nmr studies of pigment-sunscreen interactions - a. a. parker consulting - silanes … ·...
TRANSCRIPT
A. A. Parker Consulting, LLC
Portions of this presentation were given at The University of Akron NMR Users Meeting
(Akron, OH November 2009); and at the 6th International Symposium on Silanes and Other
Coupling Agents (Cincinnati, OH June 2007).
NMR Studies of Pigment Sunscreen Interactions
©2019 A. A. Parker – Page 1; www.aaparkerconsulting.com
A new type of hydrophobic surface treatment for sunscreen pigments produces lipophilic
dispersion properties, and simultaneously mitigates unwanted chromophore side-
reactions between organic UV absorbers and Fe compounds. This surprising benefit can
facilitate the formulation of new families of sunscreen products that contain both iron-
based pigments and organic UV absorbers (U.S. Patent 7,387,795).
n-octylphosphonic acid (NOPA)
vs.
n-octylsilane (NOS)
TONY PARKER
A. A. Parker Consulting LLC
Newtown, PA
BACKGROUND CONCEPTS
Hydrophobic Coatings for Inorganic Powders
Hydrophobic / Lipophilic inorganic powders are desirable for use in certain
cosmetics formulations.
Powders are surface treated to facilitate the selective dispersion of
particles in the oil-phase of either oil-in-water or water-in-oil based
systems (i.e. sunscreen formulations that contain particulates of TiO2
and/or ZnO).
Alkyltrialkoxysilanes such as n-octyltriethoxysilane (NOS) are used
extensively in these types of applications. Alkylphosphonic acids such as
n-octylphosphonic acid (NOPA) are possible alternatives (U. S. Patent
7,387,795).
Si
OHHO
HOP
OOH
OH
NMR Studies of Pigment Sunscreen Interactions
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Alkyltrialkoxysilanes vs. Alkylphosphonic Acids
Further Background Reading:
Parker, A. A., et al., “Comparative Studies of Hydrophobic Surface Treatments
for TiO2: n-Octylphosphonic Acid and n-Octyltriethoxy silane,” in Silanes and
Other Coupling Agents, Volume 4, K.L. Mittal, Editor, VSP BV, The Netherlands,
2007 pp. 399-409.
Parker, A. A., Wagler, T., “Solid State NMR Studies of a Hydrophobic Surface
Treatment for TiO2: n-Octylphosphonic Acid,” in Silanes and Other Coupling
Agents, Volume 5, K.L. Mittal (Ed.), VSP/Brill, Leiden, The Netherlands, 2009
pp. 323-331.
"Cosmetic Compositions Containing Organophosphonic Acid Coated
Particulates and Methods of Producing the Same," U. S. Patent 7,387,795, June
17, 2008.
NMR Studies of Pigment Sunscreen Interactions
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This presentation will focus on three comparative phenomena with each being
related to the end-use application (i.e., sunscreen formulation & performance):
• Yellow iron oxide & avobenzone degradation
• TiO2 dispersions in water & in oil
• Self-assembled monolayer (SAM) formation
What are the similarities and differences between NOS and NOPA?
NMR Studies of Pigment Sunscreen Interactions
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Hydrophobic surface
treatments enable
these surface-treated
iron oxide pigments to
float on water.
NOS vs. NOPA
Yellow iron oxide &
avobenzone degradation
NMR Studies of Pigment Sunscreen Interactions
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Organic UV absorbers such as avobenzone are often used in sunscreens.
When certain inorganic pigments are added to these types of sunscreen formulations,
unwanted side reactions can occur. For example, yellow iron oxide (Fe2O3 hydrate)
will cause the carrier liquid (caprylic triglyceride) to turn orange in the presence of
avobenzone.
Heat / time
Inorganic Pigments with Sunscreens – Side Reactions
O
O
O
O
O
O
Fe
3
NMR Studies of Pigment Sunscreen Interactions
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Discoloration of avobenzone will occur in the presence of treated and “as-
received” yellow iron oxide pigment.
These images depict discoloration after heat aging for 50 days at 55oC.
The NOPA treated powder mitigates this undesirable side reaction.
