Process for reducing the levels of nitrite contaminants in amine oxide surfactants

Amine oxide surfactants contaminated with nitrite are treated under acidic conditions with materials such as maleic acid and diethylenetriaminepentaacetate (DTPA) to reduce nitrite levels below 1 ppm. Thus, C.sub.12-13 dimethyl amine is oxidized with hydrogen peroxide in the presence of maleic acid and DTPA to reduce nitrite levels. The resulting nitrite-free amine oxide is employed in various detergent compositions.

FIELD OF THE INVENTION 
The present invention relates to an improvement in a chemical process for 
preparing amine oxide surfactants, whereby the content of nitrite 
contaminants is reduced or substantially eliminated for an extended period 
of time. 
BACKGROUND OF THE INVENTION 
The preparation of amine oxide surfactants by the oxidation of tertiary 
amines is of considerable commercial interest. Such surfactants are widely 
used in commercial cleaning compositions, especially high sudsing 
dishwashing detergents. However, it has now been discovered that some 
sources of amine oxide surfactants may be contaminated with residual 
amounts of nitrite materials, especially inorganic nitrites. Contamination 
by such nitrites may be tolerable under many circumstances. For some uses, 
however, the presence of nitrites may be undesirable, since they 
prospectively can react with other ingredients which may be present in 
fully formulated detergent compositions. 
Detergent formulators presumably could arrange for special care to be taken 
during the manufacture of amine oxides in order to minimize the formation 
of undesirable nitrite contaminants. However, the manufacturer 
[formulator] of high volume, low-cost chemicals such as home-use 
detergents can ill-afford surfactants and other raw materials requiring 
special reaction techniques or special reactants, due to their expense. 
The present invention solves the problem of contamination of amine oxide 
surfactants by nitrite materials using simple, cost-effective techniques. 
The invention herein thus affords access to a high quality supply of this 
class of surfactants. 
BACKGROUND ART 
The synthesis of amine oxides is described in Kirk-Othmer "Encyclopedia of 
Chemical Technology" 3rd Edition Vol. 2 pp. 264-266, John Wiley and Sons 
1978, in "Surfactant Science Series" Vol. 1 Nonionic Surfactants, M. J. 
Schick, editor, Marcel Dekker, Inc. 1966 pp. 403-407, and in U.S. Pat. No. 
3,283,007, issued Nov. 1, 1966 to A. F. Chadwick. 
SUMMARY OF THE INVENTION 
The present invention encompasses, in a process for preparing amine oxide 
surfactants of the formula R R.sup.1 R.sup.2 N.fwdarw.O wherein R is a 
C.sub.8 to C.sub.18 alkyl or alkenyl substituent and R.sup.1 and R.sup.2 
are each C.sub.1 -C.sub.4 hydroxyalkyl substituents by the oxidation of 
the corresponding amine of the formula R R.sup.1 R.sup.2 N, the 
improvement which comprises conducting the oxidation in the presence of 
polycarboxylate material, whereby contamination of the amine oxide 
surfactant by nitrites is substantially decreased for an extended period 
of time. The polycarboxylate material is preferably citrate or a chelating 
compound comprising a chelating active selected from the group consisting 
of water-soluble ethylenediamine tetraacetates (EDTA), 
diethylenetriaminepentaacetates (DTPA), nitrilotriacetates and mixtures 
thereof and a chelating co-active selected from the group consisting of 
malates, maleates, succinates, and mixtures thereof. In a preferred mode, 
the amine reactant is a C.sub.10 -C.sub.18 alkyl dimethylamine. 
The oxidation step herein is preferably conducted using hydrogen peroxide 
or source of hydrogen peroxide, or by hydrogen peroxide generated in situ. 
When conducted in the presence of carboxylate chelants, the process is 
preferably carried out at a pH in the range of from about 6 to about 8. 
The process can also advantageously be carried out in the presence of 
sulfamic acid, which typically can comprise 0.2%-5% of the amine being 
oxidized. 
