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Journal of Colloid and Interface Science 331 (20 09) 335342

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science

www.elsevier.com/locate/jcis

Gelling nature of aluminum soaps in oils

Xiaorong Wang , Mindaugas Rackaitis

Bridgestone Americas, Center for Research and Technology, 1200 Firestone Parkway, Akron, OH 44317, United States

a r t i c l e i n f o a b s t r a c t

Article history:

Received 11 July 2008

Accepted 13 November 2008

Available online 20 November 2008

Keywords:

Aluminum soap

Soap-hydrocarbon gels

Rheology

Electron microscopy

Fractal network

Colloidal micelle particle

Jamming

Aluminum soaps are notable for their ability to form soap-hydrocarbon gels of high viscosity. For

more than half a century, it has been believed that the gelling mechanism is due to a formation of

polymeric chains of aluminum molecules with the aluminum atoms linking along the axis and with thefatty acid chain extended sideways. Here we report results from an investigation using high-resolution

electron microscopy and rheology measurements that clearly resolve the ambiguity. Our results reveal

that the gelling mechanism stems from the formation of spherical nano-sized micelles from aluminum

soap molecules, and those colloidal micelle particles then aggregate into networks of highly fractal

and jammed structures. The earlier proposed polymer chain-like structure is definitely incorrect. The

discovery of aluminum soap particles could expand application of these materials to new technologies.

2008 Elsevier Inc. All rights reserved.

1. Introduction

Soap is a surfactant used with water for washing and clean-ing. The earliest record of producing such materials dates back

to 2800 BC, when a process known as saponification [1] was

used to produce sodium or potassium-fatty carboxylates. At mod-

ern time, soaps are usually manufactured via metathesis processes

in aqueous solutions. A systematic investigation of their chemical

and physical properties only started from the late of 1930s [2],

when metal carboxylates in groups 1 to 8 of the periodic table

were well prepared in pure state for the first time. Many metal

carboxylates, as we know them today, cannot be used for wash-

ing and cleaning, though they are still called as soaps. Examples

are metal carboxylates formed from groups 3 to 8 in the peri-

odic table. They are mostly covalently bonded and typically water-

insoluble, however, are rather soluble in organic solvents. Among

them, aluminum soaps have received the most noticeable atten-

tion because they can effectively thicken many organic solvents

and oils [26]. The excellent thickening power of the aluminum

soaps can be illustrated by the fact that with 0.3% aluminum soap,

the relative viscosity of hydrocarbons can rise as high as 30. In

addition, a rigid gel can be obtained with the soap concentration

as low as 3% [3]. Because of chemically inert, odorless and non-

toxic, aluminum soaps have been used extensively in manufacture

of greases, paints, gels, cosmetics, drugs and foods [7,8]. During

World War II, they were used as the thickening agents for mak-

ing stick fuels for flame-throwers and incendiary gels for napalm

* Corresponding author.E-mail address:[email protected](Xr. Wang).

bombs [9]. They were also used in petroleum industrials for clos-

ing pores in water-sensitive zones [10].

Aluminum soaps of industrial grade usually contain variousamounts of aluminum hydroxide, free fatty acids, aluminum mono-

and di-carboxylates [4]. Among them the effective components

are the aluminum di-carboxylates (or di-soaps) [11]. Many stud-

ies [2] have shown that aluminum di-soaps from dibutyrate to

distearate have very similar properties in organic solutions, differ-

ing in degree but not in kind. All of them can swell and dissolve in

hydrocarbons to form transparent gels. Addition of small amount

of polar chemicals, such as alcohol, cellosolves, or xylenols, can

change the gel to jelly or even to viscous sol[1220]. It would thus

appear that the aluminum soaps might exist as polymers of high

molecular weight. Sheffer[3] measured both viscosity and osmotic

pressure in diluted solutions (104 molar) in order to determine

apparent molecular weights. He found that the aggregation num-

ber of aluminum di-soaps, from the octanote to the octadecanoate,in benzene solution was about between 500 and 1000 molecules.

