Dissolution study of salt of long chain fatty acids (Soap Scum) in surfactant solutions. Part I: Equilibrium dissolution

Article abstract of DOI:10.1007/s11743-010-1208-5

Dissolution of calcium salt of a long chain fatty acid or soap scum is a major challenge for hard surface cleaners since soap scum forms when soap is exposed to hard water and has very low water solubility. In this paper, the aqueous equilibrium solubility of calcium octadecanoate (or calcium stearate) was measured as a function of pH as well as chelating agent (ethylenediaminetetraacetate disodium salt) and surfactant concentrations. Anionic, nonionic, and amphoteric surfactants were studied. The highest soap scum solubility was observed at high pH with an amphoteric surfactant. Under this condition, the chelant effectively binds calcium, and the stearate anion forms mixed micelles well with the amphoteric surfactant, which is in zwitterionic form at high pH. AOCS 2010.

Full text of DOI:10.1007/s11743-010-1208-5

J Surfact Deterg (2010) 13:367–372
DOI 10.1007/s11743-010-1208-5
ORIGINAL ARTICLE
Dissolution Study of Salt of Long Chain Fatty Acids (Soap Scum)
in Surfactant Solutions. Part I: Equilibrium Dissolution
•
Sukhwan Soontravanich Heyde E. Lopez
•
•
John F. Scamehorn David A. Sabatini
•
David R. Scheuing
Received: 24 June 2009 / Accepted: 13 April 2010 / Published online: 1 June 2010
Ó AOCS 2010
Abstract Dissolution of calcium salt of a long chain fatty
acid or soap scum is a major challenge for hard surface
cleaners since soap scum forms when soap is exposed to
hard water and has very low water solubility. In this paper,
the aqueous equilibrium solubility of calcium octadeca-
noate (or calcium stearate) was measured as a function of
pH as well as chelating agent (ethylenediaminetetraacetate
disodium salt) and surfactant concentrations. Anionic,
nonionic, and amphoteric surfactants were studied. The
highest soap scum solubility was observed at high pH with
an amphoteric surfactant. Under this condition, the chelant
effectively binds calcium, and the stearate anion forms
mixed micelles well with the amphoteric surfactant, which
is in zwitterionic form at high pH.
creates the well-known stain in the bathroom (e.g., ring
around the bathtub). There have been extensive efforts to
formulate bathroom cleaners and other hard surface
cleaners to remove soap scum stains. Requirements for
good formulation generally include effective, rapid
removal of the scum with little or no mechanical force
involved. One common approach in hard surface cleaners
used for soap scum removal involves an aqueous solution
of a chelant for calcium or magnesium complexation with a
micelle-forming surfactant. Simultaneous chelation of
calcium and mixed micellization of the fatty acid with the
added water-soluble surfactant are hypothesized to be
responsible for soap scum dissolution.
The relationship between the structure of different sur-
factants and performance in the equilibrium dissolution of
soap scum in the solution containing chelating agent and
the effect of solution pH are main focuses in this study. The
solution pH can have an effect on both the effectiveness of
the chelating reaction and the structure of the fatty acid due
to protonation [1]. Generally, the solubility of soap
decreases or the Krafft temperature increases as its alkyl
chain length increases [2]. Mixture of soaps are more sol-
uble than pure soaps [2].
Keywords Detergent formulation Á
Application of surfactants
Introduction
Soap scum is the calcium or magnesium salt of a long chain
fatty acid. Many personal care products consist of soaps,
which are salts of carboxylic acids or fatty acids. Soaps are
unstable in hard water containing metal ions, especially
calcium and magnesium, because soaps form insoluble
precipitates with those metal ions. Insoluble soap scum
Calcium di-stearate (Ca(C18)2), [CH3(CH2)16CO2]2Ca or
the calcium salt of octadecanoic acid was used as a model
soap scum in this study. Actual soap scum can be a mixture
of other metal salts (e.g., magnesium) or other fatty acids
such as palmitic acid. Calcium stearate is normally con-
sidered ‘‘insoluble’’ in water, alcohol, and ether, but solu-
ble in mineral oils and hot pyridine. The solubility of
calcium stearate is only about 0.04 g/l of water at 15 °C
[3]. Long chain fatty acid salts are weak ionizable surfac-
tants [4] with the charge varying with pH. Calcium stearate
has a pKa of 4.5 [5]. So, at low pH (e.g., 2), the stearate
anion protonates and forms nearly insoluble stearic acid.
