Interfacial surface tension

The Gibbs equation relates the extent of adsorption at an interface (reversible equilibrium) to the change in interfacial tension qualitatively, Eq. (4.3) predicts that a substance which reduces the surface (interfacial) tension [(Sy/8 In aj) < 0] will be adsorbed at the surface (interface). Electrolytes have the tendency to increase (slightly) y, but most organic molecules, especially surface active substances (long chain fatty acids, detergents, surfactants) decrease the surface tension (Fig. 4.1). Amphi-pathic molecules (which contain hydrophobic and hydrophilic groups) become oriented at the interface. 

Hence the surface adsorption of surfactant 1 and 2, and their surface mole fractions can be obtained from the surface (interfacial) tension-concentration relationships (Fig.1 and fig.2) by applying the Gibbs adsorption equation. 

Here, tr, is the liquid-to-surface interfacial tension l , u the vapor apparent velocity directing the droplets and S characterizes the wire mesh By ignoring the secondary effects, a simplified force balance can be written for the disengaging droplet size ... 

The short-range intermolecular forces which are responsible for surface/interfacial tensions include van der Waals forces (in particular, London dispersion forces, which are universal) and may include... 

Consider a thin film of thickness D that uniformly covers a solid substrate except for a cylindrical hole of radius R where the solid is exposed to the air. What is the free energy of this film compared to the case of a perfect film without the hole in terms of D, / , and the relevant surface/interfacial tensions What is the critical radius at which this hole will grow and why ... 

Techniques for Measuring Contact Angle and Surface/Interfacial Tension... 

The molecular structure of surfactants controls not only the concentration of the surfactants at the interface and the resulting reduction in surface/interfacial tensions, but also affects the orientation of the molecules at the interface. The hydrophilic group is either ionic in nature or highly polar. Based on the nature of the polar group, surfactants can be classified as anionic, cationic, non-ionic or amphoteric. Among these types, anionic and non-ionic surfactants are preferably employed in enhanced oil recovery processes (EOR) due to their low adsorption on reservoir rocks. Therefore, these surfactants are briefly described. 

Experimental investigation of fluid mechanics and mass transfer under conditions as close as possible to operating conditions (droplet formation and size, droplet size distribution, droplet movement, coalescence, separation behavior, mass transfer, HETS or HTU with respect to surface (interfacial) tension and mixing effects), selection of the optimum operating point (Chapter 6.2.4)... 

The interplay between these various factors is complex and often requires experimental measurement under as realistic conditions as possible to appropriately determine the impact of surfactant on wettability. It is the migration to, and the adsorption of, the surfactant at the fluid and solid interfaces along with the orientation and density of the adsorbed surfactant molecules that modifies the fluid-surface interfacial tension/ wettability. Surfactant adsorption at an interface is a necessary, but not a sufficient condition for wettability alteration. Although details of adsorption will be covered in Chapter 4, this section includes a brief treatise on it with the other known variables that can affect wettability modification with surfactants. 

Thermocapillary Diffusion. Temperature induced Marangoni flow. The movement of suspended drops or bubbles when subjected to a temperature gradient, due to the resulting surface/interfacial tension gradient. 

In ordinary drying, the liquid in a specimen evaporates, and the resulting surface (interfacial) tension can distort the structure. In critical point drying [425], heating a specimen in a fluid above the critical temperature to above the critical pressure permits the specimen to pass through the critical point (that temperature and pressure where the densities of the liquid and vapor phases are the same and they coexist and thus there is no surface tension). By definition, a gas cannot condense to a liquid at any pressure above the critical temperature. The critical pressure is the minimum pressure required to condense a liquid from the gas phase at just... 

If there is a free interface between fluids, gradients in concentration and/or temperature parallel to the interface cause gradients in the surface (interfacial) tension, which cause convection [85]. This convection, also known as Marangoni convection, is especially noticeable in thin layers (or weightlessness) in which buoyancy-driven convection is greatly reduced. 

The variation of surface/interfacial tension, due, for example, to the formation of a new interfacial area, is a consequence of adsorption dynamics. This transient phenomenon involves different time steps exchange between the interface and the adjacent bulk layer, reorganisation in the interfacial layer and diffusion in the bulk. However, for most of the surfactants studied so far, the first two steps are much faster than the diffusion in the bulk, so they can be considered locally in equilibrium with respect to diffusion, which is, thus, controlling the adsorption process. 

The Wilhelmy Method This is a technique that can be used for both surface/interfacial tension and contact angle measurements (Fig. 7b). To measure the surface tension, a plate with known perimeter P is attached to a balance. 

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