Volumetric and Transport Properties of Water–

Colloid Journal, Vol. 65, No. 3, 2003, pp. 394–397. Translated from Kolloidnyi Zhurnal, Vol. 65, No. 3, 2003, pp. 429–432. Original Russian Text Copyright © 2003 by Kartsev, Shtykov, Sineva, Tsepulin, Shtykova.

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Volumetric and Transport Properties of Water–n-Octane–Sodium Dodecyl Sulfate–n-Pentanol Microemulsions V. N. Kartsev*, S. N. Shtykov*, A. V. Sineva**, V. V. Tsepulin*, and L. S. Shtykova* *Saratov State University, Moskovskaya ul. 155, Saratov, 410026 Russia **Moscow State University, Vorob’evy gory, Moscow, 119899 Russia Received February 16, 2002

Abstract—The density, isothermal compressibility, and thermal expansion coefficient of monophase water–noctane–sodium dodecyl sulfate–n-pentanol microemulsions were measured in a wide range of the water-to-oil ratios. The internal pressure and molar volume of the investigated microemulsion systems were calculated. The conductivity of microemulsions was determined and its percolation character was revealed.

INTRODUCTION Typical property of microemulsions (ME) that is of interest to chemists and production engineers is their ability to concentrate and redistribute the components of chemical reactions between phases with simultaneous dissolution of hydrophilic and hydrophobic substances [1]. The microemulsions are used in separation and concentration [2], polymerization reactions [3], organic and biorganic synthesis [4], electrochemical processes [5], in the synthesis of the nanoparticles of metals, oxides, sulfides, and silicates [6], as well for the enhanced oil recovery [7]. The most important and unique property of microemulsions as a special type of reaction medium is their microheterogeneity. This property manifests itself as a sharp change in dielectric permittivity, polarity, viscosity, and acidity when passing through the interface between the dispersion medium and dispersed phase (microdroplets) in ME [1]. The effect of the aforementioned microparameters on chemical reactions is generally studied by optical spectroscopy and NMR [1, 8]. Important information on the ME macroscopic properties is provided by studying their transport [9– 15] and dielectric behavior [11, 16]. It is shown that, analyzing the concentration dependence of microemulsion conductivity, one can disclose the transition of ME from one to another type and refine the region of existence of a bicontinuous structure [10]. The number of investigations devoted to the thermodynamic parameters of microemulsions is comparatively small [11–15]. As a rule, the main attention is paid to measuring two thermodynamic parameters such as density and adiabatic compressibility of microemulsions, the latter property being determined by an ultrasonic method [12–15, 17]. We have not found any information about the experimental determination of such parameters as isothermal compressibility and ther-

mal expansion coefficient. Particularly little attention is paid to the changes in the ME thermodynamic parameters upon the variation in the water content within a wide concentration range, guaranteeing the transition of one microemulsion type to another. For comparison, it should be noted that a large number of works had been devoted to studying the ME transport properties (electrical conductivity, viscosity), part of which was cited in a review [10]. Earlier, we determined the density, isothermal compressibility, and thermal erxpansion coefficient for two water–n-heptane–sodium dodecyl sulfate–n-pentanol microemulsions with a markedly different water-toheptane ratio [18]. In this work, we investigated the concentration dependences of density, isothermal compressibility, temperature expansion coefficient, electrical conductivity, and viscosity of the water–n-octane– sodium dodecyl sulfate–n-pentanol microemulsion on varying the water-to-oil ratio within a wide range. EXPERIMENTAL Commercial preparation of sodium dodecyl sulfate (SDS), chromatography pure grade, (NPO SintezPAV), containing 99% of base substance, was used without additional purification. In accordance with [19] prior to use, n-pentanol (cosurfactant) was distilled with a drying agent; n-octane was purified also according to [19]. Microemulsions were prepared by weighing and mixing components such as SDS, bidistilled water, octane, and pentanol. The proportion of SDS in all MEs was 7.77 wt %, n-pentanol 15.49 wt %; the water-to-octane weight ratio was varied from 0.05 to 4. The investigated microemulsions remained macroscopically singlephase and transparent at 25°ë throughout the studied concentration range.

