This chapter deals with the determination of Hansen solubility parameters

(HSP) for the solid surface of cellulose acetate phthalate (CAP) by inverse gas

chromatography in the temperature range 333.15 - 393.15K. The number average

molar mass of the polymer was K - 2534.12 and density at 25 C? was 0.266 glcm).

Cellulose acetate phthalate is produced by reaction of phthalic anhydride with a

partial acetate ester of cellulose, in presence of sulphuric acid. CAP contains 21.4-

26.0% of acetyl groups, and 30.0-36.0% of phthalyl groups. The structure of CAP is

shown below in Fig.6.1.

Fig.6.1 The structure of cellulose acetate phthalate (CAP)

Cellulose esters have been extensively used as binders, additives, and coating

applications.' Cellulose acetate phthalate (CAP) belongs to cellulose ester family,

which presents large field of biotechnoiagical applications due to its

biocompatibility.' For instance, it has been used as entwic coating of drugs, which

should survive in the acidic environment of the stomach, but should be released and

absorbed in the intestine.' Cellulose acetate phthalate is commonly applied to solid-

dosage forms either by coating from organic or aqueous solvent systems or by direct

compression. Concentrations generally used are 0.5-9.P! of the core weight. The

addition of plasticizers improves the water resistance of this coating material, and

formulations using such plasticizers are more effective than when cellulose acetate

phthalate is used alone. Applications of HSP data of cellulose acetate phthalate and

polyethylene glycols were discussed in tablet film coating3 The biodegradability of

Polymethylmethacrylate - cellulose acetate phthalate blend has been studied by four

different methods namely, soil burial test, enzymatic degradation, degradation in

phosphate buffer and activated sludge degradation followed by water absorption

tests to support the degradation studies.' The blends of PMMA with CAP have

pharmaceutical appl i~at ions.~ Therapeutically, cellulose acetate phthalate has

recently been reported to exhibit experimental microbicidal activity against sexually

transmitted disease pathogens, such as the HIV-1 retrovirus?'

Inverse gas chromatography (IGC) is one of the most versatile, fast and

reliable technique to determine the Hansen solubility parameters of non volatile

substances? The determination of Hansen solubility parameten for the solid surface

using IGC data has been proposed by Voelkel 'O based on the adsorption model

described by Snyder and ~ a r ~ e r . " In the IGC method the polymer which is to be

characterized is placed in the column and the volatile solute probes are injected so

that the probe is at infinite dilution in the stationary phase. Hildebrand solubility

parameter and Hansen three component solubility parameters are usehl in

predicting the solubility behavior of a polymer in various solvents and plasticizers.

'"I4 Hansen solubility parameter data have been found lo be applicable to examine

different intermolecular interactions at the solid surface^.'^ The solubility parameter

data is also useful to correlate polymer compatibility with other polymers and in

designing coating formulations. HSP data are particularly useful in solid phase

extraction for the selection selection of a suitable ~olvent . '~ ~umerous papers have

been published on the characterization, based on the HSP data, of non volatile

materials such as inorganic salts, fibers, clays, pharmaceutical powders, cellulose,

pigments, hydrocarbons, polymers and polymer blends."~*@

6.1. Specific retention volumes V:

In IGC the retention is mainly governed by the surface and bulk interactions

of the solute with the stationary phase. This interaction can be characterized with the

measured specific retention volume,^:, which can be related to thermodynamic

quantities. The specific retention volumes for the interaction of a solute on a solid

surface can be calculated using the relation

where, t, is the retention time of the probe solute and t,is the retention time of

methane measured at the column temperature. F is the flow rate of the carrier gas

measured at room temperatureT,, w is the mags of the stationary phase. P, is the

water vapour Pressure at T, . The flow rate of the carrier was measured at the

column outlet at T,using a soap film flow meter. The carrier was saturated with

water vapour existing in the soap film flow meter before flow rate measurement.

Therefore P, is subtracted from P, in Eq.6.1. J is the James and Martin correction

factor which depends on the inlet, P,, and outlet, 6 , pressures calculated wing the

following relation.

The specific retention volumes, v:, for 21 solute probes are calculated

according to Eq.6.1 in the tempmature range 333.15 - 393.15 K . The V , values are

listed in Table 6.1. The V; values are decreasing with increase of temperature for all

the solutes. The retention diagrams drawn between In Y: versus 1/T are shown in

Figs. 6.2 -6.9 for all the solutes. The retention diagrams for all the solutes are linear

which indicate that in the temperature range studied there was no phase change in

CAP. In the measured temperature range CAP is in the solid phase. Therefore the

retention of the solute probes is mainly due to adsorption of the solute on the solid

surface of CAP.

