Macromolecular surfactant

N Garti, A Aserin. Double emulsions stabilized by macromolecular surfactants. Advances in colloid and interfaces science 65 37-69, 1996. 

Another class of surfactants that are used in cosmetics and personal care products is the phosphoric acid esters. These molecules are similar to the phospholipids that are the building blocks of the stratum corneum (the top layer of the skin, which is the main barrier for water loss). Glycerine esters, in particular, triglycerides, are also frequently used. Macromolecular surfactants of the A-B-A block type [where A is PEO and B is polypropylene oxide (PPO)] are also frequently used in cosmetics. Another important naturally occurring class of polymeric surfactants is the proteins, which can be used effectively as emulsifiers. 

The experiments with noncolloidal CdS suspensions revealed that the latter feature of the systems under study as well as the establishing of the stationary quantum yields of the process cpst are observed only in the presence of a macromolecular surfactant. 

The stabilizing function of macromolecular surfactants in solid-liquid systems is exercised through protective colloid action. To be effective, they must have a strong solution affinity for hydrophobic and hydrophilic entities. In liquid-liquid systems, surfactants are more accurately called emulsifiers. The same stabilizing function is exercised in gas-liquid disperse systems where the surfactants are called foam stabilizers. 

This need not be true for macromolecular surfactants, since these are much more surface active than amphiphiles (see Figure 10.13), and the diffusion coefficients are also smaller, say, by a factor of 4. From Figure 10.13 we derive that /l-casein at a concentration as small as 900 mg m 3 can provide a surface excess of 2.5 mg -m-2 at the A-W interface. This then would lead to tads x 10(2.5/900)2/7 10 n x 106s or about 12 days. Very long times can indeed be observed. 

While the reduction of the interfacial tension coefficient, v, is relatively easy by introduction of a macromolecular surfactant , the stabilization of morphology and improvement of the interphasial adhesion in the solid state, may not be so. One may use either a single compatibilizer that can perform all three compatibilization tasks, or a combination of agents, each playing one or two different roles. For example, stabilization of the desired dispersion (accomplished by addition of surfactant to mechanically mixed compound), may be accom-... 

C. Fujimoto, Y. Fujise and S. Kawaguchi, Macromolecular surfactant as a pseudo-stationary phase in micellar electrokinetic capillary chromatography, J. Chromatogr. A, 871, 415-425, 2000. 

LAC Lacroix-Desmazes, P., Andre, P., Desimone, J.M., Ruzette, A.-V., and Boutevin, B., Macromolecular surfactants for supercritical carbon dioxide applications Synthesis and characterization of fluorinated block copolymers prepared by nitroxide-mediated radical polymerization (experimental data by P. Lacroix-Desmazes), J. Polym. Sci. Part A Polym. Chem., 42, 3537, 2004. 

In addition to recoalescence and increased droplet size in the presence of insufficient surfactant, the phe nomenon of bridging flocculation, in which the emulsion droplets form clusters during homogenization, can be observed. For the bridging to occur, it is neces sary to have macromolecular surfactants with at least two sites by which they can adsorb to interfaces, and at low surfactant concentrations such molecules can become adsorbed to two separate oil... 

The most common problems encountered in any application of macromolecular surfactants are the matching of the commercially available products to a required end effect, according to Hancock (270). Manufacturers have assembled product combinations and provided end-use categories such as desalting chemicals, oil-slick dispersants, oil-well water-flooding viscosity improver, oil-well... 

Nevertheless, it is important to point that, though the use of a compatibilizer (e.g., DBCs acting as a macromolecular surfactant and form a layer at the interface between the two homopolymers, thereby reducing the interfacial tension between the two domains and stabilizing the microscopic morphologies) can improve the phase stability, but it often result in micrometer to sub-micrometer size discontinuous (islands of minor phase in pool of major phase) dispersion of phases (as shown schematically in... 

Another method of reducing coalescence is the use of macromolecular surfactants such as gums, proteins and synthetic polymers, e.g. A-B, A-B-A block and BA graft copolymers. Examples of such molecules are poly(vinyl alcohol) and polyethylene oxide-polypropylene oxide block copolymers. 

This lack of desorption under practical situations is one of the reasons for the effective stabilization produced by macromolecular surfactants for suspensions and emulsions. 

