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1972

Adsorption of dichromate at the air-solution interface Adsorption of dichromate at the air-solution interface

Josephine Juch Wang

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ADSORPTION OF DICHROMATE AT THE AIR-SOLUTION INTERFACE

BY

JOSEPHINE JUCH WANG, 1944-

A THESIS

Presented to the Faculty of the Graduate School of the

UNIVERSITY OF MISSOURI-ROLLA

In Partial Fulfillment of the Requirements for the Degree

MASTER OF SCIENCE IN CHEMISTRY

1972

Approved by

~ l IJ/~_/J!,

'{:'. Lt.

T2734 45 pages c.l

ABSTRACT

The adsorption of dichromate ion with the surfactants, tetrade­

cy1pyridinium bromide and tetradecyltrimethylammonium bromide at the

air-solution interface was studied by surface tension and turbidity

measurements in order to better understand the process of ion fl ota­

tion.

Surface tension measurements were used to study the effect of

dichromate ion concentration on precipitation in dilute surfactant

solutions. At higher concentration, near and above the erne, light

scattering was used to detect precipitation. The concentration of

dichromate required to cause preci pita ti on decreases \~i th increasing

surfactant concentration until the erne is approached, after which

increasing concentrations of dichromate are required with increasing

concentration of surfactant.

There is a minimum surfactant concentration necessary for

formation of a stable foam network. This minimum concentration is

i i

1. 8 x w- 3 t-1 for TPB and 3. 5 x 10-3 t1 for TTt1AB. The amount of

dichromate removed in a given length of time does not vary greatly

with surfactant concentration until the concentration of surfactant

exceeds the erne. At higher concentrations flotation efficiency

decreases with increasing surfactant concentration probably because

the micelles are competing with the air-solution interface as adsorp­

tion sites for dichromate ions.

In these batch type experiments it Has possible to recover as

much as 63% of the dichromate with TPB and 96% with TH1AB.

iii

AC KNm~L EDGEMENT

The author wishes to express sincere appreciation to Dr. Raymond

L. Venable for his help throughout her research. She also thanks t·1r.

Robert Ballad, a graduate student in a similar area of study, for his

helpful suggestions and assistance. She wishes to thank the University

of l~issouri-Rolla for the use of the facilities and supplies necessary

for this work.

iv

TABLE OF CONTENTS

Page

ABSTRACT. • • . • • . • . • . • • . . . . • . . . . . . • . . . . . . . • . . . . . . . • . . • . . . . . . . . . . . . . . i i

ACKNOWLEDGH1ENT •••••••••..••••••••••..•••• . . . . . . . . . . . . . . . . . . . . . . . . ; ; ; LIST OF FIGURES •.•..••••••• ....................................... v

LIST OF TABLES •••.••• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vi

I. INTRODUCTION •••••• .......................................... 1

II. LITERATURE REVIEW •• ......................................... 3

I I I. REAGENTS • •••••••••••••••••••••••••••••••••••••.••••••••••••• 5

A. PREPARATION OF SURFACTANTS ••••••••••••••.••••••••••••.•• 5

B. DISTILLED WATER. . ...................................... . 5

IV. METHODS • •••••••••••••••••••••••••••••••••••••••••••••••••••• 6

A. SURFACE TENSION MEASUREMENTS .••••.•.•.•••••.••••..••..•• 6

B. TURBIDITY MEASUREMENTS ..•.•••••.••.••••• . .............. . 7

c. FLOTATION OR ADSORPTION AT THE AIR-SOAP BUBBLE INTERFACE ........••...............•.......••..... 8

v. RESULTS AND DISCUSSIONS •••••••••••••••••..••••••••••••••••.• 12

VI. COt~CLUS ION ••• ••••••••••••••••••••••••.•••.••••..•..•••••.•.• 37

BIBLIOGRAPHY. • • • • . • • • • • • • • . • • • • • • • • • . • • • . • . • • . • • . . . • • . • • • • • • • • . . • • 38

VITA.............................................................. 39

v

LIST OF FIGURES

Figure Page

1. The ion flotation apparatus ••••••••••••••••••••••••••••••••••• 9

2. Surface tension vs. logarithm of the concentration for TPB in water solution ••••••••••••••••••••••••••••••••••.• 14

3. Surface tension vs. logarithm of the concentration for TTMAB in water solution •••••.•••••••••••••••••••••••••••• 15

4. Effects of dichromate ion concentration on surface tension of different concentrations of TPB solution •••••••••• 17

5. Effects of dichromate ion concentration on surface tension of different concentrations of TTf~B solution •••••••• 19

6. Effects of dichromate ion concentration on turbidity of TPB solution ••••••••••••••••••••••••••.••••••••••••..••••• 23

7. Effects of dichromate ion concentration on turbidity of TTf1AB so 1 uti on . ........................................... 24

8. Effects of TPB concentration on the precipitation •••••••••.•• 25

9. Effects of TTMAB concentration on the precipitation •••••••••• 26

10. Amount of dichromate floated at different times in TPB and TT~~B solutions •••••••••••••••••••.•••••••••••••••••• 29

11. Amount of dichromate floated at different concentrations of TPB and TTt~U solutions ••••••••••••••••••••.•••••.•••••••• 35

vi

LIST OF TABLES

Table Page

1 T . 2 -4 • ransm1 ttance at 59 m]..l of 10 I~ aqueous TPB solutions after addition of K2cr2o7 ••••.•••.•.••••••••••• 11