S. Soontravanich Á H. E. Lopez Á J. F. Scamehorn (&) Á
D. A. Sabatini
Institute for Applied Surfactant Research,
University of Oklahoma, Norman, OK, USA
e-mail: scamehor@ou.edu
D. R. Scheuing
Clorox Technical Center, Pleasanton, CA, USA
123

368
J Surfact Deterg (2010) 13:367–372
The solubility of stearic acid is only 0.0029 g/l of water at
20 °C [6]. At higher pH level, calcium cannot dissociate so
the Ca(C18)2 remains mainly insoluble. At intermediate pH
level, protonation is only partial so there is a mixture of
stearate anion and stearic acid, both of which have low
solubility. So, without some way to remove the calcium
from the stearate or some aggregates like micelles to
increase the solubility of the stearate anion or stearic acid
in solution, Ca(C18)2 is nearly insoluble throughout a wide
pH range.
stability constants of the ligand with the metal. There are
five possible forms of EDTA in the absence of calcium
depending on pH, and there are two forms that can form
complexes with calcium ions [9]. At high pH, the major
species found is Y4-, which is the most effective form of
EDTA in chelating calcium. The molecule of EDTA in
acidic solution is less effective than in a basic solution due
to protonation of active sites on the EDTA.
At high pH, as calcium is removed from solid Ca(C18)2,
the stearate anion solubility increases to the point where all
the solid dissolves, or it forms micelles, or forms precipi-
tate with some other counterions (e.g., Na?) in solution in
the absence of a co-surfactant acting as a micelle-pro-
moting agent. The presence of a co-surfactant forming
micelles further improves solubility synergistically with
the chelant. At a low enough pH, the protonation reaction
should cause complete dissolution of the solid calcium
stearate to nearly insoluble protonated fatty acid, which can
co-micellize (but doesn’t have enough hydrophilicity in the
nonionic head group to form micelles independently). So at
low enough pH, the chelant should not have any effect on
the equilibrium solubility of Ca(C18)2.
The solubilities of either the nonionic stearic acid at low
pH and the Ca(C18)2 at high pH are so low that micelles do
not form; i.e., as surfactant concentration increases, the
solubility limit is reached before a CMC is reached. One
strategy of increasing the solubility of Ca(C18)2 at any pH
is to add a ‘‘micelle promoting agent’’ [7], which forms
micelles with which the protonated stearic acid or stearate
anion can co-micellize. This is a common general strategy
in increasing the solubility of surfactants. Since the ten-
dency to co-micellize or form mixed micelles will depend
on the charge of the added surfactant, we will study added
anionic, nonionic, and amphoteric surfactants here.
A chelating agent is sometimes added into cleaning
products to prevent precipitation of active ingredients with
metal ions naturally found in the hard water by forming a
water soluble complex with the metal ions. In the system
studied here, the chelating agent was added to complex
with and promote the dissociation of calcium ion from the
solid calcium stearate so the dissociated stearate anion can
form micelles. There are several types of chelating agents
or complexing agents such as phosphates and aminocar-
boxylates (e.g., EDTA) [8]. Ethylenediaminetetraacetate
disodium salt, Na2EDTA, was used here. It is a common
complexing agent, which has four main active sites that can
form a water-soluble complex with metal ions. One mol-
ecule of Na2EDTA can chelate one molecule of calcium
ion stoichiometrically [9]. The effectiveness in metal
complex formation depends on the equilibrium constants or
Experimental
Materials
Sodium dodecyl sulfate (SDS) (99? % purity), obtained
from Fisher Scientific (Fair Lawn, NJ), was further purified
by recrystallization from water and then from methanol,
followed by drying under a vacuum at 30 °C. Octyl poly-
glycoside (C8APG) was obtained from Akzo Nobel and
used as received. Disodium ethylenediaminetetraacetate
(Na2EDTA) (100% purity) was obtained from Fisher Sci-
entific and used without further purification. Stearic acid
(99% purity), obtained from Alfa Aesar (Heysham, Lan-
caster), was used without further purification. Calcium
hydroxide (99.995% purity) was obtained from Sigma-
Aldrich (St. Louis, MO). Absolute ethyl alcohol (100%
purity) was obtained from AAPER (Shelbyville, KY).
