Surface pressure adsorption

Virial Isotherm Equation. No isotherm equation based on idealized physical models provides a generally valid description of experimental isotherms in gas-zeolite systems (19). Instead (6, 20, 21, 22) one may make the assumption that in any gas-sorbent mixture the sorbed fluid exerts a surface pressure (adsorption thermodynamics), a mean hydrostatic stress intensity, Ps (volume filling of micropores), or that there is an osmotic pressure, w (solution thermodynamics) and that these pressures are related to the appropriate concentrations, C, by equations of polynomial (virial) form, illustrated by Equation 3 for osmotic pressure ... 

As noted above, an isothemi plots the muiiber of molecules adsorbed on the surface at some temperature in equilibrium with the gas at some pressure. Adsorption gives rise to a change in the free energy which, of... 

When a gas comes in contact with a solid surface, under suitable conditions of temperature and pressure, the concentration of the gas (the adsorbate) is always found to be greater near the surface (the adsorbent) than in the bulk of the gas phase. This process is known as adsorption. In all solids, the surface atoms are influenced by unbalanced attractive forces normal to the surface plane adsorption of gas molecules at the interface partially restores the balance of forces. Adsorption is spontaneous and is accompanied by a decrease in the free energy of the system. In the gas phase the adsorbate has three degrees of freedom in the adsorbed phase it has only two. This decrease in entropy means that the adsorption process is always exothermic. Adsorption may be either physical or chemical in nature. In the former, the process is dominated by molecular interaction forces, e.g., van der Waals and dispersion forces. The formation of the physically adsorbed layer is analogous to the condensation of a vapor into a liquid in fret, the heat of adsorption for this process is similar to that of liquefoction. 

Let us suppose that we have a solution A in contact at one side with a surface of adsorption ab separating it from another phase -B which, for simplicity, we shall first take to be the vapour of the solvent. At the other side the solution is in contact wTith pure liquid solvent C through a semipermeable piston c, exposed to an osmotic pressure P. 

The simplest way to predict the lipid/ water partition coefficient, Kiw, of a drug is based on measurements of the surface pressure, ttd, of the drug as a function of its concentration in the aqueous subphase (Gibbs adsorption isotherm). The Gibbs adsorption isotherm provides the air/water partition coefficient, Kaw, and the cross-sectional area, Ad of the drug and allows calculation of the lipid/water partition coefficient, K]w, according to Eq. (6) [59] ... 

Fig. 20.1. Correlation between the air/water partition coefficient, Kaw, determined from measurements of the surface pressure as a function of drug concentration (Gibbs adsorption isotherm) in buffer solution (50 mM Tris/HCI, containing 114 mM NaCI) at pH 8.0 and the inverse of the Michaelis Menten constant, Km obtained from phosphate release...

The differences in time-dependent adsorption behavior between 99% PVAC at 25° and 50°C demonstrate the influence of intra- and intermolecular hydrogen bonding in the adsorption process. The limiting surface pressure of the hydrophobic water-soluble polymer appears to be 33 mN/m, approximately 7 mN/m below that of commonly used surfactants. The rate of attainment of equilibrium surface pressure values is faster if there is uniformity of the hydrophobic segments among the repeating units of the macromolecule. 

An HM-HEC monolayer at the air/aqueous interface was formed by adsorption from an aqueous solution of the polymer placed in the Langmuir trough overnight. In "stress-jump" experiments, HM-HEC monolayers were placed under rapid compression to a large degree and surface pressure was measured as a function of time after compression was stopped. (The compressional "jumps" required a minute or two to complete, and in some cases were on the order of the polymer monolayer relaxation times. See later section for discussion). In hysteresis experiments, the adsorbed monolayers were subjected to continuous compression-expansion cycles at a specific speed, while surface pressure was determined as a function of surface area. 

Figure 8. Surface pressure - area isotherms for stearylamine before (a) and after (b) adsorption of IgG at pH 8, 20 °C, and schematic representation for IgG molecule adsorbed to stearylamine monolayer.

On the other hand, inclusion and/or adsorption of NaphSOsNa molecules from the aqueous subphase to the spread monolayer of p-CDNHC12H25 were examined by using the multicompartment trough. When the P-CDNHC12H25 mono-layer spread on the distilled water surface was compressed to the prescribed initial surface pressures of 5, 10, 20, and 30 mN/m and transferred onto the aqueous subphase containing 10 3 M NaphSCbNa, the surface pressure increased with time, air/aqueous solution interface under the suggesting the... 

In the preparation of the MOPPV LB films, the polyion complex monolayer were formed by the adsorption of the precursor onto the anionic amphiphile monolayer (route b-2 in Fig.5) [36,37]. The monolayer of 2C12SUC was spread on a aqueous solution of the MOPPV precursor polymer (repeating unit concentration about 10"4 M). The monolayer was allowed to stand for 4 hours to adsorb the precursor after being compressed to a surface pressure of 30 mNm 1. The monolayer was deposited on fused quartz substrate by the LB technique. At last, the deposited polyion complex was converted to MOPPV by the heat treatment at 200 C in vacuo (about 10 2 torr). 

Gibbs adsorption equation phys chem A formula for a system involving a solvent and a solute, according to which there Is an excess surface concentration of solute if the solute decreases the surface tension, and a deficient surface concentration of solute if the solute increases the surface tension. gibz ad sorp shan i.kwa-zhon Gibbs adsorption isotherm physchem An equation for the surface pressure of surface [< ... 

Thus, the data in Table II can be readily explained if one considers the overall sorption process to consist of both adsorption and absorption. At low pressures, adsorption makes a relatively large contribution to the overall sorption process, and the values reflect the relative surface areas of the extracts. At higher pressures, absorption of benzene becomes relatively more important, and the equilibrium sorption values reflect the solubilities of benzene in the extract. It is interesting to note that the O-octylated extract sorbs more benzene than the O-butylated extract at the higher pressure, in spite of the fact that the O-butylated extract has a higher surface area. We conclude that benzene is more soluble in the O-octylated extract. 

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