Polymer-Surfactant Systems

J.C.T. Kwak

• Publisher: CRC Press
• Year: 2020
• eBook ISBN: 9781000146059
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

"Chronicles recent advances in our knowledge of polymer-surfactant systems, combining authoritative reviews of new experimental methods, instrumentation, and applications with fundamental discussions of classical methodologies and surveys of specific properties."

1 Polymer-Surfactant Systems: Introduction and Overview

ANDREW P. RODENHISER and JAN C.T. KWAK Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada B3H 4J3

I. Introduction

II. Experimental Methods

A. Classical physical chemical methods

B. Spectroscopic methods

III. Chapter Overview

References

I. INTRODUCTION

Compared to the ever increasing variety of synthetic polymer structures reported, or even in relation to the number of polymers commercially available the group of water-soluble polymers is relatively small. If we include the common water-soluble biopolymers and their synthetic derivatives, this combined group, although still not numerous in comparison to all synthetic polymers available, has a large number of industrial, environmental, household, and medical applications, and is of significant commercial importance [1,2]

The solubility of a polymer in water is determined by the balance between the interactions of the hydrophilic and hydrophobic polymer segments with themselves and with the solvent. Similarly, the aggregation of surfactants in aqueous solution is governed by the subtle balance of hydrophilic, hydrophobic, and ionic interactions. As a result, aqueous solutions containing both polymers and surfactants display an apparently infinitely varied and indeed sometimes bewildering pattern of properties, due to the many variations in molecular structures available to the investigator or formulator. Research in the area of polymer-surfactant interactions has accelerated rapidly over the last few decades, driven in part by the many current or foreseen applications for instance in pharmaceutical formulations, personal care products, food products, household and industrial detergents, paints and coatings, oil drilling and enhanced oil recovery fluids, etc., but inspired also by fundamental interest in intermolecular interactions and hydrophobic aggregation phenomena. Quantitative aspects of such studies may include the direct measurement of properties such as viscosity, conductance, volumetric and thermochemical parameters, the determination of surfactant binding isotherms by various methods, and the elucidation of the often highly complex phase diagrams. Qualitatively, from the early studies on investigators have relied on model descriptions and the systematic understanding of polymer-surfactant interactions to guide them in designing new systems and new applications. Modem spectroscopic tools such as NMR and fluorescence methods have been highly instrumental in increasing our understanding of the molecular basis for such models.

The number of reviews of polymer-surfactant research reflects the growing number of published studies. Early reviews by Breuer and Robb [3], Robb [4], Goddard [5,6], and Saito [7] were instrumental in pointing out the many research opportunities. More recent reviews highlight both applications and fundamental studies [8-14], Many early polymer-surfactant studies involved proteins, including what is possibly the first report, by Bull and Neurath [15]. Much of this early work on protein solutions may be found in the reviews of Steinhardt and Reynolds [16] and Lapanje [17], More recent overviews are presented by Jones [18], Ananthapadmanabhan [10] and Dickinson [19], The topic of surfactant-protein interactions, although not a primary focus, is also addressed in chapters 3, 4, and 9 of this book.

It has been customary to classify polymer-surfactant interactions according to polymer or surfactant charge, and according to concentration region. Most studies concerned with the determination of surfactant binding to polymers are carried out at low polymer concentration, with surfactant concentrations determined by the binding region. On the other hand, phase equilibria and phase diagrams are normally studied at higher concentrations. Although the findings of low concentration binding studies may be expected to be applicable to phase equilibrium studies at higher concentration, one has to be careful in making such assumptions. Indeed, the concepts of binding and interactions in general should be distinguished with some care. In addition, much current research focuses on systematic variations of polymer structure, including in particular the introduction of hydrophobic modifications in polymers which are normally considered hydrophilic. In many such systems concepts traditionally used in polymer-surfactant studies, such as the critical aggregation concentration, cac (sometimes still referred to as T₁) and the degree of binding are not applicable or need modification. Already some of the earliest studies in polymer-surfactant systems, especially those involving nonionic surfactants, point out that considerable changes in system properties can be observed even though there is no observable change in critical micelle concentration, cmc [7]. On the other hand, for polyelectrolytes and surfactants of opposite charge, surfactant binding is clearly observable and may start at concentrations two or three orders of magnitude below the cmc.

