Viscous Water Meniscus under Nanoconfinement - Lab Work | CH 210C, Lab Reports of Organic Chemistry

Download Viscous Water Meniscus under Nanoconfinement - Lab Work | CH 210C and more Lab Reports Organic Chemistry in PDF only on Docsity! PRL 96, 177803 (2006) P H Y S I C A L R E V I E W L E T T E R S week ending 5 MAY 2006Viscous Water Meniscus under Nanoconfinement R. C. Major,1 J. E. Houston,2 M. J. McGrath,1 J. I. Siepmann,1 and X.-Y. Zhu1 1Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, USA 2Sandia National Laboratory, Albuquerque, New Mexico 87185, USA (Received 5 November 2005; published 5 May 2006)0031-9007=A dramatic transition in the mechanical properties of water is observed at the nanometer scale. For a water meniscus formed between two hydrophilic surfaces in the attractive region, with
1 nm interfacial separation, the measured viscosity is 7 orders of magnitude greater than that of bulk water at room temperature. Grand canonical Monte Carlo simulations reveal enhancement in the tetrahedral structure and in the number of hydrogen bonds to the surfaces as a source for the high viscosity; this results from a cooperative effect of hydrogen bonding of water molecules to both hydrophilic surfaces. DOI: 10.1103/PhysRevLett.96.177803 PACS numbers: 62.10.+s, 61.46.w, 66.20.+dFIG. 1 (color). Schematic: experimental system. The scanning electron microscope image shows the parabolic Au tip with radius of curvature of 500 nm. (a)–(e) Normal force (FN , solid curves and left axis) and friction force (F
, dashed curves and right axis) profiles as a function of relative displacement between the tip and the sample at the indicated RH values for the following interface combinations: (a) CH3 tip on CH3 surface; (b) COOH tip on CH3 surface; and (c)–(e) COOH tip on COOH surface. In each of the above panels, the thin pink and blue curves are individual FN profiles during approach and retract, respectively; the thick red (approach) and blue (retract) curves are averaged data. Only averaged data are shown for F
profiles (dashed brown curves for approach and dashed green curves for retract). A, B, andC represent positions for water nucleation, capillary condensation, and film contact, respectively.Water molecules confined between interfaces with nano- scopic separation are of critical importance in many fields. Examples include, among others, hydration forces in biol- ogy and colloid science [1], swelling of layered clays [2], and capillary forces in scanning probe microscopy and nanolithography [3,4]. Despite the broad interest, little is known about the properties of water confined between two surfaces with nanoscopic separation, where macroscopic theories for capillary condensation are expected to break down [3]. ‘‘Structured’’ water at interfaces has long been thought to explain a wide range of physical, chemical, bio- logical, and geological processes. Because of experimental difficulties in probing a nanoscopic solid-water-solid inter- face, most studies on interfacial water have dealt with solid-water interfaces using, e.g., vibrational spectrosco- pies [5–7], atomic force microscopy (AFM) [8], and x-ray diffraction [9]. In principle, AFM may be used to probe the nanoscopic water meniscus, but quantitative measurements are difficult because sensor instability results in the well- known ‘‘jump-to-contact’’ in the attractive region. Similar difficulties are encountered in experiments based on the surface force apparatus (SFA) [10], which accordingly has been used to probe confined molecules only in the repul- sive region. SFA studies by Granick and co-workers showed very high effective viscosity of liquid films, in- cluding water, confined between two mica surfaces [11,12]. This is in agreement with a shear force microscopy study which showed the rapid rise in viscosity of confined water as interfacial separation decreases below 1 nm [13]. By contrast, Klein and co-workers reported the fluidic nature of water confined between mica surfaces at <3:5 nm interfacial separation, with bulk-water–like vis- cosity [14]. An earlier measurement showed bulklike vis- cosity of water confined between two mica surfaces with interfacial separation  2 nm [15]. An AFM study also suggested the lubricative effects of adsorbed water [16]. Here we probe the mechanical properties of the water meniscus in the attractive region between two chemically distinct surfaces using interfacial force microscopy (IFM) (schematic illustration in Fig. 1, detailed in Ref. [17]) whose self-balancing, force-feedback sensor eliminates06=96(17)=177803(4) 17780the mechanical instability problem and allows quantitative measurements of normal and lateral (shear) forces through- out the entire range of interfacial separation. We use a single-crystal Au(111) and an electrochemically etched parabolic Au tip (R  500 nm). Each Au surface is made hydrophilic or hydrophobic by the chemisorption of a COOH or CH3 terminated alkanethiol self-assembled monolayer (SAM). After standard cleaning, the Au sample is immersed in ethanol solutions of mercaptoundecanoic acid [HS-CH210-COOH, 0.5 mM, 5% acetic acid] or hexadecanethiol [HS-CH215-CH3, 0.5 mM] to form3-1 © 2006 The American Physical Society PRL 96, 177803 (2006) P H Y S I C A L R E V I E W L E T T E R S week ending 5 MAY 2006each SAM. In the case of -COOH terminated SAM, it is necessary to remove weakly adsorbed bilayers [18] by sonicating in 5% acetic acid/ethanol, rinsing in a 1 mM ethanolic solution of CuNO32 which displaces weakly adsorbed molecules [19], reprotonating the surface in 5% acetic acid/ethanol, and finally drying with N2. The water contact angle on the -COOH terminated surface is
