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Dual Action of Hydrotropes at the Water/oil Interface Andrei A Novikov, Anton P. Semenov, Viviana Monje-Galvan, Vladimir N Kuryakov, Jeffery B. Klauda, and Mikhail A. Anisimov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05156 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 14, 2017

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The Journal of Physical Chemistry

Dual Action of Hydrotropes at the Water/Oil Interface

Andrei A. Novikov1, Anton P. Semenov1, Viviana Monje-Galvan2, Vladimir N. Kuryakov3, Jeffery B. Klauda2, and Mikhail A. Anisimov2,3,* 1

Gubkin University, Moscow, 119991, Russia Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742, USA 3 Oil and Gas Research Institute of the Russian Academy of Sciences, Moscow, 117333, Russia 2

Abstract

Hydrotropes are substances containing small amphiphilic molecules, which increase solubility of nonpolar (hydrophobic) substances in water. Hydrotropes may form dynamic clusters (less or about 1 ns lifetime) with water molecules; such clusters can be viewed as “pre-micelles” or as “micellar-like” structural fluctuations. We present the results of experimental and molecular dynamics (MD) simulation studies of interfacial phenomena and liquid-liquid equilibrium in the mixtures of water and cyclohexane with the addition of a typical nonionic hydrotrope, tertiary butanol. The interfacial tension between the aqueous and oil phases was measured by Wilhelmy plate and spinning drop methods with overlapping conditions in excellent agreement between techniques. The correlation length of the concentration fluctuations, which is proportional to the thickness of the interface near the liquid-liquid critical point, was measured by dynamic light scattering. In addition, we studied the interfacial tension and water-oil interfacial profiles by MD simulations of a model representing this ternary system. Both experimental and simulation studies consistently demonstrate a spectacular crossover between two limits in the behavior of the water-oil interfacial properties upon addition of the hydrotrope: at low concentrations the hydrotrope acts as a surfactant, decreasing the interfacial tension by adsorption of hydrotrope molecules on the interface, while at higher concentrations it acts as a co-solvent with the interfacial tension vanishing in accordance to a scaling power-law upon approach to the liquid-liquid critical point. It is found that the relation between the thickness of the interface and the interfacial tension follows a scaling law in the entire range of interfacial tensions, from a “sharp” interface in the absence of the hydrotrope to a “smooth” interface near the critical point. We also demonstrate the generic nature of the dual behavior of hydrotropes by comparing the studied ternary system with systems containing different hydrocarbons and hydrotropes.

clusters have a size order of 1 nm and lifetime from dozens to hundreds picoseconds, being stabilized by hydrogen bonds between hydrophilic parts of hydrotrope molecules and water (see ref.6 and a movie in the supporting material, showing the simulation of the formation and dissipation of dynamic micellar-like clusters of TBA in an aqueous solution with 7 mol% TBA). Moreover, hydrotropes may stabilize mesoscopic (100-200 nm) droplets of “oil” preventing or significantly delaying their coalescence.6-8

Introduction Hydrotropes are substances consisting of small amphiphilic molecules. Examples of nonionic hydrotropes are lowmolecular-weight alcohols and amines. Hydrotropes may be completely or significantly water-soluble and can increase solubility of hydrophobic substances (“oil”) in water, serving as a co-solvents.1 Like surfactants, small addition of a hydrotrope reduces the interfacial tension between water and oil.2 Hydrotropes are effectively used as co-surfactants for stabilization of microemulsions.3 However, unlike surfactants, hydrotropes do not form stable micelles in aqueous solutions because the hydrophobic parts of their molecules are too small.4-5 Instead, some nonionic hydrotropes (tertiary butanol (TBA) is one of the most characteristic examples) may form dynamic non-covalent molecular clusters in aqueous solutions. Molecular Dynamics (MD) simulations show that these

In this work, we present the results of experimental and MD simulation studies of interfacial phenomena and liquid-liquid equilibrium in the mixture of water and cyclohexane (CHX), also referred to as “oil,” with the addition of tertiary butanol. TBA is known as “perfect amphiphile” with hydrophobic (hydrocarbon) and hydrophilic (hydroxyl) parts precisely divided by the water and “oil” interface.9 TBA is completely soluble in both water and

