Molecular-scale hydrophilicity

G. S. Shi, Y. Shen, J. Liu, C. L. Wang, Y. Wang, B. Song, J. Hu and H. P. Fang, Molecular-scale hydrophilicity induced by solute Molecular-thick charged pancakes of aqueous salt solution on hydrophobic carbon-based surfaces, Sci. Rep. 4,6793 (2014). 

Besides water and ions, other substances can also be adsorbed on both the external and internal surfaces. Since the surface of rocks and soils is strongly hydrophilic on a macroscopic scale, hydrophilic substances are more strongly adsorbed than hydrophobic substances. Some authors, however, say that the hydrophilic-hydrophobic character strongly varies on molecular scale. So the oxygen atoms next to the isomorphic substitutions are hydrophilic, while the oxygen atoms far from them are hydrophobic. The hydrophobic oxygen atoms are, for example, responsible for the adsorption of hydrophobic pesticides (Laird 2004). 

Figure 11A shows an optical micrograph of water drops preferentially condensed on hydrophilic SAMs terminated by carboxylic (COOH) groups no water condensed on the hydrophobic SAMs terminated by methyl (CH3) groups [98]. This process shows how the functionality of a SAM influences the eondensation of water vapor on a SAM-derivatized surface. It uses self-assembly at two scales the formation of SAMs at the molecular scale and the directed condensation of water vapor at the macroscopic scale. The organization of liquids into patterned arrays illustrates one of the uses of self-assembly in microfabrication [99]. 

Atomic ions, such as Li (aq), are common and chemically the simplest hydrophilic aqueous solution species. The hydration free energies are known to be in the range of — lOOkcalmoP (Friedman and Krishnan, 1973). This magnitude is comparable to chemical bond energies, and therefore the molecular-scale... 

Lipopeptides present three main advantages. The two parts of the lipopeptide molecules (lipidic and peptidic chains) are present in many biological molecules and macromolecules and one can expect a good compatibility with biological fluids and tissues (4) and an absence of toxicity of their degradation products. The HLB of lipopeptides can be easily adjusted by varying the degree of polymerization p of the peptidic chains and the nature of the amino-atid side chains R. The incompatibility between the hydrophobic paraffinic chains and the hydrophilic peptidic chains leads to a phase separation at the molecular scale and to the existence of mesophases (7). 

COLLOIDS (SECTION 13.6) Particles that are large on the molecular scale but stiU small enough to remain suspended indefinitely in a solvent system form colloids, or colloidal dispersions. Colloids, which are intermediate between solutions and heterogeneous mixtures, have many practical apphcations. One useful physical property of colloids, the scattering of visible hght, is referred to as the Tyndall effect. Aqueous colloids are classified as hydrophilic or hydrophobic. Hydrophilic colloids... 

The Cassie-Baxter mode also yielded a minimized footprint of the deposited droplets, hence allowing the production of high-density noncontact printed arrays (14,000 spots/cm ) (Ressine et al. 2007). The fact that the surface is hydrophilic on a molecular scale is highly beneficial since the antibody has a lower risk of being denatured and lose its biospecific activity in contrast to what would be the case for, e.g., a highly hydrophobic fluoro-terminated surface. It should also be noted that the macro-/nanoporous silicon used for antibody microarrays displays a very low unspecific binding and hence a low background fluorescence. 

Biomaterials. Adsorbed polymers find many apphcations as surface modifiers in biomedical apphcations. By choosing a combination of hydrophobic and hydrophilic copolsrmers, surfaces can be modified to make them biocompatible (65) (see Biomolecules at Interfaces). In the area of tissue engineering (qv), adsorbed layers with specihc amino acid sequences can be used to promote cell adhesion and proliferation. The recent developments in the design of biochips to analyze specihc DNA molecules also take advantage of this technology. Polymer adsorption on patterned surfaces can be used to mimic pattern recognition. This effect can be used to develop sensors and molecular-scale separation processes (66). 

Figure 5.3 Ellipsometric profile of a TK microdroplet on a bare, oxidized silicon wafer. Vertical scale, nm horizontal scale, mm baseline, silica layer lateral resolution, 30 pm mesoscopic scale, above 10 nm, smooth profile, controlled by van der Waals interaction (Eqs. 5.9 and 5.10b) molecular scale, below 10 nm, stepped profile. In the present case, the first layer is thinner and smoother than the next ones the conformation of the molecules changes in contact with hydrophilic silica. The molecular structures are calculated ones. Abbreviation TK, tetrakis-2-ethyl hexoxysilane.

S. Villette, M. P. Valignat, A M. Cazabat, L. Jullien, and F. Tiberg, Wetting on the molecular scale and the role of water. A case study of wetting of hydrophilic silica surfaces, Langmuir, 12,825 [1996],... 

The membrane properties of interest would be flux, retention (pore size and distribution), charge, wearing, pinholes, fouling and hydrophilicity, just to mention a few. Membrane stability during the process would contribute to a sustainable process. The best monitoring processes would be in real time, non-invasive, in situ/in vivo, at a molecular scale and using pattern recognition approaches. 

Currently, these molecular systems are studies as SAMs on a metal surface. An additional method of preparing ordered monolayers of molecular devices is the use of Langmuir-Blodgett (LB) films. Therefore, a compound with hydrophilic and hydrophobic subunits with the central nitroaniline core similar to 70 was synthesized as in Scheme 3.40. n-Hexylbenzene was easily brominated on neutral alumina and coupled to TMSA followed by silyl removal and coupling to the nitroacetanilide core intermediate, 68, to afford 88. The methyl ester, intermediate 90, was synthesized by the coupling of methyl 4-ethynylbenzoate (89) to 88. The amine was unmasked and the methyl ester was saponified with lithium hydroxide to afford molecular scale device 91. Compound 91 is suitable for the formation of a LB film due to its hydrophilic carboxylic acid end-group and the hydrophobic n-hexyl end-group. 

FTIR and other analyses confirmed the existence of intermolecular hydrogen bonding between PBI and PA the carbonyl band shifts from 1741 to 1730 cm in an 80/20 blend. Also, miscibility and compatibility were indicated further by the clarity (visual and microscopic examination) of films prepared from the 80/20 blend TGAs showed blends conforming to the Rule of Mixtures, and X-ray diffraction (XRD) analysis confirmed that PBI and PA interact and are compatible on a molecular scale blends were found to have physical properties adhering to the Rule of Mixtures values the thermal stability of the blends at 550°C increased proportionately as the relative PBI concentration increased and after blending, the hydrophilicity of the PBI decreased dramatically. Thus, PAs and aromatic PBIs were found to be miscible in all proportions, and synergistic effects of one polymer over the other s weak properties can be achieved. 

---