Surfactants cloud point test

Ethoxylated materials exhibit water solubility which is lower, rather than higher, at higher temperature (43). This is the basis of the cloud point test, an identity check for nonionic surfactants which reflects the extent of ethoxylation (44). A solution of the surfactant is warmed until it becomes cloudy, and the temperature recorded. For a family of materials, cloud point is directly proportional to extent of ethoxylation. Usually, the cloud point is not much affected by concentration in the 1-10% range. Highly ethoxylated materials (EO > 75%) often do not have a cloud point below the boiling point of water, although there may be a measurable cloud point in 1 M sodium chloride solution. 

Cloud Point Measurements Cloud points were recorded by the visual observation of aqueous solutions containing 1% W/V surfactant. The measurement defines the temperature at which the system under test shows a characteristic transitional change from a clear solution to an opalescent or cloudy state. All cloud points were recorded in both ascending and descending temperature cycles to ensure data confidence. The influence of salt and/or oils on the cloud point were systematically evaluated. 

Aqueous Solutions The transition temperature above which a non-ionic surfactant or wax loses some of its water solubility and becomes ineffective as a surfactant. The originally transparent surfactant solution becomes cloudy because of the separation of a surfactant-rich phase. Cloud points are typically reported on the basis of tests for a specified surfactant concentration such as 1 mass%. See also Coacervation. 

The temperature at which a 1% surfactant solution became turbid ( cloud point ) in pure water or in the presence of additives was determined by visual observation of the solution contained in a test tube as the temperature was increased. For this purpose, the test tubes were placed in a transparent water bath equipped with a control unit which permitted temperature increases at the rates of 3°/min and 0.2°/min. A dark background permitted a sharper estimation of cloud points. 

Cloud points were initially measured by heating 1% solutions of surfactant in distilled water at a rate of 1°C per minute in a test tube immersed in a water bath. When cloudiness was observed the heat source, was removed and the solution cooled until it was clear. The cloud point was taken as the midpoint between the observed temperature while heating and that during cooling. These were within l.O C of each other for pure surfactant. 

Noninstramental analysis of ether sulfates is complex because of the various types of organic material which must be distinguished. Puschmaim and Wickbold have described comprehensive procedures for quantitative determination of the components in alkyl ether sulfates by wet chemical methods (68,69). Nowadays, these tests are usually performed by HPLC. Cloud point determination, a measure of EO content, is described in Chapter 2 with characterization of nonionic surfactants. 

American Society for Testing and Materials, Cloud point of nonionic surfactants, D2024-65 (1997). West Conshohocken, PA 19428. 

---