The use of a single polycarboxylate material, such as DTPA, decreases the 
nitrite level in amine oxide to acceptable levels, under 3. ppm. However, 
it has now been discovered that, with the exception of citrate, when only 
a single polycarboxylate material is used to decrease nitrite levels, the 
nitrite level rebounds within a few days at ambient temperature. It is 
therefore preferable to use either citrate or polycarboxylate material 
that is a chelating compound comprising specific chelating active and 
specific chelating co-active or mixtures thereof, as set forth above. 
An overall process according to this invention is wherein: 
a. the amine is a C.sub.12 -C.sub.16 alkyl dimethylamine; 
b. the oxidation is conducted using hydrogen peroxide; 
c. the oxidation is conducted in the presence of citrate or water-soluble 
DTPA, EDTA, nitriloacetate, or mixture thereof in combination with 
water-soluble maleate, malate, succinate, citrate, or mixtures thereof; 
d. the oxidation is conducted at a pH in the range of from about 6 to about 
8; and 
e. the oxidation is conducted over a temperature range of from about 
50.degree. C. to about 75.degree. C.; 
whereby the resulting amine oxide has a nitrite content below about 3 ppm 
for at least about 4 weeks. 
It is to be understood that citric acid alone (without an additional 
chelating co-active ) gives the desired result. With maleate alone, 
optimal, low nitrite levels are approached, but not consistently achieved 
without DTPA. Excluding citrates, it is necessary to combine specific 
chelating active and specific chelating co-active in order to obtain 
long-term nitrite level below about 3 ppm. 
The invention thus provides stable detergent compositions (especially 
liquid compositions) comprising amine oxide detergent surfactant prepared 
in the foregoing manner. 
All percentages, ratios and proportions herein are by weight, unless 
otherwise specified. All documents cited are incorporated here by 
reference. 
DETAILED DESCRIPTION OF THE INVENTION 
For purposes of clarity, the following defines the terms used herein. 
By "nitrite" herein is meant the NO.sub.2 - species, including all forms of 
nitrous acid or its salts or reactive derivatives such as HNO.sub.2, 
NaNO.sub.2, N.sub.2 O.sub.3, H.sub.2 NO.sub.2.sup.+, N.sub.2 O.sub.4, 
NO.sup.+, or related species. 
By "diminished in", "substantially diminished in", or "substantially free 
of" nitrite, or like terms used herein is meant that the 
nitrite-contaminated amine oxide surfactant which has been treated 
according to the present invention thereafter contains, "as made", no more 
than about 3 ppm, preferably no more than about 1 ppm, most preferably 
zero ppm nitrite, as measured by standard nitrite analyses. 
By "polycarboxylates" such as "citrate", "malate", "maleate" or the like 
herein is meant the free acid, the soluble salt, the anhydride, or any 
other suitable source of the polycarboxylate material. For example, maleic 
anhydride is more economically available on a commercial scale than the 
free acid and is preferred herein. 
Amine Oxide Synthesis 
As described in the references cited above, alkyldimethylamine oxide 
surfactants are produced by oxidizing the desired alkyldimethylamine with 
hydrogen peroxide, which is generally in the form of a 5% to 70% aqueous 
solution. Although less can be used, typically the hydrogen peroxide is 
used at about a 5% to 10% excess of the stoichoimetric amount when 
conversion of more than 99% of the tertiary amine is required. It is 
preferable in the present invention to use peroxide levels of from about 
95% to about 125% of the stoichiometric amount, more preferably from about 
100% to about 115%, of the stoichiometric amount, and to conduct the 
process at a temperature of from about 40.degree. C. to about 100.degree. 
C., preferably from about 50.degree. C. to about 85.degree. C., and more 
preferably from about 50.degree. C. to about 75.degree. C. 
The reaction can be carried out in the presence of an organic solvent such 
as ethyl or isopropyl alcohol. Use of an organic solvent enables 
production of higher concentrations of amine oxide while avoiding gel 
formation. A catalyst can be used in the reaction of the present invention 
to accelerate the reaction rate. Total reaction time is typically 2 to 20 
hours depending on reaction stoichiometry, catalyst, solvent system and 
degree of tertiary amine conversion required. Commercially, either batch 
or continuous reaction processes are used. If desired, peroxide reaction 
residues can be reduced or eliminated by adding a reducing agent, such as 
sodium sulfite, or other agent to promote peroxide decomposition. However, 
such peroxide residues can be beneficial for the reduction of nitrite 
levels under acidic conditions. In fact, it has now been discovered that 
color stability can be affected by residual peroxide levels. In order to 
obtain a color stability benefit, residual peroxide levels should be from 
about 0.02% to about 0.09%, preferably from about 0.03% to about 0.08%, 
more preferably from about 0.04% to about 0.07%, by weight of the total 
H.sub.2 O.sub.2. 
