Surface Forces in the Thin Liquid Films (TLFs) of Water Confined between n-Alkane Drops and Hydrophobic Gold Surfaces.

In flotation, air bubbles are commonly used to selectively collect the hydrophobic particles suspended in water, with the collection efficiencies strongly affected by contact angles and the properties of wetting films. Knowing that oil drops form substantially larger contact angles than air bubbles on a hydrophobic surface, many investigators explored the possibility of using oil drops rather than air bubbles for flotation. In the present work, the surface forces present in the thin liquid films (TLFs) of water confined between n-alkanes of different chain lengths and thiol-coated gold surfaces have been measured to better understand the drop-surface interactions, which are controlled by surface forces. The force measurements are conducted by monitoring the changes of local curvatures as an oil drop approaches a flat surface slowly, while monitoring film thicknesses by microinterferometry, constructing spatiotemporal film profiles, and analyzing the profiles to derive disjoining pressure and free energy isotherms using the Frumkin-Derjaguin isotherm. The results show that drop-hydrophobic surface interactions are driven by the hydrophobic force, which was first measured by Israelachvili and Pashley in 1982, and that both short- and long-range hydrophobic forces increase with contact angle and decrease with the chain lengths of n-alkane oils. It has been found also that the hydrophobic forces measured with oil drops have longer decay lengths than those measured with air bubbles, which may provide a kinetic advantage over the method of using air bubbles for flotation.

Thermodynamics of solvophobic interaction between hydrophobic surfaces in ethanol.

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.

Thermodynamics of hydrophobic interaction between silica surfaces coated with octadecyltrichlorosilane.

Surface force measurements conducted with thiolated gold surfaces showed previously that hydrophobic interaction entails a decrease in excess film entropy, suggesting that hydrophobic force originates from changes in the structure of the medium (water) confined between hydrophobic surfaces. As a follow-up work, surface force measurements have been conducted in the present work using an atomic force microscope (AFM) with silica surfaces coated with octadecyltrichlorosilane (OTS) at temperatures in the range of 10-40°C. A thermodynamic analysis of the results show that both the excess film entropy (ΔS(f)) and excess film enthalpy (ΔH(f)) decrease with decreasing thickness of the water films between the hydrophobic surfaces. It has been found also that |ΔH(f)|>|TΔS(f)|, which represents a necessary condition for the excess free energy change (ΔG(f)) to be negative and hence the hydrophobic interaction be attractive. Thus, the results obtained with both the thiolated and silylated surfaces show that hydrophobic forces originate from the structural changes in the medium. It is believed that the water molecules in the thin liquid films (TLFs) of water form clusters as a means to reduce the free energy when they cannot form H-bonds to neighboring hydrophobic surfaces.

Surface forces in thin liquid films of n-alcohols and of water-ethanol mixtures confined between hydrophobic surfaces.

An atomic force microscope (AFM) was used to measure the surface forces in thin liquid films (TLFs) confined between gold surfaces hydrophobized by chemisorption of alkylthiols. The measurements were conducted in different H-bonding liquids and in water-ethanol solutions. Attractive forces stronger and longer-ranged than the van der Waals force were observed in water, ethanol, and 1-butanol in a descending order. The attractive force measured in methanol was much weaker. The surface forces measured in ethanol solutions varied with concentration. Initially, the attractive forces decreased rapidly upon ethanol addition, passed through a broad minimum centered around mole fractions in the range of 0.15-0.20, and then increased as the ethanol mole fraction was further increased above ~0.5. Thermodynamic analysis of the data suggests that clusters of water and ethanol may form in TLFs. Thus, the attractive forces may arise from changes in liquid structure. It appears that structuring is a consequence of H-bonding liquid to minimize its free energy in the vicinity of hydrophobic surfaces and in the presence of foreign species in solution.

Excess thermodynamic properties of thin water films confined between hydrophobized gold surfaces.

Surface forces between gold surfaces were measured in pure water at temperatures in the range of 10-40 °C using an atomic force microscope (AFM). The surfaces were hydrophobized by self-assembly of alkanethiols (C(n)SH) with n=2 and 16 in ethanol solutions. The data were used to determine the changes in excess free energies (ΔG(f)) of the thin water films per unit area by using the Derjaguin approximation [1]. The free energy data were then used to determine the changes in excess film entropy (ΔS(f)) and the excess film enthalpy (ΔH(f)) per unit area. The results show that both ΔS(f) and ΔH(f) decrease with decreasing film thickness, suggesting that the macroscopic hydrophobic interaction involves building some kind of structures in the intervening thin films of water. It was found that |ΔH(f)|>|TΔS(f)|, which is a necessary condition for an attractive force to appear when the enthalpy and entropy changes are both negative. That macroscopic hydrophobic interaction is enthalpically driven is contrary to the hydrophobic interactions at molecular scale. The results obtained in the present work are used to discuss possible origins for the long-range attractions observed between hydrophobic surfaces.

Hydrophobic forces in the wetting films of water formed on xanthate-coated gold surfaces.