2. Transmittance at 259 m]..l of TPB solutions at different concentrations in methanol and in water ........................................................ 11

3. Surface tension in different concentrations of TPB solutions ••••••••••••••••••••••••••••••••••••••••••••• 13

4. Surface tension in different concentrations of TTt4AB solutions........................................... 13

5. Surface tension of different concentrations of TPB solutions after potassium dichromate was added.................................................... 16

6. Surface tension of different concentrations of TT~~B solutions after potassium dichromate was added ••••••••••••••••••••••••••••••••••••••••••••••.•••.• 18

7. Turbidity of different concentrations of TP~ solutions after potassium dichromate was added •..••••••••..•• 21

8. Turbidity of different concentrations of TTr1AB solutions after potassium dichromate was added ••••••••••••••• 22

9. Amount of dichromate floated at different times in different concentrations of TPB solutions ••••••.•••••••••• 30

10. Amount of dichromate floated at different times in different concentrations of TTMAB solutions •••.•••.••••••••.•

11. Amount of dichromate floated at different concen­trations of TPB and TTMAB solutions after 2300 minutes . ..................................................... .

32

34

1

I. INTRODUCTION

There are a wide variety of materials that can be classified as

surface active in aqueous solution. One type, ionic surfactants,

will attract ions of opposite charge (counterions) and this combina­

tion of surface activity and ionic character can be used to remove

both the surfactant and the other ionic material from water simply by

bubbling air through the solution. If the counterion is in true

solution, the process is often called ion flotation.

There are several factors which might affect the adsorption of

the surfactant and its counterions at the air-solution interface.

Among these factors are valence and concentration of the counterions,

molecular structure and concentration of the surfactant, pH of the

solution, and state of solution of the counterionic species.

fiexavalent chromium in the form of chromate or dichromate is

found in varying concentration in many industrial waste streams. This

work has been an introductory study of the factors affecting the

adsorption of dichromate at the air-solution interface. Surfactant

concentration should be important because, if it is too high, surfac­

tant ions will cluster together in solution to form aggregates called

micelles. These micelles do not adsorb at the air-solution interface

but do attract counterions and actually compete vdth the air-sol uti on

interface as adsorption sites for counteri ons. Therefore, surfactant

concentration was one of the variaiJles studied. THo surfactants,

tetradecyl pyri di ni urn bromide (TP[3) and tetradecyl trir:1ethyl ammoni urn

bromide (TTMAB) were used. Tllus the length of the alkyl chain was

held constant while changing the nature of the charged head group.

In addition, precipitate formation was observed in some instances.

While the precipitate can often be removed by flotation, the physical

and chemical processes involved are somewhat different to those

involved in ion flotation. Since this was a study of the adsorption

of ions at the air-solution interface, it was necessary to determine

the concentration range in which precipitation could be avoided.

2

II. LITERATURE REVIEW

Surfactant solutions have been studied by a variety of methods.

This research project involved the use of three such methods;

surface tension, turbidity, and ion flotation.

3

General investigations of surface behavior of surface active

solutions have been concerned with the application of the Gibbs

adsorption law below the critical micelle concentration of the sur­

factant solution. If surface active impurities are present, there is

a minimum in the surface tension vs. concentration of sodium dodecyl

sulfate curve as shown by Brady (1). He used continuous foam­

fractionation to make this minimum disappear. Addison (2) also

found that such minima appeared in the surface tension vs. logarithm

of the concentration plots. Miles and Shedlovsky (3) observed that

no minimum occurred if the surfactant solution were entirely pure.

Precipitation of calcium oleate by measuring surface tension of

potassium oleate solutions after mixing with calcium nitrate was

shown by Hatijevic (4). An increase in surface tension was observed

when precipitation took place. This precipitation v.Jas also studied

by light scattering or turbidity measurements (4). A sharp increase

in turbidity was observed when precipitation occurred.

Another method of investigating the surface behavior is direct

measurement of surface adsorption by the foam fractionation technique.

Actually, foam fractionation may be classified in many different

ways depending upon the purpose or according to the mechanism of

removal, the type of substance to be removed, and the type of equip­

ment as discussed by Rubin (5).

4

In the present work the foam fractionation technique that was

used involved adsorption of dissolved ions at the air-solution inter­

face and will be called ion flotation. This technique has been used

to remove as much as 99% of the strontium (6) from dilute solution.

Approximately 80% of the copper II was removed (5) from solution using

sodium dodecyl sulfate as the surfactant. In other cases (7), 71-88%,

42-73%, 17-52% removals have been achieved for cerium, strontium and

cesium, respectively, using a variety of surfactants. Adsorption of

other ions, such as Na+ and Ca++ by PMT (palmitoylmethyl taurine) are

discussed by Walling (8).

Grieves and coworkers have reported several investigations of the

flotation of dichromate ions (9,10,11,12). In their work the only

surfactant used was ethylhexadecyldimethylammonium bromide (EHDA-Br).

