Adsorption of N-decanoyl-N-methylglucamine at the Interface Electrode − NaClO 4 Solution . Comparison of Adsorption Properties of Different Surfactants

The electrosorption behaviour of non-ionic surfactant: N-decanoyl-N-methylglucamine on mercury electrode in 1 mol dm−3 NaClO4 solution was determined. The values of the relative surface excess were determined on the basis of double layer differential capacity. A set of parameters of maximal adsorption and the constants of Frumkin, modified Flory-Huggins and virial adsorption isotherms were obtained. It was stated that the adsorption of this surfactant is determined by the adsorption energy, however here is no simple relation between a surface excess and the values of repulsive interactions parameter A. Adsorption properties of three surfactants: cationic, anionic and non-ionic were compared.


INTRODUCTION
[3][4] The adsorption of surfactants on charged surfaces is a topic of both practical and academic interest.[7][8][9] Inhibitors adsorb on the metal surface, thus resulting in an adsorption-film acting as a barrier between the metal surface and the corrosive medium, and blocking the active sites. [10][17][18][19][20][21] The thermodynamic analysis of surface concentrations is still the most frequently used route to gain an insight into the mode of organic compounds adsorption. [22]This entails the description of experimental data by means of an adsorption isotherm, and the construction of an adsorption model based on the derived adsorption parameters.
The aim of this work was to analyze the adsorption properties of N-decanoyl-N-methylglucamine (MEGA-10, C17H35NO6) at the mercury electrode from the 1 mol dm -3 NaClO4 solution.In addition, the adsorption properties of cationic, anionic and non-ionic surfactants were compared.The choice of NaClO4 solution resulted from the fact that ClO4 -ions cause the strongest disruption in water structure. [23]In our studies on adsorption from solutions, the double-layer capacitance was usually chosen as the primary experimental quantity.Such a measurement is far more accurate when performed on liquid rather than solid metal surfaces, which is why mercury played a dominant role as an electrode material in our studies.This is due to the highly reproducible, readily renewable and smooth surface of mercury drops.

T EXPERIMENTAL
Analytical grade C17H35NO6 and NaClO4 (Fluka, Switzerland) were used without further purification.Water and mercury were double distilled before use.The C17H35NO6 solution of concentrations ranging from 1 × 10 -6 mol dm -3 to 7.5 × 10 -4 mol dm -3 were prepared.The chosen surfactant concentrations were lower than the surfactant critical micelle concentration (CMC).The solutions were deaerated by passing high purity nitrogen over the solution during the measurements at 298 ± 0.1 K.The three-electrode system was used, with a dropping mercury electrode as a working electrode, and Ag|AgCl as a reference electrode, to which all potentials in this paper are referred.A controlled growth mercury drop electrode (CGMDE M165, MTM ANKO, Poland) was used.The differential capacity, C, of the double layer was measured with an Autolab frequency response analyzer (AutolabIII/FRA2, Metrohm Autolab B. V., The Netherlands) using the AC impedance technique.The measurements were carried out at several frequencies in the range from 400 to 2000 Hz, with an amplitude of 5 mV.The equilibrium capacities were obtained by extrapolation of the dependence of the measured capacity versus square root of frequency to zero frequency.
The potential of zero charge, Ez, was measured using a streaming electrode.The interfacial tension, γz, at Ez was measured by the maximum bubble pressure method according to Schiffrin.The charge density and surface tension were obtained by the back integration of differential capacity-potential dependences.6]

