Surface properties of butanol phosphate esters in alkali solutions

Article abstract of DOI:10.1007/s11743-009-1145-3

Phosphate esters of two butanol isomers were synthesized by esterification of n-butanol and isobutanol with phosphorus pentoxide. The surface activity and penetrability of the esters were analyzed by way of surface chemistry and canvas disc wetting tests in different alkaline solutions. The surface tension of the butanol phosphate esters and their sodium salts was found to decrease with increasing alkaline concentration up to 250 g/L NaOH. Furthermore, the products exhibited a good surface activity with proper penetrability even in highly alkaline solutions. The stable surface activity of the esters in concentrated alkali lye interpreted by the adsorption of the molecule from solution owing to the common ion effect. AOCS 2009.

Full text of DOI:10.1007/s11743-009-1145-3

J Surfact Deterg (2010) 13:201–206
DOI 10.1007/s11743-009-1145-3
ORIGINAL ARTICLE
Surface Properties of Butanol Phosphate Esters
in Alkali Solutions
Zong-liang Du Æ Dong-liang Zhou Æ Yong Chen Æ
Min Chen Æ Pu-xin Zhu
Received: 3 December 2007 / Accepted: 8 May 2009 / Published online: 9 July 2009
Ó AOCS 2009
Abstract Phosphate esters of two butanol isomers were
synthesized by esterification of n-butanol and isobutanol
with phosphorus pentoxide. The surface activity and pen-
etrability of the esters were analyzed by way of surface
chemistry and canvas disc wetting tests in different alkaline
solutions. The surface tension of the butanol phosphate
esters and their sodium salts was found to decrease with
increasing alkaline concentration up to 250 g/L NaOH.
Furthermore, the products exhibited a good surface activity
with proper penetrability even in highly alkaline solutions.
The stable surface activity of the esters in concentrated
alkali lye interpreted by the adsorption of the molecule
from solution owing to the common ion effect.
Introduction
Alkyl phosphates, a class of anionic surfactants with good
surface activity, good stability, antistatic property, wetta-
bility, compatibility, and little toxicity and irritation, play
an important role in many fields such as chemical fiber,
textile, leather, plastic, papermaking, cosmetic, and so on.
Phosphorus pentoxide is usually used as a phosphatating
agent for phosphatization of alcohols to prepare the product
that is composed of monophosphate and diphosphate esters
[1–3]. Generally, the monophosphate ester has a better
water-solubility, emulsifier, and antistatic ability, while the
diphosphate ester results in a better softness. Up to now,
researches into alkyl phosphates have dealt with how to
optimize the structure [4, 5] and how to improve their
biodegradation [2, 3]. In addition, surface properties of
phosphate ester solutions have been studied extensively,
such as the phase behavior, micelle formation, surface
adsorption and surface activity [6–8].
Keywords Butanol phosphate ester ꢀ Surface tension ꢀ
Surface adsorption ꢀ Penetrability ꢀ Alkaline solution
Abbreviations
C
Number of carbon atoms in the hydrophobic chain
of the phosphate molecule
Previous studies mainly focused on the alkyl phosphates
with alkyl chain length in the C7–C18 range, and less
attention was paid to the alkyl phosphates with less than
seven carbon atoms in their alkyl chain. Many industrial
applications involve a high concentration of electrolyte in
solution, such as strong alkali and/or salt. For example, in
the mercerizing treatment of cotton fabric before dyeing,
an alkali solution of 220–260 g/L NaOH is always used,
and it is difficult to find a suitable surfactant to promote the
penetration of the alkali solution into the fabric because the
solubility of conventional surfactants decreases greatly in
highly concentrated electrolyte solutions. In this study, n-
butanol and isobutyl alcohol phosphate esters (n-BPE and
iso-BPE) were synthesized by phosphatization of alcohols,
as a means to improving the surfactant stability and the
penetration of alkali solutions. The surface tension of the
BPE
tex
Butanol phosphate ester
A unit of measure for the yarn mass density of
fibers, defined as the mass in gram per 1,000
meters
CMC Critical micelle concentration
Z. Du ꢀ D. Zhou ꢀ Y. Chen ꢀ M. Chen ꢀ P. Zhu (&)
Textile Institute, Sichuan University, Chengdu 610065,
Sichuan Province, China
e-mail: zhupxscu@163.com
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J Surfact Deterg (2010) 13:201–206
Neutralization of BPEs
resulting solutions at various surfactant concentrations was
measured (at different alkali concentrations) before and
after neutralization, to evaluate the surface properties.
