Thin Film Water on NaCl

Peter T (1999) Physico-chemistry of polar stratospheric clouds. In Ice Physics and the Natural Environment. Wettlaufer JS (ed) p 143-167. Berlin Springer-Verlag Peters SJ, Ewing GE (1997a) Thin film water on NaCl(lOO) imder ambient conditions An infrared study. Langmuir 13 6345-6348... 

Salmeron and his group, realizing the comphcation introduced by adlayer water on the AFM tip, have developed a technique they call scanning polarization force microscopy (SPFM) [60,61]. In this form of noncontact AFM tip imaging, the metalized AFM tip is electrically biased and responds to the dielectric properties of the water film and any accompanying ions. The method requires the tip to be of the order of 10 nm from the film, so the higher resolution of contact AFM is lost. On the other hand, the electrical properties of the film can be explored. An example of their investigation of thin film water on NaCl (001) [34] is shown in Fig. 6. 

Fig. 8 Infrared spectra of thin film water on NaCl (001). The number of NaCl (001) crystal faces for these experiments, performed at 24 °C, was 28. From Foster and Ewing [76]...

Fig. 11 Pair distribution functions for thin film water on NaCl (001). The distribution function, g(z), is given as a function of the separations, z, of the water molecules from the NaCl (001) surface at coverages of 0 = 0.5 and 3.0. Adapted from Engkvist and Stone [46]...

The surfaces obtained by cleaving single crystals of NaCl are clearly (001), and a variety of techniques show them to be remarkably defect-free. However, the nanocrystallites produced by sublimation methods, while they appear cubic, their surfaces are poorly defined [18]. The diffuse infrared bands of CO adsorbed to these crystallite faces is suggestive of the heterogeneity of the surfaces [53]. Water adsorbed onto NaCl crystallites yields infrared band shapes [54,55] quite distinct from those we shall discuss in a later section for thin film water on defined NaCl (001) faces. 

Foster and Ewing [76] have measured a family of isotherms. From the changing coverage values with pressure and temperature, they have been able to extract thermodynamic quantities (AE, AH, AS) that characterize thin film water for H2O on NaCl(lOO). For example, the enthalpy of vapor condensation to form the monolayer film is AH = - 50 kj mol or more exothermic than for the condensation to liquid water that is - 44 kJ mol [79]. The mono-layer film entropy is 15 JK mol lower than that of 70 JK mol for liquid water [79]. In summary, water molecules are more strongly bound to the NaCl(lOO) surface than in the liquid, and are more ordered. 

Thin Film Water in and on Undefined NaCl Surfaces... 

In an infrared study of laboratory produced NaCl aerosols, Weis and Ewing [83] offered models on the morphology of water in (or on) the salt particles. It has been known for a long time that salt crystals in mines can be easily deformed without fracturing [12]. Thin film water that somehow penetrates crystals of salt has been implicated in inducing its plasticity in a humid environment [84]. Finally, thin film water between grains of salt has recently been modeled to understand geological processes [85]. 

The predominant RO membranes used in water applications include cellulose polymers, thin film oomposites (TFCs) consisting of aromatic polyamides, and crosslinked polyetherurea. Cellulosic membranes are formed by immersion casting of 30 to 40 percent polymer lacquers on a web immersed in water. These lacquers include cellulose acetate, triacetate, and acetate-butyrate. TFCs are formed by interfacial polymerization that involves coating a microporous membrane substrate with an aqueous prepolymer solution and immersing in a water-immiscible solvent containing a reactant [Petersen, J. Memhr. Sol., 83, 81 (1993)]. The Dow FilmTec FT-30 membrane developed by Cadotte uses 1-3 diaminobenzene prepolymer crosslinked with 1-3 and 1-4 benzenedicarboxylic acid chlorides. These membranes have NaCl retention and water permeability claims. 

Fig. 3.68. Analytical HPLC chromatograms with detection of diode array of 4.7 x 10"5mol/l of R3R dye curve (1) before and curve (2) after 180 min of photoelectrocatalysis on the Ti02 thin-film electrode biased at +1.0 V in NajSCT, 0.025 mol/l. Curve (4) before and curve (3) after photoelectrocatalysis in NaCl 0.022 mol/l and curve (5) after bleaching of 4.7 X 10-5 mol/l of R3R dye submitted to a chemical treatment by active chlorine addition. The mobile phase was methanol-water 80 20 per cent with a flow rate of 1 ml/min and controlled temperature at 30°C. The column was a Shimpack (Shimadzu) CLC-ODS, 5 /an (250 mm X 4.6 mm). Reprinted with permission from P. A. Cameiro el al. [138].

