Spreading coefficient interfacial tension

The Spreading process is governed by the spreading coefficient S defined as in equation 4 (30) where c is the surface tension of the foaming medium, C the surface tension of the defoamer, and C. the interfacial tension between them. 

The better interaction observed with the unmodifled clay was also explained in terms of surface energy. The values of surface energy of the fluoroelastomer and the clays, along with work of adhesion, spreading coefficient and interfacial tension are reported in Table 2.4. 

Spreading Coefficient. The spreading coefficient is defined as the difference of the surface tension of the foaming medium cry, the surface tension of the defoamer aj, and the interfacial tension of both materials a /. 

The effect of mutual saturation on the L-V and L-L interfacial tensions is effectively illustrated by considering the spreading coefficient of one liquid on another using both the initial (unsaturated) and equilibrium values of 7. Use the following data to calculate Se/, (equilibrium) and S B/A (nonequilibrium) ... 

In the group with positive spreading coefficients (e.g., toluene-in-water and oleic acid-in-water emulsions), the values ofkj a in both stirred tanks and bubble columns decrease upon the addition of a very small amount of oil, and then increase with increasing oil fraction. In such systems, the oils tend to spread over the gas-liquid interface as thin films, providing additional mass transfer resistance and consequently lower k values. Any increase in value upon the further addition of oils could be explained by an increased specific interfacial area a due to a lowered surface tension and consequent smaller bubble sizes. 

These agents may operate via a number of mechanisms, but the most common ones appear to he those of entry and/or spreading. The defoamer must first of all he insoluble in the foaming liquid for these mechanisms to function. Second, the surface tension of the defoamer must be as low as possible. The interfacial tension between defoamer and foamer should be low. but not so low that emulsification of the defoamer may occur. Third, the defoamer should be dispersible in the foaming liquid. It was first shown in I fM8 that thermodynamically the entry of the defuamcr droplet into a bubble surface occurs when the entering coefficient has a positive value. The physics of bubbles is described in entry on Foam. 

When setting an immiscible liquid on top of another liquid, a macroscopic thick film or a drop forms, depending on the interfacial tensions. This is quantified using the spreading coefficient. 

Surfactants assist wetting because they lower surface and interfacial tension. The energy balance that determines spreading is expressed by the spreading coefficient, S, and is illustrated in Figure 6.21. 

Antonoff s rule agrees with the result that the interfacial tension 0 12 is less than the larger (ori) of the two surface tensions, arid it requires that when two partly miscible liquids are in equilibrium the spreading coefficient (contact angle. The value of (7i2 decreases with rise of temperature. 

The interfacial (excess) heat capacity is another Interfacial characteristic that we decided to disregard. The reason for doing so is not in its intrinsic interest. On the contrary, as with bulk heat capacities, they reflect the structure, or ordering (see e.g. FIGS I, sec. 5.3c). However, it is very difficult to establish these values experimentally. Basically the second derivative of the interfacial tension with respect to the temperature at a constant pressure is needed (see sec. 1.2.7), and to obtain this extremely preeise measurements are needed. The spread in the quadratic coefficient B in [1.12.1) indicates the uncertainty, even for a well-studied liquid like water. Yang and Li showed, by a thermodyncimic ancdysis, that for LL interfaces this heat capacity is related to the two bulk heat capacities, the inter-... 

Some workers have interpreted the emulsification of fountain solution in an ink from the point of view of surface energetics and colloidal behavior. Surface measurements in the form of contact angles, spreading coefficients, interfacial tensions and surface tensions have been widely used to explain the interactive behavior of inks and fountain solutions. 

The elaboration of a model to describe cotton wicking is very complicated, although the effect of quaternaries on the wetting of fibers is easily seen the softener enhances the interfacial tension strongly. Since the fiber surface energy and the surface tension of water are not affected, the spreading coefficient is decreased, and so is the wetting of the fiber surface. 

The Contact Angle When the substrate is a solid, the spreading coefficient is usually evaluated by indirect means, since surface and interfacial tensions of solids cannot easily be measured directly. The method of doing this involves measuring the contact angle the substrate makes with the liquid in question. 

