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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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