Self-assembled monolayers formed by adsorption of 4-methyl-4‘-mercaptobiphenyl, CH3-(C6H4)2−SH (BPT), on Au(111) have been studied using scanning tunneling microscopy. The results show that with increasing coverage (resulting from longer immersion times) BPT forms a series of different structural phases with different molecular arrangements, closely resembling the behavior reported previously for n-alkanethiolate adlayers. For short immersion times, striped structures are observed (α and β), where the molecules are orientated with their axes parallel to the surface. Longer immersion times yield additional phases, namely, an ordered χ-phase, where the molecules are proposed to be oriented with their molecular axis tilted away from the surface, a disordered δ phase, and, finally, a densely packed (2√3 × √3) ε phase where the molecular axes are orientated almost upright. Unexpectedly, for BPT after adsorption small islands are seen on the Au substrate instead of the etch pits commonly observed after formation of organothiolate adlayers.

Here, we present the results of a detailed characterization of rigid-rod thiolate adlayers on Au(111) made by self-assembly of organothiols containing a terphenyl backbone, namely, p-terphenylthiol (TPT, C6H5-(C6H4)2-SH) and p-terphenylmethanethiol (TPMT, C6H5-(C6H4)2-CH2-SH) on Au(111). The results of low energy electron diffraction measurements reveal the presence of long-range order for adlayers prepared from both molecules. From the body of data obtained by X-ray photoelectron spectroscopy, infrared reflectionabsorption spectroscopy, near-edge X-ray absorption fine structure measurements, and scanning tunneling microscopy, a structure model for adlayers of TPT and TPMT on Au(111) is proposed.

In conclusion, numerous mechanisms like floating, sinking, effervescence, swelling, bioadhesion, magnetic, etc. have been proposed over the years. While most of the proposed systems show promising dissolution profiles and in-vitro retention, only few of them have also shown success in-vivo. Currently, the polymeric swelling monolithic systems are the most prominent marketed forms. The possibility to combine different mechanisms in order to ensure true gastric retention even in the fasted state should be further investigated.