Article pubs.acs.org/jced
Physicochemical Characterization of Anionic and Cationic Microemulsions: Water Solubilization, Particle Size Distribution, Surface Tension, and Structural Parameters Achinta Bera,*,† Ajay Mandal,*,‡ and T. Kumar‡ †
Department of Civil & Environmental Engineering, School of Mining and Petroleum Engineering, University of Alberta, Edmonton, Alberta Canada T6G 2W2 ‡ Department of Petroleum Engineering, Indian School of Mines, Dhanbad, 826004, India ABSTRACT: The composition, oil type, and thermodynamic parameters influence the water solubilization capacity, particle size distribution, surface tension, and also the structural parameters of microemulsion systems. In the present study sodium lauryl sulfate (SLS) and hexadecyltrimethylammonium bromide (HTAB) have been used as surfactants and 3-methyl-1-butanol as cosurfactant to prepare microemulsions. Four n-alkanes (hexane, heptane, decane, and dodecane) were chosen as oil phases. Water solubilization capacities of anionic (SLS) and cationic (HTAB) microemulsions were investigated in both the presence and the absence of NaCl salt. A detailed study of particle size analysis for both the surfactants with different composition has been made from laser light scattering measurement. Surface tensions of microemulsions and surfactant solutions were also measured for investigation of their surface activities. Surface tensions have been reduced remarkably in the case of microemulsion systems compared to simple aqueous solutions of surfactants. Different structural parameters like water droplet and effective microemulsion droplet size including interface, aggregation numbers of surfactant, and cosurfactant have been determined assuming monodispersity of the droplets from dilution experiment. The effects of temperature on the above parameters have also been studied.
1. INTRODUCTION Microemulsions have attracted noticeable attention in the petroleum oil industry, pharmaceuticals, cosmetic formulation, photoredox reactions, and organic synthesis, lubrication, ultrafine cleaning, synthesis of nanoparticles, drug delivery systems and organ prevention fluids due to their produced ability of ultralow interfacial tension (IFT), high solubilization, transparency, thermodynamic stability, and distribution of size uniformity.1−6 Microemulsions are thermodynamically stable, optically isotropic and transparent dispersions of oil and water (or brine) stabilized by surfactant (and/or cosurfactant).7−9 Considerable attention is given to the application of microemulsions in enhanced oil recovery (EOR) because of the highlevel extraction efficiency of petroleum oil from natural oil reservoirs as injected fluids.10−13 The characterization of microemulsions is very important in several industrial applications. For optimization processes an understanding of water solubilization, internal structure, phase behavior, and the dynamic nature of microemulsions has been stressed in laboratory investigations using a variety of techniques.14−18 The water solubilization capacity and phase equilibria of microemulsion systems are determined by two phenomenological properties such as the spontaneous curvature and the elasticity of the interfacial film.19−21 The addition of electrolytes can enhance the water solubilization capacity of microemulsions, and the solubilization limit reaches © XXXX American Chemical Society
to a maximum by changing the concentration of the electrolyte.22,23 The surface tension at the plain air−water interfaces is typically of the order of about 72 mN·m−1. Microemulsions are characterized by far lower interfacial tensions typically in the range of 20 mN·m−1 to 25 mN·m−1, and generally can be of the order of 10−3 mN·m−1 to 10−6 mN· m−1 in the case of the oil−microemulsion interface.24−26 These latter values reflect the absence of direct oil−water (o/w) contact at the interface due to orientation of surfactant molecules present in the liquid. The addition of excess dispersed phase or change in thermodynamic parameters can alter the phase behavior of microemulsions which is interestingly related to particle size distribution.6,27−29 Microemulsion particle sizing methods generally have focused on the use of light scattering techniques including photon correlation spectroscopy,30 time average light scattering,31 and small angle neutron scattering.32 Upon dilution with excess aqueous phase o/w microemulsions are inverted through some proposed intermediates into w/o microemulsions and/or mixed microemulsions like w/o/w with number of liquid crystalline phases.33−35 However, the internal structural parameters and thermodynamic properties of Received: March 21, 2014 Accepted: June 18, 2014
A
