Thermodynamics of Solvophobic Interaction between Hydrophobic

Article pubs.acs.org/Langmuir

Thermodynamics of Solvophobic Interaction between Hydrophobic Surfaces in Ethanol Zuoli Li and Roe-Hoan Yoon* Center for Advanced Separation Technologies, Virginia Tech, Blacksburg, Virginia 24061, United Sates ABSTRACT: AFM surface force measurements were conducted in pure ethanol using gold surfaces hydrophobized with alkanethiols (CnSH) with n = 2−16. The forces measured at 5−35 °C were net attractive and became stronger with decreasing temperature and with increasing surface hydrophobicity. Thermodynamic analysis of the experimental data showed that the macroscopic solvophobic interactions were enthalpic but exhibited significant enthalpy−entropy compensations. The enthalpy decreases may represent the energy gained in forming H-bonded structures of ethanol, while the entropy decreases represent the thermodynamic costs for building structures. These results are consistent with those obtained previously in pure water.

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Considine and Drummond11 reported the surface forces measured between Teflon-coated surfaces in n-alkanes and various H-bonding liquids using an AFM. The forces measured in the former (hexane and hexadecane) showed no attractive solvophobic forces, while those measured in the latter (water, glycerol, formamide, ethanol, and methanol) showed evidence for attractive forces with large jump-in distance in the range of 9−500 nm. The authors concluded that the attractive solvophobic forces were due to coalescence of the nanobubbles preexisting on the Teflon-coated surfaces. In the present work, we have conducted a series of AFM force measurements in ethanol with gold surfaces hydrophobized with alkanethiols (n-CnSH) with varying chain lengths at temperatures in the range of 5−35 °C. The measured forces were net attractive and varied with temperature, film thickness, and surface hydrophobicity. The force data obtained at different temperatures were analyzed using the thin film thermodynamics. The results showed that the solvophobic interactions in pure ethanol were enthalpic as has been the case in water.5,6 The results are discussed in view of the possible structural changes of the H-bonding liquid confined between hydrophobic surfaces.

INTRODUCTION Hydrophobic interactions play an important role in biology, colloid assembly, detergency, wetting, etc., and their mechanisms vary with length scales involved. At molecular scale, hydrophobic interactions are entropic as is well-known,1−3 while those at macroscopic scale are enthalpic. Evidence for the latter has been obtained from the thermodynamic analysis of the surface forces measured using the atomic force microscope (AFM).5,6 That the thermodynamics of hydrophobic interactions change from entropic to enthalpic with increasing length scale is consistent with the results of molecular dynamic (MD) simulations that hydrophobic hydration, which is the reverse of hydrophobic interaction, where it becomes enthalpic when the radius of hydrophobe extends over ∼1 nm at ambient conditions.7−9 Analysis of the AFM force data obtained in water with thiolated gold6 and silylated silica5 surfaces showed that hydrophobic interactions entail decreases in both the excess film enthalpy and entropy. It was suggested that the enthalpy decrease represents the energy gained in building H-bonded structures while the entropy decrease represents the thermodynamic cost for building structures. These results seemed to support the notion that hydrophobic force may be a solventmediated (or solvophobic) force, which prompted the authors of the present investigation to conduct AFM force measurements in several different H-bonding liquids other than water.10 The forces measured in short-chain alcohols (methanol, ethanol, and butanol) and ethanol−water mixtures were netattractive, both the strengths and ranges varying with alcohol type and the hydrophobicity of confining surfaces. In ethanol− water mixtures, the measured forces were strongest in pure solvents and became weaker in mixtures of the two, suggesting that the H-bonded structures of water and ethanol may be compromised in the presence of the other. © 2014 American Chemical Society

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THERMODYNAMICS OF THIN LIQUID FILM For a pure liquid such as ethanol, one can derive the free energy change (Δγf) associated with film thinning as follows: dΔγ f = −ΔS f dT + h dP − Π dh

(1)

where γf is the film tension in units of energy per unit area, ΔSf the corresponding entropy change, T the absolute temperature, Received: August 18, 2014 Revised: October 16, 2014 Published: October 19, 2014 13312

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and Π is the disjoining pressure in a thin liquid film (TLF) which varies with film thickness (h). At a constant pressure (P), eq 1 simplifies to dΔγ f = −ΔS f dT − Π dh

