Polymeric Surfactants: Dispersion Stability and Industrial Applications pdf by Tharwat F. Tadros

Contents

Preface
Polymeric surfactants are essential materials for the preparation of many industrial disperse systems and their stabilization, of which we mention dyestuffs, paper coatings, inks, agrochemicals, pharmaceuticals, personal care products, ceramics and detergents. The most effective polymeric surfactants for stabilization of disperse systems are those of the A–B, A–B–A block and BAn (or ABn ) graft types. The chain B (referred to as the “anchor” chain) is chosen to be highly insoluble in the medium and has a strong affinity (strong adsorption) to the surface of the particle or droplet. The chain A (referred to as the “stabilizing chain”) is chosen to be highly soluble in the medium and strongly solvated by its molecules. These block and graft polymers provide effective steric stabilization thus preventing any flocculation and/or coalescence. One of the most important applications of polymeric surfactants is in the preparation of oil-in-water (O/W) and water-in-oil (W/O) emulsions as well as solid/liquid (S/L) dispersions. In this case, the hydrophobic portion of the surfactant molecule should adsorb “strongly” at the O/W or the S/L interface, leaving the hydrophilic components in the aqueous medium where they become strongly solvated by the water molecules, thus providing effective steric stabilization. There are generally two methods for preparation of suspensions, referred to as the condensation and dispersions methods. In the first case, one starts with molecular units and builds up the particles by a process of nucleation and growth, e.g. the preparation of polymer lattices. In the dispersion methods, preformed particles (usually powders) are dispersed in an aqueous solution containing a surfactant. The latter is essential for adequatewetting of the powder (both external and internal surfaces of the powder aggregates and agglomerates must be wetted). This is followed by dispersion of the powder using high speed stirrers and finally the dispersion is “milled” to reduce the particle size to the appropriate range. Polymeric surfactants are also effective here for the long-term stability of the resulting suspension.

This book starts with a general introduction (Chapter 1) highlighting the importance of polymeric surfactants for the preparation and stabilization of various disperse systems. Chapter 2 gives a general classification of polymeric surfactants. It starts with a section on homopolymers, which are made from the same repeating units, e.g. poly(ethylene oxide), poly(vinylpyrrolidone) and poly(acrylic acid). The next section deals with block and graft copolymers which are essential for stabilizing disperse systems.

Examples of several block and graft copolymers are given to illustrate their use as stabilizers. A section is devoted to polymeric surfactants based on natural materials such as hydroxyethyl cellulose and polysaccharides. The final section in this chapter describes block and graft copolymers that are used in nonaqueous disperse systems. Chapter 3 deals with the solution properties of polymeric surfactants. It starts with the thermodynamic treatment of the free energy of mixing a homopolymer with a solvent on the basis of the Flory–Huggins treatment of the free energy. The modification of this treatment when using block and graft copolymers is described. Chapter 4 deals with the adsorption and conformation of polymeric surfactants at interfaces with particular reference to block and graft copolymers.Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to describing how these molecules act as stabilizers. Theories of polymeric surfactant adsorption are described with particular reference to the adsorption parameters and their experimental determination.

Several examples of polymeric surfactant adsorption are given in this chapter. Chapter 5 deals with stabilization of disperse systems using polymeric surfactants. The interaction between particles or droplets containing adsorbed or grafted polymeric surfactants is described in terms of the unfavourable mixing of the chains (when these are in good solvent conditions) and the loss of configurational entropy (elastic interaction) on considerable overlap of the chains. Combining steric repulsion with van der Waals attraction results in energy-distance curves that show very steep repulsion when the surface-to-surface distance between the particles or droplets become smaller than twice the adsorbed layer thickness. Chapter 6 describes the flocculation of disperse systems containing polymeric surfactants. Four different types of flocculation mechanisms are described: (i) weak and reversible flocculation obtained when using “thin” adsorbed layers (< 5 nm) that results in an attractive minimum at separation distances compared to twice the adsorbed layer thickness; (ii) incipient (strong) flocculation obtained when the solvency of the medium for the chains becomes worse than a theta solvent; (iii) depletion flocculation induced by the addition of “free” nonabsorbing polymer chains in the continuous medium; (iv) bridging flocculation obtained by simultaneous adsorption of the polymer chain on two or more particles or droplets. Chapter 7 deals with applications of polymeric surfactants in emulsion and suspension polymerization for use in the preparation of polymer colloids (latexes).

The theories of emulsion and suspension polymerization are described at a fundamental level with particular emphasis on the use of polymeric surfactants for stabilization of the resulting particles against aggregation. Chapter 8 describes the role of polymeric surfactants in stabilizing preformed particles. The importance of strong adsorption that results in a high affinity adsorption isotherm is described at a fundamental level. Chapter 9 deals with the use of polymeric surfactants for stabilizing emulsions and nanoemulsions. The origin of stability against coalescence is discussed in terms of the disjoining pressure of the thin film produced on approach of two emulsion droplets. Another important use of polymeric surfactants is to reduce Ostwald ripening, in particular with nanoemulsions. Chapter 10 gives a summary of applications of polymeric surfactants in pharmacy. Several polymeric surfactants that have been approved by the Food and Drug Administration (FDA) are described. These polymeric surfactants are used for formulating various pharmaceutical systems, such as suspensions, emulsions and multiple emulsions. A section is devoted to applications involving polymeric surfactants in the preparation of biodegradable nanoparticles for targeted delivery of drugs. Chapter 11 deals with applications of polymeric surfactants in cosmetics and their advantages in formulating emulsions in hand creams.

Polymeric surfactants are also used to stabilize liposomes and vesicles that are incorporated in many cosmetic formulations to efficiently deliver active ingredients such as anti-wrinkle agents. Chapter 12 gives examples of applications of polymeric surfactants in paints and coatings. Particular attention is paid to the steric stabilization obtainedwhen using polymeric surfactants in nonaqueous paint systems,where electrostatic stabilization is not possible. Chapter 13 deals with applications of polymeric surfactants in agrochemical formulations, in particular their use in formulating concentrated suspensions and emulsions. Chapter 14 gives examples of applications of polymeric surfactants such as gums and proteins in the food industry. The interaction between proteins and polysaccharides is also described.

The present text gives a comprehensive description of polymeric surfactants, their properties, adsorption at interfaces and their applications for the stabilization of various industrial disperse systems. The book is valuable for formulation scientists and chemical engineerswho are engaged in formulating various disperse systems. It is also extremely valuable for academic researchers who are studying the role of polymeric surfactants in the preparation and stabilization of model disperse systems.