Virtual organisations (VO) are a set of independent organisations sharing resources to achieve common goals. Security is a main concern for the adoption of such a collaboration model over IT technologies like Grid. Security issues are not only related to the protection from the outside world but also to the protection of the integrity and confidentiality of each organisation against potentially conflicting goals within the VO. To tackle this problem, this paper details how the Chinese wall security model can be adapted in the VO framework. The model is expressed as a reusable pattern over a VO ontology. The pattern formalization is also proved using the Alloy SAT-based technology.