Quantitative assessment of alkyl chain branching in alcohol-based surfactants by nuclear magnetic resonance

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73
Quantitative Assessment of Alkyl Chain Branching
in Alcohol-Based Surfactants
by Nuclear Magnetic Resonance
J.P.M. van Duynhoven*, A. Leika, and P.C. van der Hoeven
Unilever Research and Development, 3130 AC Vlaardingen, The Netherlands
ABSTRACT: Surfactants with branched hydrophobes have
gained considerable interest, since these can be used in formulations for laundry cleaning at a wide range of conditions. The
claims range from improved dissolution rate to hardness tolerance and stain removing efficacy. In contrast to the historically
known heavily branched surfactants, novel branched surfactants
are less compromised by increased biodegradability. These
properties find their basis in the structural characteristics of the
hydrophobe, such as number, position, and type of alkyl chain
branches. Our current understanding of structure–property relations, however, is hampered by the lack of generic methodology needed to obtain structural data on hydrophobe branching. A nuclear magnetic resonance (NMR) approach was
developed by which we could obtain a comprehensive set of
quantitative hydrophobe branching parameters in alcoholbased surfactants. The 13C and 1H NMR spin systems of hydrophobe branched species were assigned by means of twodimensional NMR techniques. These assignments allowed the
quantitative assessment of these branched species by straightforward signal integration in the 1H and 13C NMR spectra. The
quantified NMR data can be used to understand product performance and the biodegradation of surfactants with branched
hydrophobes.
Paper no. S1451 in JSD 8, 73–82 (January 2005).
KEY WORDS: Alcohol, alkyl chain, branching, feedstocks, hydrophobe branching, mid-chain branching, NMR, nuclear magnetic resonance spectroscopy.
Since the switch by the industry from the poorly biodegradable, tetrapropylene-derived, and heavily-branched alkylbenzene sulfonates to the more quickly biodegradable linear
alkylbenzene sulfonates (LAS) in the early 1960s, the surfac*To whom correspondence should be addressed at Unilever Research
and Development, P.O. Box 114, 3130 AT Vlaardingen, The Netherlands. E-mail: [email protected]
Abbreviations: 1-D, one-dimensional; 2-D, two-dimensional; AE, alcohol ethoxylate; BI, branching index; BI(Et), ethyl branching index; COxo, conventional Oxo process (see Ref. 4); ESMS, electrospray mass
spectrometry; FID, flame ionization detection; GC, gas chromatography; HB, highly branched; HMBC, heteronuclear multiple-bond correlation; HSQC, heteronuclear single-quantum coherence; LAS, linear
alkylbenzene sulfonates; LC, liquid chromatography; MB, mild midchain branched; M-Oxo, modified Oxo process (see Ref. 4); MS, mass
spectrometry; NMR, nuclear magnetic resonance; Oxo, RMB, random
moderate branching; TCAI, trichloroacetyl isocyanate; TOCSY, total
correlation spectroscopy.
COPYRIGHT © 2005 BY AOCS PRESS
tants used for laundry applications have differed mainly only
in the type of hydrophilic group. Thus, the range of surfactant families available to the product developer has traditionally been limited to those based on sulfonates (LAS), carboxylates (soaps), sulfates, ethoxylates, or ethoxysulfates (1).
In the last 10 yr, the range of hydrophiles has been extended
even further, and new surfactants have come onto the market (2,3). With the exception of alkylbenzene-based materials, most surfactants are derived from olefins and alcoholbased materials that originate from petrochemical or oleochemical feedstocks. Until recently, there was limited room
to maneuver with the hydrophobe structure. Alcohol feedstocks with branching at the β-position of the alkyl chain
could be produced efficiently by the Oxo process, and they
provided a good compromise between cleaning performance
and biodegradability (5). In the conventional and modified
Oxo processes (C-Oxo and M-Oxo, respectively), the degree
of β-branching typically varies between 55 and 20%, respectively. Also, materials were available with high branching
(HB) or random moderate branching (RMB) at the midchain positions of the alkyl chains, but owing to their unfavorable biodegradability, they were applicable only in niche
applications. A rough overview of the current commercially
available alcohol feedstocks is presented in Table 1, together
with a classification according to their branching characteristics (6,7). In recent years, surfactants with hydrophobes having a limited number of methyl groups alongside the linear
chain have been introduced, also denoted as mild, mid-chain
branching (MB). It has been reported that products containing these MB surfactants have a rate of biodegradation hardly
distinguishable from that of the purely linear ones (3,8). The
benefits claimed from these MB surfactants, when applied in
laundry applications, are an improved dissolution rate at low
temperatures, improved hardness tolerance, and more efficacious stain removal.
