Contact interactions colloid stability

Clays of the montmorillonite family are lamellar aluminosificates [46] used in many industrial processes and in products such as paints, softeners, and composite materials [47]. They swell when brought into contact with water, which is due to the insertion of water molecules between the sheets. Complete exfoliation can be induced leading to dispersions of disk-like particles of 10 A thickness and 300-3000 A in diameter, depending on the variety of clay used. These clay platelets bear a rather large surface electrical charge so that electrostatic interactions between them must be considered and are actually responsible for the colloidal stability of these dispersions. These suspensions have been widely studied as model colloids and also because they form physical thixotropic gels. 

Consequently, for symmetric films the molecular component of disjoining pressure is always negative, which corresponds to a tendency of dispersion medium layer separating identical phases to decrease its thickness. At the same time, one should emphasize that in such systems in the absence of non-dispersion interactions the lower the value of complex Hamaker constant is, the more similar in nature the interacting phases (dispersed phase and dispersion medium) are. If contacting phases are essentially similar in structure and chemical composition, the value of A may be as low as 10 21 J or even much lower. The so low Hamaker constants result in changes in the nature of colloidal stability. 

Two diverse views of non-specific adhesion processes form the bases for contemporary theories introduced to rationalize observations of colloidal stability and flocculation in solutions of macromolecules (see 16-18 for general reviews). The first view is based on adsorption and cross-bridging of the macromolecules between surfaces. Theories derived from this concept indicate a strong initial dependence on concentration of macromolecules there is a rapid rise in surface adsorption for infinitesimal volume fractions (32) followed by a plateau with gradual attenuation of surface-surface attraction because of excluded volume effects in the gap at larger volume fractions (19-20). The interaction of the macroinolecule with the surface is assumed to be a snort range attraction proportional to area of direct contact. The second - completely disparate - view of non-specific adhesion is based on the concept that there is an exclusion or depletion of macromolecules in the vicinity of the surface, i.e. no adsorption to the surfaces. Here, theory shows that attraction is caused by interaction of tne (depleted) concentration profiles associated with each surface which leads to a depreciated macrornolecular concentration at the center of the gap. The concentration... 

Atomic force microscopy (AFM) [20] has recently been used to image interfacial aggregates directly, in situ and at nanometer resolution [21, 22], The key to this application lies in an unusual contrast mechanism, namely a pre-contact repulsive force ( colloidal stabilization force ) between the adsorbed surfactant layers on the tip and sample. In contrast to previous adsorbate models of flat monolayers and bilayers, AFM images have shown a striking variety of interfacial aggregates - spheres, cylinders, half-cylinders and bilayers - depending on the surface chemistry and surfactant geometry. I review the AFM evidence for these structures and discuss the possible inter-molecular and molecule-surface interactions involved. 

Let us now address an interesting problem in colloid science and physical-chanical mechanics related to the contact interactions in disperse systems, namely, the possibility of spontaneous dispersion and the formation of thermodynamically stable colloid system. Originally, this problem was formulated by Max Volmer in 1927 [60,61] and later addressed by Rehbinder and Shchukin. Shchukin has made two principal contributions to the analysis of this problem [33,62-69]. The first is the detailed analysis of the conditions that make the process of spontaneous dispersion (at constant volume of the disperse phase, constant particle size, or constant number of particles) possible. Second, he proposed incorporating the entropy of mixing into the description of the conditions of spontaneous dispersion. The latter allows one to quantitatively estimate the concentration of the disperse phase in the disperse system formed. The analysis of the thermodynamics of spontaneous dispersion has important implications in the analysis of colloidal stability and in the control of various technological processes. 

Two brushes repel each other as they are brought into contact. This repulsion, which is a result of the osmotic interaction between the polymer segments, is the basis for colloidal stabilization. It has also been utilized to probe the brush structure. " Most of these studies have been on... 

The presence of shielding compounds interferes with subsequent processes, as the formation of metal-support interactions is able to stabilize supported particles. Moreover, the shielding effect of the colloid protectors prevents the contact of metal particles with the reacting molecules, thus avoiding the use of unsupported colloidal particles as a catalytic system [11]. 

The steric stabilization, which is imparted by polymer molecules grafted onto the colloidal particles, is extensively employed.3 Amphiphilic block copolymers are widely used as steric stabilizers. The solvent-incompatible moieties of the block copolymer provide anchors for the polymer molecules that are adsorbed onto the surface of the colloidal particles, and the solvent-compatible (buoy) moieties extend into the solvent phase. When two particles with block copolymers on their surface approach each other, a steric repulsion is generated bet ween the two particles as soon as the tips of the buoy moieties begin to contact, and this repulsion increases the stability of the colloidal system.4-6 Polymers can also induce aggregation due to either depletion 7-11 or bridging interactions.12 15... 

Besides equilibria in the liquid phase (proteolytic, complex forming, etc.) that influence directly the values of effective mobilities of compounds to be separated, it is necessary to also establish, in the electrophoretic system, equilibria between the liquid and solid phase. In electrophoretic techniques which use solid stabilizing media adsorption of solutes on the sorbent surface is the main consideration. In capillary methods, and with colloid particles, similar effects have also to be considered (the surface of the solid phase that is in contact with the liquid phase is, with respect to the volume of the liquid, rather large). In both these latter cases the interaction between the solid and liquid phases participates in the formation of the electric double layer that conditions the electro-osmotic flow, and attributes the electric charge to colloid particles. 

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