ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2015, Vol. 89, No. 8, pp. 1504–1506. © Pleiades Publishing, Ltd., 2015. Original Russian Text © R.Kh. Dadashev, R.S. Dzhambulatov, D.Z. Elimkhanov, 2015, published in Zhurnal Fizicheskoi Khimii, 2015, Vol. 89, No. 8, pp. 1338–1340. SHORT COMMUNICATIONS Features of the Concentration Dependences of the Surface Tension of Water Suspensions of Bentonites R. Kh. Dadasheva, R. S. Dzhambulatovb, and D. Z. Elimkhanova a bIbragimov Chechen State University, Grozny, Russia Complex Scientific Research Institute, Russian Academy of Sciences, Grozny, Russia e-mail: [email protected] Received October 2, 2014 Abstract—The concentration dependence of the surface tension of water suspensions of bentonites is studied experimentally. Possible reasons for the emergence of the minima on isotherms of the surface tensions of dispersed systems are analyzed. Keywords: surface tension, adsorption, bentonite suspensions, extrema, exchange cations, hydration, nanoparticles, dispersion. DOI: 10.1134/S0036024415080075 Studies in the field of the physical chemistry of bentonites provide extensive information on the crystal structure of clays. At the same time, the physicochemical and adsorption properties of bentonite suspensions remain poorly studied. The adsorption processes that occur on the surfaces of bentonites largely determine the final properties of their products and the goods made from them. In this respect, studies of the interphase phenomena in dispersed systems are of great theoretical and applied importance. Unlike solutions, suspensions of bentonites have specific features. In particular, isotherms of the surface tension (σ) of suspensions are characterized by minima at 3–5 wt % of the solid phase. The nature of these minima has not yet been clarified, though a variety of opinions on them exist [1, 2]. The authors of [1] attributed these features to the effects of exchange sodium cations, while the authors of [2] believed extrema on the σ isotherms of water solutions containing sodium and potassium ions were also associated with the hydration energy of these ions in water solutions. Experimental data [1, 2] indicate that in the region of low concentrations of potassium and sodium ions, the isotherms of surface tension are not in agreement with the classical representations [3–5], since one Na+ or K+ component or another can be either active (at low concentrations) or inactive (at high concentrations). In [6], we attempted to relate these features to the spatial orientation of nanoparticles on the surfaces of suspensions. We performed a chemical analysis of a disperse medium after 15–20 days of exposing its natural bentonite. Figure 1 shows the dependence of the concentration of K+ and Са2+ ions in an extract on the content of solid phase in bentonite suspensions. Similar results were also obtained by analyzing the concentration dependences of Fe3+ ions via colorimetric titration (Fig. 2) and Na+ via acid-base titration (Fig. 3). In doing so, we studied the effect potassium, calcium, sodium and iron ions had on the course of the concentration dependence of the suspension’s surface tension. It can be seen in Figs. 1–3 that the concentration dependences of the K+, Са2+, Fe3+, and Na+ ions in a dispersion medium of water suspensions of bentonites are straight lines with no features. Judging from these data, the concentrations of hydrated ions grow linearly upon an increase in the concentration of the bentonite solid phase. To reveal the effect sodium and potassium ions have on the σ isotherms of water suspensions of bentonites and their dispersion media, we studied the concentration dependence of the surface tension of suspensions. Measurements were performed on a DSA-100 tensiometer using the pendant drop method. Solutions of six different concentrations were prepared for our experimental studies. Measurements were performed at Т = 293 К. Our results are shown in Fig. 4. It can be seen in Fig. 4 that a slight (1 mN/m) minimum in the concentration region of 3 wt % of the solid phase is observed in the σ isotherms of the bentonite suspension. A slight increase in σ is observed after a further increase in the solid phase concentration. Analysis of the σ isotherm of a extract obtained from the dispersion medium containing the solid 1504 FEATURES OF THE CONCENTRATION DEPENDENCES Concentration of K+ and Ca2+, mol-eq./L 0.0030 Concentration of Fe3+, mol/L 0.00030 0.0026 0.00025 0.0022 0.00020 1505 0.00015 0.0018 0.00010 0.0014 0.00005 0.0010 1 2 3 4 5 6 Bentonite extract, % 0 1 2 3 4 5 6 Bentonite extract, % Fig. 2. Concentration dependence of Fe3+ ions in an extract on the content of solid phase in bentonite suspensions, obtained via colorimetric titration. Fig. 1. Concentration dependences of K+ and Са2+ ions in an extract on the content of solid phase in bentonite suspensions, obtained via complexometric titration. Concentration of Na+, mol/L 0.0009 0.0008 0.0007 0.0006 0.0005 1 2 3 4 5 6 Bentonite extract, % Fig. 3. Concentration dependence of Nа+ ions in an extract on the content of solid phase in bentonite suspensions, obtained via acid–base titration. phase of natural bentonite revealed no features, indicating that the increase in the concentration of ions had no appreciable effect on the surface tension of the extract. Our experimental studies thus indicate that sodium, potassium, calcium, and iron ions do not affect the value of the surface tension. mN/m 72 Suspension Extract 70 68 0 1 2 3 4 5 6 Bentonite, % Fig. 4. Dependence of surface tension on the concentration of the solid phase in water suspensions of natural bentonites and their dispersion media at 293 К. RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A It was shown in [6] that the minimum in the isotherm of the surface tension disappears after ultrasound dispersion. In this work, we used thermal impact for our method of dispersion; i.e., the water suspensions were subjected to long-term boiling. We measured the σ value of water suspensions of bentonites prepared from samples of different deposits before and after boiling. It can be seen in Fig. 5 that the isotherms of the σ dependence on the solid phase concentration are curves with minima in the region of 3.5 wt % of the solid phase. No such minima were observed on the σ isotherms of extracts. Vol. 89 No. 8 2015 1506 DADASHEV et al. Katayama Nal’chik Katayama after 30 min of boiling Nal’chik after 60 min of boiling Katayama after 30 min of boiling Nal’chik after 30 min of boiling mN/m 72 70 68 66 1. V. D. Shantarin and V. S. Voitenko, Physicochemistry of Dispersed Systems (Nedra, Moscow, 1990) [in Russian]. 2. O. Ya. Samoilov, Structure of Aqueous Electrolytes and Hydration of Ions (Akad. Nauk SSSR, Moscow, 1957) [in Russian]. 3. A. I. Rusanov, Phase Equilibria and Surface Phenomena (Khimiya, Leningrad, 1967) [in Russian]. 64 62 60 REFERENCES 4. R. Kh. Dadashev, Thermodynamics of Surface Phenomena (Fizmatlit, Moscow, 2008) [in Russian]. 0 1 2 3 4 5 6 Bentonite, % Fig. 5. Concentration dependences of the surface tensions of suspensions of natural bentonites of different origins before and after boiling at 298 К. The experimental data obtained after the ultrasound [6] and thermal dispersion of bentonite suspensions thus indicate that the minimum on σ isotherms is directly associated with the particles’ sizes. 5. S. I. Popel’, A. I. Sotnikova, and V. I. Boronenkov, Theory of Metallurgical Processes (Metallurgiya, Moscow, 1986) [in Russian]. 6. R. Kh. Dadashev, V. Kh. Mezhidov, R. S. Dzhambulatov, and D. Z. Elimkhanov, Bull. Russ. Acad. Sci.: Phys. 78, 288 (2014). RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A Translated by L. Mosina Vol. 89 No. 8 2015
