Dipolar, also zwitterionic, headgroup of DPPC

Marrink, Siewert-Jan and Herman J. C. Berendsen. "Simulation of Water Transport through a Lipid Membrane." The Journal of Physical Chemistry 98, no. 15 (1994): 4155--4168.

Marrink and Berendsen describe the headgroups of DPPC as "dipolar" (p. 4161).  Phosphatidylcholine contains both a negatively charged phosphate and a positively charged choline, hence the descriptor.  Another name would be zwitterion, wherein a molecule contains both a positively charged and negatively charged group but whose net charge is neutral.

Because of the polar nature of the the DPPC headgroup, they observed a "very diffuse interface between the dipolar headgroups and water" (p. 4161).

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