Subtle Effects of Aliphatic Alcohol Structure on Water Extraction and

Article pubs.acs.org/Langmuir

Subtle Effects of Aliphatic Alcohol Structure on Water Extraction and Solute Aggregation in Biphasic Water/n‑Dodecane Andrew W. Knight,† Baofu Qiao,‡ Renato Chiarizia,§ Geoffroy Ferru,‡ Tori Forbes,† Ross J. Ellis,‡ and L. Soderholm*,‡ †

Department of Chemistry, E373 CB, University of Iowa, Iowa City, Iowa 52246, United States Chemical Sciences and Engineering Division and §Argonne Associate of Seville, Argonne National Laboratory, Lemont, Illinois 60439, United States

‡

S Supporting Information *

ABSTRACT: Organic phase aggregation behavior of 1-octanol and its structural isomer, 2-ethylhexanol, in a biphasic ndodecane−water system is studied with a combination of physical measurement, small-angle X-ray scattering (SAXS), and atomistic molecular dynamic simulations. Physical properties of the organic phases are probed following their mixing and equilibration with immiscible water phases. Studies reveal that the interfacial tension decreases as a function of increasing alcohol concentration over the solubility range of the alcohol with no evidence for a critical aggregate concentration (cac). An uptake of water into the organic phases is quantified, as a function of alcohol content, by Karl Fischer titrations. The extraction of water into dodecane was further assessed as a function of alcohol concentration via the slope-analysis method sometimes employed in chemical separations. This method provides a qualitative understanding of solute (water/alcohol) aggregation in the organic phase. The physical results are supported by analyses of SAXS data that reveals an emergence of aggregates in n-dodecane at elevated alcohol concentrations. The observed aggregate structure is dependent on the alcohol tail group geometry, consistent with surfactant packing parameter. The formation of these aggregates is discussed at a molecular level, where alcohol−alcohol and alcohol−water H-bonding interactions likely dominate the occurrence and morphology of the aggregates.

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INTRODUCTION Oil-in-water (o/w) or water-in-oil (w/o) microemulsions play an important role in a variety of fields, ranging from drug delivery to food chemistry to separation sciences.1−5 Microemulsions are defined as thermodynamically stable solutions composed of two immiscible liquids in which the minor constituent is dispersed as small, drop-like clusters stabilized by a surfactant. Surfactants, which serve to decrease the system free energy, are amphiphilic molecules composed of two componentsa hydrophilic headgroup and hydrophobic tail group6−8that organize themselves in immiscible solvents so as to decrease the interfacial tension between the two dissimilar liquids.9,10 As a whole, aggregate structures minimize electrostatic interactions and are understood to be a stabilizing mechanism for the formation of microemulsions.1,7 Unlike the more broadly studied oil-in-water microemulsions, interactions between surfactant molecules in nonpolar organic solvents play a significant role in the formation and stability of amphiphile aggregates.9 Hydrophobic interactions are not a factor, but instead, interactions between the headgroups of the amphiphiles dominate the aggregate structures as well as their formation energetics and stability. Hydrogen bonding has been shown to play a major role in the © XXXX American Chemical Society

formation, shape, size, and stability of water-in-oil aggregates.11,12 The aggregates themselves, sometimes referred to as premicelles,13 tend to be small with stabilization energies oftentimes on the order of kBT (where kB is Boltzmann’s constant, 1.38 × 10−23 J/K, and T the absolute temperature). As such, small changes in the molecular details of the amphiphile can have a significant impact on aggregation formation and solution behavior. For example, previous studies have shown that small changes in the tail group of similar surfactants can lead to significant differences in the physical properties and structure of the aggregates formed.14,15 Factors underlying this behavior may include differences in proton donor/acceptor strength and destabilization through steric crowding. Specifically, electron withdrawing/donating groups can lead to the observed differences in aggregation through influencing the strength of H-bonding whereas steric effects may involve destabilization of surfactant interactions through crowding. The combination of these effects influences the size and shape of the aggregate as discussed in detail, for example, in Received: December 28, 2016 Revised: March 23, 2017