Analysis of Experimental Data
Figure 1 presents the selected differential capacity-potential curves extrapolated to zero frequency.This procedure assumes that the impedance of the double layer is equivalent to the combination of a capacity-resistance series and that the rate of adsorption is diffusion-controlled.
By introducing C17H35NO6 to the 1 mol dm -3 NaClO4 solution a decrease of differential capacity occurred in a wide range of potential from E = −150 mV to E = −1100 mV.This decrease is also detected even at lower surfactant concentrations.For the highest surfactant concentrations the desorption peaks developed whose potentials shifted with the increase of the C17H35NO6 concentration towards negative values.
The addition of C17H35NO6 to the solution shifts the potential of zero charge from Ez = -461 mV for 1 mol dm −3 NaClO4 to Ez = -459 mV for the solution containing 7.5 × 10 -4 mol dm -3 C17H35NO6.The obtained results show the mechanism of surfactant adsorption; the positive pole, that is the radical -C10H21 is directed to the mercury, while the hydrophilic atoms of oxygen and nitrogen are directed towards the solution.The value of surface tension γz at Ez decreases from 421 mN m -1 to 391 mN m -1 with the increase of concentration of C17H35NO6 (Table 1).
The capacity versus potential data were numerically integrated from the point of Ez to obtain the values of electrode charge, σ.
Figure 2 presents the dependences of electrode charge on electrode potential.The common intersection point marks the maximum adsorption parameters: Emax = -466 mV and σmax = 0.It also confirms that the equilibrium for the measured capacities is reached. [27]

Adsorption Isotherms
Knowing the change in surface tension for the base electrolyte solution, γ0, as well as the corresponding changes in the presence of C17H35NO6, γ, the surface pressure, Φ, can be calculated for given values of electrode potential and surfactant concentration: Φ = (γ0 -γ)E.According to the Gibbs adsorption isotherm, the relative surface excess, Γ', of C17H35NO6 is given by equation: where c is the bulk concentration of C17H35NO6.
Positive values of surface pressure, Φ, were obtained in the potential range from E = -300 mV to E = -800 mV subsequently for the above potential range, the values of Γ' were calculated.
Figure 3 presents the dependence of Γ' values obtained for the studied concentrations of surfactant in the bulk as a function of electrode potential.The maximum Γ' corresponds to the electrode potential E = -400 mV, that is potentials close to Emax (Table 2).The shape of curves in Figure 3 shows competitive electrostatic interactions: organic molecules-water dipoles.
Using the obtained Γ' values for the determined electrode potentials and C17H35NO6 concentrations, the linear Frumkin isothermal test was determined based on the equation: where x is the mole fraction of C17H35NO6 in solution, β is the adsorption coefficient, which is defined as β = exp(-ΔG°/ RT) and θ is the surface coverage, θ = Γ' / Γs. [4] The surface excess at saturation, Γs, was estimated by extrapolation of the 1 / Γ' vs. 1 / c(C17H35NO6) lines at different electrode potential to 1 / c(C17H35NO6) = 0.The surface occupied by one C17H35NO6 molecule, S, (S = 1 / Γs) was 0.249 nm 2 .Such a small S value indicates the perpendicular orientation of the adsorbed molecule.The values of interaction parameter, AF, were calculated from the standard Gibbs energy of adsorption, ΔGF ° values were determined by extrapolation of linear dependences ln(1 -θ ) / θ vs. θ to the value θ = 0. Table 3 presents the values of the Frumkin isotherm constants.ΔG° values are strongly dependent on the Γ' values (Figure 3): the highest Γ' value with the potential of electrode E = -400 mV corresponds to the highest adsorption energy.At the same time, with this potential the   most important finding is the repulsive interaction between the adsorbed molecules.Thus, the size of adsorption is determined by the changes of adsorption energy, ΔG°.The adsorption of C17H35NO6 was further analyzed on the basis of constants obtained from the modified Flory-Huggins isotherm for long-range particle-particle interactions: [28,29]     where n = 2.02 is the number of water molecules replaced by one C17H35NO6 molecule.
In the presented studies the projected area for water molecule is 0.123 nm 2 . [30]As the ClO4 -ions cause the strongest disruption in water structure, [23] the surface of one water molecule was used in calculations instead of water clusters.
Figure 4 shows the test of the modified Flory-Huggins isotherm for electrode potential from E = -300 mV to E = -800 mV.The values of ΔGH° and AH were calculated in the same way as presented previously.The trends of constant changes in the modified Flory-Huggins isotherm are similar to constant changes of the Frumkin isotherm.However, the values of adsorption energy are slightly higher and are accompanied by weaker repulsive interactions.Because the above isotherms are burdened with some inaccuracy connected with the determination of Γs for their verification the virial isotherm was applied, the use of which does not require the knowledge of Γs.The virial isotherm equation is: where B is a 2-dimensional (2D) second virial coefficient.The linear test for the virial isotherm was performed in the system: log(Γ' / c) vs. Γ' using the standard state 1 mol dm -3 in the bulk solution and 1 mol cm -2 on the surface.The values of the obtained virial isotherm constants are presented in Table 3.
The trend of changes of these values is similar to the above-described isotherms.Noteworthy is the fact that there is a virtual lack of dependences of repulsive interactions in the potential function.