NOPA treatmentNOS treatmentNo treatment2.5% Avobenzone
NMR Studies of Pigment Sunscreen Interactions
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No treatmentNOS treatmentNOPA treatment
These images depict discoloration of yellow iron oxide pigment after heat aging
for 60 days at 55oC, followed by continued ambient aging for 4 months.
The longer-term visual benefit afforded by NOPA can be corroborated
analytically via NMR studies of the caprylic triglyceride supernatants.
NMR Studies of Pigment Sunscreen Interactions
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ChemNMR 13C Estimation
124.9
155.7
124.9
128.4
133.6
128.4
34.2
31.3
31.3
31.3
190.4
93.5
190.4129.0
129.8
114.2
165.0
114.2
129.8
55.8
O
O O
124.9
150.5
124.9
127.2
132.1
127.2
34.2
31.3
31.3
31.3
183.593.0
186.7
130.2
130.9
114.8
166.4
114.8130.9
55.8O
O O
H
Water
Tautomerisation of Avobenzone and Anticipated Effects on 13C Chemical Shifts
Symmetric carbonyls
Asymmetric carbonyls
NMR Studies of Pigment Sunscreen Interactions
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Chemical shift
calculations predict that
Fe(III) chelation should
produce a moderate
upfield chemical shift for
one of the 13C carbonyl
carbons, and a strong
upfield shift for the
second carbonyl
carbon.
124.9
150.5
124.9
127.2127.2
127.2
34.2
31.3
31.3
31.3
180.3
91.2
52
133.9129.1
114.5
159.5114.5
129.1
55.8
124.9
150.5
124.9
127.2127.2
127.2
34.2
31.3
31.3
31.3
180.3
91.2
52
133.9129.1
114.5
159.5114.5
129.1
55.8
O
O
O
Fe
O
O
O
NMR Studies of Pigment Sunscreen Interactions
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ppm20406080100120140160180
13C sample 7-2solvent: CDCl3d1=20s dm=nny nt=256400 MHZ 5mmSW probe 01/08/2007
Archive directory: /export/exec/vnmrsys/data Sample directory:
Pulse Sequence: s2pul
13C solution NMR spectrum of caprylic triglyceride containing 2.5% avobenzone (w/w)
124.9
150.5
124.9
127.2
132.1
127.2
34.2
31.3
31.3
31.3
183.593.0
186.7
130.2
130.9
114.8
166.4
114.8130.9
55.8O
O O
H
enone
NMR Studies of Pigment Sunscreen Interactions
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ppm145150155160165170175180185190195
13C sample 7-2solvent: CDCl3d1=20s dm=nny nt=256400 MHZ 5mmSW probe 01/08/2007
Archive directory: /export/exec/vnmrsys/data Sample directory:
Pulse Sequence: s2pul
185.5
44
183.8
60
172.6
7917
2.965
172.3
16
163.0
79
155.5
08
13C solution NMR spectrum of caprylic triglyceride containing 2.5% avobenzone (w/w)
(expansion of carbonyl region)
NMR Studies of Pigment Sunscreen Interactions
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avobenzone
tautomer
Caprylic
triglyceride
ppm145150155160165170175180185190195
13C sample 7-3solvent: CDCl3d1=20s dm=nny nt=608400 MHZ 5mmSW probe 01/09/2007
Archive directory: /export/exec/vnmrsys/data Sample directory:
Pulse Sequence: s2pul
185.6
12
183.9
50
172.8
3717
3.341
172.4
61
163.1
17
155.6
02
13C solution NMR spectrum of caprylic triglyceride containing 2.5%
avobenzone (w/w) after exposure to yellow iron oxide
NMR Studies of Pigment Sunscreen Interactions
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In the absence of
treatment, we see
the gradual
appearance of new
peaks, upfield from
the original carbonyl
carbons.
ppm145150155160165170175180185190195
13C Sample 7-5solvent: CDCl3d1=20s dm=nny nt=608400 MHZ 5mmSW probe 01/09/2007
Archive directory: /export/exec/vnmrsys/data Sample directory:
Pulse Sequence: s2pul
185.5
99
183.9
33
174.9
09
172.8
0317
3.303
172.4
31
163.1
13
155.5
85
13C solution NMR spectrum of caprylic triglyceride containing 2.5%
avobenzone (w/w) after exposure to NOS-treated yellow iron oxide
NMR Studies of Pigment Sunscreen Interactions
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Even with NOS
treatment, we still see
the pronounced
appearance of new
peaks, upfield from the
original carbonyl
carbons.