A

DOI: 10.1021/acs.langmuir.6b04657 Langmuir XXXX, XXX, XXX−XXX

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Following the mixing stage, the aqueous and organic phases were separated by centrifugation (Model 614 B laboratory centrifuge, The Drucker Co., Port Matilda, PA) for 10 min at 3150 rpm and subsequently removed and stored in separate glass vials. Each solution was analyzed by tensiometry and Karl Fisher titration, and their structures probed with small-angle X-ray scattering (SAXS) and atomistic molecular dynamics (MD) simulations as described in more detail below. Interfacial Tension Measurements. To measure the interfacial tension (γ) between the organic and aqueous phases, a drop shape analyzer (DSA 100 Krüss tensiometer) coupled with a high-speed 1/2 in. CCD sensor camera and DSA4 software was used. Following contact with water, organic solutions were loaded into a 5 mL disposable syringe equipped with a 2.0 mm reverse pendant drop hook (Krüs NE97). The syringe was placed onto a mechanical arm with an injection pump and slowly immersed into a cuvette containing fresh distilled−deionized water to create the organic/aqueous phase interface, which was centered in the sight of the camera. The camera was focused, and the magnification scale was determined; then a drop was partially dispensed from the syringe and equilibrated for 30 min. The software extracted the bubble outline, the shape and volume of the drop were analyzed with the DSA4 software, and the mass (m) was determined using the known density of the organic solution.21 From this experiment, γ was calculated via

the investigation of 1-decanol and a number of alkylphenoxy alcohols relevant for metal separation applications.14 In that work it was shown that when the −OH group is attached to an aromatic ring, inductive effects reduce the basicity of the oxygen atom and, consequently, weaken the tendency to form hydrogen bonds, a result consistent with other published literature.16 The overlapping of steric and inductive effects is very important also for aliphatic alcohols. In these compounds, the length of the alkyl chain has no appreciable influence on aggregation. However, the H-bonding properties are very sensitive to branching in the immediate proximity of the −OH group, leading to reduced self-association and formation of smaller aggregates.17,18 For these reasons, simple aliphatic alcohols dissolved in a nonpolar solvent represent an ideal yet relatively simple system to observe how small tail group differences subtly impact water solubility, interfacial tension, and size and composition of the resulting aggregates. Lipophilic alcohols, like 1-octanol and 2ethylhexanol, are amphiphilic molecules; however, these molecules are considered to be weak surfactants with weak dipole−dipole, H-bonding, and van der Waals interactions.19 In general, a single hydroxyl group is a weakly polarizing functional group, and therefore the ability for lipophilic alcohols to decrease the interfacial tension between phases is relatively poor and the aggregates that form are relatively small. It has been observed on a molecular level, in solutions consisting of 1-octanol and water and ethanol, that the 1octanol molecules form unique nanodomains described as “loose agglomeration of more than one aggregate”.20 It has also been concluded that these large, diffuse agglomerations of aggregates are dynamic, relatively short-lived, and composed of numerous smaller, stable aggregates. Further, these authors observed an influence of the packing parameter on the morphology, similar to the behavior of stronger surfactants, producing aggregates that diverge from perfectly spherical governed by the tail group geometry.20 The motivation for this study was to investigate the relationship between the differences in organic phase aggregation resulting from tail group substituents and the ability to dissolve water into immiscible phases. To achieve this goal, we investigated 1-octanol and 2-ethylhexanol in biphasic systems consisting of n-dodecane and water. The results show that small tail group differences between these two molecules bring about different interfacial and bulk properties and different behaviors in the extraction of water. The simple cases investigated in this study shows how slightly different Hbonding interactions have a large impact on the aggregation and result in distinctly different physical properties. The results are cast in terms of a transitional regime in which molecularscale electrostatic, hydrogen bonding, and steric forces are of similar magnitude and compete for control over the observed physical behaviors.

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γ=

mg 2πr

(1)