These industrial developments make it necessary to have
methods available that define the structures of the surfactant
hydrophobes. This is an essential tool for elucidating and predicting the effect of hydrophobe branching on surfactant
performance and environmental safety. Nuclear magnetic
resonance (NMR) already has a track record of elucidating
surfactant structures (9) and is also particularly suited to detecting, identifying, and quantifying the alkyl chain branches.
JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 1 (JANUARY 2005)
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J.P.M. VAN DUYNHOVEN ET AL.
TABLE 1
Overview of Commercially Available Alcohol-Based Feedstocksa
Product
(examples)
Lutensol AN/AT
Novel 1012-6.2
Lialet 123-7
Lutensol AO
Neodol 1-7
Safol 23
Neodol 67
Lutensol TO5
Exxal 13
Producersb
(examples)
BASF
Sasol
Sasol
BASF
Shell
Sasol
Shell
BASF
Exxon
Process technology
Branching type
Oleochemical
NO
≥99% linear
Conventional Oxo
C-Oxo
ca. 55% β-branched
Modified Oxo
Fischer–Tropsch
M-Oxo
MB
ca. 20% β-branched
Mildly mid-chain branched
Trimer butene + Oxo
Tetramer propene + Oxo
RMB
HB
Random moderately branched + Oxo
Highly branched + Oxo
a
Products in italics were assessed quantitatively in this work. The Oxo processes are outlined in the text.
Sasol (Houston, TX); BASF (Florham Park, NJ); Exxon (Houston, TX); Shell (Houston, TX).
b
The most detailed structural description of branching in surfactants and feedstocks is available for Oxo-alcohol feedstocks
(5,10). To date, only a few structural descriptors for the novel
MB surfactants are known. One can roughly estimate the
number of methyl moieties per alkyl chain by simple integration of a region in the 1H or 13C NMR spectrum. So far, no
general NMR approach has been described that can assess
hydrophobe branching in a detailed (site-specific) and quantitative manner. However, it is well recognized that such detailed descriptions are required to make rational predictions
and explanations of the properties (11–13) and environmental behavior (8,14,15) of surfactants with branched hydrophobes. In this study an NMR approach is described that
provides more structural detail than the methods currently
available. The NMR method is demonstrated on six material
examples from the previously described hydrophobe branching classes.
EXPERIMENTAL PROCEDURES
Samples. The alcohol feedstocks and ethoxylated materials described in this work are all commercial products; the suppliers are listed in Table 1.
NMR sample preparation. The NMR samples were prepared
by adding 10% of deuterated methanol (CD3OD, 99.8%
deuteration) to the products. For the quantitative assessment
of alcohol ethoxylates (AE), recourse should be taken to derivatization (16). These samples were prepared by dissolving
a few milligrams of the product in deuterated chloroform
(99.5% deuteration; Merck, Darmstadt, Germany) and subsequently adding a few drops of trichloroacetyl isocyanate
(TCAI) (Aldrich, Milwaukee, WI).
Gas chromatography (GC) sample preparation of alcohol-derived
materials. Alkyl bromides are prepared by heating alcohol
samples in an excess of hydrogen bromide in glacial acetic
acid in a sealed tube. (Warning: The procedure can be dangerous! Glassware can explode during the heating cycle. Take
proper precautions!) For sample preparation, approximately
5 to 10 µL of alcohol sample is dispensed into a 5-mL glass
ampoule. Approximately 200 µL of 50% HBr in glacial acetic
acid is added to the ampoule. The ampoule is closed and
placed in a freezer. When frozen, the sample is connected to
a vacuum line and evacuated to 600 mbar. Finally, a flame is
applied to the stem of the ampoule to close it. The sealed ampoule is then placed in an oven and heated to 115°C for 3 h.
