Water monolayer

FIG. 26 Optimized structure of a water monolayer on mica obtained from molecular dynamic simulations by Odelius et al. The water molecules and the first layer of sihca tetrahedra of the mica substrate are shown in a side view in the top. K ions are the large dark balls. The bottom drawing shows a top view of the water. Oxygen atoms are dark, hydrogen atoms light. Notice the ordered icelike structure and the absence of free OH groups. All the H atoms in the water are involved in a hydrogen bond to another water molecule or to the mica substrate. (From Ref. 73.)... 

Nagy, G. and Heinzinger, K. (1992) A molecular dynamics study of water monolayers on charged platinum walls. J. Electroanal. Chem., 327, 25-30. 

The ionic profile of the metal was modeled as a step function, since it was anticipated that it would be much narrower than the electronic profile, and the distance dx from this step to the beginning of the water monolayer, which reflects the interaction of metal ions and solvent molecules, was taken as the crystallographic radius of the metal ions, Rc. Inside the metal, and out to dl9 the relative dielectric constant was taken as unity. (It may be noted that these calculations, and subsequent ones83 which couple this model for the metal with a model for the interface, take the position of the outer layer of metal ion cores to be on the jellium edge, which is at variance with the usual interpretation in terms of Wigner-Seitz... 

Nucleic acids, DNA and RNA, are attractive biopolymers that can be used for biomedical applications [175,176], nanostructure fabrication [177,178], computing [179,180], and materials for electron-conduction [181,182]. Immobilization of DNA and RNA in well-defined nanostructures would be one of the most unique subjects in current nanotechnology. Unfortunately, a silica surface cannot usually adsorb duplex DNA in aqueous solution due to the electrostatic repulsion between the silica surface and polyanionic DNA. However, Fujiwara et al. recently found that duplex DNA in protonated phosphoric acid form can adsorb on mesoporous silicates, even in low-salt aqueous solution [183]. The DNA adsorption behavior depended much on the pore size of the mesoporous silica. Plausible models of DNA accommodation in mesopore silica channels are depicted in Figure 4.20. Inclusion of duplex DNA in mesoporous silicates with larger pores, around 3.8 nm diameter, would be accompanied by the formation of four water monolayers on the silica surface of the mesoporous inner channel (Figure 4.20A), where sufficient quantities of Si—OH groups remained after solvent extraction of the template (not by calcination). 

The second stage of modeling is the introduction of solvated ionic species into the model double layer. Coadsorption of HF and water yields adsorbed HgO ions the solvation stoichiometries of ions in the first monolayer and in subsequent layers are determined. The third stage of modeling is establishment of potential control in UHV. Hydrogen coadsorption is used to deflect the effective potential of the water monolayer below the potential of zero charge. The unique ways in which UHV models can contribute to an improved molecular-scale understanding of electrochemical interfaces are discussed. 

The UHV data suggest that 20% of saturation hydrogen atom coverage must be added to the water monolayer to shift the potential below the pzc. Electrons can be added to the interface either by adsorption of... 

A water-alone monolayer potential above the pzc is in accordance with an absolute work function measurement for the water monolayer on Pt(lll) of 4.8 eV (29). Comparing this to the hydrogen electrode (4.7 eV below vacuum (30) for the normal hydrogen electrode NHE) corrected by 7x0.059 V for a nominaI pH 7 yields a water-alone mono-layer potential of +0.5 V vs. RHE at pH 7. This lies 0.3 V above our proposed pzc of 0.2 V RHE. This relatively high apparent potential of the water monolayer has been noted previously (Sass, J.K., private communication), and has raised concern about the relevance of the UHV monolayer to real electrochemical conditions, since most electrochemical measurements of the pzc of polycrystalline Pt have been closer to 0.2 V than to 0.5 V (31). By showing that the water monolayer lies above, not at, the pzc, the present H.+H-O data remove part of the apparent discrepancy between the electrochemical and UHV results. If future UHV work function data show a large ( 0.3 V) decrease in the water monolayer work function upon addition of small (<20X saturation) amounts of hydrogen, all of the apparent discrepancy could be quantitatively accounted for. 

