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Section 9. Chemistry
https://doi.org/10.29013/ESR-21-5.6-69-74
Maryaskin Yuriy, Sc. D, Assistant Professor, Dnepr Chemical-Engineering University, Ukraine E-mail: iryna190856@gmail.com Stanislav Derman, Sc. M Chemistry Development Leader, Addionics, Israel E-mail: stanislav.derman@gmail.com
INFLUENCE OF INTERNAL PROCESSES IN AQUEOUS SOLUTIONS OF SURFACTANTS ON THEIR TECHNOLOGICAL PROPERTIES
Abstract. It is known that the ability of surfactants to remove impurity is associated with their adsorption activity at the corresponding phase boundaries.
In this work, it is suggested that the achievement of conditions conducive to an effective purification process is realized under the influence of adsorption and micelle formation processes, which simultaneously occur in aqueous surfactant solutions, when a certain concentration of the latter is reached.
A number of experimental data are presented that confirm this assumption.
Keywords: adsorption, micelle, surface-active agent, critical micelle formation concentration, surface tension, micelle formation, adsorption capability.
Detailed study of the processes occurring in of the contact angle is achieved and the subsequent
aqueous solutions surface-active agents (surfactants) removal of contamination (oil) due to the sedimen-
is very important for predicting their technological tation force takes place [4]. As defined in [2], the
properties, such as their detergent. realization of such a state, like other technological
In work[l], the possibility ofcorrelation between properties (emulsification, dispersion, solubiliza-
the processes of adsorption and micelle formation, tion), depends on the ratio of water- and oil-soluble
occurring in aqueous solutions of surfactants. The parts of the surfactant, that is, their hydrophilic-lipo-
suggestion was made that such a relationship can af- philic balance. Also worthy of note are works [5; 6],
fect the technological properties of surfactant solu- which relate the surfactant adsorption at oxidized
tions and, in particular, their effect of washing. solid (for instance, cleanable) surfaces with the ion-
According to [2], the ability of aqueous surfac- ization potentials of the adsorbed surfactant. tant solutions to eliminate contamination is associ- In our view, except for the specified factors, ac-
ated with their adsorption activity at the correspond- count should be taken of the circumstance that the
ing interface. Consequently, as follows from the washing process is carried out in a surfactant solution,
well-known Young's equation [3], the required value where adsorption and micelle formation processes
occur simultaneously. As shown in [1], this affects such features as the amount of surfactants in the surface layer and the surface tension at the interface, which, therefore, forms the thermodynamic conditions necessary to eliminate contaminants from the surface to be cleaned.
It can be assumed that the simultaneous occurrence of adsorption and micelle formation impacts not only on thermodynamics, but also the kinetics of technological processes in surfactant solutions. Let us assume that, as a result of numerous collisions with each other, large aggregates (micelles), and other components of the solution, some surfactants acquire a certain storage of energy. Hence, they will not just accumulate at the interface, but also actively influence the state of the latter, thereby destroying the continuous film of contamination and accelerating the transition of the oil droplet to the equilibrium value of the contact angle. The number of such collisions depends on the nature and concentration of the surfactant, the number, size, and type of micelles.
Taking into account the kinetic factor seems to be important, since all technological processes have strict time limits. This explains the use of external factors increasing their speed. Such factors, leading to an increase in the energy storage in surfactants, include, for example, stirring the washing solution and increasing its temperature. This can also be associated with the use of an electric field and ultraviolet radiation in the cleaning process (in addition to the effects of gas release, cavitation, etc.).
The suggested approach is identical to one of the basic laws of chemical kinetics, according to which only the so-called "active" molecules with a sufficient supply of energy participate in the chemical process [7]. The same approach is used when describing diffusion, where particles must have enough energy to move in a stationary medium [7].
It follows from the above considerations that washing solutions having similar adsorption characteristics, but differing in the energy storage of the
surfactant, can provide different cleaning rates, that is, differ in their effectiveness.
The quantitative assessment of the effectiveness of washing solutions is carried out mainly by two methods: as regards the mass ofremoved oils to their initial amount (effect ofwashing, EW) [8] or in terms of the part of the surface cleaned in a given time (active surface, Sact ) [9]. The second method seems to be preferable, since the main purpose of cleaning is precisely to free the surface from contamination, to carry out further technological operations on it (painting, applying a phosphate film or electroplating, etc.).