Table 6.1. The specific retention wlums, v,Y , (m31g) of solutes on cellulo~

acetate phthalate in the ternpmture range 333.15 - 393.1 5K.

Solutes

n-octane

Acetone

Ether

Methanol

Ethanol

Benzene

Toluene

2-Propanol

2-Butanol

Dichloromethane

Trichloromethane

Methyl acetate

Ethyl acetate

1-Butyl acetate

1 ,4-dioxane

THF

Fig.6.2 Retention diagrams of n-pentane (*), n-hexane (o), n-heptane ( A ) nnd n-

octane (*) on CAP in the temperature range 333.15-393.15K.

and J -butan01 (7) on CAP in the temperature range 333.15-393.15K.

Fig.6.4 Retention diagrams of esters: methyl acetate (I), ethyl acetate (e) and I-butyl

acetate (A) on CAP in the temperature range 333.1 5-393.15K.

Fig.6.5 Retention diagrams of dichloromethane (r), trichloromethane (o) on CAP in

the temperature range 333.15-393.15K.

Rg.6.6 Retention diagrams of 2-alcohols: 2-propanal (0 ) nnd 2-butanol (9) on CAP

in the temperature range 333.15-393.15K.

Fig.6.7 Retention diagrams of acetone (r) and diethyl ether (*) on CAP in the

temperature range 333.15-393.1 5K.

Fig.6.8 Retention diagrams of benzene (I), toluene (a) on CAP in the temperature

range 333.15-393.15K.

Fig.6.9 Retention diagrams of I, 4-dioxane (o), tetrahydrofuran ((I) on CAP

in the temperature range 333.1 5-393.15K.

6.2. Weight fraction activity wffdent, a,"

The partial molar weight fraction activity coefficient, R; for the interaction

of a solute at infmite dilution on CAP surface cm b nl.4 to V: by the following

equation2'

where MI is the molar mass of the solute and R (8.314 J ~ ' m o l . ' ) is gas constant.

9 4 and are molar volume, saturated vapour pressure and second virial

coefficients of the solute at temperature T. Vl and 4 were estimated in the

temperature range 333.15 - 393.15K following the standard methopds reported in

the litmature and the necessary parameters required in the evaluation were taken

from the literature. 2' The second virial coefficients, B,, were calculated by

utilizing the Tsonopoulos m e t h ~ d . ~ V, , f: and R,, values are given in Tables. 2.5,

2.6 and 2.7 in chapter 11.

The weight fraction activity coefficients at infinite dilution, Inn" , for 21

solute probes were calculated according to Eq.6.3 in the temperature range 333.15 - 393.15 K. The Innm are listed in Table.6.2. The Inn" values are decreasing with

increase of temperature. The Inn" values are increasing with increase of chain

length in the n-alkane series. The magnitude of In 0" is a measure of interaction of

the solute probe with the solid surface of CAP. The various types of molecular

interactions viz. dispersive, polar-induced polar, dipolar and hydrogen bonding

contribute to the values of 1nR". In the n-alkane series only dispersive interactions

are present between CAP and n-alkane. The dispersive interactions are increases

with increase of hydrocarbon chain length and hence InR" values increased with

increase of n-alkane chain length. In the case of 1-alcohols and 2- alcohols both

hydrogen bonding and dispersive interactions are present in between CAP and

alcohols. As the chain length increases, the strength of hydrogen bonding decreases

and dispersive interactions increases. Therefore the variation of Innm with chain

length in alcohols depends on the resultant of the two interacting effects. The results

indicate that the dispersive interactions dominate over hydrogen bonding in alcohols.

Esters can form hydrogen bonds with -OH groups present on CAP surfaces. The

variation of ha" with chain length in esters depend on hydrogen bonding, dipolar

and dispersive interactions. The results indicate that the dispersive interactions

dominate and hence InQm values are increasing with increase of chain l a & in

esters. The variation of 1nRm with carbon number in n-alkanes, alcholos and esters

is shown in Figs. 6.10 to 6.13. The increase in Inn" with chain length is almost

linear in n-alkanes and esters at all t e m p e r a m . However in 1-alcohols the

variation of In Q" with chain length is linear at lower temperatures rmd nonlinear at

higher temperatures.

Table 6.2. The partial molar weight fraction activity coefficients, InRm, of solutes on

cellulose acetate phthalate in the temperature range 333.1 5-393.1 5K.

