An Introduction to Surfactants

Description

Surfactants are surface active agents, molecules that have a significant role in emulsions, suspensions, and foams. They find widespread application in personal care, cosmetics, pharmaceuticals, agrochemicals and the food industry. The main objective of this graduate level textbook is to present an overview of the classification, physical properties, phase behavior, their effects and applications of surfactants, e.g. as emulsifiers, foam stabilizer, in nano- and microemulsions and as wetting agents. Highly practical in approach, introducing briefly the science, followed by industruial applications. Preface 1 General introduction 2 General classification of surfactants 2.1 Anionic surfactants 2.1.1 Carboxylates 2.1.2 Sulfates 2.1.3 Sulfonates 2.1.4 Isethionates 2.1.5 Taurates 2.1.6 Phosphate-containing anionic surfactants 2.2 Cationic surfactants 2.3 Amphoteric (zwitterionic) surfactants 2.4 Nonionic surfactants 2.4.1 Alcohol ethoxylates 2.4.2 Alkyl phenol ethoxylates 2.4.3 Fatty acid ethoxylates 2.4.4 Sorbitan esters and their ethoxylated derivatives (Spans and Tweens) 2.4.5 Ethoxylated fats and oils 2.4.6 Amine ethoxylates 2.4.7 Amine oxides 2.5 Speciality surfactants 2.5.1 Fluorocarbon and silicone surfactants 2.5.2 Gemini surfactants 2.5.3 Surfactants derived from mono- and polysaccharides 2.5.4 Naturally occurring surfactants 2.5.5 Biosurfactants 2.5.6 Polymeric surfactants 3 Aggregation of surfactants, self-assembly structures, liquid crystalline phases 3.1 Thermodynamics of micellization 3.1.1 Kinetic aspects 3.1.2 Equilibrium aspects: Thermodynamics of micellization 3.2 Enthalpy and entropy of micellization 3.3 Driving force for micelle formation 3.4 Micellization in surfactant mixtures (mixed micelles) 3.5 Surfactant self-assembly 3.5.1 Structure of liquid crystalline phases 3.5.2 Hexagonal phase 3.5.3 Micellar cubic phase 3.5.4 Lamellar phase 3.5.5 Bicontinuous cubic phases 3.5.6 Reversed structures 3.6 Experimental studies of the phase behavior of surfactants 4 Surfactant adsorption at interfaces 4.1 Introduction 4.2 Adsorption of surfactants at the air/liquid (A/L) and liquid/liquid (L/L)interfaces 4.2.1 The Gibbs adsorption isotherm 4.2.2 Equation of state approach 4.2.3 The Langmuir, Szyszkowski and Frumkin equations 4.3 Interfacial tension measurements 4.3.1 The Wilhelmy plate method 4.3.2 The pendent drop method 4.3.3 The Du Nouy’s ring method 4.3.4 The drop volume (weight) method 4.3.5 The spinning drop method 4.4 Adsorption of surfactants at the solid/liquid interface 4.4.1 Adsorption of ionic surfactants on hydrophobic surfaces 4.4.2 Adsorption of ionic surfactants on polar surfaces 4.4.3 Adsorption of nonionic surfactants 5 Surfactants as emulsifiers 5.1 Introduction 5.1.1 Nature of the emulsifier 5.1.2 Structure of the system 5.1.3 Breakdown processes in emulsions 5.2 Physical chemistry of emulsion systems 5.2.1 The interface (Gibbs dividing line) 5.2.2 Thermodynamics of emulsion formation and breakdown 5.2.3 Interaction energies (forces) between emulsion droplets and their combinations 5.3 Mechanism of emulsification 5.3.1 Methods of emulsification 5.3.2 Role of surfactants in emulsion formation 5.3.3 Role of surfactants in droplet deformation 5.4 Selection of emulsifiers 5.4.1 The Hydrophilic-Lipophile Balance (HLB) concept 5.4.2 The Phase Inversion Temperature (PIT) concept 5.5 Stabilization of emulsions 5.5.1 Creaming or