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Table 1. Effect of evacuation on the bulk properties of microemulsions No. ME

State

1

ME–

ρ, kg/m3

ME+ 2

α × 105, K–1

β × 1011, Pa–1

25°C

50°C

25°C

50°C

25°C

50°C

754.20 754.19

733.50 733.52 958.67 958.70

108 108 49.1 49.0

116 116

114 114 56.5 56.2

140 140

ME– ME+

Table 2. Density ρ, isothermal compressibility βT, thermal expansion coefficient α, molar volume Vm, and internal pressure Pi of water–n-octane–n-pentanol–sodium dodecyl sulfate microemulsion containing various amounts of water and its individual components at 25°C ϕ of water, wt % 3.89 7.77 15.55 27.20 38.88 46.64 54.42 62.19 Water n-Pentanol n-Octane

ρ, kg/m3

α × 105, K–1

β × 1011, Pa–1

Vm × 105, m3/mol

Pi × 10–8, Pa

744.81 754.7 774.3 806.0 843.19 863.44 891.07 918.49 997.04 811.52 698.58

112 109 104 94.1 86.2 79.2 69.5 61.4 25.7 92.4 118

119 115 110 100 91.2 84.4 75.6 67.2 45.2 90.1 131

12.74 10.73 8.077 5.787 4.410 3.794 3.285 2.881 1.807 10.86 16.35

3.1 3.1 3.0 3.0 3.1 3.0 2.8 2.8 1.69 3.06 2.68

The peculiar feature of dilatometric experiment consisted in the simultaneous measurement of all thermodynamic parameters on the same original unit [20]. A large dilatometer volume (about 50 cm3), a small capillary length-to-diameter ratio (0.001), the careful thorough thermostatting of samples and control of temperature inside the reaction vessel allow to determine density ρ = m/V, isothermal compressibility βT = −(∆V/∆P)T /V, and the thermal expansion coefficient α = (∆V/∆T)P/V with errors of 5 × 10–3, 1, and 2%, respectively. The pressure drop ∆P and temperature drop ∆T amounted to 4 × 105 Pa and 0.4°C, respectively. The technique of measuring the bulk properties of liquids suggests the evacuation of the dilatometer cell followed by filling with the liquid under study in vacuum [21]. The evacuation can change the component ratio in the initial ME, and, hence, its bulk properties. A preliminary experiment was carried out in order to extimate the effect of this operation on measurement results. The experiment consisted in measuring the bulk properties of evacuated (ME+) and unevacuated (ME–) microemulsion samples at 25 and 50°ë. The microemulsions differed in the water-to-oil weight ratio, while the concentrations of n-pentanol and SDS were fixed. The results of preliminary experiment are listed in Table 1 and prove that the differences in the bulk propCOLLOID JOURNAL

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erties of ME+ and ME− samples lie within the limits of experimental error. Thus, the evacuation does not substantially change the microemulsion composition, and there is no necessity to change the standard measuring technique of the bulk properties of liquids [21]. Therefore, in performing main experiments the dilatometer was filled in vacuum. The ME conductivity was measured at 25 ± 0.1°ë on an OK-102/1 conductometer (Radelkis, Hungary). The conductivity was calculated by the formula k = σλ, where σ is the cell constant, λ is the conductivity of the cell filled with ME. The cell constant σ = 0.8 cm–1 was determined using the aqueous potassium chloride solutions. The ME kinematic viscosity was measured using an Ostwald capillary viscometer placed into a thermostat, at a constant temperature of 25°ë. RESULTS AND DISCUSSION The water-to-oil ratio was varied in MEs at the SDSto-pentanol fixed weight ratio equal approximately to 1 : 2. Hence, the composition–property diagrams of microemulsions are similar to those for binary systems, and they can easily be represented graphically and simplify the study of ME properties and occurring structural changes.

396

KARTSEV et al. η, cSt 15

logk [S/m] 1 0 –1

40

60

80 ϕ, wt %

–2

10

5

–3 0

–4

20

40

60

80 ϕ, wt %

Fig. 1. The conductivity of the water–n-octane–SDS–npentanol microemulsion vs. water content.

Fig. 2. Kinematic viscosiity of the water–n-octane–SDS–npentanol microemulsion vs. water content.

Thermodynamic Parameters of the Water–Octane–SDS–Pentanol System

Transport Properties of the Water–Octane–SDS–Pentanol System

The results of measuring density, isothermal compressibility, and thermal expansion coefficient of microemulsions are listed in Table 2, together with similar parameters for water, n-octane, and n-pentanol, calculated by the equations proposed in [22]. The molar volume and the internal pressure of ME were calculated from the measured parameters (Table 2). The bulk properties of MEs were studied in a fairly wide range of water:oil ratios. Hence, one could assume that the microemulsion type can be changed with increasing water concentration.

