Interaction forces, between membrane surfaces

Berkowitz and Raghavan Interaction Forces between Membrane Surfaces 5... 

Berkowitz and Raghavan Interaction Forces between Membrane Surfaces 11 For k h >> 1, the repulsion follows the exponential law ... 

TTHE MOST IMPORTANT FORCES ACTING BETWEEN MEMBRANE SURFACES are van der Waals, electrostatic, and hydration. The first two forces are explained by the Deijaguin-Landau-Verwey-Overbeek (DLVO) theory (I) the existence of the hydration force was anticipated before it was measured (2). The van der Waals force is always attractive and displays a power law distance dependence, whereas the electrostatic and hydration forces are repulsive and exponentially decay with distance. The electrostatic force describes the interaction between charged membrane surfaces when the separation between surfaces is above 10 molecular solvent diameters. The hydration force acts between charged and uncharged membrane surfaces and at distances below 10 molecular solvent diameters its value dominates the values of van der Waals and electrostatic forces (3). The term hydration reflects the belief that the force is due to the structure of water between the surfaces. Electrostatic and hydration forces are similar in some respects both are exponential and repulsive and their theoretical description involves coupling electrostatic concepts and ideas borrowed from statistical mechanics. Although the nature of the electrostatic force is solidly established, this is not the case for the hydration force. To illustrate the role the electrostatic... 

The silicon nitride tip is used mostly for C-AFM. Measurements can be done in ambient air, controlled atmospheres, or in non-aggressive liquids. AFM also allows surface forces, and even molecular forces, to be directly quantified [23]. For example, the interaction forces between a silicon tip and microfiltration and ultrafiltration membranes in an electrolyte solution can be measured [24]. The geometry of the cantilever is not simple, and in some cases not even known, so comparison with theory is difficult. However, attaching a sphere to the cantilever instead of a tip enables the measurement of interaction between surfaces of known geometry [25]. This technique has been used to measure interactions between different materials in air... 

Recently, Bowen et al. [27,28] and Hilal and Bowen [29] and Hilal et al. [30] applied the APM technique to study adhesion at the membrane sinface. The measurement of interaction forces between a colloid probe and a membrane smface allows quantification of the electrostatic double layer interactions when the colloid probe approaches the membrane surface, and of the adhesion force (van der Waals interaction force) when the colloid probe is withdrawn after it has been in contact with the membrane surface. Quantification of the interaction forces involved in fouling and chemical cleaning of fouled membranes is very important in order to imderstand the mechanism of fouling and to develop a favorable membrane for water treatment. 

Perhaps the least-understood set of interactions involving membranes is that between bilayers or other amphiphilic surfaces. Yet, no discussion of membrane functions would be complete without considering these forces because of their essential role in cell-cell interactions and membrane fusion. The forces acting between membranes are often called hydration forces . This name originates in the tendency at one time to ascribe many unexplained or ubiquitous effects seen in various colloidal systems to the effects of aqueous solvation. However, it is still unclear, despite recent experimental and theoretical advances, whether or not these forces are mainly due to hydration , as understood in the conventional sense. In this section, we describe some of the major issues involving hydration forces between membranes, and provide a summary of selected theoretical and experimental works. More complete reviews, with different emphases, can be found in articles by Israelachvili, et al. [136,137] and Leikin, et al. [138]. 

In the earliest theory of cell adhesion or aggregation, Tyler (1947) and P. Weiss (1947) proposed an antigen-antibody interaction as the cause of attraction. The requirement of destruction of the antigen or antibody site for detachment (Roseman, 1974) and the probable absence of antibodies at the surface of cells (Roth, 1973) makes this concept unlikely. Curtis (1960 1962) based his theory of cell adhesion or aggregation on the Verwey and Overbeck (1948) theory applied to the adhesion of two parallel membranes at about 150 A of distance. This theory is based on the existence of van der Waals forces between membranes if the constitutions of these membranes are similar. It is evident that the sialic acid residues, which have a flexible chain of alcohol groups (C7 to C9), could play an important role in this type of interaction. 

With an AFM adhesion force measurement technique, Bowen et al. [42] characterized an interaction force between a colloidal silica probe and a rough membrane surface. It was found that membrane surface roughness significantly reduced electrostatic repulsion between the colloid and the surface, and the valley regions experienced a greater adhesion force. 

The surface molecules are under a different force field from the molecules in the bulk phase or the gas phase. These forces are called surface forces. A liquid surface behaves like a stretched elastic membrane in that it tends to contract. This action arises from the observation that, when one empties a beaker with a liquid, the liquid breaks up into spherical drops. This phenomenon indicates that drops are being created under some forces that must be present at the surface of the newly formed interface. These surface forces become even more important when a liquid is in contact with a solid (such as ground-water oil reservoir). The flow of liquid (e.g., water or oil) through small pores underground is mainly governed by capillary forces. Capillary forces are found to play a very dominant role in many systems, which will be described later. Thus, the interaction between liquid and any solid will form curved surface that, being different from a planar fluid surface, initiates the capillary forces. 

It is important to note that the concept of osmotic pressure is more general than suggested by the above experiment. In particular, one does not have to invoke the presence of a membrane (or even a concentration difference) to define osmotic pressure. The osmotic pressure, being a property of a solution, always exists and serves to counteract the tendency of the chemical potentials to equalize. It is not important how the differences in the chemical potential come about. The differences may arise due to other factors such as an electric field or gravity. For example, we see in Chapter 11 (Section 11.7a) how osmotic pressure plays a major role in giving rise to repulsion between electrical double layers here, the variation of the concentration in the electrical double layers arises from the electrostatic interaction between a charged surface and the ions in the solution. In Chapter 13 (Section 13.6b.3), we provide another example of the role of differences in osmotic pressures of a polymer solution in giving rise to an effective attractive force between colloidal particles suspended in the solution. 

Possible Role of Long-Range VDW Forces Between Cell Membrane Surfaces. The above discussion has been based on the model of macro-molecular bridging, and the Eb is caused by the interaction between the bridging macromolecule and the cell surface. Recent theoretical and experimental studies (41,42) have shown that VDW attractive forces can be exerted between cell surfaces over distances in the range found in RBC rouleaux. 

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