TY - JOUR
T1 - Solvent dependence of the activation energy of attachment determined by single molecule observations of surfactant adsorption
AU - Honciuc, Andrei
AU - Baptiste, Denver Jn
AU - Campbell, Ian P.
AU - Schwartz, Daniel K.
PY - 2009/7/7
Y1 - 2009/7/7
N2 - Single-molecule total internal reflection fluorescence microscopy was used to obtain real-time images of fluorescently labeled hexadecanoic (palmitic) acid molecules as they adsorbed at the interface between fused silica and three different solvents: hexadecane (HD), tetrahydrofuran (THF), and water. These solvents were chosen to explore the effect of solvent polarity on the activation energy associated with the attachment rate, i.e., the rate at which molecules were transferred to the surface from the near-surface layer. Direct counting of single-molecule events, made under steady-state conditions at extremely low coverage, provided direct, model-independent measurements of this attachment rate, in contrast with conventional ensemble-averaged methods, which are influenced by bulk transport and competing detachment processes. We found that the attachment rate increased with increasing temperature for all solvents. Arrhenius analyses gave activation energies of 5 ± 2 kJ/mol for adsorption from HD, 10 ± 2 kJ/mol for adsorption from THF, and 19 ± 2 kJ/mol for adsorption from water. These energies increased systematically with the solvent polarity and, therefore, with the expected strength of the solvent-substrate interaction. We hypothesize that the adsorption of amphiphilic solute molecules from solution can be regarded as a competitive exchange between solute molecules and surface-bound solvent. In this scenario, adsorption is an activated process, and the activation energy for attachment is associated with the solvent-substrate interaction energy.
AB - Single-molecule total internal reflection fluorescence microscopy was used to obtain real-time images of fluorescently labeled hexadecanoic (palmitic) acid molecules as they adsorbed at the interface between fused silica and three different solvents: hexadecane (HD), tetrahydrofuran (THF), and water. These solvents were chosen to explore the effect of solvent polarity on the activation energy associated with the attachment rate, i.e., the rate at which molecules were transferred to the surface from the near-surface layer. Direct counting of single-molecule events, made under steady-state conditions at extremely low coverage, provided direct, model-independent measurements of this attachment rate, in contrast with conventional ensemble-averaged methods, which are influenced by bulk transport and competing detachment processes. We found that the attachment rate increased with increasing temperature for all solvents. Arrhenius analyses gave activation energies of 5 ± 2 kJ/mol for adsorption from HD, 10 ± 2 kJ/mol for adsorption from THF, and 19 ± 2 kJ/mol for adsorption from water. These energies increased systematically with the solvent polarity and, therefore, with the expected strength of the solvent-substrate interaction. We hypothesize that the adsorption of amphiphilic solute molecules from solution can be regarded as a competitive exchange between solute molecules and surface-bound solvent. In this scenario, adsorption is an activated process, and the activation energy for attachment is associated with the solvent-substrate interaction energy.
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U2 - 10.1021/la900307f
DO - 10.1021/la900307f
M3 - Article
AN - SCOPUS:67650078515
SN - 0743-7463
VL - 25
SP - 7389
EP - 7392
JO - Langmuir
JF - Langmuir
IS - 13
ER -