We studied the formation of phenylalkyltrichlorosilane self-assembled monolayers on native oxide covered silicon. After a first chemisorption step in the monolayer growth, the presence of the short alkyl chain (3-4 carbon atoms) is responsible for a second growth step which corresponds to the arrangement between molecules. We found that this packing step is accelerated by replacing phenyl by pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated cycles. Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an even faster packing step which is accounted for by hydrogen bonding CH...FC in a face to face phenyl/pentafluoro-phenyl arrangement. We believe these results allow improving charge delocalization over conjugated molecular domains. In a second part, we studied the phase separation between phenyl-alkyltrichlorosilane and octadecyltrichlorosilane (OTS) molecules. Improving the phase separation was studied using ring to ring interactions afore-analyzed. We show phase separation is improved and OTS islands are smaller with phenyl species that involve stronger ring to ring interactions. The best case is obtained with mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for packing together the aromatic species. These results demonstrate improved control of SAM composition and morphology essential to further use the obtained islands for building molecular devices.