Molecular Interactions in Binary Surfactant Solutions: Effect of Ph

https://doi.org/10.3311/PPch.13975 Creative Commons Attribution b |437 Periodica Polytechnica Chemical Engineering, 64(4), pp. 437–445, 2020 Molecular Interactions in Binary Surfactant Solutions: Effect of pH Olga Kochkodan1*, Victor Maksin2, Nadiya Antraptseva1, Tetyana Semenenko1 1 Department of General, Organic and Physical Chemistry, National University of Life and Environmental Sciences of Ukraine, Geroiv Oborony Str. 15, 03041, Kyiv, Ukraine 2 Department of Analytical and Bioinorganic Chemistry and Water Quality, National University of Life and Environmental Sciences of Ukraine, Geroiv Oborony Str. 15, 03041, Kyiv, Ukraine * Corresponding author, e-mail: [email protected] Received: 03 March 2019, Accepted: 07 June 2019, Published online: 25 July 2019 Abstract By using surface tension and conductivity measurements, the colloid-chemical properties of the mixtures of cationic hexadecylpyridinium bromide with nonionic Triton X-100 surfactants were investigated both in the bulk solution and at air/solution interface at different pH values. The composition of mixed micelles and adsorption layers, parameters of molecular interactions in mixed micelles βm and σ 0 0 adsorption layers β , as well as standard free energies of micelle formation ΔG mic and adsorption ΔG ads were calculated. It was found that molecules of the nonionic surfactant presumably dominate in the mixed micelles and adsorption layers. It was shown that βm and βσ have negative values, which indicate the strengthening of intermolecular interactions in the mixed micelles and adsorption layers. Based on the data obtained, it was suggested that ion-dipole interactions are involved in the formation of intermolecular structures between nonionic and cationic surfactants in aqueous solution and at the air-solution interface. It was shown that βm, βσ as 0 0 well as ΔG mic and ΔG ads parameter depends on the solution pH value. The complex interplay of ion-dipole, protonation and chelation processes, which occur in the surfactant mixtures at different pH and affect the strength of intermolecular interaction, should be taken into account for data analysis. Keywords intermolecular interactions, surfactants mixtures, adsorption, Triton X-100, hexadecylpyridinium bromide 1 Introduction Due to surface tension reduction and micelle formation prop- One of the most widely accepted and used to study the erties, surfactants are used as stabilizers, emulsifiers and nonideal intermolecular interactions in surfactant mix- foam forming agents in numerous industrial and domestic tures, is the Rubin-Rosen model, which is based on the the- applications, including mineral flotation, oil recovery, sur- ory of regular solutions [2, 8, 9]. In this model the intermo- face coating, wetting, detergency, synthesis of nanoparticles, lecular interactions between the surfactants molecules at catalysis, cosmetic and food formulations [1, 2]. Usually, the interfaces or at micelle formation in the solution are eval- mixtures of the surfactants are employed in these processes. uated by using molecular interaction β parameters, which To that end different surfactants are mixed deliberately to can be estimated from surface tension (βσ) or critical micelle optimize their formulations and performance by using syn- concentration (βm) data [10]. ergetic or antagonistic interactions between the components The effect of surfactant type, molecular structure, of the mixture [1-4]. Therefore, understanding the main length of hydrophobic/hydrophilic chains of the surfac- features of surfactants interactions in the mixed solutions tants and the concentration ratios between the components and at interfaces is of vital importance for prediction of the in the mixture on molecular interactions in surfactant sys- properties and designing the surfactant systems with opti- tems has been a rich field of research [8-22]. It was found mal performance for specific application. that the mixtures of structurally homologous surfactants Different theoretical models were suggested to describe usually behave similar to ideal solutions [1, 12, 13], while properties and interactions in the surfactant systems [5-7]. the mixtures of structurally different compounds such as Cite this article as: Kochkodan, O., Maksin, V., Antraptseva, N., Semenenko, T. ″Molecular Interactions in Binary Surfactant Solutions: Effect of pH″, Periodica Polytechnica Chemical Engineering, 64(4), pp. 437–445, 2020. https://doi.org/10.3311/PPch.13975 Kochkodan et al. 438|Period. Polytech. Chem. Eng., 64(4), pp. 437–445, 2020 ionic and nonionic surfactants often show nonideal behavior 2 Materials and methods [8-10, 13-22]. In many cases due to a complex interplay of Cationic surfactant HDPBr of the general for- intermolecular forces