Tension interfacial

Interfacial tension is the tension that is present at the interface of two immiscible phases and it has the same units as surface tension. The value of interfacial tension generally lies between the surface tension of two immiscible liquids as seen in Table 2.2, although it could also be lower than the surface tension of both liquids (water-diethyl ether). The interfacial tension between phases A and B, pab is expressed by  

The measurement of interfacial tension (ASTM D-971, ASTM D-2285) between oil and water is a sensitive method for determining traces of polar contaminants, including products of oil oxidation, and minimum values will sometimes be found in insulating oil specifications. The test is frequently 

Whereas the surface tensions refer to the liquid-vapor interface, interfacial tension is defined as surface free energy per unit area between two immiscible liquids. It has the same unit as the surface tension, and it results from 

Methods for measuring surface tension (a) capillary rise (b) pendant drop (c) du Noiiy ring (d) Wilhelmy plate. 

For pure liquids, the work of cohesion is defined as the work required pulling apart a volume of cross-sectional area. 

Both the work of cohesion and the work of adhesion are defined for a reversible process. Surface fension can be interpreted as half of the work of cohesion. Similar to the surface tension measurements, interfacial tension can also be measured both by the du Noiiy ring and the Wilhelmy plate surface tensiometers. 

The next section describes measurements of interfacial tension and surfactant adsorption. The sections on w/c and o/c microemulsions discuss phase behavior, spectroscopic and scattering studies of polarity, pH, aggregation, droplet size, and protein solubilization. The formation of w/c microemulsions, which has been achieved only recently [19, 20], offers new opportunities in protein and polymer chemistry, separation science, reaction engineering, environmental science for waste minimization and treatment, and materials science. Recently, kinetically stable w/c emulsions have been formed for water volume percentages from 10 to 75, as described below. Stabilization and flocculation of w/c and o/c emulsions are characterized as a function of the surfactant adsorption and the solvation of the C02-philic group of the surfactant. The last two sections describe phase transfer reactions between lipophiles and hydrophiles in w/c microemulsions and emulsions and in situ mechanistic studies of dispersion polymerization. 

The drop image is recorded with a video camera and digitized in order to calculate y from the Young-Laplace equation [13]. Independent measurements of the densities of the two phases are made wiA a vibrating tube densitometer. 

This inward attraction acts to minimize the interfacial area and the force which causes this decrease in area is known as the interfacial tension (y). If one phase is air, the interfacial tension is referred to as surface tension. Interfacial tension can be expressed as force per unit length (N m ) or the energy needed to increase the interfacial area by a unit amount (Jm or Nm ). 

In addition to temperature (which decreases y), the properties of interfaces are governed by the chemistry of the molecules present, their concentration and their orientation with respect to the interface. Solutes adsorbed at an interface which reduce interfacial tension are known as surface active agents or surfactants. Surfactants reduce interfacial tension by an amount given, under ideal conditions, by the Gibb s equation  

Interfacial tension may be measured by a number of techniques, including determining how far a solution rises in a capillary, by measuring the weight, volume or shape of a drop of solution formed at a capillary tip, measuring the force required to pull a flat plate or ring from the surface or the maximum pressure required to form a bubble at a nozzle immersed in the solution. Ring or plate techniques are most commonly used to determine y of milk. 

Reported values for the interfacial tension between milk and air vary from 40 to 60Nm, with an average of about 52Nm at 20°C (Singh, McCarthy and Lucey, 1997). At 20-40°C, the interfacial tension between milk serum and air is about 48Nm while that between sweet cream, buttermilk and air is about 40Nm (Walstra and Jenness, 1984). Surface tension values for rennet whey, skim milk and 25% fat cream are reported to be 51-52, 52-52.5 and 42-45Nm , respectively (Jenness and Patton, 1959). 

Surface forces exist at both gas-liquid and liquid-liquid interfaces. The latter are called interfacial tensions. They can be measured most easily with a Du Nouy tensiometer (Fig. 17.7), if the denser fluid wets the ring. In that case the force required to pull the ring from the lower fluid up into the upper fluid depends on the interfacial tension. 

In general, interfacial tensions are greater for liquid pairs with low mutual solubilities than for those with high ones. Thus, hexane-water (very low mutual solubility) has an interfacial tension two-thirds that of air-water, whereas butanol-water (reasonably large mutual solubility) has an interfacial tension only a few percent of that of air-water. For miscible liquid pairs such as ethanol-water, there can be no interfacial tension because there can be no interface. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

As the latter is comparatively simple to use it can be regarded as the most suitable method to measure low and ultra-low interfacial tensions. In the following the general features of interfacial tensions in microemulsion systems are presented. The dramatic decrease of the water/oil interfacial tension upon the addition of surfactant, the correlation of interfacial tension and phase behaviour, the variation of the water/oil interfacial tension with the respective tuning parameter and the scaling of the interfacial tension will be discussed in detail. All data presented have been determined using the spinning drop technique [17]. 

