Surface Behavior of Boronic Acid-Terminated Silicones

Article abstract of DOI:10.1021/acs.langmuir.5b02143

Silicone polymers, with their high flexibility, lie in a monolayer at the air-water interface as they are compressed until a critical pressure is reached, at which point multilayers are formed. Surface pressure measurements demonstrate that, in contrast, silicones that are end-modified with polar groups take up lower surface areas under compression because the polar groups submerge into the water phase. Boronic acids have the ability to undergo coordination with Lewis bases. As part of a program to examine the surface properties of boronic acids, we have prepared boronic acid-modified silicones (SiBAs) and examined them at the air-water interface to better understand if they behave like other end-functional silicones. Monolayers of silicones, aminopropylsilicones, and SiBAs were characterized at the air-water interface as a function of end functionalization and silicone chain length. Brewster angle and atomic force microscopies confirm domain formation and similar film morphologies for both functionalized and non-functionalized silicone chains. There is a critical surface pressure (10 mN m^-1) independent of chain length that corresponds to a first-order phase transition. Below this transition, the film appears to be a homogeneous monolayer, whose thickness is independent of the chain length. Ellipsometry at the air-water interface indicates that the boronic acid functionality leads to a significant increase of film thickness at low molecular areas that is not seen for non-functionalized silicone chains. What differentiates the boronic acids from simple silicones or other end-functionalized silicones, in particular, is the larger area occupied by the headgroup when under compression compared to other or non-end-functionalized silicones, which suggests an in-plane rather than submerged orientation that may be driven by boronic acid self-complexation. (Figure Presented)

Full text of DOI:10.1021/acs.langmuir.5b02143

Article
pubs.acs.org/Langmuir
Surface Behavior of Boronic Acid-Terminated Silicones
Erum Mansuri,† Laura Zepeda-Velazquez,‡ Rolf Schmidt,† Michael A. Brook,*,‡
and Christine E. DeWolf*,†
†Department of Chemistry and Biochemistry and Concordia Centre for NanoScience Research, Concordia University, 7141
́ ́
Sherbrooke Street West, Montreal, Quebec H4B 1R6, Canada
‡Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4M1,
Canada
S
* Supporting Information
ABSTRACT: Silicone polymers, with their high flexibility, lie
in a monolayer at the air−water interface as they are
compressed until a critical pressure is reached, at which
point multilayers are formed. Surface pressure measurements
demonstrate that, in contrast, silicones that are end-modified
with polar groups take up lower surface areas under
compression because the polar groups submerge into the
water phase. Boronic acids have the ability to undergo
coordination with Lewis bases. As part of a program to
examine the surface properties of boronic acids, we have
prepared boronic acid-modified silicones (SiBAs) and
examined them at the air−water interface to better understand
if they behave like other end-functional silicones. Monolayers of silicones, aminopropylsilicones, and SiBAs were characterized at
the air−water interface as a function of end functionalization and silicone chain length. Brewster angle and atomic force
microscopies confirm domain formation and similar film morphologies for both functionalized and non-functionalized silicone
chains. There is a critical surface pressure (10 mN m−1) independent of chain length that corresponds to a first-order phase
transition. Below this transition, the film appears to be a homogeneous monolayer, whose thickness is independent of the chain
length. Ellipsometry at the air−water interface indicates that the boronic acid functionality leads to a significant increase of film
thickness at low molecular areas that is not seen for non-functionalized silicone chains. What differentiates the boronic acids from
simple silicones or other end-functionalized silicones, in particular, is the larger area occupied by the headgroup when under
compression compared to other or non-end-functionalized silicones, which suggests an in-plane rather than submerged
orientation that may be driven by boronic acid self-complexation.
