Title page for ETD etd-11497-104420

Type of Document Dissertation
Author Aksoy, Bayram Suha
Author's Email Address baksoy@umr.edu
URN etd-11497-104420
Title Hydrophobic Forces in Free Thin Films of Water in the Presence and Absence of Surfactants
Degree PhD
Department Mining and Minerals Engineering
Advisory Committee
Advisor Name Title
Yoon, Roe-Hoan Committee Chair
Davis, Richey M. Committee Member
Karmis, Michael E. Committee Member
Luttrell, Gerald H. Committee Member
Wightman, James P. Committee Member
  • none listed
Date of Defense 1997-05-08
Availability unrestricted
In this study, two different system were studied to investigate the forces

interacting in thin liquid films. In the former, direct force measurements

were conducted with bitumen-coated mica surfaces using a Mark IV surface

force analyzer. Besides long-range electrostatic repulsive forces at

separation distances above approximately 70 nm, unexpectedly strong

repulsive forces were measured at shorter separation distances. These

non-DLVO forces may be attributed to the steric repulsion between

asphaltenes extracted on the bitumen surface. The steric forces increased

with pH and temperature, which may be explained by the increased

solubilization of asphaltenes in water. The steric force also increased

with electrolyte concentration, possibly due to a decrease in the mobility

of the tails of the asphaltenes on the surface.

The kinetics of coalescence of bitumen droplets was studied by measuring

the induction time. There was a general agreement between the force data

and the kinetic information, suggesting that bitumen suspensions are

stabilized by asphaltene. The steric forces identified by the direct force

measurements may have profound implications on the hot water processing of

Alberta Tar Sands.

In the later, a thin film balance (TFB) of Scheludko and Exerowa-type was

used to determine equilibrium film thicknesses of dodecylammonium chloride

and sodium dodecyl sulfate solutions. The hydrophobic force as a third

component of the extended DLVO theory was represented as a power law. The

results showed that at low surfactant concentrations, the hydrophobicity

parameter, K232 is positive and decreases with increasing surfactant

concentration. By extrapolating this data for the zero surfactant

concentration, K232 was found about 10-17 J, which is approximately 270

times larger than the Hamaker constant, A232. These results suggest that

air bubbles are hydrophobic and the hydrophobicity decreases with

increasing surfactant concentration..

The disjoining pressure isotherm for both surfactant systems in the

presence of inorganic electrolyte (NaCl) were also obtained. The results

indicated that the K232 values estimated from the equilibrium film

thickness measurements can be used to fit the experimentally obtained

disjoining pressure isotherm. 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.

To compare the hydrophobicity of air bubbles and solid surfaces, an air

bubble was simulated by using a hydrophobic solid surfaces. The

equilibrium contact angle of aqueous surfactant solutions on hydrophobic

surfaces was measured using a contact angle goniometer. The results

indicated that the nature of both the air-water interface and the

solid-water interface is altered gradually from hydrophobic to hydrophilic

with increasing surfactant concentration. The results also suggested that

the hydrophobicity of both systems are comparable, i.e., K232 is about

10-17 J, which is estimated using the extended DLVO theory, and K131 is in

the order of 10-16 J, which is obtained by direct force measurements

between the hydrophobic surfaces. It was showed that hydrophobic forces

much higher than the London-van der Waals forces for both systems need to

be included when the stability of foams and colloids is considered.

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