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volumes of oil were added using an appropriate microsyringe. After that, desired amount of water was added drop by drop to the mixtures and the samples were stirred by magnetic stirrer until all the surfactant dissolved and a clear single-phase microemulsion was obtained. 2.2.2. Determination of Water Solubilization Capacity. The solubilization of water in the microemulsion region was determined by a conventional titration method of microemulsion with brine or water under satisfied conditions until the opacity of the microemulsions was just obtained. Here the opacity is especially defined as the turbid, dense milky appearance of that system through which nothing can be seen. For different cases a different colored translucent mixture was not considered as the required opacity of the system. However, the end point of the titration was considered the actual transition point of the clear, transparent, and isotropic microemulsions to a birefringent phase where the boundary was determined as the onset of the cloudiness due to a lack of a strong turbidity. In all cases, more than three successive measurements were carried out, and the standard deviation did not exceed ± 0.01·10−3 kg. All of the experiments were performed at three different temperatures such as (303 ± 1) K, (313 ± 1) K, and (323 ± 1) K. 2.2.3. Measurement of Particle Size. The particle size distributions of the microemulsions with different formulations were measured approximately 5 h after the preparation by a laser diffraction method of Zetasizer version 6.00 (Malvern Instruments Ltd., Worcestershire, UK). Drops of microemulsion were introduced into the sample containing cell until the volume reached to the optimum one, indicated by the instrument. The particle size distribution can be obtained by the built-in software of the instrument. The temperature was kept constant at (303 ± 1) K throughout the experiments. 2.2.4. Surface Tension Measurement. Measurement of surface tension is very much useful supplementary test method for characterization of the surface activity of a surfactant. In the present study surface tensions of the surfactant solutions and microemulsions were measured with the help of a programmable tensiometer (Kruss GmbH, Germany, Model: K20 EasyDyne) at (303 ± 1) K, (313 ± 1) K, and (323 ± 1) K by the Du Noüy ring method. The platinum ring was thoroughly cleaned with acetone and flame-dried before each measurement. Before starting measurements the density of the liquid was entered in the menu for the ring method. The surface tension of the test solution was measured by the Du Noüy Ring method by pressing the start button after immersing the ring about 1 mm deep in the liquid. The measurement runs automatically. During the measurement the individual surface tension values are shown. The instrument was set at 10 readings and at the end of the test run, the display shows a page with the mean value for the surface tension, the standard deviation, and the experimental settings. In this measurement the standard deviation did not exceed ± 0.1 mN·m−1. 2.2.5. Dilution Experiments. The structural parameters of the microemulsions were evaluated by the method of dilution. In the dilution method, 1.5·10−3 mol of surfactant was placed in a dry beaker, and a mixture of 0.055 mol of water and 0.04 mol of oil was added to the surfactant. The beaker was then placed in a thermostatic water bath and mixing was conducted by magnetic stirrer. The cosurfactant was added slowly to the initially turbid mixture from a microsyringe until the solution became just clear at the experimental temperature. Sufficient time was allowed for equilibrium to be reached. The volume of
microemulsions in solubilization processes are important, but only the effect of alkanol chain length and mole fraction of surfactant have been reported in few of the articles until now.36−38 In the present study 3-methyl-1-butanol has been used as cosurfactant for preparation of a microemulsion. It has been found that the surfactants used under the study have good solubilities in 3-methyl-1-butanol.10 It also improves the viscosity of the microemulsion significantly because of its branched structure compared to other conventional alcohols. The structural parameters are very helpful in understanding the mechanism of microemulsion formation and spontaneity of the reaction, and to prepare the microemulsion for a high solubilization capacity in the EOR technique. Water-soluble molecules are assumed to be located within very small spherical droplets of water surrounded by surfactant molecules in a continuous oil medium. The aqueous phase is thus an internal or dispersed phase, whereas the oil makes up the external or continuous phase. The alkyl chain lengths of oil and cosurfactant strongly influence the interfacial composition, formation, and various physicochemical properties of microemulsions and distribution of cosurfactant in the oil and water phases.39−42 The objectives of the present study are to investigate the capacity of anionic and cationic microemulsion systems to solubilize water in both the presence and absence of NaCl with different concentrations. The effect of NaCl on the solubilization capacity of the SLS microemulsion was studied by the dropwise addition of brine in different concentrations. The particle size distributions of cationic and anionic microemulsions with different n-alkanes have been analyzed. Surface tensions of different microemulsions and surfactant solutions have been measured to compare their surface activities at the air−liquid interface. The dilution experiments were carried out with microemulsion systems, using four nalkanes (hexane, heptane, decane and dodecane) as oil phase, SLS, and HTAB as surfactants, and 3-methyl-1-butanol as cosurfactant, at different temperatures (303, 313, and 323 K) to determine structural parameters of the microemulsions. The structural parameters, effective radii of the droplets, aggregation numbers of surfactants, and cosurfactant in the interface have been calculated assuming monodispersity of the droplets from dilution experiments.
