Step-wise isothermal

Thermomechanical, Dynamic Mechanical and Dielectric Methods Step-wise Isothermal Frequency Scans... 

The shape of adsorption isotherms measured on lamellar halides [102], on graphite [103] and on NaCl [101] strongly depends on strrface homogeneity. Poor substrates exhibit broad distributions of adsorption energies whereas uniform strrfaces allow the observation of step wise isotherms. The steps are clear signatures of first order two-dimensional phase transitions [6,8,14]. 

In order to increase the resolution of TG curves, it is necessary to change the heating rate in coordination with the decrease in mass. This technique is called controlled rate thermogravimetry (CRTG). Several kinds of technique for controlling the temperature, such as step-wise isothermal control, dynamic rate control and constant decomposition rate control, are employed. The above technique is mainly achieved using software with commercial TG apparatus (2). 

The procedures are based on an imaginary emptying of the pores by the step-wise lowering of relative pressure, from the point already referred to where the mesopore system is taken as being full up a relative pressure of 0-95po is frequently adopted as starting point with isotherms having a hysteresis loop of Type A or Type E. (With Type B, as will appear later, the validity of pore size calculations is doubtful.)... 

Fig. 18 77 -< A > Isotherms for PVP/PVAc binary monolayers on water at 25 °C. Surface pressure 77 for a variety of poly(vinyl palmitate)/poly(vinyl acetate) mixtures as defined in the plot are shown as a function of area per monomer. Surface concentration was controlled by step-wise compression. The incorporation of PVP, which does not form stable monolayers alone, condenses the film and also increases the instability of the film. < A > = average area per monomer...

As already indicated, a weU-defmed step-wise (Type VI) isotherm is obtained when a noble gas or lower hydrocarbon is adsorbed on a basal graphitic surface at an appropriate temperature [7, 11]. The regular steps can extend up to four or five molecular layers, but become less sharp with increased distance from the adsorbent surface. An increase in temperature also produces a progressive blurring of the layer-by-layer adsorption [7]. The appearance of such regular multilayer steps in isotherms on uniform surfaces supports the view that (a) the influence of the surface structure can extend well beyond the first adsorbed layer and (b) the multilayer steps are associated with a form of localized physisorption. 

To illustrate the multitudes of possible modulations, the top row of Fig. 4.86 illustrates five segments that can be linked and repeated for modnlation of the temperature. Examples of quasi-isothermal sinusoidal and step-wise modulations are... 

Figure 4.67. Analysis of the crystallisation kinetics. [A] Isothermal analysis with a DSC by quick cooling to the crystallisation temperature and analysis after steady state has been reached. [B] Step-wise analysis by heating after different times of crystallisation of PEEK by direct measurement from the crystallisation temperamre.

Adsorption of phenol and its derivatives from aqueous solutions on active carbons and carbon blacks has been the subject matter of a large number of investigations. Jaroniec and coworkers,Enrique et al. Worch and Zakke, and Magne and Walker studied the adsorption of several phenols from aqueous solutions and found that the adsorption was partly physical and partly chemical in character. Aytekin, ChapUn, and Kiselev and Krasilinkov observed that the adsorption isotherms of phenol from aqueous solutions were step-wise, suggesting the possibility of rearrangement of phenol molecules in the adsorbed phase and their interaction with active sites on the carbon surface. Morris and Weber, however, found that the adsorption isotherms of phenols on active carbons show two plateaus, even... 

The adsorption of short-chain aliphatic alcohol molecules on to the clay mineral kaolin has been elucidated by considering the heterogeneous surface sites of the clay and the self-association of alcohol molecules in apolar decane solutions [28]. The adsorption isotherms are shown in Fig. 4. The isotherms of ethanol, propanol, and butanol are of L-type, showing large initial slopes that readily decrease with increasing alcohol concentration, while the isotherm of methanol appears as a step wise function. 

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