Gray [12] proposed a polymeric structure of aluminum molecules

in explanation of their ability to thicken hydrocarbons based on

an octahedral coordination. This concept was later extended by

McGee [13] with adjacent chains held together by van der Waals

forces and hydrogen bondings. Bauer and co-workers[14,15]based

upon their infrared studies advanced this proposal in which they

suggested that aluminum had a coordination of six, and the octa-

hedral was jointed by coordinate bonding through both hydroxyl

groups and carboxylate groups of the fatty acid from which the

soap is derived. Leger et al. [18] conducted a light scattering study

of aluminum distearate in diluted benzene. They found the ratio

of molecular weight to the square of the size of the soap aggre-

0021-9797/$ see front matter 2008 Elsevier Inc. All rights reserved.

doi:10.1016/j.jcis.2008.11.032
http://www.sciencedirect.com/http://www.elsevier.com/locate/jcismailto:[email protected]://dx.doi.org/10.1016/j.jcis.2008.11.032http://dx.doi.org/10.1016/j.jcis.2008.11.032mailto:[email protected]://www.elsevier.com/locate/jcishttp://www.sciencedirect.com/

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336 Xr. Wang, M. Rackaitis / Journal of Colloid and Interface Science 331 (2009) 335342

Fig. 1. Earlier proposed polymer chain-like structure[318,20]of aluminum soap molecules. The jelly-like transparent material in the bottle is a toluene solution containing

2.5 wt% aluminum di-laurate.

gate is approximately constant and then interpreted the results as

formation of linear chains of random coil shapes in the solution.

Since then, for more than half a century, such a centipede

chain-like structure with the aluminum atoms along the axis and

with the fatty acid chain extended sideways, as shown in Fig. 1,

has received wide acceptance among theorists and experimental-

ists[2022], though a rigorous prove is lacking. To the best of our

knowledge, at present there is no conclusive evidence that alu-

minum soaps in hydrocarbons do take this polymeric chain-like

structure. In this contribution we report an experimental inves-tigation of aluminum di-laurate soap in an attempt to verify the

proposed gelling mechanism. Here we report our new discovery

based on this study and briefly consider their implication on the

nature of aluminum-soap solutions.

2. Materials preparation

Lauric acid (C12H24O2, 98% purity), sodium hydroxide (NaOH,

99+% purity), aluminum potassium sulfate dodecahydrate

(AlK(SO4)212H2O, 99+% purity) and squalane (C30H62, bp 285C/

25 mm, 99% purity) were purchased from Aldrich. Hexane (C6H14,

bp 69 C/760 mm, 99.6% purity) and toluene (C7H8, bp 110C/

760 mm, 99.8% purity) were used as supplied under nitrogen from

the Firestone Polymer Company. Squalane was purged with nitro-

gen before use.

In literature there are two ways to synthesize aluminum di-

laurate. One is the wet method in which aluminum di-laurate

was prepared from the corresponding fatty acid by a metathe-

sis reaction of its sodium soap with an aluminum salt in aque-

ous solutions. This method has been used by a number of in-

vestigators in the past, including Sheffer [3], McBrain et al. [4,5],

Gray [12], McGee [13], Bauer and co-workers [14,15] and Leger

et al. [18], and has been well documented [218]. There is also

a dry method suggested by Mehrotra [19] in which aluminum di-

laurate was prepared by reacting aluminum isopropoxide with a

fatty acid in an organic solvent under nitrogen atmosphere, then

subsequently distilling off the isopropanol under reduced pres-

sure to yield aluminum-soap materials. This method is more ef-

fective in making aluminum tri-soaps. In this study, we adoptedthe wet method instead of the dry method to make aluminum

di-laurate. This is because in industry all aluminum di-soaps are

currently manufactured via a similar process in aqueous solutions

[7,8]. Their structures and their gelling nature in oils are the most

interesting because they are used daily in manufacture of greases,

paints, gels, cosmetics, drugs and foods.