Multicolumn apparatus was used with high gas flow rates and variable

surfactant feed rates. A molar feed ratio of EHDA-Br to dichromate

of 2.1-3.0 to 1 gave most efficient operation and 80-95% removal of

dichromate. Whenever a precipitate was observed, the formula v>~as

found to be approximately (EHDA) 2cr2o7• Use of a polymer flocculant

aid appears promising in this type of operation. Hhile it can be

seen that their work is related to the present work, there is no

overlap.

III. REAGENTS

A. Preparation of Surfactants

The surfactants used in this research were tetradecylpyridinium

bromide (TPB) and tetradecyltrimethylammonium bromide (TTt1AB). TPB

was prepared by refluxing tetradecyl bromide with about a 30% excess

of pyridine in methanol solution for eight to ten hours. After

refluxing, the solution was evaporated down to a syrupy consistency

and diethyl ether added until the solution became turbid. The flask

was placed in an ice bath until crystallization was complete.

5

Further purification was accomplished by dissolving the crystals in a

small amount of methanol, adding ether and chilling as before. This

process was repeated until a plot of surface tension versus concentra­

tion contained no minimum. TH1AB was purchased from K&K Laboratories

and purified as described above. Purified surfactants were stored in

a desiccator.

The pyridine, methanol, and ether used in the preparation and

purification of the surfactants were purchased from Fisher Scientific

Company and used without further purification.

B. Distilled Water

Softened \'later as supplied by the ohysical plant (UI1R) was

distilled from a Corning AG lb still. This VJas followed by two dis­

tillations from alkaline permanganate solution. This treatment was

sufficient to give water with a surface tension of 72.0 dynes per

centimeter which showed no surface aging effects.

6

IV. t·1ETHODS

A. Surface Tension Measurements

All surface tension measurements \A/ere made by the Wilhelmy plate

method. This method was chosen because the calculations are straight

forward and corrections are not necessary except in the most precise

\'Jork. The necessary rel ati onshi ps are:

where:

~W = W - W = vP 2 1 l

w2 = weight required to detach plate from the surface of the solution

w1 = ~1/eight of dry plate

y = surface tension in dynes cm- 1

r = perimeter of plate in em

For the particular plate used the working equation becomes

y = 157.3 flW

The plate used was made of platinum with platinum support wires.

The plate was soaked in chromic acid for at least 15 minutes, rinsed

with water from the Corning still and then with water from the second

permanganate still and dried with hot air before each measurement.

The sample cell was cleaned by the same procedure except it was not

dried before use.

The weighing device was a Christian Becker chainomatic balance

adapted for surface tension measurements. It was equipped with a

platform which could be raised and lowered as needed. The sample

container was designed so that the temperature could be controlled by

pumping constant temperature water through a jacket surrounding the

cell. By this method it was possible to maintain the sample at

25°C ± O.l°C.

The surface tension was measured by the following procedure.

7

Solutions were stored in the water bath at 25°C for at least an hour

before measurements were made. After placing a solution in the

sample container, the platform on which the sample container rested

was raised until the solution contacted the plate hanging on one arm

of the balance. The platform was then lowered until the lower edge

of the plate began to rise above the liquid level. The beam arrest

was slowly released so that the beam rested on the knife edge. The

pointer was brought to the zero mark and kept essentially at this

position by adding weights to the other arm of the balance or by

lowering the platform and putting more pull on the plate as needed.

In this way the weight required to pull the plate free of the surface

was determined. The balance was flushed with nitrogen for thirty

minutes before and then during each surface tension measurement to

eliminate the possibility of aging effects.

B. Turbidity r~easurements

The light scattering photometer used in this work was a Brice­

Phoenix model 2000 m1 manufactured by the Phoenix Precision Instrument

Company. This instrument uses an AH-3 mercury arc as a light source

and is equipped with filters for isolating the 436 and 546 mu wave­

lengths. The 436 m~ line was used throughout this work. Output of the

1P21 photomul ti pl i er tube used as a detector cou·l d be registered on a

recorder, on a galvanometer, and with this particular instrument on an

oscilloscope. A spotlight galvanometer was used in this work.

8

Filtration for the removal of dust from the TPG or TTr1AB solution

was done with a Gelman type Vr1-6 metri eel filter of 0.45 micron pore

size supported in a glass filtration funnel from the Gelman Instrument

Company of Ann Arbor, Michigan. The filters Here soaked in concen­

trated nitric acid for about a week prior to use. This treatment

seemed to remove any undesirable organic matter from the filters so

that none was leached from the filters by the surfactant solutions.

The filter funnel was mounted on top of a vacuum desiccator arranged

in such a way that solutions could be filtered directly into the

sample cells for the light scattering photometer if desired.

Actual turbidity measurements were made by placing 50 ml of dust

free TPB or TTMAB solution in a cell and making progressive additions

of small volumes of 0.15 M potassium dichromate with micro pipettes.

Mixing was accomplished by swirling the cell. The turbidity Has

determined after each addition of dichromate for the purpose of

determining the concentration of dichromate that would initiate pre­

cipitation at a given surfactant concentration.

