Membrane surface tension

Analytical pervaporation is the process by which volatile substances in a heated donor phase evaporate and diffuse through a porous hydrophobic membrane, the vapour condensing on the surface of a cool acceptor fluid on the other side of the membrane. Surface tension forces withhold the fluids from the pores and prevent direct contact between them. A temperature difference that results in a vapour pressure difference across the membrane provides a strong driving force for the separation, which also occurs in the absence of a temperature gradient. Evaporation will occur at the sample surface if the vapour pressure exceeds that at the acceptor surface. One important feature of pervaporation modules used for analytical purposes is the air gap between the donor phase and the hydrophobic membrane, which avoids any contact between them and reduces the problems associated with fouling of the membrane. 

Young s equation [Eq. (2)] shows that high contact angles are achieved when the solid (membrane) surface tension, y, is low, and the interfacial solid-liquid tension, ysi, and liquid surface tension are both high. Accordingly, the most suitable membranes for use in... 

Solid (membrane) surface tension (Nm ) Interfacial solid-liquid tension (Nm )... 

Machado et al. [25] reported solvent fluxes for alcohols, paraffins, acetone, and water and presented a model describing solvent transport through solvent-resistant NF membranes (MPF-50, M.W.C.O. 700 Da, Koch, USA). They concluded that both viscosity and surface tension are major parameters that influence the solvent flux. Yang et al. (2001) showed that the transport mechanism by NF polymer membranes with organic solvents is not based solely on viscous flow through pores or simple molecular diffusion, and there must be some interaction between the solvent and the membrane (surface tension, sorption, and hydrophilicity or hydrophobicity of interfaces), dependent on the membrane material and properties of the solvent, which are important in determining the solvent flux. 

These observations may well be reconciled with classic membrane biophysical dynamics increased fluidity does not necessarily imply enhanced enzyme function -since many enzymes manifest a given range of membrane surface tensions for optimal activity. Values exceeding or below optimum prevent optimal enzyme substrate recognition. This particularly applies to membrane/cytosol interface located enzymes such as phospholipases. 

Pressure Decay Test (PDJ)—Filled This test involves applying pressurized air to the feed side at a predetermined level below the bubble point and then isolating. When the pores of a membrane are flUed with liquid and air pressure is applied to one side of the membrane, surface tension prevents the liquid in the pores from being blown out by the air pressure below a certain minimum pressure known as the bubble point. The predetermined pressure directly relates to the size of defect under investigation [Drinking Water Inspectorate (DWI), 2001]. 

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

Toffoli and Margolus [tofF86] point out that what appears on the macroscopic scale is a good simulation of surface tension, in which the boundaries behave as though they are stretched membranes exerting a pull proportional to their curvature. Vichniac adds that the behavior of such twisted majority rules actually simulates the Allen-Cahn equation of surface tension rather accurately ... 

Concerning function integration, for example, micro-flow membrane reactors can exhibit similar process intensification, as shown already for their large-scale counterparts [75]. Separation columns for proteomics, immobilizing enzymes, utilize the large surface-to-volume ratios. Surface tension differences can guide and transport liquids selectively. 

Bubble Point Large areas of microfiltration membrane can be tested and verified by a bubble test. Pores of the membrane are filled with liquid, then a gas is forced against the face of the membrane. The Young-Laplace equation, AF = (4y cos Q)/d, relates the pressure required to force a bubble through a pore to its radius, and the interfacial surface tension between the penetrating gas and the liquid in the membrane pore, y is the surface tension (N/m), d is the pore diameter (m), and P is transmembrane pressure (Pa). 0 is the liquid-solid contact angle. For a fluid wetting the membrane perfectly, cos 0 = 1. 

Surface tension The attraction between molecules that tends to pull the molecules at the surface of a liquid down. This makes the surface become as small as possible and makes certain substances—water, for instance—act as though a thin membrane was stretched across the surface. 

Surface tension contribution defined by a stress [Pg.86]

It follows from Eqs. (73) and (74) that the only stabilizing force for a-modes at long X is the membrane tension, and critical voltage vanishes as cr 0. In experiments with black lipid membranes the surface tension a arises from the contact of the bilayer with the bulk phase contained in the surrounding rim and is typically < 0.002 N/m. Then choosing... 

Koryta et al. [48] first stressed the relevance of adsorbed phospholipid monolayers at the ITIES for clarification of biological membrane phenomena. Girault and Schiffrin [49] first attempted to characterize quantitatively the monolayers of phosphatidylcholine and phos-phatidylethanolamine at the ideally polarized water-1,2-dichloroethane interface with electrocapillary measurements. The results obtained indicate the importance of the surface pH in the ionization of the amino group of phosphatidylethanolamine. Kakiuchi et al. [50] used the video-image method to study the conditions for obtaining electrocapillary curves of the dilauroylphosphatidylcholine monolayer formed on the ideally polarized water-nitrobenzene interface. This phospholipid was found to lower markedly the surface tension by forming a stable monolayer when the interface was polarized so that the aqueous phase had a negative potential with respect to the nitrobenzene phase [50,51] (cf. Fig. 5). 

If the surface of a liquid is regarded as an elastic membrane, then the surface tension is the breaking force of this membrane. Water has one of the highest surface tensions of all liquids. For example, the surface tension of ethanol at 20°C is 22 mN/m, while that of water is 72.75 mN/m. The surface tension of water decreases with temperature. 

The attempt to show that surface tension phenomena were the cause of osmotic pressure was first made by Jager, and his theories were vigorously supported and developed by Traube, whose conclusions we shall state and examine briefly. He finds that the more a dissolved substance reduces the surface tension of water the greater is the velocity of osmosis of the solution. Hence he concludes that it is the difference in the surface tensions of solvent and solution which determines the direction and velocity of osmosis. The direction of flow Traube obtains by the following consideration let M (Fig. 7) be a membrane separating two liquids A and B. The molecules of each liquid are then drawn into its interior by the cohesion or intrinsic pressure. If the intrinsic... 

This is the definition of the surface tension according to the Gibbs surface model [1], According to this definition, the surface tension is related to an interface, which behaves mechanically as a membrane stretched uniformly and isotropically by a force which is the same at all points and in all directions. The surface tension is given in J m-2. It should be noted that the volumes of both phases involved are defined by the Gibbs dividing surface X that is located at the position which makes the contribution from the curvatures negligible. 

The surface tension defined above was related to an interface that behaved mechanically as a membrane stretched uniformly and isotropically by a force which is the same at all points on the surface. A surface property defined this way is not always applicable to the surfaces of solids and the surface energy of planar surfaces is defined to take anisotropy into account. The surface energy is often in the literature interchanged with surface tension without further notice. Although this may be useful in practice, it is strictly not correct. 

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