2. EXPERIMENTAL SECTION 2.1. Materials Used. Anionic surfactant, SLS (0.98 mole fraction purity) procured from Fisher Scientific, India, and cationic surfactant, HTAB of 0.98 mole fraction purity procured from Merck, India, were used in the present study. Different nalkanes (C6 to C12) were used as synthetic oils. Two lower nalkanes, namely, hexane (0.98 mole fraction purity) and heptane (0.98 mole fraction purity), were purchased from Loba Chemie Pvt. Ltd., India. Other n-alkanes such as decane and dodecane with mole fraction purity of 0.99 each were analytical reagent grade products of Otto-Kemi Pvt. Ltd., India. 3-Methyl-1-butanol with 0.98 mole fraction purity (CDH Pvt. Ltd., India) was used as the cosurfactant. Reverse osmosis water from a Millipore water system (Millipore SA, 67120 Molshein, France) was used for the preparation of microemulsions. All the chemicals were used without further purification. 2.2. Methods. 2.2.1. Preparation of Microemulsion. The general preparation procedure of the microemulsion started with weighing the required amount of each surfactant and cosurfactant into a clean, dry beaker, and then the required B
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cosurfactant at that point was recorded. At this stage, 0.04 mol of oil was added to the system to destabilize it and the solution turned turbid. The turbid solution was then made clear by the addition of cosurfactant, and the quantity of cosurfactant added was again noted. This procedure was repeated several times to get the quantity of the oil and the cosurfactant at each step. The entire procedure was followed at three different temperatures: 303 K, 313 K, and 323 K for different n-alkanes (hexane, heptane, decane, dodecane) with a fixed cosurfactant (3methyl-1-butanol).
Vcsi =
ncsiMcs ρcs
(3)
where, Mcs, and ρcs are the number of moles of cosurfactant at the interface, molar mass of cosurfactant, and density of cosurfactant, respectively. The term Vs can be estimated by considering the contributions of various groups like volume of surfactant tail (Vtail) and of head (Vhead) as suggested by Hirata and his co-workers.43 nics,
3. THEORETICAL CONSIDERATION FOR THE CALCULATION OF STRUCTURAL PARAMETERS Different structural parameters of the w/o microemulsions could be properly computed from the results of dilution experiments. The microemulsion droplets are assumed to have an approximately spherical shape and monodispersed with a surface monolayer comprising surfactant (SLS/HTAB) and cosurfactant (3-methyl-1-butanol) present at the interface as shown in Figure 1. The total volume of the dispersed phase
Vs = (Vtail + Vhead)nsNA
(4)
Vtail = [VCH3 + (nc − 1)VCH3]
(5)
Vhead = 2(2VCH3 + VN) + sVCH3 + 2VBr
(6)
where the values of VCH3, VCH3, VN, VBr are (0.0426, 0.0282, −0.0305, and 0.0500) nm3 respectively and NA (6.023 × 1023) is Avogadro’s number; nc is the number of carbon atom in the surfactant molecule and s is the number of −CH2 groups in the surfactant. Now the droplet surface area (Ad) per unit volume is given by the following relationship: Ad = 4πR e2Nd = (nsA s + ncsiAcs)NA
(7)
where As and Acs are the cross sectional area of the polar headgroups of the surfactant and cosurfactant molecules, respectively, and NA (6.023 × 1023) is the Avogadro’s number. The values of As and Acs are considered to remain unchanged with variations in droplet size and interfacial composition.42 The values of As for SLS and HTAB are 50.5 Å2 and 35 Å2, respectively.9,44 The value of Acs for 3-methyl-1-butanol is taken as 20 Å2.42 The combination of eqs 1 and 7 gives the expression of Re as follows:
R e = 3Vd /Ad
(8)
Now rearranging eq 1, one can get the value of Nd as follows: Nd = 3Vd /4πR e3
The average aggregation numbers of surfactant (N̅ s) and the cosurfactant (N̅ cs) on the droplet surface are given by the relations:
Figure 1. Schematic diagram of structural distribution of w/o microemulsion: Rw, the radius of the water pool; Re, the effective radius including the interface; d, the thickness of the interfacial layer including surfactant and cosurfactant.