in the aqueous subphase and the methyl groups extending into
the air. As the molecular area is reduced, molecules are forced
to come into contact with one another and adopt a zigzag
conformation, where the backbone becomes less-solvated as
some oxygen and silicon atoms are no longer in contact with
water.7 Further compression leads to a phase transition. In early
work, Lenk et al.9 proposed the formation of helices during this
phase transition for both functionalized and non-functionalized
PDMS. Later work on PDMS by Kim et al.7 suggested a
multilayering rather than helix formation on the basis of
vibrational sum frequency spectroscopy measurements. It is
now generally accepted that the chains slide above one another
and form odd numbered multilayers. If they do form helices, it
is most likely that the helices form on top of a monolayer and
not just directly at the air−water interface.7,11
INTRODUCTION
■
An important consequence of the highly flexible Si−O
backbone of silicone polymers is an intrinsically low surface
energy and high surface activity. When further modified with
hydrophilic groups, silicone surfactants have applications
ranging from agricultural adjuvants1 to foam stabilizers in
polyurethane.2 While polyethers are frequently the hydrophiles
of choice, silicones bearing other surface-active groups,
including phosphates, sulfates,3 amines, carboxylic acids, and
amino acids,4,5 have been developed for specific applications,
particularly in personal care applications. The surface activities
of some of these materials can be modified at pH values away
from neutrality, however, usually at conditions under which the
silicones are known to undergo depolymerization.
The interfacial behavior of silicone polymers [typically
poly(dimethylsiloxane) (PDMS)], including a range of end-
functionalized PDMS, has been widely studied.2,6−10 At high
molecular areas, PDMS chains are generally assumed to adopt a
caterpillar conformation, where molecules are stretched and
lying at the water surface with the siloxane backbone immersed
Received: June 11, 2015
Revised: July 21, 2015
© XXXX American Chemical Society
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Figure 1. Chemical synthesis and structure of boronic acid-functionalized silicones. The ratio of terminal/internal isomers in the SiBAs is ∼70:30.
The abbreviated name indicates the total number of silicon atoms in the central silicone block (i.e., SiBA-10; n = 8).
Table 1. Details of the Functionalized Silicones Used in These Studies
*
Note that the terminal and internal isomers are formed in a ∼70:30 ratio.2
The impact of end functionalization of both PDMS and
carbon-based backbone polymers has also been studied at the
air−water interface.9,12 Functional groups typically serve to
anchor the chain to the subphase, forcing a more horseshoe-
type conformation, as also reported for long-chain bolaform
amphiphiles.13 The strength of this type of anchoring has been
shown to impact the surface pressure at small molecular areas
and the surface pressure at film collapse.9,12
Boronic acids are known to provide a pH-sensitive binding
site for diols, specifically the 1,2- and 1,3-diols that are common
in saccharides. The binding affinity for different diol-containing
compounds varies significantly (over orders of magnitude),
which enables differentiation of sugars.1 On the basis of this
selective binding, boronic acids have been proposed as drug
delivery agents,14 artificial lectins,15 and tunable saccharide
sensors.16,17 Any changes to the local environment at boron can
be “locked in” by forcing the boron center to tetracoordination,
for example, at higher pH.
Recently, we reported the preparation of boronic acid-
modified silicone polymers.18,19 The surface activity of boronic
acids at the air−water interface has been reported to be affected
by the size and structure of boronate.20−22 Silicone boronic acid
polymer behavior should be tunable by modification of the
subphase constituents, including pH, presence of sugars, etc.,
under conditions that do not affect the silicone backbone. That
is, unlike other silicone-based surfactants, it should be possible
to dynamically modify the interfacial behavior of silicone
boronic acids under mild conditions. This paper describes an
initial study on the behavior of silicone boronates at the
air−water interface compared to related surface-active silicone
polymers, which will allow for specific applications to be
identified. The silicone boronates under consideration are α,ω-
boronic acid-functionalized silicones (Figure 1) of various
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combined thickness of the LB film and silicon dioxide after LB film
deposition. All values are reported as average film thicknesses.
Atomic force microscopy (AFM) was carried out using a
Nanoscope IIIa MultiMode AFM from Veeco (Santa Barbara, CA).
All images were obtained using tapping mode, using V-shaped silicon
nitride tips with a nominal spring constant of 0.58 N m−1. Images were
analyzed using the NanoScope Analysis Version 1.40 software.
molecular weights that were examined as a function of chain
length, pH, and temperature.
