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Probing hydrophilic interface of solid/liquid-water by nanoultrasonics.

Mante PA, Chen CC, Wen YC, Chen HY, Yang SC, Huang YR, Chen IJ, Chen YW, Gusev V, Chen MJ, Kuo JL, Sheu JK, Sun CK - Sci Rep (2014)

Bottom Line: To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale.We developed a new experimental technique that extends the classical acoustic technique to the molecular level.Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT
Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

No MeSH data available.


Related in: MedlinePlus

(a) The laterally-homogenous experimental system can be considered as an ultrasonic A-scan system. (b) Amplitude of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (c) Phase of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (d–f) The spatial distribution of the density, elastic modulus and viscosity for water at the interface with amorphous Al2O3 obtained by fitting the complex reflection spectrum. All data are normalized to the values of bulk water. The origin of the x-axis corresponds to the center of the atomic plane terminating the Al2O3 surface.
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f3: (a) The laterally-homogenous experimental system can be considered as an ultrasonic A-scan system. (b) Amplitude of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (c) Phase of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (d–f) The spatial distribution of the density, elastic modulus and viscosity for water at the interface with amorphous Al2O3 obtained by fitting the complex reflection spectrum. All data are normalized to the values of bulk water. The origin of the x-axis corresponds to the center of the atomic plane terminating the Al2O3 surface.

Mentions: It is known that the intermolecular interaction between substrate and liquid will generally make the properties of the interfacial liquid different from those of bulk1234567891011121314. Our observed acoustic reflection spectra can thus help to discover the viscoelastic properties of interfacial water and unravel the intermolecular interactions inside. In order to be able to describe our experimental results we use a discrete model to represent interfacial water. Since the acoustic pulse is a quasi-plane wave and the studied system is laterally homogeneous, the experimental system can be considered as a 1-dimensional problem. Here, a typical ultrasonic analysis algorithm is adopted to determine the acoustic properties of water as a function of distance from the solid surface. In this algorithm, water at the interface is divided into thin layers with variable density, ρ, elastic modulus, A, and viscosity, b as can be seen in Fig. 3a. Then, the effective acoustic impedance can be calculated in a similar way to the electromagnetic transmission line theory27: with: and where Δzj is the thickness of the j-th segment, Zj is its acoustic impedance and τ = b/A is the viscous relaxation time. In our study, the term viscosity refers to the phenomena responsible for the loss of the acoustic signal. The possible mechanisms responsible for sound attenuation are thermal conduction and molecular motion28. At j = 0, the input acoustic impedance, , is trivially equal to the acoustic impedance of bulk water, Z0 = ZBulk. We assume that the fast sound phenomenon29 is still invalid in water at our studied frequency and simply take the values of acoustic properties of bulk water from Ref. 24. Here the thickness of every segment, Δzj, is set to half of the spatial resolution of the acoustic pulse, which also corresponds to the half water molecule size. Since the penetration depth of our acoustic wave pulse is less than 2 nm, considering a 3nm-thick water layer in the calculation is sufficient. In the fitting process, ρ, A, and b of each segment within 10 Å from the solid surface can vary freely under a converging constraint and the properties of water behind 10 Å from the solid surface asymptotically transform to the bulk-state values in an exponential way (see supplementary information). Under this setup, the totally 27 parameters can be uniquely determined by finding the best fit of the full complex reflection spectra, shown in Fig. 3b and c, with a number of data points much over 27. Using this method, we can have access to the density, elastic modulus and viscosity profile of water. The profiles obtained for the Al2O3/water interface are shown in Fig. 3(d), (e) and (f). In this profile the x-axis corresponds to the distance from the surface, and the origin is defined as the center of the atomic plane that is terminating the Al2O3 surface.


Probing hydrophilic interface of solid/liquid-water by nanoultrasonics.

Mante PA, Chen CC, Wen YC, Chen HY, Yang SC, Huang YR, Chen IJ, Chen YW, Gusev V, Chen MJ, Kuo JL, Sheu JK, Sun CK - Sci Rep (2014)

(a) The laterally-homogenous experimental system can be considered as an ultrasonic A-scan system. (b) Amplitude of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (c) Phase of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (d–f) The spatial distribution of the density, elastic modulus and viscosity for water at the interface with amorphous Al2O3 obtained by fitting the complex reflection spectrum. All data are normalized to the values of bulk water. The origin of the x-axis corresponds to the center of the atomic plane terminating the Al2O3 surface.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150100&req=5

f3: (a) The laterally-homogenous experimental system can be considered as an ultrasonic A-scan system. (b) Amplitude of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (c) Phase of the acoustic reflectivity of the GaN/air interface, simulation of the reflectivity using continuum elasticity theory and best fitted result using our algorithm. (d–f) The spatial distribution of the density, elastic modulus and viscosity for water at the interface with amorphous Al2O3 obtained by fitting the complex reflection spectrum. All data are normalized to the values of bulk water. The origin of the x-axis corresponds to the center of the atomic plane terminating the Al2O3 surface.
Mentions: It is known that the intermolecular interaction between substrate and liquid will generally make the properties of the interfacial liquid different from those of bulk1234567891011121314. Our observed acoustic reflection spectra can thus help to discover the viscoelastic properties of interfacial water and unravel the intermolecular interactions inside. In order to be able to describe our experimental results we use a discrete model to represent interfacial water. Since the acoustic pulse is a quasi-plane wave and the studied system is laterally homogeneous, the experimental system can be considered as a 1-dimensional problem. Here, a typical ultrasonic analysis algorithm is adopted to determine the acoustic properties of water as a function of distance from the solid surface. In this algorithm, water at the interface is divided into thin layers with variable density, ρ, elastic modulus, A, and viscosity, b as can be seen in Fig. 3a. Then, the effective acoustic impedance can be calculated in a similar way to the electromagnetic transmission line theory27: with: and where Δzj is the thickness of the j-th segment, Zj is its acoustic impedance and τ = b/A is the viscous relaxation time. In our study, the term viscosity refers to the phenomena responsible for the loss of the acoustic signal. The possible mechanisms responsible for sound attenuation are thermal conduction and molecular motion28. At j = 0, the input acoustic impedance, , is trivially equal to the acoustic impedance of bulk water, Z0 = ZBulk. We assume that the fast sound phenomenon29 is still invalid in water at our studied frequency and simply take the values of acoustic properties of bulk water from Ref. 24. Here the thickness of every segment, Δzj, is set to half of the spatial resolution of the acoustic pulse, which also corresponds to the half water molecule size. Since the penetration depth of our acoustic wave pulse is less than 2 nm, considering a 3nm-thick water layer in the calculation is sufficient. In the fitting process, ρ, A, and b of each segment within 10 Å from the solid surface can vary freely under a converging constraint and the properties of water behind 10 Å from the solid surface asymptotically transform to the bulk-state values in an exponential way (see supplementary information). Under this setup, the totally 27 parameters can be uniquely determined by finding the best fit of the full complex reflection spectra, shown in Fig. 3b and c, with a number of data points much over 27. Using this method, we can have access to the density, elastic modulus and viscosity profile of water. The profiles obtained for the Al2O3/water interface are shown in Fig. 3(d), (e) and (f). In this profile the x-axis corresponds to the distance from the surface, and the origin is defined as the center of the atomic plane that is terminating the Al2O3 surface.

Bottom Line: To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale.We developed a new experimental technique that extends the classical acoustic technique to the molecular level.Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical Engineering and Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei 10617, Taiwan.

ABSTRACT
Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

No MeSH data available.


Related in: MedlinePlus