Interactions of Surfactants with Polymers and Proteins

E. Desmond Goddard

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9781315894683
• eBook ISBN:
  9781351090681
• Topic: Scienze fisiche
• Subtopic: Chimica

Interactions of Surfactants with Polymers and Proteins covers work done in this area over the last 30 years and examines in detail the physico-chemical, microstructural, and applications aspects of interactions of surfactants with polymers and proteins in bulk surfaces and at interfaces. The physical chemistry of individual components (surfactants, polymers, and proteins) is discussed, and extensive coverage of interactions of surfactants with uncharged, oppositely charged, and hydrophobe modified polymers is provided. Other topics addressed include water soluble and insoluble keratinous proteins, the principles and applications of fluorescence spectroscopy, the physical properties and microstructural aspects of polymer/protein-surfactant complexes, and implications of surfactant interactions with polymers and proteins in practical systems. Interactions of Surfactants with Polymers and Proteins provides a wealth of information for chemists involved in a number of different research areas, including cosmetics, pharmaceutics, foods, paints, pigments, lubrication, ceramics, minerals/materials processing, and biological systems.

Chapter 1

Introduction

E.D. Goddard

Two areas of active and continuing research are concerned with the physical chemistry of aqueous surfactant solutions and of aqueous polymer solutions. Both aforementioned solute species have unique properties and it is not surprising that their mixed solutions can reveal rather unusual interaction effects. Historically, interest in the properties of mixtures of polymers and surfactants in aqueous solution is quite old. The formation and existence of lipoprotein aggregates in biological fluids were, for example, well recognized in the early part of this century.¹ Likewise, in the foods, cosmetics, and other industrial sectors, e.g., mineral processing, it has long been appreciated that interesting and unusual effects can be obtained by employing mixtures of surfactants and polymers.

The foundations of today’s activities on mixed polymer/surfactant systems were laid in work carried out in two separate areas. The first, in the 1940s and 1950s, involved protein (and, to a lesser extent, acidic polysaccharide)/synthetic ionic surfactant pairs. In these, the importance of electrical forces of attraction was easy to recognize, the interaction was generally referred to as “binding” of the charged surfactant by the macromolecule, and an awareness of changes in the conformation of protein molecules during the binding process was developed. The second, in the 1950s and 1960s, involved water-soluble synthetic polymers which were uncharged and surfactants which were again charged. Though the sites for binding the surfactant molecules on such polymers were less easy to identify, the notion of “binding” of the former persisted in this case also. It should be pointed out that interest in charged pairs has again developed in the 1970s, 1980s, and early 1990s, in systems in which the polyelectrolyte is either synthetic or natural, including various acidic and basic polypeptides.

Five years ago this author prepared a review^2,3 of the field of polymer/surfactant research covering the previous two decades of activity. In this compilation the investigative methods employed and the factors influencing the associative reactions were reviewed in detail, and an overview of extant theories of complex formation was offered. Two reasons have prompted the undertaking of the present larger review. The first is that there has been a significant increase in research activity in this field in the last five years and it has seemed desirable to broaden the previous compilation to include an account of these recent developments. The second reason concerns the decision, in the interest of limiting the length of the 1986 review, to omit the important field of protein/surfactant interactions. For the present compilation this decision has been reversed in the hope of expanding the potential utility and interest of the work to embrace other fields, in particular the biosciences.

Furthermore, to set the stage for a better appreciation and understanding of the properties of polymer/surfactant pairs it was decided to add chapters on the physical chemistry of the separate components themselves, i.e., surfactants and polymers in solution. While each of these chapters covers its respective subject in some detail, it is hoped that the latter one, in particular, will help to inspire more thought and research work on polymer/surfactant systems as viewed from the vantage point of the polymer since, to date, it is true to say that most interpretations have emphasized the role and fate of the surfactant member of the pair. Proteins are recognized as being sufficiently important in their own right that separate chapters are devoted to them and to their mixtures with surfactants. In consequence of the foregoing, the undertaking has increased considerably in scope over the original reviews (which appear in their original form as Chapter 4, Parts 1 and 2) and a very practical decision was taken to expand authorship to include several other contributors who are recognized experts in their respective fields.