EXPERIMENTAL SECTION
■
Materials. Boronic acid-terminated PDMS (difunctional SiBAs and
monofunctional SiBAM; Figure 1) were synthesized as previously
described.19 The hydride-terminated silicone precursors, (H-PDMS-m,
where m = number of silicon atoms in the chain, 10, 16, or 69) and
aminopropyl-terminated polydimethylsiloxane (H2N-PDMS-10; m =
10) were obtained from Gelest, Inc. (Morrisville, PA). Details of the
functionalized silicones are provided in Table 1. The average number
of repeat units and corresponding molecular weights were determined
RESULTS AND DISCUSSION
■
Surface pressure−area isotherms of silicone boronic acids with
varying chain lengths (SiBA-10, SiBA-16, and SiBA-69; Figure
1) at the air−water interface are shown in Figure 2, along with
1
by H nuclear magnetic resonance (NMR).
Spreading solutions were prepared in chloroform [high-perform-
ance liquid chromatography (HPLC) grade, Fisher Scientific], with
concentrations ranging from 0.5 to 1 mM. All solutions were stored at
−4 °C and brought to room temperature before use. Ultrapure water
with a resistivity of 18.2 MΩ cm was obtained from an EasyPure II LF
system (Barnstead, Dubuque, IA). Silicon wafers were purchased from
Wafer World, Inc. (West Palm Beach, FL) and cleaned using the
following procedure: substrates were immersed in a 1:1 mixture of
concentrated hydrochloric acid [American Chemical Society (ACS)
grade, EM Science] and methanol (HPLC grade, Fisher Scientific) for
30 min, rinsed abundantly with ultrapure water, immersed in
concentrated sulfuric acid (reagent grade, J.T. Baker) for 30 min,
rinsed abundantly with ultrapure water, and stored in ultrapure water.
Storage times never exceeded 7 days. Prior to use, silicon substrates
were blown dry with nitrogen. Mica sheets were purchased from Ted
Pella, Inc. (Redding, CA) and were cleaved by separating thin layers
from either side of the sheet to expose a clean surface for deposition.
Methods. Surface pressure−area isotherms were obtained on
thermostated Langmuir film balances (140 cm2, 6:1 length/width
aspect ratio, Nima Technology, Ltd., Coventry, U.K.) at varying
temperatures with a compression speed of 5 cm2 min−1. Surface
pressure measurements were made using a filter paper (Whatman No.
1) Wilhelmy plate. Monolayers of each surfactant were spread from
chloroform solutions on the aqueous subphase, and the solvent was
allowed to evaporate for approximately 3 min before beginning
compression. All isotherms and transfers were carried out under
symmetric compression. All subphases were made using ultrapure
water (pH ≈ 5.5), and where noted, the subphase pH was adjusted by
the addition of sodium hydroxide (ACS reagent, Sigma-Aldrich).
Monolayers were transferred at a constant pressure onto clean silicon
wafers and mica using the Langmuir−Blodgett (LB) technique with a
dipping speed of 2 mm min−1.
Figure 2. Isotherms for silicone boronic acids and hydride-terminated
silicones on ultrapure water at 23 °C: (A) SiBA-10 (solid blue), SiBA-
16 (solid red), H-PDMS-10 (dashed blue), and H-PDMS-16 (dashed
red) and (B) SiBA-69 (solid green) and H-PDMS-69 (dashed green).
Isotherms for the high molecular weight polymer films are displayed
separately for clarity given the large difference in molecular area.
isotherms for the corresponding hydride-terminated poly-
(dimethylsiloxane) precursors (H-PDMS-10, H-PDMS-16,
and H-PDMS-69) for comparison. The isotherms for the
hydride-terminated PDMS are very similar to those reported
for methyl-terminated PDMS,2,6,7,9 while those for the silicone
boronic acids show similar transitions to other α,ω-difunction-
alized PDMS.9
In all isotherms, irrespective of the end group (H versus
boronic acid), there is a plateau corresponding to a first-order
phase transition starting at pressures between 8 and 10 mN
m−1, with a slight increase for the low-molecular-weight
compounds as previously observed.9 Only the molecular areas
for transitions and the length of the plateaus vary with chain
length. The isotherms for larger molecules have a longer, flatter
phase transition plateau, as a result of greater conformational
freedom of the chain (i.e., more isoenergetic conformations).