Another distinction of interest which has been widely discussed concerns the difference between anionic and cationic surfactants. In particular, the observation that anionic surfactants display a relatively strong and cooperative interaction with nonionic hydrophilic polymers such as poly(ethyleneoxide) (PEO) and polyvinylpyrrolidone (PVP), while cationic surfactants do not exhibit interactions, remains relevant. This may be another example where one has to be careful in applying apparently simple concepts such as polymer charge or induced charge or dipolar effects. A better understanding of the microscopic structure of the aggregates and the role of hydrophobic interactions may be needed, as exemplified by the difference in micellar surface structure of anionic and cationic surfactants, evident from NMR and SANS studies on alkylsulfate micelles [20,21] and on alkylammonium surfactants [22],

The precise structure of a polymer-surfactant complex will of course depend on the structures of the molecules involved, in particular the hydrophobicity and molecular weight of the polymer, and the charge and shape of the surfactant. For linear polymers, a general model has emerged, often referred to as the "necklace" or "beads-on-a-string" model, in which one or more small surfactant micelles reside within the random coil of the polymer [23], There are proven exceptions to this structure, particularly for surfactants which form rod-shaped micelles or vesicles [24], and with hydrophobically modified polymers; however, the model has become accepted as the typical structure of a polymer-surfactant complex. In this introduction we can only present a brief history of the development of this model, and of polymersurfactant studies in general. The reader is referred to earlier reviews [3-14] and subsequent chapters in this volume for more complete citations.

It is interesting to note that interactions of surfactants with proteins have been known since the 1930s [15,25], well before the first studies for synthetic polymers were published. Anionic surfactants such as sodium dodecylsulfate (SDS) were found to bind to positive charge sites on proteins in stoichiometric proportions, in some cases opening up the protein conformation, and at higher surfactant concentrations with much higher ratios of surfactant molecules per protein molecule. The possibility that this excess binding to proteins was micelle-like was discussed nearly twenty years before the "necklace" model was proposed for binding of surfactants to synthetic polymers [26,23], Isemura and co-workers observed that poly(vinyl formal), poly(vinyl butyral), poly(vinyl acetate) (PVAc) and poly(vinyl alcohol) (PVA), which have poor solubility both in organic solvents and in water, could be dissolved in an SDS solution [27,28], Such solutions could be diluted well below the cmc of SDS without precipitation of the polymer, indicating that these were not the result of polymer being solubilized in SDS micelles. Electrophoresis experiments demonstrated the negative charge of the complex, attributed to bound surfactant. The electrophoresis measurements also indicated Langmuir-type binding, with the number of adsorbed anions increasing with increased hydrophobicity of the polymer. The authors considered several modes of binding and decided that the binding was driven by hydrophobic interactions. Studies by Saito and co-workers, summarized in chapter 9 of this book, were among the first to discuss the role of headgroup charge and nature of the counterion [29], A few years later, White and co-workers examined the binding of cationic surfactants to anionic cellulosic polymers, and concluded that the initial binding of surfactant cations was due to ion exchange, and was followed by clustering of surfactant-counterion pairs on those binding sites [30-32], Decreased adsorption was noted in hydroxyethyl celluloses, where some fraction of the anionic carboxyl sites had been removed.

We may speak of the start of a new era of polymer-surfactant studies with the work of Jones on PEO-SDS and PVP-SDS systems [33,34], He defined the critical aggregation concentration (cac or T₁) as observed by conductance and surface tension measurements, and the saturation point (T₂), which was found to increase linearly with polymer concentration. Although Jones postulated a polymer-nucleated micelle, the structure suggested followed the model of anionic azo dye-PVP complexes presented by Eirich and co-workers [35], involving individually bound surfactant molecules with their tails parallel to the polymer chain and their headgroups well separated. Lewis and Robinson observed a critical concentration for binding of SDS to several polymers, which they found consistent with a hydrophobic bonding mechanism similar to micelle formation [36], They also observed that surfactant binding broke up existing interpolymer aggregation which allowed aggregation of further surfactant to fresh polymer surface. They considered the saturated complex to be a mixed micelle.

Shinoda and co-workers noted the cooperativity of polymer-surfactant binding [37]. They made surface tension, dialysis and dye solubilization measurements of the cmc and cac for sodium alkyl sulfates in the absence and presence of PVP, finding a stoichiometric...

Table of contents

• Cover
• Half Title
• Series Page
• Title Page
• Copyright Page
• Preface
• Contents
• Contributors
• 1. Polymer-Surfactant Systems: Introduction and Overview
• 2. Applications of Polymer-Surfactant Systems
• 3. Phase Behavior of Polymer-Surfactant Systems
• 4. The Nature of Polymer-Surfactant Interactions
• 5. Models of Polymer-Surfactant Complexation
• 6. NMR Studies of Polymer-Surfactant Systems
• 7. Fluorescence Methods in the Study of Polymer-Surfactant Systems
• 8. Isothermal Titration and Temperature Scanning Calorimetric Studies of Polymer-Surfactant Systems
• 9. Interactions of Polymers and Nonionic Surfactants
• 10. Polyelectrolyte-Surfactant Interactions: Polymer Hydrophobicity, Surfactant Aggregation Number, and Microstructure of the Systems
• 11. Solubilization of Dyes by Polymer-Surfactant Complexes
• Index