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CHX at ambient conditions. The physics of water-mediated hydrophobic interactions has been recently reviewed by BenAmotz10; also, Rankin et al11 suggest that there is essentially no free energy driving force associated with the aggregation of TBA at concentrations up to a mole fraction ~0.02. However, semidilute aqueous solutions of TBA exhibit pronounced thermodynamic anomalies associated with hydrogen-bond induced water-TBA molecular clustering,6, 12 which was also previously suggested in literature.2 MD simulations6, 13-14 of TBA dilute and semi-dilute aqueous solutions show the formation of small aggregates of TBA with a preference of tail-to-tail configurations to minimize exposure of the hydrophobic portion of the molecule to water (see the simulation movie as a supporting material). While the mutual solubility of water and CHX is very low, at high TBA concentrations, the ternary water-CHX-TBA system demonstrates complete mutual solubility of all three components. We focused our attention on water-“oil” interfacial phenomena and phase equilibria in this ternary system. In particular, our goal was to investigate the dual role of the hydrotrope, namely, surfactant-like behavior dominating at small concentrations and co-solvent, near-critical behavior at higher concentrations upon approach to the critical point of liquid-liquid coexistence. Our study consistently demonstrates a spectacular crossover between two universal limits in the behavior of the water-oil interfacial properties upon addition of the hydrotrope: surfactantlike decrease of the interfacial tension at low hydrotropes concentration (Gibbs – von Szyszkowski isotherm, see section 2 in supplemental material), caused by adsorption of hydrotrope on the interface, and power-law vanishing of the tension upon approach to the liquid-liquid critical point as followed from the scaling theory. This dual behavior was already suggested in literature,5 but has not been examined in detail through experiment or simulation. MD simulations were used to expand the study of the ternary system of TBA-water-cyclohexane. These simulations help to interpret the results from experiments by providing details of the components’ interactions at the atomic level. Extensive studies have been made on this ternary system to examine the surfactantlike character of the hydrotrope to shield a hydrocarbon droplet from the water solvent.6, 12, 14

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Measurements of surface and interfacial tensions Heterogeneous, containing two liquid phases, systems were prepared by mixing of the components at room temperature, following by stirring for at least 6 hours to ensure phase saturation. Since the phases were emulsified as a result of the stirring, the samples were further equilibrated for at least 16 hours for complete stratification of phases. Probes, taken with a pipet from the aqueous and oil phases, were used for the measurements of the refractive index, density, concentration, surface tension between air and each liquid phase, and interfacial tension between two liquid phases. All the measurements were performed at 20 °С using water circulation baths MPC-E (Huber, Offenburg, Germany) and F25ME (Julabo, Seelbach, Germany). Concentrations of TBA and CHX were determined by gas chromatography with Crystal-5000.2 apparatus (SKB Chromatek, Yoshkar-Ola, Russia; automated injection of 1 µL; stainless steel column 2 m × 2 mm with HayeSep D 80/100 mesh was heated from 90 °C to 210 °C at 10 °C/min; carrier gas was argon from NIIKM, Moscow, Russia of 99.998% purity at 45 mL/min). Measurements of the density, air-liquid surface tension, and liquid-liquid interfacial tension (at the values >10-3 N/m) were performed with a force tensiometer K20 (Krüss, Hamburg, Germany) by the Wilhelmy plate method. The results of our measurements of the density and surface tension CHX/air and water/air at 20 °С with the force tensiometer are in good agreement with the literature.15 Interfacial tensions were also measured with a spinning drop tensiometer SVT-20N (DataPhysics, Filderstadt, Germany). The results of the measurements of the interfacial tension of three samples in the range where these two methods are overlapping (between 1 and 2·10-3 N/m) are in good mutual agreement. Measurements of the correlation length Nine two-phase samples with the states close the criticalpoint composition were investigated by dynamic light scattering (DLS) using a Photocor Complex spectrometer (Photocor, Moscow, Russia) (see Table S4). The purpose of these measurements was to examine the correlation length of critical fluctuations in the systems with an anomalously low interfacial tension. The measurements were carried out in the aqueous (heavier) phase at 20 °С; the samples for this portion of the study were held at this temperature for more than 10 hours before taking the measurements. The light-scattering angle was 90° a laser wavelength of 654 nm, a laser power of 25 nW, and a correlation function accumulation time of 100 s. The correlation length of critical fluctuations can be found from the exponential decay of the time-dependent lightscattering intensity correlation function,16

