A Lecture Demonstration SISTER M. IGNATIA Marygrove College, Detroit
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symbol O/W means an oil-in-water emulsion, one in which tiny droplets of oil, colloidal in dimensions, are dispersed in water. Since an emulsion has approximately the same solubility as its continuous phase, an O/W emulsion can be diluted with more water (a commercially valuable property) but is insoluble in more oil or in other nonpolar solvents. Similarly, W/O symbolizes a water-in-oil emulsion in which the contimuous phase is nonpolar in character. Such an emulsion can be diluted with more oil but not with more water. To form stable emulsions of these two types, surfaceactive agents are needed, compounds whose molecules are not only complex, but in which one part of each molecule is hydrophiic or water-loving, and another part is lipophilic, or oil-loving. Sometimes surfaceactive agents are more effective when used in pairs. This is true if one is strongly attracted to water, the other to oil, and the agents to each other. Among the snrface-active agents commercially available are the Span and Tween products of the Atlas Powder Company. The Spans tend to be oil-soluble and the Tweens water-soluble. Properly selected, a Span and a Tween together make an effective emulsifying agent. According to the booklet, "Atlas Surface Active Agents," the "Span type materials are essentiauy partial esters of the common fatty acids (lauric, palmitic, stearic, and oleic) and hexitol anhydrides (hemtans and hexides), derived from sorbitol. The Tween type materials are derived from the Span products by addmg polyoxyethylene chains to the non-esterified hydroxyls. Both types are actuaUy complex mixtures of several compounds." To illustrate the formation and the solubility of O/W and W/O emulsions, I have worked out Dr. George E. F. Brewer's idea for a simple demonstration experiment in general chemistry. We prepare an O/W emulsion by putting 100 ml. of distilled water into the semimicro head of a Waring blendor, starting the blendor, and adding 6.6 ml. of Tween 81, 7 ml. of Span 85, and, slowly, 100 ml. of level oil. Mmeral oil can be used instead of level oil, but the latter is less expensive and its flash point (120°F.) is not likely to be reached. In preparing the W/O emulsion, 100 ml. of level oil are put into the blendor, the hlendor started, and 7 ml. of Span 85, 6.6 ml. of Tween 81, and, slowly, 100 ml. of distilled water are added. Both emulsions are poured into beakers and their volume, color, texture, and viscosity compared. m e n a few milliliters of the O/W emulsion are added
to a beaker of water, the emulsion dissolves readily, but it is notably insoluble in level oil, benzene, and other nonpolar solvents. The W/O emulsion, according to prediction, dissolves readily in more oil and other nonpolar solvents, but is insoluble in water. Dreft acts as an emulsifying agent for an O/W emulsion but is not satisfactory for the W/O type. Onefifth of a gram of Dreft is sufficient to hold 100 rnl. of oil in 100 ml. of water. Other pairs of Spans and Tweens, for example, 20 and 20 or 60 and 60, may be substituted for Span 85 and Tween 81, but the last pair seems to give the most stable emulsions of the W/O type and is quite satisfactory for demonstration of O/W emulsions. If the students need to be convinced tbat emulsions formed without the aid of emulsifying agents are unstable, 50 ml. of level oil may he added slowly to 50 ml. of water in the blendor, or vice versa. The emulsion formed is so unstable tbat it separates into layers even before it can be tested as to type. If time permits, the emulsions prepared during the demonstration may be reversed in phase. If water is put into the blendor and part of the W/O emulsion added while the blendor is running, the emulsion is readily reversed to a stable O/W emulsion. O/W emulsions formed with the Spans and Tweens reverse, but the resulting W/O colloidal suspensions tend to be unstable. O/W emulsions formed with Dreft do not reverse in phase to W/O, probably because Dreft is not a satisfactory emulsifying agent for the latter type. If the blendor containing oil and an O/W emulsion formed with Dreft is stopped and the oil layer allowed to come to the top before the motor is started again, a stable and stiffer O/W emulsion is formed. Students are frequently amazed to learn that a stable emulsion may be formed in which the continuous phase is considerably smaller in volume than the dispersed phase. If, for example, 20 ml. of level oil are put into the blendor, the blendor started, and 4 ml. each of Span 85 and Tween 81, and 80 ml. of water added, the last very, very slowly, a stiff and stable W/O emulsion is formed, in spite of the fact that the dispersed phase has four times the volume of the continuous phase. Similarly, a stable O/W emulsion in which the volume ratio of dispersed to continuous phase is four to one can he formed by putting 20 ml. of water and 0.5 g. of Dreft into the hlendor and adding 80 ml. of oil very, very slowly while the blendor is mnning.
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