Stabilization by nonionic surfactants

Carpenter, J. F., Kreilgaard, L., Jones, L. S., Webb, S., Randolph, T. W. Mechanisms of protein stabilization by nonionic surfactants. Freeze-Drying of Pharmaceuticals and Biologicals, presented by National Science Foundation, Industry/University Cooperative Research Center for Pharmaceutical Processing, CPPR, Brownsville, Vermont USA, 1998... 

Many different types of interaction can induce reversible phase transitions. For instance, weak flocculation has been observed in emulsions stabilized by nonionic surfactants by increasing the temperature. It is well known that many nonionic surfactants dissolved in water undergo aphase separation above a critical temperature, an initially homogeneous surfactant solution separates into two micellar phases of different composition. This demixtion is generally termed as cloud point transition. Identically, oil droplets covered by the same surfactants molecules become attractive within the same temperature range and undergo a reversible fluid-solid phase separation [9]. 

Later we discover another parameter, the phase inversion temperature(PIT), which helps us to predict the structure of emulsions stabilized by nonionic surfactants. The PIT concept is based on the idea that the type of an emulsion is determined by the preferred curvature of the surfactant film. For a modern introduction into the HLB and PIT concepts see Ref. [546],... 

A.T. Florence, F. Madsen and F. Puisieux, Emulsion stabilization by nonionic surfactants the relevance of surfactant cloud point, J. Pharm. Pharmacol. 27 (1975) 385-394. 

Poly(vinyl acetate-co-acrylic), stabilized by nonionic surfactant. 

Pena, A.A. and Miller, C.A., Kinetics of compositional ripening in emulsions stabilized by nonionic surfactants, J. Colloid Interface ScL, 244, 154, 2001. 

Chem C-S, Liou Y-C (1999) Kinetics of styrene miniemulsion polymerization stabilized by nonionic surfactant/alkyl methacrylate. Polymer 40 3763-3772... 

Kusters et al. [42] showed that the desorption rate constant for monomeric radicals decreases signihcantly, compared to anionic surfactant, in the emulsion polymerization stabilized by n-nonylphenoxypolyethoxy ethanol with an average of 30 ethylene oxide units per molecule. This result implies that there exists a steric barrier surrounding the latex particle that retards the entry (or exit ) of free radicals for the emulsion polymerization system stabilized by nonionic surfactant. More research efforts are required to reconcile this controversial issue. 

Unzueta and Forcada [24] carried out semibatch emulsion copolymerizations of methyl methacrylate and n-butyl acrylate stabilized by mixed anionic and nonionic surfactants (sodium dodecyl sulfate and dodecyl polyethoxyl-ate) under the monomer-starved condition. The polymerization system stabilized by nonionic surfactant alone results in a slower rate of polymerization... 

Potential determining salts, also referred to as phase transfer agents for a future objective of electrochemistry at the oil-water interface in microemulsions are considered. Reasearchers have studied these salts, composed of a hydrophilic and a hydrophobic ion, in microemulsion stabilized by nonionic surfactants with an oligo ethylene oxide head-group. NMR measurements show that the salts preferentially dissoc. across the surfactant interface between the oil and water domains, and hence create a potential drop across the surfactant film, and back to back diffuse double layers in the oil and water phases. These observations are also supported by Poisson-Boltzmann calcns. ... 

Florence AT, Whitehill D. 1985. StabUity and stabilization of water-in-oil-in-water multiple emulsions. ACS Symp Ser 272 (Macro- and Microemulsions). 359-380. Florence AT, Law TK, Whateley TL. 1985. Nonaqueous foam structures form osmoti-cally swollen W/O/W emulsion droplets. J Colloid Interface Sci 107(2) 584-588. Florence AT, Rogers JA. 1971. Emulsion stabilization by nonionic surfactants Experiment and theory. / Pharm Pharmacol 23 153-169. 