Temperature (K) Solutes

333.15 343.15 353.15 363.15 373.15 383.15 393.15

n-Pentane 2.736 2.627 2.463 2.360 2.300 2.252 2.288

n-Hexane 3.264 3.094 2.894

n-Haptane 3.637 3.493 3,306

n-octane ' 3.974 3.892 3.640

Acetone 2.956 2.791 2.668

Ether 2.792 2.716 2.624

Methanol 2.144 2.085 1.989

Ethanol 2.219 2.141 2.073

I -Propanot 2.340 2.256 2.173

1-Butanol 2.412 2.434 2.261

Benzene 2.823 2.733 2.671

Toluene 3.166 3.015 2.912

2-Propanol 2.625 2.561 2.430

2-Butanol 2.768 2.672 2.515

Dichloromethane 1.585 1.499 1.406

Trichloromethane 1.822 1.687 1.605

Methyl acetate 2.089 1.923 1.772

Ethylacetate 2.522 2.305 2.128

1-Butylacetate 3.012 2.926 2.904

1,4-dioxane 2.424' 2.242 2.048

THF 2.405 2.347 2.200

Fig.6.10 The variation of In Qm with hydrocarbon chain length, of n-alkanes at seven

temperatures on CAP.

Fig.6.11 The variation of Inn" with carbon nuniber, Z, of I-alcohols at seven

temperatures on CAP.

Fig.6.12 The variation of Innm with carbon nurnbcr, Z, of 2-alcohols at seven

temperatures on CAP.

Fig.6.13 The variation of In R" with carbon number, Z, of n-acetates at seven

temperatures on CAP.

6.3. Flory-Huggins interaction parameter,

The Flory-Huggins interaction parameter, x;, for the solute on the solid

surface of CAP is related to In R" as fol~ows.~'

M,u* ~ ; = l n R , ~ +In- - 1 v, (6.4)

where u2 is the specific volume of the polymer. For CAP polymer specific volume is

given by 3.759 cm31g. The Flory-Huggins interaction parameter, xP,, was evaluated

using Eq. 6.4. The XI", values are given in Table.6.3. The z; is a measure of the

interaction of solute, I, with the polymer CAP, 2, at infinite dilution of solute on the

CAP. For n-alkanes X; values are positive and grater than 0.5. The variation of

X; with temperature is shown in Figs.6.14 to 6.19 for eight youps of solute probes.

The X; values are decreasing linearly with increase of temperature. As shown in

Fig.6.20 at any one temperature x;", values are increasing with increase of chain

length in the n-alkane series. The trend for aromatic compounds was similar to n-

alkanes, where xP; values are increasing with increase of size of the compound. As

shown in Fig.6.21, the xP; is not linear with the chain length of I-alcohols. This

behaviour may be interpreted primarily using the values of and p, as well as

how these values are changing with size of the solute. The X; is inversely

proportional to V; and p, of the solute. In n-alkanes x,", increased with increase of

chain length; here p, is crucial, which is higher for lower n-alkanes. In 1- alcohols,

due to hydrogen bonding, the vapour pressure is relatively less and the X; values

only slightly decreased with increase of chain length. The V i values are importnnt in

determining the trend in 1- alcohols. On the other hand, in esters, x;", values are

slightly increased with increase of chain length, which is due to slightly higher

vapour pressures in esters compared to I - alcohols. The variation of x,*, for aters

was intermediate between n-alkanes and 1- alcohols.'

Table 6.3. Floty-Huggns interaction parameter, X; , of solutes at infinite

dilution, on cellulose acetate phthalate in' the temperature range 333.15 - 393.15K.

Temperature (K) Solutes

333.15 343.15 353.15 363.15 373.15 383.15 393.15

n-Pentane

n-Hexane '

n-Haptane

n-octane

Acetone

Ether

Methanol

Ethanol

I -Propano1

1 -Butan01

Benzene

Toluene

2-Propanol

2-Butanol

Dichloromethane

Trichloromethane

Methyl acetate

Ethyl acetate

I-Butyl acetate

1,4-dioxane

THF

Fig.6.14 The variation of xT2 values with temperature on CAP for n-alkanes : n-

pentane ($1, n-hexane (I), n -heplane (*) and n- octane (A) .

Fig.6.15 The variation of xr2 values with temperature on CAP for I-alcohol:

methanol (I), ethanol (a), 1 -propano1 (*) and I - butanol(o).

T(K)

Fig.6.16 The variation of ~ 7 ; values with temperature an CAP for esters: methyl

acetate (m) ethyl acetate ( 0 ) and n- butylacetate (0).

Fig.6.17 The variation of xrz values of benzene (8) and toluene (o) on CAP in the

temperature range 333.15-393.1 5K.