C. Flotation or Adsorption at the Air-Soap Bubble Interface

The flotation apparatus consisted of three identical double

walled glass cells designed by Oko (13) as shown in Fig. 1. Each was

equipped with a magnetic stirrer, a 120 em column with foamtrap, and

a rotameter for measuring the volume of gas. tJi trogen v~as bubbled

through solutions in these cells at the rate 22 ml/min from an approx­

imately 1 mm diameter capillary tube immersed to a depth of approxi­

mately 10 em. Each solution was agitated with a magnetic stirrer bar.

The solutions were prepared by placing 500 ml of TPB solution of the

FOAM TRAP

120cm

MAGNETIC STIRRER

THERMOSTATED /WATER

Figure 1. The ion flotation apparatus

9

PURIFIED ..,_N 2

WATER

10

desired concentration into the cell and adding 60 ~of 0.15 11 (1.94 mg)

potassium dichromate to give an initial dichromate concentration of

1.8 x 10-5 M. The initial TPB concentrations were varied from -3 -3 2.0 x 10 ~1 to 8.0 x 10 t·1. TH1AB solutions were prepared by plac-

ing 310 ml of surfactant solution with the desired concentration into

the cell and adding 50 ~of 0.15 N (1.62 mg) potassium dichromate to

give an initial concentration of 2.42 x 10- 5 1'1. The reason for using

a smaller volume of TTr~AB solution was because this surfactant was

in very limited supply. The initial TH1AB concentrations were varied

from 3. 55 x 10-3 r~ to 7. 58 x 10-3 ~1.

The foam was collapsed in the trap vlith methanol. After flotation

the methanol was removed, the flask rinsed and the collapsed foam

analyzed for dichromate with a Perkin Elmer 14odel 303 Atomic Absorp­

tion Spectrophotometer. Attempts \'/ere made to analyze for the

surfactant TPB making use of the ultraviolet absorption band of the

pyridine ring, but this did not prove feasible in the presence of

dichromate because the dichromate drastically reduced the transmittance

of the TPB by some mechanism that was dependent on the concentration

of dichromate as shown in Tables 1 and 2.

T/\BLE 1

Transmittance at 259 ml-1 of 10-4 11 aqueous TPl3 solutions after addition of K2cr2o7

Volume of 0.15 M K2cr2o7 added Transmittance

0 A 38.7 1 A 36.5 2 A 35.9 3 A 35.1 4 A 34.1 5 A 32.7

lOA 27.2 20 A 19.7 30 A 17.7

40 A 10.6

50 A 10.0

60 A 7.2

TABLE 2

Transmittance at 259 ml-1 of TPB solutions at different concentrations in methanol and in water

Concentration of TPB in methanol Transmittance

5.0 X 10- 5 r~ 60.5

9.0 X 10-5 t1 40.5

l.Ox 10-4 ~1 38.7

2.5 X 10-4 r~ 8.5

Concentration of TPB in water Transmittance

5.0 X 10-5 M 61.2

9.0 X 10-5 r~ 41.5

1.0 X 10-4 r~ 37.5

2.5 X 10-4 M 9.1

11

12

V. RESULTS AND DISCUSSIONS

Results of the measurement of surface tension as a function of

TPB concentration are presented in Table 3 and graphically in Figure

2 as surface tension vs. the logarithm of the concentration of TPB.

From the figure it can be seen that there is no minimum in the curve.

As previously discussed this is indicative that the TPB was in a

satisfactory state of purity. The abrupt change in slope of the curve

takes place at the erne and gives a value of 3.0 x 10-3 t1. Similar

data are presented for TTMAB in Table 4 and Figure 3, and from these

data it can be seen that the erne is 3.5 x 10-3 M. Again the absence

of a minimum indicates a satisfactory degree of purity.

Surface tension measurements were also used to determine the

concentration of potassium dichromate required to start formation of

a precipitate at a given TPB or TTf.1AB concentration. Addition of

dichromate to a solution of either of the surfactants caused a sharp

decrease in surface tension until precipitation began, as confirmed by

visual observations, after which an increase in surface tension

occurred. Data are presented in Table 5 and Figure 4 for TPB and

Table 6 and Figure 5 for TTr1AB. These results were required in this

work because this was a study of the adsorption of ions at the air­

surfactant solution interface. Therefore it was necessary to know the

concentration ranges in which it is possible to work in the particular

system under study, without interference by precipitate formation.

At higher surfactant concentrations, above the erne, the addition

of dichromate to solutions of the surfactants produces only minor

changes in surface tension and it is not possible to determine the

13

TABLE 3

Surface tension in different concentrations of TPB solutions

Concentration Logarithm of the of TPB concentration Surface tension

( t1) (dynes cm- 1)

2.0 X 10-4 3".6990 52.33 6.0 X 10-4 3.2218 47.72

8.0 X 10-4 3".0969 45.88 1.0 X 10-3 3".0000 44.26 1.4 X 10-3 2. 8599 43.79

2.0 X 10-3 2.6990 40.58

2.4 X 10-3 2. 6198 39.30

2.6 X 10-3 2.5850 39.21

3.0 X 10-3 2. 5229 38.94

4.0 X 10-3 2.3979 38.80

1.0 X 10-2 2.0000 38.53

TABLE 4

Surface tension in different concentrations of TTt1AB solutions

Concentration Logarithm of the of TH1AB concentration Surface tension

0~) (dynes cm- 1)