There is the appearance of an additional transition (smaller
plateau starting around 550 Å2 molecule−1) for the higher
molecular weight SiBA-69. Such transitions are commonly
observed when the PDMS chain is sufficiently long, in both the
presence and absence of end functional groups.9 This
secondary transition is not limited to silicones and has also
been observed in the isotherms of high-molecular-weight, end-
functionalized polyisobutylenes.12 The second transition may
also be occurring for the oligomers SiBA-10 and SiBA-16 but
may be obscured because the entire isotherm occurs over a
much smaller range of molecular areas.
Brewster angle microscopy (BAM) and ellipsometry measurements
were carried out with an I-Elli2000 imaging ellipsometer (Nanofilm
Technologie GmbH, Gottingen, Germany) equipped with a 50 mW
̈
Nd:YAG laser (λ = 532 nm) using a 20× magnification with a lateral
resolution of 1 μm. BAM experiments were performed at an incident
angle of 53.15° (Brewster angle of water) and a laser output of 50 and
100% (analyzer, compensator, and polarizer were all set to 0).
Ellipsometric measurements at the air−water interface were carried
out at an incident angle of 50.00° and a laser output of 100%, with the
compensator set to 20.00°. The reported thickness is an average of 10
measurements each taken at a different location on the same film and
is consistent for multiple samples. The ellipsometric isotherm is
reported in terms of δΔ, which is independent of the optical model.
δΔ is defined as the difference between the ellipsometric angle Δ of
the film on the subphase and the subphase alone (δΔ = Δfilm
−
Δ
subphase). Measurements were also carried out on LB films deposited
onto silicon substrates using an incident angle of 65.00° and a laser
output of 1% with the compensator set to 45.00°. To determine the
optical thickness of the monolayers at the air−solid interface, the
silicon dioxide layer thickness was determined on clean substrates with
the following two-box optical model: silicon as the substrate (n = 4.15;
κ = 0.04) and silicon dioxide as the layer (n = 1.46; κ = 0), assuming an
isotropic film. The same box model was used to determine the
Finally, there is a sharp increase in the surface pressure at low
molecular areas for the SiBAs that is not observed for the H-
terminal silicones but which is observed for aminopropyl- and
other end-functionalized silicones.9 This transition must
correspond to an additional conformational change for
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Figure 3. Ellipsometric isotherms at the air−water interface [surface pressure, blue; ellipsometric measurement (δΔ), red]: (A) SiBA-10, (B) SiBA-
16, (C) SiBA-69, and (D) H-PDMS-69.
molecules that are terminated with polar tethering groups.
Without polar end functionalization, the molecules are free to
slide over one another as thicker (non-monolayer) organic
films are formed, while the surface pressure remains constant.
With the polar end functionalization, this migratory layering
process is hindered as the molecule is tethered to the subphase.
Longer chains may still provide enough flexibility for some
layering,7,11 but at some point, the tethering limits this process
and an alternative conformation is adopted, which leads to a
pressure increase at smaller molecular areas (see below). The
pressure continues to increase as the film is compressed until a
collapsed state is reached at approximately 38−40 mN m−1.
The high collapse pressure observed for SiBAs is consistent
with strong tethering of the terminal group to the subphase9,12
and is comparable to values obtained for amine- and carboxylic
acid-terminated PDMS.9
BAM images were used to follow film morphology changes
throughout the compression isotherms. The formation of very
bright domains toward the end of the plateau is attributed to
the autophobic dewetting and/or localized collapse of the film
(Supporting Information). Autophobic dewetting arises when a
fluid cannot spread or wet a thin film comprising the same
material;23 such dewetting processes have been reported for
polymer films, such as PDMS, and result in the coexistence of
regions of different thicknesses.24,25 Schull et al. reported
polymer dewetting from a surface-adsorbed (end-tethered)
polymer film and noted that this process occurs more readily
when the film is tethered at both ends (loops) rather than one
end (tails).23 Dewetting of droplets from a thin film of the same
material has been observed for films where the first layer is
more ordered than the bulk.25 Given that domains were
observed for both the end-functionalized and non-end-
functionalized materials, this must be a property inherent to
the silicone backbone that is not impacted by tethering and is
attributed to the underlying monolayer being more ordered
(zigzag conformation)24 than the overlying layers, which then
spontaneously dewet to a lens of disordered material. The
dewetting process only occurs after a critical film thickness is
achieved, i.e., upon high compression.