⎛ ∂(F /2πR ) ⎞ ΔSsf = −⎜ s ⎟ ⎝ ⎠P,h ∂T

(2)

=−

which can be used to determine the changes in film tension (or Gibbs free energy) as a function of T and h. At a given film thickness, eq 2 gives the following relation: ⎛ ∂Δγ f ⎞ ⎟ ΔS f = −⎜ ⎝ ∂T ⎠h

ΔGsf = −

which can be used to determine entropy changes provided that the temperature gradient of Δγ f is known. Derjaguin approximation12 relates the changes in film tension to the surface force (F) and the radius of curvature (R) of interacting surfaces as follows:

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One can, therefore, measure surface forces at different temperatures to obtain the temperature gradient of Δγf and subsequently the entropy change using eq 3.6 At a given temperature, eq 2 becomes h

∫∞ Π dh

∫∞ (Πe + Πd + Πs) dh

(5)

in which Πe, Πd, and Πs are the disjoining pressures due to the electrical double layer (Fe), van der Waals dispersion (Fd), and solvophobic (Fs) forces, respectively. Equation 5 suggests that ΔGf due to film thinning is determined by the three surface forces. In the present work, the role of Fs in determining ΔGf is studied. The solvophobic (or hydrophobic) forces are frequently represented by a single-exponential empirical expression13−16 ⎛ h⎞ Fs = −C exp⎜ − ⎟ ⎝ D⎠ R

(6)

in which C and D are fitting parameters. Equation 4 gives a relation between the total force (F) and the total free energy change (ΔGf) associated with film thinning. It can be rewritten for the contribution from the Fs as follows: Fs = 2π ΔGsf R

(7)

in which ΔGfs represents the free energy change due to Fs. Likewise, eq 3 may be rewritten as ⎛ ∂ΔG f ⎞ s ⎟⎟ ΔSsf = −⎜⎜ ∂ T ⎝ ⎠h

ΔSfs

(10)

ΔHsf = ΔGsf + T ΔSsf

(11)

EXPERIMENTAL SECTION

Materials. Thin films of gold were deposited on silicon wafers using an electron beam physical vapor deposition (EBPVD) system, model-250. The thicknesses of the gold coatings were ∼50 nm. Prior to the gold coatings, the silicon wafers had been coated with thin (∼5 nm) layers of chromium to strengthen the gold coatings. Gold spheres were produced as described by Raiteri et al..17 In this method, gold wires (0.0127 mm radius, 99.9% pure, Alfa Aesar) were short circuited at 120 V AC to melt the thin wires and produce gold spheres of various radii. The gold spheres of ∼10 μm radii were selected for AFM surface force measurements. For each AFM force measurement, a gold sphere was glued onto the tip of a cantilever with a spring constant of ∼0.48 N/m. The cantilever was calibrated using the thermal tuning function18 of the AFM used for the force measurement, while its deflections were monitored by means of the AFM Nanoscope V 7.3 software. The diameters of the gold spheres were measured by means of an optical microscope. The gold-coated plates were cleaned in boiling piranha solutions (7:3 by volume of H2SO4/H2O2) for 20 min, thoroughly washed with ultrapure water, and then dried in an ultrapure N2 gas stream. The gold spheres were cleaned after gluing them onto cantilever springs using a resin (EPON 1004F). Since the piranha solution attacks the resin, the sphere-cantilever assemblies were cleaned by exposing them to UV irradiation at a wavelength of 254 nm for 1 h to remove organic contaminants. The resin was insoluble in ethanol. Alkanethiols (CnSH) of n = 2, 4, 12, and 16 were obtained from TCI America (97-98% purity) and used to hydrophobize the macroscopic gold surfaces. Ethanol (200 proof, >99.5% pure, Decon Laboratories, Inc.) was used as solvent for the thiols and as the medium for surface force measurements. Hydrophobization of Gold Surfaces. After cleaning, the goldcoated plates and the sphere-cantilever assemblies were washed thoroughly with ethanol and then subjected to surface hydrophobization. For a set of force measurements, a pair of gold-coated plate and gold sphere were immersed in a thiol-in-ethanol solution simultaneously for a period of time, so that both surfaces could have an identical hydrophobicity. The thiolated gold surfaces were then washed thoroughly with pure ethanol before use. In the present work, the surface hydrophobicity was controlled by using alkanethiols (CnSH) of different chain lengths and by varying the concentrations of thiols-in-ethanol solutions and the immersion times. AFM Forces Measurement at Various Temperatures. The surface forces acting between the thiolated gold plate and sphere were measured in pure ethanol using a Nanoscope V atomic force microscope (AFM). In a given set of measurements, the temperature

