Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Solvent relaxation membranes

Fluorophores which have been used to study solvent relaxation processes in membranes include 2-(p-toluidinyl)naphthalene-6-sulfonate (TNS)/130,132,I33 I35) dansylated lipids, and drugs/128,136)... [Pg.259]

A more detailed study of transport processes in solvent polymeric membranes was initiated recently.72 One aim was to get information on the distribution within the membrane of the carrier and the cation transported after a steady state has built up during an electrodialysis experiment. A further objective was the demonstration of a relaxation of the concentration gradients of both carrier and cation. To this end the transport properties of solvent polymeric membranes containing the carrier l4C-valinomycin (66 wt.% dioctyladipate, 33 wt.% polyvinyl chloride, 1 wt.%, JC-valinomycin) in contact with aqueous solutions of -,H-a-phenylethylammonium chloride were studied. [Pg.307]

The cylinder stretching protocol appears to work very well for simple solvent-free membrane models [111, 113, 114], but with more refined models this method suffers from two drawbacks, both related to the equifibration of a chemical potential. First, the cylinder separates the simulation volume into an inside and an outside. If solvent is present, its chemical potential must be the same in these two regions, but for more highly resolved models the solvent permeability through the bilayer is usually too low to ensure automatic relaxation. Second, the chemical potential of lipids also has to be the same in the two bilayer leaflets, and again for more refined models the lipid flip-flop rate tends to be too low for this to happen spontaneously. [Pg.247]

Benz, Frohlich, and Lau r have used the charge-pulse relaxation technique to determine the rate constants of association (A r) and dissociation (A d) of the valino-mycin-Rb+ complex as well as the rate constants of translocation of the complex (A ms) and the free carrier (Ars) in experiments with a series of lipid bilayers ( black membranes ). For membranes made from glycerol mono-oleate dissolved in different n-alkanes (ranging from n-decane to n-hexadecane) and for solvent-free membranes of the same lipid, the translocation rate constants Ars and A ms were both found to differ by less than a factor of two (Table 1). On the other hand, fairly large changes of the rate constants were observed (Table 2) if the structure of the fatty acid residue... [Pg.298]

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. [Pg.308]

First of all, the capacitance of lecithin-hexadecane membrane is about 0.62 yF/cm. This value is smaller than the capacitance of biological membranes, i.e., 1 yF/cm2. The difference is perhaps partially due to the absence of proteins in artificial membranes. In addition, it is known that the presence of solvents decreases the values of membrane capacitance. For example, membranes formed by the Montal-Mueller Method (21), which are believed to be free of solvents, have a capacitance of 0.7 yF/cm2 (22). Thus, the capacitance of bilayer membranes shown in this figure may be in error by about 0.1 yF/cm2 because of the presence of solvent molecules. However, it is more important to note that membrane capacitance is independent of frequency, which provides unequivocal evidence that there is no relaxation process in lipid membranes in this frequency range. Coster and Smith (23) reported that they observed a frequency dispersion of membrane capacitance of artificial layers at very... [Pg.135]

Asymmetrical flow-FFFA (A-Fl-FFF) was introduced by Wahlund and Giddings in 1987 [47]. The same system was independently suggested slightly earlier by Granger et al. [237,248], but their application of the technique suffered from a lack of a primary relaxation step preceding separation [47]. A-Fl-FFF is notable for a channel which has only one permeable wall so that the solvent can leave the channel only via the accumulation wall and thus generates a cross-flow. The permeable wall is usually a sintered metal plate or ceramic frit covered by an ultrafiltration membrane (see Fig. 20). [Pg.120]

It is noteworthy that the S-To conversion rate is given by Qn for a radical pair with 7-0 J as shown in Chapter 3. When 7 - 0 J,Ag = 0.01, B = 1 T, and A/g tB = Ai/gfis = 0 T, the rate becomes 4.4 x 10 s from problem 3-5. If such S-To conversion rate is comparable to the escape rate of two radicals from a solvent cage, appreciable MFEs and MIEs can be observed. In some cases, this condition can be satisfied in homogeneous solvents. If two radicals are confined with membranes, micelles, or chemical bonds, the escape rate of the two radicals becomes much smaller than the S-To conversion rate. In this case, the S and To states attain equilibrium and the T i-To and T i-S relaxations become important under sufficiently high fields as shown in Fig. 7-4(b). In 1984, the author s group proposed the relaxation mechanism (RM) [2] in order to explain MFEs and MIEs on chemical reactions in confined systems. When st(0 T), stCB) fcp in the RM as shown in Fig. 7-4, the rate equations of the populations of the singlet and triplet radical pair ([S] and [T ] for n = +1,0, and -1) produced from a triplet precursor can be represented as follows [2] ... [Pg.101]

There a been a number of interesting applications of the framework developed in the studies of the simple ions were MD simulations of the quadrupolar relaxation has been performed on counterions in heterogeneous systems. Studies of a droplet of aqueous Na embedded in a membrane of carboxyl groups [54], showed that the EFG was strongly effected by the local solvent structure and that continuum models are not sufficient to describe the quadrupolar relaxation. The Stemheimer approximation was employed, which had been shown to be a good approximation for the Na ion. Again, the division into molecular contributions could be employed to rationalize the complex behavior in the EFG tensor. Similar conclusions has been drawn from MD simulation studies of ions solvating DNA... [Pg.306]

Din and Deq are the diffusion coefficient of the solvent in the membrane at the beginning of the swelling phenomenon and at swelling equilibrium (C = Ceq), respectively g and k are the two adjustable parameters Teq is the membrane relaxation time when C = Ceq... [Pg.436]

Although most aspects of the formation of asymmetric skin type membranes can satisfactorily be rationalized by applying the basic thermodynamic and kinetic laws of phase separation processes, there are other parameters, such as surface tension (16), polymer relaxation (J ), solvent loss by evaporation (Jg), etc., which are not directly related to the phase separation process, but nevertheless will have a strong effect on the membrane structure and properties. [Pg.194]


See other pages where Solvent relaxation membranes is mentioned: [Pg.324]    [Pg.231]    [Pg.258]    [Pg.258]    [Pg.146]    [Pg.147]    [Pg.201]    [Pg.211]    [Pg.237]    [Pg.1132]    [Pg.190]    [Pg.299]    [Pg.321]    [Pg.79]    [Pg.466]    [Pg.114]    [Pg.132]    [Pg.51]    [Pg.592]    [Pg.137]    [Pg.348]    [Pg.153]    [Pg.284]    [Pg.382]    [Pg.230]    [Pg.16]    [Pg.431]    [Pg.530]    [Pg.85]    [Pg.190]    [Pg.161]    [Pg.216]    [Pg.501]    [Pg.332]    [Pg.920]    [Pg.33]    [Pg.201]   
See also in sourсe #XX -- [ Pg.257 ]




SEARCH



Membrane solvent

© 2024 chempedia.info