Surfactants are used at high temperatures as additives for steamfloods, and in high-temperature surfactant floods. Most surfactants must be above the critical micelle concentration (CMC) in order to have appropriate properties for forming foams or mobilizing oil. The CMC of a surfactant increases dramatically at high temperatures; thus, it is important to know this value at the temperature of application for economical engineering design.

The CMC's of a series of surfactants as a function of temperature in the range 77°-392° F (298-473 K) have been determined at high temperatures from heats of dilution, a novel application of calorimetry. Enthalpies of dilution were determined at ambient and elevated temperatures. Surface tension measurements at ambient temperatures served to confirm the assignment of CMC from the heat of dilution data.

The CMC's of a pure surfactant were determined in distilled water, in aqueous solutions of NaBr up to 10% salinity, in n-butyl alcohol solutions up to 4% alcohol concentration, and at temperatures of 122° through 347° F (323-448 K). In general, CMC increased with an increase in temperature. The presence of electrolyte at a fixed temperature lowered the CMC and increased the sharpness of the onset of micellization. At 212° F (373 K), the presence of n-butyl alcohol caused the CMC to be lowered but had little effect on the sharpness of micellization. At 347° F (448 K), the alcohol had very little effect on the CMC, apparently because of the change in water properties at high temperatures.

The CMC's and solution behavior of three commercial surfactants were determined at 77°, 257°, and 392° F (298, 398, and 473 K), with no added salt. The CMC for these commercial surfactants also increased with temperature.

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