droplets (Vd) per unit volume in milliliters can be expressed by the following relationship: 4 Vd = πR e3Nd (1) 3
Ns̅ = nsNA /Nd
(10)
Ncs̅ = ncsiNA /Nd
(11)
where ns is the total number mole of surfactant and nics is the total number of moles of cosurfactant. The required data sets of ns and nics have been taken from our previous paper45 for calculation of the structural parameters of the microemulsion systems. The effective radius of the water droplet, Rw, including the contributions of the anchored amphiphiles head groups is obtained from the relationship:
where Re and Nd represent the effective radius (including the interface) and the total number of droplets in the solution, respectively. Vd can be calculated from another point of view; it is equal to the sum of the volume contribution of water (VH2O), surfactant (Vs), and the interfacial cosurfactant molecules (Vics) at the interface. Thus, one could write Vd = VH2O + Vs + Vcsi
(9)
R w = {(VH2O + Vsh + Vcsh)/Vd}1/3R e
Vhs
(12)
Vics
where the new terms and are the volumes of the surfactant and cosurfactant head groups, respectively, according to the relationships:
(2)
Vics can be computed from the parameter of dilution experiment as follows: C
Vsh = (4/3π 1/2)A s3/2 Ns̅
(13)
Vcsh = (4/3π 1/2)Acs3/2 Ncs̅
(14)
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The effective thickness of the interfacial layer of the droplet “d” also is calculated as follows: d = Re − R w
length of dodecane is higher than that of decane in the case of the HTAB/dodecane system, because of steric hindrance its water solubilization is a little bit smaller than that of the decane/HTAB system. The effect of NaCl concentration on the water solubilization capacity of a microemulsion has been depicted in Figure 3a.
(15)
With help of the equations herein, a set of complete structural parameters has been calculated from the dilution experiments.
4. RESULTS AND DISCUSSION 4.1. Effect of Salt on Water Solubilization Capacity. The water solubilization capacity of SLS and HTAB microemulsions in the hexane, heptane, decane, and dodecane systems has been shown in Figure 2. The water solubilization
Figure 3. (a) Effect of NaCl concentration (S, mole fraction of NaCl) on water solubilization capacity of microemulsions containing 0.04 mol of alkane, 1.5·10−3 mol of SLS, and 0.011 mol of 3-methyl-1butanol at 303 K. Symbols: ■, n-hexane; ●, n-heptane; ▲, n-decane; ▼, n-dodecane. (b) Effect of temperature on water solubilization capacity of microemulsion systems at 1.5 salinity. Symbols: ▲, nhexane; ●, n-heptane; ■, n-decane; ▼, n-dodecane.
