Surface forces have been measured in situ between gold surfaces hydrophobized by self-assembly of xanthates from aqueous solutions using an atomic force microscope (AFM). The measurements conducted in potassium amyl xanthate (KAX) and potassium ethyl xanthate (KEX) solutions showed long-range hydrophobic forces, with the force curves showing no evidence for nanobubbles on the surfaces. The strongest hydrophobic force was observed at contact angle above 90 degrees and in pure water, with a decay length of 29.5 nm. Both the contact angle and hydrophobic force increased with increasing xanthate concentration. At an excessively high concentration, the hydrophobic force diminished substantially. When the xanthate solution was replaced with pure water, however, a strong hydrophobic force reappeared, suggesting that the presence of residual xanthate ions in solution is detrimental to obtaining a strong and long-range hydrophobic force. This finding is consistent with the observation that hydrophobic force becomes weaker in the presence of NaCl in solution. (C) 2013 Elsevier B.V. All rights reserved.