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J. Phys. Chem. C 2010, 114, 394–402
Behavior of Organic Liquids at Bare and Modified Silica Interfaces Pearl Horng,† Michael R. Brindza,† Robert A. Walker,†,‡,⊥ and John T. Fourkas*,†,‡,§,| Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742, Chemical Physics Program, UniVersity of Maryland, College Park, Maryland 20742, Institute for Physical Science and Technology, UniVersity of Maryland, College Park, Maryland 20742, Maryland NanoCenter, UniVersity of Maryland, College Park, Maryland 20742, and Center for Nanophysics and AdVanced Materials, UniVersity of Maryland, College Park, Maryland 20742 ReceiVed: September 1, 2009; ReVised Manuscript ReceiVed: October 21, 2009
Static contact-angle measurements have been used to study water and five organic liquids (n-decane, benzene, acetonitrile, octyl cyanide, and sebaconitrile) on bare silica and on a range of silanized silica surfaces. We have used these measurements to determine the polar and dispersive components of the surface energies of these liquids and of the modified silica surfaces. These data offer clues into the microscopic structuring of the liquids and how this structuring is influenced by solid interfaces. We have also observed a strong relationship between the polar and dispersive components of the surface energies of the modified silica surfaces. I. Introduction Organic liquids at solid interfaces are important in a wide range of technological applications, including chromatography, lubrication, and heterogeneous catalysis. The structure and dynamics of an organic liquid at a solid interface can vary considerably from those of the bulk liquid due to a complex interplay of molecular-scale interactions.1-3 These microscopic interactions are ultimately manifested in the form of macroscopic properties of the liquid/solid system. One commonly studied property of liquid/solid interfaces is the contact angle.3 Static contact angles are often explained from an empirical perspective by dividing the surface energies of the phases into “polar” and “dispersive” components.4 This type of model can yield predictions for contact angles that are reasonably accurate. While this approach does not offer a direct picture of the molecular structuring at solid/liquid interfaces, structural information can often be inferred from the partitioning of the surface energy. Computer simulations are also beginning to yield important insights into the microscopic origin of contact angles, as it is becoming possible to simulate drops of physically meaningful dimensions.5-8 A powerful aspect of such simulations is that it is possible to tune the liquid/solid interactions at will in order to unravel the connections between microscopic and macroscopic behavior.8 However, there is still a vast amount to be learned about solid/liquid interfaces, and for now experiment remains the most efficient means of investigating interactions in these interfacial systems. Here we present the results of contact-angle experiments for organic liquids ranging from nonpolar (n-decane and benzene) to polar (acetonitrile, octyl cyanide, and sebaconitrile). Water, which has a contact angle that is highly sensitive to the nature of the surface with which it is interacting,8 is used as a reference * To whom correspondence should be addressed. E-mail: fourkas@ umd.edu. † Department of Chemistry and Biochemistry. ‡ Chemical Physics Program. § Institute for Physical Science and Technology. | Maryland NanoCenter. ⊥ Current address: Department of Chemistry and Biochemistry, Montana State University.
liquid. We have studied these liquids on bare silica and on silica functionalized with groups that mimic the properties of the liquids themselves: saturated hydrocarbons for n-decane, aromatics and perfluorinated aromatics for benzene, and cyanides for the nitriles. We use Owens-Wendt9 analysis to estimate the polar and dispersive contributions to the surface energy for liquid and each modified surface. Our results allow us to infer some of the microscopic structural details of the interfaces studied here. Additionally, for the modified silica surfaces we have discovered a strong and unexpected connection between the polar and dispersive portions of the surface energy. II. Theory Young’s description10 of the equilibrium interfacial tension of a liquid droplet on a solid surface (γSL) relates the surface tensions of the solid and liquid phases (γS and γL, respectively) to the static contact angle at the solid-liquid-vapor interface:
γL cos θ ) γS - γSL
(1)
The contact angle θ can range from 0° (completely wetting) to 180° (completely nonwetting). On the basis of this understanding of surface tension as the attraction between the surface layer of the solid and the liquid phase, Fowkes4 proposed that surface tension was composed of additive polar and dispersive portions. These components characterize the non-London interactions and the London dispersion interactions between the two phases, respectively.
