[Paper Review] Polysiloxane surfactants for the dispersion of carbon nanotubes in non-polar organic solvents

Yali Ji, Youqing Huang+2 more|ArXiv.org|Jul 18, 2009

Porphyrin and Phthalocyanine Chemistry4 citations

TL;DR

This study develops two polysiloxane-based surfactants—mono-pyrene-siloxane (mPSi 70) and porphyrin-siloxane (PhrSi 60)—that enable effective dispersion of multi-walled (MWNTs) and single-walled carbon nanotubes (SWNTs) in non-polar solvents like petroleum ether and siloxane matrices. The surfactants use pyrene and porphyrin as CNT-philic heads and long siloxane chains (n ≈ 60–70) for steric stabilization, achieving MWNT dispersion at >10 mg/mL and SWNT solubility up to 0.5 mg/mL.

ABSTRACT

We develop two new amphiphilic molecules that are shown to act as efficient surfactants for carbon nanotubes in non-polar organic solvents. The active conjugated groups, which are highly attracted to graphene nanotube surface, are based on pyrene and porphyrin. We show that relatively short (C18) carbon tails are insufficient to provide stabilization. As our ultimate aim is to disperse and stabilize nanotubes in siloxane matrix (polymer and crosslinked elastomer), both surfactant molecules were made with long siloxane tails to facilitate solubility and steric stabilization. We show that pyrene-siloxane surfactant is very effective in dispersing multi-wall nanotubes, while the porphyrin-siloxane is making single-wall nanotubes soluble, both in petroleum ether and in siloxane matrix.

Motivation & Objective

• To develop amphiphilic surfactants that enable stable dispersion of carbon nanotubes (CNTs) in non-polar organic solvents.
• To address the challenge of achieving effective CNT dispersion in siloxane-based matrices for use in elastomer and polymer composites.
• To investigate the role of surfactant architecture—specifically the CNT-affinity of the head group and the length of the solvent-compatible tail—in determining dispersion efficiency.
• To demonstrate scalability and practical applicability of the surfactants through simple one-step organic synthesis.

Proposed method

• Synthesis of mono-pyrene-siloxane (mPSi 70) via one-step esterification between 1-pyrenebutyric acid and hydroxy-terminated polydimethylsiloxane (PDMS, Mn ≈ 5600 g/mol, n ≈ 70).
• Synthesis of porphyrin-siloxane (PhrSi 60) using a similar one-step reaction with a porphyrin-based acid and PDMS with n ≈ 60.
• Characterization of surfactants using FTIR (to confirm ester bond formation at 1740 cm⁻¹), ¹H NMR (to determine substitution ratio, x ≈ 0.89 for mono-substitution), and thermogravimetric analysis (TGA).
• Dispersion testing of CNTs in petroleum ether and in siloxane matrix using sonication and visual/UV-Vis analysis to assess solubility and stability.
• Rheological and sedimentation studies to evaluate long-term dispersion stability in neat silicone oils (5 cSt and 100 cSt) and in PDMS reactants.
• Theoretical estimation of the radius of gyration (Rg ≈ 1.3 nm) of the siloxane chain using Rg = √n × a_SiO, with n ≈ 60–70 and a_SiO ≈ 1.6 Å.

Experimental results

Research questions

• RQ1 Can pyrene- and porphyrin-based surfactants with long siloxane tails effectively disperse MWNTs and SWNTs in non-polar solvents like petroleum ether?
• RQ2 Why do short alkyl chains (C18) fail to stabilize CNTs in non-polar solvents, while long siloxane chains succeed?
• RQ3 Does the siloxane chain interact directly with the CNT surface, or does it act solely through steric stabilization in solution?
• RQ4 What is the optimal chain length for siloxane tails to maximize dispersion stability without reducing surfactant mobility or surface coverage?

Key findings

• The pyrene-siloxane surfactant (mPSi 70) achieved a stable dispersion of multi-walled carbon nanotubes (MWNTs) in petroleum ether at concentrations exceeding 10 mg/mL.
• The porphyrin-siloxane surfactant (PhrSi 60) enabled the dispersion of single-walled carbon nanotubes (SWNTs) in petroleum ether with a provisional solubility limit of 0.5 mg/mL.
• Neat siloxane oils (5 cSt and 100 cSt) and PDMS reactants showed no inherent ability to disperse CNTs, indicating that the siloxane chains alone do not interact favorably with CNT surfaces.
• The NMR analysis confirmed that approximately 89% of the surfactant molecules were mono-substituted (mPSi 70), with the remainder being di-substituted, indicating high selectivity in the reaction.
• The radius of gyration (Rg) of the siloxane chain was estimated at ~1.3 nm, which exceeds the van der Waals interaction distance (~1 nm), providing effective steric stabilization.
• The study demonstrates that long siloxane chains (n ≈ 60–70) are essential for steric stabilization, and that the surfactant synthesis is scalable via a simple one-step reaction.