1.2x 10-3 2.9208 47.36

1.6 X 10-3 2. 7959 44.59

2.0 X 10-3 2. 6990 42.65

2.2 X 10-3 2.6576 41.25

2.4 X w-3 2.6198 40.78

2.6 X 10-3 2.5850 39.38

3.4 X 10-3 2.4685 38.28

4.0 X 10-3 2. 3970 38.03

5.0 X 10-3 2.3010 37.46

6.0 X 10-3 2.2218 38.01

1.0 X 10-2 2.0000 37.61

14

- 51 -I E 0

tJ) Q) c:: ~

"'0 -z 0 (/)

z w t-

w u <l lL a:: ::::> 41 (/)

37_--~---~~---~~---~~---~~---~--~-~

4.2 3.8 3.4 3.0 2.6 2.2 1.8

LOG [TPB] Figure 2. Surface tension vs. logarithm of the concentration for

TPB in water solution

15

--I

E u

f/) Q) c: ~

"0 .....,

z 0 - 43 en z UJ 1-

41 IJJ u ~ a:: ::> (/)

37

35_~-------~-----~---~~---~~---~--~-~ 3.0 2.8 2.6 2.4 2.2 2.0 1.8

LOG [TTMAB]

Figure 3. Surface tension vs. logarithm of the concentration for TTMAB in water solution

TABLE 5

Surface tension of different concentrations of TPB solutions after potassium dichromate was added

Concentration of TPB

(r4)

2.5 X 10-4

3.0 X 10-4

5.0 X 10-4

Log [K2cr2o7]

4.6478

if. 5229

4.3468

l.2218

if.1249

if.0458

3.9788

3.9208

4.3468

4.2218

4.1249

4.0458

3.9788

if. 8239

4.5229

4.3468

4.2218

4.1269

4.0458

Surface tension (dynes em - 1)

42.10

39.26

34.37

32.29

31.36

32.49

33.31

33.66

39.18, 42.40

34.44, 36.16

33.72, 34.37

34.63

35.64

48.84, 47.19

38.22, 40.55

34.38

35.34

36.82

37.88

16

17

* 2.5xi0-4M TPB (3.6021) • 3.0x 104 M TPB (3.5229) • 5.0 )(10-4 M TP.B (3.3010)

-•e u

(/) Q) c: ~

"0 .....,.

z 0 -CJ)

z 38 w ~

w u 36 <( LL.. a:: ::> CJ) 34

30~~--~~~--~~--~~--._~--~~

- - - - - - -5.0 4.8 4.6 4.4 4.2 4.0 3.8

figure 4. Effects of dichromate ion concentration on surface tension of different concentrations of TPB solution

TABLE 6

Surface tension of different concentrations of TTMAB solutions after potassium dichromate was added

Concentration of TTMAB

(t'1)

1.2 X 10-3

1.6 X 10-3

-3 2.2 X 10

Log [K2cr2o7]

4. 8239 4.3468

4.1249 3. 9788

3.8697

3. 7825 3. 7100 3. 5935

4.8239 4. 3468

4.1249

3. 9788

3. 8697 3. 7825

3. 7100

4.8239

4.5229

4.3468

4. 2218

4.1249

4.0458

3.9788

Surface tension ( -1 dynes em )

47.12 43.54

39.29 38.50 38.79

39.05 39.34

39.89

43.08 38.60

36.82 37.01

37.72 38.17 38.47

40.52

37.53

36.60

36.30 36.49

36.72

36.46

18

--I E (.)

f/) Q) c >-

"C ~

z 0 -(/) z w 1:-

w u ~ 0::: ::::> (/) 34

32

A 1.2 x 103M T TMAB (2.9208) • 1.6 x 10-3M TTMAB (2.7959) e 2.2 x 10-3 M TTMAB (l.6576)

19

30_~_.--_._~---~~---~~---~~~~--~~

5.0 4.6 4.2 3.8 3.4 3.0 2.6

LOG [K 2 Cr20 7J Figure 5. Effects of dichromate ion concentration on surface tension

of different concentrations of TT!1AB solution

20

concentration of dichromate which will cause precipitate formation.

An alternate way of detecting precipitate formation is by

turbidity measurements. This method could be used at the higher

surfactant concentrations. At surfactant concentrations below the erne

the method did not prove successful because, for some as yet undis­

covered reason, the measurements were very erratic at these low

surfactant concentrations.

The results of the turbidity measurements are presented in Tables

7 and 8 and Figures 6 and 7 for TPB and TTf1AB, respectively. The

sharp increase in slope is indicative of precipitate formation, which

can be verified by visual observations.

Figures 8 and 9 summarize the results of the precipitation studies

as plots of the logarithm of the dichromate concentration that will

cause precipitation vs. the logarithm of the surfactant concentrations.

The rationalefor presenting the results in this Hay will be discussed

later. As can easily be seen these curves are approximately in the

shape of a V. At concentrations of surfactant and dichromate which

1 ie outside the V, no problems are encountered vJith precipitation.