The water solubilization capacity of every microemulsion system increases initially with increased NaCl concentration, reaches a maximum, and decreases sharply at high concentrations. The increase in the NaCl concentration in solution diminishes the effective polar area of the surfactant by decreasing the thickness of the electrical double layer around the polar group, which trends to increase the natural negative curvature of the surfactant monolayer. Therefore, the aggregation number would be reduced to be compatible with the decreased area of head groups which causes a decrease in water solubilization capacity of the microemulsion. The latter effect should play a dominant role as the concentration of NaCl becomes higher, which explains the further decrease in water solubilization capacity. In the water/SLS/3-methyl-1-butanol system, since 3-methyl-1-butanol is a polar solvent, SLS micellization driven by the hydrophobic effect is not as effective as that in alkane systems. However, small aggregates may be formed by the interaction between the hydroxy group of 3-methyl-1-butanol and the polar groups of SLS, which leads to smaller water solubilization capacity. It is again confirmed from Figure 3 that as oil chain length increases, the peak of the solubilization curve moved to a higher salinity region. This may be due to increasing fluidity and decreasing packing ratio of the microemulsion systems. In Figure 3b the effect of temperature on water solubilization capacity of microemulsions has been shown. As the temperature of the system increases, water solubilization capacity also increases. This occurs because with increasing temperature the curvature and interfacial elasticity as well as interfacial area also increases, which promote the water solubilization of microemulsion systems. 4.2. Particle Size Distribution. Particle size distributions (intensity wise) of different microemulsion systems are shown
Figure 2. Water solubilization capacity of different types of microemulsions in both the presence and absence of NaCl at 303 K. Symbols: dark shade, with NaCl; light shade, without NaCl.
capacity and phase equilibria of microemulsions are determined by the phenomenological parameters (the spontaneous curvature and the elasticity of the interfacial film).45 The maximum solubilization capacity was observed in the HTAB microemulsion in both presence and absence of NaCl. The possible reason for high water solubilization capacity of HTAB microemulsion is due to micellar structural differences that exist because of headgroup size and steric requirements. As the positive charge residing on the quaternary nitrogen atom of HTAB surfactant is less exposed than the negative charge of SLS surfactant, the proximity of the counterions to the headgroup is smaller in cationic surfactant and as a result, it affects interactions with water or water solubilization capacity. It is clear from the figure that water solubilization capacity of a microemulsion system increases in the presence of NaCl. The addition of NaCl decreases the attractive interaction between the droplets by making the interfacial layer more rigid, which increases the water solubilization capacity.46 Another interesting fact is that in the case of SLS and HTAB, heptane and dodecane show the highest solubilization capacity among the alkanes. According to the chain length compatibility effect (also known as BSO [Bansal, Shah, and O’Connell] the solubilization reaches a maximum when the alkane or oil chain length (lo), plus that of the alcohol (la) is equal to that of the surfactant (ls) (i.e., lo + la = ls).47 In the present study SLS/heptane and HTAB/decane systems are under the aforementioned rule, and therefore they show the highest solubilization. As the chain D
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surfactant molecules increase, owing to the weak presence of solvent molecules in the tail region of the droplets. The net effect results in an increase in micellar exchange rate with the increase in chain length of solvent. Shorter oil molecules penetrate more easily, resulting in additional interfacial area, interfacial rigidity, and larger drop size. Therefore, in the case of heptane, the size of the particle is larger. But in the case of SLS, the compatibility effect reduces the particle size to a noticeable extent, offering the particle size very close to that of the decane system. In the case of HTAB Z-average particle size decreases more pronouncedly with an increase in the alkane carbon number (ACN) of the oil than that of SLS. This phenomenon can be explained on the basis of the chain length compatibility effect. The water solubilization capacity of the HTAB microemulsion is highest for decane. Owing to higher water solubilization capacity, the interactions between the droplets are very strong, which offers smaller Z-average particle size. In the case of larger oil chain length the packing with surfactant molecule is very tight, which influences the particle size distribution in the microemulsion systems. It is important to mention that we have measured particle size at different temperatures (such as 303 K, 313 K, and 323 K), but as microemulsion systems are thermodynamically stable, no variation in particle size was obtained. 4.3. Surface Tension. The main mechanism of improvement of microscopic displacement efficiency in chemical EOR is the reduction of surface tension of the displacing fluid or reduction of IFT between the displacing and displaced fluid. An interesting surface tension behavior of aqueous surfactant solutions and water−oil microemulsions has been investigated in the present study. Surface tensions of SLS and HTAB surfactant solutions (above CMC) and different microemulsions have been shown in Figure 5a. With the addition of SLS and HTAB into water surface tensions have been reduced up to 32 mN·m−1 and 33 mN·m−1 respectively. On the other hand reduction of surface tensions for the microemulsion systems in the presence of the same concentration of surfactants is much higher than that of simple water−surfactant systems. The reduction of surface tensions of microemulsions
in Figure 4. The mean particle diameter and polydispersity have been calculated from intensity, mass, and number bimodal
Figure 4. Particle size distributions of SLS and HTAB microemulsion systems at 303 K: (a) SLS/water/3-methyl-1-butanol; (b) HTAB/ water/3-methyl-1-butanol. Symbols: ■, n-hexane; ●, n-heptane; ▲, noctane; ▼, n-dodecane.