h

=−

⎛ h⎞ C exp⎜ − ⎟ ⎝ D⎠ 2π

that can be used to determine the free energy changes from the C and D parameters. Finally, one can determine the enthalpy change from the entropy and free energy changes obtained using eqs 9 and 10, respectively, as follows

(4)

Δγ f = ΔGf = −

(9)

which can be used to obtain the changes in excess film entropy from the C and D parameters of the solvophobic forces measured at different temperatures. From eqs 6 and 7, one can derive an expression

(3)

F = 2π Δγ f R

Fs ⎛ dln C h dln D ⎞ ⎜ ⎟ + 2πR ⎝ dT D dT ⎠

(8)

ΔGfs

in which and represent the changes in excess film entropy and free energy, respectively, due to solvophobic interactions. From eqs 6 to 8, one obtains5 13313

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of the ethanol in the AFM liquid cell was ramped up from 5 °C to 15, 25, and 35 °C using the heater/cooler assembly placed under the cell. A thermal applications controller (TAC) was used for temperature control. All of the force measurements were carried out using the colloidal probe technique as described previously.19 Measured forces (F) were normalized by the radius of the gold sphere (R) and plotted vs the closest separation distance (h) between the sphere and plate.

to the artifacts created by bubbles and/or cavities. At temperatures above 40 °C, however, bubbles were visible to the naked eyes. Therefore, no force measurements were conducted above 35 °C to avoid the possibility of bubbles interfering with force measurement. Decreasing gas solubility with temperature can cause bubbles to nucleate on hydrophobic surfaces. Note, however, that the advancing and receding contact angles of ethanol on the C16SH-coated gold surface are 32° and 24°, respectively,10 eliminating the possibility of cavitation affecting the force measurement. Thermodynamically cavitation should occur at contact angles above 90°. The measured forces were net attractive, increased with decreasing film thickness, and increased with increasing surface hydrophobicity. As shown in Table 1, water contact angles increased with increasing chain lengths of the thiols (CnSH) used to hydrophobize the gold surfaces. Also, the attractive forces increased with decreasing temperature. Note also that the measured forces were substantially stronger and longer-ranged than the van der Waals force (Fd), which was calculated using the following relation:

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RESULTS AND DISCUSSION As shown previously, the conditions under which gold and silica substrates are hydrophobized by long-chain alkanethiols and silanes critically affect surface force measurements.5,19 In general, immersion in highly concentrated solutions (e.g., 10−3 M) for a long period of time (>15 h) shows discontinuities (or steps) in AFM force curves, which are usually attributed to the coalescence of preexisting nanobubbles.20 In the present work, surface force measurements were conducted in ethanol, in which air dissolves more readily than in water; N2 and O2 are 8.4- and 7-times more soluble in ethanol than in water at 20 °C.21 At higher temperatures, the solubility differences become larger; therefore, the probability of gas bubbles affecting force measurement in ethanol would be higher than in water. To minimize this problem, the gold surfaces used in the present work were hydrophobized by immersing them in a relatively low (10−5 M) concentration of CnSH-in-ethanol solution. With a shorter-chain thiol, it was necessary, however, to employ a longer contact time to obtain a desired contact angle. The contact angle measurements were conducted using the sessile drop technique. Table 1 shows the water contact angles of the

A Fd = − 131 R h3

(12) −20

with a Hamaker constant of A131 = 0.94 × 10 was obtained from the combining rule A131 = ( A11 −

A33 )2

J. This value (13)

‑20

where A33 (= 4.2 × 10 J) is the Hamaker constant of ethanol22 and A11 (= 9.11 × 10−20 J) is that of gold. The latter was calculated using eq 13 from the value of A131 = 1.2 × 10−20 J that was obtained from the AFM surface force measurement conducted with gold surfaces in pure water.19 The A11 value used in the present work was considerably lower than those (20 to 50 × 10‑20 J) obtained using the Lifshitz theory.22 Contributions from the double layer forces (Fe) to the measured forces were calculated using the following:22

