Forces colloidal

Ducker W A, Senden T J and Pashley R M 1991 Direct measurement of colloidal forces using an atomic force microscope Nature 353 239... 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

Ducker, W. A., T. J. Senden, and R. M. Pashley (1991), "Direct Measurement of Colloidal Forces Using an Atomic Force Microscope", Nature 353, 239-241. 

F. Leal-Calderon, T. Stora, O. Mondain Monval, P. Poulin, and J. Bibette Direct Measurement of Colloidal Forces. Phys. Rev. Lett. 72, 2959 (1994). 

The formation of complexes is not restricted to mixtures of polyectrolytes and surfactants of opposite charge. Neutral polymers and ionic surfactants can also form bulk and/or surface complexes. Philip et al. [74] have studied the colloidal forces in presence of neutral polymer/ionic surfactant mixtures in the case where both species can adsorb at the interface of oil droplets dispersed in an aqueous phase. The molecules used in their studies are a neutral PVA-Vac copolymer (vinyl alcohol [88%] and vinyl acetate [12%]), with average molecular weight M = 155000 g/mol, and ionic surfactants such as SDS. The force measurements were performed using MCT. The force profiles were always roughly linear in semilogarithmic scale and were fitted by a simple exponential function ... 

D.C. Prieve and N.A. Frej Total Internal Reflection Microscopy A Quantitative Tool for the Measurement of Colloidal Forces. Langmuir 6, 396 (1990). 

In the past decade, much development has taken place in regard to measuring the forces involved in these colloidal systems. In one method, the procedure used is to measure the force present between two solid surfaces at very low distances (less than micrometer). The system can operate under water, and thus the effect of addictives has been investigated. These data have provided verification of many aspects of the DLVO theory. Recently, the atomic force microscope (AFM) has been used to measure these colloidal forces directly (Birdi, 2002). Two particles are brought closer, and the force (nanoNewton) is measured. In fact, commercially available apparatus are designed to perform such analyses. The measurements can be carried out in fluids and under various experimental conditions (such as added electrolytes, pH, etc.). 

AFM has been used to study surface molecules under different conditions. Colloidal system studies by AFM AFM has allowed scientists to be able to study molecular forces between molecules at very small (almost molecular size) distances. Further, it is a very attractive and sensitive tool for such measurements. In a recent study, the colloidal force as a function of pH of Si02 immersed in the aqueous phase was reported using AFM. The force between an Si02 sphere (ca. 5 mm diameter) and a chromium oxide surface in the aqueous phase of sodium phosphate were measured (pH from 3 to 11). The Si02 sphere was attached to the AFM sensor as shown in Figure 10.3. 

The main objective of the present section, however, is to begin with a very standard technique such as optical microscopy and to use it to illustrate why colloids are difficult to see and what modern developments have emerged in recent years to allow us to see and do things that were considered impossible until a decade ago. We also use this opportunity to review briefly some new techniques that are currently available to measure interaction forces between particles directly. We appeal to some of these techniques in other chapters when we discuss colloidal forces. 

The final group deals with colloidal forces and their applications to colloid stability and deposition phenomena. These include... 

Our objective in this chapter is modest, namely, to provide a general discussion of the electroviscous effects and to present a few equations that serve as guidelines for understanding the effects of colloidal forces on the viscosity of dispersions. The underlying theories are rather complicated and fall outside our scope. 

While in the case of noninteracting dispersions one needed to consider only the effect of the particle concentration, in interacting dispersions one needs to consider the time over which the flow behavior is observed and its magnitude relative to the time scales over which either shear or colloidal forces alter the local structure of the dispersions. What the flow behavior is, which interaction effects dominate the behavior, and how they do depend on the competing influences of the applied shear and interaction effects. In this section, we outline some of the important parameters one can formulate to judge the relative effects of various colloidal interactions and the physical significance of those parameters. 

Elimelech, M., Gregory, J., Jia, X., and Williams, R., Particle Deposition and Aggregation Measurement, Modelling and Simulation, Butterworth-Heinemann, Oxford, England, 1995. (Graduate and research levels. A state-of-the-art treatment of deposition of colloidal particles and their dependence on colloidal forces. Includes theoretical, computational, and experimental approaches.)... 

Kruyt, H. R. (Ed.), Colloid Science. Vol. 1. Irreversible Systems, Elsevier, Amsterdam, Netherlands, 1952. (Graduate and undergraduate levels. A classic reference on colloids. Chapters 6-8, by Professor J. Th. G. Overbeek, present the classical DLVO theory of colloidal forces and their application to kinetics of coagulation.)... 

This rearrangement makes the presentation of colloidal forces, colloid stability, and electrokinetic phenomena more logical and pedagogically more appealing. 

See D. C. Prieve, "Measurement of colloidal forces with TIRM," Adv. Colloid Interface Sci., 82, 93-125 (1999), for a clear description of technique as well as references also S. G. Bike, "Measuring colloidal forces using evanescent wave scattering," Curr. Opin. in Colloid Interface Sci., 5, 144-50 (2000). 

There are some qualitative difficulties when the specific ion effects are explained via the dispersion forces of the ions. Particularly the anions, for which the dispersion coefficients / , are large, affect the double layer interactions. However, experiments on colloid stability [6] or colloidal forces [11] revealed strong specific ion effects especially for cations. Furthermore, the ions which affect most strongly the solvating properties of the proteins are those from their vicinity, since they perturb mostly the structure of water near the proteins. However, the van der Waals interactions of ions predict that the cations remain in the vicinity of an interface, and the anions are strongly repelled, while Hofmeister concluded that anions are mainly responsible for the salting out of proteins. 

Since the first report by Ducker et al. on the direct measurement of colloidal forces using AFM [43], a number of investigations have been carried out to measure attractive van der Waals forces, electrostatic forces (double-layer forces) [44], hydrophobic forces [45-50], intermolecular forces between ligands and receptors [51,52], such as the avidin-biotin complex... 

We have also to say a few words about a smeared out analogue utilized in our consideration to represent the surface charge distribution. There was an indication in the literature that the image interaction within this adsorption layer may influence the interaction on a macroscopic scale, i.e., which is being commensurate with the range of colloidal forces [29]. However, later results... 

These trajectory methods have been used by numerous researchers to further investigate the influence of hydrodynamic forces, in combination with other colloidal forces, on collision rates and efficiencies. Han and Lawler [3] continued the work of Adler [4] by considering the role of hydrodynamics in hindering collisions between unequal-size spheres in Brownian motion and differential settling (with van der Waals attraction but without electrostatic repulsion). The results indicate the potential significance of these interactions on collision efficiencies that can be expected in experimental systems. For example, collision efficiency for Brownian motion will vary between 0.4 and 1.0, depending on particle absolute size and the size ratio of the two interacting particles. For differential... 

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