Solvents surface forces

Film pressure is often measured directly by means of a film balance. The principle of the method involves the direct measurement of the horizontal force on a float separating the film from clean solvent surface. The film balance has been considerably refined since the crude model used by Langmuir and in many... 

While evidence for hydration forces date back to early work on clays [1], the understanding of these solvent-induced forces was revolutionized by Horn and Israelachvili using the modem surface force apparatus. Here, for the first time, one had a direct measurement of the oscillatory forces between crossed mica cylinders immersed in a solvent, octamethylcyclotetrasiloxane (OMCTS) [67]. 

Dhinojwala A and Granick S 1997 Surface forces In the tapping mode solvent permeability and hydrodynamic thickness of adsorbed polymer brushes Macromoiecuies 30 1079-85... 

A stainless steel cannula with a 2-mm. inside diameter and both ends sharpened is inserted through the septum into the receiving flask above the surface of the liquid, and a stream of nitrogen is passed briefly through the stopcock and out the cannula to remove air. The other end of the cannula is then inserted through the septum on the reaction vessel, the end of the cannula in the receiver is pushed below the surface of the liquid, and the solvent is forced into the reaction vessel with nitrogen pressure. 

In this paper we briefly describe the apparatus and experimental method, then consider the interactions between i) layers of polystyrene in cyclohexane under poor-solvent and ii) 0 - solvent conditions,iii) the interactions between adsorbed PEO layers in a good (aqueous) solvent and iv) the surface forces between layers of adsorbed poly-L-lysine, a cationic polyelectrolyte, in aqueous salt solutions. We consider briefly the implications of our results for the current theoretical understanding. 

Gas adsorption into the liquid falling down a wetted wall column is of considerable interest. The flow of liquid down the surface of such a tube is essentially laminar if Re < 1200, where Re is defined as iu/vl, u being the volume flow rate of liquid, I the perimeter of the tube, and v the kinematic viscosity of the solvent. Under these conditions, if there are no surface forces acting, the velocity of the air-water surface of the... 

The quality of solvent (i.e., solvent-polymer interactions) can also clearly affect the interaction forces. In a good solvent, polymer segments favor contacts with the solvent. Since the compression of the polymer layer by an approaching surface tends to squeeze out the solvent and force segment-segment interactions, the net result is a repulsion. In contrast, poor solvents produce an opposite effect, and a net attraction is possible for certain range of compression (see Section 13.6b). 

When two such surfaces approach each other, layer after layer is squeezed out of the closing gap (Fig. 6.12). Density fluctuations and the specific interactions then cause an exponentially decaying periodic force the periodic length corresponds to the thickness of each layer. Such forces were termed solvation forces because they are a consequence of the adsorption of solvent molecules to solid surfaces [168], Periodic solvation forces across confined liquids were first predicted by computer simulations and theory [168-171], In this case, however, the experimental proof came only few years afterwards using the surface forces apparatus [172,173]. Solvation forces are not only an important factor in the stability of dispersions. They are also important for analyzing the structure of confined liquids. 

The force between surfaces which are coated with polymers is mainly determined by two factors. The first one is the quality of the solvent. In good solvents the force tends to be repulsive, in bad solvents attractive. Moreover, in good solvents polymer tends to remain in solution rather than adsorbing to surfaces. 

In certain cases - solutions of electrolytes, sugars, etc. - small increases in surface tension due to negative adsorption are noted. Here, because the solute-solvent attractive forces are greater than the solvent-solvent attractive forces, the solute molecules tend to migrate away from the surface into the bulk of the liquid. 

As shown in Equation 10.4, the depression of the melting point of a given confined solvent is related to the geometry of the pores of the confining material. In principle, measurement of AT can give access to the pore size. Three main techniques have been developed to measure porosity in solids via the use of the Gibbs-Thomson equation thermoporosimetry, NMR cryporometry and surface force apparatus. These techniques are secondary methods since they require pre-... 

Good quality RO membranes can reject >95-99% of the NaCl from aqueous feed streams (Baker, Cussler, Eykamp et al., 1991 Scott, 1981). The morphologies of these membranes are typically asymmetric with a thin highly selective polymer layer on top of an open support structure. Two rather different approaches have been used to describe the transport processes in such membranes the solution-diffusion (Merten, 1966) and surface force capillary flow model (Matsuura and Sourirajan, 1981). In the solution-diffusion model, the solute moves within the essentially homogeneously solvent swollen polymer matrix. The solute has a mobility that is dependent upon the free volume of the solvent, solute, and polymer. In the capillary pore diffusion model, it is assumed that separation occurs due to surface and fluid transport phenomena within an actual nanopore. The pore surface is seen as promoting preferential sorption of the solvent and repulsion of the solutes. The model envisions a more or less pure solvent layer on the pore walls that is forced through the membrane capillary pores under pressure. 

Large proteins or polystyrene beads assembled on a glass plate or Hg surface on evaporation of a solvent Capillary forces nm to (Jim 2-4, 32, 33, 43, 44... 

The shape of the experimental isotherm (Fig. 3.38) is qualitatively similar to that found by van Vliet [226] with foam films from aqueous solutions of polyvinyl alcohol. It is also characteristic of the interaction energy as a function of the distance between two mica surfaces bearing grafted polymer brushes in good solvent as determined with Surface Force Apparatus (SFA) [e.g. 242],... 

Water and often fine sand and silt are held in various crude oils in permanent emulsions. Particularly crudes obtained by secondary methods and those from tar sands where water or steam are used contain water and mineral matter emulsified therein by the surface forces on small particles and drops. Azeotropic distillation removes the relatively small amount of water, using the solvent as an entrainer which dilutes the crude. This allows the mineral matter to be separated easily without using centrifuges with their substantial cost and wear, free of organic material, so it may be discarded with-out hazards of fire or odors the bitumen to be recovered for such use or cracked to give volatile fractions and coked to an ash-free coke the water to be obtained as distilled water for reuse. 

In a liquid binary solution, this accumulation is accompanied by the corresponding displacement of another component (solvent) from the surface region into the bulk solution. At equilibrium a certain amount of the solute will be accumulated on the surface in excess of its equilibrium concentration in the bulk solution, as shown in Figure 2-6. Excess adsorption E of a component in binary mixture is defined from a comparison of two static systems with the same liquid volume Vo and adsorbent surface area S. In the first system the adsorbent surface considered to be inert (does not exert any surface forces in the solution) and the total amount of analyte (component 2) will be no = VoCo. In the second system the adsorbent surface is active and component 2 is preferentially adsorbed thus its amount in the bulk solution is decreased. The analyte equilibrium concentration Ce can only be measured in the bulk solution, so the amount VoCe is thereby smaller than the original quantity no due to its accumulation on the surface, but it also includes the portion of the analyte in the close proximity of the surface (the portion U Ce, as shown in Figure 2-6 note that we did not define V yet and we do not need to define... 

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