Macromolecular surfactants or high-molecular-mass surfactants (HMMS), butyl acrylate-butyl meth-acrylate-methacrylic acid copolymers (BMMAs), and sodium 10-undecylenate (SUA) oligomer have been introduced as PSs for MEKC. Since a HMMS forms a molecular micelle, which consists of one molecule, and the CMC value is essentially zero, one can expect a higher reproducibility in a HMMS-MEKC system compared with a low molecular mass surfactant (LMMS)-MEKC system. 

A macromolecular surfactant based on poly(acrylic acid sodium salt) (HMWSP-A2) was used to prepare LDPE dispersions. The emulsion droplet size decreased with increasing surfactant concentration up to about 20 wt% of the oil phase. The first phase inversion from a water-in-polymer-melt emulsion to a polymer-melt-in-water emulsion occurred at a critical water phase volume of 20%. After phase inversion and subsequent dilution of the emulsion, if solidification of the melt was carried out during mixing, a second phase inversion occurred and water-in-solid polymer aggregates were formed even if the phase volume of the aqueous phase was well above the critical value. These aggregates contained an aqueous phase encapsulated by the polymer. The kinetics of the phase inversions were studied and the use of the technique to obtain microcapsules from aqueous solutions was discussed. 20 refs. 

Proteins, naturally occurring macromolecular surfactants with amphiphilic nature, are adsorbed onto interfaces, thereby affecting the physical states of interfaces. Many enzymes are involved in catalytic reaction at interfaces. For enzymatic reaction at interfaces, different from the reaction in homogeneous systems, interfacial contact and subsequent conformational change of enzymes are important events determining their catalytic activity. In this chapter, I will describe the conformation of proteins and their interaction (protein-protein and protein-surfactant) at interfaces (mainly liquid-liquid interfaces). The characteristics of enzymatic reaction at liquid-liquid and solid-liquid interfaces, especially lipase reaction, wiU also be described. 

Systems like ternary blends of immiscible A and B homopolymers and of a macromolecular surfactant such as an AB diblock copolymer are especially well described by the Teubner-Strey equation, which reproduces scattering experimental data by just three parameters, ao, Ci and Cj. 

FLUORINATED MACROMOLECULAR SURFACTANTS FOR WATER/CARBON DIOXIDE EMULSIONS... 

As depicted in this chapter, specific structural and electronic properties of fluo-ropolymers have made them the polymers of choice to be used in SC-CO2. The study of different families of fluoropolymer demonstrated that, apart from the effect of polymer architecture and CO2 density, the lowering of polymer-polymer interactions appeared as the main critical parameter to improve polymer solubility. Moreover, the use of those polymers as building blocks to design macromolecular surfactants allows the successful formation of micelle-like structures or the formation of emulsions in the presence of additional water. Based on this knowledge, development of new fluoropolymer families will surely pave the way for the preparation of stable W/SC-CO2 emulsions with great promise for the development of environmental-friendly chemical processes in diverse fields of organic and inorganic synthesis and polymerization. 

With emulsifiable concentrates, emulsions and microemulsion, the surfactant adsorbs at the oil/water interface, with the hydrophilic head group immersed in the aqueous phase, leaving the hydrocarbon chain in the oil phase. Again, the mechanism of stabilization of emulsions and microemulsions depends on the adsorption and orientation of the surfactant molecules at the liquid/liquid interface. As we will see, macromolecular surfactants (polymers) are nowadays used to stabilize emulsions and hence it is essential to understand their adsorption at the interface. Suffice to say that, at this stage, surfactant adsorption is relatively simpler than polymer adsorption. This is because surfactants consist of a small number of units and they are mostly reversibly adsorbed, allowing one to apply some thermodynamic treatments. In this case, it is possible to describe the adsorption in terms of various interaction parameters such as chain/surface, chain solvent and surface solvent. Moreover, the configuration of the surfactant molecule can be simply described in terms of these possible interactions. In contrast, polymer adsorption is fairly complicated. In addition to the usual adsorption considerations described... 

The enhanced stability using high molecular weight surfactants (polymeric surfactants) can be understood by considering the steric repulsion, which produces more stable films. Films produced using macromolecular surfactants resist thinning and disruption, thus reducing the possibility of coalescence. 

The rate of coalescence is measured by following the droplet number n or average droplet size d (diameter) as a function of time. Plots of log(droplet number) or average diameter versus time give straight lines (at least in the initial stages of coalescence), from which the rate of coalescence k can be estimated using Eq. (6.73). In this way, one can compare the different stabilisers, e.g. mixed surfactant films, liquid crystalline phase and macromolecular surfactants. 

Macromolecular surfactants possess considerable advantages for use in cosmetic ingredients. The most commonly used materials are the ABA block copolymers. 

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