, where he researched various fields of surfactants, emulsions, suspensions, microemulsions, wetting spreading and adhesion, and rheology. During that period he was also appointed visiting professor at Imperial College London, Bristol University and Reading University. In 1992, he was elected President of the International Association of Colloid and Interface Science. Since leaving ZENECA, Dr Tadros has worked as a consultant for various industries and also given several courses in his specialized field. He is the recipient of two medals from the Royal Society of Chemistry in the UK, and has more than 250 scientific papers to his name.

From the Back Cover

Colloidal chemistry involves a great number of disciplines, including chemistry, physics, materials sciences, and biology, with colloidal and surface systems having tremendous importance for nearly all industrial processes.
The first modern approach to relate fundamental research to the applied science of colloids, this series bridges academic research and practical applications, thus providing the information vital to both. Written by the very top scientists in their respective disciplines, this volume discusses the nature of various forces, as well as the influence of surface forces on the stability of dispersions, their measurement and role in adsorbed polymers and liquid films.
For surface, polymer and physicochemists, materials scientists, and chemical engineers.

Excerpt. © Reprinted by permission. All rights reserved.

Colloid Stability

The Role of Surface Forces, Part I

John Wiley & Sons

Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
All right reserved.
ISBN: 978-3-527-31462-1

Chapter One

General Principles of Colloid Stability and the Role of Surface Forces

Tharwat Tadros

Abstract

In this overview, the general principles of colloid stability are described with some emphasis on the role of surface forces. Electrostatic stabilization is the result of the presence of electrical double layers which, on approach of particles, interact, leading to repulsion that is determined by the magnitude of the surface or zeta potential and electrolyte concentration and valency. Combining this electrostatic repulsion with van der Waals attraction forms the basis of theory of colloid stability due to Deyaguin-Landau-Verwey-Overbeek (DLVO theory). This theory can explain the conditions of stability/instability of colloidal particles and it can predict the Schulze-Hardy rule. Direct confirmation of the DLVO theory came from surface force measurements using cross mica cylinders. Particles containing adsorbed or grafted nonionic surfactant or polymer layers produce another mechanism of stabilization, referred to as steric stabilization. This arises from the unfavorable mixing of the stabilizing layers when these are in good solvent conditions and the loss of entropy of the chains on significant overlap. The criteria of effective steric stabilization have been summarized. The flocculation of sterically stabilized dispersions can be weak and reversible or strong and irreversible depending on the conditions. Weak flocculation can occur when the adsorbed layer thickness is small (<5 nm), whereas strong (incipient) flocculation occurs when the solvency of the medium for the stabilizing chains become worse than that of a theta-solvent. The effect of addition of "free" non-adsorbing polymer is described in terms of the presence of a polymer-free zone (depletion zone) between the particles. This results in weak flocculation and equations are presented to describe the free energy of depletion attraction.

1.1 Introduction

The stability of colloidal systems is an important subject from both academic and industrial points of views. These systems include various types such as solid–liquid dispersions (suspensions), liquid–liquid dispersions (emulsions) and gas–liquid dispersions (foams). The colloid stability of such systems is governed by the balance of various interaction forces such as van der Waals attraction, double-layer repulsion and steric interaction. These interaction forces have been described at a fundamental level such as in the well know theory due to Deryaguin and Landau and Verwey and Overbeek (DLVO theory). In this theory, the van der Waals attraction is combined with the double-layer repulsion and an energy–distance curve can be established to describe the conditions of stability/instability. Several tests of the theory have been carried out using model colloid systems such as polystyrene latex. The results obtained showed the exact prediction of the theory, which is now accepted as the cornerstone of colloid science. Later, the origin of stability resulting from the presence of adsorbed or grafted polymer layers was established. This type of stability is usually referred to as steric stabilization and, when combined with the van der Waals attraction, energy–distance curves, could be established to describe the state of the dispersion.

This overview summarizes the principles of colloid stability with particular reference to the role of surface forces. For more details on the subject, the reader is referred to two recent texts by Lyklema.

1.2 Electrostatic Stabilization (DLVO Theory)

As mentioned above, the DLVO theory combines the van der Waals attraction with the double-layer repulsion. A brief summary of these interactions is given below and this is followed by establishing the conditions of stability/instability describing the influence of the various parameters involved.

1.2.1 Van der Waals Attraction

There are generally two ways of describing the van der Waals attraction between colloids and macrobodies: the microscopic and the macroscopic approach [8]. The microscopic approach is based on the assumption of additivity of London pair interaction energy. This predicts the van der Waals attraction with an accuracy of ~80%–90%. The macroscopic approach, which gives a more accurate evaluation of the van der Waals attraction, is based on the correlation between electric fluctuations of two macroscopic phases. However, this approach requires quantification of the dielectric dispersion data, which are available for only a limited number of systems. For this reason, most results on the van der Waals attraction are based on the microscopic approach that is briefly described below.

