Dodecane, adsorption

The configuration-bias Monte Carlo (CB-MC) technique (112) has also been extensively applied to characterize the sorption of alkanes, principally in silicalite (111, 156, 168-171) but also in other zeolites (172-174). Smit and Siepmann (111, 168) presented a thorough study of the energetics, location, and conformations of alkanes from n-butane to n-dodecane in silicalite at room temperature. A loading of infinite dilution was simulated, based on a united-atom model of the alkanes and a zeolite simulation box of 16 unit cells. Potential parameters were very similar to those used in the MD study of June et al. (85). As expected, the static properties (heat of adsorption, Henry s law coefficient) determined from the CB-MC simulations are therefore in close agreement with the values of June et al. The... 

Figure 6 shows the isotherms of the samples using different alkanes as expander. Except the sample obtained with nonane, the adsorption-desorption isotherms of all other compounds are type IV, characteristic of mesoporous materials according to the BDDT classification [21], Isotherms can be decomposed in three parts the formation of the monolayer, a sharp increase characteristic of the capillary condensation of nitrogen within the mesopores and finally a plateau indicating the saturation of the samples. From pentane to decane the relative pressure at which the capillary condensation occurs, increases from 0 30 to 0.60, indicating that the value of the pore diameter increases when the alkane chain length is raised since the p/po position of the inflection point is related to the pore diameter. From undecane, this value decreases to reach 0.40 for dodecane We can conclude that the value of the pore diameter drops from decane to dodecane... 

The adsorption and desorption kinetics of surfactants, such as food emulsifiers, can be measured by the stress relaxation method [4]. In this, a "clean" interface, devoid of surfactants, is first formed by rapidly expanding a new drop to the desired size and, then, this size is maintained and the capillary pressure is monitored. Figure 2 shows experimental relaxation data for a dodecane/ aq. Brij 58 surfactant solution interface, at a concentration below the CMC. An initial rapid relaxation process is followed by a slower relaxation prior to achieving the equilibrium IFT. Initially, the IFT is high, - close to the IFT between the pure solvents. Then, the tension decreases because surfactants diffuse to the interface and adsorb, eventually reaching the equilibrium value. The data provide key information about the diffusion and adsorption kinetics of the surfactants, such as emulsifiers or proteins. 

The decrease in interfacial tension is related to the amount of extractant adsorbed at the interface through the Gibb s adsorption equation (46). The molecular areas of the extractant at the interface can thus be directly obtained from this equation. As an example, an area of 104 8 A2 is obtained for the. V,.V -dimc(hyl dibu-tyltetradecylmalonamide (DMDBTDMA) at the dodecane/water interface (4, 34). For classical surfactants, it should be noted that a nearly constant area per molecule with the addition of salt strongly suggests that anions and cations are adsorbed and extracted as pairs (47). Thus, the variation of the area per molecule with added salt can provide information on the mechanism of extraction. 

Fuerstenau, D.W., and Wakamatsu T. (1975). Effect of pH on the adsorption of sodium dodecane sulphonate at the alumina/water interface. Faraday Discussions of the Chemical Society, 59, 157-168. 

As will be shown, the equation rates obtained for o-HAP formation in the case of strong (polar sulfolane) and weak (nonpolar dodecane) solvent adsorption (Figure 2.3) explain the inhibiting effect of sulfolane as well as the differences in reaction order with respect to PA in the polar (order 1) and nonpolar (order 0.5)... 

Zeolite polarity and reaction rate The competition between sulfolane, PA and product molecules for the adsorption on the active protonic sites is sufficient enough to explain the differences in reaction orders and catalyst stability and selectivity between PA transformation in sulfolane and in dodecane. However, the competition for the occupancy of the zeolite micropores plays a significant role as well. This was demonstrated by studying a related reaction the transformation of an equimolar mixture of PA with phenol in sulfolane solvent on a series of H-BEA samples with different framework Si/Al ratios (from 15 to 90).[49] According to the largely accepted next nearest neighbour model,[50,51] the protonic sites of these zeolites should not differ by their acid strength, as furthermore confirmed by the... 

A demonstration of the single molecule detection at the liquid-liquid interface was reported for the fluorescent probe of l,l/-dioctadecyl-3,3,3/,. 3 -tetramethyI i ndoearboeyan i ne (Dil), which is a monovalent cation with two Cig alkyl chains. Thus, it has high adsorptivity at the dodecane-water interface. 

The adsorption of PNP to a wide variety of interfaces has been studied by SHG. The adsorption isotherm at the dodecane/water interface is shown in Figure 1.4. The adsorption isotherms typically used to fit the surface tension and SHG data at the fiquid/liquid interfaces are in terms of the interfacial coverage 0 and the bulk phase concentration c, the Langmuir (13) and the Frumkin isotherms (14)... 

The addition of tributyl phosphate (TBP) to the dodecane acts to reduce the SHG signal from an initial (partial) monolayer of PNP at the dodecane/water interface the dependence shown in Figure 1.5 can be fitted to a Langmuir-like equation (15) from which an effective free energy of adsorption for TBP, AadsG Bp. can be extracted [48]. 

For comparison, the measured adsorption rate for PNP to the air/water interface gave a rate constant of it = 4.4 0.2 x 10 s but a desorption rate of 6 2 s [52]. The rapid adsorption to the air/water interface is quite consistent with the very rapid establishment of the SHG signal from PNP at the dodecane/water interface observed in these flow cell experiments. However, the observed decay rate constant in the presence of TBP of ca. 0.5 s is much faster than the desorption rate constant that would be implied from the air/water experiments. This further implicates a reorganization process involving bonding between TBP and PNP as the cause of the loss of SHG intensity, which results in an overall loss of orientational order. 

The addition of TBP in the dodecane increases the free energy of adsorption of PNP in a manner consistent with the formation of an interfacial complex between TBP and... 

In the example of the adsorption of TBP from the dodecane phase (density p = 766 kg m and molar mass of 0.17 kg mol" ) to die dodecane/water interface, the conversion to the mole fraction scale gives... 

DMPC was dissolved in chloroform, and the solution was mixed with pure diethyl ether at a ratio of 1 19 (chloroform/diethyl ether) by volume. Pure water was placed in the lower container, and the DMPC solution was subsequently (5 mm ) spread carefully on the water. After evaporation of chloroform and diethyl ether, the Dil dodecane solution was added on the DMTC layer. Since Dil has a high adsorptivity at the interface, no Dil... 

Pioneers in measuring polymer bound fractions by IR were Fontana and Thomas 1), who studied poly(alkyl methacrylates) in n-dodecane on silica. Upon adsorption, part of the carbonyl stretching bemd (originally at 1736 cm ) shifted to a lower frequency (1714 cm ) due to formation of bound carbonyls. Fontana and Thomas also observed shifts in the vibration of silanol groups from the substrate and used these to obtain the train density. The IR technique has been exploited by several authors using different poljrmer systems, mostly with non-aqueous media and silica or tltanla as the substrates ll. The ATR approach has been used in several more recent studies ). 

A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of 0- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no diffusion limitations in the zeolite pores. 

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