distribution. Usually size effects are due to significant colloidal interaction between particles, that is to say when the particles are considerably smaller than 10 nm. Two different factors are acting behind the particle size distribution: first the particle deformability decreases with droplet size and second the width of the particle size distribution usually also decreases with droplet size. In the present study the particle size distribution of microemulsion was found to be influenced by oil chain length as well as alkyl chain length of surfactants. For both the surfactants (SLS and HTAB) as oil chain length increases Zaverage particle size decreases, and the results have been summarized in Table 1. The result is well supported by other Table 1. Results of the Particle Size Distributions of the Microemulsions Which Are Composed of 1.5·10−3 mol Surfactant, 0.011 mol Cosurfactant and 0.04 mol Oil in Each Casea microemulsion compositions
d/nm
PDI
I
PV
hexane/SLS/3-methyl-1-butanol/water heptane/SLS/3-methyl-1-butanol/water decane/SLS/3-methyl-1-butanol/water dodecane/SLS/3-methyl-1-butanol/water hexane/HTAB/3-methyl-1-butanol/water heptane/HTAB/3-methyl-1-butanol/water decane/HTAB/3-methyl-1-butanol/water dodecane/HTAB/ 3-methyl-1-butanol/water
19.1 18.1 20.2 22.2 50.3 52.3 21.5 22.5
0.266 0.252 0.043 0.063 0.304 0.314 0.178 0.188
0.957 0.955 0.958 0.968 0.924 0.904 0.956 0.976
49 50 50 50 49 49 50 50
a
Notation: Z-average diameter (d/nm), polydispersity index (PDI), intercept (I) and peak volume % r.nm (PV).
research works.48,49 An increase in oil chain length results in an increase in the value of coefficient of intermolecular exchange rate due to increased coiling in the oil molecule, and therefore its penetration in the surfactant layer becomes more difficult. As a result, the mutual interaction between surfactant tails is stronger than that between the surfactant tail and oil molecules. On the other hand, interdroplet tail−tail interactions of two
Figure 5. (a) Surface tension of SLS, HTAB, and different microemulsion (ME) systems at 303 K and (b) effect of temperature on surface tension of SLS, HTAB, and different microemulsion (ME) systems. Symbols: ■, SLS; ●, HTAB; ▲, SLS/hexane; ▼, SLS/ heptane; ◀, SLS/decane; ▶, SLS/dodecane; ◆, HTAB/hexane; □, HTAB/heptane; ○, HTAB/decane; △, HTAB/dodecane. E
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radius of a microemulsion droplet (including interface) (Re) increases with an increase in temperature up to a certain temperature, and then at high temperature it decreases. The values of Rw are smaller than those of Re in all cases. The values of Re and Rw obtained from the present study are comparable with other works.9,38,52 No systematic dependence of Nd on temperature has been found. The values of Nd decrease for hexane and heptane with increase in temperature but opposite trends are found for decane and dodecane in a few cases. The aggregation numbers of surfactant (N̅ s) and cosurfactant (N̅ cs) with each droplet and their dependence on temperature are important aspects of microemulsion stability. In all the cases the aggregation numbers of surfactant (N̅ s) and cosurfactant (N̅ cs) both are dependent on the temperature and alkane chain length which are shown in Tables 2 and 3 for SLS and HTAB microemulsion systems, respectively. Initially the aggregation numbers of surfactant (N̅ s) and cosurfactant (N̅ cs) both increase as temperature increases but at higher temperature the values are found to be decreased. The ratio of the aggregation numbers of cosurfactant (N̅ cs) to surfactant (N̅ s), that is, N̅ cs/N̅ s, shows the independency of oil chain length at a particular temperature. The values decreases with an increase in temperature, but at higher temperature the values are again found to be increased as shown in Figures 6 and 7. Therefore, at low surfactant composition, for the dispersion of a fair amount of water, the requirement of cosurfactant is more than that of the surfactant.
