Entropy of Branching Out: Linear versus Branched Alkylthiols Ligands on CdSe Nanocrystals

Abstract

Surface ligands of semiconductor nanocrystals (NCs) play key roles in determining their colloidal stability and physicochemical properties and are thus enablers also for the NCs flexible manipulation toward numerous applications. Attention is usually paid to the ligand binding group, while the impact of the ligand chain backbone structure is less discussed. Using isothermal titration calorimetry (ITC), we studied the effect of structural changes in the ligand chain on the thermodynamics of the exchange reaction for oleate coated CdSe NCs, comparing linear and branched alkylthiols. The investigated alkylthiol ligands differed in their backbone length, branching position, and branching group length. Compared to linear ligands, lower exothermicity and entropy loss were observed for an exchange with branched ligands, due to steric hindrance in ligand packing, thereby justifying their previous classification as “entropic ligands”. Mean-field calculations for ligand binding demonstrate the contribution to the overall entropy originating from ligand conformational entropy, which is diminished upon binding mainly by packing of NC-bound ligands. Model calculations and the experimental ITC data both point to an interplay between the branching position and the backbone length in determining the entropic nature of the branched ligand. Our findings suggest that the most entropic ligand should be a short, branched ligand with short branching group located toward the middle of the ligand chain. The insights provided by this work also contribute to a future smarter NC surface design, which is an essential tool for their implementation in diverse applications.

Substituent effects on the mechanochemical response of zinc dialkyldithiophosphate

“…Previous studies of other systems have also shown lower reactivity for surfaces protected by branched alkyl monolayers compared to linear ones. 48,66 For the secondary alkyl ZDDPs at 80 °C and 1000 N, the reactivity increases in the order: oct-2-yl < 4-methylpent-2-yl < but-2-yl. The trend is similar to the primary ZDDPs in that the secondary ZDDPs with longer alkyl chains give slower tribofilm formation rates, which can again be attributed to the formation of more densely packed SAMs.…”

“…47 Therefore, for most of the ZDDPs studied here, variation in the adsorption behavior is not expected to significantly affect the tribofilm formation rate on steel. However, for the ZDDPs containing longer branches 48 (2-ethylhexyl) and particularly the bulky cycloaliphatic substituents 49 (2-cyclohexylethyl), SAMs with lower packing densities are expected to form, which may affect their mechanochemical response.…”

Substituent Control of Tribofilm Growth under Mechanochemical Conditions

“…For the primary ZDDPs with C8 alkyl groups, the tribofilm growth rate increases in the order: 2-cyclohexylethyl < 2ethylhexyl < oct-1-yl. This suggests that the increased steric bulk has a larger effect on the rate than the lower packing density expected for the ethyl-branched 45 and particularly cyclohexylcontaining 46 ZDDPs compared to the linear one. Previous studies of other systems have also shown lower reactivity for surfaces protected by branched alkyl monolayers compared to linear ones.…”

“…Previous studies of other systems have also shown lower reactivity for surfaces protected by branched alkyl monolayers compared to linear ones. 45,61 For the secondary alkyl ZDDPs at 80 °C and 1000 N, the reactivity increases in the order: oct-2-yl < 4-methylpent-2-yl < but-2-yl. The trend is similar to the primary ZDDPs in that the secondary ZDDPs with longer alkyl chains give slower tribofilm formation rates, which can again be attributed to the formation of more densely packed SAMs.…”

“…However, for the ZDDPs containing longer branches 45 (2-ethylhexyl) and particularly the bulky cycloaliphatic substituents 46 (2-cyclohexylethyl), SAMs with lower packing densities are expected to form, which may affect their mechanochemical reactivity.…”