An ideal study of support effects requires model catalysts with metal particles that are identical in size and shape (so that only the support oxide varies). This is difficult to achieve for impregnated catalysts, but identical metal particles can be prepared via epitaxial model catalysts [36]. Well-faceted Rh nanocrystals were grown on a 100-cm area NaCl(OOl) thin film at 598 K. One half of a Rh/NaCl sample was covered with Al Oj, and the other half with TiO. The preparation of Rh particles for both Al Oj- and TiO -supported model catalysts in a single step prevents any differences in particle size, shape, and surface structure which could occur if the samples were prepared in separate experiments. Three model catalysts were prepared, with a mean Rh particle size of 7.8, 13.3, and 16.7 mn (the films were finally removed from the NaCl substrate by flotation in water). Activation was performed by O /H treatments, with the structural changes followed by TEM (Fig. 15.6). Oxidation was carried out in 1 bar O at 723 K prodncing an epitaxially grown rhodium oxide shell on a Rh core (cf Fig. 15.5e), whereas the hydrogen reduction temperature was varied. 

There are several types of methods to obtain an IR spectrum depending on the nature of the sample to be analyzed. Transmission spectroscopy is based on the absorption of IR radiation at specific wavelengths as the radiation passes through a sample. With this technique, it is possible to analyze liquids, solids, or gaseous samples. This is the most common method the samples can be placed in a cell in solution, dispersed in NaCl, KBr, Cap2 (for water-soluble samples), or CsBr. Liquid thin films can also be analyzed using a drop of the sample, which will be sandwiched between two IR KBr cells and placed in a holder. When samples are analyzed in a solvent solution, several factors must be considered the solvent has to dissolve the whole sample with a minimum of solvent-solute chemical interactions and it should not strongly absorb IR radiation. 

Vadimsky [23] described a useful method for preparation of thin films from the melt or solution. The method involves the evaporation of carbon onto freshly cleaved mica or fractured NaCl crystal substrates. The thin polymer film is cast onto the carbon coated substrate or the substrate is dipped into a polymer solution. After solvent evaporation, the film is scored and removed from the glass by floating it onto a water surface. The specimen can also be deposited directly onto the substrate followed by carbon coating. Geil [24,25], in a variation of this method, deformed PE single crystals by deposition on a Mylar substrate and drawing it before carbon coating and TEM examination. 

The close similarity of the liquid water profile in Fig. 7 with the 13 mbar spectrum of H2O on NaCl(OOl) in Fig. 8 is good evidence that the thin film is liquid-like. Use of the optical constants of liquid water and the Beer-Lambert relation has enabled the determination of the coverage values as listed on Fig. 8 and the construction of the isotherm in Fig. 9. (A monolayer, (9 = 1, corresponds to each Na" Cl" surface ion pair covered, on average, by an H2O molecule.) In order to provide a context with an ambient environment, the water pressure needed to produce a monolayer on NaCl(lOO) corresponds to 40% relative humidity [77,78], a rather arid condition. In addition, the pristine appearing surfaces of the salt crystals in Fig. 2, photographed at 50% RH, were coated with several water layers. 

A series of papers originating in the laboratory of Finlayson-Pitts [22] and continuing in the group of Hemminger [24] describe water adlayers on NaCl as quasi-liquid. They, and others, demonstrate that this thin film plays a profound role in physical and chemical transformations of salt particles in the... 

M. Foster and G. E. Ewing. An infrared spectroscopic study of water thin films on NaCl (100). Surf. Sci. A21-A2%, 1999, 102. 

The sample may be a thin unsupported film (usually about 2 mm thick) a film evaporated on a nonabsorbing substrate such as NaCl a solution held in a nonabsorbing cell (often made of NaCl) a pressed, clear wafer of the material to be analyzed mixed with KBr powder or a mull of the material with a heavy paraffin oil held between NaCl flats. Other alkali and alkaline earth halides are used as substrates. Water and alcohols are not used as solvents generally, because they absorb strongly in the IR region and may corrode the substrates. 

In ncmnar FTIR die polymer is either ground into a powdo widi KBr and pressed into a disc (high-Tg polymers) or dissolved in a suitable solvent (not water), and applied on to a KBr or NaCl plate, which after evaporation of the solvent leads to a thin polymer film, Wang and Poehlein [55] used FTIR in combination with carbon Nh to analyse the composition of oligomers in latexes formed from styrene (S)-(meth)aciylic acid and S-methyl methacrylate (MMA) cmnonomer mixtures. 

DLVO theory was applied (63,64) for the description of spontaneous and forced thinning of the liquid film separating the droplets. These experimental results and DLVO theory were used (63) for the interpretation of the reported visual study of coalescence of oil droplets 70—140 pm in diameter in water over a wide pH interval. A comparison based on DLVO theory and these expermental data led the authors to condlue (63) that if the total interaction energy is close to zero or has a positive slope in the critical thickness range, i.e., between 30 and 50 nm, flie oil drops should be expected to coalesce. In the second paper (64), where both ionic strength and pH effects were studied, coalescence was observed at constant pH values of 5.7 and 10.9, when the Debye thickness was less than 5 nm. The main trend in our experiments and in Refs 63 and 64 were in accordance, because it was difficult to establish the decrease in t.d.c at NaCl concentrations lower than 5 x 10 M, i.e., double-layer (DL) thicknesses larger than 5 nm. An almost quantitative coincidence in the double-layer influence on... 

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