For concentrations which lie inside the V, precipitation occurs and

it is not possible to investigate ionic processes in these concentra­

tion ranges. Data for concentrations near the point of the V show

poor reproducibility and the dotted portions of the curves should be

considered as only very approximate guidelines to solubility. Some

possible reasons can be given for the general shape of the curves.

For the left hand portion of the curve the surfactant concentration

is such that the molecules are not in micellar aggregates and the

equilibrium might be expected to be of the simple solubility product

21

TABLE 7

Turbidity of different concentrations of TPB solutions after potassium dichromate was added

Concentration 104 of TPB Log [K2cr2o7] Turbidity x

(I~) (cm- 1)

2.5 X 10-3 4. 5229 18.86

4. 2218 17.18

4.1249 32.92

4.0458 229.25

3.9788 675.54

4.o x w-3 4.5229 15.47

4.0458 17.10

J. 9208 27.31

3.8239 52.04

3. 7825 192.70

3.6778 392.16

s.o x w- 3 4.5229 18.74

4.0458 19.59

3. 9208 27.56

3. 8239 28.54

3. 7825 30.63

3. 7447 30.18

3. 7100 38.72

3.6478 102.32

3. 6198 226.4

22

TABLE 8

Turbidity of different concentrations of TTMAB solutions after potassium dichromate was added

Concentration 104 of TTt1AB Log [K2cr2o7] Turbidity x

01) (cm- 1)

3.0 X 10-3 4.5229 6.92

4.3468 10.80

4.2218 22.90

4.1249 82.04

3.9788 394.34

4.0 X 10-3 4.5229 9.01

4.0458 30.90

3.9788 8.38

3.9208 17.36

3.8697 20.34

3. 7825 17.16

!. 7447 18.70

"!.6478 157.18

J.6364 164.48

·~ 6001 • 2.5 x 103M TPB (2.6021}

• 4.0 x 10-3M TPB (2.3979)

• 5.0 x l0-3M TPB (2.3010)

¢0 500 -)(

>- 400 t--c -m ~ 300 ::::> t-

z 0 200 -t-:::> ....J 0 100 (/)

I ~ ~ .,.. . I 0 I I I I •. I I *' • I I I I I I

- - - - - - - - -5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

LOG [K2 Cr2 0 7] Figure 6. Effects of dichromate ion concentration on turbidity of TPB solution

N w

600 --' E 500~

_._ 3.0 x 10-3 M TTMAB (2.5229) u - • 4.0 x I0-3 M TTMAB (2.3979) • 0 -)(

400 >-~ -0 - 300 m cr :::> ~

z 200

0 -t-:::> lOO ..J 0 (/)

0 - - - - - - - - - -5.0 4.8 4.6 4.4 4.2 4.0 3.8 3.6 3.4 3.2

LOG [K2Cr2 0 7J Figure 7. Effects of dichromate ion concentration on turbidity of TTr1AB sol uti on N

~

r-1 ,.... 0

N '-u N ~

L.....J

<.!) 0 --'

-4.2

-4.6

-4.8

-

0 - BY SURFACE TENSION

b- BY LIGHT SCATTERING

PRECIPITATION NO

25

PRECIPITATION I

I

PRECIPITATION,' I CMC

I I

I I

' I ' NO ' I

' I PRECIPITATION', I

' I ' I "

- - -5.0 ...___.__..___--'-_..____._.......,ji---&...___.I~...L-___.1~-'---' --3.6 3.2 2.8 2.4 2.0 -4.0 1.6

LOG [TPB] Figure 8. Effects of TPB concentration on the precipitation

r;J 0

N NO

a - BY SURFACE TENSION

b - BY LIGHT SCATTERING

26

PRECIPITATION NO PRECIPITATION

'-UN 4.2 PRECIPITATION I

I I I I I~ I

(.!)

0 ...J

-4.6

-4.8

-

I \ I

"

s.o~~~~~~--._~~~~_. __ ._~~ - - - - - - -3.2 3.0 2.8 2.6 2.4 2.2 2.0

LOG [T TMAB]

Figure 9. Effects of TT~~B concentration on the precipitation

27

type since the concentration of dichromate required to initiate

precipitation decreases with increasing surfactant concentration.

More will be said about this later. The erne of an ionic surfactant

depends on the concentration of added electrolyte in a way that has

to be determined for each electrolyte-surfactant pair. No detailed

data of this type are available for these surfactants and potassium

dichromate. Therefore it is not possible to say with certainty that

the minima in the curves occur at the respective critical micelle

concentrations. However, there seems to be little doubt that the

upward turn in the curves is related to formation of micelles within

the solutions. The presence of these micellar aggregates could

enhance the solubility of tetradecylpyridinium or tetradecyltrimethyl­

ammonium dichromate by incorporation into the interior of the micelle.

Such a system undoubtedly involves quite complex equilibria and no

straightforward interpretation is possible.

On the other hand, if the precipitation equilibria involved for

the left hand portion of the curves is a simple solubility product

type equilibrium, it should be possible to interpret the slope of the

left hand portion of the V shaped curves in a rather simple fashion.

