CC BY
0
ISSN 2310-5577
r
ppublishing.org
PREMIER
Publishing
Section 3. Chemistry
DOI:10.29013/ESR-24-5.6-30-33
THE INFLUENCE OF CERTAIN FUNCTIONAL GROUPS ON ADSORPTION PROCESSES AND MICELLE FORMATION IN AQUEOUS SOLUTIONS OF SURFACTANTS
Derman S.1, Maryaskin Y. B. 2, Maslak H. 2, Netronina O. 2
1 Senior Electrochemical Technologies Researcher, Addionics, Israel
2 Biochemistry and Medical Chemistry Department, Dnipro State Medical University, Ukraine
Cite: Derman S, Maryaskin Y. B., Maslak H, Netronina O. (2024). The Influence of Certain Functional Groups on Adsorption Processes and Micelle Formation in Aqueous Solutions of Surfactants. European Science Review 2024, No 5-6. https://doi.org/10.29013/ESR-24-5.6-30-33
Abstract
This work investigates the processes of adsorption and micelle formation in aqueous solutions of surfactants (SAS), which determine their technological properties. Through theoretical analysis based on a method proposed in the literature, the contribution of various functional groups to the change in Gibbs free energy of these processes was assessed. The results show that the change in free energy during micelle formation can be represented as the sum of changes in energy of the hydrophilic and hydrophobic components of the SAS molecules. A linear dependence between the change in free energy and the number of hydrocarbon groups was revealed, allowing the calculation of the energetic contribution of a single hydrocarbon group. The analysis showed no direct dependence between the number of hydrophilic groups and the change in free energy, indicating that only a certain portion of these groups integrate into the micelles. The study also emphasizes the difference in the technological properties of solutions when using anionic surfactants and offers a new understanding of the thermodynamic aspects of adsorption and micelle formation processes in aqueous solutions of molecular SAS. Keywords: surfactants, adsorption, micelle formation, Gibbs free energy, functional groups, hydrophilic and hydrophobic components, technological properties
Introduction
Within the framework of studying aqueous solutions of surfactants (SAS), in which the key roles are played by the processes of
adsorption and micelle formation, determining their technological properties, an analytical work aimed at evaluating the contribution of functional groups to the reduction of
Gibbs free energy in these processes was conducted. The analysis is based on a method mentioned in sources (Lin I. J., Marszall L., 1978; Maryaskin Y., Derman S., 2019), which allows for a first approximation to represent the change in Gibbs free energy during micelle formation as the sum of changes in energy for the hydrophilic and hydrophobic components of the SAS molecules:
AG0 = AG0' P + AG0' R , (1)
m m m7 v y
where AG0m denotes the change in Gibbs free energy upon forming micelles from one mole of SAS, AG0, P and AG0, R respectively reflect
mm
the changes in Gibbs energy for the hydro-philic and hydrophobic parts of the molecules.
Research Part
In the context of the homologous series of SAS, the value of AG0, P is considered a constant, while AG0, R can be calculated as the change in free energy upon integrating one hydrocarbon group into the micelle (AGCH2m), considering the number of such groups in the molecule (n) and Avogadro's number (NA).
From (1):
AG0 = const + nAGCH2 N , (2)
m ma7 v y
From (2), it follows that in the coordinates AG0m - f(n) a linear dependence should be observed, where the slope of the line allows for the calculation of the change in free energy upon transitioning one hydrocarbon group into the micelle, and the segment cut off on the ordinate axis corresponds to the value of AG0, P
m.
Table 1 presents the calculated values of AGO, Pm and AG0, R for a series of molecular SAS, using AG0m values from (Lin I. J., 1972).
The presented data indicate that all values of AGCH2m are approximately the same (the average value of this magnitude is -5.64*10A-- 21 J). This value of AGCH2 was used in calculating AG0, Rm for the studied SAS.
It should be noted that our value of AGCH2
m
differs from those presented in (Maryaskin Y., Derman S., 2019; Maryaskin, Y.B., Dani-lov, F.I., 2004) for other classes of surfactant compounds. Likely, the nature of the hydro-philic parts of SAS affects the magnitude of this parameter.