Table 1. Water Contact Angles and ζ-Potentials of the Gold Surfaces Treated in a 10−5 M Thiol-in-Ethanol contact angle (deg) thiol

immersion time (min)

θa

θr

θe

θa-θr

ζ-potential (mV)*

C2SH C4SH C12SH C16SH

60 17 23 45

85.5 96.0 105.0 109.0

70.0 86.0 98.0 102.5

82.0 92.0 102.0 107.0

15.5 10.0 7.0 6.5

-37.3 -38.6 -42 -43

2 ⎡ ⎤ ⎛ ⎛ eψ ⎞⎞ Fe = 2π ⎢64kTρ∞⎜tanh⎜ 0 ⎟⎟ κ −1⎥exp( −κh) ⎝ 4kT ⎠⎠ ⎝ ⎢⎣ ⎥⎦ R

*

ζ-potential of bare gold was -40.6 mV.

(14)

−1

where the Debye length (κ ) is given as

gold surfaces hydrophobized with alkanethiols of varying chain lengths (n = 2, 4, 12, and 16) and contact times (17−60 min). As shown, both the advancing (θa) and receding angles (θr) increased with increasing chain lengths. With C2SH, a 60 min immersion time was necessary to obtain an equilibrium contact angle (θe) of 82°. Note also that contact angle hysteresis decreased with increasing chain lengths, indicating that the hydrophobic surface coatings became smoother with longerchain thiols. Also shown in Table 1 are the ζ-potentials of the thiol-coated gold particles in ethanol. The gold samples obtained from Alfa Aesar were in a narrow (0.8 to 1.5 μm) particle size range. The measurements were conducted using the Zetasizer Nano ZS from Malvern. The ζ-potentials of the gold particles treated with CnSH increased with increasing chain lengths. The ζpotential of bare gold was −40.6 mV, which was not too different from those of the thiol-coated gold. Figure 1 shows the results of the AFM force measurements conducted with CnSH-coated gold surfaces at temperatures in the range of 5 to 35 °C. All of the force vs distance curves were smooth, suggesting that the long-range attractions were not due

κ −1 = (ε0εkT /2ρ∞e 2)2

(15)

where ε0 is the permittivity of vacuum, ε the dielectric constant, k the Boltzmann constant, T the absolute temperature, ρ∞ the number density of ions in bulk solution, e the electronic charge, and ψ0 is the surface potential. It has been shown that eq 14 approximates full Poisson−Boltzmann equation when ψo is less than ∼100 mV.23 That the ζ-potentials of the thiol-coated gold surfaces were in the range of −37 to -43 mV, as shown in Table 1, may justify the use of eq 14. At a higher contact angle, the number densities of proton donor and/or acceptor sites (or surface OH groups) on gold must be lower, which should in turn increase the number of free (or non-H-bonded) ethanol molecules in the TLFs confined between the hydrophobic surfaces. Equation 14 was used to calculate Fe using the ζ-potentials given in Table 1 as ψ0 and κ−1 = 40 nm at 25 °C, and the results are plotted in Figure 2. As shown, the double-layer forces were repulsive and increased with increasing chain lengths (n) of the hydrophobizing agent (CnSH). Also shown in the figure are the dispersion 13314

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Figure 1. Measured surface forces (F) plotted vs the thickness (h) of the TLF of ethanol. The AFM force measurements were conducted using gold sphere of radius R and plate hydrophobized with CnSH to obtain θe = 82.0, 92.0, 102.0, and 107.0° at n = 2, 4, 12, and 16, respectively. The lines through the data points represent the extended DLVO theory which includes solvophobic force, and the dashed lines represent the van der Waals force.4,9