For a one-component system, individual atoms or molecules attract each other at short distances due to van der Waals forces. The latter may be considered to consist of three contributions: dipole–dipole (Keesom), dipole–induced dipole (Debye) and London dispersion interactions. For not too large separation distances between atoms or molecules, the attractive energy G_a is short range in nature and it is inversely proportional to the sixth power of the interatomic distance r:

G_a = ß₁₁/r⁶ (1)

where ß₁₁ is a constant referring to identical atoms or molecules.

Since colloidal particles are assemblies of atoms or molecules, the individual contributions have to be compounded. In this case, only the London interactions have to be considered, since large assemblies have no net dipole moment or polarization (both the Keesom and Debye forces which are vectors tend to cancel in such assemblies). The London (dispersion) interaction arises from charge fluctuations within an atom or molecule associated with the motion of its electrons. The London dispersion energy of interaction between atoms or molecules is short range as given by Eq. (1), whereby ß₁₁ is now the London dispersion constant (that is related to the polarizability of atoms or molecules involved). For an assembly of atoms or molecules, as is the case with colloidal particles, the van der Waals energy of attraction between two equal particles each of radius R, at a distance h in vacuum, is given by the expression

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

where s=(2R+h)/R and A₁₁ is the Hamaker constant, which is given by

A₁₁ = π²q²ß₁₁ (3)

where q is the number of atoms or molecules per unit volume.

For very short distances of separation (h<, Eq. (2) can be approximated by

G_A = RA₁₁/12h (4)

It is clear from comparison of Eqs. (1) and (4) that the magnitude of the attractive energy between macroscopic bodies (particles) is orders of magnitude larger than that between atoms or molecules. It is also long range in nature, increasing sharply at short distances of separation. G_A is also proportional to R and A₁₁.

The above expressions are for particles in vacuum and in the presence of a medium (solvent), the Hamaker constant A₁₂ of particles of material 1 dispersed in a medium of Hamaker constant A₂₂ is given by

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

Since with most disperse systems A₁₁ > A₂₂ then a dispersion medium A₁₂ is positive and two particles of the same material always attract each other.

The London dispersion forces exhibit the phenomenon of retardation implying that for large r the attraction decreases more rapidly with distance than at small r. This means that G_a (Eq. (1)) becomes proportional to r^-7. For intermediate distances there is a gradual transition from r^-6 to r^-7. This retardation is also reflected in the attraction between particles, whereby h^-1 should be h^-2 in Eq. (4). The retardation effect is automatically included in the macroscopic approach.

1.2.2 Double-layer Repulsion

Several processes can be visualized to account for charging suspended particles such as dissociation of surface groups (e.g. OH, COOH, SO₄Na) and adsorption of certain ionic species (such as surfactants). In all cases, charge separation takes place with some of the specifically adsorbed ions at the surface forming a surface charge which is compensated with unequal distribution of counter and co-ions. This forms the basis of the diffuse double layer due to Gouy and Chapman, which was later modified by Stern, who introduced the concept of the specifically adsorbed counter ions in the fixed first layer (the Stern plane). The potential at the surface ψ₀ decreases linearly to a value ψ_d (located at the center of the specifically adsorbed counter ions) and then exponentially with decrease in distance x, reaching zero in bulk solution. The Stern potential is sometimes equated with the measurable electrokinetic or zeta potential, ζ.

The extension of the double layer, referred to as double-layer thickness, depends on the electrolyte concentration and valency of the ions, as given by the reciprocal of the Debye-Hckel parameter:

1/κ = (ε_rε₀k/2n₀ Z²e²)^1/2 (6)

where ε_r is the relative permittivity, ε₀ is the permittivity of free space, k is the Boltzmann constant, T is the absolute temperature, n₀ is the number of ions of each type present in the bulk phase, Z is the valency of the ions and e is the electronic charge.

The parameter 1/κ increases with decrease in electrolyte concentration and decrease in the valency of the ions. For example, for 1:1 electrolyte (e.g. KCl), the double-layer thickness is 100 nm in 10^-5, 10 nm in 10^-3 and 1 nm in 10^-1 mol dm^-3. As we shall see later, this reflects the double-layer repulsion, which increases with decrease in electrolyte concentration.

