Emulsifiers, nonionic tween

At the same disperse phase flux, the mean frequency of droplet formation was substantially smaller for SDS than for Tween-80 emulsifier. It can be explained by the lower interfacial tension at oil-water interfaces in the presence of SDS. In addition, under the same conditions, the mean droplet size was smaller for SDS than for Tween-80. In the case of SDS as an anionic emulsifier, the droplets were detached from the pore openings immediately after formation, due to strong electrostatic repulsions between the charged droplets and identically charged membrane surface. In the case of a nonionic Tween-80 emulsifier, the newly formed droplets were kept at the membrane surface after formation, before being pushed by another droplets forming at the same pore. 

The ingredients are nonionic surfactants. It is their application as amine replacements in steam-condensate systems that is novel, not the raw materials. These products are commonly found in skin and hair products and in cosmetics because of their mildness and their excellence as water-in-oil emulsifiers and co-emulsifiers in oil-in-water emulsions. Raw material brands include Tween (ICI, PLC), Crill , and Crillet (Croda PLC). 

Tween 80 (also known as At-Plus 109 and Polysorbate 80) is a polyoxyethylene sorbitan monooleate and was obtained from Atlas Chemical Industries, Inc. This nonionic surfactant has an HLB (hydrophile-lipophile balance) of 15.0 and is used as an emulsifier, solubilizer, and dispersant. It was used without further purification at the 0.1% (w/v) level in partitioning and greenhouse herbicidal evaluations as described below. 

When a soluble LMWE (like Tween 20) as well as a protein is present in water both components will form adsorbed films at the air-water interface. At low LMWE concentrations, protein reduces the surface tension to a greater extent than protein-LMWE mixed systems. However, the opposite was observed at high LMWE concentrations, above the critical micelle concentration (CMC), because the protein molecules are displaced from the interface by the LMWE. Over the intermediate region, close to the CMC, both protein and LMWE coexist at interface. However, tensiometric studies indicate that the compatibility of proteins and nonionic emulsifier at fluid interfaces is very poor, in contrast to mixtures of ionic-surface-active homologues. 

Primarily for toxicity reasons, work has focused on the use of nonionic surfactants, particularly Tweens and Spans. Pouton (20) and Wakericy el al. (21) have screened a range of surfactants, finding that in general molecules with unsat-uraced acyl chains were most efficiem emulsifiers, particularly the oleates with an HLB value of approximately II. The authors also reported that the sorbitan esters and ethoxylated triglycerides such as Tagat TO were more efficient than the fatty acid ethoxylates, possibly due to the polydispersity of the latter. 

The HLB concept was introduced [44,45] as an empirical scale that could be used to describe the balance of the size and strength of the hydrophilic and lipophilic groups on an emulsifier molecule. Originally used to classify Imperial Chemical Industries nonionic surfactant series of Spans and Tweens the HLB system has now been applied to many other smfactants, including ionics and amphoterics. 

V. Electric charge of the membrane. Electrostatic attraction or repulsion by the membrane enviromnent influences significantly the effectiveness of prooxidants and antioxidants. The rate of oxidation is much higher in emulsions prepared with ionic emulsifiers (SDS, potassium pahnitate) than with nonionic emulsifiers (Span 20, ethenoxylated tetradecanol. Tween 20) (Table 10.2). Oxidation is accelerated in the emulsions stabilized by anionic emulsifiers, such as SDS. In these emulsions, electrostatic attraction occurs between the negatively charged oil-water interface, and the positively charged metal ions present either as trace impurities or added. Metals in the water phase become hydrated and more reactive with polar hydroperoxides and water-soluble radicals (e.g. OH, OOH) at the oil-water interface. 

This phenomenon is driven by surfactant transfer fi-om the continuous phase toward the droplets. It manifests itself as a cyclic dimpling of emulsion films. The phenomenon was first observed [490] with aqueous films between two xylene droplets in the presence of the nonionic emulsifier Tween 20 or Tween 80 (initially dissolved in water, but also soluble in oil). The same phenomenon has been observed also with other emulsion systems [491]... 

Figure 8.27(a) Plot of W/O/W emulsion formation against the weight ratio of Span-80 to Tween-20 in W/O/W systems, calculated from the constitution of various W/O/W emulsion samples, (b) Plot of W/O/W emulsion formation against the weight ratio of Span-80 to the nonionic emulsifying agents. From [158] with permission. 

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