Acetone (A.C.S. grade) was obtained from Fisher Scien-
tific. Dimethyldodecylamine oxide (DDAO) was obtained
from Stepan and used without further purification. The
NaOH and HCl were from Fisher Scientific (Fair Lawn,
NJ) and were used as received to adjust the solution pH.
Water was double deionized.
Methods
Calcium stearate, calcium salt of stearic acid, was prepared
from stearic acid. Stearic acid was first dissolved in ethanol
and then reacted with a clear solution of calcium
Fig. 1 Equilibrium solubility of Ca(C18)2 in SDS/Na2EDTA solutions
123

J Surfact Deterg (2010) 13:367–372
369
Calcium stearate was derivatized into methyl stearate by
using a derivatizing agent, BF3-methanol (Alltech, Deer-
field, IL). The derivatized solution was diluted by hexane
before being analyzed by GC.
0.04
0.035
0.03
0.1 M DDAO/0.1 M Na2EDTA
0.025
0.02
Results and Discussion
0.1 M DDAO
0.015
0.01
0.005
0
Chelant Only Systems
0.1 M Na2EDTA
5
The equilibrium solubility of Ca(C18)2 was measured in
solutions of different types of surfactants with and without
chelating agent (Na2EDTA) at 25 °C and at varying pH.
Figure 1 shows the equilibrium solubility of Ca(C18)2 in
0.1 M SDS, 0.1 M Na2EDTA, and a solution of 0.1 M
SDS/0.1 M Na2EDTA as a function of pH. Figure 2 shows
the equilibrium solubility of Ca(C18)2 in 0.1 M DDAO,
0.1 M Na2EDTA, and a solution of 0.1 M DDAO/0.1 M
Na2EDTA as a function of pH. From both Figs. 1 and 2,
the Na2EDTA alone has no significant effect on Ca(C18)2
solubility at any pH studied. Neither the protonated non-
ionic stearic acid (low pH) or the stearate anionic can attain
a high enough solubility to form micelles without co-sur-
factant, even if free calcium is largely complexed. There is
competition for calcium between the Na2EDTA and the
precipitated Ca(C18)2 and the extremely low Ksp of the
latter maintains the solid Ca(C18)2 even at very low free
calcium concentrations in the presence of the chelant at
high pH where the Na2EDTA is most effective. At the
lowest pH studied here (pH 4), the protonated form of the
fatty acid and the solid Ca(C18)2 will both be present. We
did not attempt to ascertain the fraction of the solid present
in either form since the focus of this study is the solubility
of the stearate or stearic acid, neither of which was sig-
nificant without co-surfactants. Table 1 summarizes the
possible forms of Ca(C18)2 that could be found at different
pH levels. The stearic acid could form because of low pH
by the protonation of stearate anion. The Na2EDTA will
also be protonated into less effective chelant forms at
low pH. At intermediate pH, calcium is released by the
4
6
7
8
9
10
11
12
pH
Fig. 2 Equilibrium solubility of Ca(C18)2
solutions
in DDAO/Na2EDTA
hydroxide. The precipitate was filtered and rinsed with
water, ethanol and acetone to remove excess calcium ion
and unreacted stearic acid. Finally, the precipitate was
dried in a vacuum oven at 30 °C.
The equilibrium solubility of calcium stearate in anio-
nic, nonionic, and amphoteric surfactants was measured at
25 °C. A series of surfactant solution was prepared at pH
4–11 using HCl and NaOH solutions. Sodium dodecyl
sulfate (SDS) was used as an anionic surfactant. Octyl
polyglycoside was used as a nonionic surfactant. Dimeth-
yldodecylamine oxide (DDAO) was used as an amphoteric
surfactant. An excess amount of Ca(C18)2 was added into a
mixed solution with other surfactants and Na2EDTA at the
pH of interest. The Ca(C18)2 was forced to dissolve by
heating it up to above 70 °C or until most of Ca(C18)2 was
dissolved. Then it was equilibrated at the temperature of
interest (25 °C) in a temperature-controlled water bath.