takes place because of the high efficiency of microemulsions to accumulate surfactant on the interface region, and therefore it has increased capacity to reduce the surface tensions of the systems. In the case of the SLS microemulsion system, the surface tension reduces noticeably compared to that of HTAB.50,51 In the case of a microemulsion system, the surface tension value decreases noticeably. In general the lower alkane shows a lower surface tension value than the higher alkanes. In the present study as the microemulsion is water in oil, the surface tension value also follows the same trend; that is, the lower the alkane carbon number is in the microemulsion, the lower is its surface tension. The effect of temperature on the surface tensions of the microemulsion and SLS and HTAB surfactant solutions was also studied. It is very well-known that with an increase in temperature the surface tension of liquid decreases. As the temperature increases, the molecular interaction between the liquid molecules becomes weaker, since the hydrogen bond, which is a very weak bond, is the main factor for association between the two molecules. Therefore, the hydrogen bond can be easily broken when the temperature increases, leading to the decrease in surface tension. The effect of temperature on surface tension has been shown in Figure 5b. 4.4. Effect of Temperature on Structural Parameters. Various structural parameters obtained by employing the above equations have been shown in Tables 2 and 3. The effective Table 2. Structural Parameters for SLS [Composed with SLS (1.5·10−3 mol)/3-Methyl-1-butanol (0.011 mol)/Water/Oil (0.04 mol)] Microemulsion Systems ACN
T/K
Re/nm
Rw/nm
d/nm
Nd × 1019
N̅ s
N̅ cs
6
303 313 323 303 313 323 303 313 323 303 313 323
3.17 3.69 3.63 3.16 3.57 3.49 3.15 3.37 3.32 3.13 3.34 3.25
1.54 2.19 2.12 1.52 2.05 1.96 1.51 1.80 1.74 1.48 1.77 1.65
1.63 1.50 1.50 1.64 1.52 1.53 1.64 1.56 1.57 1.65 1.57 1.60
6.57 2.29 2.49 6.82 2.78 3.15 6.97 4.08 4.52 7.41 4.27 5.35
30 72 67 24 60 53 23 40 36 22 39 31
600 725 711 582 693 675 580 639 627 574 633 607
7
10
12
Figure 6. Relation between oil chain lengths with N̅ cs/N̅ s at 303 K. Symbols: ■, SLS; ●, HTAB.
Table 3. Structural Parameters for HTAB [Composed with HTAB (1.5·10−3 mol)/3-Methyl-1-butanol (0.011 mol)/ Water/Oil (0.04 mol)] Microemulsion Systems ACN
T/K
Re/nm
Rw/nm
d/nm
6
303 313 323 303 313 323 303 313 323 303 313 323
3.93 4.27 4.09 3.85 3.96 3.79 3.78 3.91 3.42 3.77 3.82 3.38
2.39 2.74 2.56 2.30 2.42 2.24 2.23 2.37 1.82 2.21 2.27 1.78
1.54 1.53 1.53 1.55 1.54 1.55 1.55 1.54 1.60 1.55 1.55 1.61
7
10
12
Nd × 10
19
1.75 1.17 1.42 1.96 1.69 2.13 2.16 1.79 3.95 2.20 2.05 4.27
N̅ s
N̅ cs
75 113 93 67 78 62 61 73 33 60 64 30
838 944 890 811 845 792 789 831 677 785 801 664
4.5. Total Aggregation Number Behaviors of the Microemulsion Systems. The total aggregation numbers of surfactant and cosurfactant (N̅ cs + N̅ s) are important measurements in the study of microemulsion stability. In Figure 8 the total aggregation numbers of SLS and HTAB have been shown at 303 K, 313 K, and 323 K temperatures, respectively. The total aggregation numbers depend on temperature and the ACN of oil. The values have been found to be decreased as the ACN of oil increases in both the SLS and HTAB systems. Paul and his co-worker also reported the similar results with other compositions of surfactant, oil, and cosurfactant.53,54 The decrease of aggregation number with increases in ACN of oil can be explained on the basis of the size of the oil molecule. As the size of oil molecule increases the steric repulsion between the aggregate heads increases and the number also decreases. In F
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shows a higher aggregation number than SLS. Since the alkyl chain length of the HTAB is larger than that of SLS, the headgroup of HTAB is larger than that of SLS and therefore HTAB can accumulate more number of aggregates than SLS.