where γ is the interfacial tension in mN/m, m is the mass of the drop in grams, g is gravity in m/s2, and r is the radius of the tip of the syringe hook in meters. Titrations. The Karl Fischer titration method was used to determine the concentration of water transferred into the organic phase after mixing and equilibration.22 The apparatus used in these experiments was an 831 KF coulometer (Metrohm AG, Switzerland). To measure the equilibrium concentration of water in the organic phase, a known mass of the organic solution (following contact with water) was injected into the Karl Fisher apparatus, and titration was initiated at the time of injection. This method provides a concentration of water in parts per million (ppm). To determine the water concentration in molarity, the equilibrium organic phase densities were measured and used to convert ppm to molarity. Small-Angle X-ray Scattering (SAXS). Following mixing, equilibration and phase separation, the organic solutions were transferred to the Advanced Photon Source (Argonne National Laboratory) at the beamline 12-ID-C to be analyzed by SAXS. Data were obtained by injecting the organic phases through a flow cell at a fixed distance from the detector. The sample-to-detector distance was adjusted to provide a detecting range for momentum transfer of 0.04 < q (Å−1) < 2.41. The scattering vector, q, was calibrated using a silver behenate standard and incident photon energy 19.0 keV, providing sufficient X-ray transmittance for the data acquisition. Scattering profiles were obtained via 0.5 s exposure times with a MAR 165 CCD detector (Norderstedt, Germany), with a 165 mm diameter active area and resolution of 2048 × 2048 pixels. The 2D scattering images were corrected for spatial distortion and detector sensitivity and then radially averaged to produce plots of scattered intensity, I(q), versus q. The I(q) data were normalized on an absolute scale (cm−1) by calibration with deionized water and after a thorough background subtraction consisting of just the diluent, n-dodecane. SAXS data were collected at various concentrations of alcohol (1-octanol and 2ethylhexanol) in n-dodecane ranging from 0.1 mM to 2 M, with the assumption that the signal recorded at 0.1 mM corresponds to the monomers contribution. The background subtracted, normalized SAXS data were analyzed using the Percus−Yevick interacting sphere model.23 Molecular Dynamics Simulations. Classical MD simulations were performed at the all-atoms resolutions by means of the package GROMACS 4.5.5.24 The CHARMM General Force Field (CGenFF 3.0.1) was utilized.25,26 The recommended TIP3P water model was chosen with the structures constrained using the SETTLE algorithm.27

EXPERIMENTAL SECTION

Biphasic Systems. Samples used for this study were prepared by contacting aqueous phases with immiscible n-dodecane solutions containing varying amounts of 1-octanol and 2-ethylhexanol in a range of concentrations (0.1 mM to pure alcohol). The organic reagents ndodecane, 1-octanol, and 2-ethylhexanol were purchased from SigmaAldrich (St. Louis, MO). 0.5 mL volumes of organic solutions were contacted with equal volumes of distilled deionized water via vortexer (Fisher Scientific fixed speed vortex mixer) at 3200 rpm for 1 min followed by constant mixing via shaker table (IKA MS 3 Digital Shaker) for 15 min at 1200 rpm at room temperature (22 ± 1 °C). B

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Langmuir For aggregation behavior in organic solutions, two alcohols were investigated: 1-octanol and 2-ethylhexanol. For each system, the alcohol concentrations were simulated at 0.5 and 2 M, and the concentration of water molecules was set to mimic the experimental values from Karl Fischer titrations. See the Tables S1 and S2 in the Supporting Information for the detailed concentrations and number of components. The initial structures of the simulations were built using the package Packmol,28 where all the molecules were randomly distributed (Figure S1). These structures were subjected to energy minimizations and annealing simulations to speed up the aggregation behavior.12,21,29,30 In the annealing simulation, the system temperature increased from 298 to 360 K for 0.1 ns, maintained at 360 K for 0.8 ns, and then was cooled down to 298 K within 0.1 ns. The temperature was maintained at 298 K for another 2 ns. The annealing simulation was repeated four times for each system. During the annealing simulations, the system density, the potential energy, and the radial distribution function (RDF) between the alcohol oxygen atoms were calculated to assess and ensure convergence (Figure S2). In the annealing simulations, all the parameters were the same as those in the production simulations below, except the system temperature. In the production simulations, the isothermal−isobaric ensemble (constant number of particles, temperature, and pressure) was employed. The reference temperature of 298 K was employed using the Nosé−Hoover algorithm31 with the relaxation time of 0.5 ps. The system pressure was coupled to 1 bar using the Parrinello−Rahman algorithm32 with the compressibility of 44.6 × 106 bar−1 and the relaxation time of 4 ps. Three-dimensional periodic boundary conditions were employed, and both the short-range Coulomb and van der Waals interactions were calculated up to 1.2 nm. Long-range Coulomb interactions were included using the smooth particle mesh Ewald method,33,34 in addition to the long-range dispersion correction for energy and pressure. All chemical covalent bonds were constrained by means of LINCS algorithm, which supported the stable simulations with an integration time of 2 fs.24,35 Each simulation was performed for 50 ns, with a saving frequency of 10 ps per frame to collect the simulation trajectory. Simulated SAXS data were calculated by means of nMoldyn.36 Note that nMoldyn actually calculates the atomic number weighted structure factor for SAXS.37,38 The atomic form factor f(q) is related to the atomic number (Z) via eq 2:

fi (q)|q → 0 = Zi for atom i, where q is the wave vector.

