Surface forces colloids

For monodisperse or unimodal dispersion systems (emulsions or suspensions), some literature (28-30) indicates that the relative viscosity is independent of the particle size. These results are applicable as long as the hydrodynamic forces are dominant. In other words, forces due to the presence of an electrical double layer or a steric barrier (due to the adsorption of macromolecules onto the surface of the particles) are negligible. In general the hydrodynamic forces are dominant (hard-sphere interaction) when the solid particles are relatively large (diameter >10 (xm). For particles with diameters less than 1 (xm, the colloidal surface forces and Brownian motion can be dominant, and the viscosity of a unimodal dispersion is no longer a unique function of the solids volume fraction (30). 

Chin, Ch.J.. Yiacoumi. S.. and Tsoui is. C., Influence of metal ion sorption on colloidal surface forces measured by atomic force microscopy. Environ. Sci. Technol., 36, 343, 2002. 

The forces between colloidal surfaces in solution can be measured by a number of clever techniques. One important method involves x-ray diffraction from an ordered... 

R. J. Hunter, Foundations of Colloid Science, Vol. 1, Clarendon Press, Oxford, 1987. J. Israelachvili, Intermolecular and Surface Forces, 2nd ed., Academic, San Diego, CA,... 

Craig V S J 1997 An historical review of surface force measurement techniques Colloids Surf. A Physicochem. Eng. Aspects 129-30 75... 

Dan N 1996 Time-dependent effects in surface forces Current Opinion Colloid Interface Sol. 1 48-52... 

Chowdhury P B and Luckham P F 1995 Interaction forces between kappa-casein adsorbed on mica Colloids Surfaces B 4 327-34... 

Holmberg M ef al 1997 Surface force studies of Langmuir-Blodgett cellulose films J. Colloid Interface Sci. 186 369-81... 

Pashley R M and Israelachvili J N 1981 A comparison of surface forces and interfacial properties of mica in purified surfactant solutions Colloids Surf. 2 169-87... 

Surfaces can be characterized using scaiming probe microscopies (see section B1.19). In addition, by attaching a colloidal particle to tire tip of an atomic force microscope, colloidal interactions can be probed as well [27]. Interactions between surfaces can be studied using tire surface force apparatus (see section B1.20). This also helps one to understand tire interactions between colloidal particles. 

Tabor, D., Surface forces and surface interaction. J. Colloid Interface Sci., 58, 2-13 (1977). 

Molecularly motivated empiricisms, such as the solubility parameter concept, have been valuable in dealing with mixtures of weakly interacting small molecules where surface forces are small. However, they are completely inadequate for mixtures that involve macromolecules, associating entities like surfactants, and rod-like or plate-like species that can form ordered phases. New theories and models are needed to describe and understand these systems. This is an active research area where advances could lead to better understanding of the dynamics of polymers and colloids in solution, the rheological and mechanical properties of these solutions, and, more generally, the fluid mechaiucs of non-Newtonian liquids. 

The surface force apparatus (SFA) is a device that detects the variations of normal and tangential forces resulting from the molecule interactions, as a function of normal distance between two curved surfaces in relative motion. SFA has been successfully used over the past years for investigating various surface phenomena, such as adhesion, rheology of confined liquid and polymers, colloid stability, and boundary friction. The first SFA was invented in 1969 by Tabor and Winterton [23] and was further developed in 1972 by Israela-chivili and Tabor [24]. The device was employed for direct measurement of the van der Waals forces in the air or vacuum between molecularly smooth mica surfaces in the distance range of 1.5-130 nm. The results confirmed the prediction of the Lifshitz theory on van der Waals interactions down to the separations as small as 1.5 nm. 

Surface forces measurement directly determines interaction forces between two surfaces as a function of the surface separation (D) using a simple spring balance. Instruments employed are a surface forces apparatus (SFA), developed by Israelachivili and Tabor [17], and a colloidal probe atomic force microscope introduced by Ducker et al. [18] (Fig. 1). The former utilizes crossed cylinder geometry, and the latter uses the sphere-plate geometry. For both geometries, the measured force (F) normalized by the mean radius (R) of cylinders or a sphere, F/R, is known to be proportional to the interaction energy, Gf, between flat plates (Derjaguin approximation). 

FIG. 1 Schematic drawings of (a) the surface forces apparatus and (b) the colloidal probe atomic force microscope. 

The process of adsorption of polyelectrolytes on solid surfaces has been intensively studied because of its importance in technology, including steric stabilization of colloid particles [3,4]. This process has attracted increasing attention because of the recently developed, sophisticated use of polyelectrolyte adsorption alternate layer-by-layer adsorption [7] and stabilization of surfactant monolayers at the air-water interface [26], Surface forces measurement has been performed to study the adsorption process of a negatively charged polymer, poly(styrene sulfonate) (PSS), on a cationic monolayer of fluorocarbon ammonium amphiphilic 1 (Fig. 7) [27],... 

Israelachvili, J, Intermolecular and Surface Forces, 2nd ed. Academic Press London, 1992. Ivory, CF, Transient Electrophoresis of a Dielectric Sphere, Journal of Colloid and Interface Science 100, 239, 1984. 

Israelachvili, J.N. (1985) Intermolecular and Surface Forces with Applications to Colloidal and Biological Systems, Academic Press, London. 

T.D. Dimitrova, F. Leal-Calderon, T.D. Gurkov, and B. Campbell Surface Forces in Model Oil-in-Water Emulsions Stabilized by Proteins. Adv. Colloid Interface Sci. 108-109, 73 (2004). 

In the past decades, it has become more and more obvious that students and scientists of chemistry and engineering should have some understanding of surface and colloid chemistry. The textbooks on physical chemistry tend to introduce this subject insufficiently. Modern nanotechnology is another area where the role of surface and chemistry is found of much importance. Medical diagnostics applications are also extensive, where both microscale and surface reactions are determined by different aspects of surface and colloid chemical principles. Drug delivery is much based on lipid vesicles (self-assembly structure) that are stabilized by various surface forces. 

In 1991, the same author helped to develop a new experimental procedure, called the colloid probe technique , which is now widely used to measure the interaction forces between colloidal surfaces (see Ducker et ah, 1991). 

PC Hiemenz. Principles of colloid and surface chemistry. New York Marcel Dekker, 1986. J Israelachvili. Intermolecular and Surface Forces. Orlando, FL Academic Press, 1992. 

In this section, we present a few examples of instruments available for visual observation and imaging of colloids and surfaces, for measurement of sizes and for surface force measurements. Such a presentation can hardly be comprehensive in fact, that is not our purpose here. Throughout the book, we discuss numerous other techniques such as osmotic pressure measurements, light and other radiation scattering techniques, surface tension measurements,... 

A number of techniques have been developed over the years to determine colloidal and surface forces as well as interatomic and intermolecular forces of interest in colloid and surface chemistry. These methods can be divided into two groups (a) indirect methods and (b) direct methods. We discuss examples of both in other chapters. It is therefore useful to consider these methods here briefly. 

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