Hydrocarbons surface tension

Surface tension accounts for a number of everyday phenomena. For example, a droplet of liquid suspended in air or on a waxy surface is spherical because the surface tension pulls the molecules into the most compact shape, a sphere (Fig. 5.14). The attractive forces between water molecules are greater than those between water and wax, which is largely hydrocarbon. Surface tension decreases as the temperature rises and the interactions between molecules are overcome by the increased molecular motion. 

The kinetic regularities of heterogeneous defoaming, i.e. the acceleration of foam breakdown with the decrease in hydrocarbon molecular weight (respectively, the decrease in hydrocarbon surface tension and the increase in the contact angle between the meniscus and the substrate) and with the increase in both foam expansion ratio and reduced pressure, indicate that foam breakdown can occur due to the lowered stability of film borders at their contact with the substrate (worsened wetting). The fact that the foam situated between the cuvette wall and the capillary decayed at a considerably slower rate is in benefit to this statement. 

Such mobile behavior has also been noted for drops of squalane and isopropyl-1,9-diphenylnonane supported, respectively, on the adjacent faces of a polished metal disk and a glass cover slip 1 mm. above it. If the drop of squalane (surface tension 27.8 dynes per cm.) is centered on the disk and the drop of the aromatic hydrocarbon (surface tension 34.4 dynes per cm.) on the underside of the cover slip above it is displaced from... 

Hydrocarbon Surface tension (mN m ) Interfacial tension with water (mN m )... 

Adsorption may occur from the vapor phase rather than from the solution phase. Thus Fig. Ill-16 shows the surface tension lowering when water was exposed for various hydrocarbon vapors is the saturation pressure, that is, the vapor pressure of the pure liquid hydrocarbon. The activity of the hydrocarbon is given by its vapor pressure, and the Gibbs equation takes the form... 

Fig. Ill-16. Surface tension lowering of water at 15°C due to adsorption of hydrocarbons. , n-pentane A, 2,2,4-trimethylpentane O, n-hexane x, n-heptane A, n-octane. (From Ref. 133.)...

It has been pointed out [138] that algebraically equivalent expressions can be derived without invoking a surface solution model. Instead, surface excess as defined by the procedure of Gibbs is used, the dividing surface always being located so that the sum of the surface excess quantities equals a given constant value. This last is conveniently taken to be the maximum value of F. A somewhat related treatment was made by Handa and Mukeijee for the surface tension of mixtures of fluorocarbons and hydrocarbons [139]. 

Eq. IV-9 would use the surface tensions that liquids A and B would have if their inter-molecular potentials contained only the same kinds of interactions as those involved between A and B (see Refs. 20, 22-24). For the hydrocarbon-water system, Fowkes [20] assumed that Uh arose solely from dispersion interactions leaving... 

The higher members of the series decrease the surface tension of aqueous solutions well below the point possible with any type of hydrocarbon surfactant, although in practice because of their strong acid character and solubiUty characteristics, more commonly salts and other derivatives are employed. A 0.1% solution of C F COOH has a surface tension of only 19 mN/m (dyn/cm) at 30°C (6). 

Emulsifiers. The chemical stmctures of emulsifiers, or surfactants (qv), enable these materials to reduce the surface tension at the interface of two immiscible surfaces, thus allowing the surfaces to mix and form an emulsion (33). An emulsifier consists of a polar group, which is attracted to aqueous substances, and a hydrocarbon chain, which is attracted to Hpids. 

The atoms and molecules at the interface between a Hquid (or soHd) and a vacuum are attracted more strongly toward the interior than toward the vacuum. The material parameter used to characterize this imbalance is the interfacial energy density y, usually called surface tension. It is highest for metals (<1 J/m ) (1 J/m = N/m), moderate for metal oxides (<0.1 J/m ), and lowest for hydrocarbons and fluorocarbons (0.02 J /m minimum) (4). The International Standards Organization describes weU-estabHshed methods for determining surface tension, eg, ISO 304 for Hquids containing surfactants and ISO 6889 for two-Hquid systems containing surfactants. 

