Role of Counterion in the Adsorption Behavior of 1:1 Ionic Surfactants at Fluid Interfaces—Adsorption Properties of Alkali Perfluoro-n-octanoates at the Air/Water Interface

Abstract

Equilibrium surface tension (σ) versus bulk concentration (c) isotherms of aqueous, surface-chemically pure solutions of various alkali perfluoro-n-octanoates were measured at 295 K. These 1:1 ionic surfactant systems belong to the pseudo nonionic ones. Evaluating the different σ vs log c isotherms by basic adsorption equations reveals that they follow ideal surface behavior. The novelty of this investigation exists in the fact that the surface area demand per molecule adsorbed calculated from the experimental σ vs log c isotherms is identical to that of the hydrated alkali cation. Thus, as long as the counterion's cross-sectional area is greater than that of its amphiphilic anion, the amphiphile's total surface area demand will exclusively be governed by that of its alkali counterion. This, in turn, means that the counterion is nonrandomly bound to the amphiphilic anion in the adsorption layer. Furthermore, the size of the hydrated alkali counterion in the adsorption layer does not differ from that in the bulk phase.

Quantifying the Counterion-Specific Effect on Surfactant Adsorption Using Modeling, Simulation, and Experiments

“…like Cs + make the surfactant ions less repulsive to each other, so that more surfactant ions can be pushed into the adsorption layer with the same surface area. 8,18 Figure 3 highlights the difference between the Li + ion and the Cs + ion, as it shows the density distribution and electrostatic potential profile of 1 mM SDS in the presence of 10 mM LiCl and CsCl. The concentration profile was calculated by eq 4, and the potential profile was calculated by eq 1 using the parameters in Table 1.…”

“…For some surfactants like sodium laurate, this interaction might explain why the COO − headgroup prefers small counterions, 73 but large counterions still decrease the surface tension more effectively. 18 Large ions may have genuine affinities to the air/water interface, which could contribute to the overall counterion adsorption energy. 74,75 An accurate determination of this ion/ interface affinity is very challenging with a classical nonpolarizable force field as QM interactions need to be included accurately.…”

“…16,17 The most recent counterion-specific adsorption model points out that small ions such as Li + will have larger hydrated radii than large ions such as Cs + , which will prevent them from coming into close contact with the surfactant headgroup. 12,18 The distance between the surfactant headgroup and the counterion determines the surface potential, which in turn affects the surface tension of the solution. 12 All theories can explain the counterion-specific surface tension because this property has only limited information.…”

“…Experimentally, larger monovalent cations decrease the surface tension of anionic surfactants (e.g., sodium dodecyl sulfate or SDS) more effectively (Cs + > K + > Na + > Li + ), and this trend is the same for anions with cationic surfactants such as dodecyl trimethyl ammonium chloride. ,,, However, different explanations have been proposed for the counterion-dependent surface tension. Each counterion was believed to have different adsorption energies, which will either enhance the surfactant adsorption , or result in a higher surface excess directly. , Some adsorption models assume that the adsorption energy was caused by the ion-pair formation between the headgroup and counterions, while some believe that it was from the counterion affinity to the air/water interface. , The most recent counterion-specific adsorption model points out that small ions such as Li + will have larger hydrated radii than large ions such as Cs + , which will prevent them from coming into close contact with the surfactant headgroup. , The distance between the surfactant headgroup and the counterion determines the surface potential, which in turn affects the surface tension of the solution . All theories can explain the counterion-specific surface tension because this property has only limited information.…”

Thermodynamic Behaviour of Mixed Films of an Unsaturated and a Saturated Polar Lipid. (Oleic Acid-Stearic Acid and POPC-DPPC)

“…Bibliographic references to previous studies can be found in [1][2][3]. Articles on single component fatty acids or related amphiphiles published in the last years are those of [4][5][6][7][8][9][10][11][12][13][14][15][16], and articles on single component phospholipids are those of [17][18][19][20][21][22].…”