If one assumes that the precipitate formed by mixing TTf1AB and

potassium dichromate is tetradecyltrimethylammonium dichromate,

(TTMA) 2cr2o7, one can write the solubility equilibrium as

+ = (TTMA) 2cr2o7 t 2TTMA + Cr2o7

and

Taking the logarithm of both sides of this expression gives

which can be rearranged to give

+ = log Ksp - 2 log[TTMA ]

From this it is obvious that a plot of

28

should have a slope of negative two if this is the only equilibrium

that is involved. The observed slope for TTMAB is approximately -1.3.

From this it is concluded either that (1) there are more complex

equilibria involved or (2) that perhaps the precipitate is not

(TTMA) 2cr2o7 or (3) the solutions are not ideal as is implicitly

assumed in the above treatment. Insufficient data are available to

allow further attempts at interpretation of the equilibria. The

slope of the plot for TPB and potassium dichromate is very near to

that for TTMAB.

Some typical results of the experiments on adsorption at the air­

solution interface, or flotation, are presented in Figure 10 as mg of

dichromate floated at different flotation times for a surfactant

concentration of 4.0 x 10-3 M for both TTMAB and TPB. All available

data are presented in Tables 9 and 10. As \1/ould be expected, flota­

tion for longer periods of time results in more dichromate being

removed until the supply of surfactant is depleted in these batch type

experiments. There is a minimum surfactant concentration necessary

for foam stability. This has been found to be approximately

........ 0 E -0 UJ

~ 0 ....J u..

.. ,... 0

N .... u

.8

.6

.4

.2

__.._ 4.03 x 10·3 M TTMAB

-e- 4.0 x 10'"3 M TPB

29

o~----.__. __ .__. __ ~~~~~~~--~~--1600 2000 2400 o 400 800 1200

TIME (min) Figure 10. Amount of dichromate floated at different times in TPl3

and TTf1AB solutions

TABLE 9

Amount of dichromate f1oated at different times in different concentrations of TPB so1utions

Concentration of TPB ( t~)

2.0 X 10-3

-3 2.2 X 10

-3 3.0 X 10

Tota1 time Tota1

(min)

120 400

695 970

1220

1520

1820

2120

350

710

990

1320

1620

1860

2160

2670 2960

270

510

750

1050 1310

1610

1910

2190

cr2o7- f1oated

(mg)

o. 210 0.580

0.770 0.840

0.894 0.952

1.087

1.102

0.402

0.526

0.638

0.675

0. 712

0.745

0. 775

0.796 0.806

0.354

0.588

0.797 0.967

1.072

1.157

1.200

1. 231

30

31

TABLE 9 - (continued)

Concentration of TPB Total time Total cr2o7= floated (!-~) (min) (mg)

4.0 X 10-3 300 0.344 550 0.456 850 0.642

1150 0.749 1450 0.836 1810 0.887 2170 0.924 2450 0.951

5.0 X 10 -3 360 0.062 760 0.165

1090 0.227

1430 0.301

1760 0.375

2050 0.445

2290 0.519

2580 0.593

2855 0.665

8.0 X 10-3 300 0.124

560 0.182

850 0.252

1150 0.306

1450 0.351

1810 0.384

2170 0.413

2450 0.439

2780 0.470

32

TABLE 10

Amount of dichromate floated at different times in different concentrations of TTMAB solutions

Concentration of TH1AB Time cr2o7- floated (t~) (min) (mg)

3. 55 X 10-3 815 1.340 1535 1.460 2155 1.476

3. 71 X 10-3 835 1.265

1555 1.526

2175 1.559

4.03 X 10-J 835 1.207

1555 1.476

2175 1.513

5. 97 X 10-3 610 0.664

1260 1.058

2060 1.399

2420 1.544

7. 58 X 10-3 610 0.506

1260 0.820

2060 1.165

2420 1.310

33

-3 3 1.8 x 10 11 for TPB and 3.5 x 10- r·1 for TTt-1AB. This depletion of

surfactant is reflected in the flattening out of the curves, and when

the surfactant concentration becomes low enough, less than the con­

centration required for a stable foam, flotation ceases. It would

appear from these curves that TH1AB is more efficient at removing

dichromate than TPB.

The curves in Figure 10 are for one surfactant concentration and

are representative of the kind of data obtained. Several surfactant

concentrations were used and plotted in a similar fashion for each

surfactant. From these curves an attempt was made to determine the

amount of dichromate floated. The dichromate concentration at the

asymptotic values (dc/dt=O) were plotted as a function of surfactant

concentration. There was considerable spread in the data. Difficul­

ties were encountered in controlling gas flow rates for long flotation

periods. Since flotation efficiency depends on gas flow rate, any

fluctuation in flow would effect the results obtained. The data are

presented in Table 11 and Figure 11 and the curves are presented as

dashed lines for comparison to previous \1/0rk by Oko (13) and Coleman

(14). This general curve shape is expected, although with the small

number of data points and the scatter in these points, this is not

obvious from the present data. A trend toward a decrease in flotation

effectiveness with increasing surfactant concentration can be observed.