In preparation, the sodium soap solution of 10 wt% solute was

made by reacting a stoichiometric amount of the sodium hydroxide

with the fatty acid in presence of distilled water. The aluminum

salt solution of 10 wt% solute was made by dissolving weighted

aluminum potassium sulfate in distilled water. Both solutions werethen heated to 75 C. Under vigorous stirring, the aluminum salt

solution was then added slowly into the sodium soap solution un-

til the precipitate was completed [11]. The precipitate was then

washed with water many times until free from sulfate and dried

in vacuum at 90 C. This raw product was then soaked in dried

acetone for 24 h to extract out the free fatty acid [3]. The final

product was then washed with acetone 2 times, and dried at vac-

uum at 50 C. On the basis of ash values the aluminum di-laurate

prepared in this manner gave ratios of the moles of aluminum to

the moles of fatty acid of 1:2 within 2% error bar. The aluminum

di-laurate synthesized according to present method was analyzed

by 1H-NMR, 13C-NMR, and 27Al-NMR techniques. The experiments

were carried out in CDCl3 solutions. 1H-NMR experiments showed

that the content of free fat acid (chemical shift at 9.8 ppm) inthe compound was less than 1%. 13C-NMR spectrum showed that

the ratio of free carbonyl group (chemical shift at 180.74 ppm) to

hydrogen-bonded carbonyl group (chemical shift at 179.99 ppm)

was 1:1 within 1% error bar, which was a result of that the di-

soap molecule had two carbonyl groups and one hydroxyl group

in its structure. 27Al-NMR experiments were little bit complicated,

because the spectrum was sensitive to the viscosity of the solution,

particularly the gel formation. To significantly reduce the viscosity,

we added a small amount of isopropanol into the Al-soap/CDCl 3solution. The resultant spectrum showed that the aluminum di-

laurate synthesized according to present method displayed only

a single peak at the chemical shift of 6.9 ppm. Based on these

results, we estimated that the purity of the product was about

95+%, and concluded that the product prepared was aluminum di-laurate.

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Xr. Wang, M. Rackaitis / Journal of Colloid and Interface Science 331 (2009) 335342 337

Fig. 2. EM photographs of the morphological structure of aluminum di-laurate from a toluene gel containing 2.5 wt% solute.

Solutions of the aluminum soap in hydrocarbon solvents of

various ratios were prepared by dissolving the soap into hydro-

carbon solvents of measured weights in sealed bottles. Aluminum

di-laurate swells in cold hydrocarbons, however, complete solution

can only be made by heating the mixture to an elevated temper-

ature for some time. Hexane solutions were prepared under vig-

orous stirring for 2 h at 70 C. Toluene solutions were prepared at

100 C and squalane solutions at 140 C. Once the solutions were

clear, they were placed at room temperature (23C) for at least

2 days prior to use. Depending on the soap concentration, a solu-

tion at 23 C could be the gel, jelly or viscous sol.

3. Experiments

Electron microscopy (EM) observations were carried out with

a Hitachi S-4800 electron microscope in scanning electron mi-

croscopy (SEM) and scanning transmission electron microscopy

(STEM) modes. This equipment has the ability to visualize objects

of 1 nm sizes. To investigate the morphological structures of dried

aluminum soaps, thin films of samples were made by cutting or by

casting the corresponding soap solutions on a carbon-coated cop-

per micro-grid. In the case of casting, the aluminum soap solution

was first diluted to about 102 wt%. Then, a small drop of the di-

luted solution was deposited on a carbon coated micro-grid. After

the solvent was evaporated, the grid was directly examined under

the electron microscopy. To observe the morphological structures

of aluminum soaps in solution, we selectively used squalane forthis investigation because it is non-volatile and can survive in the

high vacuum compartment of an electron microscopy. In sample

preparation, we first make an aluminum soap/squalane solution

containing about 2.5 wt% the soap. The solution was then diluted

by adding hexane to about 102 wt%. The diluted solution was

deposited on a carbon coated micro-grid. After the hexane evapo-

ration, the squalane formed a thin liquid film on the grid. The grid

was examined under the microscopy.