When two particles each with an extended double layer with thickness 1/κ approach to a distance of separation such that double-layer overlap begins to occur (i.e. h<2/κ), repulsion occurs as a result of the following effect. Before overlap, i.e. h>2/κ, the two double layers can develop completely without any restriction and in this case the surface or Stern potential decays to zero at the mid-distance between the particles. However, when h<2/κ, the double layers can no longer develop unrestrictedly, since the limited space does not allow complete potential decay. In this case the potential at the mid-distance between the particles ψ_h/2 is no longer zero and repulsion occurs. The electrostatic energy of repulsion, G_el, is given by the following expression which is valid for κR<3:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (7)

Equation (7) shows that Gel decays exponentially with increase of h and it approaches zero at large h. The rate of decrease of G_el with increase in h depends on 1/κ: the higher the value of 1/κ, the slower is the decay. In other words, at any given h, G_el increases with increase in 1/κ, i.e. with decrease in electrolyte concentration and valency of the ions.

1.2.3 Total Energy of Interaction (DLVO Theory)

The total energy of interaction between two particles G_T is simply given by the sum of G_el and G_A at every h value:

G_T = G_el + G_A (8)

A schematic representation of the variation of G_el, G_A and G_T with h is shown in Fig. 1.1. As can be seen, G_el shows an exponential decay with increase in h, approaching zero at large h. G_A, which shows an inverse power law with h, does not decay to zero at large h. The G_T-h curve shows two minima and one maximum: a shallow minimum at large h that is referred to as the secondary minimum, G_sec, a deep minimum at short separation distance that is referred to as the primary minimum, G_primary, and an energy maximum at intermediate distances, G_max (sometimes referred to as the energy barrier). The value of G_max depends on the surface (Stern or zeta) potential and electrolyte concentration and valency.

The condition for colloid stability is to have an energy maximum (barrier) that is much larger than the thermal energy of the particles (which is of the order of kT). In general, one requires G_max >25kT. This is achieved by having a high zeta potential (>40 mV) and low electrolyte concentration (<10^-2mol dm^-3 1:1 electrolyte). By increasing the electrolyte concentration, G_max gradually decreases and eventually it disappears at a critical electrolyte concentration. This is illustrated schematically in Fig. 1.2, which shows the G_T-h curves at various 1/κ values for 1:1 electrolyte. At any given electrolyte concentration, G_max decreases with increase in the valency of electrolyte. This explains the poor stability in the presence of multivalent ions.

The above energy–distance curve (Fig. 1.1) explains the kinetic stability of colloidal dispersions. For the particles to undergo flocculation (coagulation) into the primary minimum, they need to overcome the energy barrier. The higher the value of this barrier, the lower is the probability of flocculation, i.e. the rate of flocculation will be slow (see below). Hence one can consider the process of flocculation as a rate phenomenon and when such a rate is low enough, the systems can stay stable for months or years (depending on the magnitude of the energy barrier). This rate increases with reduction of the energy barrier and ultimately (in the absence of any barrier) it becomes very fast (see below).

An important feature of the energy–distance curve in Fig. 1.1 is the presence of a secondary minimum at long separation distances. This minimum may become deep enough (depending on electrolyte concentration, particle size and shape and the Hamaker constant), reaching several kT units. Under these conditions, the system become weakly flocculated. The latter is reversible in nature and some deflocculation may occur, e.g. under shear conditions. This process of weak reversible flocculation may produce "gels", which on application of shear break up, forming a "sol". This process of sol<->gel transformation produces thixotropy (reversible time dependence of viscosity), which can be applied in many industrial formulations, e.g. in paints.

1.2.4 Stability Ratio

One of the useful quantitative methods to assess the stability of any dispersion is to measure the stability ratio W, which is simply the ratio between the rate of fast flocculation k₀ (in the absence of an energy barrier) to that of slow flocculation k (in the presence of an energy barrier):

W = k₀/k (9)

The rate of fast flocculation k₀ was calculated by Von Smoluchowski, who considered the process to be diffusion controlled. No interaction occurs between two colliding particles occurs until they come into contact, whereby they adhere irreversibly. The number of particles per unit volume n after time t is related to the initial number n0 by a second-order type of equation (assuming binary collisions):

n = n₀/1 + k₀n₀t (10)

where k₀ is given by

k₀ = 8πDR (11)

where D is the diffusion coefficient, given by the Stokes-Einstein equation:

D = kT/6πηR (12)

Combining Eqs. (11) and (12):

k₀ = 8kT/6η (13)

(Continues...)

Excerpted from Colloid Stability Copyright © 2007 by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Excerpted by permission of John Wiley & Sons. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.

Edition details

• Publisher: Wiley-VCH
• Publication date: 2006
• Language: English
• ISBN 10: 3527314628
• ISBN 13: 9783527314621
• Binding: Hardcover
• Edition number: 1
• Number of pages: 448
• Editor: Tadros Tharwat F.