It is appropriate to comment briefly on some recent important developments, all of which will be elaborated upon in the text. First, in the category of interacting nonionic polymer/ionic surfactant pairs the notion of site binding of the surfactant, still useful conceptually as regards detailed binding behavior, has given way progressively to mechanisms based on a perturbation of the micellization of the surfactant by the polymer. If one examines the structure of a typical ionic surfactant micelle (spherical) one can easily appreciate that two factors unfavorable to the formation of the micelle are the electrostatic repulsion of the assembled headgroups and the residual contact of the first few peripheral carbon atoms of the hydrocarbon tails with water (see Figure 1).

The current view of polymer/surfactant interaction is that, in the complex, segments of the polymer are wrapped around the micelle to relieve both of these stresses. Refined models presented in terms of the various free energy contributions to the overall aggregation process continue to be developed, and a summary of the current status is given in Chapter 5. Another growing recognition has been that of the great importance of hydrophobic groupings in the polymer in promoting interaction with surfactants. These hydrophobic entities can be as small as methyl groups. With such hydrophobic substitution the reactivity of certain polymers can be increased to the point of promoting interaction even with (certain types of) nonionic surfactants, which have heretofore been considered relatively inert. In some polymers fluorescent probe groups, introduced covalently for mechanistic studies, themselves provide the hydrophobic centers which favor interaction with surfactants and, indeed, their site binding. A noteworthy point is the greatly expanded use of the technique of fluorescence spectroscopy, in general, to study these and related systems. In view of this development an appendant section, which describes the underlying principles and use of fluorescent dyes for studying aggregation in solution, and polymer/surfactant interaction in particular, is included as the next to last chapter of the book. It augments and expands the many references to the use of the fluorescent dye technique found in several of the other chapters.

FIGURE 1. Schematic diagram of an anionic surfactant micelle.

A great deal of renewed interest is being shown in what can be termed “polymeric surfactants”, i.e., soluble polymers in which the hydrophobic groups are long (>C₁₀) alkyl groups and which are akin to preformed polymer/surfactant complexes. A separate chapter is devoted to this important field with an emphasis on “polysoaps”. Several more references to unionized, hydrophobically modified water-soluble polymers (“associative thickeners”) can be found in Chapter 10 on Applications.

Another recent development has been the detailed study, and the modeling, of the phase behavior of certain nonionic polymers, viz., those possessing cloud points, in combination with selected surfactants. Other developments include the determination, with an analysis, of detailed phase diagrams of selected polyion/surfactant counterion pairs and the demonstration of unusual rheological behavior of many of these systems. In this second category of complex, viz., that comprising a polyion and an oppositely charged surfactant, the notion of binding of the surfactant ion, reinforced by hydrophobic association, has persisted. Chapter 4, Part 2 presents an overview of this field, which is augmented in Chapter 5 by a description of new developments.

Finally, mention is made again of the new subject category which represents the final chapter of the book. This recognizes the growing practical and commercial importance of polymer/surfactant complexes under the title “Applications”. It should be pointed out that this chapter does not include the category of proteins: in their case the utility and applications of their interactions with surfactants are so closely linked to the physical chemical aspects that they are woven directly into the text of Chapters 7 and 8.

A concluding word about the structure of the book, and the choices which seemed available, is offered in summary. One approach was, while maintaining the old format, to update and considerably expand the original review. The second was to augment it with new chapters involving new contributors. We, the authors, chose the second course. In this way the utility of the earlier review, with its extensive coverage of the early work, methods and principles, is retained while new perspectives, a broader foundation, and a comprehensive treatment of newer developments in this field have been made possible. We believe the overall compilation offered now represents a rather complete and up-to-date coverage of the field. One consequence, or “penalty”, of the above approach is that some overlap between the chapters became inevitable. If this is considered a drawback we apologize. For our part we view this feature to be something of a positive attribute which can add to the utility, convenience, and readability of the work as a whole.