During the sizing process, the rubber phase is becoming increasingly finely dispersed in the SAN matrix. During this process, the surface area is increased. This process requires less energy when the interfacial tension is low. A reduction of the interfacial tension can be achieved by  

As the induction period can be affected profoundly by so many external influences, it cannot be regarded as a fundamental property of a system. Nor can it be relied upon to yield basic information on the process of nucleation. Nevertheless, despite its complexity and uncertain composition, the induction period has frequently been used as a measure of the nucleation event, making the simplifying assumption that it can be considered to be inversely proportional to the rate of nucleation  

The classical nucleation relationship (equation 5.9) may therefore be written 

The temperature dependence of interfacial tension has been demonstrated using induction period data for nickel ammonium sulphate recorded over a short temperature range. The salt was precipitated by quickly mixing equimolar solutions of nickel and ammonium sulphates after which the system was allowed to remain static until nucleation occurred. Plots of log/md versus T (log S) , in accordance with equation 5.38, gave a family of straight lines 

A comprehensive review of the general subject of solid material surface energy has been made by Linford (1972). 

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The Parachor is a parameter used to determine the interfacial tension. It can be estimated by a simple method proposed by Quayle in 1953 ... 

The interfacial tension is usually expressed in mN/m or dynes/cm. It measures the tendency for a liquid to form an interface having the least area. The interfacial tension decreases as the temperature Increases. 

The interfacial tension of a petroleum fraction at 20°C can be estimated by the following formula, cited by the API ... 

Soave m coefficient Solubility parameter at 25°C 0iJ/m ) /2 Temperature n °c Interfacial tension at mN/m Lee Kesier acentric factor... 

The relationship between the pressure drop across the interface AP, the interfacial tension o, and the radius of the droplet, r, is... 

A zero or near-zero contact angle is necessary otherwise results will be low. This was found to be the case with surfactant solutions where adsorption on the ring changed its wetting characteristics, and where liquid-liquid interfacial tensions were measured. In such cases a Teflon or polyethylene ring may be used [47]. When used to study monolayers, it may be necessary to know the increase in area at detachment, and some calculations of this are available [48]. Finally, an alternative method obtains y from the slope of the plot of W versus z, the elevation of the ring above the liquid surface [49]. 

Fig. 11-13. Apparatus for measuring the time dependence of interfacial tension (from Ref. 34). The air and aspirator connections allow for establishing the desired level of ftesh surface. IV denotes the Wilhelmy slide, suspended from a Cahn electrobalance with a recorder output.

Princen and co-workers have treated the more general case where w is too small or y too large to give a cylindrical profile [86] (see also Refs. 87 and 88). In such cases, however, a correction may be needed for buoyancy and Coriolis effects [89] it is best to work under conditions such that Eq. 11-35 applies. The method has been used successfully for the measurement of interfacial tensions of 0.001 dyn/cm or lower [90, 91]. 

Another oscillatory method makes use of a drop acoustically levitated in a liquid. The drop is made to oscillate in shape, and the interfacial tension can be calculated from the resonance frequency [113]. 

Molten naphthalene at its melting point of 82°C has the same density as does water at this temperature. Suggest two methods that might be used to determine the naphthalene-water interfacial tension. Discuss your suggestions sufficiently to show that the methods will be reasonably easy to cany out and should give results good to 1% or better. 

Figure III-l depicts a hypothetical system consisting of some liquid that fills a box having a sliding cover the material of the cover is such that the interfacial tension between it and the liquid is zero. If the cover is slid back so as to uncover an amount of surface dJl, the work required to do so will he ydSl. This is reversible work at constant pressure and temperature and thus gives the increase in free energy of the system (see Section XVII-12 for a more detailed discussion of the thermodynamics of surfaces).

The classic theory due to van der Waals provides an important phenomenological link between the structure of an interface and its interfacial tension [50-52]. The expression... 

The theoretical treatments of Section III-2B have been used to calculate interfacial tensions of solutions using suitable interaction potential functions. Thus Gubbins and co-workers [88] report a molecular dynamics calculation of the surface tension of a solution of A and B molecules obeying Eq. III-46 with o,bb/ o,aa = 0.4 and... 

Fig. III-9. Representative plots of surface tension versus composition, (a) Isooctane-n-dodecane at 30°C 1 linear, 2 ideal, with a = 48.6. Isooctane-benzene at 30°C 3 ideal, with a = 35.4, 4 ideal-like with empirical a of 112, 5 unsymmetrical, with ai = 136 and U2 = 45. Isooctane-

Van Oss and Good [148] have compared solubilities and interfacial tensions for a series of alcohols and their corresponding hydrocarbons to determine the free energy of hydration of the hydroxyl group they find -14 kJ/mol per —OH group. 

Application of 150 MPa pressure increases the interfacial tension for w-hex-ane-water from 50.5 to 53.0 mN/m at 25°C. Calculate AV. What is AV for that area corresponding to a molecular size (take a representative molecular area to be 20 A ) Convert this to cm /cm mol. 

The gradient model for interfacial tension described in Eqs. III-42 and III-43 is limited to interaction potentials that decay more rapidly than r. Thus it can be applied to the Lennard-Jones potential but not to a longer range interaction such as dipole-dipole interaction. Where does this limitation come from, and what does it imply for interfacial tensions of various liquids ... 

A. W. Neumann et al.. Applied Surface Thermodyrmmics. Interfacial Tension and Contact Angles, Marcel Dekker, New York, 1996. 

This rule is approximately obeyed by a large number of systems, although there are many exceptions see Refs. 15-18. The rule can be understood in terms of a simple physical picture. There should be an adsorbed film of substance B on the surface of liquid A. If we regard this film to be thick enough to have the properties of bulk liquid B, then 7a(B) is effectively the interfacial tension of a duplex surface and should be equal to 7ab + VB(A)- Equation IV-6 then follows. See also Refs. 14 and 18. 

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