C O M M U N I C A T I O N S
cycles). Moreover, all additional compressions of the alcohol, once
the first compression/relaxation sequence has been completed, now
show a more normal rise at 22 Å/mol instead of 40 Å/mol (see
Figure 4B for compression no. 2).
The hysteresis is understandable in terms of horizontal-to-vertical
molecular reorientation at the air/water interface. Thus, during
compression no. 1 the diketopiperazine rings hydrogen-bond to each
other as they initially lie flat on the air/water interface and, thereby,
occupy an unusually large area for surfactant with only a single
tail. When pressures near 25 mN/m are reached, the rings flip so
that they are now more-or-less perpendicular to the interface where
they can also hydrogen-bond but in a much smaller space. This is
a stable ring orientation that remains vertical when the monolayer
is expanded during relaxation no. 1, and hence the hysteresis. Since
the vertical orientation is already in place throughout compression
no. 2, the first phase of the compression seen in compression no.
1 is avoided. The model implies the absence of a rapid equilibrium
between the vertical and horizontal states. Rearranging film
molecules by a chemist, like rearranging puzzle pieces by a child,
can obviously be an engaging activity.
Figure 4. (A) Compression no. 1 and relaxation no. 1 of monolayer film
of 1A (n ) 12) alcohol showing hysteresis. (B) Repeat cycle upon
completion of cycle A. All subsequent cycles are identical to cycle B!
33 °C there is an abrupt change of slope at a cmc of 16 mM (the
same value obtained by conductivity) (Figure 3, right). Normally,
surface tension plots level off above the cmc, but in our case the
slope turns positive. Although such behavior has in the past been
attributed to a surface-active impurity,15 quantitative NMR of our
1B (n ) 9) sample shows that decanoic acid, if present at all, cannot
exceed 0.5 wt % (an impurity level that was shown to have no
effect on SDS). We believe the aberrant surface tension plot results
from formation of suspended aggregates destined to become
precipitated solid (an explanation consistent with a haziness that
develops in the solutions just above the cmc). Thus, the picture
that emerges for 1B (n ) 9) is a rather insoluble surfactant that is
monomeric at low concentrations, micellar at the cmc, and a solid
phase soon thereafter. Rather than hydrophobicity and hydrogen-
bonding acting in concert, they seem to play independent roles,
with hydrophobicity important in micellization, and hydrogen-
bonding dominating precipitation (i.e., the solid state) as the
concentration is increased above the cmc.
Acknowledgment. We thank Dr. Kenneth Hardcastle and Dr.
Shaoxiong Wu for their assistance with the X-ray analyses and
diffusion NMR, respectively. We also thank Mr. Lei Shi, Dr. Dan
Lundberg, and Dr. Syed Rizvi for useful discussions and Prof. Kevin
Caran and Stephanie Torcivia for use of a Nima Tech DST 9005
tensiometer. This work was supported by the National Institutes of
Health.
Supporting Information Available: Analytical and synthetic
procedures including spectroscopic data and instrumentation. This
References
The question remained as to the nature of the micellar phase.
Since 1H NMR spectra showed no significant line-broadening above
the cmc, the micelles are likely small and spherical as opposed to
elongated assemblies. Pulse-gradient spin-echo NMR (i.e., diffusion-
NMR)16 in D2O at 33 °C, coupled with the Stokes-Einstein
equation, gave a hydrodynamic radius for 1B (n ) 9) of 3.29 nm
as compared with 2.45 nm for SDS. The modest increase in the
diameter of 1B (n ) 9) over SDS is likely related to the former’s
longer and bulkier headgroup as well as hydration of the dike-
topiperazine ring.
The most revealing data came from compression of monolayer
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4A upon the relaxing the monolayer back to its original low
pressure. (By way of comparison, stearic acid monolayers show
only a minor 2 Å/mol shift between compression and relaxation
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