Measurement of dynamic moduli (G and G) of the solutions

were carried out at various strain amplitudes using a Rheomet-

rics ARES strain-controlled rheometer equipped with dual 200 and

2000 g cm force rebalance transducers. Strain sweeps were made

at 1 Hz. A cone and plate geometry was used to ensure a homoge-

neous strain field. The plate diameter/cone angle combination used

was 25 mm/0.02 radian. An aluminum soap gel was first heated to

above its melting point, and then was loaded in the cone/plate fix-

ture. The setup was then cooled down to a desired temperature

and was allowed to equilibrate for 1 h. After the normal force re-

laxed to zero, a strain sweep from 0.01% to 30% was performed in

logarithmic increments. The strain during oscillatory shear varied

as sin t, where is the strain amplitude and is the angu-lar frequency. The experiments were only conducted on squalane

solutions because hexane and toluene are too volatile for testing.

Turbidity measurements of the aluminum soap solutions were

carried out with a turbidimeter (DRT-15CE) from USEPA & HF

Scientific. The turbidity was recorded in Nephelometric Turbidity

Units (NTU). Standard formazin solutions certified by HF Scientific

were used for the calibration. Optical glass vials (Liquid Scintil-lation) with 28 mm outside diameter and 55 mm height from

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Fig. 3. Morphological structure of aluminum di-laurate casted on an amorphous carbon film surface from a toluene solution containing 0.02 wt% the solute.

Wheaton Scientific were used for holding the samples. Before thevial was placed into the equipment, the outside surface of the

vial was carefully cleaned with a lint free wiper moistened with

tetrahydrofuran and hexane. A measurement was taken from an

average of three to five readings. Reported data are within a 5%

error bar.

4. Results and discussion

For a toluene solution that is loaded with 2.5 wt% aluminum di-

laurate, the solution at 23 C is jelly-like transparent material (see

Fig. 1). The gel can withstand its own weight without flow, but

it can be easily broken upon shaking. Increasing the soap content

can improve the gel strength. Nevertheless, if the aluminum soap

molecules indeed line up to form a chain of centipede structure asthat shown in Fig. 1, the diameter of the chain due to the exis-

tence of fatty centipedes should be larger than 1 nm. Under cur-

rent microscopic technology it might be possible to visualize the

aluminum soap structure.Fig. 2shows EM photographs of the mor-

phological structure of the aluminum di-laurate from the toluene

gel. The sample was cut from the gel. At first glance, the results

shown in the picture are very surprising. On micro-scale, the ma-

terial cannot collapse completely after solvent evaporation and the

film is of highly porous nature. On nano-scale, the aluminum soap

molecules do not organize themselves into linear chains as that

proposed inFig. 1. Instead, they form mono-dispersed and spheri-

cal shaped micelles with size of about 68 nm.

Generally speaking, organic materials are composed of low

atomic number materials and thus they exhibit little variation inelectron density. That makes difficult to get contrast in conven-

tional TEM or STEM images. Staining with electron dense metalatoms is a technique commonly used. During the staining metal

atoms are incorporated into the polymer material and thus in-

crease contrast of different areas depending on their reactivity. In

our case the aluminum soap already has metal atom that in it-

self indicates increased concentration of aluminum atoms in the

image. Therefore images did not involve any additional staining

in order to detect structure of the particles. The darker spots in

the images show increased concentration of aluminum atoms and

thus inner structure of particles. If the aluminum atoms would be

evenly distributed across all sample there would not be notice-

able contrast increase in any placeelectron scattering from evenly

dispersed aluminum atoms would just increase opaqueness of the

overall sample. In this study images do show that there is a defi-

nite structure of particles.To confirm this result, we dilute the gel solution to concen-

tration of 0.02 wt% of the solute. This dilution was carried out

at 23 C and homogenized using a mechanical shaker. The rea-

son is that, according the Leger et al. [18], performing dilution

at cold condition does preserve the characteristic structure in the

starting solution. After the dilution, a small drop of the solution

was deposited on a carbon-coated micro-grid. After the solvent

was evaporated, the grid was examined under the electron mi-

croscopy.Fig. 3shows the microscopy pictures for the morphologi-

cal structure of aluminum di-laurate soap from the diluted toluene

solution. Again, we find that the aluminum soap molecules self-

assemble themselves into micelles of size about 68 nm. Those

primary particles then link together as colloidal aggregates to form

a percolated network. Clearly, the network is not formed by chain-like objects, rather by nano-sized particles.