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Figure 1. Interfacial tension (γ) as a function of the alcohol concentration for (A) 1-octanol and (B) 2-ethylhexanol in n-dodecane following equilibration with H2O.

where R is the universal gas constant (8.3143 J/(mol K)), T is the temperature (K), γ is the surface tension (N/m), and c is the alcohol concentration (M). The slopes of the two data sets, determined by linear regression, are y = −1.99x + 12.50 and y = −1.54x + 14.1 for 1-octanol and 2-ethylhexanol, respectively. From these values the surface excess for 1-octanol was found to be 8.035 × 10−7 mol/m2, and that for 2-ethylhexanol was 6.225 × 10−7 mol/m2. The experimentally determined surface excesses were used to calculate the surface area occupied by each surfactant molecule through eq 4: 1 σ= NA Γ (4) where NA is Avogadro’s constant (6.023 × 1023 mol−1). The headgroup area was thus calculated for each molecule to be 2.07 and 2.67 nm2 for 1-octanol and 2-ethylhexanol, respectively. Extraction of H2O into n-Dodecane. In these ternary, biphasic solutions containing structural isomers 1-octanol or 2ethylhexanol in n-dodecane contacted with water, we observed a rapidly increasing water concentration in the organic phase with increasing initial alcohol concentration (Figure 2A). As a general trend, when the alcohol concentration was less than 1 M, relatively little water was extracted into n-dodecane. For instance, the concentration of water in the organic phase was 0.02 ± 0.02 M for a 0.5 M concentration of either alcohol. For a 5 M alcohol concentration, however, 1-octanol extracted nearly twice the amount of water as 2-ethylhexanol did ([H2O]org = 1.62 ± 0.04 and 0.92 ± 0.06 M for 1-octanol and 2-ethylhexanol, respectively). The extraction of water by the two alcohols can be analyzed in terms of classic solvent extraction (SX) theory, with alcohol acting as the extractant in the n-dodecane diluent and water as the extracted species.45 In this context, the extraction of water can be described by the chemical equilibrium

(2) 39

RESULTS Tensiometry. The interfacial tension (γ) results, obtained from tensiometry data for both alcohols as a function of their concentrations, are presented in Figure 1. With the exception of low octanol concentrations, the plots are linear across the entire wide range of concentrations, steadily approaching γ of the pure alcohol/water systems (this study: 7.7 ± 0.4 mN/m).40 The γ in 1-octanol decreased from 24.3 ± 2.1 mN/m (0.5 mM) to 7.7 ± 0.4 mN/m (pure 1-octanol) with comparable values of 25.3 ± 1.2 (0.5 mM) to 13.1 ± 0.5 mN/m for the 2-ethylhexanol solutions. The deviation of linearity in the slope at low alcohol concentrations is consistent with the requirement that the surface excess must gradually decrease toward zero as the alcohol content approaches zero.9,41 The slopes of the γ vs ln[alcohol] plots for both solutions can be used to extract the surface excess, Γ, an expression of the concentration deviation of the dodecane surface relative to the bulk42 and a comparative indicator of free energy changes. Γ is calculated from the Gibbs adsorption equation:42−44 dγ 1 Γ=− RT d ln(c)

K ex

qROH + nH 2O ⎯→ ⎯ (ROH)q (H 2O)n

(5)

where the bar denotes organic phase species and whose equilibrium constant can be written as

(3) C

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Figure 2. (A) Equilibrium concentration (M) of H2O extracted into the organic phase as a function of the initial alcohol concentration (M) in ndodecane at 27 °C. (B) A log−log plot showing DH2O versus the equilibrium concentration of alcohol (M).