Figure 5.5 Limiting zero-shear-rate relative viscosity as a function of volume fraction at 25 C for fine emulsions stabilized by nonionic surfactant (Tween 20) or anionic surfactant (SDS) at surfactant oil ratio 1 30 (wt basis) , Tween 20 (d 2= 0.55 ftm) , SDS (dj2 = 0.44 ftm)...

Two main interaction potentials are considered in systems stabilized by nonionic surfactants. The emulsion droplets are attracted by van der Waals interaction, which can be coimteracted by an energy barrier because of steric repulsion. These potentials are represented schematically in Fig. 6. 

V. Schmitt, C. Cattelet, and F. Leal-Calderon Coarsening of Alkane-in-Water Emulsions Stabilized by Nonionic Poly(Oxyethylene) Surfactants The Role of Molecular Permeation and Coalescence. Langmuir 20, 46 (2004). 

Liz-Marzan, L.M.L. and Tourind, I. 1996. Reduction and stabilization of silver nanoparticles in ethanol by nonionic surfactants. Langmuir, 12 3585-9. 

There are an enormous variety of commercial emulsifiers that are employed in emulsion polymerization. Emulsifiers are generally categorized into four major classes anionic, cationic, nonionic and zwitterionic (amphoteric). The anionic and nonionic emulsifiers are the most widely used. In addition, mixtures of emulsifiers are also often used. Since the effects of the molecular structme and chemical and physical properties of an emulsifier on particle formation are still far from being well understood, numerous experimental investigations on particle formation have been carried out to date with various nonionic emulsifiers [99-102], mixed emulsifiers (ionic and nonionic emulsifiers) [18,103-106] and reactive surfactants [33, 107-110]. Recently, polymeric surfactants have become widely used and studied in emulsion polymerizations [111-116]. A general review of polymeric surfactants was published in 1992 by Piirma [117]. Recently, emulsion polymerization stabilized by nonionic and mixed (ionic and nonionic) emulsifiers was reviewed by Capek [118]. 

Double-layer potential effects, however, cannot be responsible for the stabilization conferred by nonionic surfactants, nor can they explain the stability of water-in-oil emulsions where the double layer would be exceedingly extensive and the potential drop would be gradual. These emulsions are stabilized by a stable, thick, liquid-crystalline film at the interface or by steric stabilization. 

Steric Stabilization. A third stabilization mechanism that is normally achieved by amphipathic polymers or by nonionic surfactants is steric stabilization. To be a candidate for steric stabilization, the polymers must consist of long segments, which are soluble in the liquid medium, interspersed by short segments, usually called anchors which are strongly adsorbed at the oil-water interface (Figure 17). 

Alkyl ethylene oxide condensates and nonionic polymers such as poly(vinyl alcohol) or poly(propylene oxide)-poly(ethylene oxide) block copolymers become insoluble in water above a certain temperature, usually designated as the cloud point. The polymer chains collapse at this temperature, and, consequently, flocculation of aqueous dispersions or emulsions stabilized by these surfactants occurs when the system is heated above the cloud point. 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

A flow chart of this general process is illustrated in Figure 5.11. Surfactant and alkaline coupler solutions are combined to form a solution wherein surfactant and coupler form mixed micelles. This solution is then acidified and the coupler is reprotonated to create metastable coupler (nano)particles. Excess salt is then removed by washing (ultrafiltration or dialysis). Steric stabilization may be imparted by adding polymeric stabilizers or nonionic surfactants at the initial or final process stages. 

Salad dressings and mayonnaise can be stabilized by ionic surfactants, which provide some electrostatic stabilization as described by DLVO theory, or by nonionic surfactants which provide a viscoelastic surface coating. The protein-covered oil (fat) droplets tend to be mostly stabilized by steric stabilization (rather than electrostatic stabilization) [34,126,129], particularly at very high levels of surface protein adsorption, in which case the adsorption layer can include not just protein molecules but structured protein globules (aggregates). In some cases, lipid liquid crystal layers surround and stabilize the oil droplets, such as the stabilization of O/W droplets by egg-yolk lecithins in salad dressing [34,135]. 

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