Ellipsometric measurements at the air−water interface
provide the means to monitor film thickness changes during
compression. The ellipsometric isotherms for all three SiBAs
are shown in Figure 3, where δΔ is the change in ellipsometric
angle Δ and correlates to a change in optical thickness. In all
cases, the films show a δΔ of −0.5 just before the critical area
for the onset of pressure. This step change in δΔ corresponds
to the transition from a backbone caterpillar (Si−O submerged
in the subphase) to a zigzag conformation, where the Si−O
backbone partially desolvates and forms a thicker film on the
surface.7 The films then exhibit a slow, progressive increase in
thickness (more negative δΔ values) until the end of the
plateau, where δΔ reaches approximately −1.0. For all three
SiBA molecules, there is a sharp increase in the average
thickness of the film after the plateau (Figure 3D). A much
smaller, less drastic increase is observed for H-PDMS-69. The
differences between SiBAs and H-PDMSs observed by
ellipsometry at these low molecular areas must therefore be
attributed to thickness increases as a result of conformational
restriction in the underlying SiBA film induced by the boronic
acid interaction with the subphase.
The absolute thicknesses of the film below the transition
pressure, where the film is homogeneous by BAM, were
established by depositing LB films onto silicon wafers. The
thicknesses of the deposited films were determined by fitting
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ellipsometric angle changes to a two-box model and are shown
in Table 2. All three SiBA films have approximately the same
was used to more accurately determine the height differences
between the domains and the surrounding film (Figure 4).
Films of SiBA-10 were deposited onto silicon and mica from
the air−water interface using LB. The films were deposited at
surface pressures above the transition plateau (25 mN m−1),
where the onset of domain formation (BAM) was observed.
The films exhibit the formation of domains of varying lateral
and vertical dimensions. Small sub-200 nm domains are
distributed throughout the background matrix (note that
these domains would not be observed by BAM, which has a
limit of resolution of 1−2 μm). The majority of these domains
vary between 1 and 3 nm above the background film, although
a second population of domains with heights from 7 to 11 nm
was also observed. Additionally, larger domains of the order of
2−3 μm in diameter were observed; these are likely the
domains that are observed by BAM. Upon closer examination,
it can be seen that these domains comprise clusters of domains
(ranging from 15 to 30 nm above the background film) often
surrounded by a thick corona (2−7 nm above the background
film). These extremely high domains are likely due to the
autophobic dewetting mentioned earlier.
Table 2. Film Thickness of LB-Deposited Monolayers on
Silicon Wafers at 5 mN m−1 Measured by Ellipsometry at the
Air−Solid Interface
surfactant
average film thickness (nm)
SiBA-10
SiBA-16
SiBA-69
0.5 0.1
0.6 0.1
0.6 0.1
thickness, 0.5−0.6 nm; i.e., the thickness is independent of the
chain length and correlates well with the thickness determined
by Mann and Langevin26 for a PDMS monolayer. This
thickness is comparable to the silicone chain dimensions and
confirms that the SiBA chains are lying flat at the air−water
interface and gradually rise above the surface as the area is
compressed, as might be expected for a polymer with a flexible
backbone. These observations are quite different from end-
functionalized polymers with a less flexible carbon backbone,
which were reported to be standing at an angle to the surface
and show a defined chain-length dependence.12
Ellipsometric measurements on PDMS films by Mann and
Langevin revealed an average thickness of 1.4 nm over the
phase transition plateau,26 which correlates well with the
ellipsometric isotherms (Figure 3) at the air−water interface,
showing a significant change in δΔ. Ellipsometry measurements
on the solid substrate are not reported here for higher pressures
given the inhomogeneity of the film. For these systems, AFM
Figure 5 shows isotherms obtained from repeated
compression−expansion cycles for SiBA-10 (similar results
were also obtained for SiBA-16; Supporting Information) and
SiBA-69. Recompression yielded the same isotherm but shifted
to smaller molecular areas. This shift was greater when the film
was compressed to pressures above the plateau. The isotherms
for other bis-end-functionalized silicones have been reported to
display little or no hysteresis for molecular weights above 1200
Figure 4. AFM images and corresponding height profiles of SiBA-10 transferred onto a solid substrate at 25 mN m−1: (A) film transferred on mica
and (B) film transferred on a silicon wafer. The width of both images is 5 μm.