Furthermore, the mechanism of the surface activity for the
resultants at high alkali concentration was interpreted by
using the Gibbs adsorption equation.
An exact amount of an aqueous solution of 30% NaOH was
used to neutralize the BPEs at 50 °C for 2 h, and to adjust
the pH of the resultant to 7.0. The product was put into a
tapped glass bottle.
Surface Tension Testing
Experimental Procedures
Surface tension was measured by the Wilhelmy plate
technique with a Sigma 703 tensiometer (KSV, Finland). A
cylinder made of Teflon with an inside diameter of 70 mm
was used as the container of tested solutions.
Materials
Chemicals, such as n-butanol, isobutyl alcohol, phosphorus
pentoxide (P2O5), sodium hydroxide (NaOH), absolute
alcohol, and other analytical indicators were of analytical
reagent grade. Freshly re-distilled water was used in all
experiments.
The container was cleaned three times with anhydrous
ethanol, then was rinsed with re-distilled water, and air-
dried. Next 50 ml of BPE solution was slowly poured into
the container. The surface tension was measured at equi-
librium. The critical micelle concentration (CMC) and
corresponding surface tension were obtained from the plot
of the surface tension versus BPE concentration. The
relationships between surface tension and concentration of
n-BPE or iso-BPE solution before and after neutralizing,
and at different alkaline concentrations, were also
determined.
Unscoured cotton canvas with yarn number 28 tex 9
4/28 tex 9 4 and set 140 9 110/10 cm was used for the
canvas disc wetting test.
Phosphatization of Butanol
A certain amount of either n-butanol or isobutyl alcohol
was placed in a dried three-necked flask equipped with a
stirrer and a thermometer. The flask was heated at 50 °C,
then P2O5 was gradually added to the flask under agitation
within a period of 1.5 h with a 1:3 P2O5:butanol mole ratio.
The system was then heated to 70–75 °C and maintained at
this temperature for 5 h. The product, n-butanol phosphate
(n-BPE) or isobutyl alcohol phosphate (iso-BPE), which
was a mixture of alkyl mono and diesters, was kept in a
tapped glass bottle for use in neutralization and testing
experiments. Since it is the kind of mixture used directly in
industry, the product was not purified further.
Penetrability Testing
The canvas subsidence time was used to characterize the
penetrability of BPEs, according to the procedure used by
Yang et al. [9]. This method repeatability was improved as
follows: 800 ml of BPE solution at different concentrations
were prepared in a 1-L beaker (8.75 cm height), and the
canvas was cut into (30 mm diameter) wafers. A pin
weighting 92.12 mg was placed at the center of the wafer.
The time was recorded from the moment at which the
canvas wafer contacted the solution surface to the moment
at which the pin sank to the bottom of the beaker. The test
was repeated ten times for each sample and the average
subsidence time was estimated. The shorter the time is, the
better is the penetrability of the surfactant solution.
Component Analysis of Monophosphate and
Diphosphate Esters
The product of the reaction of P2O5 with a fatty alcohol is
always a mixture of monophosphate and diphosphate esters
with a small quantity of free phosphoric acid, which can be
quantified by the potentiometric titration on the basis of 1-,
2-, and 3-stage ionization constants of the phosphoric acid
[1]. The titration results indicated that the mole ratio of
monophosphate to diphosphate was about 2:1, and that the
esterification yield was over 90%. For the n-BPE, the
contents of monophosphate and diphosphate esters were
66.7 and 33.3%, respectively; for the iso-BPE, they were
60.7 and 35.7%, respectively, together with 3.6% of free
phosphoric acid.