Kex =

water data against the free equilibrium alcohol concentrations. Based on the preliminary q value, these were calculated by subtracting twice the organic phase water from the initial alcohol concentrations. The results are shown in Figure 2B and provide for ethylhexanol a slope of 2.0 which, from eq 5, provides an alcohol to water association of 2:n. It is important to note that the analysis performed does not provide the value of n, i.e., the number of water molecules in the aggregates. The complete stoichiometry of equilibrium (5) remains undetermined as we can only claim that in the organic phase species the ratio of alcohol to water is 2:n. It can be seen from the data in Figure 2B that 1-octanol system is more complex. Whereas the slope is similar to that of the 2-ethylhexanol data at lower concentrations, it increases sharply for alcohol concentrations higher than ∼1.5 M, suggesting the probable formation of larger aggregates at these higher concentrations. It is also worth noting that the extraction of water is associated with minimal energy difference between the two alcohol isomers. For example, it can be seen in Figure 2 that at equilibrium concentrations of 1 M the D values are 1.15 (D1) and 0.75 (D2) for 1-octanol and 2-ethylhexanol, respectively. Since, as shown in eq 8, D is proportional to Kex, it holds that

[(ROH)q (H 2O)n ] q

n

[ROH] [H 2O]

(6)

In eq 6, concentrations have been used instead of activities because the activity of water in the aqueous phase can be considered constant and close to one. Within this context, we attribute to aggregation any deviation from ideal behavior of organic phase species. It follows that the distribution ratio of water, D (in SX D is defined as the ratio of the organic and aqueous concentrations of the distributing species in all its chemical forms) can be written as D=

[H 2O] = [H 2O] = n(ROH)q (H 2O)n [H 2O]

(7)

By introducing eq 6 into eq 7, we arrive at the following expression for D: q

D = nKex[ROH]

(8)

Equation 8 can be used to apply to the water extraction data the technique of slope analysis, a graphic analysis technique commonly used in SX to obtain information on the composition of the extracted species.46 Taking the logarithms of each side of eq 8 provides eq 9: log D = log n + log Kex + q log[ROH]

Δ(ΔG0) = −RT ln(D1/D2)

(10)

Equation 10 provides a value of ∼1.1 kJ/mol for the free energy difference between water extraction by the two alcohols at 300 K, favoring the 1-octanol complex. This is a very small energy difference, corresponding to a fraction of the thermal energy kBT (2.5 kJ/mol) at 300 K. Small-Angle X-ray Scattering. SAXS was used to probe the size and morphology of the putative solution aggregates.47−49 The data were obtained for the organic-phase samples of biphasic systems with alcohol concentrations of 0.1 mM and 0.1, 0.5, and 2 M. The scattering patterns from samples containing 0.5 and 2 M alcohol suggest aggregation, as seen in Figure 3. A robust background subtraction was required for our fitting approach, so that only scattering from the oxygen atoms (alcohol and water) contributed to the data. A consequence of this fitting approach is that only samples with

(9)

Assuming that the value of n (the number of water molecules associated with an alcohol/water aggregate) is constant, the resulting log−log plot of the organic phase water concentration (which is equal to D, see eq 7) versus the equilibrium alcohol concentration will result in a straight line with slope q, which as seen from eq 9 represents the average number of alcohol molecules involved in the extraction of water according to equilibrium (5). In order to obtain an initial, approximate value of q, the organic water concentration data were plotted against the initial concentrations of the alcohols (plot not shown for brevity). Slope values of ∼1.8 were derived for both alcohols. A more precise value of q was then acquired by plotting the organic D

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Table 1. Fitting Parameters of the Percus−Yevick Fit of the SAXS Data at 2 M Alcohol fitting parameter

1-octanol

2-ethylhexanol

sphere radius (Å) interaction radius (Å) volume fraction reduced χ2 a

5.5 ± 0.3 6.4 ± 0.1 0.11 ± 0.01 0.122

4.16 ± 0.04 5.58 ± 0.02 0.204 ± 0.003 0.351

a Generally it is considered that reduced χ2 values significantly less than 1 are a result of overfitting data. In this case, we observe small χ2 values largely as result of a small signal-to-noise ratio in the high-q region (Q/ (A−1) > 0.55), from background subtracting hydrocarbon interactions. The observed noise is small variations in the scattering of hydrocarbons among different measurements. Conclusions regarding aggregations are made from observation in the low-q region.