γi ) γiP + γiD
(2)
Here i denotes a specific phase (solid or liquid) and the superscripts P and D denote the polar and dispersive components, respectively. Using Young’s relationship (eq 1) and Fowkes’ description of surface tension (eq 2), Owens and Wendt9 used a geometric mean to formulate the solid-liquid interfacial tension.
10.1021/jp908444x 2010 American Chemical Society Published on Web 11/05/2009
Behavior of Organic Liquids at Silica Interfaces
J. Phys. Chem. C, Vol. 114, No. 1, 2010 395
γSL ) γS + γL - 2√γSDγLD - 2√γSPγLP
(3)
By combining eqs 1 and 3, the individual polar and dispersive surface free energy components of one phase can be extracted, assuming that the contact angle of the liquid on the surface and the surface free energy components of the other phase are known:
γSDγLD γSPγLP √ √ 1 + cos θ ) 2 +2 γL
(4)
γL
Applying eq 4 to two different liquids of known γLD, γLP, and γL allows us to determine the contributions to the surface energy of a solid interface via
1 γSD ) 4
and
γSP
1 ) 4
( (
γL(2)(1 + cos θ2) - γL(1)(1 + cos θ1)
√
γLD(2) -
γLD(1)γLP (2)
√
-
γLP (1)
γLP (1)
γL(2)(1 + cos θ2) - γL(1)(1 + cos θ1)
γLP (2)
γLP (2)
γLD(2)γLP (1) γLD(1)
γLP (2) γLP (1)
) )
2
(5)
2
(6)
Kaelble rearranged the Owens-Wendt equation so that a linear regression can be used to determine the polar and dispersive components of a solid or liquid if contact angles can be measured for multiple partners and the surface energy parameters of one of the phases are known.11 For determining the polar and dispersive components of the surface energy of a liquid, Kaelble’s equation is given by
(
γL(1 + cos θ) 2√γSD
)
)
√
γLD
+
√
γLP
γSP γSD
(7)
In this equation, the square of the slope gives the polar component and the square of the y-intercept gives the dispersive component. III. Experimental Section Surface Modification. The silicate substrates employed were microscope coverslips (Corning #2). Prior to surface modification, the slides were rinsed with ethanol and treated with an O2 plasma under vacuum conditions for 3 min. Substrates modified using monochlorosilanes were placed in a desiccator immediately following plasma treatment and were exposed to the vapor from 2-3 mL of silane for 24 h,
after which they were rinsed with ethanol. In the case of dichloro- and trichlorosilanes, plasma-cleaned substrates were refluxed in a solution of 2-3 mL of silane in 30 mL of dry toluene at 130 °C for 24 h. This procedure helps to inhibit polymerization of the silanes. Upon removal from the reflux solution the slides were rinsed thoroughly with toluene, then benzene, and then methanol. The silanes used for surface modification are shown in Tables 1-4. We do not have an absolute measurement of the surface coverage of functional groups for these modified surfaces, but experiments were performed multiple times to ensure reproducible results. Contact-Angle Measurements. Static contact angles between the liquids and the prepared surfaces were measured at room temperature using the sessile drop method,12 providing a relative measure of wetting behavior. Using a contact-angle goniometer (Rame´-Hart Instrument Co., 250-00), a drop of liquid was dispensed from an ethanol-cleaned syringe onto the functionalized surface and a drop image was captured by a drop-shape analysis program (DROPImage, Rame´-Hart Instrument Co.). The contact angle was measured between the solid-liquid interface and the tangent line to the drop shape at the liquid-vapor interface (Figure 1). The organic liquids used in these measurements, benzene (Fisher Scientific), acetonitrile (Fisher Scientific), n-decane (Alfa Aesar), octyl cyanide (Sigma Aldrich), and sebaconitrile (1,8dicyanooctane, Sigma Aldrich), were purchased in the highestpurity form available and were used as received. The water employed was deionized, and had a measured surface tension of 69.3 ( 2.0 mJ/m2, in good agreement with the literature value.13 The drop shape images for the different liquids on a surface treated with dimethyldichlorosilane are shown in Figure 2. Benzene and n-decane have relatively high vapor pressures, and so contact angles were measured as quickly as possible after the drop of liquid was delivered onto the substrate to prevent evaporation of the liquid from distorting the initial drop shape and thereby affecting the measurement of the contact angle. Because contact angles of less than 5° are difficult to measure, such angles were recorded as