Regarding the values of AG0, Pm, there is no correlation between the number of hydro-philic groups and the change in the system's
free energy. Likely, not all hydrophilic groups are integrated into the micelles, but only a certain part that ensures the maximum reduction of contact between the hydrocarbon radical and water. This limitation seems reasonable since the inclusion of hydrophilic groups in micelles is accompanied by an increase in Gibbs energy. Thus, the spontaneity of the micelle formation process in solutions of molecular SAS is conditioned by the fact that the absolute value of AG0, Rm is greater than AG0' P . m
m
It should also be noted that, unlike the solutions mentioned, the use of anionic SAS is accompanied by a decrease in the value of AG0, Pm. Possibly, this is one of the reasons for the difference in the technological properties of these solutions.
The assumption that the properties of SAS molecules are composed of the properties of their hydrophilic and hydrophobic parts can also be applied to the adsorption process. In this case:
AG0 = AG0, P + AG0, R, (3)
a a a7 v y
where AG0a - is the change in free energy upon adsorbing one mole of SAS; AG0, Pa and AG0, Ra - are the changes in free energy upon adsorbing the hydrophilic and hydrophobic parts of SAS, respectively.
For the homologous series of SAS, AG0, Ra consists of the change in Gibbs free energy upon adsorbing one -CH2- group of the hydrocarbon chain, considering their number in the molecule (n) and the number of molecules in one g-mole of adsorbate, and AG0, Pa is a constant magnitude:
AG0 = const + nAGCH2 N.. (4)
a a A v J
Values for the components of equation (4) are provided in Table 2. The calculations used of AG0a values from (Lin I. J., 1972).
From this table, it is evident that all values AGCH2a are roughly the same (the average value of this magnitude is -4.19*10A-- 21 J). This value AGCH2 was used in calculating AG0, Ra for the studied SAS.
Regarding the magnitude of AG0, Pa - it is little dependent on the number of oxyeth-ylene groups in the molecules of SAS. It can be assumed that approximately the same hy-drophilic part of SAS molecules is placed at the phase boundary in all cases.
It is also worth noting that, unlike micelle formation, the adsorption of hydrophilic
groups of the studied SAS molecules is accompanied by a reduction in Gibbs free energy. This fact is what makes the adsorption process more thermodynamically favorable than micelle formation from a thermodynamic point of view.
Furthermore, it was shown in (Maslak, H. S., Maryaskin, 2023) that at the critical micelle concentration, the number of molecules that formed micelles is greater than those absorbed on the surface. There, it was also suggested that the presence of micelles in the solution volume might act as a barrier, hindering the delivery of molecules to the phase boundary, which affects the technological properties of SAS solutions.
Conclusions
Thus, through the conducted study, the contribution of functional groups to the change in Gibbs free energy during adsorption and micelle formation in aqueous solutions of surfactants was assessed.
The analysis demonstrated that the change in free energy during micelle forma-
tion can be represented as the sum of changes in energy for the hydrophilic and hydro-phobic parts of SAS molecules. The obtained data indicate a linear dependency between the change in free energy and the number of hydrocarbon groups in the molecule, confirming the significance of hydrophobic interactions in the process of micelle formation.
It was also established that the magnitude of the change in free energy for hydro-philic groups does not show a direct correlation with their number, indicating the selective inclusion of these groups into micelles. The results emphasize the thermodynamic advantage of adsorption compared to micelle formation and suggest that the presence of micelles in the solution might affect the technological properties of SAS by limiting the delivery of molecules to the phase boundary.
Thus, the study provides new insights into the thermodynamic aspects of SAS behavior in aqueous solutions, important for understanding and optimizing their technological applications.