ppm145150155160165170175180185190195
13C Sample 7-6solvent: CDCl3d1=20s dm=nny nt=608400 MHZ 5mmSW probe 01/09/2007
Archive directory: /export/exec/vnmrsys/data Sample directory:
Pulse Sequence: s2pul
185.5
95
183.8
90
172.8
0717
2.436
163.1
09
13C solution NMR spectrum of caprylic triglyceride containing 2.5%
avobenzone (w/w) after exposure to NOPA-treated yellow iron oxide
NMR Studies of Pigment Sunscreen Interactions
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The NOPA surface
treatment appears to
mitigate the formation
of the upfield carbonyl
species.This finding corroborates
with visual observations
(i.e., less discoloration of
dispersions containing
the NOPA-treated
pigment).
2.5% avobenzone
2.5% avobenzone after
exposure to yellow oxide
2.5% avobenzone after exposure
to NOS-treated yellow oxide
2.5% avobenzone after exposure
to NOPA-treated yellow oxide
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After heat aging for 60 days at 55oC, followed by ambient aging for 4 months
NOS vs. NOPA
TiO2 dispersions in water & in oil
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Relative Hydrophobicity
0.05 g aliquots of each powder were individually weighed into separate glass jars
together with 10 g of distilled water. The mixtures were then vigorously shaken by hand,
and the closed containers were set on a horizontal surface for visual observation.
No treatment 2% octanoic 2% NOPA 2% NOS
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NOS and NOPA surface treatments impart hydrophobicity – an attribute desirable for
creating oil-phase dispersions for water-in-oil or oil-in-water sunscreen formulations.
Relative Hydrophobicity vs. Surface Concentration – NOPA
2% NOPANo treatment 0.5% NOPA 1% NOPA 4% NOPA
surface
bilayer
formation
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• Hydrophobicity is observed to improve up to a point, but a reversal occurs when the NOPA
treatment exceeds 2% by weight dry powder – i.e., the powder becomes hydrophilic.
• NMR results suggest that this phenomenon is related to the presence of excess NOPA
(i.e., weakly bonded NOPA that is not assembled within the chemisorbed self-assembled
monolayer or “SAM”). Excess or “free” NOPA would be available to phase-invert, and to
form a second surface layer with charged groups that extend into the water phase.
• These weakly bonded, “free” species are observed to appear when the total NOPA
concentration exceeds the critical adsorption concentration (CAC) for SAM formation.
Relative Lipophilicity via Sedimentation(14.28% solids in decamethylcyclopentasiloxane carrier, DC245 silicone from Dow Corning)
The (CAC) can
be interpolated
between these
two
concentrations
Adsorbate Concentration (mmoles/m2)
Settling
Density
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• When high density powders are dispersed in lower density liquids, they tend to settle under
static conditions. Less agglomeration (i.e., better dispersion) is accompanied by slower
sedimentation rates and by higher sediment densities.
• The critical adsorption concentration (CAC) can be approximated by means of these types
of sedimentation experiments.
• Solid state NMR can be used to study this phenomenon at a deeper level.
2% NOPANo treatment 0.5% NOPA 1% NOPA 4% NOPA
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 5 10 15 20 25
NOPA
NOS
Se
dim
en
t D
en
sit
y (
g/c
m3)
Surface Concentration
(x 10-6
moles/m2)
Relative Lipophilicity via Sedimentation
NOS and NOPA appear to impart similar degrees of lipophilicity up to their respective
critical surface adsorption concentrations, but beyond these concentrations, the two
surface treatments behave differently.
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Sedimentation density (g/cm3) of TiO2 powder dispersed in DC245 silicone at 14.28%
solids by wt. as a function of NOPA and NOS surface concentrations (mole/m2)
The CAC’s for NOS and
NOPA occur at
approximately 6 to 10
mmoles/m2, a range
which is typical for
many chemically
bonded surface-
treatments on many
types of inorganic
powder surfaces.
NOPA
Self-assembled monolayer (SAM) formation
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Solid state 13C CP/MAS NMR
spectra
Neat NOPA, NOPA-treated TiO2
powders, and neat TiO2.
