Ice-like water supports hydration forces and eases sliding friction

Abstract

The nature of interfacial water is critical in several natural processes, including the aggregation of lipids into the bilayer, protein folding, lubrication of synovial joints, and underwater gecko adhesion. The nanometer-thin water layer trapped between two surfaces has been identified to have properties that are very different from those of bulk water, but the molecular cause of such discrepancy is often undetermined. Using surface-sensitive sum frequency generation (SFG) spectroscopy, we discover a strongly coordinated water layer confined between two charged surfaces, formed by the adsorption of a cationic surfactant on the hydrophobic surfaces. By varying the adsorbed surfactant coverage and hence the surface charge density, we observe a progressively evolving water structure that minimizes the sliding friction only beyond the surfactant concentration needed for monolayer formation. At complete surfactant coverage, the strongly coordinated confined water results in hydration forces, sustains confinement and sliding pressures, and reduces dynamic friction. Observing SFG signals requires breakdown in centrosymmetry, and the SFG signal from two oppositely oriented surfactant monolayers cancels out due to symmetry. Surprisingly, we observe the SFG signal for the water confined between the two charged surfactant monolayers, suggesting that this interfacial water layer is noncentrosymmetric. The structure of molecules under confinement and its macroscopic manifestation on adhesion and friction have significance in many complicated interfacial processes prevalent in biology, chemistry, and engineering.

Keywords: coordinated water; friction, surfactants; hydration forces; sum frequency generation.

Fig. 1. Contact mechanics measurements.

(A) Comparison of the underwater COF on the left axis and the work of adhesion (W_ad) obtained using zero-load JKR (33) on the right axis. With zero-load JKR, the values were not measurable after 0.005 mM CTAB. The lens did not make adhesive contact past 0.005 mM. (B and C) The amount of adsorbed CTAB was measured by interfacial energy calculated from contact angle measurements (B) and QCM-D measurements (C), both as a function of surfactant concentration. QCM-D measurements of the mass adsorbed were analyzed using the Sauerbrey equation, assuming a rigid adsorbed film. At a CTAB concentration of 0.4 mM, the surface coverage of CTAB is 125 ± 15 ng/cm² on PDMS and 199 ± 45 ng/cm² on PETS (text S3). The SDs were determined using a sample size of n = 3 for COF and adhesion and n = 2 for the QCM-D measurements. Propagated error for γ_s,l was determined from contact angle measurements of multiple samples and contact spots. The roughness measured by atomic force microscopy is within good agreement for the silanization of glass with PETS (0.7 nm). The root mean square roughness of glass and a PETS self-assembled monolayer (PETS-SAM) on glass was 2.8 and 3.6 nm, respectively. The dashed lines denote the three regions described in the main text, highlighting the differences between friction and surface coverage. C* is the concentration at which the monolayer formation is complete, and C* ≠ C*_COF. (D) The COF in air and underwater in the absence of CTAB serves as control experiments. (E) The schematic of a PDMS lens sliding on a PETS substrate is shown to measure the COF (not to scale). The flattened contact under the compressive load is accentuated for clear visualization.

Fig. 2. SFG spectra for CTAB adsorption on PETS.

(A and B) SFG spectra collected using SSP (A) and SPS (B) polarization as a function of CTAB concentration, in which we expect hydrocarbon signatures. Measuring both polarizations provides complementary data for the spectral interpretation of CTAB on PETS. (C) The presence of D₂O was observed by scanning the region where we expected only D₂O spectral features. The characteristic regions (1 to 3) are indicated by the vertical bars. The weakly coordinated −OD peak (represented by dashed line with 2) and the strongly coordinated −OD (represented by dashed line with 1) peak evolve with increasing surfactant concentration. (D) The spectra were fitted using the Lorentzian function, and the changes in amplitude CH_2,asym (A_q,2935) are shown as a function of surfactant concentration. SPS is more sensitive to the orientational changes of CTAB than SSP. a.u., arbitrary unit. (E) Model of the CTAB interface in contact with water. For simplicity, this model assumes 100% dissociation of CTAB head groups, although it has been reported that the dissociation can be as low as 21% (34). The surface density of −OH groups on sapphire surface (35), molecular dimensions for CTAB (11, 36), water (37), and PETS (38, 39) were estimated on the basis of published literature. The orthogonal packing represented is consistent with the area per molecule based on the interfacial energy measurement, 52 Ų per molecule, which is similar to the theoretical area calculated for orthogonal packing (text S2).

Fig. 3. Underwater static contact observed using SFG spectroscopy.

(A and B) Contact interface observed using SFG spectroscopy for PDMS-PETS underwater contact using D₂O with increasing CTAB concentration in SSP for the hydrocarbon (A) and D₂O (B) regions. (C) An increase in ordering of strongly coordinated interfacial water (A_q,2390) is compared with COF with surfactant addition, in which the water peak continues to increase past C*. The characteristic regions (1 to 3) from the adsorption isotherm are indicated by the vertical bars. (D) Schematic for probing the contact interface in total internal reflection geometry. The flattened contact under the compressive loads is accentuated for clear visualization. ω_Vis and ω_IR indicate the frequencies of visible and infrared laser beams, respectively. (E) Depiction of confined D₂O arranged between two saturated hydrophobic surfaces with CTAB in region 3. The arrangement captures confined strongly coordinated water at the repulsive contact interface between cationic head groups.

Fig. 4. The underwater contact interface probed during sliding using SFG spectroscopy.

The contact interface probed during sliding using SFG spectroscopy for PDMS-PETS underwater contact using D₂O in the presence of CTAB. The measurements were done in SSP polarization at an incident angle of 8°. (A) A schematic for a sliding experiment where the PDMS lens is initially outside the contact zone of the interrogating aligned laser beams. The lens is then slid into contact and then out of contact at 5 μm/s. (B and C) Traces for the changing absolute SFG intensity, not the Lorentzian peak amplitude, for PETS (3050 cm⁻¹), CTAB (2935 cm⁻¹), and ice-like D₂O (2410 cm⁻¹) peaks with sliding in the presence of (B) 0.1 mM CTAB and (C) 5.0 mM CTAB. (D) Comparison of the normalized SFG intensities of CTAB and D₂O peaks by PETS peak intensity in static and sliding contact. The intensity as a function of time during sliding is averaged when the laser beams are overlapped with the center of contact area. Error bars are evaluated from at least three static spectral intensities and two sliding scans.