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Fig. 4. Morphological structure of aluminum di-laurate casted an amorphous carbon film surface from a hexane solution containing 0.02 wt% the solute. The bottom panel

on the right with magnification of 300 nm was taken on a sample prepared under a restricted evaporation of hexane for 4 h. Other panels were taken on a sample prepared

under open evaporation condition, in which completing the evaporation took about 5 min.

The existence of a network of aggregated colloidal particles in

aluminum soap gels is very surprising. In order to determine sol-

vent influence, we changed the toluene with hexane and made a

solution that has approximately the same concentration. The mor-

phological structure of aluminum di-laurate in hexane is shown in

Fig. 4. Again, we see the same structure: a highly porous network

composed of nano-sized particles. Further, we restricted the evap-

oration rate of the solvents by cover the wet with a lid that had

only a small hole over the center through which the solvent vapor

could escape. Completing the evaporation of the solution at room

temperature could take from 0.1 to 4 h, depending on size of the

hole. However, there is no significant change in the morphological

structure of the aluminum di-laurate as that shown inFig 4.Another possible reason of formation of this structure could

be due to drying artifacts, because a phase separation of solvent

and solute could occur on the micro-grid surface if the two com-

ponents have different affinities to the surface during the evap-

oration. To confirm that our observation is actually occurring in

hydrocarbon solutions, squalane was selected for this investigation

because it is non-volatile and can survive in the high vacuum of

the electron microscope. In this case, we first made a squalane

solution that contains about 2.5 wt% soap. Then we diluted the

solution by adding hexane to about 102 wt% at room tempera-

ture. After that, a small drop of the diluted solution was deposited

on a carbon coated micro-grid. After the hexane was evaporated,

the squalane formed thin films on the carbon-coated grid and was

examined under the electron microscopy. Fig. 5 displays the mor-phological structure of aluminum di-laurate inside the squalane

solution. Apparently, the squalane cannot completely wet carbon

surface and has broken up as small flat droplets of various thick-

nesses. For droplets that are thin enough or of thin edges, one can

see the aggregated structures of the aluminum di-laurate in the

liquid media. To the best of our knowledge, this is the first time

that experiments under well-defined conditions provide such clear

picture that aluminum soap molecules form spherical nano-sized

micelles in hydrocarbon solutions, and those colloidal micelle par-

ticles then aggregate into networks of highly fractal and jammed

structures.

Experiments on rheological measurement also support this con-

clusion. The dynamic moduli (G and G ) of the aluminum di-

soap/squalane solutions as functions of the di-soap concentrationare presented in Fig. 6. Similar to many gel forming systems, the

sol-gel transition of the system is characterized by a crossover be-

tween G and G . At low concentrations (e.g., when C

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Fig. 5. Morphological structure of aluminum di-laurate inside a squalane solution. Aggregated structure of the aluminum di-laurate can be seen in the liquid droplets.

Fig. 6.Dynamic moduli (G and G ) of the aluminum di-soap/squalane solutions as functions of the di-soap concentration. The insert shows that the relationship between G

and (C Cgel) may be described by a simple scaling relation G (C Cgel)

m .

suggest that the gel structure may have remarkably different na-

ture from any chain-like structure.

In addition, the dynamic storage modulus G for those gels is

very sensitive to the change of strain amplitude, as shown in Fig. 7.

At low strains (when

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Fig. 7. Nonlinear behavior of the aluminum di-soap/squalane solutions as functions

of strain amplitude. All gels yield beyond a sort of critical strain ( y 3%).

rubbers[2730], where particle (or filler) networks are prevailing

in these systems.