Experimental Methods and Procedures Measurements of phase equilibria Tertiary butanol (Carl Roth, Karlsruhe, Germany, >99% purity), cyclohexane (Ecos-1, Moscow, Russia, >99.5% purity), and distilled water were used without further purification. The transitions from two-phase liquid-liquid coexistence to a onephase homogeneous solution in the ternary system was determined by volumetric and gravimetric titration at 20 °С. Since the simulation studies were performed at 25 °С, we checked the temperature dependence of the interfacial tension in the range 2025 °С and found that for the TBA concentrations below 20 wt% the changes in the interfacial tension do not exceed experimental uncertainties. Only simulation results with TBA concentrations smaller than 20 wt% were compared with experimental data. At the gravimetric titration, small titration portions of tertiary butanol (0.1–0.2% of mass) were added to heterogeneous mixtures of water and cyclohexane. The existence of remaining immiscibility was checked after having the sample stirred for 3 min. The titration continued until the disappearance of the meniscus (the signature of complete mutual solubility) was detected.

() = 〈() ( + )〉 = ⁄ ,

(1)

where I is the intensity, t is time, τ is the diffusive relaxation time. We obtain the correlation length ξ from the expression for the rate of relaxation,

= =

,

(2)

#

where is the effective diffusion coefficient, = sin is the light-scattering wavenumber, n is the refractive index, λ is the wavelength of light, θ is the light-scattering angle, and $

2


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The Journal of Physical Chemistry every simulation step in bars. The reported ( values on Table S6 were block-averaged every 1 ns. Hydrotrope concentration in the aqueous phase was determined from the alcohol and water atom number density profiles (see Figure S2), computed using CHARMM32 by dividing the simulation box along the zdirection in equidistant bins and counting the number of molecules of a given species per bin. The symmetric molecule density profiles (MDP; atom density profile (ADP) divided by the corresponding number of atoms per molecule) were used to estimate the interfacial thickness by fitting a hyperbolic tangent function to the total density profile using Matlab cftool33:

is the viscosity. Equation (2) is valid for the condition % ≪ 1 which is always satisfied in our experiments. DLS measurements require a known viscosity value of the medium. The viscosity of the aqueous phase was assumed to be equal to the viscosity of binary TBA solutions in water with the same TBA concentration.17-18 In order to ensure this assumption was reasonable, the viscosity in the aqueous phase of two samples additionally containing SiO2 nanoparticles of known size (R=55 nm) were obtained from DLS measurements of the Brownian diffusion of these nanoparticles. In these samples the light scattering intensity was dominated by the contribution from the nanoparticles. The viscosity was obtained through an equation equivalent to Eq. (2) in which % was replaced by R. The values of viscosity were in good agreement with the estimates used for the correlationlength measurements. Computational Methods The systems studied using MD simulations are summarized on Table S5 and have corresponding overall compositions to the experimental studies. Each system has triplicate simulation trajectories to allow for proper average and standard error calculations. Each replica started from a different configuration to ensure they were independent runs and the statistical analysis is unbiased. Initial system setups were built with Packmol software19-20 with a box of water-alcohol mixture on top and bottom of a box containing cyclohexane (see Figure S1; all systems were visualized using VMD,21 and simulation snapshots were generated using this software). A system of the binary mixture water-cyclohexane was also simulated to compare against experimental values. The TIP4P-EW water model22 was used in these systems due to its superior accuracy compared to TIP3P model for liquid-liquid equilibrium studies. All systems were initially run for 100 ps to allow a quick thermal equilibration at 25 °C and 1 bar using the NAMD Software Package.23 The NPAT ensemble (constant number of molecules, pressure, temperature, and fixed area) was then used during all production runs to complete trajectories of 75 ns for each system using the CHARMM general force field parameters.24 This setup allows changes in the z-dimension, but x and y dimensions were fixed. Systems with high TBA content (S15-S18 in Table S5) were simulated using the NPT ensemble because the use of an area constraint in the NPAT ensemble resulted in negative interfacial tension values. The simulation time-step was 2fs, and the SHAKE algorithm was used to constraint hydrogen atoms.25 We used Langevin dynamics26-27 to keep the temperature constant, and a Langevin piston coupled to a temperature bath controlled by Langevin dynamics23, 28-29 to keep the pressure constant in NAMD. VDW and electrostatics interactions were computed with a Lennard-Jones force-switching function over 8 to 10 Å.30 Long-range electrostatics were accounted for using the Particle Mesh Ewald method,31 and all runs used periodic boundary conditions.

