Water adsorption 710 INDEX

It should be noted that optical humidity sensors as a rule use similar effects, which were discussed above (Russell and Flecher 1985 Ballantine and Wohltjen 1986 Boltinghouse and Abel 1989 Wang et al. 1991 Kharaz and Jones 1995 Ando et al. 1996 Zhou et al. 1998 Skrdla et al. 1999 Alvarez-Herrero et al. 2004). The water adsorption in a porous matrix produces a variation in the optical response of the device, because the refractive index of the layer changes when the hydration of sensing material takes place and the pores are filled or emptied. The water adsorption isotherms and, therefore, the sensor response depend on the size and shape of the pores. One can find in Posch and Wolfbeis (1988), Otsuki and Adachi (1993), Papkovsky et al. (1994), Costa-Femandez et al. (1997), Costa-Femandez and Sanz-Medel (2000), Choi and Tse (1999), Choi and Shuang (2000), and Bedoya et al. (2001, 2006) a description of optical humidity sensors used and other principles. 

A variation of the water adsorption isotherm method was then developed (Yeatman, 1994 Dawnay, 1995) called Molecular Probe Ellipsometry (MPE). In this technique, refractive index readings are made in dry nitrogen, and then in nitrogen saturated with an adsorbate solvent vapor. The porous structure is modelled as an effective medium according to the Lorentz-Lorenz relation, giving a relation between the measured film index f and the indices of the material in the pores and of the solid skeleton, p and s, respectively ... 

Benezet et al. (2012) reported that the addition of fibers increased the expansion index and led to a reduction in water adsorption of extruded potato starch foams. They also reported that the fiber additions at a 10% level generally improved the starch foam s properties. 

Moisture capacity of the ore is a index which shows the ability of water adsorption the moisture capacity has the positive relation with the optimal water content of the granulation process. The measurements of the moisture capacity was introduced in the previous papers[5-6]. The results of the moisture capacity are shown in Figure 1. 

Molinari M, Parker SC, Sayle DC, Islam MS Water adsorption and its effect on the stability of low index stoichiometric and reduced surf es of Ceria, J Phys Chem C 116... 

Once initiated, and provided the surface continues to be exposed to the environment, the process of hydration continues at a slow, but measurable rate. The adsorption of the water is accompanied by changes in the physical properties of the obsidian. The refractive index of the obsidian, for example, is altered as it becomes hydrated. If the obsidian was subjected to alternative wet and dry periods, successive hydrated layers are formed on the surface. The differences in refractive index between the bulk and the hydrated layer (or layers) creates an interface between the bulk and the hydrated layer, and between the layers, that stands out sharply when observing a cross-cut section of obsidian under a microscope (see Fig. 23). Thus the thickness of the hydrated layer, or layers, can be measured. 

Figure 22. Calculated waveguide effective index shift due to the presence of a 4 nm thick layer with refractive index n = 1.5 on top of the waveguide core. This layer is intended to emulate the the adsorption of a layer of large organic molecules. The index shift is shown for SOI and glass waveguides of varying core layer thickness. The uppermost layer is assumed to be water (n = 1.3). The inset shows an expanded view of the SOI data.

In view of the great importance technically and in natural processes of adsorption in presence of water, especial attention has been directed to the detection of possible hydrogen bond interaction in aqueous media, for which the refractive index method [4] used with model compounds is useful. Table 1 summarizes some of the conclusions reached by this means, or by tests in monolayers (cf. [5]), and includes also some parallel data for interactions in non-aqueous solution. 

Especial points which emerge from these studies include (a) the almost complete absence of reactivity of the hydroxy-groups of simple carbohydrates in water, which is attributed to their powerful solvation by water preventing a close approach of any other solute and (b) the ability of ester groups to interact with proton-acceptors. The refractive index tests, examination of m.p. or b.p., and infrared spectra of certain mono- and poly-esters appear to be interpreted most simply by assuming the formation of weak CH bonds by ester groups under the activating influence of the adjacent (5—0 double bond. These bonds can account for certain properties of l 2-diesters and for the adsorption of proton-acceptor solutes by cellulose acetates. 