The gold surfaces became more hydrophobic with increasing chain lengths, as shown in Table 1. It can be said, therefore, that the solvophobic forces increased with increasing water contact angles or surface hydrophobicity. At a given chain length or surface hydrophobicity, Fs increased with decreasing temperature. In general, the results obtained in ethanol showed the same trends as obtained in water.5,6 In the present work, the solvophobic forces are represented as a single-exponential force law (eq 6). Figure 4 shows its C and D parameters obtained from the curve-fitting exercise for the solvophobic forces given in Figure 3. Both parameters are shown to increase with increasing chain lengths (n) of CnSH and decreasing temperature. Note also that the slopes of the C vs temperature (t) plots tend to increase with increasing n. On the other hand, the slopes of the D vs t plots are about the same for all of the CnSH-coated surfaces. Based on the data presented in Figure 4, the changes in excess Gibbs free energy (ΔGsf ) due to the attractive solvophobic force (Fs) present in the TLFs of ethanol have been calculated using eq 10. The results are plotted vs film thickness (h) at different temperatures in Figure 5a. Corresponding changes in excess film entropy (ΔSfs) and enthalpy (ΔHfs) have been calculated using eqs 9 and 11, respectively, and are plotted in Figure 5b,c. As shown, ΔGfs becomes increasingly negative with decreasing temperature. Note also that both ΔSfs and ΔHfs decrease with decreasing temperature. When both enthalpy and entropy decrease at a given temperature, it is necessary that |ΔHfs| > |TΔSfs| for ΔGfs to be negative. In Figure 5d, all three thermodynamic functions obtained at 25 °C are plotted vs h. Indeed, ΔHfs is more negative than the TΔSfs term at a given h, indicating that the solvophobic interaction in ethanol is driven by enthalpy changes, as has been the case with the macroscopic hydro-

Figure 2. Changes in the van der Waals (Fd), double-layer (Fe), and solvophobic (Fs) forces vs the thickness of the TLFs of ethanol at 25 °C. Fs becomes more attractive with increasing chain lengths of the CnSH due to increased contact angles; θe = 82.0, 92.0, 102.0, and 107.0° at n = 2, 4, 12, and 16, respectively. The forces were normalized by the radius (R) of the gold sphere used for the AFM force measurement.

(Fd) and solvophobic (Fs) forces. The value of the latter were obtained from the values of Fe and Fd using following relation: F = Fe + Fd + Fs

(16)

and are plotted in Figure 2. As shown, the solvophobic forces increased with increasing hydrophobicity (or water contact angle) of the CnSH-coated gold surfaces. Figure 3 shows the solvophobic forces (Fs) obtained in the manner described above from the force data presented in Figure 1. The solvophobic forces were substantially stronger and longer ranged than the van der Waals force and increased with the chain lengths of the hydrophobizing agents (CnSH). 13315

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Figure 3. Effect of the temperature on the solvophobic forces (Fs) measured in the TLFs of ethanol confined between hydrophobic gold surfaces. Fs increased with decreasing film thickness, decreasing temperature, and increasing chain lengths of the CnSH. Solid lines represent the solvophobic forces (eq 6), while the dashed lines representing the van der Waals force (eq 12) with A131 = 0.94 × 10‑20 J. The hydrophobicity of gold surfaces increased with CnSH; θe = 82.0, 92.0, 102.0, and 107.0° at n = 2, 4, 12, and 16, respectively.

Figure 4. Effects of temperature (t) on the C and D parameters of the solvophobic forces (eq 8) measured between hydrophobic gold surfaces using an AFM. The solvophobic forces increased with the hydrocarbon chain length of CnSH: θe = 82.0, 92.0, 102.0, and 107.0° at n = 2, 4, 12, and 16, respectively.

phobic interactions in water.5,6 Since both ethanol and water are H-bonding liquids, the attractive forces observed in these liquids may be of the same origin, that is, antipathy between Hbonding liquids and the hydrophobic surfaces confining them. For the molecular-scale hydrophobic interactions in water, e.g., micellization and adsorption of long-chain surfactants, enthalpy changes are negative but are small in magnitude relative to the entropy term so that |ΔH| < |TΔS|,24−26 which is contrary to what has been observed with the macroscopic hydrophobic interactions in water5,6 and also in ethanol as discussed above. When a small hydrophobe of less than 1 nm radius is placed in water, the solvent molecules can efficiently wrap around it and build a stronger H-bonded network, resulting in a significant entropy decrease. This process known as hydrophobic hydration involves a significant entropy decrease and, hence, is entropic with ΔG > 0.7,27 The molecular-scale hydrophobic interaction, which is the reverse

