Molecular permeation

V. Schmitt, C. Cattelet, and F. Leal-Calderon Coarsening of Alkane-in-Water Emulsions Stabilized by Nonionic Poly(Oxyethylene) Surfactants The Role of Molecular Permeation and Coalescence. Langmuir 20, 46 (2004). 

To further elucidate the nature of methanol permeability through ion exchange membranes it is helpful to consider the theoretical basis for the understanding of molecular permeation. The equilibrium condition for a compound which is distributed between two phases will have an identical chemical potential in the two phases ... 

Itoh et al. attempted to control molecular permeation using the system shown in Fig. 7(a) [126]. Acrylic acid was plasma polymerized... 

Moisture absorptivity, 17-45 Molar conductivity AMPS, 305-6 PAMPS, 305-6 temperature dependence, 306 Molecular imprinting resins, 293, 295, 296 Molecular imprinting technique, 289, 290, 291 Molecular orientation, 395 Molecular permeation, 135 Molecular recognition capability, 290 Molecular recognition information, 290 Molecular recognition molecules, 289 Molecular recognition sensor, 296 Molecular stifiening concept, 38 Molecular valves, stimuli-responsive sur ce as, 135-6... 

Gel permeation chromatography, exclusion chromatography. gel filtration chromatography. A technique for separating the components of a mixture according to molecular volume differences. A porous solid phase (a polymer, molecular sieve) is used which can physically entrap small molecules in the pores whilst large molecules pass down the column more rapidly. A solvent pressure up to 1000 psi may be used. 

Permeation process of small molecules across lipid membranes studied by molecular dynamics simulations. J. Phys. Chem. 100 (1996) 16729-16738. 

Two classes of micron-sized stationary phases have been encountered in this section silica particles and cross-linked polymer resin beads. Both materials are porous, with pore sizes ranging from approximately 50 to 4000 A for silica particles and from 50 to 1,000,000 A for divinylbenzene cross-linked polystyrene resins. In size-exclusion chromatography, also called molecular-exclusion or gel-permeation chromatography, separation is based on the solute s ability to enter into the pores of the column packing. Smaller solutes spend proportionally more time within the pores and, consequently, take longer to elute from the column. 

Next let us consider the differences in molecular architecture between polymers which exclusively display viscous flow and those which display a purely elastic response. To attribute the entire effect to molecular structure we assume the polymers are compared at the same temperature. Crosslinking between different chains is the structural feature responsible for elastic response in polymer samples. If the crosslinking is totally effective, we can regard the entire sample as one giant molecule, since the entire volume is permeated by a continuous network of chains. This result was anticipated in the discussion of the Bueche theory for chain entanglements in the last chapter, when we observed that viscosity would be infinite with entanglements if there were no slippage between chains. 

At first glance, the contents of Chap. 9 read like a catchall for unrelated topics. In it we examine the intrinsic viscosity of polymer solutions, the diffusion coefficient, the sedimentation coefficient, sedimentation equilibrium, and gel permeation chromatography. While all of these techniques can be related in one way or another to the molecular weight of the polymer, the more fundamental unifying principle which connects these topics is their common dependence on the spatial extension of the molecules. The radius of gyration is the parameter of interest in this context, and the intrinsic viscosity in particular can be interpreted to give a value for this important quantity. The experimental techniques discussed in Chap. 9 have been used extensively in the study of biopolymers. 

This chapter contains one of the more diverse assortments of topics of any chapter in the volume. In it we discuss the viscosity of polymer solutions, especially the intrinsic viscosity the diffusion and sedimentation behavior of polymers, including the equilibrium between the two and the analysis of polymers by gel permeation chromatography (GPC). At first glance these seem to be rather unrelated topics, but features they all share are a dependence on the spatial extension of the molecules in solution and applicability to molecular weight determination. 

Both preparative and analytical GPC were employed to analyze a standard (NBS 706) polystyrene sample. Fractions were collected from the preparative column, the solvent was evaporated away, and the weight of each polymer fraction was obtained. The molecular weights of each fraction were obtained usmg an analytical gel permeation chromatograph calibrated in terms of both and M. The following data were obtained ... 

Gel-permeation media are extremely versatile and may be used for separation of particles such as vimses (Fig. 11) as well as proteins (34). Separations of proteins and other particles having sizes equivalent to a molecular weight of 40 x 10 are possible using the agar-based Sepharose-type gel. This particular gel has a limited temperature range for operation, however. It melts upon heating to 40°C (34). 

Gases and vapors permeate FEP resin at a rate that is considerably lower than that of most plastics. Because FEP resins are melt processed, they are void-free and permeation occurs only by molecular diffusion. Variation in crystallinity and density is limited, except in unusual melt-processing conditions. 

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