Surfactants polyethylene block copolymers

Poly(ethylene oxide) (PEO), 10 665, 673-674 13 540, 542-543, 731. See also Ethylene oxide polymers association reactions of, 10 682 behavior in solution, 10 685 commercial block copolymers, 7 648t crystallinity of, 10 690 as a flocculating agent, 11 630-631 low molecular weight, 14 259 oxidation of, 10 682 in paper manufacture, IS 117 preparation of, 20 462 Polyethylene oxide chains, in cationic surfactants, 24 147... 

Alexandridis P, Hatton TA. Polyethylene oxide)-poly(propylene oxide)- poly(ethylene oxide) block copolymer surfactants in aqueous solutions and at interfaces thermodynamics, structure, dynamics, and modeling (review). Colloid Surf A Physicochem Eng Aspects 1995 96 1 16. 

Recently, a new class of inhibitors (nonionic polymer surfactants) was identified as promising agents for drug formulations. These compounds are two- or three-block copolymers arranged in a linear ABA or AB structure. The A block is a hydrophilic polyethylene oxide) chain. The B block can be a hydrophobic lipid (in copolymers BRIJs, MYRJs, Tritons, Tweens, and Chremophor) or a poly(propylene oxide) chain (in copolymers Pluronics [BASF Corp., N.J., USA] and CRL-1606). Pluronic block copolymers with various numbers of hydrophilic EO (,n) and hydrophobic PO (in) units are characterized by distinct hydrophilic-lipophilic balance (HLB). Due to their amphiphilic character these copolymers display surfactant properties including ability to interact with hydrophobic surfaces and biological membranes. In aqueous solutions with concentrations above the CMC, these copolymers self-assemble into micelles. 

Extensive neutron reflectivity studies on surfactant adsorption at the air-water interface show that a surfactant monolayer is formed at the interface. Even for concentration cmc, where complex sub-surface ordering of micelles may exist,the interfacial layer remains a monolayer. This is in marked contrast to the situation for amphiphilic block copolymers, where recent measurements by Richards et al. on polystyrene polyethylene oxide block copolymers (PS-b-PEO) and by Thomas et al. on poly(2-(dimethyl-amino)ethylmethacrylamide-b-methyl methacrylate) (DMAEMA-b-MMA) show the formation of surface micelles at a concentration block copolymer, where an abrupt change in thickness is observed at a finite concentration, and signals the onset of surface micellisation. 

Poloxamer is the generic name for a series of block copolymers that are composed of one polypropylene oxide block sandwiched between polyethylene oxide blocks. For example, poloxamer 188 can be written as (PEO)75-(PPO)3o-(PEO)75. The poloxamers serve as high molecular weight surfactants because the PEG blocks are hydrophilic, whereas the PPO blocks are hydrophobic. 

This is produced by using nonionic surfactants or polymers, for example alcohol ethoxylates, or A-B-A block copolymers PEO-PPO-PEO (where PEO refers to polyethylene oxide and PPO refers to polypropylene oxide), as illustrated in Eigure 10.11. 

The primary W/O emulsifier various low-H LB surfactants are available of which the following may be mentioned decaglycerol decaoleate mixed triglycerol trioleate and sorbitan trioleate ABA block copolymers of polyethylene oxide (PEO) and polyhydroxystearic acid (PHS). 

Secondary O/W emulsifiers High-HLB surfactants or polymers may be used, such as Tween 20, polyethylene oxide-polypropylene oxide (PPO) block copolymers (Pluronics) may be used. 

Another improvement of the solubilisation has been accomplished by adding short amphiphilic block copolymers in low concentration (see Section 4.2 for details). Briefly, these polymers have a polyethylene-propylene hydrophobic block and a polyethylene oxide head group and are thus similar to the ethoxylated non-ionic surfactants to which these are added. The main difference is that the two blocks of the amphiphilic polymer are several times larger than the corresponding low molecular weight surfactant. The role of these polymers is to increase the reach of the amphiphilic layer such that it extends deeper into both the oil and the aqueous phase in accordance with Winsor s premise. As a consequence, they are found to notably increase solubilisation [50]. As seen in Fig. 3.9(c), these additives could be called amphiphilic linkers since they act upon both sides of the interface. 

The type of emulsion stabilized by polymeric surfactants, charged or uncharged, was not investigated in detail, although it has long been understood that it was dependent of the structure of the surfactant macromolecules. In a series of papers, Riess and his coworkers [143-145] have investigated the effect of the composition and architecture of PS-PEO-based block copolymers (PS polystyrene PEO polyethylene oxide) on stability and emul-... 

At higher concentrations block copolymers form lyotropic liquid-crystalline phases. Their range of stability can depend strongly on temperature. In aqueous solutions polyethylene oxide (PEO) is usually the soluble block. An increase of temperature reduces the solubility of the PEO block which can result in phase transitions into different phases. Most of the present knowledge on lyotropic phase behavior of block copolymers was obtained from studies of Pluronics,i.e., poly(ethyleneoxide-h-propyleneoxide-h-ethyleneoxide) (PEO-PPO-PEO) [31]. Phase diagrams of block copolymers with shorter chains resemble those of low-molecular surfactants. 

Nonionic surfactants such as polyoxyethylated fatty alcohols (such as Emul-phor ON-870 from GAP), alkyl phenyl polyethylene glycol ethers (such as the Tergitols from Union Carbide) and polyoxyethylated octylphenol may be used as protective colloids along with anionic surfactants or, in some cases, as emulsifiers in their own right. The block copolymers of polyoxyethylene and polyoxypropylene (Pluronics) solubilized vinyl acetate. Polymerization takes place at the interface of the surfactant-monomer droplet and the aqueous phase [151]. 

Stable double emulsions, based on various block copolymers of polyethylene oxides and polypropylene oxides known as Pluronies, have been used. In a recent example. Cole and Whateley (84) have used complexes of Pluronic F127 PAA (polyaerylie aeid) in the internal aqueous phase. In the oil phase, Span 80 and Pluronie LlOl (5 wt %) were used. The outer interfaee was stabilized by xanthan gum (0.25 wt %) and Tween 80 (1 wt %). Theophylline and I-insulin (iodinated insulin) were ineorporated in the internal aqueous phase of the stabilized multiple emulsion, and the release rates were studied. The release rates were found to be related to the droplet sizes of the emulsion whieh were dependent on the partiele size of the pluronie F127 PAA eomplex in the internal aqueous phase and the type of the lipophilie surfaetant in the oil phase. The authors have used the eomplex between the poloxamer surfactant and PAA that occurred at pH 2 and at low molar ratio as a barrier for the release of active matter from the inner to the outer phase. 

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