Limited Area of a BPE Molecule Adsorbed at the Air/
Water Interface
The surface excess concentration (C) was calculated by the
Gibbs adsorption equation (Eq. 1) and the data of surface
tension versus concentration of BPE. Next the area occu-
pied by an adsorbed molecule, a, was calculated using
Eq. 2.
ꢀ
ꢁ
1
dc
RT d ln c
C ¼ ꢁ
ð1Þ
T
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203
1
Table 1 Adsorption data of BPE aqueous solutions
a ¼
ð2Þ
C ꢀ 6:023 ꢂ 1023
2
˚
Sample
dc/dln c
U (10-6 mol/m2)
a (A )
where c (in mol/L) is the mean concentration of the BPE
solution calculated with the average ratio of monophos-
phate to diphosphate; c (in mN/m) is the equilibrium sur-
face tension at c; R is the gas constant; T is the absolute
temperature (K); dc/dln c is the slope of c-ln c plot at
concentrations just before CMC, which is characteristic
of the efficiency of a surfactant to reduce surface tension;
Unneutralized n-BPE
Unneutralized iso-BPE
Neutralized n-BPE
-5.58
-4.53
-5.38
-3.81
2.25
1.83
2.17
1.54
73.7
90.9
76.5
Neutralized iso-BPE
108.0
As seen from Table 1, the adsorption area of iso-BPE is
larger than that of n-BPE. This is probably related to the
structure isomerization; in other words, the branched
hydrophobic chains of iso-BPE inhibit the molecule com-
pactation at the air/water surface. In addition, the data
indicate, as expected, that the neutralized BPEs have a
larger absorption area. This is certainly due to the inter-
molecular electrostatic repulsion due to a full ionization of
BPE after neutralization, which also increase the solubility
in water.
2
2
˚
C (mol/m ) and a (A ) are the mean surface excess con-
centration and the area of a BPE molecule adsorbed at the
surface, respectively.
Results and Discussion
The Variation of Surface Tension Versus BPE
Concentration
Figure 1 showed typical c–ln c curves for esters with
less than seven carbon atoms in their alkyl group. Because
of their high solubility in water, both n-BPE and iso-BPE
do not exhibit a CMC in the figure concentration range. In
other words, BPEs have a weak surface activity, no matter
whether neutralized or unneutralized.
Figure 1 shows the variation of surface tension, c, versus
BPE concentration, as ln c, at 25 °C. The figure, indicates
that the variation trend is the same for all cases, i.e., the
surface tension decreases with increasing the BPE con-
centration. At the same concentration along the curves, the
surface tension of n-BPE solution is slightly lower than
those of iso-BPE solution, which means that linear BPE has
a better surface activity than the branched one. On the other
hand, the surface tension of each neutralized BPE solution
is higher in comparison with the corresponding unneu-
tralized case, indicating a better solubility and lower
adsorption at the air/water interface for the ionized BPEs.
The data in Table 1 indicate the mean surface excess
concentration and molecular area of BPE adsorbed at
solution surface, calculated from Eqs. 1 and 2 and Fig. 1.
Surface Activity at Different Alkaline Concentrations
Generally speaking, the solubility of an anionic surfactant
would decrease to a great extent in a highly concentrated
alkaline solution. This is due to the salting out effect of
electrolyte, even resulting in a failure to dissolve in water.
On the contrary, BPEs exhibit a distinct surface property in
alkali lye. Figures 2, 3, 4 and 5 indicate the variation of
surface tension versus BPE concentration at different
Fig. 2 Relationship between concentration and surface tension of
unneutralized iso-BPE at different NaOH concentrations
Fig. 1 Relationship between surface tension and concentration of
different BPE solutions
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Fig. 5 Relationship between concentration and surface tension of
neutralized n-BPE at different NaOH concentrations
Fig. 3 Relationship between concentration and surface tension of
neutralized iso-BPE at different NaOH concentrations
the other hand, the results in Table 2 indicate that the
surface excess concentration of iso-BPE increases along
with the augmentation of alkaline concentration, which is
favoring the surface activity.