Some amine oxide surfactants prepared in the foregoing, standard manner 
have now been found to be contaminated with as much as 60 ppm of nitrite. 
By the present procedures, these nitrite levels can be lowered to 1-3 ppm, 
or less. 
Present Process 
In the process of this invention the oxidation of the amine is carried out 
using otherwise conventional procedures, as noted above, but with the 
addition of the polycarboxylate material. 
Polycarboxylate Material 
The use of a single polycarboxylant material, with the exception of 
citrate, such as DTPA, decreases the nitrite level in amine oxide to 
acceptable levels, under 3. ppm. However, it has now been discovered that 
when only a single polycarboxylate material is used to decrease nitrite 
levels, the nitrite level rebounds within a few days at ambient 
temperature. It is therefore preferable to use citrate or polycarboxylate 
material that is a chelating compound comprising specific cheating active 
and specific chelating co-active. 
Said chelating active is selected from the group consisting of 
water-soluble (EDTA), (DTPA), nitrilotriacetates and mixtures thereof and 
said chelating co-active selected from the group consisting of malates, 
maleates, succinates, and mixtures thereof. Preferably said chelating 
active is selected from the group consisting of EDTA and DTPA and said 
chelating co-active is selected from the group consisting of malates and 
maleates, more preferably said chelating active is DTPA and said chelating 
co-active is maleic acid. 
Typically, the amount of citrate or chelating compound, (chelating 
active+chelating co-active) is from about 0.1% to about 6%, preferably 
from about 0.25% to about 5%, more preferably from about 0.5% to about 4%, 
by weight of the amine. The ratio of chelating active to chelating 
co-active typically is from about 1:20 to about 2:1, preferably from about 
1:15 to about 1:5, more preferably from about 1:10 to about 1:1. 
Thus, in a preferred process DTPA and maleic acid (typically 1:5 to 5:1 by 
weight) are dissolved in the reaction mixture. The pentasodium form of 
DTPA, available as DOW VERSENEX 80, is especially useful herein, but 
either acid or salt forms work. 
Storage 
It has been discovered that storage temperature is also responsible for the 
rebounding of nitrite levels as well. Elevated temperatures, i.e., above 
about 50.degree. C., cause increased nitrite levels in the production 
materials of the present invention. Therefore, it is preferably to keep 
the resulting product of the present invention at typical ambient 
temperature levels, i.e., from about 25.degree. C. to about 49.degree. C. 
Nitrite Analyses 
The presence of nitrites in the amine oxide surfactants, both before and 
after use of the procedures of this invention, can be measured in several 
ways. The following are illustrative. 
a) Nitrite test strips, sold under the trademark E. M. Quant, Catalog 
number 10007-1 (Division of EM Industries, Inc., Gibbstown, N.J.) are used 
in standard fashion as directed by the manufacturer. These test strips are 
convenient and quite suitable under most conditions. 
b) If more precise measurement is needed, nitrite can be assayed 
spectrophotometrically (Varian Spectrophotometer Model 219) using a 
colorimetric assay with sulfanilamide reagent (5 g. sulfanilamide, 60 ml 
conc. HCl diluted to 500 ml in H.sub.2 O) and 
N-(1-naphthyl)-ethylene-diamine dihydrochloride "NED" (500 mg diluted to 
500 ml in H.sub.2 O; stored in a dark bottle and replaced if the solution 
develops a strong brown color). The test employs a 5 ml test solution of 
the amine oxide surfactant at 3-30% concentrations, depending on nitrite 
concentration. In the test, 0.1 ml of the sulfanilamide reagent is added 
to the amine oxide solution. After 5 minutes, the pH is adjusted to 
1.0-1.5 using HCl. Then, 0.1 ml of the NED solution is added. Color 
develops fully within a few hours. The absorbance is measured at 543 
nanometer (nm) wavelength. The results are compared with samples to which 
known amounts of nitrite have been intentionally added. 