The solution was allowed to equilibrate for at least 1 week
with routine shaking to ensure that equilibrium was
reached. The solution was filtered using a 0.22-micron
Durapore hydrophilic membrane. Then the supernatant was
analyzed for the concentration of Ca(C18)2 by a gas chro-
matography (GC) (Varian 3300), using an SPB 20 column
(Supelco) with a FID detector following derivatization.
Table 1 Forms of soap scum, DDAO, ability of Na2EDTA to complex calcium, and solubility of Ca(C18)2
different pH levels
in surfactant/chelant system at
pH
Dominant form of soap scum
Dominant charge
on DDAO
Ability of Na2EDTA Solubility of Ca(C18)2 in
to complex calcium
surfactant/chelant system
SDS
DDAO
C8APG
Low
Protonated stearic acid (nonionic) Cationic
Low
Medium Low
Low
Intermediate (6–9) Protonated stearic acid (nonionic) Cationic and zwitterionic Medium
and stearate (anionic)
Low
Low
High
Medium
High
Stearate (anionic)
Zwitterionic
High
Very high High
123

370
J Surfact Deterg (2010) 13:367–372
protonation of Ca(C18)2. The calcium is also complexed
with chelant to some extent. The other molecules formed
from Ca(C18)2 are stearic acid and stearate anion. At higher
pH, even though the calcium is complexed effectively, the
dissolution of Ca(C18)2 is still considered very low since
the stearate anion has limited solubility, and at the tem-
perature of study (25 °C), it cannot form micelles alone to
increase its solubility.
above the CMC, the effective pKa is that in micellar form.
At the pH = pKa, half of the surfactant is in cationic form
and half in zwitterionic form. At pH below the pKa, the
cationic form dominates, and at pH above the pKa, the
zwitterionic form dominates. In addition to protonation of
the stearate and the DDAO, the effectiveness of com-
plexation of Na2EDTA varies with pH as previously dis-
cussed, with higher pH being more effective.
In explaining the pH effects on Ca(C18)2 dissolution,
three effects dominate as pH decreases; the protonated
nonionic stearic acid/anionic stearate anion ratio increases;
the cationic DDAO/zwitterionic DDAO ratio increases;
and complexation of calcium by Na2EDTA decreases. The
effect of micellar composition on the effective pKa of the
stearate is probably a secondary effect in explaining trends.
Charge repulsion of the head groups when anionic stearate
is incorporated into an SDS micelle would inhibit co-
micellization of stearate with SDS, so the increase in sol-
ubility of Ca(C18)2 at pH below 5 in a chelant-free SDS
solution is due to a higher fraction of the fatty acid being in
nonionic, protonated form. The highest synergism in mixed
micellization is for a cationic/anionic surfactant mixture
[14], explaining why the highest solubility in a chelant-free
surfactant system is DDAO at low pH where it is below its
pKa and primarily in cationic form, co-micellizing with
anionic stearate and (less synergistically) with protonated
stearic acid. The synergism between a zwitterionic and an
anionic surfactant is between that of an anionic/anionic and
a cationic/anionic surfactant mixture [14], explaining why
the solubility of Ca(C18)2 at high pH (where the DDAO is
primarily zwitterionic) is greater than that for SDS, but less
than that of DDAO at low pH in the absence of chelant.
Table 1 shows the ionic form of possible formed soap
components and of micellar DDAO at low, intermediate,
and high pH.
Surfactant Only Systems
In the absence of additives, the CMC of SDS is
6.7 9 10-3 M [10]. Extrapolating from reported CMC
values with added electrolyte [11, 12] to pure surfactant
values, the DDAO at low pH where it is in cationic form
has a CMC of 7.0 9 10-3 M, and the DDAO at high pH
where it is in zwitterionic form has a CMC of
5.4 9 10-3 M. The dissolved calcium and stearate ions
will reduce the CMC values below these pure surfactant
values.
