Citrus oil—aqueous phase interfaces

Interfaciai Tension Procedure. IFT measurements were made by the Wilhelmy plate method. The apparatus was the same as that described previously (2). A standard protocol was followed for all IFT determinations. The desired interface was formed at a specified temperature by partially filling a thermostatted sample holder with the desired aqueous phase. This phase, distilled water (mono triple) or a supernatant aqueous phase isolated from a complex coacervate system, completely covered the Wilhelmy plate (roughened platinum). The desired citrus oil was carefully layered onto the aqueous phase. It had been preheated (or cooled) to the same temperature as the aqueous phase. Once the citrus oil/aqueous phase interface was formed, the Wilhelmy plate was drawn completely through the interface and into the oil phase where it was zeroed. 

The plate was then placed at the citrus oil/aqueous phase interface. 

The pronounced reduction in IFT aging observed at low temperatures is attributed to a marked reduction in the rate at which interfacially active species congregate at citrus oil/aqueous phase interfaces when such mixtures are stored cooled. This is believed to reflect primarily a reduction in rate at which these species are produced in systems that are kept at low temperatures. If cooling simply reduced the solubility of interfacially active species that existed initially in these systems, IFT should decrease, since reduced solubility favors adsorption at an interface. 

When a citrus oil and aqueous phase are equilibrated together under static conditions for prolonged periods at 30-50°C (e.g., 12 hours), a film or precipitate is often seen at the interface. If an interfacial film forms, it is transparent and clearly visible only when the Wilhelmy plate is pulled away from the citrus oil/ aqueous phase interface. Such films appear to be continuous and are located on the oil side of the interface. They are very thin and cannot be seen on the plate or hanging from the plate once the plate is removed from the citrus oil. The film acts as though it dissolves as the plate is slowly pulled away from the interface and through the citrus oil phase. 

A discontinuous precipitate often was observed at citrus oil/ aqueous phase interfaces kept at 50 C for prolonged periods. The precipitate particles congregate on the water side of the citrus oil/aqueous phase interface and disperse into the aqueous phase upon agitation. If the aqueous phase is distilled water, or a supernatant phase, the precipitate particles cause the aqueous phase to become noticeably cloudy. Precipitate particles or interfacial films were not detected at citrus oil/complex coacervate phase interfaces. However, such interfaces normally were not kept for prolonged periods, because their IFT values rapidly decayed to a value too low to measure. 

Duplicate and triplicate IFT aging curves were obtained at one or two temperatures for most of the interfaces characterized in this study. The replicate IFT data reported in Figures 1,3,4,7,8 and 10-14 show that many IFT aging curves for citrus oil/aqueous phase interfaces differ by a maximum of 1.7mJ/m2. Replicate curves often differ by less than lmJ/m2. Because each IFT aging experiment involved formation and separation of a new complex coacervate and supernatant phase, replicate IFT aging curves measure the combined effect that several factors have on reproducibility. These factors include variability of the complex coacervation procedure, protocol followed for separation of the coacervate and supernatant phases, and the IFT measurement process itself. The variability in solids content of replicate coacervate and supernatant phases shown in Table 1 could contribute to the observed IFT variability. 

Citrus oils readily form oxygenated products that are likely to congregate at oil/water interfaces and thereby cause a detectable change in IFT. The aldehydic components of citrus oil could react with the amine groups of the gelatin molecules present in the aqueous phases formed by complex coacervation and thereby affect IFT. In addition to chemical reactions, physical changes can occur at an interface and alter IFT. A visible interfacial film can form simply due to interfacial interactions that alter the interfacial solubility of one or more components. No chemical reactions need occur. An example is the formation of a visible interfacial film when 5 wt. per cent aqueous gum arabic solutions are placed in contact with benzene (3). Interfacial films or precipitates can also form when chemical reactions occur and yield products that congregate at interfaces. 

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