After the reaction, the ampoule is broken open and 200 µL
of water is added. The sample is then partially neutralized by
adding a saturated sodium hydrogen carbonate solution.
Solid sodium hydrogen carbonate is then added to complete
the neutralization. The aqueous sample is extracted using
200 µL of hexane, an aliquot of which is injected.
NMR acquisition. All NMR experiments were performed at
303 K. One-dimensional (1-D) 1H NMR measurements, as
well as two-dimensional (2-D) total correlation spectroscopy
(TOCSY) and heteronuclear single-quantum coherence
(HSQC) experiments (17,18) generally were performed with
a Varian Unity400 NMR spectrometer, operating at 400 MHz
for 1H. 13C NMR measurements were taken with a Bruker
DMX600 spectrometer, running at 150 MHz. Samples for the
13
C NMR measurements were placed in 10-mm tubes, and a
30-s relaxation delay was used (to prevent signal saturation).
Liquid chromatography (LC)-NMR analyses were performed
using a 120-µL LC-NMR probe head, interfaced to the LC by
means of a Bruker Peak Sampling Unit, which was under the
control of an NT Workstation (Hystar software; Bruker
BioSpin, Rheinstetten, Germany). An Eclipse XDB-C8 RP column (5 µm × 250 mm × 4.6 mm; Agilent Technologies, Palo
Alto, CA) was used, at a temperature of 35°C. The injection
volume was 20 µL. The sample concentration was approximately 100 mg/mL, and an isocratic (A: 20% D2O; B: 80%
MeCN) gradient was used.
NMR processing. The NMR data sets were processed with
Bruker XWINNMR software (Bruker BioSpin), operating on
Unix workstations. Linear forward prediction was applied to
the t1 dimension of the 2-D NMR data sets to increase the
spectral resolution. Time-domain data were processed by the
XWINNMR package. The processed NMR data were compressed and subsequently stored by means of a Bruker AMIX
software package (Bruker BioSpin), also running on a Unix
workstation. NMR spectral predictions were performed using
the Sadtler Searchmaster & Predictor (Informatics/Sadtler
Group, Philadelphia, PA) and the ACDLabs C13 Predictor
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HYDROPHOBE BRANCHING IN ALCOHOL-BASED SURFACTANTS BY NMR
′
FIG. 1. Overview of branched structures (i, B) in alcohol-based feedstocks or surfactants. m,
mid-chain.
(Advanced Chemistry Development, Inc., Toronto, Ontario,
Canada) packages.
GC–flame ionization detection (FID)/GC–mass spectrometry (MS)
acquisition and processing. GC–FID experiments were carried
out on a Chrompack CP 9000 gas chromatograph equipped
with a flame-ionization detector, split injection, and a
CPSil5CB*MS low-bleed column (50 m × 0.25 mm internal
diameter, film thickness = 0.4 µm; Varian, Palo Alto, CA). The
carrier gas was helium, the amount injected was 1.0 µL, and
the gas pressure was 240 kPa. The oven was programmed at
60°C (2 min) at 5°C/min to 250°C (5 min). GC–MS data
were acquired on both the MD800 and HP5973 instruments.
GC–FID data were processed using Turbochrom Client/
Server software (PerkinElmer Life and Analytical Sciences,
Inc., Boston, MA). GC–MS data were processed by AMDIS
and NIST98 software (National Institute of Standards and
Technology, Gaithersburg, MD).
Assignment and nomenclature of branched species. The known
manufacturing routes for feedstocks with branched hydrophobes produce a multiplicity of molecular structures, as
presented in Figure 1. This figure also explains the nomenclature for branched structures used throughout this study.
The branched structures can be characterized by branch
length (B, i.e., the carbon number of the branch) and position (i). In our study we were unable to identify branches
longer than propyl chains. Branch positions were referred to
relative to either the “first” oxygen (α, β, or γ), or the endmethyl moiety (ω1 or ω2), and all remaining positions were
JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 1 (JANUARY 2005)
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J.P.M. VAN DUYNHOVEN ET AL.
SCHEME 1. Nomenclature of protons and branching positions in alcohol ethoxylates. m, mid-chain; c, number of ethoxylates; n, alkyl
chain length.
denoted mid-chain (m) (see also Scheme 1). The positional
numbering in Figure 1 does not fully comply with IUPAC
conventions, but instead conforms to the designations used
in the practice.
