Water spreading coefficient

Initial versus Final Spreading Coefficients on Water [9,10] ... 

For practical purposes, if the contact angle is greater than 90° the liquid is said not to wet the solid (if the liquid is water one speaks of a hydrophobic surface) in such a case drops of liquids tend to move about easily and not to enter capillary pores. If 8 = 0, (ideal perfect wettability) Eq. (A.4.3) no longer holds and a spreading coefficient, Sls(V). reflects the imbalance of surface free energies. 

Spreading (Eq. (iii) of Fig. A.4.4) occurs when the oil adheres to the water more strongly than it coheres to itself this is generally the case when a liquid of low surface tension is placed on one of high surface tension. This mineral oil spreads on water, but water cannot spread on this oil. The initial spreading coefficient does not consider that the two liquids will, after contact, become mutually saturated. The addition of surfactants which lower yow and ysw (cf. Fig. A.4.3) cause the dispersion of the oil into droplets. 

Sow spreading coefficient of organic liquid at air-water table interface (dyn/cm)... 

However, the value of yab needs to be considered as the equilibrium value, and therefore if one considers the system at nonequilibrium, then the spreading coefficients would be different. For example, the instantaneous spreading of benzene is observed to give a value of Sa/b as 8.9 dyn/cm, and therefore benzene spreads on water. On the other hand, as the water becomes saturated with time, the value of water decreases, and benzene drops tend to form lenses. The short-chain hydrocarbons such as hexane and hexene also have positive initial spreading coefficients, and spread to give thicker films. Longer-chain alkanes, on the other hand, do not spread on water (e.g., the Sa/b for C16(hexadecane)/water is -1.3 dyn/cm at 25°C. 

In the following table are given a few values, chiefly from the data of Harkins, of the spreading coefficients for a number of liquids which spread,upon water and for liquids which do not spread but form lenses, in this latter case the value of cr [o + o- z is evidently negative. 

This view of the process of spreading and the conditions of equilibrium possesses certain advantages because many solids such as the fatty esters and higher fatty acids spread upon water and their spreading coefficients can only be determined by a direct measure of the film pressure which exists when equilibrium is attained. 

In the following table are given a few spreading coefficients for solid materials on a water surface at the melting point determined by Cary (Proc. Roy. Soo. A, cix. 318, 1925) by the above method. 

The ranges of surfacse tension and spreading coefficient for several oils on water as determined by Gray and Bent (BS) are shown in Table III. 

Spreading coefficient as used here is surface tension of water minus surface tension of the oil minus inter-facial tension. 

Jobbins 26) of the New Jersey Agricultural Experiment Station, has suggested the need for a list of adjuvants and the quantities to be used in oil sprays to be applied from aircraft that will permit control of particle size on the ground under various temperature conditions. He would also like to include in oil specifications some reference to their natural spreading coefficients when they are applied on clear water. 

Use these data together with the surface tensions of the pure liquids from Table 6.1 to calculate the spreading coefficients for the various liquids on carbon black. Use your results to interpret the authors observations About equal quantities of water and organic liquid were put into a test tube with a small amount of the finely divided solid and shaken. It was noted that the carbon went exclusively to the organic liquid phase. ... 

With these facts in mind, let us examine the fate of the drop of solution placed on the surface of water. The initial spreading coefficient S0/fV (Equation (6.61)) for the organic layer on water is positive. This is primarily because yQ/w is unusually low and yw is high, even with an adsorbed layer of the organic solvent. After spreading, we allow sufficient time to elapse for all the solvent to evaporate from the spread layer. At this point the surface will contain a... 

In the group with negative spreading coefficients (e.g., kerosene-in-water and paraffin-in-water emulsions), the values of in both stirred tanks and bubble columns decrease linearly with an increasing oil fraction. This effect is most... 

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. 

Harkins defined the term initial spreading coefficient (for the case of oil on water) as... 

Table 4.6 Initial spreading coefficients (in mN m— ) for liquids on water at 20°C54 (By courtesy of Academic Press Inc. )...

The initial spreading coefficient does not consider the mutual saturation of one liquid with another for example, when benzene is spread on water,... 

More complex with respect to molecular interaction is the case of formation of non-aqueous films on the surface of aqueous solutions from non-ionic surfactants [528], Films from octane were obtained by adsorption from drops of octane/non-adsorbing diluent (squalane) mixture. Occasionally the spreading of alkanes on aqueous surfactant solution gives stable thin oil films (e.g. on solutions of the anionic surfactants Aerosol OT) [529,530], Some evidence about the stability of asymmetric films can be derived from the data about the surface pressure and spreading coefficients of liquids on water surface. These data are known for many organic liquids [531,532], It should be also noted that the techniques for determination of the spreading coefficients have improved considerably [533,534]. Most precise values were obtained by measuring the surface pressure of a monolayer with a special substance introduced as an indicator [533]. 

It is important, however, to point out that, because of the existence of metastable equilibrium films, even large values of the classical spreading coefficient cannot ensure enter of oil drops into the water surface (rupture of water film on an oil substrate) which is confirmed by direct measurements [528]. 

In this context it should be noted that the surface tension O yw will not, in general be the tension existent in the absence of oil, due to the solubilization of oil on the air/water surface. This point is particularly critical for determining the classical spreading coefficient... 

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