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Atomic Force Microscopy
Published in Thomas M. Nordlund, Peter M. Hoffmann, Quantitative Understanding of Biosystems, 2019
Thomas M. Nordlund, Peter M. Hoffmann
Hydrophobic forces are forces that act between two hydrophobic surfaces in an aqueous solution. The origin of hydrophobic forces is subject to much controversy, but scientists generally agree that they are an example of entropic forces. Entropic forces are forces that are not due to a change in a potential, but rather due to a change in average arrangement of particles, which lowers entropy. In water, nonpolar molecules cannot form hydrogen bonds with water. Because water likes to form hydrogen bonds, the presence of a nonpolar object restricts the motion of water molecules around it, decreasing its entropy. A lower entropy implies increased free energy, and therefore hydrophobic materials expel water from their surfaces. If two hydrophobic particles or surfaces approach each other in water, it is entropically favorable to bring them closer together to minimize the disruption to hydrogen bonds in the water. Therefore, an attractive force between the hydrophobic surfaces is generated. This force can be quite strong for a macroscopic object, like an AFM tip, and can make it difficult to image a hydrophobic surface in water using tapping or non-contact mode.
Nanoparticles in Fluids
Published in Chun Huh, Hugh Daigle, Valentina Prigiobbe, Maša Prodanović, Practical Nanotechnology for Petroleum Engineers, 2019
Chun Huh, Hugh Daigle, Valentina Prigiobbe, Maša Prodanović
In random copolymers, monomers have no definite order or arrangement. Block polymers have a long segment or block of one monomer followed by a block of a second monomer. The result is that different homopolymer chains are joined in a head-to-tail configuration (Ellerstein and Ullman 1961). So, a block polymer is a linear arrangement of blocks of different monomer composition. A diblock copolymer is poly-A-block-poly-B, and a triblock copolymer is poly-A-block-poly-B-block-poly-A. If A is a hydrophilic group and B is hydrophobic group, the result can be regarded as polymeric surfactant (Piirma 1992). If two NPs with adsorbed polymer layers reach a distance from each other which is less than twice the thickness of the adsorbed polymer layer, the two layers can interact. If the resulting Gibbs free energy upon interaction (ΔG) is negative, NPs will aggregate. Following Sato and Ruch (1980), the entropic stabilization theory assumes that a second surface approaching the adsorbed layer is impenetrable. Therefore, the adsorbed polymer layer of one of the interacting NP is compressed and the polymer in this compressed zone can occupy fewer configurations, hence, losing configurational entropy. Considering ΔG=ΔH−TΔS and neglecting the change in enthalpy (ΔH), a reduction of entropy (ΔS) translates into an increase in ΔG and an overall effect of NP repulsion. Finally, the fundamental understanding of the entropic forces can help to control the self-assembly (Coalson et al. 2015), which can be used for applications in biomedical and process engineering.
Role and importance of surface heterogeneities in transport of particles in saturated porous media
Published in Critical Reviews in Environmental Science and Technology, 2020
Chongyang Shen, Yan Jin, Jie Zhuang, Tiantian Li, Baoshan Xing
While the VDW and DL interactions are long-range forces, some non-DLVO forces can also arise when the separation distances between particle and collector surfaces are short. Hydration and steric repulsion are the most common short-range repulsive forces encountered by particles in aqueous media (Petosa et al., 2010). For a hydrophilic particle, a quasi-discrete water layer (i.e. in an oscillatory density profile extending several molecule diameters into the solution) can form around the particle surface in aqueous media. When the particle approaches a collector surface, the pressure of the water in the boundary layer will increase, resulting in a repulsive interaction (i.e. hydration) (Grasso et al., 2002; Israelachvili & Wennerstrom, 1996). If the particle and/or collector surfaces carry chain molecules, the entropy of the chains can result in a repulsive entropic force (i.e. steric repulsion) when the two surfaces approach each other (Israelachvili, 2011; Marra, 1985). Traditionally, the influence of short-range repulsion such as hydration and steric interaction has been included in DLVO energy calculations by assigning a minimum separation distance or calculating the Born (BR) potential energy (Ruckenstein & Prieve, 1976). The former method means that the potential energy is infinitely repulsive at the minimum separation distance and zero at all other separation distance. The Born potential is derived from the empirical Lennard-Jones 6–12 potential model using Hamaker’s approach. Both methods are highly speculative for estimating the effects of short-range repulsion. In contrast to the repulsive hydration and steric interactions, hydrophobic interaction can cause attractions at short separation distances which can play a critical role in attachment of particles such as viruses and nanobubbles (Meyer, Rosenberg, & Israelachvili, 2006; Song et al., 2011). Hydrophobic interaction is viewed as the consequence of the thermodynamically unfavorable interaction of hydrophobic substances with water molecules, which is not due to interactions between hydrophobic surfaces themselves (Schijven & Hassanizadeh, 2000).