(c) In cases where a large excess of nitrite scavenger is present, nitrite 
concentration can be determined by chromatographic separation methods, 
either by ion exchange chromatography or capillary zone electrophoresis. 
The following illustrates the nitrite reduction procedure of the present 
invention.

EXAMPLE I 
10 grams of maleic acid and 3.75 grams of VERSENEX 80, a 40% by weight 
aqueous solution of the sodium form of DTPA, are added to 523 grams of 
distilled water. 157 grams of 30% aqueous hydrogen peroxide are added and 
stirred. The mixture is heated to 50.degree. C., 219 grams of C.sub.12 
-C.sub.16 alkyldimethylamine are added. The pH of the mixture at this 
point is 8.0. Stirring is continued at 50.degree. C. for two hours 
followed by an additional five hours at 70.degree. C. The 963 grams of 
reaction product contain 28% amine oxide. The pH of the reaction product 
is 6.9. No nitrite is detected with E. M. Quant test strips. After a 
week's storage at ambient conditions, the reaction product has no 
detectable nitrite using the test strip method and 0.6 ppm measured using 
ion chromatography. 
EXAMPLE II 
7.4 grams of sulfamic acid and 4.4 grams of the acid form of DTPA are added 
to 1659 grams of water. 471 grams of 30% hydrogen peroxide are then added 
and stirred. The mixture is heated to 50.degree. C. while stirring. Upon 
reaching 50.degree. C., 867 grams of C.sub.12 -C.sub.16 alkyldimethylamine 
are added. Stirring is continued for two hours at 50.degree. C. followed 
by 5 hours at 70.degree. C. The reaction product contains 31% amine oxide 
and no nitrite is detected with test strips. The pH of the reaction 
product is 7.6. After a week's storage at ambient conditions, the reaction 
product contains 5 ppm nitrite measured using the test strip method and 
6.5 ppm measured by ion chromatography. 
EXAMPLE III 
10 grams of maleic acid are added to 527 grams of distilled water. 157 
grams of 30% aqueous hydrogen peroxide are added and stirred. The mixture 
is heated to 50.degree. C. while stirring. Upon reaching 50.degree. C., 
219 grams of C.sub.12 -C.sub.16 alkyldimethylamine are added. Stirring is 
continued at 50.degree. C. for two hours followed by an additional five 
hours at 70.degree. C. The reaction product contains 28% amine oxide. The 
pH of the reaction product is 6.8. After completion of the reaction no 
nitrite is detected with the test strips. After a week storage at ambient 
conditions, the reaction product contains 5 ppm nitrite measured using the 
test strip method and 3.2 ppm measured by ion chromatography. 
EXAMPLE IV 
157 grams of 30% aqueous hydrogen peroxide are added to 537 grams of 
distilled water and stirred. The mixture is heated to 50.degree. C. while 
stirring. Upon reaching 50.degree. C., 219 grams of C.sub.12 -C.sub.16 
alkyldimethylamine are added. Stirring is continued at 50.degree. C. for 
two hours followed by an additional five hours at 70.degree. C. The pH of 
the reaction product is 7.6. After completion of the reaction, 20 ppm 
nitrite is measured using the test strip method. 
The following data illustrate the results secured in the manner disclosed 
above. 