-

∆5 = ∆5 tanh 9 ;, :

where ζ is the characteristic length of the thickness of the interface (thickness=2ζ), ∆ρ is the density profile, and ∆ρo is the difference between the bulk densities of the liquids. All the profiles were re-centered around the interface prior to the fitting. Results and Discussion Ternary phase diagram

Figure 1. Phase diagram of the ternary system water-cyclohexanetertiary butanol at 20 °С in terms of mass fractions. The red circle indicates the critical point of liquid-liquid coexistence; the rainbow colored lines are the tie-lines (red to violet colors correspond to the change from low to high interfacial tension); the solid blue line shows the phase separation boundary; the dotted blue line conjugates the mid-points of the tie-lines. Detailed experimental results are summarized in Tables S1-S3

While the obtained liquid-liquid transition boundaries are qualitatively in agreement with earlier reported studies,7 the determination of the component concentrations in the coexisting phases enabled us to accurately locate the liquidliquid critical point with the composition of 82.61 mol% water (53.36 wt%), 16.22 mol% TBA (43.10 wt%), and 1.17 mol% CHX (3.54 wt%). The tie-lines connecting the equilibrated phases show that TBA initially enriches the aqueous phase (the tie-lines have slope to the right), but then its distribution drastically changes, and near the critical point TBA enriches the oil phase (the tie-lines have slope to the left). The ternary phase diagram at 20 °С and ambient pressure is shown in Figure 1.

The interfacial tension (γ) was computed from the NAMD pressure output using eqn. (3), ( = 0.5,- ./-- 0 0.5(/11 + /22 )3

(4)

(3)

where Lz is the size of the simulation box normal to the interface and Pii are the average pressure tensor components at

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interface is saturated by TBA, the bulk solubility of TBA starts to play an increasingly important role and a correction is needed; the difference in bulk densities of the liquids starts to become noticeable and changes in a linear fashion (refer to Figure S3). This adjustment, performed as a rescaling of the TBA concentration axis, < = < ∗ (1 0 >< ∗ ), (5) where x* is the TBA concentration in the simulated model and a=0.07, works well for the samples from S1 through S10. For larger TBA concentration this simple rescaling is not sufficient. The effect of the hydrotrope on the interfacial tension varies between two extremes. At low concentrations, TBA acts as a typical surfactant with the surface tension and the interfacial tension decreasing in accordance to the Gibbs– von Szyszkowski isotherm34 (see more details in Figures S7 and S8, and the discussion in section 2 in the supporting material):

Interfacial tension The interfacial tension between the aqueous and oil phases was measured by Wilhelmy plate and spinning drop methods with excellent agreement between techniques in the overlapping range of concentrations each method can measure. In addition, we studied the interfacial tension with MD simulations of a model that adequately represents this system. The results of the experimental measurements and simulations are presented in Figure 2 and Tables S4, S5 and S6.

?

?@

= 1 0 Aln(1 + C < + ⋯ is a function representing

the surfactant-like behavior in the dilute solution limit < → 0. Since 1 0 A ln(1 + C = AC. We note that in limit < → 0 this function represents the universal result of the Gibbs thermodynamics of adsorption.34 The other function, H = 1 1 N

> 9 M ; , is also asymptotically universal.35-36 In the limit 1 (
= 0.0083 and xc = 16.22 mol% of TBA in the aqueous phase (fixed experimental value)