In this study, the SDS and DDAO are present at least an
order of magnitude above their CMC, so a high fraction
([93%) of these surfactants are present as micelles. As
shown in Figs. 1 and 2, in the absence of chelating agent,
both SDS and DDAO increased the solubility of Ca(C18)2
slightly, with DDAO causing higher solubility than SDS at
all pH levels. The SDS effect had little pH dependence
except at pH below 5 where solubility of Ca(C18)2
increased. The DDAO caused a monotonic increase in
solubility in Ca(C18)2 with decreasing pH, exhibiting an
order of magnitude greater solubility at pH of 4 compared
to SDS. At high pH (pH 7–11), the solubility of Ca(C18)2 in
DDAO is approximately five times higher than that in SDS.
The charge on the stearate/stearic acid molecule
becomes less negative (high fraction of protonated or
nonionic surfactant) as pH decreases as already discussed
with a pKa around 4.5. The effective pKa can alter upon
incorporation of the stearate into mixed micelles depending
on the charge of the co-surfactant, increasing when co-
micellizing with an anionic surfactant like SDS [13].
The DDAO is an amphoteric surfactant, which can exist
in the form of a cationic or a zwitterionic surfactant
depending on the solution pH as shown by the protonation
reaction:
At low pH, the main soap scum component that forms
mixed micelles with the added soluble surfactant is non-
ionic stearic acid. The formation of mixed micelles is
expected to be most effective in either cationic (DDAO at
low pH) or anionic surfactant micelles since cationic/non-
ionic or anionic/nonionic surfactant synergism is present.
At intermediate pH, stearic acid and stearate anion form
mixed micelles with added surfactant. The DDAO is
composed of both cationic and zwitterionic surfactants.
The formation of mixed micelles when DDAO is used can
be enhanced due to the mixed cationic (DDAO) and
anionic (stearate) surfactants. However, the solubility of
Ca(C18)2 is lower than at low pH because stearate solubility
is so low in equilibrium with Ca(C18)2 when no calcium is
complexed. Also, less protonated stearic acid is formed
than at low pH.
The pKa of DDAO monomer is reported at about 5 [12],
but the effective pKa can be much higher in micelle form,
even higher when co-micellized with an anionic surfactant
[12]. Since 0.1 M DDAO is about two orders of magnitude
At high pH, the soap scum mostly remains as an
undissolved solid precipitate since the solubility of soap
123

J Surfact Deterg (2010) 13:367–372
371
scum is very low. Even if there is any stearate anion, there
is still an electrostatic repulsion between stearate and SDS
in micelles so the solubility of Ca(C18)2 is low. The DDAO
is in completely zwitterionic form and would form micelles
synergistically with stearate, but stearate solubility is so
low in equilibrium with Ca(C18)2 with no chelation.
lower solubility for the DDAO/Na2EDTA system com-
pared to DDAO only at pH 4 (Fig. 2) defies apparent
explanation since the chelant might be expected to have no
effect on solubility at low pH, but not to cause it to
decrease.
For the SDS system, the synergism for the chelant/sur-
factant mixture increases as pH decreases (Figs. 1, 3) in
contrast to DDAO. Here, the more efficient co-micelliza-
tion of the nonionic, protonated stearic acid into the anionic
SDS micelles relative to co-micellization of anionic stea-
rate is more important than the decrease in complexation
effectiveness of the chelant as pH decreases. However, it is
important to note that the maximum solubility of Ca(C18)2
in the SDS/chelant system is over an order of magnitude
less than for the DDAO/chelant system, so lack of effec-
tiveness of chelation at low pH greatly inhibits freeing of
the stearic acid/stearate molecule from the solid Ca(C18)2
to permit them to co-micellize.
