Rheology of surfactant systems

The next molecular organisation is more complex. This involves many layers of alternating molecules, intercalated with layers of water—i.e. lamellar phase. It is possible for rafts of these layers to float around in water, but for most real products, they present themselves as fuUy enclosed multi-layered onion-like droplets. The number of layers can nm into hundreds, and these so-called lamellar drops (or vesicles) can be up to several microns in diameter. 

The distinctive feature of these structures is the very large amount of water that they trap between the surfactant layers. For a surfactant concentration of aroimd five percent, the total phase volume of the drops can be greater than 70%, 

What are the rheological properties of surfactant-base liquids necessary to fulfil our expectations of them The following examples might be cited  

These desired properties are best brought about by the manipulation of the micelle structures formed by the proper choice of mixtures of surfactants as well as the correct processing procedures needed to assemble the more complex structures. Other chemical species present in real surfactant formulations (perfumes, hydrotropes etc.) can also play a major role in manipulating micellar form if present at significant levels. 

Exercise Produce a prescription for the rheology of a liquid abrasive cleaner that has to suspend solid particles, but has to readily drain down a vertical surface. 

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From R D to quality control, rheology measurements for each phase of the product development life cycle involve raw materials, premixes, solutions, dispersions, emulsions, and full formulations. Well-equipped laboratories with stress- and strain-controlled oscillatory/steady shear rheometers and viscometers can generally satisfy most characterization needs. When necessary, customized systems are designed to simulate specific user or process conditions. Rheology measurements are also coupled with optic, thermal, dielectric, and other analytical methods to further probe the internal microstucture of surfactant systems. New commercial and research developments are briefly discussed in the following sections. 

While dynamic mechanical and steady shear measurements are frequently used in rheology studies of surfactant systems, extensional viscosity measurements are lacking. This can be attributed to the difficulties associated with such measurements and the lack of commercial laboratory instrumentation since the discontinuance of the Rheometric Scientific RFX rheometer. For many detergent compositions, the relatively low viscosity further complicates such measurements. There appear to be very few data on extensional or elongation viscosity for detergent consumer products and actives in the technical literature at this time. 

In the trimer series, the hydrophobic domains in the fluorescence data reflect isocyanurate associations. The hydrophobe associations responsible for effective thickening occur at higher concentrations. Probe studies similar to those conducted in oxyethylene-oxypropylene block polymers (12, 13) are warranted. The difference of importance to the rheology of dispersed systems is likely related to the cohesiveness of the surfactant hydrophobe interaction (21). 

Some of these studies indicate that HLBis not the only property of the chemical which determines the demulsifier power. Cooper et al. (285) indicated that water reduetion was dependent on the chemical structure of the surfaetant when two surfactants with similar HLBs gave opposite results. The effects of the interaction of the chemical stmcture with emulsion interfaces are the more important factors in demulsification, as these influence the film rheology of the system. 

To conclude, in this work we present a review and explanation of the main contributions of Professor Kunieda in the field of rheology of worm-like micelles. It can be remarked that his works have extended the number and types of surfactant systems able to present worm-like micelles, on the one hand, and have deepened the elucidation of structure and formation mechanism, on the other hand. 

The proceedings cover six major areas of research related to chemical flooding processes for enhanced oil recovery, namely, 1) Fundamental aspects of the oil displacement process, 2) Microstructure of surfactant systems, 3) Emulsion rheology and oil displacement mechanisms, 4) Wettability and oil displacement mechanisms, 5) Adsorption, clays and chemical loss mechanisms, and 6) Polymer rheology and surfactant-polymer interactions. This book also includes two invited review papers, namely, "Research on Enhanced Oil Recovery Past, Present and Future," and "Formation and Properties of Micelles and Microemulsions" by Professor J. J. Taber and Professor H. F. Eicke respectively. 

It is difficult to summarize all the phenomena discussed in this volume. However, major topics include ultralow interfacial tension, phase behavior, microstructure of surfactant systems, optimal salinity concept, middle-phase microemuIsions, interfacial rheology, flow of emulsions in porous media, wettability of rocks, rock-fluid interactions, surfactant loss mechanisms, precipitation and redissolution of surfactants, coalescence of drops in emulsions and in porous media, surfactant mass transfer across interfaces, equilibrium dynamic properties of surfactant/oil/brine systems, mechanisms of oil displacement in porous media, ion-... 

These are suspensions of solid actives in a surfactant vehicle. Other ingredients such as polymers that provide good skin feel are added. The rheology of the system should be controlled to avoid particle sedimentation (see Chapter 7). This is achieved by addition of thickeners. Shear thinning of the final product is essential to ensure good spreadability. In stick application, a semi-solid system is produced. 

Many food formulations contain mixtures of surfactants (emulsifiers) and hydrocolloids. Interaction between the surfactant and polymer molecule plays a major role in the overall interaction between the particles or droplets, as well as the bulk rheology of the whole system. Such interactions are complex and require fundamental studies of their colloidal properties. As discussed later, many food products contain proteins that are used as emulsifiers. Interaction between proteins and hydrocolloids is also very important in determining the interfacial properties and bulk rheology of the system. In addition, the proteins can also interact with the emulsifiers present in the system and this interaction requires particular attention. 

The rheology of surfactant (otherwise called detergent) systems is important for such products as... 

Two applied areas are then covered, first the role of rheology in the surfactant systems chapter 18) found in so many everyday products, and then we give a short overview of the rheology of food systems chapter 19). 

Interactions between surfactants and natural and synthetic polymers have been studied for many years because they are vitally important to the success of product formulations in many areas. Although the basic mechanisms of interaction are reasonably well understood, there still exists disagreement on the details of some of the surfactant-polymer interactions at the molecular level. Observations on changes in the interfacial, rheological, spectroscopic, and other physicochemical properties of surfactant systems containing polymers indicate that such interactions, regardless of the exact molecular explanation, can significantly alter the macroscopic characteristics of the system and ultimately its end-use functionality. 

In modem industrial practice, compositions often contain pigments, reinforcements, rheological modifiers, surfactants, and other materials in addition to fillers. These materials can function synergisticaHy in the system. Hence, more complex models are needed to predict the optimal filler loading. ExceUent discussions of filler loading and selection in plastics are given (9,10). 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

Lu B, Zheng Y, Scriven LE, Davis HT, Talmon Y, Zakin JL (1998) Effect of variation counterion-to-surfactant ratio on rheology and micro-structures of drag reducing cationic surfactant systems. Rheol Acta 37 528-548... 

Rehage H, Hoffmann H. Rheological properties of viscoelastic surfactant systems. J Phys Chem 1988 92 4712-4719. 

Grant, J., Cho, J., Allen, C. (2006). Self-assembly and physicochemical and rheological properties of a polysaccharide-surfactant system formed front the cationic biopolymer chitosan and nonionic sorbitan esters. Langmuir, 22,4327- 4335. 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

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