Tablel. Values of AG CH2 , AG 0 R and AG 0 P for molecular surfactants (T=298)
J m m m J j K s
Surfactant Dependency equation AGCH2 *1021 m ' AG0- R *10-3, m AG0- P *10-3, m
formula AG°m- f(n) J J/mol J/mol
C8H170(C2H40)3H -27.16
C^H^O^H^H AG0 = 1850 - 3210n m - 5.33 -33.95 1.85
C12H25°(C2H4°)3H -40.74
C8H170(C2H40)ôH -27.16
C10H21°(C2H4°)6H AG0 = 6960 - 3510n m -5.83 -33.95 6.96
C12H250(C2H40)ôH -40.74
C8H170(C2H40)8H -27.16
C12H250(C2H40)8H AG0 = 7569.3 - 3509.6n m -5.83 -40.74 7.57
C14H290(C2H40)8H -47.53
C8H170(C2H40)10H -27.16
C10H210(C2H40)10H C12H250(C2H40)10H AG0 = 6060 - 3280n m -5.45 -33.95 -40.74 6.06
C14H290(C2H40)10H -47.53
C8H170(C2H40)12H -27.16
C10H210(C2H40)12H C12H250(C2H40)12H AG0 = 6933 - 3375.5n m -5.61 -33.95 -40.74 6.93
C14H290(C2H40)12H -47.53
C8H170(C2H40)14H -27.16
C12H250(C2H40)14H AG0 = 9280 - 3480n m -5.78 - 40.74 9.28
C14H290(C2H40)14H - 47.53
Table 2. Values of AGCH2 , AG0,R and AG0,P for molecular surfactants (T=298)
Surfactant formula
C8H170(C2H40)3H
C10H21O(C2H4O)3H
C^O^O^H
C8H170(C2H40)ôH
C10H21O(C2H4O)ôH
C12H250(C2H40)ôH
C8H170(C2H40)8H
C12H250(C2H40)8H
C14H290(C2H40)8H
C8H170(C2H40)10H
C10H210(C2H40)10H
C12H250(C2H40)10H
C14H29O(C2H4O)10H
C8H170(C2H40)12H
C10H210(C2H40)12H
C12H250(C2H40)12H
C14H290(C2H40)12H
C8H170(C2H40)14H C12H250(C2H40)14H
C14H290(C2H40)14H
Dependency equation AGCH2a*1021,
AG0 — f(n)
AG0 = -21547-2375n
a
AG0 = -20258-2437.5n
a
AG0 = -15371-2854.6 n
AG0 =-20149-2546n
AG0 = -18716-2591.5n
AG0 =-21730-2325n
J
-3.94
-4.05
4.74
4.23
4.30
-3.86
AG0- R *10-
a
J/mol
-20.18 -25.22 -30.27 -20.18 -30.27 -35.31 -20.18 -30.27 -35.31 -20.18 -25.22 -30.27 -35.31 -20.18 -25.22 -30.27 -35.31 -20.18 -30.27 -35.31
AG0- P *10-3.
a
J/mol
-21.55
-20.26
15.37
20.15
18.72
-21.73
References
Lin I. J., Marszall L. "Partition of HLB Coefficient and Effective Chain Length of Surface-Active Agents." Progress in Colloid and Polymer Science,- Vol. 63. 1978.- P. 99-104.
Maryaskin Y., Derman S. "Relation of Surface-Active Agents Solutions Surface and Volume Properties." European Science Review,- Vol. 1.- No. 11-12. 2019.- P. 136-140.
Lin I. J. "The Hydrophile-Lipophile Balance (HLB) of Fluorocarbon Surfactants and Its Relation to the Critical Micelle Concentration (CMC)." The Journal of Physical Chemistry,-Vol. 76.- No. 14. 1972.- P. 2019-2023.
Maryaskin, Y. B., Danilov, F. I. "The Relationship Between Colloidal-Chemical Properties and the Hydrophilic-Lipophilic Balance of Surface-Active Substances." Questions of Chemistry and Chemical Technology,- No. 3. 2004.- P. 136-140.
Maslak, H. S., Maryaskin, Y. B., Derman, S. "Influence of Micellar Structures on the Adsorption Capacity of Surfactants." European Science Review,- No. 9-10. 2023.
Israelachvili, J. N. "Intermolecular and Surface Forces." Academic Press, 2011.
Rosen, M. J., Kunjappu, J. T. "Surfactants and Interfacial Phenomena." Wiley, 2012.
Myers, D. "Surfactant Science and Technology." Wiley-VCH, 2005.
Volkov, V. A. "The Influence of Molecular Structure on Micelle Formation in Solutions of Surface-Active Substances." Colloid Journal,- Vol. XXXVII.- No. 5. 1975.- P. 845-852.
submitted 17.07.2024;
accepted for publication 12.06.2024;
published 26.06.2024
© Derman S., Maryaskin Y. B., Maslak H., Netronina O.
Contact: stanislav.derman@gmail.com