According to scaling theory, the near-critical interfacial tension ( ∝ % ∝ X Y , where Z = 0.63 is the universal critical exponent of the correlation length, and X is a “distance” to the critical point along the medium line of the phase coexistence (along the rectilinear diameter in a binary-fluid). In a ternary system, a change in X at constant temperature and pressure corresponds to the path along the midpoints of tie-lines of the ternary phase diagram37 as shown in Figure 1 by the blue dotted line. However, the path along the liquid-liquid coexistence with changing TBA concentration and the difference of densities of the coexisting phases corresponds to the change of T as T ∝ X \ , where ] = 0.326 (the universal critical exponent of the density along the phase coexistence).16, 37-39 Consequently, as a function of the density or TBA concentration in the aqueous phase, the interfacial tension varies asymptotically as ( ∝ % ∝ T Y⁄\ . Hence, along this

path, H = > 9

1^ 1 N 1

; with Q = 2ν⁄] = 3.85.

From Eq. (7), H(1→`) = H , while Ha(1M 1)→`b = H . The result of the fit of this model with two adjustable systemdependent parameters, a1, a2, and the fixed experimental value xc = 16.22 mol% (TBA in the aqueous phase) to our experimental data is presented in Figure 3. The experimental results clearly show the crossover behavior of the interfacial tension in consistency with the dual role of the hydrotrope: at low concentrations, TBA acts as a surfactant-like adsorbate until the water-oil surface is saturated. Upon further addition, the co-solvent ability of TBA to smooth the difference between water and oil becomes more and more effective, ultimately resulting in the vanishing of the interface.

Figure 4. Sample MDP fits to estimate the interface thickness for the (A) S1 and (B) S10 systems

The interfacial density profiles for all the simulated interfaces are well described by the same function of the coordinate z, as given by Eq. (4). Therefore, if the density difference is normalized by the difference in the bulk densities and the distance z is scaled by 1/2 thickness of the interface, ζ, the density profile becomes universal (Fig. 5). Table S7

5



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summarizes the fitted parameters for systems S1 to S10. Figure S3 shows the thickness of the interface and Δ5 , the difference in bulk densities of the coexisting liquids as functions of the hydrotrope concentration in the aqueous phase for the simulation systems (S1 to S10). As expected, the thickness of the interface increases with decreasing interfacial tension, diverging when the interface finally vanishes (at the critical point of demixing).

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within the uncertainties of the DLS measurements, as demonstrated in Figure 6. Remarkably, the universality of the interfacial density profile suggests that the interfacial tension is scaled as ( ∝ d for any fluid interface, sharp or smooth, even if far away from the critical point the thickness cannot be identified with the correlation length of the order-parameter fluctuations. The experimental and computational data shown in Figure 6 support this conclusion. Experimental points were obtained from DLS measurements of the correlation length relatively close to the critical point, where the interface is smooth, with a thickness of the order of a few nm. The systems examined through simulations are far away from the critical point, at low to moderate hydrotrope concentration, where the interface is very sharp.

Figure 5. Universal density profile: all simulation data collapse into a single master curve in accordance with Eq. (4).

The difference in the bulk densities, on the other hand, shows a linear dependence on the hydrotrope concentration (see Figure S3.B). The linear trend may be steeper in the real system given the current simulation model does not demonstrate the proper partition of TBA between oil and water, once the interface is saturated at higher hydrotrope concentrations. A detailed discussion on our simulation model and force field and the explanation of why our simulation of the interfacial profiles is still representative of the real systems (up to system S10) can be found in section 3 of the supporting material. Scaling relations between interfacial properties The nature of the observed universality of the density profile originates from a relation between the interfacial tension and the characteristic thickness of the interface, ζ. Scaling theory of a smooth interface35-36 predicts

Figure 6. Linear relation between the inverse of the interface thickness (2d) and the square root of the reduced interfacial tension (data error bars for both variables are of the size of symbols). Red crosses are the correlation-length measurements (shown in a larger scale in Figure S6)

( ∝ de , (8) where d is the space dimensionality. The well-known result of the mean-field van der Waals theory of the near-critical interface39, ( ∝ d f ∝ ϵf⁄ , is actually accurate only for an artificial d = 4 space. For a two-dimensional system, the line tension ( ∝ 1/ζ. Consequently, for a three-dimensional space, j = 3, ( ∝ d . The correlation length of fluctuations of the order parameter, T, diverges at the critical point. In the critical region, scaling theory predicts that the thickness of the smooth near-critical interface is proportional to the correlation length of critical fluctuations of the order parameter and thus diverges at the critical point as the same function of temperature and concentration,16 d = %, thus resulting in ( ∝ % . This prediction is in agreement with experiment

Figure 7. Dependence of the normalized difference between the water and CHX bulk densities on the square root of the product of the normalized interfacial thickness and interfacial tension.

