Orientation of water molecules at the interface

The orientation of water molecules at the interface is an important ingredient in understanding the properties of the surface region. A large body of data is available on the stmcture of water at metal surfaces measured under ultrahigh vacuum (UHV) conditions, but it is expected that the orientation of water molecules under the conditions that exist at the electrode/electrolyte interface is very different. As mentioned earlier, the fact that the minimum energy required to eject an electron from the surface (the work function) is lower when the metal is in contact with water... 

Before discussing experiments a note on nomenclature is appropriate. In the literature Volta potentials are often called surface potentials but this term has other meemings as well, so we shall not use it. The usual symbol is AV, but in line with our convention (sec. 3.4.1) the appropriate s3mibol is V , i.e. it is the Volta potential of the monolayer minus the same for the blemk at the other side of the barrier. The latter is not zero and depends on the orientation of water molecules at the interface, and in the presence of electrolytes, a double layer may form, giving rise to a non-zero y/°- In sec. II, we discussed the relevant measurement and gave results for various electrolytes. In sec. II.3.9 we concluded that for pure water, according to the best experiments presently available, >0 (the -potential is the potential of water with respect to water vapour caused by the spontaneous polarization of the Interface). This means that the water dipoles at the surface are preferentially oriented with their negative sides "out". The value of x is not... 

One can identify three physical phenomena which lead to the observed values of Ax- First, an ionic double layer can be established if the distance of closest approach for cations and anions to the interface is not the same. Second, if one of the components of the solution has a dipole moment, it may assume a preferred orientation at the interface, thereby giving rise to dipolar potential drop. Finally, the presence of the solute can change the orientation of water molecules at the interface from that present in the pure solvent. The fact that Ax is usually positive is evidence that the anion approaches the surface more closely than the cation. This is not difficult to understand given that anions are more weakly solvated than... 

Nmr methods have unrivalled potential to explore interfaces, as this account has striven to show. We have been able to determine the mobility of hydrated sodium cations at the interface of the Ecca Gum BP montmorillonite, as 8.2 ns. We have been able to measure the translational mobility of water molecules at the interface, their diffusion coefficient is 1.6 10 15 m2.s. We have been able to determine also the rotational mobility of these water adsorbate molecules, it is associated to a reorientational correlation time of 1.6 ns. Furthermore, we could show the switch in preferred reorientation with the nature of the interlayer counterions, these water molecules at the interface tumbling about either the hydrogen bond to the anionic surface or around the electrostatic bond to the metallic cation they bear on their back. And we have been able to achieve the orientation of the Ecca Gum BP tactoids in the strong magnetic field of the nmr spectometer. 

Interfadal tension, however, cannot serve as the only criterion of adsorption. Typical sui ctants with asymmetric molecules or ions consisting of a polar group and a sufficiently long hydrocarbon chain are always active at water-hydrocarbon interfaces. The greater the difference in polarity between bounda ry phases the steeper the orientation of surfactant molecule at the interface and the larger the reduction in free energy of the system due to adsorption. 

In several studies, adsorption phenomena of both solvents and dissolved species have been investigated. The potential dependent orientation of water molecules at the Au(lll) interface in the presence of sulfuric acid has been reported [300] for more recent results, see [301], Suggested spatial arrangements of both water molecules and electrolyte anions at the electrochemical interface are depicted in Fig. 5.59. 

NOTE The orientation of surfactant molecules at an interface (water-solvent, water-gas, water-metal) confers performance characteristics on the molecule that permit it to be employed as an emulsifier, demulsifier, wetting agent, antifoam, lubricant, or other agent. 

Now, at the point of zero charge, equation (2.9) implies that A = 0 i.e. that the pzc corresponds to a potential drop across the interface of zero and, from equation (2.2), that M = s. This is not found in practice owing to the layer of water molecules at the electrode surface that are present even at the pzc. These water dipoles give rise to an additional contribution to A, see Figure 2.4(a). This additional potential drop, AD, will change sign according to the orientation of the water dipoles at the electrode, and equation (2.2) can thus be re-written as ... 

The tendency of water molecules to orient in such a way as to maximize hydrogen bonding with other water molecules at the interface and in the vicinity of the interface. 

The spread mixed lipid monolayer studies provide information about the packing and orientation of such molecules at the water interface. These interfacial characteristics affect many other systems. For instance, mixed surfactants are used in froth flotation. The monolayer surface pressure of a pure surfactant is measured after the injection of the second surfactant. From the change in n, the interaction mechanism can be measured. The monolayer method has also been used as a model biological membrane system. In the latter BLM, lipids are found to be mixed with other lipidlike molecules (such as cholesterol). Hence, mixed monolayers of lipids + cholesterol have been found to provide much useful information on BLM. The most important BLM and temperature melting phenomena is the human body temperature regulation. Normal body temperature is 37°C (98°F), at which all BLM function efficiently. 

