Low molecular weight surfactants

The addition of large linear blocks to dendrons with opposite polarity creates a desymmetrized structure predisposed to sequester insoluble components by aggregation rather than intramolecular hydrogen-bonding. Amphiphilic, linear-dendritic diblock (AB) and triblock (ABA) copolymers self-assemble into multimolecular micelles with CMC values that are well below those of low molecular weight surfactants. Typically, a hydrophilic linear block such as PEG is attached to the focal point... 

It is important to understand the characteristic interactions involved at an interface containing each of the main types of surface-active molecules, i.e., biopolymers (proteins, polysaccharides) and low-molecular-weight surfactants (lipids). But that is not the whole story. In real food systems there are almost always mixed ingredients at the interface. So it is necessary to understand what sorts of mixed interfacial structures are formed, and how they are influenced by the intermolecular interactions. 

In general, surface activity behaviour in food colloids is dominated by the proteins and the low-molecular-weight surfactants. The competition between proteins and surfactants determines the composition and properties of adsorbed layers at oil-water and air-water interfaces. In the case of mixtures of proteins with non-surface-active polysaccharides, the resulting surface-activity is usually attributed to the adsorption of protein-polysaccharide complexes. By understanding relationships between the protein-protein, protein-surfactant and protein-polysaccharide interactions and the properties of the resulting adsorbed layers, we can aim to... 

Low-molecular-weight surfactants ( emulsifiers ) are important ingredients in food products. The types of surfactants most commonly studied in food colloids research are phospholipids (lecithin), mono/diglycerides (particularly glycerol monostearate), polysorbates (Tweens), sorbitan monostearate or monooleate (Spans), and sucrose esters. These small lipid-based amphiphiles can typically lower the interfacial tension to a greater extent than the macromolecular amphiphiles such as proteins and certain gums (Bos and van Vliet, 2001). 

A situation that commonly occurs with food foams and emulsions is that there is a mixture of protein and low-molecular-weight surfactant available for adsorption at the interface. The composition and structure of the developing adsorbed layer are therefore strongly influenced by dynamic aspects of the competitive adsorption between protein and surfactant. This competitive adsorption in turn is influenced by the nature of the interfacial protein-protein and protein-surfactant interactions. At the most basic level, what drives this competition is that the surfactant-surface interaction is stronger than the interaction of the surface with the protein (or protein-surfactant complex) (Dickinson, 1998 Goff, 1997 Rodriguez Patino et al., 2007 Miller et al., 2008 Kotsmar et al., 2009). 

The term food colloids can be applied to all edible multi-phase systems such as foams, gels, dispersions and emulsions. Therefore, most manufactured foodstuffs can be classified as food colloids, and some natural ones also (notably milk). One of the key features of such systems is that they require the addition of a combination of surface-active molecules and thickeners for control of their texture and shelf-life. To achieve the requirements of consumers and food technologists, various combinations of proteins and polysaccharides are routinely used. The structures formed by these biopolymers in the bulk aqueous phase and at the surface of droplets and bubbles determine the long-term stability and rheological properties of food colloids. These structures are determined by the nature of the various kinds of biopolymer-biopolymer interactions, as well as by the interactions of the biopolymers with other food ingredients such as low-molecular-weight surfactants (emulsifiers). 

The association of block copolymers in a selective solvent into micelles was the subject of the previous chapter. In this chapter, ordered phases in semidilute and concentrated block copolymer solutions, which often consist of ordered arrays of micelles, are considered. In a semidilute or concentrated block copolymer solution, as the concentration is increased, chains begin to overlap, and this can lead to the formation of a liquid crystalline phase such as a cubic phase of spherical micelles, a hexagonal phase of rod-like micelles or a lamellar phase. These ordered structures are associated with gel phases. Gels do not flow under their own weight, i.e. they have a finite yield stress. This contrasts with micellar solutions (sols) (discussed in Chapter 3) which flow readily due to a liquid-like organization of micelles. The ordered phases in block copolymer solutions are lyotropic liquid crystal phases that are analogous to those formed by low-molecular-weight surfactants. 

CHANGES IN THIN FILM PROPERTIES AS A FUNCTION OF INCORPORATION OF LOW MOLECULAR WEIGHT SURFACTANT IN THE ADSORBED PROTEIN LAYER... 