RESULTS AND DISCUSSION
NMR spectral assignments. The introduction of branched structures into AE led to marked changes in their 1H and 13C
NMR spectra. This is illustrated in Figure 2. The resonance
assignments in these spectra were not trivial. In the literature
one can find scattered reports of linear and a limited number of branched structures of alcohol-based materials
(8,19–21). 2-D NMR experiments were performed to obtain
more complete signal assignments of materials with mid-
chain hydrophobe branching. Even with the spectral resolution of these 2-D NMR spectra, it was difficult to distinguish
ethoxylated from nonethoxylated species, i.e., the free alcohols. Derivatization with TCAI was undertaken as a recourse,
resulting in a deshielding moiety added at the hydroxyl end
group of both the ethoxylates and free alcohols. This led to
significant chemical shifts in the alkyl chain of the free alcohols, hence enabling their unambiguous spectral assignment.
The derivatization had to be performed in chloroform. A
major part of the NMR signals of the branched moieties
could be assigned using the through-bond connectivities in
2-D TOCSY, HSQC, and heteronuclear multiple-bond correlation (HMBC) spectra. An example in Figure 3 shows regions from an HSQC spectrum of an AE, annotated with assignments of branched moieties. Such assignment exercises
were performed for both the derivatized and the nonderivatized species.
Quantification of hydrophobe branching parameters. Although
signal overlap was reduced in the aforementioned 2-D spectra, these experiments did not lend themselves to quantification. Therefore, 1-D spectra were used, and quantitative
data were obtained by integrating the assigned and
nonoverlapping signals. The signal regions suitable for
quantification of the structural features are summarized in
FIG. 2. Typical examples of 1H (left) and 13C (right) nuclear magnetic resonance (NMR) spectra of branched alcohol ethoxylates. From top to bottom are presented spectra of typical
conventional Oxo (C-Oxo), mild mid-chain branched (MB), and highly branched (HB)
alcohol ethoxylates. The Oxo processes are outlined in the text.
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HYDROPHOBE BRANCHING IN ALCOHOL-BASED SURFACTANTS BY NMR
FIG. 3. Selected regions of the heteronuclear single-quantum coherence (HSQC) spectra of
trichloroacetyl isocyanate (TCAI)-derivatized alcohol ethoxylates. The two-dimensional spectral
regions in (A) and (B) were taken from HSQC spectra recorded for C-Oxo and MB alcohol
ethoxylates, respectively. The HSQC (region) presented in (C) corresponds to a C-Oxo alcohol
ethoxylate. Assignments of several branched species have been indicated (see also Scheme
1). EO, ethylene oxide; for other abbreviations see Figures 1 and 2.
Table 2. These regions are also indicated in the 1H and 13C
NMR spectra of typical branched materials in Figures 4 and
5, respectively. Note that in the 13C NMR spectrum, structural features other than branched moieties, such as ethoxylate oligomers, also can be observed. Their signals also enabled assessment of features such as the distribution of
short ethoxylate oligomers (20). The signals in Table 2
could be distinguished from ethoxylated and nonethoxylated species (i.e., free alcohols). Signal overlap was quite
severe in the 1H NMR spectra, and this hampered the quantification of mid- and end-chain branched species in particular. Signal overlap for 13C nuclei was less severe than for
1
H NMR; hence, these spectra were much more informative
with respect to branching. However, it should be noted that
the extra information in 13C NMR could only be obtained
at the expense of long acquisition times (2–3 h at 600 MHz,
compared with 15 min for a typical 1H NMR spectrum). The
relative abundances of the different structural elements
could be determined using the signal integral regions in
Table 2. These integrals first had to be divided by the number of underlying nuclei (also indicated in Table 2) to enable their use as a quantitative measure for the abundance
of a branched species. In this work, the abundances of
branched species were normalized with respect to the total
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J.P.M. VAN DUYNHOVEN ET AL.
