Acid-base properties and zeta potentials of self-assembled monolayers obtained via in situ transformations.
Siloxane-anchored, self-assembled monolayers (SAMs) on single crystal Si were prepared with a variety of surface functional groups using a single commercially available surfactant (1-bromo-11-(trichlorosilyl)undecane) followed by in situ transformations. Polar (thioacetate and thiol), nonpolar (methyl), acidic (sulfonic and carboxylic), basic (various amines), and ionic (alkylammonium) surface functionalities were prepared. For primary amine and sulfonate surfaces, the degree of surface charge as a function of pH was determined ex situ using X-ray photoelectron spectroscopy (XPS). Sulfonate SAMs exhibited much higher effective pKa (approximately 2) than dilute sulfonic acid (-5 to -6), and amine SAMs exhibited much lower pKa (approximately 3) than dilute organic amines (approximately 10). This is attributed to the stabilization of nonionized groups by adjacent ionized groups in the SAM. Zeta potentials of these SAMs as a function of pH were consistent with the XPS results and indicated that ionizable SAM surfaces can generate surface potentials much higher than those of nonionic SAMs (thioacetate, methyl) and typical oxide surfaces.
Shyue
Jing-Jong
JJ
Department of Materials Science & Engineering, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106-7204, USA.
De Guire
Mark R
MR
Nakanishi
Tsuyoshi
T
Masuda
Yoshitake
Y
Koumoto
Kunihito
K
Sukenik
Chaim N
CN