of the hydrophobic hydration, should, therefore, be entropic as well, but with ΔG < 0. When an extended hydrophobic surface is placed in water, solvent molecules cannot form tight H-bonded network along the extended surface due to its low curvature. To minimize the energy loss, the water molecules may retreat from the surface, leading to a layer of low-density water (or depletion layer) and a large force of attraction.28 This mechanism known as drying effect implies that a liquid vapor-like interface29 is formed around an extended hydrophobic surface. That vapor/water interface has a high interfacial tension may support the strong and long-range hydrophobic forces observed in many surface force measurements. However, recent neutron scattering studies showed that the depletion layer is very thin, less than 1 nm, if any.30,31 According to an X-ray reflectivity study,32 the thickness of the depletion layer is 2 to 4 Å on a very hydrophobic CH3-terminated surface with water contact angles 13316

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Figure 5. Effect of temperature on the changes in (a) excess Gibbs free energy (ΔGfs), (b) entropy (ΔSfs), and (c) enthalpy (ΔHfs) of the TLFs of ethanol confined between gold surfaces hydrophobized with C16SH; (d) comparison of ΔGfs, ΔSfs, and ΔHfs varying with film thickness (h) at 25 °C.

been suggested that a strong short-range hydrophobic force may be necessary to overcome the repulsive van der Waals force which varies exponentially.41 Derjaguin and Churaev44 stated that formation of large contact angles cannot be explained without considering hydrophobic (or structural) forces. The thermodynamic data presented in this communication suggest that solvophobic (or hydrophobic) forces may be the consequence of a H-bonding liquid forming H-bonded structures to compensate the energy loss associated with placing an extended surface in water.5,6 Recent studies on water structure show that liquid water at room temperature has two different structures (or fluctuations), i.e., high-density liquid (HDL) and low density liquid (LDL).45−48 In bulk water, majority of the water molecules exist as HDL, with only a minority existing as LDL. As suggested previously, the population of LDL in the TLFs of water confined between hydrophobic surfaces may be higher than in the bulk water. LDL has effectively two more H-bonds than HDL and hence can serve as a more effective vehicle to expend the excess free energy created by the water molecules denied of forming Hbonding with confining surfaces. The TLFs of ethanol confined between hydrophobic surfaces may behave similarly as those of water. The ethanol molecules that cannot form O−H···O bonds with the surface will form structures with neighbors to expend the excess free energy created due to the antipathy between the OH group and the hydrocarbons on a hydrophobic surface. Solid ethanol has a crystal structure consisting of infinitely long zigzag chains joined by O−H···O bonds of 2.7 Å in length.49 Liquid ethanol, on the other hand, consists of shorter winding chains, each O− H group facing 1.8 nearest neighbors at an O−H···H bond distance of 2.8 Å.50 A combined neutron diffraction and molecular dynamic study showed, however, that a small fraction

(θ) greater than 100°. At lower contact angles, the depletion layer is not recognizable. It may be of interest to note here that the very first measurement of hydrophobic force was made with mica surfaces coated with cetyltrimethylammonium bromide (CTAB) to have a water contact angle of >60°.16 It was a relatively short-range force with a decay length of ∼1 nm.33 The authors of the present paper measured hydrophobic forces between silylated silica surfaces with θ = 78°.5 The decay lengths varied in the range of 6.9 to 8.8 nm depending on temperature. Molecular dynamic simulations showed that drying (or dewetting) occurs on paraffin plates with θ > 115o but not on graphite plates with θ = 86o.34,35 Thus, the hydrophobic forces measured at the lower contact angles may not support the suggestion that the large forces of attraction observed in direct force measurements is due to the drying effect.28 A more recent study showed that the water density near hydrophobic surfaces provides a poor quantification of surface hydrophobicity.36 It may also be noted that the minerals recovery by flotation is being done mostly at receding contact angles (θr) well below 90°. Some investigators suggested that the long-range attractive forces observed in experiment are actually due to the coalescence of preexisting nanobubbles when two surfaces approach each other.37−40 Most of the surfaces used in their experiments exhibited large water contact angles in the range of 80° to >110°. For an air bubble to adsorb on a hydrophobic surface with such large contact angles, it is necessary that hydrophobic force be present in wetting films.41 According to the Frumkin-Derjaguin isotherm,42,43 air bubbles can adsorb on hydrophilic surfaces by control of double-layer forces alone. However, the contact angles obtainable in this manner are usually less than 15 to 16°.44 That the Hamaker constant is negative in wetting films of water means that energy is required to remove the water adsorbed by the van der Waals force. It has 13317