Note that as a liquid spreads on a surface the interfacial tensions change, with the result that the spreading coefficient changes. For example, benzene spreads on a pure water surface, 9 x 10 N/m initially. When the water is saturated with benzene and the benzene saturated with water (o- )sat — 2 x 10 N/m and any additional benzene... 

In order to cause spontaneous spreading of an aqueous surfactant solution (W) over a second immiscible liquid (O) as substrate, the definite relationship between surface and interfacial tension, known as the spreading (Harkins) coefficient Swo should be fulfilled. If Swo> as defined by the equation... 

Recently, advances in buUc-phase polymer blends have recognized the role of the interfacial tensions between the polymer phases. Hobbs et al. [33] used the concept of spreading coefficients as a guide to interpreting the morphology of three-component polymer blends. Their work showed that the two dispersed phases with the lowest volume fractions would be distributed throughout a matrix phase in a variety of different morphologies which depended upon the interfacial tensions between the three polymer components. 

These authors concluded that the differences in the hydrophobicity of the oil and the polymer turned out to be the driving force for the formation of nanocapsules. Due to the pronounced difference of polarity of PMMA and hexadecane, the system was very well suited for the formation of nanocapsules. With more hydrophobic monomers such as styrene, however, it was more difQcult to create nanocapsules as the cohesion energy density of the polymer phase was close to that of the oil, and adjustment of parameters to influence the interfacial tensions and spreading coefficients became critical in order to form nanocapsules. The parameters studied were monomer concentration, type and amount of surfactant and initiators, and the addition of functional comonomers. For example, addition of 10 wt% acrylic acid as a comonomer in the miniemulsion leads to an increase in the number of close-to-perfect nanocapsules. 

This theory is more applicable to the interactions between a mncoadhesive liquid and a substrate. It uses interfacial tensions to predict spreading and thns mucoadhesion. Spreading coefficients of the mucoadhesive material need to be positive in order to spontaneously displace the content in contact with the mncosa and create the mucoadhesive bond. By determining surface and interfacial tensions, it is possible to calculate the work done in forming the mucoadhesive bond. 

Thermodynamic considerations, based on the work of Torza and Mason [63], can predict the equilibrium morphology without the influence of kinetic factors. The studies were conducted with two immiscible organic liquids in water. Based on the interfacial tensions jiy and the respective spreading coefficients Si, the equilibrium morphology could be predicted. The spreading coefficient Si is defined as ... 

The adempts to rationalize GrifHn s HLB scale from a physicochemical point of view were made in a number of studies. Various correlations were shown to exist between the HLB numbers and the chemical structure or molecular composition of the siufactants. Correlations were also fotmd between the HLB number and physicochemical properties of surfactants and their solutions, for example, stffface and interfacial tension, solubility, and heat of solution, spreading and distribution coefficient, dielectric permittivity of the surfactant, cloud point and phase inversion point, critical micelle concenlration, foaminess, etc. These studies are reviewed in Ref. 262. However, the correlations found are not generally applicable moreover, the concept of the additivity of HLB numbers as such for mixtures of surfactants or oils cannot be proven expermentally when the surfactant characteristics are varied over a wider range (265). 

The difference in pore shape was explained by the difference of interfacial tension between the polymer solutirm and the solvent or water droplet. Indeed, spreading water droplets over a unit area of a polymer solution can be determined by the spreading coefficient (5) as follows 5 = yp (yw + Xw/pl where yp is the surface tension of the polymer solution, y is the surface tension of the water droplet, and yw/p is the interfacial tension between the polymer solution and the water droplets in this case [90]. Water has the largest surface tension compared with the two alcohols, thus the water droplet spreading is reduced and water droplets maintain their spherical shape [89]. On the other hand, the value of yw/p is assumed to be very low for alcohol droplets because both methanol and ethanol are miscible with carbon disulfide. It should be noticed that under such alcoholic vapors, the polymer concentration should be higher than under aqueous atmosphere as no regular patterns were formed for polymer concentrations less than 10 g L. ... 

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