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high that the boronic acid headgroups should be predominantly
negatively charged, which, in principle, could lead to charge
repulsion between the co-localized headgroups and/or
disruption of any potential network or dimerization occurring
between adjacent boronic acids at the air−water interface,
causing an increase in molecular area as was reported for
polymerizable surfactants with a boronic acid headgroup (these
experiments were undertaken over short reaction times, where
essentially no silicone depolymerization is anticipated).21
However, for both SiBA-10 and SIBA-69, there was no
difference in the isotherm at high molecular areas and low
pressures and the shift in the isotherm only becomes apparent
at the phase transition.
The observations described above suggest that silicone
boronic acids are rather different in their behavior at air−
water interfaces from either simple silicones or those bearing
hydrophilic functional groups at their termini. When the
isotherms for SiBA and H-PDMS are normalized for chain
length, as shown in Figure 7, the impact of the boronic acid
headgroup becomes much more apparent. It should be noted
that the specific area of the functional end group has not been
estimated or removed prior to normalization; therefore, the
areas per Si−O monomer include an additional area for these
headgroups. The isotherms and values for A0 (the critical area
defined by extrapolation of the straight line region of the initial
pressure increase to 0 mN m−1) of the polymeric forms and the
longest of the oligomers, H-PDMS-16, are comparable to the
A0 of 18.8 Å2 molecule−1 for methyl-terminated PDMS
reported by Kessel et al.,30 including the slight deviation and
broadening of the isotherm at low pressure for H-PDMS-16
that was also observed for PDMS with an average chain length
of 23 silicon atoms. The normalized isotherm of the shortest
oligomer, H-PDMS-10, shifts to significantly smaller areas per
molecule. The impact of the smaller terminal group (H versus
CH3) is much greater for a short chain, where the size
differences are amortized over a smaller number of repeat units
(monomers).
Figure 5. Isotherms for repeated compression−expansion cycles for
SiBA films: (A) SiBA-10 and (B) SiBA-69 on water at 23 °C, where
solid lines represent film compressions and dashed lines of the same
color represent the corresponding film expansion. Recompression
starts immediately after expansion without any equilibration time.
g mol−1 films,9 suggesting that multilayer formation is highly
reversible. Additionally, BAM measurements carried out during
the repeated compression−expansion cycles showed that the
domains, which only become visible at the end of the plateau
during the first compression, can still be seen when the film is
expanded. During this expansion, the domains lose brightness
abruptly; however, they can still be seen below the main phase
transition pressure. Upon further expansion into the gaseous
phase, the domains are no longer visible because either the
multilayer domains have all relaxed or the surface density and/
or size of remaining domains is sufficiently low to limit their
resolution in the field of view. Upon recompression, the
reappearance of these domains occurs even before the phase
transition (unlike the first film compression). The greater shifts
in the isotherm after multilayer formation for all chain lengths
and the BAM measurements suggest that the boronic acid end
groups may not be able to readily diffuse away from one
another, once in contact, as will be discussed in more detail
below.
