Solution Properties of Polymeric Surfactants

Long flexible macromolecules have a large number of internal degrees of freedom [4-6]. A typical primary structure of such molecules is a linear chain of units connected by covalent bonds that are referred to as the backbone. By rotating about the bonds in the backbone the molecule can change its shape, and this results in a wide spectrum of conformations. Unfortunately, the rotation may be hindered by the side groups, so that some of these conformations may be rather unfavourable. In some macromolecules (e.g., proteins), sequences of preferred orientations appear as helical or folded sections. 

Another useful parameter is the radius of gyration s P, which is a measure of the effective size of a polymer molecule (it is the root mean-square distance of the elements of the chain from its centre of gravity). 

Although, in real polymers the bonds cannot assume arbitrary directions, there are fixed angles between them. In addition, rotation about bonds is not entirely free, because the potential energy shows maxima and minima as a function of the rotation angle. Consequently, to account for these effects the above equations are modified by introducing a rigidity parameter p (stiffness persistence ) which depends on the architecture of the chain  

A useful parameter, called the characteristic ratio, was introduced by Flory [6] and is defined as 

The effect of solvency for the polymer chain has been considered in the thermodynamic treatment of Flory and Huggins [6], usually referred to as the Flory- Huggins theory. This theory considers the free energy of mixing of a pure polymer with a pure solvent, in terms of two contributions, namely the enthalpy of 

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To understand the solution behavior of polymeric surfactants of the block-and-graft type, it is essential to consider the solution properties of the more simple homopolymers. The solution behavior of homopolymers was considered in the thermodynamic treatment of Flory and Huggins. 

The attractive forces between suspension particles are considered to be exclusively London-van der Waals interactions (except where interparticle bridging by long polymeric chains occurs). The repulsive forces, as discussed in Chapter 8, comprise both electrostatic repulsion and entropic and enthalpic forces. In aqueous systems the hydrophobic dispersed phase is coated with hydrophilic surfactant or polymer. As adsorption of surfactant or polymer (or, of course, both) at the solid-liquid interface alters the negative charge on the suspension particles, the adsorbed layer may not necessarily confer a repulsive effect. Ionic surfactants may neutralize the charge of the particles and result in their flocculation. The addition of electrolyte such as aluminium chloride can further complicate interpretation of results electrolyte can alter the charge on the suspension particles by specific adsorption, and can affect the solution properties of the surfactants and polymers in the formulation. Some aspects of the application of DLVO theory to pharmaceutical suspensions and the use of computer programmes to calculate interaction curves are discussed by Schneider et al. [4]. 

The surfactant properties of polymeric silicone surfactants are markedly different from those of hydrocarbon polymeric surfactants such as the ethylene oxide/propylene oxide (EO/PO) block copolymers. Comparable silicone surfactants often give lower surface tension and silicone surfactants often self-assemble in aqueous solution to form bilayer phases and vesicles rather than micelles and gel phases. The skin feel and lubricity properties of silicone surfactants do not appear to have any parallel amongst hydrocarbon polymeric surfactants. 

Chemical modification of hydroxyethylcellulose or hydroxypropylcellulose with long-chain hydrocarbon alkylating reagents, such as C8-C24 epoxides or halides, has been reported to yield novel water-soluble compositions exhibiting enhanced low-shear-rate solution viscosities and polymeric surfactant properties [ 104,105]. Patents have also been issued for water-soluble phosphonomethylcellulose and phosphonomethylhydroxyethylcellulose [106,107]. 

The first section of this chapter describes the solution properties of polymers, and this is followed by a general classification of polymeric surfactants. Examples are provided of polymeric surfactants and polyelectrolytes that are used as dispersants and emulsifiers. 

Incorporation of long-chain hydrocarbon hydrophobes into a cellulose ether backbone leads to an interesting new class of polymeric surfactants. Their enhanced solution viscosity can be explained in terms of intermolecular associations via the hydrophobe moieties. Entropic forces cause the polymer hydrophobes to cluster to minimize the disruption of water structure. The same thermodynamic principles that are used to explain the micellization of surfactants can be applied to explain the solution behavior of HMHEC. HMHECs interact with surfactants that modify their solution viscosities. The chemical nature and the concentration of the surfactant dictate its effect on HMHEC solution behavior. The unique rheological properties of HMHEC can be exploited to meet industrial demands for specific formulations and applications. 

This chapter, will begin with a brief description of polymeric surfactants and their solution properties, followed by a description of the fundamental principles of using polymeric surfactants for stabilization of emulsions (as well as suspensions), starting with a section on the adsorption and conformation of these molecules at the interface. This is followed by a section on stabilization of dispersions by polymeric surfactants. Particular... 

This chapter described the basis principles involved in stabilization of dispersions by polymeric surfactants. The first part described polymeric surfactants and their solution properties. The second part described the adsorption of polymeric surfactants and their conformation at the interface. The methods that can be applied to determine the adsorption and conformation of polymeric surfactants were briefly described. The third part dealt with the stabilization mechanism produced using polymeric surfactants. Two main repulsive forces were considered. The first arises from the unfavorable mixing of the chains on close approach of the particles or droplets, when these chains are in good solvent conditions. This is referred to as mixing or osmotic repulsion. The second force of repulsions... 

Interactions between surfactant anions and the polymeric cation not only influence the solution properties of the two compounds, but also affect the nature of their deposition on the fiber surface. Averages (16 measurements each) of advancing wettabilities of fibers treated with 15 solutions of JR-400 containing increasing concentrations of sodium lauryl sulfate show high levels of deposition with low substantivity up to the critical micelle concentration (CMC) of the lautyl sulfate, at which point wettabihty decreases, especially after the third immersion. This decrease to values below that of the imtreated fiber indicates a reorientation of the surfactant-polymer complex resulting in a hydrophobic fiber surface. 

This chapter will start with a short account of the general classification and description of polymeric surfactants. This is followed by a summary on then-solutions properties. The adsorption and conformation of polymeric surfactants at the solid-liquid interface will be discussed at a fundamental level and some experimental results will be presented to illustrate the prediction of the theories. The interaction energies between particles or droplets containing adsorbed polymeric surfactants will be briefly described. The final section will give some applications of polymeric surfactants in suspensions, emulsions, and multiple emulsions. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

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