Dispersion of Powders

in Liquids and Stabilization of Suspensions

Tharwat F. Tadros

• Publisher: Wiley-VCH
• Year: 2012
• Print ISBN:
  9783527329410
• eBook ISBN:
  9783527656615
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physical & Theoretical Chemistry

Teaching the fundamental knowledge required for successful dispersion of powders in a liquid, this book covers a host of topics -- from recent advances to industrial applications.
In 15 chapters it supports formulation chemists in preparing a suspension in a more rational way, by applying the principles of colloid and interface science, while at the same time enabling the research scientist to discover new methods for preparing stable suspensions.
Essential reading for those working in the pharmaceutical, cosmetic, food, paint, ceramic and agricultural industries.

1

General Introduction

The dispersion of powders into liquids is a process that occurs in many industries of which we mention paints, dyestuffs, paper coatings, printing inks, agrochemicals, pharmaceuticals, cosmetics, food products, detergents, and ceramics. The powder can be hydrophobic such as organic pigments, agrochemicals, and ceramics or hydrophilic such as silica, titania, and clays. The liquid can be aqueous or nonaqueous.

The dispersion of a powder in a liquid is a process whereby aggregates and agglomerates of powders are dispersed into “individual” units, usually followed by a wet milling process (to subdivide the particles into smaller units) and stabilization of the resulting dispersion against aggregation and sedimentation [1–3]. This is illustrated in Figure 1.1.

Figure 1.1 Schematic representation of the dispersion process.

The powder is considered hydrophobic if there is no affinity between its surface and water, for example, carbon black, many organic pigments, and some ceramic powders such as silicon carbide or silicon nitride. In contrast a hydrophilic solid has strong affinity between its surface and water, for example, silica, alumina, and sodium montmorillonite clay.

1.1 Fundamental Knowledge Required for Successful Dispersion of Powders into Liquids

Several fundamental processes must be considered for the dispersion process and these are summarized below.

1.1.1 Wetting of Powder into Liquid

This is determined by surface forces whereby the solid/air interface characterized by an interfacial tension (surface energy) γ_SA is replaced by the solid/liquid interface characterized by an interfacial tension (surface energy) γ_SL [1]. Polar (hydrophilic) surfaces such as silica or alumina have high surface energy and hence they can be easily wetted in a polar liquid such as water. In contrast nonpolar (hydrophobic) surfaces such as carbon black and many organic pigments have low surface energy and hence they require a surface active agent (surfactant) in the aqueous phase to aid wetting. The surfactant lowers the surface tension γ of water from ∼72 to ∼ 30–40 mN m⁻¹ depending on surfactant nature and concentration. This is illustrated in Figure 1.2, which shows the γ–log C (where C is the surfactant concentration) relationship of surfactant solutions. It can be seen that γ decreases gradually with the increase in surfactant concentration, and above a certain concentration it shows a linear decrease with the increase in log C. Above a critical surfactant concentration γ remains constant. This critical concentration is that above which any added surfactant molecules aggregate to form micelles that are in equilibrium with the surfactant monomers. This critical concentration is referred to as the critical micelle concentration (CMC).

Figure 1.2 Surface tension–log C curve.

There are generally two approaches for treating surfactant adsorption at the A/L interface. The first approach, adopted by Gibbs, treats adsorption as an equilibrium phenomenon whereby the second law of thermodynamics may be applied using surface quantities. The second approach, referred to as the equation-of-state approach, treats the surfactant film as a two-dimensional layer with a surface pressure π that may be related to the surface excess Γ (amount of surfactant adsorbed per unit area). Below, the Gibbs treatment that is commonly used to describe adsorption at the A/L interface is summarized.

Gibbs [4] derived a thermodynamic relationship between the surface or interfacial tension γ and the surface excess Γ (adsorption per unit area). The starting point of this equation is the Gibbs–Deuhem equation. At constant temperature, and in the presence of adsorption, the Gibbs–Deuhem equation is

(1.1)

where

is the number of moles of component i and adsorbed per unit area.

Equation (1.1) is the general form for the Gibbs adsorption isotherm. The simplest case of this isotherm is a system of two components in which the solute (2) is the surface active component, that is, it is adsorbed at the surface of the solvent (1). For such a case, Eq. (1.1) may be written as

(1.2)

and if the Gibbs dividing surface is used, Γ₁ = 0 and,

(1.3)

where

is the relative adsorption of (2) with respect to (1). Since

(1.4)

or

(1.5)

then

(1.6)

or

(1.7)

where

is the activity of the surfactant in bulk solution that is equal to C₂f₂ or x₂f₂, where C₂ is the concentration of the surfactant in mol dm⁻³ and x₂ is its mole fraction.