Figure 4 shows the surface tension curves of unneu-
tralized n-BPE at different concentrations of NaOH. The
trends are similar to the iso-BPE case, and the surface
tension is also efficiently reduced at 250 g/L of NaOH.
Figure 5 shows the surface tension curves of neutralized
n-BPE at different concentrations of NaOH, which also
indicates an improvement in surface activity after neu-
tralization. From Table 3, it is seen that the molecular
adsorption areas of n-BPE and iso-BPE are different. The
n-BPE areas before and after neutralization vary little with
the change in alkaline concentration, while for iso-BPE, the
areas decrease gradually with increasing alkaline concen-
tration according to Table 2. This means that n-BPE
species have a better surface activity than iso-BPE coun-
terparts even at low alkaline concentration.
Fig. 4 Relationship between concentration and surface tension of
unneutralized n-BPE at different NaOH concentrations
alkaline concentrations and 25 °C. It is seen that the sur-
face tension decreases with an increase of alkaline con-
centration in the whole range from 0 to 250 g/L NaOH.
This implied that BPEs exhibit a high degree of resistance
to alkali which is a very suitable feature for applications.
However, the CMC values of all the BPEs at alkali con-
centration of 10 g/L are out of the range in Figs. 2, 3, 4 and
5, which is indicative of a high solubility of BPEs in the
low alkalinity solutions. The mean surface excess con-
centration and corresponding adsorption area per BPE
molecule are calculated from Figs. 2, 3, 4 and 5 data and
listed in Tables 2 and 3.
The previous discussion shows that both the CMC and
the corresponding surface tension decrease with the
increase of alkali concentration for BPEs. When the
concentration of NaOH reaches 250 g/L, the CMC values
of n-BPE and iso-BPE are 0.15 and 0.14 g/L, respectively,
along with the surface tension 23.0 and 23.1 mN/m,
respectively. In general, the surface tension of conventional
surfactants at the CMC point is in the range 30–35 mN/m,
and more importantly, the surface activity tends to fall
because of a lower solubility as NaOH concentration
exceeds 50 g/L. Therefore, the surface activity of BPEs in
concentrated alkaline solution is unusually high, maybe as
a consequence of the short hydrocarbon chain which
promotes an excellent solubility in water. Because of the
common ion effect in alkaline solution, the solubility of
BPEs gradually decreases along with the increase in
As shown in Figs. 2 and 3, both the neutralized and
unneutralized iso-BPEs exhibit a surface activity which
rise with the alkaline concentration. When the concentra-
tion of NaOH came to 250 g/L, even a small quantity of
iso-BPE is able to effectively lower the surface tension. On
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205
Table 2 Adsorption data of
iso-BPE at different NaOH
concentrations
2
˚
Sample
NaOH concentration dc/dln c U (10-6 mol/m2) a (A ) CMC ccmc
(g/L)
(g/L) (mN/m)
Unneutralized iso-BPE
10
50
-5.42
-6.16
-6.71
-6.70
-6.76
-4.76
-5.84
-6.46
-7.73
2.19
2.49
2.71
2.70
2.73
1.92
2.35
2.61
3.12
76.0
66.8
61.4
61.5
60.9
86.4
70.5
63.7
53.2
–
–
0.85
0.69
0.38
0.37
–
36.9
31.5
24.3
23.1
–
100
200
250
10
Neutralized iso-BPE
100
200
250
0.52
0.23
0.14
34.0
27.9
23.1
alkaline concentration, and the molecules are forced to
adsorb at the air/water interface, which results in an
unexpected surface activity in concentrated alkaline
solution.
measurement and the results were compared to the non-
alkaline case. Table 4 shows that the BPEs exhibited an
excellent penetrability in concentrated alkaline solutions.
In contrast, in non-alkaline solution BPEs have almost no
penetrability. In comparison with iso-BPE, n-BPE has a
slightly better penetrability. The data definitively indicate
that BPEs are a good kind of penetrant that can be used in
concentrated alkali lye for the mercerizing process in the
textile industry.
