Biomedical subjects

RH Yoon

Publications and source records attributed to RH Yoon.

3 recordsLinked to original sources

PubMed1999

Hydrophobic Forces in Thin Water Films Stabilized by Dodecylammonium Chloride.

A thin film balance of Scheludko-Exerowa type was used to determine equilibrium film thicknesses of dodecylammonium chloride (RNH3Cl) solutions. The data were analyzed in view of the extended DLVO theory, which considers electrostatic, van der Waals, and hydrophobic forces. The hydrophobic force was represented as a power law which is of the same form as for the van der Waals force, so that its constant K232 can be directly compared with the Hamaker constant, A232. The results showed that at low surfactant concentrations, K232 is positive and decreases with increasing surfactant concentration, suggesting that hydrophobic force plays an important role in thin films. When the K232 versus concentration plot was extrapolated to very dilute solutions, K232 approaches 10(-17) J, which is approximately 270 times larger than A232. When the surfactant concentration was increased above 2 x 10(-3) M, however, K232 becomes negative, indicating that hydration force appears at high surfactant concentrations. These results suggest that air bubbles are hydrophobic and the hydrophobicity decreases with increasing surfactant concentration. A TFB was used to obtain a disjoining pressure isotherm at 10(-3) M RNH3Cl in the presence of 10(-4) M NaCl. The results can be fitted to the extended DLVO theory with K232 = 6 x 10(-19) J. Consideration of hydrophobic force predicted a rupture thickness larger than predicted using the DLVO theory, but is substantially smaller than the experimental result. This discrepancy may be ascribed to the hydrodynamic force operating in the film thinning process. Copyright 1999 Academic Press.

Journal Article↗

PubMed1998

Effects of Short-Chain Alcohols and Pyridine on the Hydration Forces between Silica Surfaces.

Forces between fully hydroxalated silica surfaces were measured using an atomic force microscope. The measurements were conducted in Nanopure water and in solutions containing various organic solutes such as methanol, ethanol, trifluoroethanol (TFE), and pyridine. The results obtained in Nanopure water showed a strong short-range repulsive force at distances below 15 nm. This non-DLVO force can be fitted to a double-exponential force law with its longer decay length (D2) of 2.4 nm. On the other hand, the force curve obtained at 15% methanol by volume can be fitted to the DLVO theory perfectly, showing no signs of hydration force. These results suggest that the hydration force originate from the unique water structure in the vicinity of silica, which apparently is seriously disrupted in the presence of methanol. Methanol may adsorb on silica, displacing water molecules from the silanol groups and, thereby, breaking the H-bond network within the hydration sheath around silica. The displacement of water by methanol is thermodynamically possible because the latter is more basic than the former. In 10-20% ethanol solutions, D2 decreases to 1.1-1.2 nm, indicating that ethanol also adsorbs on silica but to a lesser extent than methanol. In TFE and pyridine solutions, the hydration force changes little, suggesting that these solutes cannot readily displace water molecules from silanol groups. The results presented in this communication may have a bearing on the intoxication of humans by alcohols, which may be related to the dehydration of lipid membranes. Copyright 1998 Academic Press.

Journal Article↗

PubMed1997

Hydrophobic Interactions between Dissimilar Surfaces

An atomic force microscope (AFM) was used to measure surface forces between a glass sphere and a silica plate. When the measurements were conducted between untreated surfaces, a "short-range" hydration force with decay lengths of 0.4 and 3.0 nm was observed. When the surfaces were hydrophobized with octadecyltrichlorosilane (OTS), on the other hand, long-range hydrophobic forces with decay lengths in the range of 2-32 nm were observed. The force measurements were conducted between surfaces having similar and dissimilar hydrophobicities so that the results may be used for deriving an empirical combining rule. It was found that the power law force constants for asymmetric interactions are close to the geometric means of those for symmetric interactions. Thus, hydrophobic force constants can be combined in the same manner as the Hamaker constants. A plot of the power law force constants versus water contact angles suggests that the hydrophobic force is uniquely determined by contact angle. These results will be useful in predicting hydrophobic forces for asymmetric interactions and in estimating hydrophobic forces from contact angles.

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