It is known from the chemical kinetics that the process rate (W) is defined as the change in the amount of the reactant for a certain period of time [7]. By analogy, the decontamination rate can be calculated as the change in the part of the cleaned surface, in a given time (t):
W = dS /dT (1)
act
From the dependency graph of Sact on t, the process rate is determined by the tangent of the angle of inclination between the segments characterizing the change in the part of the cleaned surface for the corresponding time interval [7].
According to the law of mass action, the rate of a chemical process depends on the amount of reac-tants [7]. By analogy, the rate of the cleaning process should depend on the size of the cleaned surface:
W = K(Sact) n (2)
where K is the process rate constant (a value dependent only on temperature), n is the order of the process.
In chemical kinetics, to describe activity of molecules, such feature as the activation energy (E) is used, that is, the additional energy that particles need to participate in the process [7]. According to [7], the value of E can be calculated using the following equation:
ln (K2/K1) = E(T2 - Ti)/RTiT2 (3)
where K2 and K1 are reaction rate constants at temperatures T1 and T2, respectively, R is the gas constant.
Assuming that the rate of the washing process obeys the laws of chemical kinetics, one can use the equations given earlier to characterize the effect of
surfactants on the treatment efficiency. For instance, having data on the change in Sact dependent on time, obtained in a solution of the surfactant under study at temperatures T1 and T2, it is possible to calculate the cleaning rate (W1 and W2) corresponding to the same value of Sact, using (l).
Further, using (2), we obtain the ratio of the rate constants at two temperatures:
W2/W1 = K2/K1 (4)
The obtained value of K2/K1 is substituted into (3) to determine the activation energy.
However, it should be noted that the washing process takes place in multicomponent systems containing, in addition to surfactants, a solvent (e.g., water) and, if necessary, a number of inorganic compounds. An aqueous solution of inorganic substances cannot be generally considered as an inert medium in which surfactants are dissolved. For example, in the case of degreasing metal surfaces, one should take into account the processes occurring on the surface of the substrate to be cleaned when it comes into contact with inorganic components of the solution.
In this connection, it is known that metal surfaces undergo oxidation when exposed to water and solu-
tions of inorganic electrolytes [10; 11]. We also note the possibility of particular adsorption of ions of inorganic compounds on the substrate (for instance, OH-, Cl-, PO43) [12].
The electrostatic factor associated with the presence of an electric double layer at the metal-solution interface should also be taken into account. According to [12], in some cases this contributes to the removal of organic oils from the substrate surface.
In addition, we note the possibility of direct interaction of inorganic compounds with components of contamination (for example, saponification of fats in alkaline solutions).
The listed spontaneous processes are accompanied by a decrease in free surface energy at the substrate-solution interface, which creates the prerequisites for removing contaminants.
Therefore, despite the fact that water and aqueous solutions of inorganic compounds do not exhibit surface activity at the oil-solution interface, they, along with organic surfactants, can have a certain detergency effectiveness. This is confirmed by the data shown in (Figure 1).
30 25
sp 20 H
I 15 10 5
3 5
t, min
10
Figure 1. Dependence of the steel cleanliness level on the time of degreasing in inorganic solutions (mod. Contamination - paraffin, temperature 333 K) Series 1 - H2O; Series 2 - NaOH (0.1M); Series 3 - NaCl (0.1M) Consequently, the degree of purification achieved al action of all components. And it is incorrect to relate in an aqueous surfactant solution results from the mutu- the activation energy value obtained according to (3)
0
0
1
8
only to the surfactant additive. Nevertheless, taking into account that water has a relatively low cleaning ability, it is possible, as a first approximation, to apply (3) to analyze the detergent properties of various surfactants.
To illustrate this effect, we have chosen two molecular surfactants having the general formula CH2n+lO(C2H4O)mH: synthanol DS-10 (n = 10-18, m" = 8-10) and 0m-20 (n = 14-18, m = 20). Furthermore, we followed the fact that these surfactants have almost the same effect on the state of the oil-solution, metal-oil, and metal-solution interfaces.
The state of the oil-solution interface was studied by the decrease in interfacial tension as a result of adsorption (the stalagmometry method was applied).
