Surface Tension and Adsorption Kinetics of Volatile Organic Amphiphiles in Aqueous Solution
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Amphiphiles that possess a dual character, hydrophobic and hydrophilic, are employed in many chemical, pharmaceutical and biological applications. Amphiphile molecules that include a hydrophilic head and a hydrophobic tail can easily adsorb at a liquid/vapour interface, to reach to a minimum free energy and hence a most thermodynamically stable state. Surface tension is a key parameter for understanding such behavior of an amphiphile, or a surfactant. This thesis represents a comprehensive study on adsorption and surface tension of slightly volatile, organic amphiphiles in aqueous solution. Although for a vapor-liquid interface, adsorption from both liquid and vapor phases should be considered, they have been almost always considered exclusive of one another. When a volatile surfactant is dissolved in the liquid phase, it also applies a finite partial pressure in the vapor phase. Recently, dynamic surface tension experiments showed that adsorption from both sides of a vapor/liquid interface must be studied simultaneously. It is noted that surface tension phenomena are often dynamic, in particular when the surface under consideration is perturbed. With the newly discovered importance of adsorption from both sides of a vapor/liquid interface, one may have to ask the question: how dynamic surface tension is influenced and responding to the surface perturbation and environment changes, and whether both sides of the interface play a role in surface tension responses. In this research, axisymmetric drop shape analysis-profile (ADSA-P) is used for surface tension measurement. The experiments are performed in a closed chamber where the effects of surfactant concentrations of both liquid and vapor phases on the surface tension can be studied. The partial vapor pressure of surfactant is controlled with an environment solution containing the same surfactant as the sample solution. The environment solution is to facilitate adsorption from the vapor side of the interface by creating a surfactant vapor phase. The effects of surface perturbation, environment condition (i.e., temperature and pressure) and carbon chain length on the surface tension and adsorption kinetics are studied in detail. The surface tension response of 1-octanol aqueous solution to surface area perturbation is investigated. Upon surface compression, the surface tension decreases followed by a gradual increase back to the value prior to compression. On surface expansion, two categories of surface tension response are observed: First, when the change in surface area is smaller than 5%, the behavior similar to that of conventional surfactants is observed. The surface tension increases followed by a gradual decrease back to the value prior to expansion. Second, when the change in surface area is greater than 5%, and the drop concentration is sufficiently larger than the environment concentration, the surface tension initially slightly increases, but after a time delay, it sharply decreases, followed by a gradual increase back to the value prior to expansion. Previous studies showed that at steady-state condition a network of hydrogen bonding between surfactant and water molecules near the surface is created. The unique surface tension response after large expansion might be related to the momentarily destruction of this hydrogen bonding network and gradually making a new one. The effect of temperature on the surface tension and adsorption kinetics of 1-octanol, 1-hexanol and 1-butanol aqueous solutions is studied. The steady-state surface tension is found to decrease upon an increase in temperature, and a linear relationship is observed between them. The modified Langmuir equation of state and the modified kinetic transfer equation are used to model the experimental data of the steady-state and dynamic (time-dependent) surface tension, respectively. The equilibrium constants and adsorption rate constants are evaluated through a minimization procedure for temperatures ranging from 10°C to 35°C. From the steady-state modelling, the equilibrium constants for adsorption from vapor phase and liquid phase are found to increase with temperature. From the dynamic modelling, the adsorption rate constants for adsorption from vapor phase and liquid phase are found to increase with temperature too. The influence of carbon dioxide pressure on the surface tension and adsorption kinetics of the aforementioned surfactant aqueous solutions is investigated. To consider the effect of adsorption/desorption of the two species (surfactant and carbon dioxide) from both sides of a vapor/liquid interface on the surface tension, the modified Langmuir equation of state and the modified kinetic transfer equation are derived. The steady-state and dynamic surface tension data are modelled using the modified Langmuir equation of state and the modified kinetic transfer equation, respectively. The equilibrium constants and adsorption rate constants of surfactant and carbon dioxide are evaluated through a minimization procedure for CO2 pressures ranging from 0 to 690 KPa. From the steady-state modelling, the equilibrium parameters for surfactant and carbon dioxide adsorption from vapor phase and liquid phase are found unchanged for different pressures of carbon dioxide. From the dynamic modelling, the adsorption rate constants for surfactant and carbon dioxide are found to decrease with carbon dioxide pressure. The role of carbon chain length of amphiphiles in aqueous solution is also studied. It is illustrated that the equilibrium constants for adsorption from both sides of a vapor/liquid interface increase from 1-butanol to 1-octanol. The modelling results show that the ratio of the equilibrium constant for adsorption from vapor phase to the equilibrium constant for adsorption from liquid phase declines from 260 to 26 as the chain length is increased from 1-butanol to 1-octanol. Therefore, the contribution to adsorption from liquid phase augments as the chain length is increased. The adsorption kinetics for this group of short carbon chain surfactants is modelled using a kinetic transfer equation. The modelling results show that the adsorption rate constants from vapor phase and liquid phase (kag and kal) increase from 1-butanol to 1-octanol. Steady-state and dynamic modelling also reveals that the maximum surface concentration increases with carbon chain length. These results may be due to the higher hydrophobicity character of a surfactant molecule at longer carbon chain length.