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Radical re-appraisal of water structure in hydrophilic confinement.

Soper AK - Chem Phys Lett (2013)

Bottom Line: The water in the pore is divided into three regions: core, interfacial and overlap.The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density.There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K.

View Article: PubMed Central - PubMed

Affiliation: ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, UK.

ABSTRACT

The structure of water confined in MCM41 silica cylindrical pores is studied to determine whether confined water is simply a version of the bulk liquid which can be substantially supercooled without crystallisation. A combination of total neutron scattering from the porous silica, both wet and dry, and computer simulation using a realistic model of the scattering substrate is used. The water in the pore is divided into three regions: core, interfacial and overlap. The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density. There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K. The radical proposal is made here that water in hydrophilic confinement is under significant tension, around -100 MPa, inside the pore.

No MeSH data available.


Related in: MedlinePlus

Local density calculations (Eq. (9), middle expression) for the Si–Si and O–O radial distribution functions in dry MCM41 (a), and for the OW–OW and HW–HW radial distribution functions in wet MCM41 at 298 K (b) and 210 K (c). The straight lines defined by Eq. (9), right-hand expression, were fit in the region 5–15 Å. Parameters from these fits are given in Table 2. For O–O and HW–HW it was assumed the local density was twice the Si and OW local densities respectively, and the confining dimensions for O and HW were assumed to be the same as for Si and OW respectively.
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f0040: Local density calculations (Eq. (9), middle expression) for the Si–Si and O–O radial distribution functions in dry MCM41 (a), and for the OW–OW and HW–HW radial distribution functions in wet MCM41 at 298 K (b) and 210 K (c). The straight lines defined by Eq. (9), right-hand expression, were fit in the region 5–15 Å. Parameters from these fits are given in Table 2. For O–O and HW–HW it was assumed the local density was twice the Si and OW local densities respectively, and the confining dimensions for O and HW were assumed to be the same as for Si and OW respectively.

Mentions: These observations suggest a simple way to measure the local density in the pore. Using the functional form in (3), the number of atoms out to a specified distance, r, in the uniform fluid is defined as(8)Nαβr=43πρβ(l)r31-3ar4+⋯where is the local density of atoms in the pore. From this the average density of atoms around a given atom at the origin in the range is given by(9)ραβr=Nαβr43πr3=ρβ(l)1-3ar4+⋯In the limit , so estimating from the real s, such as in Figure 6a, using (3), and linearly extrapolating these to , using the right-hand side of (9), gives us simultaneously a measure of the local density, , and the approximate size of the confining medium, , via the gradient coefficient a. These calculations are shown in Figure 8 for the Si–Si and O–O distributions in the dry MCM41, and the OW–OW and HW–HW distributions in the wet MCM41. The parameters derived from the linear fits are given in Table 2


Radical re-appraisal of water structure in hydrophilic confinement.

Soper AK - Chem Phys Lett (2013)

Local density calculations (Eq. (9), middle expression) for the Si–Si and O–O radial distribution functions in dry MCM41 (a), and for the OW–OW and HW–HW radial distribution functions in wet MCM41 at 298 K (b) and 210 K (c). The straight lines defined by Eq. (9), right-hand expression, were fit in the region 5–15 Å. Parameters from these fits are given in Table 2. For O–O and HW–HW it was assumed the local density was twice the Si and OW local densities respectively, and the confining dimensions for O and HW were assumed to be the same as for Si and OW respectively.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4376068&req=5

f0040: Local density calculations (Eq. (9), middle expression) for the Si–Si and O–O radial distribution functions in dry MCM41 (a), and for the OW–OW and HW–HW radial distribution functions in wet MCM41 at 298 K (b) and 210 K (c). The straight lines defined by Eq. (9), right-hand expression, were fit in the region 5–15 Å. Parameters from these fits are given in Table 2. For O–O and HW–HW it was assumed the local density was twice the Si and OW local densities respectively, and the confining dimensions for O and HW were assumed to be the same as for Si and OW respectively.
Mentions: These observations suggest a simple way to measure the local density in the pore. Using the functional form in (3), the number of atoms out to a specified distance, r, in the uniform fluid is defined as(8)Nαβr=43πρβ(l)r31-3ar4+⋯where is the local density of atoms in the pore. From this the average density of atoms around a given atom at the origin in the range is given by(9)ραβr=Nαβr43πr3=ρβ(l)1-3ar4+⋯In the limit , so estimating from the real s, such as in Figure 6a, using (3), and linearly extrapolating these to , using the right-hand side of (9), gives us simultaneously a measure of the local density, , and the approximate size of the confining medium, , via the gradient coefficient a. These calculations are shown in Figure 8 for the Si–Si and O–O distributions in the dry MCM41, and the OW–OW and HW–HW distributions in the wet MCM41. The parameters derived from the linear fits are given in Table 2

Bottom Line: The water in the pore is divided into three regions: core, interfacial and overlap.The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density.There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K.

View Article: PubMed Central - PubMed

Affiliation: ISIS Facility, STFC Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0QX, UK.

ABSTRACT

The structure of water confined in MCM41 silica cylindrical pores is studied to determine whether confined water is simply a version of the bulk liquid which can be substantially supercooled without crystallisation. A combination of total neutron scattering from the porous silica, both wet and dry, and computer simulation using a realistic model of the scattering substrate is used. The water in the pore is divided into three regions: core, interfacial and overlap. The average local densities of water in these simulations are found to be about 20% lower than bulk water density, while the density in the core region is below, but closer to, the bulk density. There is a decrease in both local and core densities when the temperature is lowered from 298 K to 210 K. The radical proposal is made here that water in hydrophilic confinement is under significant tension, around -100 MPa, inside the pore.

No MeSH data available.


Related in: MedlinePlus