sedimentation and its prevention 5.5.2 Flocculation of emulsions and its prevention 5.5.3 Ostwald ripening and its reduction 5.5.4 Emulsion coalescence and its prevention 6 Surfactants as dispersants and stabilization of suspensions 6.1 Introduction 6.2 Role of surfactants in preparation of solid/liquid dispersions (suspensions) 6.2.1 Role of surfactants in condensation methods. Nucleation and growth 6.2.2 Emulsion polymerization 6.2.3 Dispersion polymerization 6.2.4 Role of surfactants in dispersion methods 6.3 Assessment of wettability of powders 6.3.1 Sinking time, submersion or immersion test 6.3.2 Measurement of contact angles of liquids and surfactant solutions on powders 6.3.3 List of wetting agents for hydrophobic solids in water 6.3.4 Stabilization of suspensions using surfactants 7 Surfactants for foam stabilization 7.1 Introduction 7.2 Foam preparation 7.3 Foam structure 7.4 Classification of foam stability 7.4.1 Drainage and thinning of foam films 7.4.2 Theories of foam stability 7.5 Foam inhibitors 7.5.1 Chemical inhibitors that lower viscosity and increase drainage 7.5.2 Solubilized chemicals which cause antifoaming 7.5.3 Droplets and oil lenses which cause antifoaming and defoaming 7.5.4 Surface tension gradients (induced by antifoamers) 7.5.5 Hydrophobic particles as antifoamers 7.5.6 Mixtures of hydrophobic particles and oils as antifoamers 7.6 Assessment of foam formation and stability 7.6.1 Efficiency and effectiveness of a foaming surfactant 8 Surfactants in nanoemulsions 8.1 Introduction 8.2 Fundamental principles of emulsification 8.2.1 Methods of emulsification and the role of surfactants 8.3 Preparation of nanoemulsions 8.3.1 Use of high pressure homogenizers 8.3.2 Phase inversion principle methods (low energy emulsification) 8.4 Steric stabilization and the role of the adsorbed layer thickness 8.5 Ostwald Ripening 8.6 Examples of nanoemulsions 9 Surfactants in microemulsions 9.1 Introduction 9.2 Thermodynamic definition of microemulsions 9.3 Description of microemulsions using phase diagrams 9.4 Thermodynamic theory of microemulsion formation 9.5 Characterization of microemulsions using scattering techniques 9.5.1 Time average (static) light scattering 9.5.2 Dynamic light scattering (photon correlation spectroscopy, PCS) 9.6 Characterization of microemulsions using conductivity 9.7 NMR measurements 9.8 Formulation of microemulsions 10 Surfactants as wetting agents 10.1 Introduction 10.2 The concept of contact angle 10.3 Adhesion tension 10.4 Work of adhesionWa 10.5 Work of cohesion 10.6 The spreading coefficient S 10.7 Contact angle hysteresis 10.8 Critical surface tension of wetting 10.9 Effect of surfactant adsorption 10.10 Measurement of contact angles 11 Industrial applications of surfactants 11.1 Surfactants in the home, personal care and cosmetics 11.1.1 Shaving formulations 11.1.2 Bar soaps 11.1.3 Liquid hand soaps 11.1.4 Bath oils 11.1.5 Foam (or bubble) baths 11.1.6 After bath preparations 11.1.7 Skin care products 11.1.8 Hair care formulations 11.1.9 Sunscreens 11.1.10 Make-up products 11.2 Surfactants in pharmacy 11.2.1 Surface active drugs 11.2.2 Naturally occurring micelle-forming systems 11.2.3 Biological implications of the presence of surfactants in pharmaceutical formulations 11.2.4 Solubilized systems 11.2.5 Pharmaceutical aspects of solubilization 11.3 Surfactants in agrochemicals 11.4 Surfactants in paints and coatings 11.5 Surfactants in detergents Index