The addition of inorganic electrolyte to ionic surfactants in

aaueous solution would be expected to lower the erne. Since the erne

is 3.0 x 10-3 M for TPB and 3.5 x 10-3 M for TTMAB in the absence of

electrolyte, the erne's can be expected to be less than these values

TABLE 11

Amount of dichromate floated at different concentrations of TPB and TTMAB solutions after 2300 minutes

Concentration of TPB (~0

2.0 X 10-3

2.2 X 10-3

3.0 X 10 -3

4.0 X 10-3

5.0 X 10 -3

8.0 X 10-3

Concentration of TTMAB (t1)

3. 55 X 10-3

3. 71 X 10-3 -3 4.03 X 10 -3 5.97 X 10

7.58 x w-3

Cr2o7- floated (mg)

1.02

0.81

1.23

0.94

0.46

0.43

cr2o7- floated (mg)

1.45

1.59

1.49

1.46 1.29

34

1.6t- ' ' _._ • -- --- ... - TPB ---1.51-- • - ......... -•- TTMAB • ' .......

1.4r ............ - ' 0' 1.3 ............... E A ---- I. C.. I -- ' Cl 1.1 ....... ,

w ' t- I 0 A ' ~ . ' 0 ,. _J .9 \ lL

8 A \ .. "' . \ 0 .7 ',

(\J ~ \ u .6 \

\ '6

'...., A --., I I I I I .--- -.-----

.3 0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0

CONC. OF SURFACTANT x 103 (moles II) w

Figure 11. Amount of dichromate floated at different concentrations of TP~ and TTMAB solutions ~

36

in the presence of dichromate. These solutions are very dilute in

dichromate (-10-5 M) and the effect of this concentration of electro-

lyte is small. Because of problems encountered with precipitation,

it was not feasible to determine the actual effect of dichromate

concentration on the erne of either surfactant. However, the results

presented here are consistent with the properties of ionic surfac­

tants. The decrease in flotation effectiveness of TPB above 3.0 x

10-3 M is evident. A downward trend in flotation effectiveness is

noticeable above 3.5 x 10-3 t1 for TTf1AB. It appears that TTl4AB is

more effective than TPB at a given concentration.

VI. CONCLUSION

The effects of TPB and TTMAB concentration on the flotation of

dichromate ion have been studied. The following conclusions are

obtained from the results.

37

1) The erne is 3.0 x 10-3 t-1 for TPB and 3.5 x 10-3 M for TTt-1AB.

These values were obtained by surface tension measurements and the

absence of minima in the surface tension vs. concentration plots

indicates satisfactory purity for both surfactants.

2} For optimum flotation efficiency, the surfactant concentra­

tion must be kept below the erne. Above the erne micellar aggregates

form giving rise to adsorption of dichromate in competition with the

air-solution interface and resulting in a decrease in flotation

efficiency.

3} The effects of dichromate concentration on precipitation in

TPB or TTMAB solutions have been studied. The equilibria involved

seem to be fairly complex even at 10\'1 surfactant concentrations and

undoubtedly are more complex in the concentration ranges \'lhere

surfactant micelles exist.

4) It can be seen that TH1AB is more efficient than TPB in this

batch type flotation.

38

BIBLIOGRAPHY

1. A. P. Brady,~ Phys. and Colloid Chern.,~~ 56 (1949).

2. C. C. Addison and J. Povmey, Trans. Faraday Soc., 33, 1243 (1937). -- -

3. G. ~1iles and L. Shedlovsky, h Phys. Chern., 48, 57 (1944).

4. E. Matijevic, J. Leja and R. Nemeth, J. Colloid and Interfacial Sci., 22, 419 (1966). - ~

5. A. J. Rubin, J. D. Hohnson and J. C. Lamb, I and EC. Process Desisn and Development, ~~ 368 (1966). - --

6. B. M. Davis and F. Sebba, !L_ Appl, Chern., _!i, 297 (1966).

7. J. Oglaza and A. Siemazko, flukleonika, 11., 421 (1966).

8. C. ~~all ing, E. E. Ruff and J. L. Thornton, Jr., h Phys. Chern., g. 486 ( 1957).

9. R. B. Grieves and S. ~1. Schwartz, AIChE h g, 746 (1966).

10. R. B. Grieves, T. E. Wilson and K. Y. Shih, AIChE ~11., 820 (1965).

11. R. 8. Grieves, J. K. Ghosal and D. Bhattacharyya, J. Amer. Oil Chern. Soc., 45,591 (1968).

12. R. B. Grieves and G. A. Ettelt, AIChE hJl, 1167 (1967).

13. U. t·1. Oko, A thesis of the University of t1issouri-Rolla, 1968.

14. R. E. Coleman, A thesis of the University of r,1issouri-Rolla, 1968.

39

VITA

Josephine Juch Hang \vas born on ~1arch 3, 1944 in Shenyoung, China.

She received her primary education from Chung-Si Elementary School,

Taichung and for her high school education she attended Taipei First

Girls High School. She enrolled at the National Taiwan Normal Univer­

sity in September, 1963 and received a Bachelor of Science Degree in

Chemistry in July, 1968 after teaching one year in Kuting ~1iddle

School from 1967 to 1968. She received r~adam Yen's f1emorial Scholar­

ship from the chemistry department for the period of her junior year

in call ege.

She was enrolled in the Graduate School of the University of

Rhode Island from 1969 to 1970. She came to the University of

Missouri-Rolla to work toward a r1aster of Science in Chemistry in

January, 1971. On May 27, 1972 she married Kai-t1ing So.