Surfactant-like behavior

Finally, owing to their polyanionic (acid) nature, HEf and HSp (humic fraction >3500Da) do show surfactant-like behavior (Visser 1986 Nardi et al., 1991), as decreased surface tension of water is described when their concentration is increased (Fendler and Fendler, 1975), so HS could simply act as surface-active molecules (Samson and Visser, 1989). By decreasing the pH at the surface of the plasma mem-... 

The magical counterions in terms of conductivity and mechanical properties appear to be sulfonated aromatics,58 59 60 61 62 in particular, para-toluene sulfonate (pTS). It has been shown that the benzene sulfonates induce a degree of crystallinity28 29 that results in higher conductivity. This higher conductivity enables the polymerization process to proceed efficiently. It has also been suggested that the sulfonated aromatics exhibit surfactant-like behavior as the radicals are stabilized and presumably protected from unwanted side reactions with the solvent, oxygen, or other nucleophiles. 

In addition to short-chain (< 20 hydrocarbon groups) surfactants, long-chain polymers can sometimes show surfactant-like behavior. Examples of... 

In thin films of diblock copolymers, the interactions occurring at the air/film and fihn/substrate interfaces influence the miaophase separation process, and can be used to control the orientation of the morphology. For example, a preferential interaction of one of the blocks with the boundaries ( surfactant -like behavior) will favor uniformity of a lamellar phase on a macroscopic scale, with the lamellae of periodicity I parallel to the substrate [ 10,11]. The resulting film thickness will belong to a discrete spectrum of allowed values D , depending on the boundary conditions [10,12] given by... 

Inverted raspberry-like morphologies (the mineral particles being located at the surface of the latex spheres) have also been discussed in Sections 4.4.2.2 and 4.4.2.4 about colloidal silica and layered silicates, respectively. These are mainly a consequence of the surfactant-like behavior of the inorganic particles in specific situations. This was clearly illustrated in a recent report by Landfester, who showed that silica or clays can be used as pickering stabilizers of miniemulsion jxjlymer-izations, resulting therefore in the formation of armored latexes, the surface of which was recovered by the small inorganic particles [99,131]. 

In the microemulsion systems the primary alcohols are frequently considered as cosurfactants, which are usually weakly amphiphilic molecules that help the amphiphilic surfactants to reduce the surface tension of the interface between the immiscible components of the system. In this way they usually enhance and emphasize the internal structure of the system at the colloidal level. Remarkably, the short-chain alcohols, which are sufficiently soluble in water, themselves show surfactant-like behavior in plain binary water mixtures. As was shown by Kahlweit et al. [87], this specific behavior can be observed from the break in the curves of surface tension versus molar fraction of alcohol in water. Similar breaks were observed by Zana et al. [7] in the curves of fluorescence intensity versus molar fraction of alcohol, where changes in the environment polarity are sensed by the pyrene fluorescence probe. Interestingly, with increasing the length of the... 

Surface activity of these PAMAMOS dendrimers was determined by the Wilhelmy plate method for the water soluble dendrimers and by the Langmuir trough technique for the insoluble ones. Some preliminary data on water soluble PAMAMOS have already been published (5,42), It was shown that the best of these materials lower the surface tension of water to just below 30 mN/m at 5 wt. %, with no break to a constant surface tension that would indicate micelle formation. Thus, these PAMAMOS behave more like considerably surface active water soluble polymers than surfactants. However, it is probable that their homologues with longer siloxane dendrons than the trimethylsilyl- groups studied so far will have considerably more surfactant-like behavior. 

A second method for the formation of nanostructured surfiice-functionalized ionosilicas consists in the cocondensation reaction involving ionic precursor displaying surfactant-like behavior (Scheme 16.8) [108]. Template-directed syntheses of long chain-substituted silylated imidazolium or ammoniiun precursors in the presence of cationic surfactant such as CTAB or cetylpyridinium chloride yield highly structured ionosilica materials with surface-tethered ionic groups. 

The concentration fluctuations in our system can possibly further be subdivided into a solute (surfactant aggregates) concentration fluctuation and a solvent concentration fluctuation. The solute concentration fluctuation is similar to that of critical-like behavior observed in many W/0 microemulsion systems (28), while the solvent... 

Despite that the silicate-surfactant mesophase formation resembles the phase separation normally observed in surfactant-polyelecholyte systems, it is interesting to note that it is stiU possible to make qualitative predictions about the influence of inorganic-surfactant phase behavior based on models developed for dilute surfactant systems. The packing parameter concept - is based on a geomettic model that relates the geomehy of the individual surfactant molecule to the shape of the supramolecular aggregate structures most likely to form. N, is defined as... 

The preparation, uses, and physieal chemistry of silicone siufactants have been described in a well-known reference [38], focusing on silicone polyoxyalkylene copolymers. This book explores many aspects of SPEs as surfactants, like interfacial processes, surface viscoelasticity, and aggregation, explains the imusual wetting behavior of the trisiloxane surfactants and the ternary phase behavior of mixtures of silicone siufactants with water and silicone oils. That is why we only try to emphasize some newer insights into the subject, reported mainly after year 2000. 

Surfactant-like lipids adopt either normal (type 1) or inverted (type 2) self-assembled phases, resulting in either oil-in-water (o/w) phases with convex curvature lipid/water interface or water-in-oil (w/o) phases with a concave interface, respectively. The formation of a normal or an inverted self-assembled nanostructure in water mainly depends on the lipid s molecular shape, as discussed in the seventies by Israelachvili and co-workers [78], In this regard, the geometric shape of the lipid can be a useful tool for predicting the water-lipid interface curvature and also can be helpful in imderstanding the phase behavior of binary, ternary, and even multi-component systems [79], For this purpose, the shape factor or more commonly known in the literature as the critical packing parameter CPP) was defined [78] as ... 

Figure 1 (right) illustrates the phase behavior in a binary or ternary lipid system. The self-assembled nanostructure follows the phase sequence of La" V2 H2 I2 L2 with increasing solubilized oil content and/or temperature, ranking the inverse phases by increasing values of their mean-interfacial curvature or CPP value [83, 99] ENREF 96. The CPP increases with temperature due to the increased fluctuation of the hydrophobic chains of the investigated surfactant-like lipid [83]. 

Anionic polysaccharides respond in similar fashion to surfactants. They are relatively unaffected by anionic surfactants like sodium or ammonium lauryl sulfate. On the other hand, they form strong ionic complexes with cationic surfactants like dodecyltrimethylam-monium chloride, even at cationic surfactant concentrations below the critical micelle concentration (cmc), or concentration at which the surfactant molecules form micelles in solution (92,93). The behavior of polyelectrolytes in the presence of surfactants is summarized in Chapter 5 and has been reviewed (94). 

Natural surfactants like lecithin are of special interest for food and pharmaceutical industry. However properties of gelatin/lecithin mixtures are practically not described in current publications. Combinations of gelatin with lecithin as emulsifiers and stabilizers of traditional and new food recipes can provide new perspective products, and that is why studies of these systems are of actual importance. The interrelation between the rheological properties of interfacial absorption layers containing proteins and low-molecular-weight surfactants [14-21] and the behavior of emulsions (including its stability) is also of significant values. 

A. Macrophase separation into two layers, which commonly arises when two homopolymers are mixed. B. Microphase segregation can arise with diblock copolymers. Here, striated layers appear. C. Mixing homopolymers with the corresponding diblock copolymer gives intermediate-scale phase separation. A surfactant-like mixture occurs. Figure modified from Bates, F. S. "Polymer-Polymer Phase Behavior." Science, 251, 898 (1991). 

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