Comparison of Adsorption Properties of Various Surfactants
Comparison studies of adsorption on the mercury electrode of three surfactants: cationic (C11H26N + ) (Table 4), anionic (C10H21SO 3-), [4] and non-ionic (C17H35NO6) made possible the comparative evaluation of their adsorption values.


The greatest potential area of the decreased differential capacity occurs in the case of the cationic surfactant, whereas the narrowest one occurs for the anionic surfactant. [31]


The greatest changes of the zero charge potential, Ez, for comparable concentrations of the surfactant occur in the case of the anionic surfactant.In each case, with the increase of the surfactant concentration the Ez values shift towards less negative potentials.This indicates the adsorption with the alkyl radical directed to the mercury.In the case of ionic surfactants, depending on the electrode charge, there is the probability of the adsorbed ions reorientation. Noteworthy is the stability of maximal adsorption parameters, Emax and σmax irrespectively of the surfactant type.This may confirm the assumption that in the adsorption process the main role is played by the alkyl radical, and not by the functional groups of surfactants.


The highest values of the relative surface excess occur in the case of non-ionic surfactants.For ionic surfactants, the Γ' values are comparable.


The constant changes of the applied adsorption isotherms for each surfactant are different.For the anionic surfactant, the maximal Γ' values correspond to the lowest ΔG° parameter values and the lowest A parameter values. [4]For cationic surfactant, the ΔG° values increase with the increase of the electric charge of the electrode, but at the maximal Γ' they do not reach the maximum value.Values the repulsive interactions, A, between the adsorbed surfactant cations are the weakest.Thus, in the case of ionic surfactants their adsorption is determined by the mutual repulsive interaction.In the case of non-ionic surfactants, the maximal Γ' values are accompanied by the highest ΔG° values and the strongest repulsive interaction.Thus, the adsorption of this surfactant is determined by the adsorption energy, however here is no simple relation between a surface excess and the values of repulsive interactions parameter, A.

Figure 1 .
Figure 1.Differential capacity-potential curves at Hg/1 mol dm -3 NaClO4 for various C17H35NO6 concentrations as in figure legend.

Figure 2 .
Figure 2. Dependences of electrode charge vs. the electrode potential for the studied C17H35NO6 concentrations.

Figure 3 .
Figure 3. Relative surface excess of C17H35NO6 as a function of the electrode potential and C17H35NO6 concentration in the bulk.
On the basis of the obtained results the following conclusions could be drawn:  The changes of Ez values indicate the mechanism of the surfactant adsorption with the alkyl radical -C10H21 directed to the mercury surface. The small value of the surface occupied by one surfactant molecule, S, indicates the perpendicular orientation of its molecules. The highest Γ' values occur in the vicinity of the potential of maximal adsorption Emax They are accompanied by the highest values of the free adsorption energy (in absolute terms) which determine the adsorption size.

Figure 4 .
Figure 4.A linear test of the modified Flory-Huggins isotherm in the system 1 mol dm -3 NaClO4 + C17H35NO6 for different electrode potentials.

Table 1 .
The value of zero charge potential, Ez, and of surface tension measurements at this potential, γz