Thermodynamics of hydrophobic interaction between silica surfaces coated with octadecyltrichlorosilane Zuoli Li, Roe-Hoan Yoon ⇑ Center for Advanced Separation Technologies, Virginia Tech, Blacksburg, VA 24060, United States article info Article history: Received 30 April 2012 Accepted 13 August 2012 Available online 11 October 2012 Keywords: Hydrophobic force Excess thermodynamic properties Derjaguin approximation Structural force Atomic force microscope (AFM) Octadecyltrichlorosilane (OTS) abstract 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 [1]. As a fol- low-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 (DS f ) and excess film enthalpy (DH f ) decrease with decreasing thickness of the water films between the hydrophobic surfaces. It has been found also that |DH f |>|TDS f |, which represents a necessary condition for the excess free energy change (DG f ) to be negative and hence the hydrophobic interaction be attrac- tive. 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. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Hydrophobic particles placed in water are attracted to each other much more readily than predicted by the DLVO theory due to the presence of the hydrophobic force [2,3], which is longer ran- ged and stronger than the van der Waals force. Israelachvili and Pashley [4] reported the first direct measurement of hydrophobic force using the surface force apparatus (SFA). Many investigators [5–10] confirmed its existence and discussed possible origins, while others suggested that the hydrophobic force is an artifact due to bubbles or cavities [11–13]. Still others suggested that hydrophobic forces, particularly those of longer range, may origi- nate from mechanisms that are unrelated to surface hydrophobic- ity [14,15]. In an effort to better understand the origin(s) of hydrophobic force, one of us determined the thermodynamic functions of the hydrophobic interactions by conducting surface force measure- ments at several different temperatures [1]. The results showed that macroscopic hydrophobic interactions entail decreases in both the excess entropy (S f ) and the excess enthalpy (H f ) of the thin li- quid films (TLFs) of water confined between hydrophobic surfaces. Here, the term ‘‘excess’’ refers to the thermodynamic indicator of the thin film in question relative to that of the infinitely thick film (or bulk water). It was found also that the changes in excess film enthalpy (DH f ) are slightly larger than the absolute temperature (T) times the changes in excess film entropy (DS f ). Based on these results, it was suggested that macroscopic hydrophobic interac- tions may involve building structures of water in the vicinity of hydrophobic surfaces. In effect, DH f represents the energy gained in building the structures, while TDS f represents the thermody- namic cost of building the structure. In this previous work, the sur- face force measurements were conducted using an atomic force microscope (AFM) with gold-coated silica surfaces hydrophobized with n-ethanethiol (C 2 –SH) and n-hexadecane thiol (C 16 –SH). The thermodynamic studies described above suggested that hydrophobic force is a structural force, a term first used by Derja- guin and Kusakov [16] to describe the repulsive hydration force present in wetting films. In general, the term structural force refers to the non-DLVO force created when two boundary layers, whose structures are different from that of the bulk solution, overlap [17]. The non-DLVO forces observed between two hydrophilic sur- faces are referred to as positive structural force, while those ob- served between hydrophobic surfaces are referred to as negative structural force [18]. Eriksson et al. [19] suggested that the long- range attractive forces observed between hydrophobic surfaces may be due to the surface-induced changes in water structure. In the present work, we have conducted AFM force measure- ments using silica surfaces hydrophobized with octadecyltrichlo- 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.08.078 ⇑ Corresponding author. Address: 146 Holden Hall, Blacksburg, VA 24060, United States. Fax: +1 540 231 3948. E-mail address: ryoon@vt.edu (R.-H. Yoon). Journal of Colloid and Interface Science 392 (2013) 369–375 Contents lists available at SciVerse ScienceDirect Journal of Colloid and Interface Science www.elsevier.com/locate/jcis