In accordance with the literature data (for example, see [9, 10]), a sharp (jumpwise) change in some physicochemical properties of microemulsions such as conductivity, permittivity, and viscosity, upon varying their composition, is an indicator of structural changes in ME (including the transition of water-in-oil microemulsion to a bicontinuous system). The results of measuring conductivity of microemulsions throughout the studied water : oil ratio range are represented in Fig. 1. A sharp jump testifying structural changes of ME is seen from the dependence of conductivity k on the water weight fraction ϕ, which agrees with the data obtained earlier by Sineva [10] for MEs, whose composition was similar to that investigated in this work.

An analysis of Table 2 showed that the dependences of density, isothermal compressibility, and thermal expansion coefficient of MEs on the mass concentration of water ϕ are described by the following straightline equations: ϕ = 2.725ϕ + 735.70 (r = 0.993), βT = –8.672 × 10–12ϕ + 123 × 10–11 (r = 0.994), α = –8.473 × 10−6ϕ + 117 × 10–3 (r = 0.991). Thus, in spite of considerable variations in the thermodynamic parameters (see Table 2), they do not exhibit any jumpwise changes, testifying differences in the microemulsion structure. Note that the equations for concentration dependences of the aforementioned thermodynamic parameters of the investigated MEs obtained by using an additive scheme, have virtually the same form. Hence, the studied thermodynamic parameters are sensitive to the ME composition. We measured the conductivity and viscosity of microemulsions to clarify whether or not their structure changes in the indicated water-to-oil ratio range.

It is seen from Fig. 1 that, in the region of small ϕ values, the conductivity of water-in-oil microemulsion is fairly low. The conductivity increases by roughly three decimal orders, as the concentration of water increases from 10 to 16 wt %. Thus, in this concentration range in the water-to-oil microemulsion, the percolation transition associated with the formation of an infinite aggregate of water globules covered with a monolayer of surfactant and cosurfactant obviously takes place. The position of inflection on the conductivity curve coincides with that on the viscosity dependence as a function of water concentration in ME (Fig. 2). It is also seen from this figure that, in the region of the water concentration higher than 16 wt %, the viscosity of the system increases dramatically (by approximately five times) and reaches its maximum at water content of 27–28 wt %. On the curve of conductivity in the range of 27–28 wt % of water, the inflection point is revealed, which is probably due to enhanced aggregation of water globules and increased microemulsion viscosity, thereby reducing the mobility of globules and ions. Similar concentration dependence of COLLOID JOURNAL

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viscosity (with the maximum in the region of percolation transition) was observed in [10, 14]. The third region corresponding to the water concentration higher than 40 wt % can also be singled out on the aforementioned dependences. The conductivity in this region is virtually constant, because the continuous aqueous phase has probably been eventually formed. The viscosity of microemulsion, after lowering in the range of ϕ = 27–40 wt %, changes further by no more than 10–15%. An alternative to the formation of a cluster of spherical water globules can be the formation of a bicontinuous (mosaic) structure or a Voronoy polyhedron. Presumably, with a water content in the microemulsion of 40 wt %, precisely such a bicontinuous structure composed of oil and water domains separated by multilayer surfactant membranes is formed. A small increase in viscosity at a higher water content is obviously associated with the transformation of the bicontinuous structure into the oil-in-water microemulsion. Note that the values of water critical concentration corresponding to the transition of water-in-oil microemulsion to the oil-in-water type lie for various systems in a range from 7 to 75 vol % [10]. Therefore, refining the character of the discussed structural changes requires both a further accumulation of experimental data and use of other methods allowing to study the structure of microemulsions on the micro- and macrolevel. Thus, we measured both thermodynamic and transport parameters of the microemulsions formed in the water–n-octane–SDS–n-pentanol system on varying the water content from 3.89 to 62.19 wt %. It was shown that, in this range of water concentrations, structural changes occur, that can be described in terms of the transition from the droplet-type water-in-oil microemulsion to a percolation cluster, and further, to a bicontinuous microemulsion system. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research (project nos. 01-03-32649 and 0203-33029).

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