Oil spills represent the most damaging insult to our environment and the ever-increasing recurrence of such accidents has made the containment and cleanup responses a subject of national importance.
The "National Oil and Hazardous Substances Pollution Contingency Plan", Council on Environmental Quality, 40CFR, 1510, Federal Register, Vol. 40, No. 28, Feb. 10, 1975, was devised to protect the environment from the damaging effect of pollution discharges. Under the Plan and Annexes detailed responses are outlined for various contingencies, the most frequent being the hazardous discharge of oil to an aqueous environment. Cleanup responses are detected on a case by case basis and both mechanical and chemical cleanup agents are employed.
Chemical agents are defined as elements, compounds or mixtures that disperse, dissolve, emulsify . . . concentrate . . . entrap . . . or otherwise facilitate the removal of the pollutant from the water. The subject agents are in particular "surface collecting agents," which are a surface film forming chemical for controlling oil layer thickness and provide a key element in the most effective system for concentrating and collecting oil spills.
The basic concept of using surface active agents to drive oil from the vicinity of sinking ships was described by Zisman at the U.S. Naval Research Laboratory in 1943. After a lapse of many years, the refinement and application of monomolecular films as oil collectors to control oil pollution was re-examined at the U.S. Naval Research Laboratory in the late sixties.
Oil collectors are surface active agents that are capable of forming monolayers on a water surface and thus prevent the spreading of oil lenses, or compress thin oil films into thicker lenses. The criterion necessary to attain spontaneous spreading of two immiscible phases has been taught to Harkins et al, J. Am. Chem. Soc. 44, 2665 (1922). The measure of the tendency for spontaneous spreading or spreading pressure of an oil is defined as follows: EQU F.sub.o =.gamma..sub.w -(.gamma..sub.o +.gamma..sub.o/w)
where
.gamma..sub.w --surface tension of water
.gamma..sub.o --surface tension of oil
.gamma..sub.o/w --interfacial tension between the oil and water
The measure of the spreading pressure of a collector is defined as follows: EQU F.sub.h =.gamma..sub.w -.gamma..sub.w h
where .gamma..sub.w.sup.h is the surface tension of water in the presence of collector. F.sub.h and F.sub.o are thus the opposing spreading pressures of the collector and the oil respectively in dynes/cm. Since the surface pressures of crude oils and refined petroleum products generally vary between 10 and 33 dynes/cm, a minimum opposing pressure of 40 dynes/cm should suffice. In actuality, available oil collectors give surface pressures of 35 dynes/cm at 5 milligrams/square meter and higher spreading pressures of 43-46 dynes/cm only when applied at two to four times greater concentration. This spreading pressure is generally sufficient for clean oil in confined calm areas, but insufficient for weathered or aged oil slicks in unconfined areas especially in the presence of winds, waves and dispersants. Fluorochemical oil collectors as described herein are considerably more effective.
The oil lens thickness maintained by a collector is predicted by the Langmuir equation (J. Chem. Phys. 1, 756 (1933)) ##EQU1## where t is the thickness of the oil layer in cm F.sub.o and F.sub.h are as previously defined, and P.sub.w =density of water phase, gm/cc; P.sub.o =density of oil layer, gm/cc; g=gravitational constant, 980 cm/sec.sup.2.
Consequently for a particular oil, the higher the opposing spreading pressure of the collector, the greater the thickness.
If the oil spilled on water has a low spreading pressure, the spreading rate will be fairly slow and the oil will tend to remain in a relatively thick layer on the water. In this case of a thick oil layer, some mechanical oil skimmers will work very effectively, especially if waves are less than 1 ft. high.
If the oil spilled has a high spreading pressure, which is generally the case, the oil will spread to an extremely thin layer, on the order of microns in thickness. Thin oil slicks are literally impossible to remove in a practical sense, even with the more efficient skimming devices.
The efficiency of any oil-spill pick-up scheme for use offshore is directly related to the thickness of the oil layer and the severity of the weather. There are many mechanical skimming devices available that operate with good efficiency on thick oil layers (say 1/4 to 1 in.) in calm water. There are no mechanical skimming devices that work effectively on thin oil slicks. Since offshore weather conditions generally preclude a calm sea and since there usually is little time to respond effectively to an oil spill, it appears that mechanical skimming alone will not suffice except in very special cases, and more efficient collectors could be quite useful.
Other practical limitations effecting the use of prior-art oil collecting agents are noted below:
Since the evaporation of volatile components is a significant mechanism of aging of crude oil slicks and the residual slicks are difficult to collect, it has been suggested that natural surface active materials are present. This suggests the need of applying surface collecting agents only on fresh spills of such oils. Presently available collectors are ineffective in the presence of such natural competing surfactants, while the subject fluorochemical collectors are far more effective.
Prior-art collectors initially spread rapidly (about 40 cm/sec) and then more slowly due to the progressive effects of gravity and inertial flow, gravity and viscous flow, and surface and viscous flow. The initial high spreading rate is usually reduced to 3 to 4 cm/sec after about a minute, but then remains fairly constant, reducing inversely as the square root of time until the film approaches a thickness of several molecules. As the thickness approaches monomolecular dimensions the spreading rate approaches zero.
Since spreading velocity generally increases with spreading pressure, and the subject collecting agents have such high spreading pressures, they spread very rapidly. This is true in general with fluorochemical agents in contrast to the conventional hydrocarbon based collecting agents. Differences in spreading velocity are also expected within any homologous series of candiate agents depending on the chemical functionality, solubility and molecular weight of the compounds in question.
The ability of collecting agent to spread against wind also increases with increasing spreading pressure.
Finally, prior-art oil collectors are minimally effective on dispersant oil mixtures, particular at high dispersant levels. The subject collectors are generally effective even on such contaminated oil mixtures.
In 1974 a patent was issued to Shell Internationale for compositions suitable for herding oil or preventing it from spreading (South African Pat. No. ZA 7300 346), which comprise at least one primary long chain aliphatic alcohol having 10 to 20 carbon atoms together with a liquid and/or solid diluent material. U.S. Pat. No. 3,810,835 issued in 1974 to Chevron Research Co. discloses aliphatic oil slick containment agents selected from dialkyl amides, n-alkyl and n-alkenyl polyethylene glycol ethers, polyethylene glycol monoesters of n-alkanoic acid, and n-alkyl and n-alkenyl monoesters of propylene glycol. Japanese Pat. No. 49-4675 discloses the use of poly (oxyalkylene) alkyl ethers.
The use of fluoroalkyl group containing surfactants or aqueous solutions thereof are claimed in Japanese Pat. No. 50-22783 issued in 1975. The patent fails to teach the use of low melting, water-insoluble fluorochemicals useful at practical levels below 10 mg/m.sup.2. The disclosed compositions and examples are water soluble, high melting and ineffective.