between the components, the compo- mula С16Н33NС5Н5Вг and nonionic oxyethyl- sition of the mixed micelles and mixed adsorbed layer at the ated octylphenol TX100 of the molecular formula air/solution interface is notably differ compared to the com- С8Н17С6Н4О(СН 2СН2O)nH with the degree of oxyethyla- position of the bulk solution [1, 2, 8-10, 17-21]. tion n = 9-10, were used in the experiments. The surfac- Though different parameters, which affect molecular tants were purchased from Sigma-Aldrich (USA). TX100 interactions in the surfactants mixtures, have been widely was used without additional purification, while HDPBr investigated, there are only a few studies related to the was purified by recrystallization from methylethylke- influence of solution pH on micelle formation in multicom- tone before the experiments. The degree of purification ponent aqueous mixtures and to composition of the mixed was controlled by the absence of minima at the isotherms surfactant layers at the air/solution interface [23-25]. of surface tension close to critical micelle concentration Rosen and Zhao [23] evaluated the molecu- (CMC) region. The surfactants solutions were prepared lar interaction parameters for mixtures of nonionic with ultrapure water (Milli-Q Plus, Millipore). The chem- C12H25(OC2H4)4OH(C12EO4) and C12H25(OC2H4)8OH(C12EO8) ical structures of the used surfactants are shown in Fig. 1. surfactants with anionic sodium alkylsulphates and sodium Tensiometric and conductometric methods were used alkanesulphonates at different concentration and pH of the to study the micelle formation in the bulk solution and solutions. It was found that interaction of polyethylenated the interface adsorption from single and mixed surfac- nonionic surfactants is stronger with anionic surfactants tant solutions. Surface tension (σ) in the solutions was than with cationic compounds with the same alkyl hydro- determined by Wilhelmy method by balancing a platinum phobic group. It was shown that the β interaction parame- plate [26] using the tensiometer BT-500 (Analytprilad, ter slighly increase with increasing solution pH from 3.1 to Ukraine). The measurements were conducted three times 10.1 for the anionic/nonionic system due to a week cationic for each solution and the average value was reported. The charge of the polyoxyethylene chains [23]. In contrast, it measurement error was ± 0.5 mJ m−2. Before measuring was reported later that pH of the mixed solution did not the surface tension, the surfactants solutions were kept in notably affect molecular interaction parameters for anionic the sealed flasks for 24 h. C12SO3Na or C12H25(OC2H4)2SO4Na mixtures with nonionic The specific conductivity (k) of the surfactant solutions surfactants [24]. was measured by a L-Micro conductivity meter (Chemlab, Goloub et al. [25] studied the micelle formation in Ukraine). The volume of the solution was 15 mL, the stan- amphoteric dodecyldimethylamine oxide/anionic sodium dard measurement deviation was ± 2%. dodecyl sulfate and dodecyldimethylamine oxide/nonionic The mole fraction of TX100 in the mixed surfactant hexa(ethyleneglycol) mono-n-dodecyl ether mixtures at solutions was calculated as: different pH. Strong interactions between the surfactants C 100 α = TX were found for the amphoteric/anionic mixtures at pH 8 TX100 (1) CCHDPBr + TX100 when dodecyldimethylamine oxide was uncharged. On the other hand, almost ideal surfactants’ behavior was observed where αTX100 is the TX100 mole fraction in the mixed solu- at pH 8 for the amphoteric/nonionic mixture. The attractive tion, while CHDPBr and CTX100 are the concentrations of interactions were shown at pH 2 when the amphoteric sur- TX100 and HDPB in the surfactant mixture. factant exists in its cationic form in the solution. The CMC value in the surfactant solutions was eval- This study investigates the surface active properties of uated by measuring and plotting the surface tension (σ) the nonionic/cationic mixtures of Triton X-100 (TX100) with hexadecylpyridinium bromide (HDPBr). The goal was to evaluate the molecular and thermodynamic interaction parameters in the mixed surfactant solutions at the air/water interface to provide better insight into the molecular interactions between TX100 and HDPBr at different pH values of the solutions. Fig. 1 Chemical structures of HDPBr (a) and TX100 surfactants (b) Kochkodan et al. Period. Polytech. Chem. Eng., 64(4), pp. 437–445, 2020| 439 and specific conductivity k( ) of the solutions versus the irrespective to pH value of the mixed solutions. It should equilibrium surfactant concentration (C). The CMC value be mentioned that after adding TX100 to HDPBr solution, is defined as the concentration, which corresponds to the CMC value of the binary mixture was lower compared

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