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Figure 6. Effects of chain lengths on the (a) excess Gibbs free energy (ΔGfs), (b) entropy (ΔSfs), and (c) enthalpy (ΔHfs) in the TLFs of ethanol confined between hydrophobic gold surfaces. The surfaces were hydrophobized with CnSH to obtain θe = 82.0, 92.0, 102.0, and 107.0° at n = 2, 4, 12, and 16, respectively.

solvent. Note here that the enthalpy data obtained at different temperatures (Figure 6c) show that the constant pressure heat capacity (Cp = ∂H/T) is positive and becomes larger with decreasing film thickness, which provides important evidence that the TLFs of ethanol confined between hydrophobic surfaces are structured and the structuring increases with decreasing film thickness, decreasing temperature, and increasing contact angle. However, the structuring may be subtle, involving small changes in energy per solvent molecule. For the case of using water as an H-bonded solvent, the changes are in the range of 10−5 to 10−3 kT per molecule,15 which represents a tiny fraction of the average H-bond energy (∼7 kT) for water. Yet, such small changes can add up and lead to macroscopic hydrophobic interactions that are much stronger than the van der Waals attractions but substantially weaker than covalent bonding.

of the winding structures form clusters, e.g., hexamers and tetramers, by closing the loops of the winding chains.51,52 In the TLFs of ethanol, the cluster formation may be easier due to the excess free energies associated with the ethanol molecules that cannot from O−H···H bonds with the confining surfaces. Another way to minimize free energy may be to locate the gauche rather than trans form of ethanol at the interface, as the former is the more stable form.53 Note in Figure 5d that the structuring entails a high degree of enthalpy−entropy compensation, but the free energy change (ΔGfs) remains negative. It appears, therefore, that structuring aids rather than retards the solvophobic interactions in ethanol. Figure 6 shows the changes in ΔHfs, ΔSfs, and ΔGfs with a separation distance (h) for the forces measured with C2SH-, C4SH-, C12SH-, and C16SH-coated gold surfaces at 25 °C. The equilibrium water contact angles (θe) varied from 82°, 92°, 102°, to 107° with increasing hydrocarbon chain (Table 1). Both ΔHfs and ΔSfs became more negative with increasing chain length and θe. These findings suggest that the higher the hydrophobicity, the more likely it is for ethanol molecules to rearrange themselves to form a more extensive H-bonded network. As a consequence, ΔGfs becomes increasingly negative as shown in Figure 6a. The results obtained in ethanol are similar to those obtained in water, which is not surprising as both are H-bonding liquids. In a TLF confined between hydrophobic surfaces and in the absence of solutes, one way to reduce the excess free energy would be to form H-bonded structures with neighboring solvent molecules, resulting in a larger number of OH···O bonds or a larger number of winding structures and hexamers, Another way may be to adopt a lower potential energy conformation for the case of using ethanol as an H-bonded

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SUMMARY AND CONCLUSION AFM force measurements were conducted in ethanol at different temperatures using gold surfaces hydrophobized with alkanethiols. Thermodynamic analysis of the experimental data showed that the solvophobic interaction causes decreases in both the excess film enthalpy and entropy, with the enthalpy changes representing the energy gained for building structures and the entropy decrease representing the thermodynamic cost for building structures. That the attractive forces increased with decreasing temperature suggests that structuring becomes easier at lower temperatures. The results obtained with ethanol are similar to those obtained with water, indicating that solvophobic (and hydrophobic) forces are the consequences of the H-bonding liquids forming structures in confined spaced between hydrophobic surfaces to minimize free energy. 13318

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AUTHOR INFORMATION

Corresponding Author

*Phone: 540-231-7056. Fax: 540-231-3948. E-mail: ryoon@vt. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

Authors express sincere appreciation to the sponsor, National Energy Technology Center (NETL), for providing a financial support (DE-FE-0000699). They are also thankful to Professor Jan Christer Eriksson and Dr. Lei Pan for helpful discussions.

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