There are numerous properties of materials which can be used as measures of composition, e.g. preferential adsorption of components (as in chromatography), absorption of electromagnetic waves (infra-red, ultra-violet, etc.), refractive index, pH, density, etc. In many cases, however, the property will not give a unique result if there are more than two components, e.g. there may be a number of different compositions of a particular ternary liquid mixture which will have the same refractive index or will exhibit the same infra-red radiation absorption characteristics. Other difficulties can make a particular physical property unsuitable as a measure of composition for a particular system, e.g. the dielectric constant cannot be used if water is present as the dielectric constant of water is very much greater than that of most other liquids. Instruments containing optical systems (e.g. refractometers) and/or electromechanical feedback systems (e.g. some infra-red analysers) can be sensitive to mechanical vibration. In cases where it is not practicable to measure composition directly, then indirect or inferential means of obtaining a measurement which itself is a function of composition may be employed (e.g. the use of boiling temperature in a distillation column as a measure of the liquid composition—see Section 7.3.1). 

The first ellipsometric measurement of the thickness of the adsorbed layer and the adsorbance of a polyelectrolyte and a negative adsorbance of salt onto a solid surface was reported by Takahashi et al.U4) They measured the adsorption of sodium poly(acrylate) (M = 950 x 103) onto a platinum plate as a function of the concentration of added sodium bromide. In an aqueous polyelectrolyte solution with an added simple salt, the bulk phase is a three-component system which consists of a polyelectrolyte, a simple salt, and water. The adsorbed layer on the solid surface is a three-component phase as well. The adsorbance of polyelectrolytes thus cannot easily be determined from measurements of the refractive index nf of the adsorbed phase. Hence, it was assumed that the adsorbed layer is a homogeneous layer of thickness t and further that nf is represented by the Lorenz-Lorentz equation as follows ... 

Index Entries Succinic acid sorbent adsorption hot water regeneration glucose fermentation broth. 

Morrall2 used a HPLC system with two columns. The first column was loaded with the controlled pore glass (CPG) to be modified. The second column was used for separation of the reaction effluents. This column was coupled to a refractive index detector, allowing for quantitative detection of the effluents. The reaction was initiated by injecting an APTS/toluene mixture and stopped by injection of pure toluene. With this so-called stop-flow mechanism reaction times down to 18 seconds could be used. From these analyses it became evident that upon mixing of the aminosilane with the silica, a very rapid physisorption occurs. The initial adsorption of the APTS (from toluene solution on dried CPG) occurred before the 18 second minimum time delay of the stop-flow apparatus. For non-aminated silanes the adsorption proved to be much slower. This study also revealed the pivotal role of surface water in the modification of siliceous surfaces with alkoxysilanes, as discussed in the previous chapter. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

For instance, a high porosity can increase the extent of adsorption of certain molecules, but at the same time, the internal surface of the pores is not fully irradiated so that the density of photoproduced active species inside the pores can be lower than on the external surface. Photons are not only absorbed but also reflected and scattered by the semiconducting particles, whether they are in the form of powders or films. Consequently, the texture, surface rugosity, and agglomeration of particles affect the fraction of photons that are absorbed and therefore are potentially useful for photocatalytic chemical transformations. In addition, scattering depends on the refractive index of the medium and is therefore very different depending on whether Ti02 is exposed to air or liquid water. 

Antioxidant activity of silica nanocomposites with immobilized vitamin C was tested using the polyphenolic activity index.8 After adsorption of ascorbic acid on the silica surface and centrifugation, the excess solution was removed to obtain the suspension of a fixed volume (2 ml). Distilled water, sodium carbonate solution, and Folin-Ciocalteu s phenol reagent were subsequently added to suspensions and to the reference Vitamin C solution. The suspensions were then stored for 30 min, and the optical density of supernatant was measured at X = 750 nm. The reference solution of ascorbic acid was used to compare antioxidant activity of vitamin-containing nanocomposites with the activity of dissolved vitamin C. 

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