Surfactant-Chelant Systems
In the presence of Na2EDTA, the highest solubility is with
DDAO at high pH (Figs. 2, 3, 4), where it is several orders
of magnitude greater than in the chelant-free system
(Fig. 2). The solubility increases monotonically with pH,
the opposite trend than for the chelant-free system. So as
long as there are zwitterionic or cationic micelles with
which the stearate can co-micellize, the complexation of
calcium by a chelant is crucial to improving solubility of
the Ca(C18)2, and the increasing effectiveness of com-
plexation of calcium by Na2EDTA at high pH dominates
pH effects, leading to maximum solubility at high pH. The
More limited studies were carried out using the nonionic
surfactant C8APG compared to SDS and DDAO as shown
in Figs. 3 and 4 for the C8APG/chelant system (chelant-
free C8APG was not studied). The solubility of Ca(C18)2 in
the C8APG system shows increasing solubility of Ca(C18)2
with increasing pH as with the DDAO system, although at
high pH where maximum solubility is observed, total sol-
ubility is about a factor of five less for C8APG than DDAO.
Since anionic/nonionic mixed micelles are less synergistic
than anionic/zwitterionic mixed micelles, the stearate/
C8APG synergism is less than the stearate/DDAO at these
high pH conditions, explaining this large amphoteric sur-
factant effectiveness. Aiding this performance difference is
that the DDAO has a much larger hydrophobe than the
C8APG, forming micelles that co-micellize more effec-
tively. Similarly, at low pH, the SDS system shows higher
Ca(C18)2 solubility than the C8APG system, explainable by
the greater synergism of the nonionic/anionic mixed
micelles composed of protonated nonionic stearic acid/
SDS compared to the nonionic/nonionic protonated stearic
acid/C8APG mixed micelles. Table 1 summarizes the
effects of pH on the chelation, surfactant structure and
equilibrium solubility of the soap scum in surfactant/che-
lant systems.
0.012
0.01
0.1 M C8APG / 0.1 M Na2EDTA
0.008
0.006
0.1 M SDS / 0.1 M Na2EDTA
0.004
0.002
0
4
5
6
7
8
9
10
11
12
pH
Fig. 3 Equilibrium solubility of Ca(C18)2 in solutions of 0.1 M of
SDS/0.1 M Na2EDTA and 0.1 M C8APG/0.1 M Na2EDTA
Hydrolysis of SDS
Since equilibration experiments took 1 week at a pH as low
as 4, the possible effect of the well-known hydrolysis
reaction of SDS (to dodecanol) [15–17] on our results
needs to be addressed. At 25 °C and pH 4 above the CMC,
using hydrolysis rate constants [15], the reaction conver-
sion is approximately 99% of that at equilibrium. However,
at equilibrium, the reaction proceeds to such a small level
as to be undetectable [15]. So the reaction is controlled by
Fig. 4 Equilibrium solubility of Ca(C18)2 in solutions of 0.1 M SDS/
0.1 M Na2EDTA, 0.1 M C8APG/0.1 M Na2EDTA, and 0.1 M
DDAO/0.1 M Na2EDTA
123

372
J Surfact Deterg (2010) 13:367–372
of synthetic anionic surfactant and soap. I. Effect of sodium
octanoate on hardness tolerance of sodium dodecyl sulfate.
J Surfact Deterg 1:321–328
equilibrium, not kinetic considerations. At pH levels below
about 3.5, hydrolysis can be significant.
14. Rosen MJ (2004) Surfactants and interfacial phenomena, 3rd edn.
Wiley, New Jersey
Acknowledgments Financial support for this work was provided by
the industrial sponsors of the Institute for Applied Surfactant
Research at the University of Oklahoma, including Akzo Nobel,
Clorox, Conoco/Phillips, Church and Dwight, Dow, Ecolab, Halli-
burton, Huntsman, Oxiteno, Procter & Gamble, Sasol, SC Johnson,
and Shell. Dr. Scamehorn holds the Asahi Glass Chair in the School
of Chemical, Biological and Materials Engineering at the University
of Oklahoma. Dr. Sabatini holds the Sun Oil Company Chair in the
School of Civil Engineering and Environmental Science at the Uni-
versity of Oklahoma.
15. Nakagaki M, Yokoyama S (1985) Acid-catalyzed hydrolysis of
sodium dodecyl sulfate. J Pharm Sci 74:1047–1052
16. Motsavage VA, Kostenbauder HB (1963) The influence of the
state of aggregation on the specific acid-catalyzed hydrolysis of
sodium dodecyl sulfate. J Colloid Sci 18:603–615
17. Lunkenheimer K, Czichocki G (1993) On the stability of aqueous
sodium dodecyl sulfate solutions.
