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

Franceska Waggett, Mohamad Danial Shafiq, Paul Bartlett

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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
Original languageEnglish
Pages (from-to)1-9
Number of pages9
JournalColloids Interfaces
Issue number51
Publication statusPublished - 22 Oct 2018


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