Study of the influence of the binary mixtures of fluorocarbon surfactants on the surface tension of water

Influence of the binary mixtures of fluorocarbon surfactants Zonyl FSO-100 (FSO100) and Zonyl FSN-100 (FSN100) on the surface tension of the water was studied. The effectiveness of adsorption process of the surfactant at the water-air interface was calculated from the obtained values of the surface tension of studied mixtures. Also the maximum surface area per molecule of surfactant in the interfacial area and the free energy at the water-air interface ∆ were determined. Moreover, the existence of synergetism or antagonism effect, which reduces the surface tension of the water was examined. On the basis of the Rosen's model parameter the values of intermolecular interactions in the mixed monolayer adsorption were calculated.


INTRODUCTION
Surface active agents (surfactants) are the substances which reduce the surface tension of the medium in which they are dissolved, and/or the interfacial tension with other phases, and, accordingly, are positively adsorbed at the liquid/vapour and/or other interfaces.The term surfactant is also applied correctly to sparingly soluble substances, which decrease the surface tension of a liquid by spreading spontaneously over its surface.The surfactants can aggregate in their solutions making micelles.This two processes adsorption and micellization are fundamental for the surfactants [1].
On the other hand, fluorocarbon amphiphile is characterized by unique properties such as stronger surface activity and formation of micelles as well as other self-assemblies at lower concentrations in comparison with the corresponding hydrogenated analogue [1,2].Perfluorocarbon chains are much more hydrophobic than the corresponding hydrocarbon ones and have a pronounced lipophobicity, which is estimated to be about one-third the free energy of transfer of a methylene group from alkane to water [3].Furthermore, the low polarizability of fluorine atoms results in very weak van der Waals interactions between perfluorocarbon chains.The combination of fluororacbon and hydrocarbon surfactants generates unusual and interesting phenomena such as miscibility, phase separation, and compartmentalization for micelles, liposomes, and Langmuir monolayers [4][5][6].Also mixtures of fluorocarbon surfactants are very important due to their high surface activity and chemical stability.Thus the purpose of the presented studies was to determine the influence of the concentration and composition of binary mixtures of two nonionic fluorocarbon surfactants, Zonyl FSN-100 (FSN100) and Zonyl FSO-100 (FSO100), on the surface tension of water at 293K.The interactions between surfactants in the surface layers were also investigated and compared to the properties of single surfactants and their mixtures at the water-air interface.
Zonyl FSN-100 (FSN100) and Zonyl FSO-100 (FSO100) are ethoxylated nonionicfluorosurfactants, which are applied as effective additives to organic polymers as well as solvent-based paints and coatings [7,8].Zonyl FSN-100 by weight of press-ready ink can improve cylinder life by 25-40% depending on the ink and cylinder, and improve the print quality by reducing ''snowflaking'' and streaking of certain inks and keeping ink viscosity [8].Zonyl FSN-100 is also used in cleaning formulations which facilitates the removal of calcium sulphate scale from a reverse osmosis membrane [7].

EXPERIMENTAL
Aqueous solutions of FSN100 and FSO100 mixtures at different molar fractions in the bulk phase, , were prepared using doubly distilled and deionized water (Destamat Bi18E).The surface tension of water was always controlled before the solution preparation.
Surface tension measurements were made at 293 K with a Krüss K9 tensiometer under atmospheric pressure by the ring method.The platinum ring was thoroughly cleaned, and the flame dried before each measurement.In all cases more than 10 successive measurements were carried out, and the standard deviation did not exceed ±0.2 mN/m temperature was controlled within ±0.1 K.