Release agents function by either lessening intermolecular interactions between the two surfaces in contact or preventing such close contact. Thus, they can be low surface-tension materials based on aUphatic hydrocarbon, fluorocarbon groups, or particulate soHds. The principal categories of material used are waxes, fatty acid metal soaps, other long-chain alkyl derivatives, polymers, and fluorinated compounds. 

OU repeUency is measured by observing a fabric s resistance to wetting by a selected series of numbered test Uquid hydrocarbons with a range of surface tensions. The fabric rating is based on the Uquid that does not wet the fabric surface in a specified time (AATCC Test Method 118 and INDA... 

Liquid-Phase Components. It is usual to classify organic Hquids by the nature of the polar or hydrophilic functional group, ie, alcohol, acid, ester, phosphate, etc. Because lowering of surface tension is a key defoamer property and since this effect is a function of the nonpolar portion of the Hquid-phase component, it is preferable to classify by the hydrophobic, nonpolar portion. This approach identifies four Hquid phase component classes hydrocarbons, polyethers, siHcones, and duorocarbons. 

Fluorocarbon soHds are rare in defoamer compositions, presumably on account of their cost. SoHd fluorine-containing fatty alcohols and amides are known. The most familiar fluorocarbon soHd is polytetrafluoroethylene [9002-84-0]. Because it is more hydrophobic than siHcone-treated siHca, it might be expected to perform impressively as a defoamer component (14). However, in conventional hydrocarbon oil formulations it works poorly because the particles aggregate strongly together. In lower surface tension fluids such as siHcone and fluorocarbon oils, the powdered polytetrafluoroethylene particles are much better dispersed and the formulation performs weU as a defoamer. 

Zanker, Adam, Estimating Surface Tension Changes For Liquids, Hydrocarbon Processing, April 1980, p. 207. 

The physical properties of saturated fluorocarbons and their analogous hydrocarbons differ in many respects [4 5] Sahirated fluorocarbons have the lowest dielectric constants, surface tensions, and refractive indexes of any liquids at room... 

Fluorocarbons with a hydrophilic functional group are very active surfactants [23]. Less than 1% of ionic or nonionic surfactants with perfluoroalkyl groups can reduce the surface tension of water from 72 to 15-20 dyne/cm, compared with 25-35dyne/cm for typical hydrocarbon surfactants [24] Perfluoroether surfactants are about as active as their perfluoroalkyl counterparts of similar chain length [25, 26], but fluorosurfactants with more polar alkyl end groups are considerably less active than their perfluoroalkyl analogues (Table 7)... 

For hydrocarbons in high-pressure fractionators Strigle [82] reports there is aeration of the rather low surface tension liquid phase. This effect increases with the lower surface tension and as the vapor density increases, thus... 

Perfluoroalkyl group-containing surfactants like fluoroalkylsuccinamic surfactant and fluoroalkylthio derivatives from the reaction of bis-2-hydroxyethyl-aminomethylphosphonic acid ester with maleic anhydride were especially useful for the preparation of foams for extinguishing burning hydrocarbon liquids. A surface tension of 19.8 mN/m in a 0.1% aqueous solution was observed [88. ... 

Replacement of gas by the nonpolar, e.g., hydrocarbon phase (or oil phase) is used to modify the interactions between molecules in a spread film of investigated long-chain substances [6,15,17,18]. The nonpolar solvent-water interface possesses the advantage over that between gas and water, that the cohesion (i.e., interactions between adsorbed molecules due to dipole and van der Waals forces) is negligible. Thus, at the oil-water interfaces behavior of adsorbates is much closer to ideal, but quantitative interpretation may be uncertain, in particular for the higher chains which are predominantly dissolved in the oil phase to an unknown activity. Adsorption of dipolar substances at the w/a and w/o interfaces changes surface tension and modifies the surface potential of water [15] ... 

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