| Summary: | 博士 === 國立陽明大學 === 醫學工程研究所 === 95 === Phase transition mechanisms of 1-dodecanol and human serum albumin at air-water interface were studied by tensiometry and ellipsometry respectively at different temperatures. The results for the adsorption of 1-dodecanol at 20 ºC indicated that the adsorption isotherm of G(C) clearly showed three abrupt increases at bulk concentration C of 1.3x10-9, 2x10-9 and 3.7x10-9 mol/ml, respectively. The 1st and the 3rd transitions were consistent with the gas to liquid expanded (G-LE) and the liquid expanded to liquid condensed (LE-LC) phase transitions observed in a previous tensiometry study. The 2nd transition observed at C=2x10-9 mol/ml was identified for the 1st time in the present study. Judging from the substantial increase in the film thickness at this transition, it is believed that the orientation change of the adsorbed molecule is responsible for this transition in the LE phase. A LEh and a LEv phase, that denoted the “lie-down” and “stand-up” types of adsorption respectively, was used to describe this transition. A cusp, instead of a typical constant surface tension region, was observed in the dynamic p(t) curves for this transition. As the effects of temperature and bulk concentration, the corresponding surface pressures for (LEh-LEv) and (LE-LC) phase transitions were independent on bulk concentrations but increased linearly with the temperature. The transition surface pressure increased in a rate of 0.64 and 0.88 mNm-1/ºC. for (LEh-LEv) transition and for (LE-LC) transition respectively. The dynamic p(t) curves and equilibrium p(C) isotherm both indicated that the lower temperature is favorable for 1-dodecanol adsorption. Due to the lower transition surface pressure and the faster adsorption rate at 15 ºC, the (LEh-LEv) transition at this temperature shifted to lower concentration at C=1.4x10-9 mol/ml when compared with the same transition at 20 ºC.
For human serum albumin, the early stage of the dynamic p(t) curves at lower bulk concentrations showed an induction period, which is characterized by an apparent increase in surface concentration at the beginning of the induction period and a nearly constant surface pressure during the induction period. Together with the film thickness analysis, it reveals that the adsorbed film experiences a phase transition, from a gaseous to a liquid expanded state at the induction period. Two different methods were applied to evaluate the diffusion coefficient of albumin at 15 ºC. The first is based on the concept that the surface concentrations at the end of the induction time for different bulk concentrations are the same. The second method according to modified Ward and Tordai’s diffusion theory, is by analyzing slopes of the G-t1/2 and p-t1/2 curves. Diffusivity obtained by both methods are consistent and remained nearly constant over a wide range of bulk concentration. The adsorption G(C) and p(C) isotherms of HSA both showed four steps phase transition behaviors. Judging from the equivalent film thickness and the dimension of the adsorbed protein molecules, it is believed that these transitions were due to the orientational of the adsorbed molecules, i.e. change from “side-on”、 “heart shape-triangular side”、 “heart shape-rectangular side” and finally turn into end-on adsorption state with the increase of the surface concentration. By applying van’t Hoff equation and the equilibrium adsorption/desorption rate constant at various temperatures, one can obtain the adsorption enthalpy and entropy change. The results indicated that the enthalpy and the entropy change for the adsorption of albumin is temperature dependent from15 ºC to 37 ºC. At lower temperatures, the adsorption is entropy driven due to the contribution of hydrophobic effect. While turns into enthalpy driven due to the diminish of the hydrophobic interactions at higher temperature.
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