5. CONCLUSIONS The maximum water solubilization capacity was observed for the HTAB microemulsion in both the presence and absence of NaCl. The water solubilization capacity in heptane/SLS system increases initially with increased NaCl concentration, reaches a maximum, and then decreases sharply at high concentrations. The experimental results show that with an increase in temperature, water solubilization of microemulsion systems also increases. The light scattering experiment shows that the Z-average diameter of the particle in a microemulsion is smaller for the SLS microemulsion systems than for the HTAB systems. Surface tension behavior of surfactant solutions and oil−water microemulsions in the presence of surfactants has been studied. Interestingly it has been found that a reduction of surface tension for the water−oil microemulsion systems is much higher than that of the simple aqueous surfactant solutions because of the higher ability of the microemulsions to accumulate the surfactants in the interface region. Microemulsions noticeably show lower surface tensions than surfactant solutions. The dilution method employed was found to be applicable to calculate the radius of the microemulsion droplet, water droplet, and aggregation numbers of surfactant and cosurfactant of the microemulsions. Initially
Figure 7. Relationship between N̅ cs/N̅ s with temperature for SLS and HTAB microemulsions in hexane. Symbols: ■, SLS; ●, HTAB.
the case of HTAB this steric repulsion is greater than SLS and thus the trend of decrease in aggregation number for HTAB is more prominent than that of the SLS microemulsion. The total aggregation number at a favorable temperature shows a maximum value, but at high temperature it decreases due to stability loss to aggregate the molecules. In all cases, HTAB
Figure 8. Plot of total aggregation number (N̅ cs/N̅ s) versus alkane carbon number (ACN) of oil for SLS and HTAB at (a) 303 K; (b) 313 K; and (c) 323 K, respectively. Symbols: ■, SLS; ●, HTAB. G
dx.doi.org/10.1021/je500274r | J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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the aggregation numbers of surfactant N̅ s and cosurfactant N̅ cs both increase as temperature increases but at higher temperature the values are found to be decreased. The ratio of the aggregation numbers of cosurfactant N̅ cs to surfactant N̅ s, that is, N̅ cs/N̅ s shows the more or less independency of oil chain length at a particular temperature, and the values decrease with an increase in temperature. Therefore, at low surfactant concentration, for the dispersion of a fair amount of water, the requirement of cosurfactant is more than that of the surfactant. The total aggregation number at a favorable temperature shows its maximum value but at a high temperature it decreases due to a stability loss of the aggregate the molecules. The total aggregation numbers are also influenced by surfactant headgroup. In all cases, HTAB shows higher aggregation number than SLS because of larger alkyl chain length of the HTAB than that of SLS. Because of a larger headgroup of HTAB than that of SLS, HTAB can accumulate more number of aggregates than the SLS.
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Corresponding Authors
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[email protected]. Tel.: 91-326-2235485. Fax: 91-326-2296632. *E-mail:
[email protected]. Funding
The authors gratefully acknowledge the financial assistance provided by University Grant Commission [F. No. 37-203/ 2009(SR)], New Delhi, to the Department of Petroleum Engineering, Indian School Of Mines, and Dhanbad, India. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank all individuals associated with the project. REFERENCES
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