TABLE 2
13
C and 1H Nuclear Magnetic Resonance (NMR) Spectral Integration Regionsa
1
Branched species
(i, B, X)b
(α + β + γ, H, OH)
(α + β + γ, H, EO1)c
(α + β + γ, H, EOc)
(β, Me, OH)
(β, Me, EOc)
(β, >Et, OH)
(β, >Et, EOc)
(β, Et, OH)
(β, Et, EOc)
(β, Pr, OH)
(β, Pr, EOc)
(β, ≥Bu, OH)
(β, ≥Bu, EOc)
(γ, Me, OH + EOc)c
(m, Me)
(ω1, Me)
(ω2, Me)
BI
BI(Et)
EOc
13
H NMR
integration region
C NMR
integration region
Nuclei
Start
End
Nuclei
Start
End
2
2
2
1
1
2
2
4.21
4.28
3.40
4.00
3.15
4.13
3.27
4.17
4.25
3.34
3.95
3.10
4.07
3.21
1
1
1
1
1
67.6
67.9
71.6
72.3
17.0
67.5
67.8
71.3
72.2
16.9
1
1
1
1
1
1
1
1
1
1
10.7
74.3
37.0
37.8
37.2
38.0
68.2
20.1
23.7
10.8
20.0
13.0
70.8
10.6
74.1
36.9
37.7
37.1
37.9
67.8
19.2
22.7
10.5
0.0
5.0
70.1
4
0.9
0.0
3.64
3.53
2
a
Assignments are valid for materials dissolved in CDCl3 and derivatized with trichloroacetyl isocyanate (TCAI).
b
i, position; B, length; X, OH and/or EOc.
c
Sometimes difficult to be resolved due to signal overlap. NMR, nuclear magnetic resonance; EO, ethylene oxide; m,
mid-chain; BI, branching index; BI(Et), ethyl branching index.
amount of hydrophobe chains. For AE, the normalization
intensities for 1H and 13C NMR integrals were given by
norm (1H, AE) = (α + β + γ, H, OH) + (α + β + γ, H, EOc) +
(β, Me, OH) + (β, Me, EOc) + (β, ≥Et, OH) + (β, ≥Et, EOc)
[1]
norm (13C, AE) = (α + β + γ, H, OH) + (α + β + γ, H, EOc) +
(β, Me, OH) + (β, Me, EOc) + (β, Et, OH) + (β, Et, EOc) +
(β, ≥Pr, OH) + (β, ≥Pr, EOc)
[2]
In Table 2, integration regions are specified for alcohols and
their ethoxylates only; however, an experienced NMR spectroscopist can easily assign corresponding regions for surfactants with other hydrophiles. So-called 1H and 13C NMR
branching indices (BI) are used in the field (6). Determination of the BI encompasses a simple integration of the
“methyl” region in the 1H or 13C NMR spectra (see Table 2)
and correction (–1) for the signal of the end-methyl moiety
of the alkyl chain. The BI values can be assessed rapidly but
cannot discriminate between the different branching types
(Me, Et, Pr, etc.) or most of the positions (α, β, γ, m, ω1, ω2).
Furthermore, the spectral 1H and 13C NMR assignments (see
previous section) showed that, on one hand, the “methyl integration” region does not cover all methyl signals, and that,
on the other hand, it also contains signals from nonmethyl
protons. This is pointed out in the HSQC spectrum presented in Figure 6. One can observe, first, that the integral
regions typically used for 1H and 13C BI contain nonbranched species, and second, that they do not cover all
branched spin systems. Hence, the BI do not accurately represent the true number of branches and can be taken only as
a rough measure. One should also be aware that the BI pertains to all spin systems that contain a methyl end group,
hence to all alkyl branches. Table 2 also gives the integration
region that corresponds to the ethyl branches. An ethyl BI,
BI(Et), can be derived from this number.
FIG. 4. 1H NMR spectra of TCAI-derivatized alcohol ethoxylates,
based on (A) C-Oxo and (B) MB alcohol feedstocks. Assignments of
structural elements are indicated (nomenclature is explained in the
text). For other abbreviations see Figures 2 and 3.
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HYDROPHOBE BRANCHING IN ALCOHOL-BASED SURFACTANTS BY NMR
FIG. 5. 13C NMR spectra of TCAI-derivatized alcohol ethoxylates, based on (A) C-Oxo and
(B) MB alcohol feedstocks. Assignments of structural elements are indicated; nomenclature
is explained in the text. For abbreviations see Figures 1–4.