To study the effect of a surfactant on the state of the substrate-oil interface, weighed portions of the surfactant were dissolved in oil. Then, a water drop was applied to the steel substrate, the sample was immersed in a glass cell filled with sample contamination, and the contact angles (0) were measured in the steel-water-oil system.
The data on the effect of surfactants on the state of the substrate-solution interface were obtained by measuring the contact angles in the three-phase steel-solution-air system. Taking into account that the free surface energy of the metal-air interface is constant, with reference to Young's equation, we can write:
s1Cos01 - s0Cos60 = DGmet/So, (5)
where s0, s1 are the surface tension of water and surfactant solution, respectively; 0O, 01 are the contact angles in water and surfactant solutions; D G is
c ' met/sol
the change in the surface free energy during the adsorption of surfactants at the metal-solution interface.
As evidenced by the data given in Table 1, micel-lar solutions ofthe selected surfactants almost equally reduce the interfacial tension at the oil-solution interface. In addition, they are not adsorbed from the oil on the substrate surface (the contact angle changes insignificantly after the introduction of these surfactants into the oil); their adsorption from an aqueous solution on the substrate surface has almost the same effect on the interface energy state.
Table 1. - Values of parameters characterizing the adsorption of aqueous solutions of the surfactants under study at different interfaces
Item grnet/so^^J/m2 0 DGmet/So,*1«3,J/mol
Water 51.1 - 0
Oil (decane) - 66.60 -
Synthanol DS-10 9.3 67.50 -14.3
0S-20 10.8 67.20 -14.5
Moreover, as evidenced by the data shown in (Figure 2) (a and b), the purification efficiency in solutions of these surfactants (achieved within the same time frame) differs significantly.
Calculations carried out using equations (1-4) showed that in the case of synthanol DS-10, the value of activation energy is approximately 35 kJ/mol, and when using 0S-20-109 kJ/mol, i.e., molecules of the first surfactant have a larger energy level. According to the previously stated considerations, this probably explains a higher rate of purification in a solution containing synthanol DS-10.
The data given in [1] demonstrate that the adsorption of molecular surfactants from their mi-cellar solutions is often accompanied not by the heat release, as it follows from the condition of a spontaneous process, but by the heat absorption. It can be assumed that the effective impact of synthanol DS-10 on the washing process results from the fact that the composition of this technical agent includes C10H210(C2H40)10H,
C12H250(C2H40)8H, C14H290(C2H40)10H
tions, the adsorption of which proceeds exother-mically, with the release of some supply of energy.
It is likely that this energy contributes to the intensification of the removal of contaminants from the substrate surface due to an increase in the energy
storage of surfactant molecules. Let us stipulate that the stated hypothesis needs further experimental verification.
80 70 60 50 40 30 20 10 0
Series 1 Series2
5
t, min
10
a)
100
80
60
40
20
Series1 Series2
5
t, min
10
6)
Figure 2. Dependence of the steel cleanliness level in aqueous surfactant solutions on time (contamination - paraffin; surfactant concentration - 2.5 g/l; a) - temperature 333K; b) - 353K): series 1 - OS-20; series 2 - synthanol DS-10
It is also worth mentioning that the effectiveness (a mixture of I-50 industrial oil and solid oil). An
of the surfactant in the cleaning solution depends on increase in temperature has little effect on the clean-
the type of contamination to be removed. As follows ing efficiency. Probably, the energy storage of this
from (Figure 3), an aqueous solution of synthanol surfactant molecules is not sufficient to destroy the
DS-10 has a low detergency effectiveness when re- oil layer. moving sample contamination from the steel surface
0
3
7
0
0
3
7
16 14 12 10
so OS
5 8
1X1
6 4 2 0
0 2 T, min 5 10
Figure 3. Dependence of the effect of washing of an aqueous solution of syntanol DS-10 on time (surfactant concentration - 2.5 g/l; the composition of the sample contamination (% vol.): solid oil - 4, industrial oil I-50-96) series 1-333K, series 2-353K
As can be seen above, the joint occurrence of ad- process, which should be taken into account when sorption and micelle formation in aqueous solutions developing new cleaning compositions. of surfactants affects the efficiency of the washing
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Series 2 Series 2