9 while that on the -CH3 surface is 115. We measure the normal force (FN) and friction force (F
) as a function of relative interfacial separation at 20 C and 1 atm with controlled relative humidity (RH)
45%. Friction is mea- sured by oscillating the tip laterally at 100 Hz and 1 nm amplitude, and recording the resulting force signal syn- chronously. We have used larger oscillating amplitudes (up to 20 nm) and found the friction force to scale linearly with shear velocity. Figure 1 shows normal- and friction-force profiles for (a) -CH3 tip on -CH3 surface, (b) -COOH tip on -CH3 surface, and (c)–(e) -COOH tip on -COOH surface. Note that there are 12 independent measurements as well as the averaged data for each force profile. The excellent agree- ment from measurement to measurement shows that the experiment is highly reproducible. The zero point of rela- tive displacement (RD) refers to the FN value (1
N) at which the tip begins its retraction. Also shown are the averaged friction data indicated by the heavy dashed lines. When both surfaces are hydrophobic [panel (a)], the force profiles are virtually independent of RH (data not shown). There is a small attraction well before contact indicated by point C. After contact between the two films (C, at RD 2:5 nm) the normal force begins its rise to the repulsive region and the friction force increases rapidly above zero. The repulsive region of the FN profile can be analyzed within the Johnson-Kendall-Roberts (JKR) model [10] which describes deformable interfaces in the presence of an attractive potential. The JKR fit gives a composite modulus of 13 GPa. For comparison, fits to the COOH=CH3 and COOH=COOH combinations give com- posite moduli of 29 and 33 GPa, respectively. The com- posite modulus increases in the order CH3=CH3 < COOH=CH3 < COOH=COOH, owing to different total molecular lengths of the SAM combinations [17]. The differences in the composite modulus are responsible for the different relative RD scales for the three panels. Compared to CH3=CH3, the maximum attractive force for COOH=CH3 increases about threefold [panel (b)]. The higher adhesion is due to the addition of a dipole— induced dipole component to the van der Waals (vdW) interaction due to the polar nature of the COOH surface (2 D per COOH group), as well as capillary condensa- tion of water on the COOH surface. The later serves to decrease the effective distance for the vdW interaction between the two surfaces. As the two surfaces approach, there is an attractive jump in FN around point A (RD 4:2 nm). The scatter in this point is consistent with nuclea- tion and capillary condensation; this is supported by simu-17780lation below. As the tip continues its approach, FN begins a slow decrease up to approximately point C (RD 2:0 nm), where FN enters the repulsive region and F
rises rapidly. This point, again, corresponds to contact between the two films. When both surfaces are hydrophilic [panel (c)], the magnitudes of the initial jump in FN and the attrac- tive force in the slow growth region (from point A to point B) are higher than those seen for the COOH=CH3 combination. This is consistent with the nucleation and growth of water meniscus at the hydrophilic interface. A quantitative understanding of the jump in and the slow growth of attractive normal force requires realistic simu- lation at a scale not possible with current theoretical methods [3]. Here we focus on the most provocative aspect of experimental observation for confinement within 1 nm of interfacial separation. The unique aspect of the COOH=COOH data is the rapid increase in attractive force beginning near point B (to a maximum value an order of magnitude larger than that in CH3=CH3). At virtually the same distance, the friction force begins a rapid rise. This is very different from the CH3=CH3 and COOH=CH3 combi- nations, where the friction force only rises sharply at film contact (point C). This behavior strongly suggests capillary condensation to form a water meniscus, which is respon- sible for both FN and F
. As expected for capillary con- densation and water-meniscus formation, the FN and F
profiles for the COOH=COOH interface depend on RH [panels (c)–(e) in Fig. 1]. At RH  10%, presence of the peak in F
profile shows up as a shoulder (*), which grows as RH is increased to 45%. Note that FN continues to rise until film/film contact (point C) whereas F
peaks (*) and then decreases to near baseline at point C, before rising again after film contact. This is because the meniscus is squeezed or swept away from the nanogap as the interfacial separation decreases beyond a critical point (*). The cross sectional area (Am) of the water meniscus can be obtained from the peak attractive normal force and the Kelvin radius (rK) of the meniscus given by [10] rK  H2OVM RT lnp=ps ; (1) where H2O, VM, R, T, and p=ps are the surface tension, the molar volume of the water meniscus, the gas constant, the absolute temperature, and the relative humidity, respec- tively. The Laplace pressure of the water meniscus is PL  H2O=rK and the capillary force is FC  AmPL. When the attractive normal force reaches a minimum, the maximum size of the meniscus is Am  1000, 1600, and 2400 nm2 for RH  10%, 26%, and 45%, respectively. The growth of the meniscus with RH is also shown by the outward shift of the point of meniscus nucleation (A) or capillary conden- sation (B) with increasing RH (right panels in Fig. 1). The viscosity of the water meniscus must be signifi- cantly larger than that of bulk water since a submerged tip in water shows negligible friction force under the same3-2