Henry region

Since the concentration of singlet oxygen in the experiment described is low, one may consider the adsorption to take place in the Henry region. Then... 

Restricting ourselves to the region of low pressure (the Henry region), we have, instead of (38) and (39) ... 

In case of injection of a very small amount of analyte, its concentration is in the linear region of adsorption isotherm (Henry region of linear variation of adsorption with the equilibrium concentration of the analyte) and the derivative could be substituted with the slope of excess adsorption isotherm, also known as Henry constant, Kh, to get... 

In the Henry region the Langmuir and Volmer isotherms differ only with respect to their affinity constants this is the only way in which the two modes of adsorption can be distinguished here. In practice the distinction is not always easy because of surface heterogeneity the most energetical sites are filled first. However, the equations of state are identical (both reduce to nA = NkT) because such equations do not include adsorbent-adsorbate interactions. 

The gravimetric method has in its favour the fact that all variables adsorbed mass, gas pressure and temperature are measured independently. Apparatus are more complicated and more expensive on account of the additional vacuum microbalance. Balance operating requires some skills. The sample is not in direct contact with the thermostat and must be shielded against false heat radiation. Measures to avoid the influence of eddy gas flow may be required. At low pressure the Knudsen pressure difference causes thermal gas flow which seriously interferes in measurements in the Henry region. 

For the simple Henry region everything is linear. The isotherm is N° / A = K x... 

Outside the Henry region calculation of the permeation from adsorption and diffusion data requires knowledge of the value of Especially for weakly adsorbing gases the value is not always known nor can be Ccisily determined from experiments. As discussed by Kapteyn et eil. [88] the value of tjsat can be estimated from the molar volume which is obtained from extrapolation of the liquid state [90] or from volume filling theory [91]. Some results will be discussed below (binary gas permeation). In the Henry regime separate values of q at are not necessary as discussed above and the product K-t/gat = b (Henry coef.)... 

Molecular statistic and gas chromatographic study of hydrocarbons adsorption on the modified layer silicates and silica in the Henry region... 

The differential molar variations of the entropy and heat capacity can also be expressed via the Jjj values. Clearly the expressions (3)-(4) are valid within the limit of negligible adsorption, i.e. the Henry region. 

A sharp increase of the retention volumes Vn, and heats of adsorption of saturated hydrocarbons on tetramethylammonium montmorillonite in the Henry region as compared with natural mineral (Table 5) shows that the adsorption and separation of these molecules take place in chink-like micropores having the width of 0.45 nm on the (internal) surface of (CH3)4N - montmorillonite [39]. 

In the Henry region p is rather high and independent of chain... 

Fig. II-6. The surface tension, a(c), and two-dimensional pressure, tt(.vm), isotherms of three surfactants, the consecutive members of a homologous series. The concentration range corresponds to the Henry region...

A linear drop in the surface tension with increasing surfactant concentration (the Henry region) in agreement with eq. (11.12) corresponds to a linear increase in adsorption ... 

The Szyszkowski equation (II. 18) satisfies both limiting conditions it is consistent with the linear dependence of the surface tension on concentration within the Henry region, and agrees with eq. (11.17) at sufficiently high concentrations. At the same time, the criteria which define low and sufficiently high concentrations are set. Indeed, at low (as compared to a = IA) concentrations the logarithm can be expanded in series, yielding... 

Within the Henry region the dependence of the adsorption on concentration is given by... 

This form is particularly useful as it provides the basis for a convenient way to extract the Henry constant from data at higher loadings (beyond the Henry region). A plot of ln(p/g) vs q should yield a linear asymptote with slope 2Ai and intercept - IniC (see Fig. 4) [14]. 

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