______________________________________ 
Nitrite 
(ppm) Ion 
"As Made" Test Strip 
Chroma- 
Description Test Strip 
Days Later 
tography 
______________________________________ 
Laboratory Control #1 
20 
(0.25% sulfamic acid) 
Laboratory Control #2 
30 
(0.25% sulfamic acid) 
Laboratory Control #3 
20 
(0.25% sulfamic acid) 
Anion exchange treated 
ND 
control #2 
3% sulfamic acid 
10 
5% ethanol (35% AO 
5 
active) 
1% maleic acid, no 
&lt;2 5 3.2 
sulfamic acid 
0.5% maleic acid, no 
2 
sulfamic acid 
0.375% maleic acid, 
5 
no sulfamic acid 
0.25% maleic acid, 
10 
no sulfamic acid 
1% maleic acid, 0.25% 
&lt;2 
sulfamic acid 
0.06% DTPA, 0.25% 
ND 3 
sulfamic acid 
0.06% DTPA, 0.25% 
ND &lt;2 3.3 
sulfamic, city water 
0.6% DTPA, no sulfamic 
2 7-8 
acid, city water 
0.15% DTPA, no sulfamic 
ND 2 5.7 
acid, city water 
0.15% DTPA, 0.25% 
&lt;2 2-5 6.5 
sulfamic acid, city water 
0.25% citric acid, no 
2 
sulfamic acid 
1% citric acid, no sulfamic 
ND ND 
acid 
2% citric acid, no sulfamic 
ND ND ND 
acid 
0.15% DTPA, 1% maleic 
ND ND 0.6 
acid, no sulfamic acid 
______________________________________ 
ND = none detected 
For nitrites: &lt;.about.2 ppm by test strips; &lt;0.5 ppm by ion 
chromatography) 
Detergent Formulations 
Surfactants and Suds Enhancers--The substantially nitrite-free amine oxide 
surfactants afforded by the present invention are useful in any 
circumstance where the prospective reaction of nitrite with the detersive 
ingredients is desirably avoided. The amine oxides herein are especially 
preferred for use in liquid detergents, especially those containing 
nitrogen compounds like amines and/or amides. 
The following is intended to illustrate the use of the amine oxide 
surfactants made in accordance with this invention in liquid detergent 
compositions, but is not intended to be limiting thereof. Water-soluble 
Ca.sup.++ or Mg.sup.++ salts, e.g., MgSO.sub.4, MgCl.sub.2 or the like 
can be used to introduce such cations into the compositions, typically at 
levels of 0.01%-2%, to enhance sudsing and grease removal performance. 
EXAMPLE V 
Homogeneous light duty liquid detergent compositions which are especially 
adapted for dishwashing and other hard surface cleaning operations are as 
follows. In the Examples A-D, the surfactants comprise various alkyl 
ethoxy sulfate surfactants which, using standard terminology, are 
abbreviated to indicate their average degree of ethoxylation; thus 
C.sub.12-13 EO(0.8) sulfate indicates a sulfated mixed C.sub.12 -C.sub.13 
alcohol fraction having an average degree of ethoxylation of 0.8. These 
anionic ethoxy sulfates are preferably used in their Na.sup.+ or 
NH.sub.4.sup.+ salt form. The C.sub.12 -C.sub.13 amine oxide is a mixed 
C.sub.12-14 (average) dimethyl amine oxide. The amine oxide is treated in 
the manner of Examples I, II III and IV, herein, respectively, for use in 
compositions A, B, C and D, respectively. The C.sub.12-14 AP betaine is 
C.sub.12 /.sub.14 H.sub.25 /.sub.29 CONH(CH.sub.2).sub.3 
N+(CH.sub.3).sub.2 --CH.sub.2 CO.sub.2 H. The C.sub.12-14 AP sultaine is 
C.sub.12 /C.sub.14 H.sub.25 /.sub.29 CONH(CH.sub.2).sub.3 
N+(CH.sub.3).sub.2 CH.sub.2 CH(OH)CH.sub.2- SO.sub.3 H. The C.sub.12-14 DM 
betaine is C.sub.12 /.sub.14 H.sub.25 /.sub.29 N+(CH.sub.3).sub.2 CH.sub.2 
CO.sub.2 H. The ethoxylated nonionic surfactant designated C.sub.9-1 EO(8) 
refers to C.sub.9-C.sub.11 alcohols ethoxylated with an average of 8 moles 
of ethylene oxide. The Ca.sup.++ and Mg.sup.++ cations are conveniently 
introduced into the compositions as CaCl.sub.2 and MgCl.sub.2. The Balance 
of the compositions comprises water and citrate/propylene glycol present 
in the glucamide surfactant (1-5%) and 1-3% cumene sulfonate or xylene 
sulfonate hydrotope. The pH is typically 6.8-7.4(NH4.sup.+ salts) or 7- 
8.2 (Na.sup.+ salts). 