The kinetics of thinning of water films on hydrophobic gold substrates has been studied using the thin film pressure balance (TFPB) technique. The changes in the thickness of the wetting films have been monitored by recording the profiles of the dimpled films as a function of time using a high-speed video camera. It was found that the kinetics, measured at the barrier rim of a wetting film formed on a hydrophilic silica surface, could be predicted using the Reynolds lubrication approximation with the no-slip boundary condition. However, the wetting films formed on hydrophobized gold substrates thinned much faster, and the kinetics increased with increasing hydrophobicity. The data obtained with gold surfaces of different hydrophobicities have been fitted to the Reynolds approximation to determine the hydrophobic force constants (K132) of a power law. K132 increased with increasing contact angle and decreased with electrolyte (NaCl) concentration. It was also found that the K132 values can be predicted from the hydrophobic force constants (K131) for the interaction between hydrophobic surfaces and the same (K232) for the foam films using the geometric mean combining rule that is frequently used to predict asymmetric molecular forces from symmetric ones.

Effect of pH and NaCl concentration on the stability of surfactant-free foam films.

The stability of surfactant-free foam films was studied in NaCl solutions of varying concentration and pH using the thin film pressure balance (TFPB) technique. In pure water, it was not possible to produce foam films due to weak film elasticity and the strong attractive hydrophobic force in foam films, despite the presence of strong repulsive double-layer force. In the presence of a very small amount of an electrolyte, however, the hydrophobic force was dampened, allowing metastable foam films to form. As the NaCl concentration was raised above 10(-6) M, the film stability diminished as a result of double-layer compression. The TFPB technique was also used to measure the equilibrium film thicknesses (H(e)) in 10(-5) M NaCl solutions of varying pH; H(e) reached a maximum of approximately 130 nm at pH 6.0-7.3, and decreased on either side of this pH range as a result of the increased ionic strength caused by the HCl and NaOH added to control the pH. The hydrophobic force in surfactant-free foam films was maximum at pH 7.3, where the concentration of electrolytes, including that of H(2)CO(3) species, was minimum.

AFM forces measured between gold surfaces coated with self-assembled monolayers of 1-hexadecanethiol.

An atomic force microscope (AFM) was used to measure the forces between gold surfaces with and without hydrophobizing them by the self-assembly of 1-hexadecanethiol. The forces measured between bare gold surfaces were fitted to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory with a Hamaker constant of 1.2 x 10 (-20) J, which was close to the value determined using the methylene iodide contact angle method but was lower than that calculated using the Lifshitz theory. When the surfaces were hydrophobized in a 0.01 mM thiol-in-ethanol solution for 10 min, the measured forces exhibited a long-range force with a decay length of 35 nm. Despite its high water contact angle (105 degrees ), the force curve was smooth and exhibited no steps. When the surfaces were hydrophobized in a 1 mM thiol solution for longer than 6 h, however, the force curves exhibited steps, indicating that the long-range attractions were caused by bridging bubbles. When the measurements were conducted after washing the substrates with organic solvents, the steps disappeared and long-range attractive forces appeared. In the presence of ethanol, the water contact angle decreased to below 90 degrees , the attraction became weaker, and the force curves became smooth. On the basis of the results obtained in the present work, possible mechanisms for the long-range attractions are discussed.

Effects of degassing and ionic strength on AFM force measurements in octadecyltrimethylammonium chloride solutions.

Sakamoto et al. (Langmuir 2002, 18, 5713) conducted AFM force measurements between silica sphere and fused-silica plate in aqueous octadecyltrimethylammonium chloride (C18TACl) solutions and concluded that long-range attractive force is not observed in carefully degassed solutions. In the present work, AFM force measurements were conducted by following the procedures described by Sakamoto et al. The results showed the presence of an attractive force that was much stronger than the van der Waals force both in air-saturated and degassed solutions. The force was most attractive at 5 x 10(-6) M C18TACl, where contact angle was maximum. At this concentration, which is close to the charge compensation point (ccp) of the glass sphere, the long-range decay lengths (D) were 34 and 38 nm in air-saturated and degassed solutions, respectively. At 10(-5) M, the decay length decreased from 30 to 4 nm upon degassing. This decrease in decay length can be explained by a pH increase (from 5.7 to 6.6), which in turn causes additional surfactant molecules to adsorb on the surface with inverse orientation. The attractive force was screened by an added electrolyte (NaCl), indicating that the attractive force may be of electrostatic origin. Therefore, the very long decay lengths observed in the absence of electrolyte may be ascribed to the fact that the ccp occurs at a very low surfactant concentration.

Hydrophobic forces in the foam films stabilized by sodium dodecyl sulfate: effect of electrolyte.

Further studies of the hydrophobic force in foam films were carried out, including the effect of added inorganic electrolyte. We used a thin film balance of Scheludko-Exerowa type to obtain the disjoining pressure isotherms of the foam films stabilized by 10(-4) M sodium dodecyl sulfate in varying concentrations of sodium chloride. The results were compared with the disjoining pressure isotherms predicted from the extended Derjaguin-Landau-Verwey-Overbeek theory, which considers contributions from hydrophobic force in addition to those from double layer and van der Waals dispersion forces. The double layer forces were calculated from the surface potentials (psi s) obtained using the Gibbs adsorption equation and corrected for the counterion binding effect, while the dispersion forces were calculated using the Hamaker constant (A232) of 3.7 x 10(-20) J. The hydrophobic forces were calculated from the equilibrium film thickness as described previously. The predicted disjoining pressure isotherms were in good agreement with the experimental ones. It was found that the hydrophobic force is dampened substantially by the added electrolyte.