INVESTIGATION OF THE CHARGE-TRANSFER PROPERTIES OF ELECTRODES MODIFIED BY THE SPONTANEOUS ADSORPTION OF UNSYMMETRICAL DIALKYL SULFIDES

Electrodes modified by the spontaneous adsorption of unsymmetrical dialkyl sulfides are used to investigate two monolayer properties, structural disorder and solvent compatibility, that may be of importance in determining the ability for faradaic charge transport to occur in the presence of the monolayer. Two groups of dialkyl sulfides are prepared and studied: a purely hydrocarbon dialkyl sulfide with the structure CHs(CH2)17S(CH2)nCH3, where n = 7, 9, and 17, and an acid containing dialkyl sulfide with the structure CH3(CH2)17S(CH2)mCOOH, where m = 7, 10, and 15. Reflection infrared spectroscopy of the hydrocarbon dialkyl sulfide reveals that the monolayer structure is disordered, when measured ex situ, and is not largely affected by the presence of contacting aqueous solutions. Infrared spectroscopy of the acid containing dialkyl sulfides suggests a partially ordered structure when measured ex situ, but the monolayer becomes disordered when in contact with aqueous solutions. Additional evidence of monolayer disorder for both groups of dialkyl sulfides is found in the contact angle the monolayers make with hexadecane. Aqueous contact angle measurements for the acid containing dialkyl sulfides indicate that the acid group is protected by the surrounding hydrocarbon chains, but the proximity of the acid group to the monolayer-solution interface influences the interfacial properties. Electrochemical results indicate that no solution permeation occurs into the purely hydrocarbon monolayer, while substantial permeation occurs with the acid containing monolayers. For the acid containing monolayers, solution permeation increases as the acid group is placed closer to the monolayer-solution interface. These results suggest that the presence of a polar group within the monolayer structure increases the charge transfer properties of the monolayer modified electrode.