Anionics at the air/water interface

Anionic at the Air/Water Interface. In Figure 2 we show II—1/A isotherms obtained by one of us (38) for Ci8 sulfate on several salt solutions at 20°C. We also show a few points where the A values overlap with the results on 0.01 M NaCl reported in our paper on entropies of compression (33). The favorable agreement between both sets of data at lower A is discussed in Ref. 33. The most likely explanation of the slight discrepancies is trace residual alcohol in the older experiments. No other data are available for comparison at these large A values. 

For the characterization of Langmuir films, Fulda and coworkers [75-77] used anionic and cationic core-shell particles prepared by emulsifier-free emulsion polymerization. These particles have several advantages over those used in early publications First, the particles do not contain any stabihzer or emulsifier, which is eventually desorbed upon spreading and disturbs the formation of a particle monolayer at the air-water interface. Second, the preparation is a one-step process leading directly to monodisperse particles 0.2-0.5 jim in diameter. Third, the nature of the shell can be easily varied by using different hydrophilic comonomers. In Table 1, the particles and their characteristic properties are hsted. Most of the studies were carried out using anionic particles with polystyrene as core material and polyacrylic acid in the shell. 

The interaction between the adsorbed molecules and a chemical species present in the opposite side of the interface is clearly seen in the effect of the counterion species on the HTMA adsorption. Electrocapillary curves in Fig. 6 show that the interfacial tension at a given potential in the presence of the HTMA ion adsorption depends on the anionic species in the aqueous side of the interface and decreases in the order, F, CP, and Br [40]. By changing the counterions from F to CP or Br, the adsorption free energy of HTMA increase by 1.2 or 4.6 kJmoP. This greater effect of Br ions is in harmony with the results obtained at the air-water interface [43]. We note that this effect of the counterion species from the opposite side of the interface does not necessarily mean the interfacial ion-pair formation, which seems to suppose the presence of salt formation at the boundary layer [44-46]. A thermodynamic criterion of the interfacial ion-pair formation has been discussed in detail [40]. 

Shimomura and Kunitake have reported that stable monolayers and LB films were obtained by electrostatic interaction of water soluble anionic polymers with cationic amphiphiles [58]. This polyion-complexation was also a useful method for stabilization of monolayers of unstable [59] or water soluble anionic surfactants [60]. Mixtures of water soluble cationic and anionic surfactants (1 1) also formed stable Langmuir monolayers at the air/ water interface [60]. 

As a cationic polymer and a cationic amphiphile, poly(allyl amine hydrochloride) (PAA) and octadecylamine (ODA) shown in Fig. 6 were used, respectively. The stability of the monolayers of the anionic amphiphiles was increased by polyion-complexation with PAA added in the aqueous subphase in comparison with Ca2+ salt formation. Ion complexation (1 1) of each anionic amphiphile with ODA was also performed at the air-water interface by spreading a chloroform solution of a 1 1 surfactant mixture. 

A. Asnacios, D. Langevin, and J.-F. Argillier Complexation of Cationic Surfactant and Anionic Polymer at the Air-Water Interface. Macromolecules 29, 7412 (1996). 

Figure 8.6 Comparison of the influence of non-ionic Ci2E6 (hexaoxyethyl-ene ft-dodecyl ether) or anionic SDS (sodium dodecyl sulfate) on adsorbed amount of p-lactoglobulin at the air-water interface (0.1 wt% protein, pH = 6, ionic strength = 0.02 M, 25 °C) as determined by neutron reflectivity measurements. Protein surface concentration is plotted against the aqueous phase surfactant concentration ( ) Ci2E6 ( ) SDS. Reproduced from Dickinson (2001) with permission.

Figure 23. (a) Schematic representation of an anionic surfactant azobenzene derivative monolayer film at the air-water interface. (i>) Schematic representation of the stable monolayer film formed from the polyion complex of anionic surfactant azobenzene derivatives with a cationic polymer. Note the difference in free volume around the reactant chromophores in the two monolayers. 

One characteristic property of surfactants is that they spontaneously aggregate in water and form well-defined structures such as spherical micelles, cylinders, bilayers, etc. (review Ref. [524]). These structures are sometimes called association colloids. The simplest and best understood of these is the micelle. To illustrate this we take one example, sodium dode-cylsulfate (SDS), and see what happens when more and more SDS is added to water. At low concentration the anionic dodecylsulfate molecules are dissolved as individual ions. Due to their hydrocarbon chains they tend to adsorb at the air-water interface, with their hydrocarbon chains oriented towards the vapor phase. The surface tension decreases strongly with increasing concentration (Fig. 3.7). At a certain concentration, the critical micelle concentration or... 

The possible accumulation of negative ions at the air/ water interface was first predicted by Perera and Berkow-itz,8 who found out from molecular dynamics simulations, surprisingly, that the large anions (Cl , Br , and I ) are expelled from water clusters to their interface. Their predictions are supported by the recent large-scale molecular dynamics simulations for the air/water interface of various electrolyte solutions, which reveal that, when the polarization of ions and water molecules is explicitly taken into account, the large anions are accumulated near the interface.9... 

Figure 4. (ad) Distribution of cations (1), anions (2), and water (3) at the air/water interface. The continuous curves are calculations based on the SM/SB model for short-range ion-hydration forces. The squares (cations), circles (anions) and triangles (water) are the molecular dynamics results of ref 9. (a) NaCl (b) HC1 (c) HBr (d) NaOH. 

In a recent review of this topic [B.C. Gaietl, Science 303 (2004) 1146] the emphasis was on some recent experiments, in which it was found that some anions accumulate at the air/water interface and not in the bulk, as usually happens to the cations, and on some simulations which explained those positive surface adsorption excesses. Because a large number of these experiments could be explained on the basis of some simple physical models proposed by the authors for the interaction between the ions and the air/water interface [M. Mandu, E. Ruckenstein, Adv. Colloid Interface Sci. 105 (2003) 63 Adv. Colloid Interface Sci. 112 (2004) 109 Langmuir 21 (2005) 11312], those models ate reviewed in the present note, the goal being to draw attention to them. 

The adsorption of the anions at the mercury-water interface is not exactly in the same order as the adsorption at an air-water interface (cf. Chap. Ill, 10), for the mercury exerts specific attractions on certain ions, notably the sulphide and iodide the degree of hydration of the ions, which was all-important for the air-liquid adsorption, is here only one of the factors controlling the adsorption. Talmud1 shows marked lowering of tension and a similar shift of the maximum to the right, with soap solutions, in which of course the anions are strongly adsorbed. 

In flg. 3. lb the preference of the hydrophobic tails of the (anionic) surfactant molecules for the oil phase gives rise to the double layer. Such double layers are for instance encountered in some emulsions. They may also occur at the air-water interface then the driving force for their formation is the expulsion of the hydrocarbon tails from the aqueous phase. We speeik of ionized monolayers and return to them in Volume III. 

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