Fig. 3.5.12 Area of DDAB air-water monolayer on K2PtCL subphase as a function of time through compression of monolayer and 10 dip cycles of a quartz plate. Inset Area of monolayer as a function of time through dip cycles 2-4. (From Ref. 10.)...

Some amphiphilic molecules such as oleic acid and hexadecyl alcohol containing an alkyl chain and a polar head group form monolayers on the surface of water. The polar head groups of these molecules are attracted to and are in contact with water while their hydrocarbon tails protrude above it (Figure 15). The term monolayer implies the presence of a uniform mono-molecular film on the surface of water. Monolayer films can be classified as gaseous, liquid, or solid depending upon the degree of compression and the effective area per molecule. Clearly the liquid phase of a monolayer film and, more so, the solid represent constrained environments for individual molecules of amphiphiles. Monolayers, just like micelles, are dynamic species. 

Fig. 26. Plot of the data of Table 6 showing rate of lysozyme adsorption at air/water monolayers of different zeta potentials. Refer to Eq. (35) and text for details (from Ref.3 )...

Hageman et al. [3.13] calculated the absorption isotherms for recombinant bovine somatotropin (rbSt) and found 5-8 g of water in 100 g of protein, which was not only on the surface but also inside the protein molecule. Costantino et al. [3.72] estimated the water monolayer M0 (g/100 g dry protein) for various pharmaceutical proteins and for their combination with 50 wt% trehalose or mannitol as excipient. They compared three methods of calculating MQ (1) theoretical (th) from the strongly water binding residues, (2) from conventional adsorption isotherm measurements (ai) and (3) from gravimetric sorption analysis (gsa) performed with a microbalance in a humidity-controlled atmosphere. Table 3.5 summarizes the results for three proteins. The methods described can be helpful for evaluating RM data in protein formulations. 

Simulations of three representative Cs-smectites revealed interlayer Cs+ to be strongly bound as inner sphere surface complexes, in agreement with published bulk diffusion coefficients [78]. Spectroscopic and surface chemistry methods have provided data suggesting that in stable 12.4 A Cs-smectite hydrates the interlayer water content is less than one-half monolayer. However, Smith [81] showed using molecular simulations of dry and hydrated Cs-montmorillonite that a 12.4 A simulation layer spacing was predicted at about one full water monolayer. The results of MD computer simulations of Na-, Cs-and Sr-substituted montmorillonites also provide evidence for a constant water content swelling transition between one-layer and two-layer spacings [82]. 

Fig. 3. The model employed for the calculations based on the polarization model in silica. The first water layer is at a distance d from the middle distance between silica surfaces. The surface dipoles of magnitude p=4 Debyes, each occupying an area A =50 A2, are located at a distance A = A below the first water monolayer (e = l). The shortest distance recorded experimentally corresponds to d=6. The surface of silica is at a distance 2f=15 A from the shortest experimental distance, as obtained from the best fit of the experiments with Eq. (43b). for Ah= 8.3 x 10—21./. If 6 would be equal to t, the surface dipoles would be located on the silica surface. When 6

To be consistent with the experimental data for hydration forces measured between silica interfeces, Xm has to be of the order of 4 A, a value which is compatible with Eiq. (24) for a random orientation of the neighboring water molecules. Therefore, in what follows, we will employ only the value A, =4 A. For the van der Waals interactions, we will assume in all calculations A//-X.3 10 21 J and 2t= 15 A, as obtained previously form the fit with Eq. (43b). The magnitudes of the surface dipoles will be considered 4 Debyes (about twice that of a water molecule) and it will be assumed that each dipole occupies on the surface an area of 50 A" and that they are located at a distance A =1 A below the first water monolayer. 

