According to W. M. Lee, et.al. (see the link) , non-ideal gas model for describing nonlinearity of colloidal nanosuspensions can be explained using the higher order nonlinearities n2, n4, ... proposed in virial expansion of exponential model. However, they have not presented any estimation of the orders of n4 while n2 can be found in experimental reports. Can someome redirect me to some valid orde of n4 in contrast with n2?
http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-17-12-10277