6


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Figure 8. Interfacial tension dependence on hydrotrope concentration using (A) different oil phases (toluene, benzene, cyclohexane and heptane), and (B) a different hydrotrope (isopropanol). The mol fractions for the literature data40-41 were linearly rescaled with respect to our system by Eq. (5) (>k lmnonpqr = 4.25, >snotnonpqr = 0.75, >vnwkxonpqr = 5.55, >k lmnonyz w{ w = 3.75, >snotnonyz w{ w = 4.60, >|}~yz w{ w = 2.85).

function that describes the behavior of the real systems doesn’t require an adjustable parameter either, = a1 + 1/2(1 0 ) b (12) Comparison with other water/hydrocarbon systems containing hydrotropes To conclude our study, we compared the behavior of our ternary system with other water-oil mixtures for which data is available in the literature.40-42 TBA exhibits similar behavior regardless of the “oil” in the mixture (Figure 8.A). Comparing our results with ternary systems of different hydrophobes as well as a different hydrotrope (isopropanol) shows the same trend. Isopropanol still shows a clear dual action at the water and oil interface, but there is more delay in the reduction of the interfacial tension, i.e. the smaller the alcohol, the slower the reduction of the interfacial tension. Even smaller alcohols (like methanol41 and ethanol40, not shown in Fig. 8) exhibit this dual behavior in reducing or vanishing the interfacial tension between water and oil phases; as surfactants at low concentrations and as co-solvents at larger concentrations. Conclusion The presented results of experimental and MD simulation studies on the effects of a typical hydrotrope on water-oil interfacial phenomena unambiguously demonstrate the dual role played by small amphiphilic molecules in aqueous solutions. At diluted concentrations, hydrotropes act as adsorbates, reducing the water-oil interfacial tension in accordance to the Gibbs adsorption theory. In this surfactantlike regime, the hydrotrope is mainly accumulated on the interface while the bulk densities of the coexisting phases remain almost unchanged. After the interface is saturated, the hydrotrope acts as a co-solvent smoothing the difference between the aqueous and oil phases, ultimately resulting in criticality with vanishing interface. We describe this dual behavior by a crossover function (Eq. 7) that interpolates two theoretically well-defined limits, and show that the thickness of the interface and the difference between the densities of the

Another relation between parameters of the universal profile, which can shed additional light on the dual role of TBA as a surface adsorbate and a co-solvent, is the density difference of the bulk phases. The gradient theory of interfaces35-36 calculates the interfacial tension as G e

( ∝ 9 ; j

(9)

e-

which satisfies the mean field (d = 4) result for smooth interfaces, namely, ( ∝ d f and (∆5)-→ ∝ 1⁄d , as well as the scaling prediction for a smooth interface at d = 3: ( ∝ 1⁄d , d = %, and (∆5)-→ ∝ 1⁄% ; the latter being valid in the Ornstein-Zernike approximation, which corresponds to the first-order 4-d expansion in the renormalization-group theory of critical phenomena.35 Linearization of the integral in Eq. (9) gives the following estimate of the interfacial tension:

(∝

()F→ :

.

(10)

Therefore, in the critical region one expects (Δ5)-→ ∝ (d)⁄ . However, our simulation data on the water-oil interface show that while both ( and d strongly depend on the addition of small amounts of TBA, the difference in the bulk densities, for the sharp interface, does not. The dependence of (Δ5)-→ on ((d)⁄ for simulation data is presented in Figure 7, and it can be approximated by an empirical function without any adjustable parameters, = a1 0 (1 0 ) b (11)

with = (Δ5)-→ /(Δ5) and = (d(/d ( )⁄ . This function satisfies the linear correlation (Δ5)-→ ∝ (d)⁄ near the critical point, predicted by the gradient theory. Since in the entire range of TBA concentrations examined in this study the product (d ≅ constant, the variable ((d)⁄ ∝ ( = ( (1 0 >

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