The special and unique role of the spacer of Gemini surfactant in self-assembly is reviewed, which is considered to benefit mainly from both the spacer length and the rigid/flexible features. These two effects lead to the synergism of two alkyl tails of a Gemini molecule, the change in the charge density of its headgroups, the variation of its molecule geometry by which rich structures and morphologies of aggregates yield, etc. More detailed, the flexible spacer influences the above functions of the Gemini mainly depending on its length. Too long flexible spacer can bend toward the alkyl tails so as to meet the chemical environment around the molecule, by which the molecule self-assembly is influenced. The effect of the short rigid spacer is almost identical with that of the flexible spacer having a similar length. However, the long rigid spacer yields quite different effects from the flexible spacer owing to the two alkyl tails are inhibited to be close, which leads to the column-like molecular shape and the identical probability for the cis/trans configuration of the two alkyl tails around the spacer. These make the Geminis with long rigid spacer form network-like aggregates at low concentrations, which can be transformed into threadlike micelles or vesicles with low surface curvature with increasing the Gemini concentration or adding a few additives. Under suitable conditions, a few molecules in the aggregates can be hastened to yield trans-form, resulting in the cohesion between the aggregates through the hydrophobic interactions between the extended alkyl tails. The chemical modification for the spacer is also discussed, which is expected to promote molecular self-assembly, or give some new functions to the aggregates. The cases listed here well indicate the specialty and uniqueness of the Gemini-structure of molecule and impress us for the complicated self-assembly behavior and diversified aggregates of Gemini surfactants.

Contents
1 Introduction
2 Basic understanding for the role of spacer
2.1 Hydrophobic synergism between alkyl tails and inhibition for headgroup separation
2.2 Strong salt-effect
2.3 Adjustment for molecular geometry
3 Adaptive behavior of flexible spacer
4 Gemini surfactants with short rigid spacer
5 Gemini surfactants with long rigid spacer
5.1 Aging effect of interfacial adsorption
5.2 Column-like molecular geometry
5.3 Network-like aggregates in dilute solution
6 Cis/trans configuration of two alkyl tails brought by spacer
7 Functioning about spacer
7.1 Hydroxyl modification
7.2 Light-sensitive group modification
7.3 Part fluocarbon modification
8 Other consideration about spacer role
9 Summary