An impression of the amount of information that can be
gathered from the 1H and 13C NMR spectra is presented in
Table 3 for representative examples from the six branching
classes. Note that for β-branching, the structural information
can be given for both the ethoxylated and the nonethoxylated alcohol. The information on hydrophobe branching
given in Table 3 matches our expectations from the theoretical classification in Table 1 well. For example, the C-Oxo and
M-Oxo materials contain no mid- or end-chain branching.
According to our expectations, these branched species are
increasingly abundant in the MB, RMB, and HB materials.
This is also reflected in the increasing BI in this series. Although both 13C and 1H BI show the same trend, their absolute values differ. In general, the 13C BI is in better agreement with the site-specific branching assessment. The BI(Et)
listed in Table 3 is rather small in most classes, except for the
RMB and HB species. An interesting observation is the distribution of branching in the MB material, which is low in β, γ,
and ω1 branches, typically 6, 12, and 4%. The mid-chain
branches seem higher (34%), but when an estimated number of eight mid-chain methylene moieties are considered,
the average value is approximately 4%. The variation of
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J.P.M. VAN DUYNHOVEN ET AL.
tion of complex surfactant mixtures (22). This technique was
used to add chain length (c) information to the bulk branching parameters that could be obtained by the 1H NMR
method previously outlined. A sample of an alcohol feedstock was measured in the so-called loop-storage mode using
1
H NMR detection. In this mode, peaks containing different
alkyl chain lengths are stored in loops until NMR spectrometer machine time becomes available, at which time they are
analyzed in automated mode. The LC separation could not
separate the different branched species, but one could still
derive the branching pattern per alkyl chain by means of the
1
H spectral characteristics. The degree of methyl and ethyl
branching increased with alkyl chain length (from C12 to
C15), as shown in Figure 7. This increase in degree of branching was observable as a decrease in the ratio between the signals of linear (B = H) and branched species (B = Me, Et). Attempts also were made to obtain additional structural detail
using LC-NMR, such as quantitative information on midchain branching. This required LC separation in combination with 13C NMR detection. It was possible to record goodquality 13C NMR spectra in the loop storage mode, but at present it is not possible to make this information chain-length
specific.
Comparison of methods for assessing hydrophobe branching. To
put the NMR methodology in a broader perspective, all
known relevant analytical capabilities are summarized in
Table 4. When used in conjunction with derivatization (23),
electrospray MS (ESMS) also can be used to assess surfactant
materials. This method can provide alkyl chain length and
ethoxylate number distributions rapidly, but it is somewhat
less accurate and precise for the quantification of free alcohols and short ethoxylate numbers in comparison with the
FIG. 6. 13C and 1H connectivities in the HSQC spectrum of an alcohol
ethoxylate. Horizontal and vertical dashed boxes correspond to integral regions for the 13C and 1H branching indices, respectively.
1-D, one-dimensional; for other abbreviations see Figure 3.
branching per methylene over the chain between 4 and 12%
is in agreement with the supposed random distribution (7).
Chain-length specific assessment of branching parameters by LC–
NMR. The NMR methodology previously described can yield
only “bulk” branching parameters. Consequently, chain
length specificity cannot be obtained. It is necessary to “hyphenate” NMR with a chromatographic separation dimension, such as LC, to add chain length information. LC–NMR
is already recognized as a versatile tool for the characterizaTABLE 3
Typical Results of Branching Quantificationa
Novel-1012-6.2
NO
AF
Class
1
H NMR
13
C NMR
i
B
—
α
β
β
β
β
—
—
α
β
β
β
β
γ
m
ω2
ω1
BI
Me
H
Me
≥Et
EO
BI
BI(Et)
Me
H
Me
Et
≥Pr
Me
Me
Me
Me
EO
Alc
Lialet 123-7
C-Oxo
AE
EO
0
0
100
0
0
0
0
44
15
32
6.7
0
0
0
1
1
6
0
0
50
13
7
24
Alc
Safol 23
MB
AF
EO
0.2
0
79
7
7
5.2
0.5
0.08
0
0
0
0
0
0
0
0
0
EO
0.5
0
0
0
0
7.0
0
100
0
0
0
Alc
Neodol 1-7
M-Oxo
AE
0
4
1
2
0
0
75
6
7
6
0
0
0
0
a1
EO
0
0
0
0
0
93
7
0
0
0
0
0
0
EO
0
72
10
6
0
7
2
3
0
78
4
3
7
0
0
0
0
0
92
8
0
2.9
1.0
0
2
4
1
3
7
71
17
+
Alc
3.0
1.5
0.65
0
94
6
0
0
12
34
4
+
Alc
Exxal 13
HB
AF
2.3
0.4
0.08
0
4
2
0
1
0
0
0
0
Alc
0.7
0.1
0
0
0
1
1
4
Lutensol TO5
RMB
AE
0
0
0
0
0
0
96
4
0
0
41
190
9
+
H and 13C NMR results are provided for alcohol feedstocks (AF) and derived alcohol ethoxylates (AE). The material classification is explained in Table 1
and in the text. “+” denotes that a species is detected at a level too low to be quantified reliably. For abbreviations see Tables 1 and 2.