RESULTS AND DISCUSSION
From the values of the surface tension of studied binary mixtures of FSN100 and FSO100 (Fig. 1) as well as the literature data of the surface tension of single surfactants [9], it results that at a concentration in the bulk phase equal to 10 -6 M mixture M c reveals the greatest reduction of the surface tension of water ( = 57.55mN/m) while single surfactant FSO100 the lowest ( = 66.7 mN/m) (Fig. 2).At the concentration equal to 10 -4 M, that is higher than the values of the critical micelle concentration of the studied mixtures (Table 1), it is quite different that is the smallest value of (21.6 mN/m) is observed for the binary mixture of surfactants at the molar fraction FSN100 = 0.2 (M a ), and the greatest one for FSN100 ( = 28.5 mN/m).These differences in values at the same concentrations indicates that the efficiency of the adsorption process at the air-water interface of studied solutions is different.
A convenient measure of the adsorption process efficiency is the concentration of surfactant, which is required to reduce the surface tension of water by 20 mN/m, .The calculated values of para-meter of the studied mixtures and those taken from the literature [9,10,11,12] (Table 1) are arranged in the following order:  As follows the mixture M c (FSN100+FSO100, FSN100 = 0.6) shows the greatest efficiency in the reduction of the surface tension of water, as the value is achieved already at a concentration equal to 1.61•10 -6 M and the lowest efficiency is exhibited by FSO100 ( = 4.74•10 -6 M) [9,10].
Taking into account the obtained values of the surface tension of studied mixtures, it is also possible to determine the effectiveness of adsorption process of the surfactant at the water-air interface on the basis of the values of the surface excess concentration at the surface saturation, , surface excess concentration of surfactants at the water-air interface can be determined on the basis of the adsorption isotherms using the Gibbs equation [5,9,12,13].
For the dilute solution (10 -2 mol/dm 3 or less) containing a nonionic surfactant, the Gibbs equation can be written in the form: where is the concentration of surfactant and its surface tension.The concentration of each surfactant mixture at the interface at the surface saturation can be calculated from the slope of -plot.Assuming the monolayer adsorption it can be stated that the amount of adsorbed substance is inversely proportional to the surface area per molecule of surfactant in the interfacial area, which may be expressed by the equation: where is expressed in µmol/m 2 .Table 1 shows that the values are arranged in the following order: From the comparison of the values of (Table 1), it results that the binary mixture M a exhibits the highest effectiveness in reducing the surface tension of water, because it has the highest value thus the value of is the smallest.Knowing the values of the free energy, ∆ , at the water-air interface can be determined.∆ is regarded as the work needed to make an interface per mole or the free energy change accompanied by the transition from the bulk phase to the surface phase of the solution components and is defined as [10,11]: where !"! is the surface tension of the solution at the CMC.
In other words, the lower the value of ∆ , the more thermosdynamically stable surface is formed.It follows from Table 1 that the mixture M b formed the surface that is the most thermodynamically stable.
The comparison of the calculated values of ∆ and for the studied solutions and those for the single surfactants, FSN100 and FSO100 [9,12] indicates synergism in the surface tension reduction efficiency for binary mixtures.The first step to establish and explain this effect is to calculate the composition of the mixed monolayer adsorption, and on its basis the parameters of intermolecular interactions.There are many theories, which allow to determine the parameters of intermolecular interactions in the monolayer adsorption consisting of two different surfactants.One of the frequently used is the Rubingh and Rosen theory [1].This theory introduces the following relationships: where is the mole fraction of surfactant 1 in the surface mixture in solution, $ % is the mole fraction of surfactant 1 in the mixed monolayer (1-FSN100).The values % , , % are molar concentration of surfactants 1 and 2, and their mixture in solution respectively, which are necessary to reduce the surface tension of the given value; -. is a parameter of intermolecular interactions in the mixed monolayer at the solution-air interface, the subscripts 11, 22 and 12 refer to interactions between like molecules (11,22) and dissimilar molecules (12).
Using this theory the mole fractions of the surfactant in the mixed adsorption monolayer at the water-air interface were calculated, $ % and the intermolecular interaction parameter, -. for the examined mixtures with = 60.55,52.8, 50,45,40,35, and 30 mN/m and they are presented in Fig. 3 and 4. It follows from Fig. 3 that the molar fraction of the fluorocarbon surfactant FSN100 in a mixed adsorption monolayer for all tested mixtures decreases with the decrease of the values of the surface tension of the solution.The series of parameters describing efficiency and effectiveness of the adsorption process does not overlap.These differences can be probably explained on the basis of geometric shapes of surfactant particles, size of the hybridized hydrophilic groups and that of electrostatic interactions or other interactions between these groups.Comparing the values of | -. | and |ln( % / *| it can be stated that all mixtures except from M c (at low concentrations), exhibit a synergistic effect in reducing the surface tension of water, throughout the changes of the values of surface tension.

CONCLUSIONS
The measurements of surface tension of aqueous solutions of binary mixtures of the fluorocarbon surfactants, FSN100 and FSO100, and analysis of their values allow to draw the conclusion that the surface tension of aqueous solutions of the studied mixtures depends on both the composition and total concentration.Also at a concentration at which the surface tension of water is reduced by 20 mN/m, from the tested systems of binary mixture, M c has the highest effectivness in reducing the surface tension of water ( =1.61x10 -6 ), while FSO100 has the lowest efficiency in the process ( =3.74x10 -6 ).The molar fractions of the mixture of surfactants in a mixed adsorption monolayer are determined on the basis of surface tension and Rosen's model for all tested mixtures with the decreasing surface tension.Moreover the parameter values of the intermolecular interactions in the mixed adsorption monolayer calculated from Rosen's model and specific conditions of synergetizm or antagonism occurrence to reduce the surface tension of the water show that a mixture of M c throughout values of surface tension shows a synergistic effect in reducing the surface tension of water.

Fig. 2 .
Fig. 2. The relationship of the values of of the binary mixtures of fluorocarbon surfactants FSO100 and FSN100 versus log

Fig. 3 .
Fig. 3.The relationship of the values of of the binary mixtures of fluorocarbon surfactants FSO100 and FSN100 at their concentrations: 10 -4 and 10 -6 mol/dm 3 .

Fig. 4 .
Fig. 4. The relationship of the molar fraction, $ % , FSN100 in the mixed adsorption monolayer at the water-air interface mixtures formed from the solution, M a , M b , M c , M d versus surface tension of the solution.

Fig. 5 .
Fig. 5.The relationship of -. parameter of the aqueous solutions of the mixtures M a , M b , M c , M d versus surface tension of the solution.

Fig. 4
shows the values of intermolecular interactions in mixed monolayers of adsorption -. .They are negative and like the values of $ % change with the decrease of the mixture surface tension.The negative values of -. parameter, the lowest in the case of the mixture M d , indicate a synergistic effect of the studied mixtures in the reduction of the water surface tension.However, in accordance with the Rubingha and Rosen theory -. value is necessary in the presence of synergism or antagonism in reducing the surface tension of water.In the case of a mixture of two surfactants, synergism in the reduction of occurs if the surface tension of an aqueous solution of the surfactant is obtained at the lower mixture concentration that of any of the surfactants and when the | -. |>|ln( % / *| is accomplished.|ln( % / *| for 60 mN/m is equal 0.74; for 55 mN/m is 0.69; 52,8 mN/m = 0.59; |ln(C % /C *| for 45 mN/m = 0.39; for 40 mN/m is equal 0.26; 35 mN/m = 0.03 and |ln( % / *| for 30 mN/m is 0.36.

Table 1 .
Values of the calculated parameters , , ,, ∆for the mixtures under investigations.