Fig. 8 shows the response of a squalane solution to shear at

various temperatures. The solution was loaded with 15.5 wt% of

aluminum di-laurate. As expected, the modulus G of solution de-

creases with increasing temperature. The melting point of this gel

is about 130 C. For viscosity of the solution, however, it takes

a very different dependence: as the temperature increases ini-

tially it also increases, then it decreases as the temperature in-

creases further. This phenomenon was previously observed only

in some particle-filled systems [27], but not in pure polymer so-

lutions. An explanation for this phenomenon is based on particle

networks. Before the thermal energy is able to break the particle

network, increasing temperature will expand the network and thus

may loosen the jammed structure a little bit, which will result in

lower G . However, as long as this increased distance between the

particles is not enough to fluidize the particle aggregates, under

shear the expanded aggregates will jam together easily, which as

a result will increase the viscosity. There will come a point when

the thermal energy is large enough to move particles away from

the network, then further temperature increase melts the network

structure and reduces the viscosity.

Measurements of critical micelle concentrations (CMC) of alu-

minum di-soap in hexane, toluene, and squalane were carried out

by measuring the turbidity. The CMC was found by plotting the ex-

cess turbidity (the turbidity of the solution minus the turbidity of

the solvent) versus the concentration. The concentration at which

the excess turbidity is zero is the values of CMC. Fig. 9 presents

such a plot. The critical micelle concentrations for di-laurate alu-minum soap in hexane, toluene and squalane are 6.3 104,

5.5 104 , and 1.5 105 wt%, respectively. All of the solutions

investigated here have concentrations higher than those critical

values.

The present work demonstrates that aluminum di-laurate forms

micelles in the oils and that the aggregation of these micelles

forms a network that gives rise to the gel formation at concentra-

tions above approximately 2.5 wt%thereby refuting the long-held

belief that the gel formation was the result of polymeric chains of

aluminum association. This micelle colloidal network mechanism

also explains previously observed phenomena in such systems that

are hardly explained by a polymeric chain model, such as why the

viscosity or the modulus of a gel decreases with prolonged storage

time [3]. It is now understood that these micelle particles aggre-gate into highly fractal structures that are transiently jammed and

Fig. 8. Temperature dependence of the nonlinear behavior of a 15.5 wt% aluminum

di-soap/squalane solution. Top is the modulus G plot and bottom is the viscosity

plot.

Fig. 9. Critical micelle concentration (CMC) of aluminum di-laurate in hexane,

toluene, and squalane.

not stable in nature. Given sufficiently long time, they will tend to

minimize their configuration energy by re-arranging the particles

into more close packed forms. As a result, the hydrodynamic vol-

ume of these aggregates will decrease and the number of chainsthat link the network will decrease. Thus, the modulus of a gel

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Fig.10.Gelling nature of aluminum soaps in oils: spherical nano-sized micelles from

aluminum soap molecules aggregate into networks of highly fractal and jammed

structures.

will decrease and the viscosity of a solution will decrease with in-

creasing storage time.

5. Conclusion

We conclude that the gelling mechanism stems from the for-

mation of spherical nano-sized micelles from aluminum soap

molecules, and those colloidal micelle particles then aggregate into

networks of highly fractal and jammed structures, as shown in

Fig. 10. Earlier proposed polymer chain-like structure is definitely

incorrect. Our observed mechanism also allows for better expla-

nation and understanding of phenomena observed earlier though

hardly explained by a polymeric chain model. One is that the

viscosity of a solution or the modulus of a gel decreases with pro-

longed storage time[3].The other is that the relative viscosity of a

solution can sometimes increase with increasing temperature [4,5].

Now we understand that all those are the consequence of jammed

particle networks[24,27].The discovery of the mechanism and the

aluminum soap nanoparticles could expand application of these

materials to new technologies.

Supporting material

The online version of this article contains additional support-

ing information: the remarks on microscopy method used and theremarks on NMR experiments.

Please visitDOI: 10.1016/j.jcis.2008.11.032.

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