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FIG. 8. Comparison of NMR (1H and 13C) and gas chromatography
(GC) branching quantification results for a modified Oxo (M-Oxo) type
feedstock. H denotes a linear chain (H-branch). Note that for the
longer branches, NMR can give only cumulative results (e.g., ≥Pr and
≥Et for 13C and 1H NMR, respectively). For other abbreviations see
Figure 2.
FIG. 7. Loop-storage 1H liquid chromatography (LC)–NMR spectra,
obtained for a C-Oxo feedstock. The spectra obtained for C12, C13,
C14, and C15 alkyl chain lengths are shown. In the 1H NMR spectra, the
signals of the CH2O protons of β-branched alcohols are indicated. For
other abbreviations see Figures 2 and 3.
NMR method. More important, it does not provide information on hydrophobe branching. In this respect, GC with FID
or MS detection is more powerful. The GC method is considerably less expensive and is therefore more widespread than
the aforementioned NMR and ESMS techniques. It is also
very informative with respect to branching on the β-position
of the hydrophobe. When used in conjunction with bromide
derivatization, GC can resolve longer branches (up to pentyl)
on the β-position than can the 1H (methyl) and 13C (propyl)
NMR methods. However, the GC method can be applied only
to nonderivatized alcohols. The results obtained by GC and
NMR for two Oxo-type feedstocks are presented as an example in Figure 8. Good agreement between the NMR and GC
results can be observed. However, the GC method has weaknesses in resolving and identifying branches on positions
other than β. In simple materials, it was possible to identify
some branches at the γ and mid-chain positions using GC–MS,
but complete assignments were difficult to make when the
materials became more complex. Future deployment of com-
prehensive chromatography (24) (e.g., GC–GC) may be a
more useful tool in this respect.
ACKNOWLEDGMENTS
Anneke Groenewegen is gratefully acknowledged for her technical
NMR assistance. Herrald Steenbergen, Dr. Hans-Gerd Janssen, and
Dr. Wijnand Schuyl are thanked for their expert GC–MS measurements and interpretations.
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TABLE 4
Overview of Current Analytical Capabilities for Assessing the Hydrophobe Branching in Alcohol-Based Materialsa
EO number
(c)
ESMS
1
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C NMR
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[Received August 13, 2004; accepted November 12, 2004]
John van Duynhoven received his Ph.D. degree from the University
of Nijmegen. He is now responsible for the Molecular and Material
Analysis group at Unilever R&D Vlaardingen. He is the (co-)author of 65 peer-reviewed papers in the area of solid and liquid NMR
applied to detergent and food systems.
Afranina Leika received her training as a laboratory engineer. She
is now employed at Unilever R&D as an NMR spectroscopist and
has specialized in the structural elucidation of detergent materials.
Philip (Flip) van der Hoeven obtained his Ph.D. degree from the
Wageningen University and Research Center (WUR). He has 39
years of Unilever experience in the detergents area. As a lead scientist, he is now responsible for surfactant research at Unilever R&D
Vlaardingen. He has authored six peer-reviewed papers and holds
27 patents.
JOURNAL OF SURFACTANTS AND DETERGENTS, VOL. 8, NO. 1 (JANUARY 2005)
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