The behavior of guanidinium chloride at the surface of aqueous
solutions is investigated using classical molecular dynamics (MD) simulations. It
is found that the population of guanidinium ions oriented parallel to the
interface is greater in the surface region than in bulk. The opposite is true for
ions in other orientations. Overall, guanidinium chloride is depleted in the
surface region, in agreement with the fact that the addition of guanidinium
chloride increases the surface tension of water. The orientational dependence of 
the surface affinity of the guanidinium cation is
related to its anisotropic hydration. To bring the ion to the surface in 
the parallel orientation does not require hydrogen bonds to be
broken, in contrast to other orientations. The surface enrichment of 
parallel-oriented guanidinium indicates that its solvation is more
favorable near the surface than in bulk solution for this orientation. 
The dependence of the bulk and surface properties of
guanidinium on the force field parameters is also investigated. 
Despite significant quantitative differences between the force fields,
the surface behavior is qualitatively robust. The implications for the 
orientations of the guanidinium groups of arginine side chains on
protein surfaces are also outlined.