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Published in Mohammad E. Khosroshahi, Applications of Biophotonics and Nanobiomaterials in Biomedical Engineering, 2017
A depletion force often is regarded as an entropic force and is an effective attractive force that arises between large colloidal particles that are suspended in a dilute solution of depletants, which are smaller solutes that are preferentially excluded from the volume between the approaching colloidal spheres (Fig. 4.5). When colloidal spheres and polymers are mixed in a solution (e.g., water), the colloides can experience an effective attractive interaction force in the absence of polymers in the space between colloidal spheres, i.e., the excluded volume. The depletion force is defined as: Fd=-JIVd=-JI4/3πRg3 $$ F_{d} = - JIV_{d} = - JI4/3\pi R_{g}^{3} $$
Effective electrostatic interaction between columnar colloids: roles of solvent steric hindrance, polarity, and surface geometric characteristics
Published in Molecular Physics, 2023
Surface-surface interactions in solution control the stability and dynamics properties of complex fluids [1–11] that are critical for a wide range of important applications, their studies are important for developments of many techniques and processes, such as molecular recognition, folding and repair [12–14], self-assemblies [15,16], colloidal stability against aggregations [17–19], efficient separation of nanoparticles in microfluidic devices [20–22]. The surface-surface interaction potential in solution is also called effective potential [23–27], and corresponding force is called effective force, which can be decomposed into energetic and entropic components. The effective force originates from the interactions among medium particle, solute particle and surface [28]; these interaction forces include van der Waals force, electrostatic force, force existing between polar molecules (for example hydrogen bonding), and hard sphere repulsion interaction (usually called steric hindrance interaction). Correspondingly, one has the so-called solvent-mediated force, effective electrostatic force, hydrophobic force, and entropic force (also called depletion force). It should be pointed out that these various forces cannot be absolutely separated; their dependence on intermolecular interaction is quite complex, and their simple addition is not the total effective force. Generally speaking, if certain component of the total effective forces dominates, we call the total effective force by this component force, but other components of the effective force also exist. For example, if the medium particle is polar molecule, for example water molecule, and the surface is not hydrophilic, the water molecule tends to associate with each other. Thus, an attractive effective force is induced between the two surfaces when the latters get nearer to each other in the water solution; such attractive effective force can appropriately be called as hydrophobic force [29–32] because interaction between the polar molecules is stronger than the van der Waals force existing between any two neutral molecules. The effective potential in electrolyte solution is usually called as effective electrostatic potential [33–43] as the electrostatic interaction between ions is always stronger than the van der Waals force and polar force, and the steric hindrance coming from the ions is relatively very weak because of the small ion number density. However, in the electrolyte solution, in addition to ions, there are solvent molecules in large numbers. So, the steric hindrance-related effective force coming from the solvent molecules should be not negligible [44–46]. However, in the widely used primitive model (PM), the water is represented by a dielectric continuum; accordingly, only the energetic effect originating from the water molecule electric dipole is considered approximately by a relative dielectric constant. In consequence, the effective electrostatic potential based on the PM necessarily does not include the entropic force component to a considerable extent.