The methyl carbon resonance at
12.7 ppm is indicative of monolayer
formation [W. Gao, L. Dickinson, C.
Grozinger, F. G. Morin, L. Revin, Langmuir,
13, 115-118 (1997)].
*The critical adsorption concentration
from sedimentation corroborates with
a change in the composition of
surface structures.
A different chemical environment
begins to appear at a NOPA
concentration of greater than 1% and
less than 2% w/w powder (i.e., when
the CAC is exceeded for SAM
formation).
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*
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Solid state 31P MAS NMR spectra
180 second relaxation delay time
Neat NOPA, NOPA-treated TiO2 powders,
and neat TiO2 (from top to bottom).
All spectra are scaled to the same height
(scaling factors are listed on the right).
*The critical adsorption concentration
from sedimentation corroborates with a
change in the composition of surface-
bound phosphorous structures.
*
28.8 ppm - surface-bound monodentate
titaniumphosphonate ester1 is the
predominant species when NOPA is
present at near-monolayer
concentrations (less than 2% by weight
in this case).
1(W. Gao, L. Dickinson, C. Grozinger, F. G. Morin,
L. Revin, Langmuir, 12, 6429-6435 (1996))
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Solid state 31P MAS NMR spectra
10 second relaxation delay time
Shorter relaxation delay times tend to
suppress signals that have long T1 spin-
lattice relaxation times (i.e., relatively mobile
nuclei in this case).
The peak near 38 ppm is characteristic of a
neat NOPA component with relatively high
molecular mobility.
The peak near 33 ppm is also characteristic
of a neat NOPA component, but it is
relatively rigid.
The surface-bound phosphonate esters are
all relatively rigid (~ 31, ~ 28, and ~ 8 ppm).
Note that the as-received neat TiO2 also
appears to contain a surface-bound
phosphonate salt (unrelated to NOPA).
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Solid State 31P T1’s vs. NOPA Concentration
0
20
40
60
80
100
120
NOPA4% NOPA2% NOPA1% NOPA0.5% NOPA
37.6 ppm33.2 ppm28.4 ppm8.5 ppm
Neat NOPA
“rigid” crystalline
Neat NOPA
“mobile”
amorphous
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31P
NM
R T
1(s
ec)
What happens when we add water?
Aside from oils and organic UV absorbers like avobenzone,
water can be an integral component of a sunscreen formula.
It is also present in the end-use environment (e.g., at the
beach on a sunny day).
“Solid State” NMR techniques can be used to
study dispersions of solids in liquids.
In this example, we have used solid state NMR to study
50/50 (w/w) slurries of treated and neat TiO2 powders in D2O.
Recall that when NOPA is used at a concentration of 4% w/w
dry powder (i.e., above the CAC), the material becomes
hydrophilic. Why?
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Solid-state 31P MAS NMR spectra of NOPA
Experimental NMR conditions can allow for the “solid state” analysis of high-solids
slurries in deuterated water.
Signal averaging with long recycle delay times allows for the detection of mobile
(desorbed) nuclei, and rigid (chemisorbed) nuclei.
50/50 w/w slurry mixture neat NOPA & D2O
neat NOPA (dry)
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Owing to differences in mobility in the presence of water, the original NOPA peaks (~33
& ~38 ppm) appear to coalesce into a single peak with a small shoulder.
Neat NOPA/D2O
4% NOPA on TiO2/D2O
2% NOPA on TiO2/D2O
1% NOPA on TiO2/D2O
0.5% NOPA on TiO2/D2O
TiO2/D2O
• 180 second relaxation delay (for
comparison to the solid-state spectra in
slide 24).
• The rigid surface-bound ester (~28
ppm SAM) and the weakly bonded
ester (~31 ppm) remain chemisorbed at
all concentrations.
• The non-bonded NOPA species (~38
& 33 ppm) have begun to desorb.
• Some of the NOPA species may be
converted into different types of salts in
the presence of TiO2, (e.g., the
phosphonate near zero ppm, similar to
the salt that was originally present on
the as-received solid TiO2).
• The 1% to 2% treatment levels appear
to be the most stable, with complete
SAM coverage remaining intact.
