Molecular Structure of the Chloroform−Water and Dichloromethane−Water Interfaces

Hore, Dennis K.; Walker, Dave S.; MacKinnon, and Libby; Richmond, Geraldine L.

doi:10.1021/jp067176t

Cited by 56 publications

(92 citation statements)

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“…This indicates a preference for the dipole moment to point away from the bulk, similar to the orientation of water molecules next to hydrophobic surfaces found in earlier studies. [66][67][68] The probability density of θ (i.e., the distribution of θ normalized by the cone angle sin θ) in the first layer (0.80 e z e 2.50 nm) and in the bulk (0.55 g z g 2.75 nm) is shown in Figure 6b. A uniform distribution is displayed for the bulk at both temper- Figure 3a) indicates that the major contribution (∼75%) for the large negative heat capacity change of hydrophobic association arises from changes in the reorganization energy between the water molecules.…”

Section: Resultsmentioning

confidence: 99%

Temperature Dependence of Dimerization and Dewetting of Large-Scale Hydrophobes: A Molecular Dynamics Study

2008

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We studied by molecular dynamics simulations the temperature dependence of hydrophobic association and drying transition of large-scale solutes. Similar to the behavior of small solutes, we found the association process to be characterized by a large negative heat capacity change. The origin of this large change in heat capacity is the high fragility of hydrogen bonds between water molecules at the interface with hydrophobic solutes; an increase in temperature breaks more hydrogen bonds at the interface than in the bulk. With increasing temperature, both entropy and enthalpy changes for association strongly decrease, while the change in free energy weakly varies, exhibiting a small minimum at high temperatures. At around T ) T s ) 360 K, the change in entropy is zero, a behavior similar to the solvation of small nonpolar solutes. Unexpectedly, we find that at T s , there is still a substantial orientational ordering of the interfacial water molecules relative to the bulk. Nevertheless, at this point, the change in entropy vanishes due to a compensating contribution of translational entropy. Thus, at T s , there is rotational order and translational disorder of the interfacial water relative to bulk water. In addition, we studied the temperature dependence of the drying-wetting transition. By calculating the contact angle of water on the hydrophobic surface at different temperatures, we compared the critical distance observed in the simulations with the critical distance predicted by macroscopic theory. Although the deviations of the predicted from the observed values are very small (8-23%), there seems to be an increase in the deviations with an increase in temperature. We suggest that these deviations emerge due to increased fluctuations, characterizing finite systems, as the temperature increases.

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Section: Resultsmentioning

confidence: 99%

Temperature Dependence of Dimerization and Dewetting of Large-Scale Hydrophobes: A Molecular Dynamics Study

2008

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“…Our understanding of the complex assembly found in this study is aided by what we have learned over the past decade (10)(11)(12) about the molecular characteristics of this soft interface that guides the adsorption of ions and molecules. We know, for example, that weak bonding interactions between interfacial water and various hydrophobic oils is a general trait for these oil-water systems, resulting in significant molecular orientational ordering and structuring of both phases near the interfacial region (13)(14)(15).…”

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confidence: 84%

Ordered polyelectrolyte assembly at the oil–water interface

Beaman

Robertson

Richmond

2012

Proc. Natl. Acad. Sci. U.S.A.

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107

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Polyelectrolytes (PEs) are widely used in applications such as water purification, wastewater treatment, and mineral recovery. Although much has been learned in past decades about the behavior of PEs in bulk aqueous solutions, their molecular behavior at a surface, and particularly an oil-water interface where many of their applications are most relevant, is largely unknown. From these surface spectroscopic and thermodynamics studies we report the unique molecular characteristics that several common polyelectrolytes, poly(acrylic acid) and poly(methylacrylic acid), exhibit when they adsorb at a fluid interface between water and a simple insoluble organic oil. These PEs are found to adsorb to the interface from aqueous solution in a multistepped process with a very thin initial layer of oriented polymer followed by multiple layers of randomly oriented polymer. This additional layering is thwarted when the PE conformation is constrained. The adsorption/desorption process is highly pH dependent and distinctly different than what might be expected from bulk aqueous phase behavior. macromolecular assembly | surface spectroscopy | water surfaces | hydrophobic surfaces | surfactants

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“…The interfacial water structure is believed to be similar to that of the water/vapor interface, which is also deemed hydrophobic, except for the weak van der Waals interaction between water and the substance. There are quite a few theoretical studies on hydrophobic interfacial water structure (1,(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18), but not enough experimental work to test the theories.…”

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confidence: 99%

Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy

Tian

Shen

2009

Proc. Natl. Acad. Sci. U.S.A.

277

288

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We have studied the hydrophobic water/octadecyltrichlorosilane (OTS) interface by using the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS), and we obtained detailed structural information of the interface at the molecular level. Excess ions emerging at the interface were detected by changes of the surface vibrational spectrum induced by the surface field created by the excess ions. Both hydronium (H 3O ؉ ) and hydroxide (OH ؊ ) ions were found to adsorb at the interface, and so did other negative ions such as Cl ؊ . By varying the ion concentrations in the bulk water, their adsorption isotherms were measured. It was seen that among the three, OH ؊ has the highest adsorption energy, and H 3O ؉ has the lowest; OH ؊ also has the highest saturation coverage, and Cl ؊ has the lowest. The result shows that even the neat water/OTS interface is not neutral, but charged with OH ؊ ions. The result also explains the surprising observation that the isoelectric point appeared at ϳ3.0 when HCl was used to decrease the pH starting from neat water.surface charging ͉ octadecyltrichlorosilane ͉ hydronium ͉ hydroxide H ydrophobicity is a well-known phenomenon (1). It plays a crucial role in many chemical, biological, and environmental processes. Yet, despite extensive research over the years, our understanding at the molecular level is still limited. The lack of hydrogen bonding between water molecules and a substance often defines a hydrophobic interface. It is supported by the existence of the dangling OH bonds at the interface (2, 3). The interfacial water structure is believed to be similar to that of the water/vapor interface, which is also deemed hydrophobic, except for the weak van der Waals interaction between water and the substance. There are quite a few theoretical studies on hydrophobic interfacial water structure (1, 4-18), but not enough experimental work to test the theories.Recently, attention has been drawn to the question of whether a depleted water layer exists between water and the hydrophobic substrate (1, 19-21). Free energy argument suggested the existence of such a layer under the ambient condition, with a layer width that decreases with increase of the van der Waals interaction between water and the substrate (1, 7). There were reports on the observation of nanobubbles at hydrophobic interfaces by atomic force microscopy imaging (22-24), but they were not confirmed by others (25)(26)(27). Neutron reflection measurements revealed the presence of a layer of reduced water density at a hydrophobic silane/water interface that depends on the dissolved gas in the water (28-30). Removal of the dissolved gas decreases the width of the layer. Recent observation of an electron depletion layer of 1-6 Å by X-ray reflection at such an interface also suggested the presence of a corresponding water depletion layer at the interface (25-27). However, the interpretation was questioned by Galli and coworkers (14) and Ocko et al. (31). The latter noted that the preferred polar alignment of C-H and O-H bonds a...

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Molecular Structure of the Chloroform−Water and Dichloromethane−Water Interfaces

Cited by 56 publications

References 57 publications

Temperature Dependence of Dimerization and Dewetting of Large-Scale Hydrophobes: A Molecular Dynamics Study

Temperature Dependence of Dimerization and Dewetting of Large-Scale Hydrophobes: A Molecular Dynamics Study

Ordered polyelectrolyte assembly at the oil–water interface

Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy

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