Competitive Intercalation of Sulfonates into Layered Double Hydroxides (LDHs): the Key Role of Hydrophobic Interactions

Abstract

Layered double hydroxides (LDHs) Mg:Al 2:1 intercalated with straight-chain hydrocarbon sulfonates have been synthesized by anion exchange and characterized by powder XRD, FTIR, and elemental (C,H and metals) analysis. We find that the interlayer spacing of these LDHs can be explained by an interpenetrating packing model with the hydrocarbon chain tilting at an angle of 55° to the metal hydroxide layers. When two such sulfonate anions differ by only one methylene group, their simultaneous intercalation results in a single uniform LDH phase, in which the interlayer spacing varies linearly with the ratio between the two anions. In competitive intercalation experiments, the longer-chain sulfonate is preferentially or even exclusively intercalated. Semiquantitative analysis shows that the affinity of C8H17SO3- (or C7H15SO3-) for LDH is 6 kJ mol-1 stronger than that of C7H15SO3- (or C6H13SO3-). We attribute this increase to an increase in hydrophobic interactions between hydrocarbon chains. Hence, we estimate that the total contribution of hydrophobic interactions between hydrocarbon chains to the affinity of C8H17SO3- for Mg2Al−LDH is around 40−50 kJ mol-1.