160:509–510
J Colloid Interface Sci
Author Biographies
References
Sukhwan Soontravanich received a BS in Chemical Engineering
and MS in Petroleum Engineering from Chulalongkorn University in
Bangkok, Thailand, and a PhD in Chemical Engineering from the
University of Oklahoma. She is a senior chemist at the Asia
Technology Center, Ecolab, Thailand, working on product develop-
ment and applications.
1. Pandit NK, Strykowski JM (1989) Effect of pH and temperature
on the solubility of a surface active carboxylic acid. J Pharm Sci
78:767–770
2. de Mul MNG, Davis HT, Evans DF, Bhave AV, Wagner JR
(2000) Solution phase behavior and solid phase structure of long-
chain sodium soap mixtures. Langmuir 16:8276–8284
3. National Organic Standards Board Technical Advisory Panel
(TAP) Review, http://www.ams.usda.gov/nop/NationalList/TAP
Reviews/CalciumStearate2.pdf. [cited June 2007]
4. Kanicky JR, Shah DO (2003) Effects of premicellar aggregation
on the pKa of fatty acid soap solutions. Langmuir 19:2034–2038
5. Sukhija PS, Palmquist DL (1990) Dissociation of calcium soaps
of long-chain fatty acids in rumen fluid. J Dairy Sci 73:1784–
1787
6. Bagby MO (1993) Carboxylic acids (survey). In: Kroschwitz JI,
Howe-Grant M (eds) Kirk-Othmer Encyclopedia of Chemical
Technology, vol 5. Wiley, New York, pp 77–88
7. Scamehorn JF, Harwell JH (2004) Precipitation of surfactant
mixtures. In: Abe M, Scamehorn JF (eds) Mixed surfactant sys-
tems, 2nd edn. Marcel Dekker, New York, pp 601–655
8. Tai LHT (2000) Formulating detergents and personal care
products a complete guide to product development. AOCS Press,
Champaign
9. Martell AE, Motekaitis RJ (1992) Determination and use of
stability constants, 2nd edn. VCH Publishers, Inc., New York
10. Stellner KL, Scamehorn JF (1989) Hardness tolerance of anionic
surfactant solutions. I. Anionic surfactants with added monova-
lent electrolyte. Langmuir 5:70–77
Heyde E. Lopez graduated from the University of Oklahoma with a
BS in Chemical Engineering in 2005. She had two internships with
Playtex Products when a student. After graduation, she worked for
over 3 years for 3M and is currently a process engineer for the Fabrics
Division of WL Gore and Associates.
John F. Scamehorn has a BS and MS from the University of
Nebraska and a PhD from the University of Texas. He has worked for
Shell, Conoco, and DuPont, and is currently a Professor emeritus at
University of Okklahoma, where he is director emeritus of the
Institute of Surfactant Research. He has been working on surfactant
systems and applications to consumer product formulation and
separation processes.
David A. Sabatini has a BS from the University of Illinois, a MS
from Memphis State University, and a PhD from Iowa State
University. He is currently a Professor at the University of Oklahoma,
where he is the associate director of the Institute of Surfactant
Research. He is working on surfactant-based remediation and
formulation, and advanced microemulsions for cleaning systems.
11. Rathman JF, Christian SD (1990) Determination of surfactant
activities in micellar solutions of dimethyldodecylamine oxide.
Langmuir 6:391–395
12. Soontravanich S, Scamehorn JF, Harwell JH, Sabatini DA (2008)
Interaction between an anionic and an amphoteric surfactant. Part
I. monomer-Micelle equilibrium. J Surfact Deterg 11:251–261
13. Rodriguez CH, Chintanasathien C, Scamehorn JF, Saiwan C,
Chavadej S (1998) Precipitation in solutions containing mixtures
David R. Scheuing received his BS from the University of Illinois at
Chicago and is currently Research Director in the Advanced
Technology group at the Clorox Company. His research interests
include the application of vibrational spectroscopy to the character-
ization of self-assembled colloidal systems and their interactions with
solid surfaces.
123