Due to their structure (Figure 17.1), all surfactants have the tendency to accumulate at interfaces because there the hydrophobic tail can be shielded from interacting with water molecules while the hydrophilic headgroup remains solvated by water molecules. As a result of this orientation, surfactant molecules displace water molecules at the interface. Consequently, the number of hydrogen bonds decreases per unit interface area. This can be... 

Brief consideration is now given to the solvent structure at metal/aqueous electrolyte interfaces.Several molecular models have been proposed which treat a single layer of water molecules at the metal surface. Within the layer, the individual water molecules (or clusters of molecules) are allowed to have certain orientations. In the earliest and simplest molecular model, an inner-layer water molecule is oriented as a result of its dipole interaction with the charge on the metal electrode. Orientation is limited to either of the two positions in which the molecular dipole is perpendicular to the electrode surface. More realistic treatments have since been described which variously... 

The double electric layer formation occurs at the air-water interfaces owing to different reasons. First of all, the double electric layer may arise as a result of specific orientation of water molecules inside the boundary layer. So, it may be formed due to dipole-ion interactions. Second, the double electric layer may arise as a result of adsorption to the surface of hydroxyl ions, halide ions, surfactants, and others. So, it may be formed dne to ion-ion interactions [34]. For example, the hydroxyl ions, which are present in water dne to its dissociation, are hydrophobic when compared with protons. And their concentration at the interface is mnch greater than that in the water bulk. 

Jedlovszky, R, A. Vincze, and G. Horvai, New insight into the orientational order of water molecules at the water/1,2-dichloroethane interface A Monte Carlo simulation study, J Ghent Phys, Vol. 117, (2002) p. 2271. 

Structure and Orientation of Surfactant Molecules at the Air—Water Interface... 

The hrst molecular dynamics computation of single molecule orientational correlation functions at liquid interfaces was reported by Benjamin. In bulk water, the water dipole correlation time (4 0.2 ps) and the water HH vector correlation time (1.5 0.1 ps, which can be approximately deduced from the NMR line shape) are in reasonable agreement with experiments. The reorientation was found to be faster at the water liquid/vapor interface. The reorientation dynamics of water molecules at the water/l,2-dichloroethane interface is, in contrast, slightly slower (to 6 0.3 and 2.3 0.2 ps for the dipole and the HH vectors, respectively).Similar results were found in a recent study by Chowdhary and Ladanyi of water reorientation near hydrocarbon liquids having different structure (different branching). The slower reorientation was limited to water molecules immediately next to the organic phase. Slower dynamics were observed when the reorientation was calculated in the intrinsic frame (thus eliminating the effect of capillary fluctuations). 

P. Jedlovszky, A. Vincze, and G. Horvai,/. Chem. Phys., 117, 2271 (2002). New Insight into the Orientational Order of Water Molecules at the Water/l,2-Dichloroethane Interface A Monte Carlo Simulation Study. 

In Ref. 60, the surface charge itself was tuned by the amount of adsorbed ionic surfactants (l-dodecyl-4-dimethylaminopyridinium bromide — DMPB ). Charging up the interface by a small amount of DMP leads to an increase in SF intensity by a factor of ten. A further increase of the surface charge leads to stronger electric fields at the interface, however, the charge will also be more effectively screened by the counterions. This leads to an overall decreased depth of the electric field, which goes along with a decreased amount of oriented water molecules at the interface. Less oriented water molecules reduce the SFG response which was observed in the experiments. 

The Gibbs equation allows the amount of surfactant adsorbed at the interface to be calculated from the interfacial tension values measured with different concentrations of surfactant, but at constant counterion concentration. The amount adsorbed can be converted to the area of a surfactant molecule. The co-areas at the air-water interface are in the range of 4.4-5.9 nm2/molecule [56,57]. A comparison of these values with those from molecular models indicates that all four surfactants are oriented normally to the interface with the carbon chain outstretched and closely packed. The co-areas at the oil-water interface are greater (heptane-water, 4.9-6.6 nm2/molecule benzene-water, 5.9-7.5 nm2/molecule). This relatively small increase of about 10% for the heptane-water and about 30% for the benzene-water interface means that the orientation at the oil-water interface is the same as at the air-water interface, but the a-sulfo fatty acid ester films are more expanded [56]. 

Interfacial water molecules play important roles in many physical, chemical and biological processes. A molecular-level understanding of the structural arrangement of water molecules at electrode/electrolyte solution interfaces is one of the most important issues in electrochemistry. The presence of oriented water molecules, induced by interactions between water dipoles and electrode and by the strong electric field within the double layer has been proposed [39-41]. It has also been proposed that water molecules are present at electrode surfaces in the form of clusters [42, 43]. Despite the numerous studies on the structure of water at metal electrode surfaces using various techniques such as surface enhanced Raman spectroscopy [44, 45], surface infrared spectroscopy [46, 47[, surface enhanced infrared spectroscopy [7, 8] and X-ray diffraction [48, 49[, the exact nature of the structure of water at an electrode/solution interface is still not fully understood. 

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