Tables 2 and 3, and Figs. 2, 3, 4 and 5 indicate that there
are differences in CMC and ccmc between neutralized and
unneutralized products. Under a certain NaOH concentra-
tion, the CMC value of unneutralized BPE is higher and the
ccmc is lower with respect to the neutralized one. It means
that unneutralized BPEs are likely to adsorb at the air/
solution interface owing to the salt effect, rather than to
stay in the solution to form micelles. The phenomena
reflect a stronger hydrophobicity for the unneutralized
BPE, though it could be neutralized in situ at high alkaline
concentration. This may be due to the weak solubility of
diphosphate ester in concentrated alkaline solution, which
could inhibit the neutralization of BPEs at room tempera-
ture. As a consequence, it is important to neutralize BPEs
at 50 °C for 2 h.
Because of their short hydrophobic chain, BPEs are not
effectively able to form micelles in water. As a result, BPE
aqueous solutions exhibit a weak or non-existent surface
activity in pure water. In the case of concentrated alkali
lye, however, a lyophobic behavior becomes obvious
owing to the common ion effect, which promotes the
adsorption at the air/solution interface and hence decreases
the surface tension. BPEs have a favorable penetrability in
quite concentrated alkaline solutions, along with an alkali
resistance up to 250 g/L NaOH. This property can meet the
demand of industries where concentrated alkaline solutions
are employed, and can be considered of great value for
some actual processes. For instance, this feature is suitable
for a penetrant in the cotton pre-treatment process and for
cotton mercerization.
Penetrability
The penetrability of BPEs in (250 g/L NaOH) concentrated
alkaline solution was tested by using the canvas subsidence
Table 3 Adsorption data of
n-BPE at different NaOH
concentrations
2
˚
Sample
NaOH concentration dc/dln c U (10-6 mol/m2) a (A ) CMC ccmc
(g/L)
(g/L) (mN/m)
Unneutralized n-BPE
10
50
-5.63
-5.80
-5.43
-5.36
-5.94
-5.73
-6.50
-5.41
-5.48
2.27
2.34
2.19
2.16
2.40
2.31
2.97
2.26
2.21
73.1
70.9
75.8
76.8
69.3
71.8
56.0
73.6
75.1
–
–
0.93
0.77
0.53
0.44
–
31.5
29.3
24.0
22.8
–
100
200
250
10
Neutralized n-BPE
100
200
250
0.49
0.23
0.15
33.5
25.1
23.0
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Table 4 Penetrability of BPEs
Sample
Concentration NaOH
(g/L) concentration time
Subsidence
(g/L)
iso-BPE Unneutralized 2.5
250
250
0
13006
Neutralized
2.5
5.0
220059
[1.5 h
11007
200022
340420081
n-BPE
Unneutralized 2.5
250
250
0
Neutralized
2.5
5.0
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The BPEs solubility in water or alkaline lye is improved
by neutralization, so as to enlarge their application fields.
In either the presence or absence of alkali, the surface
activity of n-BPE is found to be slightly better than that of
iso-BPE. For both n-BPE and iso-BPE the adsorption
density increases gradually with the increase in alkaline
concentration, with a concomitant reduction in the area per
adsorbed molecule.
Author Biographies
Zong-liang Du is an associate professor at the Textile Institute,
Sichuan University, China. He received his Ph.D. in Materials Sci-
ence from Sichuan University in 1999. His research interests include
biosurfactants, emulsion polymerization and waterborne coatings.
Acknowledgments The authors thank the National High-tech R&D
Program (Grant No. 2007AA03Z344) and the Chinese Natural Sci-
ence Foundation (Grant No. 50673062) for their financial supports.
Dong-liang Zhou received his Master’s degree in Materials Science
in 2008 at Sichuan University, China. He is engaged in research
involving polymeric surfactants and polymer/clay composites applied
to textiles.
Yong Chen received his Master’s degree in Materials Science in 2008
at Sichuan University, China. His research interest is surfactants and
organosilicon compounds applied to textiles.
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