Figure 3. (A) Experimental and (B) simulated SAXS data of the four systems. A solvent peak is observed in the high q-region at approximately 1.4 Å−1, identified as a correlation peak between hydrocarbon atoms.21,30 Long-range correlation peaks in the low qregion were observed in systems containing 0.5 and 2 M alcohol. In the 2-ethylhexanol systems (blue and red lines) a correlation peak is observed at around 0.5 Å−1, while in 1-octanol (green and black lines) a small broad peak is observed around 0.4 Å−1 when the 1-octanol concentration is 2 M.

calculated by optimizing the parameter for sphere radius in the data fitting. This can be interpreted as the radius of the scattering component of a spherical particle that produces the “form factor” component of the scattering function (i.e., the electron-rich oxygen atoms of the water and alcohols that make up the micelle core). The structure factor models the “volume exclusion effect” of a concentrated system of “hard spheres”. The interparticle scattering is therefore related to the volume fraction of the hard-sphere particles. These hard spheres repel infinitely when they come into contact, and the point at which they repel is the interaction radius or the radius of the hard sphere. If the system is described successfully using the PY hard-sphere model, then the interaction radius of the hard spheres (structure factor component approximating interparticle scattering) should be close to the sphere radius of the scattering particle that makes up the form factor component. It can be seen from the fitting results presented in Table 1 that the interaction radius is approximately 1 Å longer then the sphere radius, suggesting that a portion of the “hard” component of the sphere (i.e., the point where the spheres repel infinitely) extends slightly beyond the scattering component of the particle (i.e., the oxygen atoms in the core). This may arise if a part of the “hard” sphere is also made up from the “invisible” hydrocarbon chains, as these would not contribute to the form factor scattering but would contribute to the structure factor scattering as it changes the center-of-mass to center-of-mass distance. Alternatively, this discrepancy may arise from the assumptions made within the model, notably those invoked to describe the aggregates as solid spheres. The interaction or “hard”-sphere radius is found to be 6.4 ± 0.3 Å for 1-octanol and 5.58 ± 0.04 Å for 2-ethylhexanol (Table 1). Using these interaction radii from Table 1, we can obtain VHS, the hard-sphere volume: 4 VHS = πr 3 (11) 3

a concentration of 2 M produced a signal strong enough to extract metrical information about the aggregates. The scattering peak in the high q-region at approximately 1.4 Å−1 is attributed to a correlation between hydrocarbon atoms.21,30 Long-range correlation peaks in the low q-region were observed in systems containing 0.5 and 2 M alcohol. While these SAXS data show that both 1-octanol and 2ethylhexanol produce aggregates under these conditions, they differ as evidenced by the details of their scattering patterns in the low q-region. For the 1-octanol sample a small broad peak is observed around 0.4 Å−1 while in 2-ethylhexanol systems a correlation peak is observed at around 0.5 Å−1. The Percus−Yevick (PY) hard-sphere model was used to evaluate the SAXS data (Figure 4).23 The PY fitting parameters include scale factor, sphere radius, interaction radius, and volume fraction (Table 1). This model, used for concentrated particulate systems, fits the data using a form factor (intraparticle scattering) and a structure factor (interparticle scattering). The form factor is that of a sphere, and this is

If r is the hard-sphere radius in nm, then VHS = 1.1 nm3 for 1octanol. The volume fraction (VF) parameter reported in Table 1 corresponds to the fraction of total sample volume (VT) taken up by the hard-sphere particles as approximated by the structure factor component of the model. This can be used in the equation for the volume fraction (VF), relating the total number of aggregates (Nagg) and VHS to VT

Figure 4. SAXS spectra of the organic phases consisting of 2 M 2ethylhexanol (blue line) and 2 M 1-octanol (red line). The background scattering are subtracted for air, the sample holder, and the solvent of dodecane. The dashed and dotted lines represent spectra fitting based upon Percus−Yevick model.

VF = E

NaggVHS VT

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Langmuir which can be rearranged to give the effective volume (Veff) of an aggregate or the average volume an aggregate takes up. Following this approach, Veff is the volume of sample that contains just one aggregate.

Veff =

VHS V = T VF Nagg

(13)

