Failure of Debye-Hückel Screening in Low-Charge Colloidal Suspensions

Derjaguin–Landau–Verwey–Overbeek (DLVO) theory remains the cornerstone of colloid stability. Electrostatic interactions dominate van der Waals attractions at large colloid-colloid separations h, unless strongly screened. Under these conditions, the potential U(h) between charged colloids is expected to be exponentially screened, U(h)∼exp(−κh)/h , with κ−1=λD where λD is the classical Debye-Hückel screening length. By measuring the force between individual charged particles at dilute electrolyte concentrations (<mM) using optical tweezers, we tested experimentally the prediction κ−1=λD in a nonpolar solvent. At low salt concentrations, we found close agreement between the directly-measured decay length κ−1 and Debye-Hückel predictions. However, above a critical electrolyte concentration (≈450 μ M), we obtained significant discrepancies between measured and predicted screening lengths, with κ−1≫λD . In marked contrast to expectations, we found that the measured screening length κ−1 appears to grow as the ionic strength of the solution is increased. The origin of this discrepancy is discussed and the importance of considering the surface is highlighted