• Higher concentrations yield the more
soluble species, i.e., those that can
assemble into inverted bi-layers,
rendering the powder as “hydrophilic.”
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NOPA NOS
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Recall that the NOPA treatment mitigates the unwanted
beta-diketone side-reaction with Fe(III) to a greater
degree than does the NOS surface treatment.
Based on the data and observations, there are several possible reasons
(hypotheses) for these differences, one of which relates to diffusion.
• In the absence of a surface treatment, surface iron species become solvated
through chelation with the conjugate base of the avobenzone diketone tautomer
(a weak acid, pKa = 9 to 10). This leads to the formation of a strong visible
chromophore in solution.
• NOS polymerizes to form a 3D polysiloxane network around the Fe oxide
particles. By contrast, NOPA does not polymerize, but instead, it bonds to the
surface to form a 2D SAM.
• Both types of protective coatings can lead to physical diffusion barriers that slow
down the solvation/chelation process.
• However, the better protection afforded by NOPA cannot be explained by its 2D
SAM diffusion barrier alone, particularly since a 3D polymer network (as formed
by NOS) would arguably provide a better physical diffusion barrier than a 2D self-
assembled monolayer. Instead, it is likely that other mechanisms are involved.
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An alternative hypothesis relates to competitive acid/base reactions.
• When the powders are pre-treated with either NOS or NOPA, a large fraction of the
surface bound molecules will exist in their conjugate base forms – i.e., bonded with Fe
species. This diminishes the availability of surface Fe for bonding with the di-ketone.
• Relative acidity: phosphonic acids (pKa ~ 2.6 and 7.9) > silanol (pKa ~ 4-5) > beta-
diketone (pKa > 9).
• Relative basicity of conjugate base forms: beta-diketone > silanol > phosphonic acid.
• As the weakest acid in the group, the beta-diketone would yield the strongest conjugate
base, giving it perhaps the greatest potential to chelate with unprotected Fe surface
moieties.
• However, in order for the chromophore to form, the surface bonded protective groups
would first have to become desorbed.
• In the presence of a proton source (e.g., even in the presence of a weak acid like a beta-
diketone), some of the surface bound protective groups will eventually become
desorbed; but in order to do so, they would first have to exchange Fe for one or more
protons (e.g., a proton from the weaker beta-diketone acid).
• Being the weaker base and the stronger acid, the surface-bound NOPA phosphonate
ester might be less likely (i.e., slower) to become protonated than comparative surface-
bound NOS species.
• These types of competitive equilibria reactions can be further complicated by the fact
that NOPA and NOS have the potential to bond in tridentate fashion with the Fe surface.
• Clearly, continued research will be needed to uncover the most likely mechanistic
reasons for the differences between the NOPA and NOS surface treatments.
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Possible Future Studies
• 13C NMR of NOS treated powders to study the self-
assembly of its alkyl chains for comparison to NOPA
• “Hydrophobicity” from the alkyl chain group’s point of
view & the effect of water on chain dynamics (via 13C
NMR)
• The influence of acid-types and acid titration on the
avobenzone-Fe interaction
• Hydrolysis of NOS together with NOPA, and co-
precipitation of these species on pigment surfaces
NMR Studies of Pigment Sunscreen Interactions
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Acknowledgments:
Dr. Todd Wagler
The University of Akron, Akron, OH
Dr. Peter Rinaldi
The University of Akron, Akron, OH
Jane Hollenberg
JCH Consulting, Red Hook, NY
Dr. Angela Parker Kaiser (artwork)
Companyia Lake Angela
NMR Studies of Pigment Sunscreen Interactions
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A. A. Parker Consulting LLC specializes in polymer science, polymer
processing, polymer adhesion, surface chemistry, silanes, and product
development in multiple fields – spanning from wood composites & cosmetics
to medical devices and surface treated musical instrument strings.
ORGANOSILANE TREATED STRINGS
(U. S. Patents 7,476,791 & 6,348,646)
2009, Starlite MacPark (ASCAP)
www.tonyparkermusic.com
A. A. Parker Consulting, LLC
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A. A. Parker Consulting, LLC
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Tony Parker is an independent consultant and musician with more than fifty technical publications,
including 30+ patents, 50+ songs, and a Ph.D. in Chemistry to his credit.