Silicone surfactants ethylene oxide

An industrial blend of ethylene oxide (EO) PEMS marketed as a personal care product was examined by positive ion FIA-APCI-MS and LC-APCI-MS-MS (Fig. 2.8.8) [41]. The FIA-APCI-MS spectrum without LC separation (Fig. 2.8.8(a)) is dominated by ions corresponding to unreacted PEG (m/z 520, 564, 608, 652,...), whilst the ions corresponding to the PEMS (m/z 516, 560, 604, 648,...) could only be clearly observed following LC separation (Fig. 2.8.8(b)). Comparison of the TIC chromatograms of PEMS and PEG (Fig. 2.8.8(c) and (h)) demonstrates the dominance of the PEG by-products in the commercial formulation. It is unclear whether the observed relative intensities are representative of the actual amounts or of the different ionisation efficiencies, due to the confidential nature of the product composition. However, the spectra indicate a trisiloxane surfactant structure of that shown in Fig. 2.8.2 (R = Ac) and FIA-MS analysis of another commercial formulation of this product showed good spectra dominated by the silicone surfactants [48], indicating that the PEG by-product composition can vary significantly in commercially available PEMS formulations. 

The surfactant properties of polymeric silicone surfactants are markedly different from those of hydrocarbon polymeric surfactants such as the ethylene oxide/propylene oxide (EO/PO) block copolymers. Comparable silicone surfactants often give lower surface tension and silicone surfactants often self-assemble in aqueous solution to form bilayer phases and vesicles rather than micelles and gel phases. The skin feel and lubricity properties of silicone surfactants do not appear to have any parallel amongst hydrocarbon polymeric surfactants. 

The high hydrophobicity of silicones can complicate their use in some applications. For example, proteins can undergo denaturation in contact with silicones [1]. In such cases, the siloxane can be modified to include a hydrophilic domain. This is typically accomplished by functionalizing the silicone with a hydrophilic polymer such as poly(ethylene oxide)(PEO). Silicone surfactants of this type have found widespread use as stabilizers for polyurethane foams, and have been investigated as a structurant to prepare siloxane elastomers for biomaterials... 

The silicone surfactants can be viewed as PDMS-polyether-copolymers which are mainly based on a combination of just three structural units the methyl substituted siloxane backbone as well as a sophisticated ratio and arrangement of ethylene oxide and/or propylene oxide forming the attached polyethers and, in some cases, additional modifications. 

Not taking cyclic molecules into account, the general structures of industrial silicone surfactants for flexible slabstock foam production can be seen in Figure 2.13. The main building blocks of these materials are a PDMS backbone and attached polyethers based on ethylene oxide and propylene oxide addition products. The siloxane backbones can either be linear or branched and can have their polyether substituents attached in an either pendant or terminal location. These four general structures are outlined in Figure 2.13). 

PDMS oil/water emulsions have been stabilized by adsorbing hybrid silox-ane polymers at the droplet surface. Examples of such additives are PDMS backbones with side chains consisting of trimethylene spacers ending with amino groups, methylated amino groups, acid groups, or an ethylene oxide decamer. Microemulsions have also been prepared directly from silicone oil with an anionic/nonionic surfactant mixture. ... 

All types of conventional non-ionic surfactants have at one time or another been recommended for use in polyester and, in certain instances, in polyether polyurethanes. However, the predominant surfactants used today are the silicones. These materials are block or graft copolymers or polydimethyl siloxanes and polyalkylene oxides. The polyether part is usually a copolymer of propylene and ethylene oxides. Variations in the commercially available surfactants are in the molecular weight and the weight ratio of the two blocks, the ratio of ethylene oxide to propylene oxide in the polyether portion, and the type of link between the silicone and... 

As an alternative to pressed powders, liquid foundations have attracted special attention in recent years. Most foundation make-ups are made of O/W or W/0 emulsions in which the pigments are dispersed either in the aqueous or the oil phase. These are complex systems consisting of a suspension/emulsion (suspoe-mulsion) formulation. Special attention should be paid to the stability of the emulsion (absence of flocculation or coalescence) and suspension (absence of flocculation). This is achieved by using specialised surfactant systems such silicone polyols or block copolymers of poly(ethylene oxide) and poly(propylene oxide). Some thickeners may be also added to control the consistency (rheology) of the formulation. 

For polyethers in polysiloxanes, allyl or butyl alcohol (R ) is commonly used as a starting molecule. The most popular monomers for polyethers are ethylene oxide (EO), propylene oxide (PO), and mixtures thereof. The effect of their ratio can be described with the hydrophilic-lipophilic balance (FILB) concept, which will be discussed later [11]. The higher the PO content, the more hydrophobic is the corresponding polyether. Thus, by varying the composition of the polyethers, sometimes denoted as polyoxyalkylene, it is possible to adjust the polarity of the polydimethylsiloxane polyether copolymer. A macroscopic measure for the polarity of both polyethers and polyethersi-loxanes is the cloud point of a 1% aq. solution. The higher it is, the more hydrophilic is the molecule. In this chapter we will denote these compounds as polyethersiloxanes. It is possible to design silicone surfactants that are suitable for aqueous and nonaqueous systems. 

Recent years have seen a great trend toward using silicone oils for many cosmetic emulsions. These silicone oils are best emulsihed using silicone surfactants such as silox-ane-poly(ethylene oxide) copolymer. 

The ability to spread and the intrinsic hydrophobidty of silicones make them among the lowest surface-energy compounds known [16]. Hence, siHcones are used as relejise agents to prevent adhesion in many applications. When silicones are chemically combined with hydrophilic species such as cationic species or poly(ethylene oxide), very interesting surfactant properties arise. For example, silicones are used as foam stabilizers in the making of polyurethane foams, while silicone copolymers are used as defoamers in the food, pulp and paper industries [13]. 

Surfactants based on block copolymers of dimethylsiloxane with poly(ethylene oxide) are unique in regulating the cell size in polyurethane foams. One route to such polymers uses reaction I.E in Table 17.6 between a polysiloxane and an allyl ether of polyethylene oxide [11], Increasing the silicone content makes the surfactant more lipophilic (oil-loving), whereas increasing the poly(ethylene oxide) content makes it more hydrophilic (water-loving). 

The most widely used surfactants are copolymers based on dimethyl polysiloxane and polysiloxanes. Some of these silicones are prepared with ethylene and propylene oxides. Some silicones contain Si-... 

Fig. 6 Schematic structure of the composite layer consisting of a silicon oxide surface with a self-assembled monolayer of octadecyl trichlorosilane and a layer of the surfactant tetraethylene glycol monododecyl ether adsorbed from aqueous solution. The diagram shows the dimensions of the layers as deduced from neutron reflection and represents the proportions of the various components in the layers. The surfactant molecules are strongly tilted with the ethylene glycol head groups pointing towards the aqueous solution...

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