To further probe the nature of the boronic acid headgroup
with the subphase, the pH was varied (Figure 6). The nominal
To examine the relative contributions to the molecular area
by different headgroups, the normalized isotherms for short-
chain oligomers of nine siloxane units with three different
terminal functionalities were directly compared. The relative
areas occupied by the molecules follow the order: terminal
hydride < aminopropyl < phenylboronic acid (Figure 7C). The
behavior of aminopropyl-terminated silicone H2N-PDMS-10
with its pronounced phase transition corresponds very well to
the literature.9 The normalized isotherm yields an A0 value of
17.5 Å2 monomer−1, which is just slightly less than that of
methyl-terminated PDMS, despite the three carbon linker
(rather than a methyl group), suggesting that the headgroup
orientation is such that the amine must be fully solvated and
extending into the subphase. Other end-functional silicones
exhibit similar or smaller areas.9
For the boronic acid-terminated silicones, only the largest
polymeric compound SiBA-69 converges to a monomer critical
area of 19 Å2 monomer−1. This is, in part, because of the lower
density of end-functional groups: the longer the chain, the
lower the impact of the terminal tethering groups on the
conformational changes of the chain and the smaller the
proportional contribution of the boronic acid to the molecular
area. For the shorter chain oligomeric surfactants SiBA-10 and
SiBA-16, A0 was found to be 28 and 30 Å2 monomer−1,
respectively. Phenylboronic acids are effectively much larger
Figure 6. SiBA isotherms on subphases with varying pH: (blue) pH
5.5 subphase and (red) pH 12 subphase. The absolute shift (in Å2·
molecule−1) to higher molecular areas (above the phase transition)
with increased pH is similar for both chain lengths but is more
prominent in the isotherms for SiBA-10 due to the scale
(solution) pKa of boronic acids is ∼8.9;27 the surface pKa may
be shifted to higher pKa.28 Furthermore, as a result of the
multiple equilibria that exist between the trigonal and
tetrahedral (charged) forms of boronic acid and their self-
complexed forms, the system may not exhibit a single sharp pKa
but the charged state may vary over a very broad range of pH
values.29 Subphase pH 12 was selected because it is sufficiently
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Figure 7. Isotherms normalized for the average chain length and reported as the area per Si−O monomer repeat unit (i.e., n − 1): (A) SiBA-10
(blue), SiBA-16 (red), and SiBA-69 (green) and (B) H-PDMS-10 (blue), H-PDMS-16 (red), and H-PDMS-69 (green). (C) Isotherms normalized
for chain length for silicones with different terminal functionalities: SiBA-10 (blue), H2N-PDMS-10 (red), and H-PDMS-10 (green). H2N-PDMS-10
= aminopropyl-terminated PDMS.
anchoring terminal groups than other functionalities, such as
aminoalkyl or carboxyl groups.
water interface.9 The smaller areas occupied by these
compounds arise because the polar groups penetrate into the
water, which allows for the silicone chains to come into closer
proximity (Figure 8C). These differences are magnified with
shorter silicones, for which the end groups represent a larger
fraction of the molecule. On the other hand, the boronic acid-
terminated silicones exhibit significant shifts to higher
molecular areas. This can only be accounted for by assuming
that the boronic acids do not extend into the subphase. Rather,
we propose that they form dimeric structures that lie on the
water surface (Figure 8D), a proposal supported by the actual
area taken up with boronic acids, which is in the same order of
magnitude and range as the estimated area from the molecular
dimensions for two phenylboronic acid end groups.
To account for these differences, it is necessary to investigate
the effect of the end groups separately from the behavior of the
silicone backbone. The role of the PDMS backbone is best seen
with high-molecular-weight compound SiBA-69. It exhibits
many of the characteristics of non-functional PDMS (panels A
and B of Figure 8), lying flat as a monolayer (0.5−0.6 nm thick)
To test this hypothesis, a monofunctional silicone boronate
SiBAM was prepared. The isotherms for SiBAM and its
mixtures with SiBA-10 are shown in Figure 9. The isotherm of
SiBAM displays the same shape as the other SiBAs, with a
transition starting at approximately 10 mN m−1, despite the
Figure 8. Schematic depictions at the air−water interface of (A)
simple silicones at high molecular areas (gas phase), (B) simple
silicones upon compression, (C) di-end-functionalized silicones with
vertical headgroup orientation observed for most end-functional
silicones (e.g., aminopropylsilicones) at all non-zero pressures and
for SiBAs at high pressures, and (D) boronic acid-functionalized
silicones with horizontal headgroup orientation at low to moderate
pressures.
when uncompressed and then increasing in thickness with
compression, forming di- to trilayers (1.1−1.4 nm thick). The
key distinguishing feature is the larger area that SiBAs exhibit
under compression (Figure 8D) when compared to non-
functional PDMS, including H-PDMS or end-functional
polymers, such as aminopropylsilicones (Figure 8C), which
actually take up smaller areas.
