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Surface Forces
Published in Arthur T. Hubbard, The Handbook of Surface Imaging and Visualization, 2022
From the previous discussions we can infer that hydration forces are not of a simple nature, and it may be fair to say that this interaction is probably the most important yet the least understood of all the forces in liquids. Clearly, the very unusual properties of water are implicated, but the nature of the surfaces is equally important. Some particle surfaces can have their hydration forces regulated, for example, by ion exchange. Other surfaces appear to be intrinsically hydrophilic (e.g., silica) and cannot be coagulated by changing the ionic conditions. However, such surfaces can often be rendered hydrophobic by chemically modifying their surface groups. For example, on heating silica to above 600°C, two surface silanol -OH groups release a water molecule and combine to form a hydrophobic siloxane -O- group, whence the repulsive hydration force changes into an attractive hydrophobic force.
Surface Forces and Microrheology of Molecularly Thin Liquid Films
Published in Bharat Bhushan, Handbook of Micro/Nano Tribology, 2020
Jacob N. Israelachvili, Alan D. Berman
From the previous discussions we can infer that hydration forces are not of a simple nature, and it may be fair to say that this interaction is probably the most important yet the least understood of all the forces in liquids. Clearly, the very unusual properties of water are implicated, but the nature of the surfaces is equally important. Some particle surfaces can have their hydration forces regulated, for example, by ion exchange. Other surfaces appear to be intrinsically hydrophilic (e.g., silica) and cannot be coagulated by changing the ionic conditions. However, such surfaces can often be rendered hydrophobic by chemically modifying their surface groups. For example, on heating silica to above 600°C, two surface silanol –OH groups release a water molecule and combine to form a hydrophobic siloxane –O– group, whence the repulsive hydration force changes into an attractive hydrophobic force.
Liposomal Drug Delivery System and Its Clinically Available Products
Published in Vladimir Torchilin, Handbook of Materials for Nanomedicine, 2020
Upendra Bulbake, Nagavendra Kommineni, Wahid Khan
A phospholipid has both a hydrophobic and a hydrophilic component. Hence, it is called amphipathic molecule. A single phospholipid molecule has a phosphate head group on one end and two fatty acids chains side-by-side of that which makes the lipid “tails.” The head is polar and hydrophilic due to negatively charged phosphate group. The phosphate groups, therefore, are attracted to the hydrophilic molecules in their environment. On the other hand, the lipid tails are nonpolar, hydrophobic, and uncharged. Hence, hydrophobic molecules are repelled by water. Some lipid tails contain saturated fatty acids and some contain unsaturated fatty acids. Phospholipid molecules as such are not water soluble, but rather, the molecules fused and align themselves in a planar bilayer form. Due to this planar arrangement hydrophilic portion of lipid bilayers has ability to interact with water. However, the hydrophobic portion will align away from water molecules.
A study on the influence of silanized clay on the barrier, hydrophobic and mechanical properties of epoxy coated steel in natural seawater
Published in The Journal of Adhesion, 2023
Joseph Raj Xavier, Raja Beryl J, Ravisankar N
One of the greatest techniques to detect whether a coated surface is hydrophobic or hydrophilic is by measuring the water contact angles (WCA). Hydrophobic materials are classified as non-polar materials with a low affinity for water, which makes them water-repellent. A contact angle of less than 90° denotes a hydrophilic interaction, whereas one of higher than 90° denotes a hydrophobic interaction. Hydrophobicity can be advantageous for the coating and substrate by reducing the amount of dirt that they retain, making them easier to clean themselves, being better at withstanding moisture and corrosion, and having longer lifespans. Images of water droplets on the surfaces of nanocoated steel in Figure 5 include pure EP, EP-Clay, EP-GPMS, and EP-GPMS/Clay coatings. The WCA of 74°, which is less than 90° for the neat epoxy-coated steel, shows that the pure epoxy coating is hydrophilic. Due to the high porosity of the basic epoxy layer, the passage of water takes place smoothly via the coating. While the EP-GPMS composite exhibits hydrophobic behaviour because of the silane’s potent adhesive properties, the epoxy-clay composite maintains its hydrophilicity due to the clay’s exceptional hydrophilicity. On the other hand, the silanized clay-based epoxy nanocomposites are hydrophobic. Due to its strong hydrophobicity, EP-GPMS/Clay exhibits a WCA of 144° (Figure 5d).
Emulsion system, demulsification and membrane technology in oil–water emulsion separation: A comprehensive review
Published in Critical Reviews in Environmental Science and Technology, 2023
Yuying Deng, Min Dai, Yanni Wu, Changsheng Peng
Most polymers are hydrophobic. Superhydrophobic membrane materials can be fabricated by constructing micro-nano structures on the surfaces of polymer substrates. The high viscosity of oil easily pollutes the surface of the superhydrophobic polymer membrane, resulting in a decline in membrane flux and performance. Therefore, improving the hydrophilicity of polymer membranes is currently a research topic of great interest. Commonly used polymer substrates include polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), Polyimide (PI), and nylon (Baig et al., 2021; Feng et al., 2022). Li et al. fabricated an underwater superoleophobic PAN membrane via electrospinning and partial hydrolysis. Hydrophobic/oleophilic ZIF-8 particles consisting of bumps were then spin-coated on the membrane to construct a beetle-like structure, as shown in Figure S3c. The research showed that, compared with the traditional single underwater superoleophobic membranes, the inverse beetle-like membrane not only improved the oil-water separation efficiency but also increased the flux in the separation process (Li et al., 2021).
Development of sludge-based activated char sorbent with enhanced hydrophobicity for oil spill cleanup
Published in Environmental Technology, 2023
Ali Zaker, Zhi Chen, Kenneth Lee, Samia ben Hammouda
The intrinsic hydrophobicity of a sorbent is critical for water/oil separation. Basically, a hydrophobic surface has a strong ability to repel water. Naturally, a water drop can easily roll off from lotus leaves without wetting them due to its no adhesive properties. This phenomenon must be addressed for developing sorbents with the purpose of oil spill recovery applications. The water CA value for AC and MAC-SS is displayed in Table 3. Surfaces with a water CA in the range of 90–150° are considered hydrophobic and over 150° superhydrophobic [43]. There is evidence that the production of char at higher temperatures has fewer polar functional groups and greater hydrophobicity [45]. Consistent with that, AC had a hydrophobic CA value of around 122.4°. Despite its hydrophobicity, AC sinks rapidly when it is placed on the water surface.