Block copolymers of polystyrene (PSt, hydrophobe) and polyoxyethylene (PEO, hydrophile) form spherical micelles in water when the length of water soluble PEO is significantly larger than that of the insoluble PSt portion of the molecule [62]. In analogy with low molecular weight surfactants, one defines the onset of intermolecular association as the critical micelle concentration (CMC). Theories of polymer micellization predict that the concentration of free, unassociated block copolymers is close to that of the CMC. 

Two models for micelle structure were identiLed in their studies (Xing and Mattice, 1998). In analogy with the structural models for systems involving low molecular weight surfactants, two kinds of aggregates of spherical shape can be pictured, depending on how the solubilizates are located inside the block copolymer micelles. Solubilization takes places in two steps in the Xing and Mattice s simulations (1998). 

If not only geometric, but also thermodynamic parameters are taken into consideration, the difference between the self-assembly of polymeric amphiphiles compared to low molecular weight surfactants is even more pronounced. The two major contributions to the free energy of the system are 1) the loss of entropy when flexible parts of the amphiphile are enforced in the restricted environment of the aggregates, and 2) the interfacial energy... 

So far these trends are analogous to the aggregation of traditional low molecular weight surfactants. It was, however, also shown by Eisenberg and coworkers that the hydrophobic... 

Generally speaking, the reason for this behavior is simple. It is known that low-molecular-weight surfactants dramatically increase the stability of polymers and are widely used to prevent aggregation in polymer solutions. In the HA model, surfactants , i.e., amphiphilic A groups, are incorporated into the polymer chain, thus ensuring the stabilizing effect. From the tempera-... 

The analogous considerations are valid for polymer systems as well. Indeed, amphiphilic monomer units also tend to occupy interfacial areas of macromolecular associates as it is normal for low molecular weight surfactants to adsorb at polymer-poor solvent boundaries. And, if such interfacial groups of the polymer associate catalyze chemical transformation of a compound which tends to adsorb at the associate interfaces, this can result in unusual kinetics effects. Okhapkin et al. [18] studied the influence of temperature-induced aggregation on the catalytic activity of thermosensitive... 

Alcohols, low molecular weight Surfactants (liquid detergents, shampoos, etc.) Alcohols, high molecular weight Fats and oils (vegetable, animal, synthetic)... 

It is well documented that in many respects PEO-PPO-PEO triblock copolymers behave like non-ionic surfactants [e.g. 225], This is also true for the interactions in foam films. The disjoining pressure isotherm in Fig. 3.39 is very much like that obtained earlier with foam films from nonylphenol eicosaoxyethylene ether (NP(EO)2o) [172], In both cases the isotherm is reversible, monotonously increasing (the barrier mechanism typical for low molecular weight surfactants is not observed) and its slope increases with decreasing film thickness. These features seem to be characteristic of surfactants having long PEO chains as already suggested in [172],... 

The resulting micellar aggregates resemble, in most of their aspects, those obtained with classical low molecular weight surfactants, but the nonergodicity of BCs allows the preparation of many different kinetically frozen morphologies. From the initial basic observations of micelle formation by Merret in 1954 [24] to the last structures of living micelles obtained by Winnik and co-workers in 2007... 

Stabilization of the interfaces between oil and water is of great importance in emulsion technology. Interfaces are usually stabilized with the help of amphiphihc molecules such as low molecular weight surfactants and biopolymers. Interfaces are two-dimensional nanoscopic spaces where amphiphilic molecules accumulate and self-assemble. Time-scales are very important in emulsion technology, as the adsorption events at the newly created interface take place in the millisecond range and are governed by the diffusion properties of the surfactant molecules (Brosel and... 

As soon as the protein is adsorbed, its structure changes. The conformation of the molecule may change such that other hydrophobic parts of the molecules become exposed to the oilAvater interface. The molecule will then be adsorbed to the interface even stronger. Typically, once a protein has become adsorbed to an oil/water interface, its adsorption wUl be irreversible (this is in contrast to low-molecular weight surfactants, that are at equilibrium with the phases surrounding the interface). The protein sometimes can form a more or less solid skin on the surface of the droplet. 

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