Synthesis and Micellization of Carbosilane Sulfonate Surfactants with Short Alkyl Chains

Carbosilane surfactants, consisting of carbosilane as a hydrophobic group linked to hydrophilic groups, are one kind of silicone surfactants. In this paper, a series of carbosilane sulfonate surfactants with short alkyl chains (C-Si₂C-SO₃Na (n = 1-6)), Me-Si₂C-SO₃Na, Et-Si₂C-SO₃Na, Pr-Si₂C-SO₃Na, Bu-Si₂C-SO₃Na, Pen-Si₂C-SO₃Na, and Hex-Si₂C-SO₃Na, were prepared and characterized by ²⁹ Si NMR, ¹H NMR, and FT-IR spectroscopies. The influence of the alkyl chain length on their micellization was studied using surface tension, dynamic light scattering, conductivity, and transmission electron microscopy. The CMC value decreases with increasing length of the short alkyl group. The γ_CMC value of C-Si₂C-SO₃Na (n = 1-6) increases as the alkyl chain increases from methyl to propyl, while the γ_CMC value gradually decreases as the alkyl chain increases from propyl to hexyl. The larger and rigid tetramethyldicarbosilane group functioned synergistically with a short alkyl chain, resulting in carbosilane sulfonate surfactants adsorbing at the air/water interface with a rugby ball shape; accordingly, the A_min values of the investigated carbosilane sulfonate surfactants increase with increasing length of the alkyl chain. The micellization process of carbosilane sulfonate surfactants is enthalpy-driven at lower temperatures and entropy-driven at high temperatures. The ΔH_m⁰ values became more negative and ΔS_m⁰ values more positive as the alkyl chain length increased. Aggregates in the range of 10-800 nm were observed for C-Si₂C-SO₃Na (n = 1-6) in an aqueous solution, and the hydrodynamic diameter (D_h) decreased with increasing length of the short alkyl group.