The most important question is, however, whether or not the polarization of water is mainly responsible for the short range interactions between silica surfaces. The fittings shown in Fig. 4 indicate that the surface dipoles are situated at a distance 25 of about 6 A from the shortest separation obtained experimentally. In contrast, the fitting of the van der Waals interactions (Eq. (43b), Fig. 2B) implies that the flat silica surfaces are at a distance It of about 15 A from the shortest separation obtained experimentally. If a value t-5 would have been obtained, it would indicate that the surface dipoles were actually situated on the flat silica surfaces. The results suggest instead that the surface dipoles, responsible for the polarization of the first water monolayer, are situated at a distance t— 5 = 5 A above each of those surfaces. This conclusion would strongly support the formation of a silica gel, with a thickness of about 5 A, on the silica surface. The possible formation of a gel is also supported by the experimental data obtained for various pHs and electrolyte concentrations, represented for convenience in Fig. 5, which exhibit an almost hard-wall-like repulsion at short separations. 

Gallas et al. (1991) noted the exceedingly high value of the silanol population (14 OH nm 2) in their sample of precipitated silica, which they attribute to the presence of many inner hydroxyls. This is consistent with the abnormally large water uptake by VN3 - as indicated in Table 10.5. This is much greater than the amount (c. 15.7 pmolm-2) required to give a close-packed water monolayer and is further evidence of the microporous nature of precipitated silicas. 

Do some measurements detect the effects of hydration beyond the end point defined above The answer is, clearly, yes. Electrostatic interactions may be longer range. Hydration forces have been discussed. Measurements of this kind are treated by use of a different model, which may not include a distinct water monolayer at the protein surface. The point to be made, however, is that water beyond the monolayer is not strongly perturbed and differs substantially from water adjacent to the surface. 

Figure 16. Surface structure the (0001) surface of sapphire (a-A Os) from Eng et al. (2000) with permission of the editor of Science, (a) Structure of vacuum-equihbrated diy surface. Al metal atoms sit on the surface, (b) Ideal bulk terminated surface with no relaxation or reconstraction. (c) Model for the wet-equihbrated surface at one atmosphere from surface scattering (ciystal truncation rod) diffraction measurements. Al metal atoms have shifted, the surface is oxygen terminated, and an organized water monolayer is required to accurately describe the observations, (d) Structure of gibbsite or y-Al(OH)3. The relaxed wet-equilibrated sapphire surface is intermediate between this structure and that of the bulk terminated stracture (B).

From Table 2 it appears that on passing from carbon black and aerosil to carbosil the thickness of the solvation shell of benzene increases and the hydration film decreases. The studies of changes of chemical potential of water molecules at the adsorbent/bonded wa-ter/ice interface depending on water layer thickness are presented in another paper [57]. For the initial silica the surface effect is confined to the adsorbent water monolayer. Poor carbonization of aerosil surface causes the increase of water layer thickness to 40-50 molecular diameters. With the increase of carbon constituent part on the complex adsorbent surface, the thickness of interfacial water layer decreases to 15 molecular diameters. 

An advantage of surface-plasmon microscopy is that great contrast between layers of different thicknesses that can be obtained in very thin films without the use of the probe molecules. It is a requirement of the method, however, that there be an underlying coating of a conductor such as silver that carries that surface plasmon field. There is a possibility that images can be obtained from the water/monolayer film that adheres to a vertical slide that has been partially drawn through the air-water interface. ... 

It has been shown that the structure of the enzyme-substrate complex undergoes rearrangement in the rate-determining step of the lysozyme reaction (29). This rearrangement may require the mobility associated with completion of the water monolayer. It is also possible that a network of water molecules participates in the catalytic process. We favor the former alternative. Regardless of the explanation, it is important that not much water is needed for enzymatic activity and that only the strongly interacting sites must be filled before activity is observable. 

The basis for the introduction of the notion of contactless flotation was the analogy with the well known phenomena of colloid particle coagulation in the secondary energetic minimum. Due to the predomination of the attractive molecular forces at large distances the particles can form aggregates in which some distance between particles is preserved. Thus, there is no direct contact between particles in this type of aggregation. However the notion of "contact" is not so simple. It is sufficient to point to the fact that a water monolayer remains on the hydrophobic surface. Thus the term "contactless" is maybe not suitable. 

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