______________________________________ 
Percent (wt.) 
Ingredient* A B C D 
______________________________________ 
C.sub.12-14 N-methyl glucamide** 
11 8 12.7 9 
C.sub.12-13 EO(0.8) sulfate 
-- 13 10.0 9 
C.sub.12-14 EO(3) sulfate 
11 -- 2.7 14 
C.sub.12-13 EO(6.5) sulfate 
-- -- -- 3 
C.sub.12-14 AP betaine 
-- -- 2 -- 
C.sub.12-14 AP sultaine 
-- -- -- 1.0 
C.sub.12-13 dimethyl amine oxide 
2.5 3.0 2.5 1.0 
C.sub.12-14 DM betaine 
-- 2.0 -- -- 
C.sub.9-1 EO(8) 0.5 8 7 -- 
Ca++ -- -- 0.5 1.0 
Mg++ 0.9 0.25 -- -- 
Balance bal bal bal bal 
______________________________________ 
*Commercial grade surfactants may be bleached to colorless (i.e., to 
provide waterclear liquids). The C.sub.12-14 Nmethylglucamide herein 
preferably has been treated with acetic anhydride in water at 60.degree. 
C.-80.degree. C. so that it contains 0.1% or less of Nmethylglucamine. 
Optionally, for highest sudsing the glucamide surfactant can also be 
treated with ethanolamine at 50.degree. C.-80.degree. C. to reduce levels 
of free fatty acids to 1% or below. The amine oxide contains less than 1. 
ppm nitrite. 
**Pretreated with acetic anhydride to decrease amine content. 
EXAMPLE VI 
A liquid detergent composition with a suds boosting fatty amide is as 
follows. Product pH is adjusted to 7.8 with NaOH. 
______________________________________ 
Ingredient % (wt.) 
______________________________________ 
Dimethyldodecyl amine oxide.sup.1 
5.0 
C.sub.12-14 EO(3) sulfate 
14.0 
Sodium cumene sulfonate.sup.2 
2.0 
C.sub.12 Monoethanolamide 
1.5 
Coconut N-methylglucamide 
7.0 
Water, dye, minors Balance 
______________________________________ 
.sup.1 Substantially nitrite free, prepared according to this invention. 
.sup.2 Introduced in acid treatment of amine oxide as sulfonic acid form 
and subsequently neutralized with NaOH. 
In addition to the foregoing, it has now further been discovered that 
concentrated solutions (i.e., 31% amine oxide, and higher, typically 
50-55% amine oxide) of amine oxides which are substantially free of 
nitrite can be prepared using the technology herein. Such concentrated 
solutions are especially useful in formulating the so-called concentrated 
or "compact" liquid detergent compositions which are now favored by many 
consumers and which, in general, contain lower amounts of water than 
conventional detergents. In the present process, the tertiary amine is 
oxidized in the presence of, for example, DTPA and maleic anhydride, 
preferably in the presence of alcohol (ethanol is preferred) or other 
suitable material to avoid amine oxide gelling at the higher 
concentrations. 
The following illustrates this concentrated process in more detail. 
EXAMPLE VII 
A 50-55% (wt.) "high active" solution of C.sub.12-14 dimethyl amine oxide 
is prepared in water/ethanol (70:30) by oxidizing the corresponding amine 
with peroxide, with the following results. 
______________________________________ 
Nitrite 
Run (ppm) 
______________________________________ 
A. High active, no additives 
7.0 
B. High active, 1% maleic acid, 0.15% DTPA 
0.5 
C. High active, 1% maleic anhydride, 0.15% DTPA 
0.2 
______________________________________ 
While the foregoing illustrates the present invention and its use in liquid 
detergents, especially dishwashing compositions, it is not intended to 
limit the scope of the invention. The amine oxide surfactants provided by 
this invention can be used in any detergent composition where high 
sudsing, good grease/oil removal and overall product stability are 
desired. The invention herein can be used with various conventional 
ingredients to provide fully-formulated fabric laundering compositions, 
hard-surface cleansers, personal cleaning products and the like. Such 
compositions can be in the form of liquids, granules, bars and the like.