For 1-octanol, Veff = 10 nm3/aggregate. This means that there is 1 aggregate in every 10 nm3 of sample. The number of water molecules per aggregate can then be calculated using this volume and the concentration of water. The concentration of water in the 1-octanol system is 0.35 mol/L. This means that there is 0.35NA molecules in every liter volume of sample (NA is Avogadro’s number) or 2 molecules of water in every 10 nm3 of sample (for the octanol system). As there is only 1 aggregate in every 10 nm3 of sample, and we assume that all of the water is in the aggregate core, then there are 2 molecules of water (on average) in every aggregate. This result corresponds to the number of water molecules/aggregate, represented in equilibrium eq 5 as n. Combined with the results from the conventional slope analysis as applied to the water extraction data, the description of the average aggregate at lower 1-octanol concentrations includes four alcohols and two water molecules. Simulated Small-Angle X-ray Scattering. Atomistic MD simulations were performed to replicate the scenarios of 0.5 and 2 M alcohol in n-dodecane contacted with water. SAXS data simulated from the results of these calculations show the presence of a high q-peak at approximately 1.4 Å−1 (Figure 3b). Long-range correlation peaks are also observable, with intensities dependent on the alcohol concentration. In the 2 M 1-octanol solution a broad peak is observed at 0.25 Å−1 while in the 2 M 2-ethylhexanol system, a similar peak is observed at around 0.5 Å−1. Consistent with the corresponding experimental SAXS, the lower-q peak in the 1-octanol pattern is broader than the equivalent 2-ethylhexanol peak, suggesting the latter has stronger correlations at a smaller, less-disperse distance. From the simulations, the average aggregation number and morphology were also obtained. In 2 M 1-octanol and 2ethylhexanol solutions, the average aggregation number was 5.3 ± 0.3 and 4.6 ± 0.2, respectively. To this end, the first minimum (3.5 Å) of radial distribution function between alcohol oxygen atoms was employed as the upper distance criteria for the aggregate definition. The program g_clustsize of the GROMACS package was then used to calculate the distribution of the aggregate sizes. This calculation was done for all the saved simulation frames so that the average number of alcohol per aggregate and its standard deviation were obtained. In the 0.5 M solutions the corresponding average aggregation numbers were 5.2 ± 0.5 and 4.1 ± 0.3 (Figure 5). The MD simulations describe the morphology and demonstrate the association of alcohol molecules through H-bond and suggest that the morphology of 1-octanol favors the elongated, nonspherical aggregates, while 2-ethylhexanol prefers curved and spherical aggregates.

Figure 5. Snapshots of the last frame for each atomistic simulation. All alcohols are presented as stick models in which the hydrocarbon chains (CHx) are green and the hydroxyl group is red (O) and white (H). Water molecules are presented as ball models with red (O) and white (H). The molecules of the solvent, n-dodecane, are omitted from the display.

range and have been used to obtain surface excess concentration values of 8.035 × 10−7 and 6.225 × 10−7 mol/ m2 for 1-octanol and 2-ethylhexanol, respectively (Figure 1). The larger surface excess for 1-octanol over 2-ethylhexanol suggests that there is a greater tendency for 1-octanol to exist as interfacial species than 2-ethylhexanol. These surface excess results are used to obtain the average headgroup area for each of the alcohols, which were found to be 2.07 and 2.67 nm2 for 1-octanol and 2-ethylhexanol, respectively. Taken together, these results confirm that 1-octanol is slightly more surface active than 2-ethylhexanol: the surface excess is larger and the headgroup area is smaller, resulting in a higher concentration at the interface. The surface areas for both alcohols are rather large with respect to the physical size of a single hydroxyl group, but the results appear to be consistent with the surfactant strength. For example, these values can be compared to the experimentally determined γ values at the water/air interface of 1-decanol solutions, an alcohol which is more hydrophobic than 1-octanol and 2-ethylhexanol. The plot of surface tension versus 1-decanol concentration provides a slope equal to −3.23 mN/m, leading to a calculated average surface excess of 1.30 × 10−6 mol/m2 and an average headgroup area of 1.28 nm2/molecule.52 Aggregation Equilibrium in the Absence of a Critical Micelle Concentration. Surfactants have the remarkable ability of readily assembling at the hydrophilic/hydrophobic interface, thus decreasing the interfacial tension, γ.7,50 Once the surfactant concentration is high enough to saturate the interface at the critical aggregate concentration (cac),50 aggregates begin to form in the bulk. (We choose the term cac to clarify the distinction between the concentration of surfactant at which reverse micelles appear and the concentration of monomer not involved in a given aggregate. For the purpose of our paper we have used the more precise term “critical aggregation

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DISCUSSION The interfacial tension (γ) for water/n-dodecane solutions as a function of 1-octanol or 2-ethylhexanol concentration reveals that both alcohols behave as surfactants because they both decrease the surface tension between two immiscible phases.6,50,51 Both plots are linear over a wide concentration F

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Langmuir concentration” (cac) to describe the concentration necessary to produce aggregates.11) Any additional surfactant molecules dissolved in solution will be in dynamic equilibrium between monomers and micelles.19,53 The strength of a surfactant can be assessed through the cac value, where a high cac (>0.1 M) indicates a weak surfactant, while a low cac (