Figure 9. Isotherms of SiBA-10, SiBAM and mixtures of SiBA-10 and
SiBAM (1:1 and 1:2 molar ratios, as indicated in the legend). The
inset shows an expanded view of the SiBAM isotherm to better see the
phase transition.
Lenk et al. proposed that end-functional silicones, such as
aminopropyl or carboxyalkyl compounds are pinned to the
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short chain. The critical area is 43 Å2 molecule−1, which
matches well the cross-sectional area of ethylphenylboronic acid
if it were lying flat at the interface (approximately 40 Å2
molecule−1). A submerged headgroup would be expected to
generate a much smaller areal footprint (approximately 16 Å2
molecule−1). Mixtures of SiBAM with SiBA-10 were prepared
in both 1:1 mole and headgroup ratios (the latter being 2
SiBAM molecules/1 SiBA-10 molecule). Both generated
that involve the formation of robust hydrophobic films at
aqueous interfaces, including coating applications. Attempts to
exploit this behavior will be the subject of future reports.
CONCLUSION
■
Boronic acid-terminated silicones show behaviors associated
with strong tethering of the end functionalization to the
subphase. Despite hindering the layering process, tethering
does not affect the PDMS autophobic dewetting. However,
unlike most bis-end-functionalized PDMS compounds that take
up less surface area than PDMS under compression, the SiBAs
occupy more space. This is ascribed to planar orientation,
possibly induced by boronic acid−boronic acid complexation at
the air−water interface, which can only be overcome at much
higher pressures that force boronic acids into water.
isotherms displaying close to ideal mixing behavior (Amix
=
x
SiBA‑10ASiBAM + xSiBA‑10ASiBA‑10, where x is the mole fraction and
A is the area in the single-component isotherm) below the
phase transition with a slight positive deviation (indicating
repulsive interactions), which increases with increasing
amounts of SiBAM.
This orientation may be driven by headgroup complexation:
the ability of boronic acids to form dimeric and higher order
complexes is well-known and has been exploited as a tool for
organization of self-assembled materials.31 At the air−water
interface, the presence of the Lewis base water can enable this
flat dimeric structure, by forming tetracoordinate boron, which
is facilitated by relief of angle strain3 and which stabilizes it
from substitution compared to the tricoordinate analogue. This
configuration (Figure 10) could explain the lack of response to
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.lang-
muir.5b02143.
BAM images of SiBA-10, SiBA-69, and H-PDMS-69
(Figure S1) and isotherms for repeated compression−
expansion cycles for SiBA-16 (Figure S2) (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: mabrook@mcmaster.ca.
*E-mail: christine.dewolf@concordia.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Natural Sciences and Engineering
Research Council of Canada (NSERC) and 20/20: NSERC
Ophthalmic Materials Network for financial support of this
research. The authors also thank Heidi Muchall for the
determination of molecular dimensions of phenylboronic acids.
Figure 10. Schematic representation of proposed boronic acid−
boronic acid complexation leading to a planar headgroup orientation at
the air−water interface.
pH changes in the subphase. Increased basicity would simply
pin boron to the water interface more efficiently (Figure 8B).
Only at very high pressures can the boron dimer be coerced
into extending into the subphase with increased solvation. Such
complexation would also explain the hysteresis exhibited by all
three SiBAs (not observed for other highly polar end
functionalizations): sufficiently strong headgroup interactions
could inhibit the complete respreading, an effect that is
compounded with each cycle as more headgroups come into
contact.
The phenylboronic acids are used to confer chemical sensing
and electrical conductivity to thin film coatings20−22,32 and are
ideal self-assembly building blocks as a result of their propensity
for self-complexation, electron deficiency, and coplanar
structure. We have demonstrated that these self-organization
properties extend to the air−water interface, where headgroup
complexation induces a planar orientation at the interface,
permitting them to take up more area than either non- or end-
functionalized silicones, which can only be disrupted through
reorientation at high surface pressures in the absence of agents
that compete for boronic acid. This different behavior when
compared to other silicone surfactants, in terms of interactions
between both the hydrophiles themselves and hydrophiles with
water, provides new opportunities to target specific applications
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