On a personal note I wish to express my appreciation to Union Carbide Corporation and Amerchol Corporation for sustained support of my research work, over many years, on the subjects dealt with in this volume.

References

1. Lipo-Proteins, Discuss. Faraday Soc. No. 6, 1949. See introductory article by D. G. Dervichian.

2. Goddard, E. D., Polymer-surfactant interaction. Part I. Uncharged water-soluble polymers and charged surfactants, Colloids Surf., 19, 255, 1986.

3. Goddard, E. D., Polymer-surfactant interaction. Part II. Polymer and surfactant of opposite charge, Colloids Surf., 19, 301, 1986.

Chapter 2

Surfactant Solutions: Adsorption and Aggregation Properties

K. P. Ananthapadmanabhan

TABLE OF CONTENTS

• Introduction
  • Scope
  • Classes of Surfactants
    1. Anionic Surfactants
    2. Cationic Surfactants
    3. Nonionic Surfactants
• Properties of Surfactant Solutions: Adsorption
  • Surface Activity at the Air-Liquid Interface
    1. The Gibbs Equation
    2. Surface Tension Minimum and the Gibbs Equation
    3. Effect of Variables
       • Chain Length Effect and Traube’s Rule
       • Effect of Ionic Strength
       • Effect of Temperature
       • Effect of pH
  • Adsorption of Surfactants at the Solid-Liquid Interface
    1. Electrostatic Forces
    2. Hydrophobic Interactions
    3. Other Interactions
    4. Effect of Variables
• Micelle Formation in Aqueous Solutions
  • Structure of Micelles
    1. Nature of Micellar Core
    2. The Micellar Surface and Counterion Binding
  • Micellar Shape and Aggregation Number
  • Factors Affecting the CMC and Aggregation Number
    1. Hydrophobic Group
    2. Hydrophilic Group
    3. Effect of Counterfoils
    4. Effect of Additives
    5. Effect of Temperature
       • Clouding of Nonionic Surfactants …
  • Thermodynamics of Micellization
    1. The Phase Separation Model
       • Nonionic Surfactants
       • Ionic Surfactants
    2. The Mass Action Model
       • Nonionic Surfactants
       • Ionic Surfactants
    3. Monomer Activity Above the CMC
    4. Other Models
• Concluding Remarks

References

I. Introduction

Surfactants are characterized by the presence of two moieties in the same molecule, one polar and the other nonpolar. The polar group may carry a positive or negative charge, giving rise to cationic or anionic surfactants, respectively, or may contain ethylene oxide chains or sugar or saccharide-type groups, as in the case of nonionic surfactants. The nonpolar part of the molecule is generally a hydrocarbon chain, but may contain aromatic groups. In addition, surfactants with fluorocarbon, polypropylene oxide, or silicone (polydimethylsiloxane) hydrophobic groups are becoming increasingly popular.

The existence of groups with opposing characteristics is responsible for all the special properties of surfactants. The behavior of surfactants in aqueous solution is determined by their tendency to seclude their hydrophobic part from solution and expose their hydrophilic part towards the solution. This dual tendency is responsible for adsorption of surfactants at interfaces and for the formation of such aggregates as micelles.

A. Scope

The purpose of this chapter is to provide a brief review of the solution and interfacial properties of surfactants for readers interested in ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• The Editors
• Contributors
• Table of Contents
• Chapter 1: Introduction
• Chapter 2: Surfactant Solutions: Adsorption and Aggregation Properties
• Chapter 3: Fundamentals of Polymer Solutions
• Chapter 4: Polymer-Surfactant Interaction
• Chapter 5: Polymer-Surfactant Interactions — Recent Developments
• Chapter 6: Hydrophobe-Modified Polymers
• Chapter 7: Proteins in Solution and at Interfaces
• Chapter 8: Protein-Surfactant Interactions
• Chapter 9: Applications of Fluorescence Spectroscopy to the Study of Polymer-Surfactant Interactions
• Chapter 10: Applications of Polymer-Surfactant Systems
• Index