Equation (1.7) allows one to obtain the surface excess (abbreviated as Γ₂) from the variation of surface or interfacial tension with surfactant concentration. Note that a₂ ∼ C₂ since in dilute solutions f₂ ∼ 1. This approximation is valid since most surfactants have low c.m.c. (usually less than 10⁻³ mol dm⁻³) but adsorption is complete at or just below the c.m.c.

The surface excess Γ₂ can be calculated from the linear portion of the γ–log C₂ curves before the c.m.c. Such a γ–log C curve is illustrated in Figure 1.2 for the air/water interface. As mentioned above, Γ₂ can be calculated from the slope of the linear position of the curves shown in Figure 1.2 just before the c.m.c. is reached. From Γ₂, the area per surfactant ion or molecule can be calculated since

(1.8)

where N_av is Avogadro’s constant. Determining the area per surfactant molecule is very useful since it gives information on surfactant orientation at the interface. For example, for ionic surfactants such as sodium dodecyl sulfate, the area per surfactant is determined by the area occupied by the alkyl chain and head group if these molecules lie flat at the interface, whereas for vertical orientation, the area per surfactant ion is determined by that occupied by the charged head group, which at low electrolyte concentration will be in the region of 0.40 nm². Such an area is larger than the geometrical area occupied by a sulfate group, as a result of the lateral repulsion between the head group. On addition of electrolytes, this lateral repulsion is reduced and the area/surfactant ion for vertical orientation will be lower than 0.4 nm² (reaching in some case 0.2 nm²). On the other hand, if the molecules lie flat at the interface, the area per surfactant ion will be considerably higher than 0.4 nm².

Another important point can be made from the γ–log C curves. At concentration just before the break point, one has the condition of constant slope, which indicates that saturation adsorption has been reached. Just above the break point,

(1.9)

(1.10)

indicating the constancy of γ with log C above the c.m.c. Integration of Eq. (1.10) gives

(1.11)

Since γ is constant in this region, then a₂ must remain constant. This means that the addition of surfactant molecules, above the c.m.c., must result in association with form units (micellar) with low activity.

The hydrophilic head group may be unionized, for example, alcohols or poly(ethylene oxide) alkane or alkyl phenol compounds, weakly ionized such as carboxylic acids, or strongly ionized such as sulfates, sulfonates, and quaternary ammonium salts. The adsorption of these different surfactants at the air/water interface depends on the nature of the head group. With nonionic surfactants, repulsion between the head groups is small and these surfactants are usually strongly adsorbed at the surface of water from very dilute solutions. Nonionic surfactants have much lower c.m.c. values when compared with ionic surfactants with the same alkyl chain length. Typically, the c.m.c. is in the region of 10⁻⁵–10⁻⁴ mol dm⁻³. Such nonionic surfactants form closely packed adsorbed layers at concentrations lower than their c.m.c. values. The activity coefficient of such surfactants is close to unity and is only slightly affected by the addition of moderate amounts of electrolytes (or change in the pH of the solution). Thus, nonionic surfactant adsorption is the simplest case since the solutions can be represented by a two-component system and the adsorption can be accurately calculated using Eq. (1.7).

With ionic surfactants, on the other hand, the adsorption process is relatively more complicated since one has to consider the repulsion between the head groups and the effect of the presence of any indifferent electrolyte. Moreover, the Gibbs adsorption equation has to be solved taking into account the surfactant ions, the counterion, and any indifferent electrolyte ions present. For a strong surfactant electrolyte such as Na⁺R⁻,

(1.12)

The factor of 2 in Eq. (1.12) arises because both surfactant ion and counterion must be adsorbed to maintain neutrally, and dγ/dln a± is twice as large as for an un-ionized surfactant.

If a nonadsorbed electrolyte, such as NaCl, is present in large excess, then any i...

Table of contents

• Cover
• Related Titles
• Title page
• Copyright page
• Dedication
• Preface
• 1 General Introduction
• 2 Fundamentals of Wetting and Spreading
• 3 The Critical Surface Tension of Wetting and the Role of Surfactants in Powder Wetting
• 4 Structure of the Solid–Liquid Interface and Electrostatic Stabilization
• 5 Electrokinetic Phenomena and Zeta Potential
• 6 General Classification of Dispersing Agents and Adsorption of Surfactants at the Solid/Liquid Interface
• 7 Adsorption and Conformation of Polymeric Surfactants at the Solid–Liquid Interface
• 8 Stabilization and Destabilization of Suspensions Using Polymeric Surfactants and the Theory of Steric Stabilization
• 9 Properties of Concentrated Suspensions
• 10 Sedimentation of Suspensions and Prevention of Formation of Dilatant Sediments
• 11 Characterization of Suspensions and Assessment of Their Stability
• 12 Rheological Techniques for Assessment of Stability of Suspensions
• 13 Rheology of Concentrated Suspensions
• Index