Fig.6.18 The variation of xT2 values of acetone (m) and diethyl ether (a) on CAP in

the temperature range 333.1 5-393.15K.

Fig.6.19 The variation of 1r2 values of 1, Cdioxane (m) and tetrahydrofuran (a) on

CAP in the temperature range 333.15-393.15K

Fig.6.20 The variation of X; with carbon number, Z, of n-alkanes at seven

temperatures on CAP.

z

Fig.6.21 The variation of xh with carbon numba, Z, of 1-alcohols at seven

temperatures on CAP.

6.4. Hansen solubility parameters

According to Hildebrand 26 the solubility parameter for a volatile

solute is related to its cohesive energy, E , and molar volume, y as

follows.

However the application of the above equation to polymers and nonvolatile

substances is not possible due to their negligible vapour pressure. Further, the

Hildebrand solubility parameter model proposed for regular solutions may not be

able to describe adequately the solubility behavior when polar and hydrogen

bonding solvents are involved in the system. Hansen addressed this problem by

introducing the three dimensional solubility parameter model.'* According to

Hansen theory the total cohesive energy is approximated as a sum of three

independent contributions arising from dispersive, E d , polar, E,, nnd hydrogen

bonding , E,, interactions respectively.

E d = Ed + EP + Eh

Therefore the total solubility parameter is derived as follows. By dividing Eq. 6.6

with molar volume V, we get

6; =6,? +6: +6,2 (6.8)

wheres,, 8, and 6, are the Hansen solubility parameters (HSP) representing

dispersive, polar and hydrogen bonding components rapectively. The determination

of HSP by IGC for materials which are in the liquid state at the temperature of the

experiment have been reported extensively in the literature. However, studies

relating to the determination of HSP for solid rr~atcrialv by IGC are scanty. Voelkel

'O developed a method for the determination of HSP for a solid surface by IGC

following the Snyder-Karger adsorptioq model." Accordingly, the energy of

adsorption of test solute '1' on solid adsorbent '2' is given by the energy balance

expression

-AE* = E~~ = ( ~ ~ ~ j ~ ~ ( ~ ~ ~ j ' ~ = Y , (d l ) (6.9)

where BEA is the of adsorption, v, is the molar volume of the test solute,

and ED are the cohesive mergis for the t a t solute '1' and the a d s o h t '2',

and Elz is the energy of cohesion for the interaction between 'I ' and '2'. S, and

are solubility parameters of solute ' 1 ' and the adsorbent '2'. Introducing Eq.6.6

in Eq.6.9 gives.

- A ~ A = ( E I Z ) d + ( E 1 2 ) p t (E12)D (6.10)

or

- a E A = V, (#&+ G f 8 + G:&) (61 1)

The energy of adsorption A,@ is related to the specific retention volume by the

following equation.

- A E A = ~ T l n t const (6.12)

From Eqs. 6.1 1 and 6.12 one can derive the following relation.

R ~ l n v ; = v,G;'&t VISPG$ + v16:6: +const (6.13)

where d, 6{ and i$ are the dispersive component, polar component and hydrogen

bonding component of Hansen solubility parameters for cellulose acetate phthalate.

Linear multiple regression on Eq.6.13 has been performed using the software

Orginpro 7.0. The specific retention volumes of 21 solutes at one temperature have

been used in the regression analysis at one time. The Hansen solubility

61 and 6: of the solid surface of CAP are obtained from the

coefficients of the multiple regression analysis. The statistical analysis has been

repeated at other temperatures and the results are yjvm in Table.6.4. The solute

&tag:, 6/' and 8; required in this evaluation are given in Table 2.9 in Chapter 11.

The Hansen solubility parameters for the 21 solutes at 298.15K were taken

from the literature.'' The d , s/' and values at the experimental temperatures

were calculated using the ,relations proposed by Hansen and Beerbower. 24 " The

HSP of all the solutes at 363.15K and their temperature gradients are given in Table

2.9 in chapter 11. The solubility parameter data can be related to many different

physicochemical characteristics and used for the interpretation of several

phenomena, such as miscibility, solubility and adsorp t i~n .~ The HSP of a CAP can

be determined using Eq.6.13. The three components of the Hansen solubility

p-&ers,,jf, &f and 8 , of the 21 solutes were fitted as a fbnction of the left

hand side of fhe E9.6.13 by multiple regression d y s i r at esch tan-. Tbc mfficients of the multiple regression war used to obfain the dispaive

componm,&, Polar component, dl, and hydrogen bonding component. 6: of the

solid surface of CAP. The three components ,$. ~f and ,$ wen used 'in Eq.6.8 to

calculate the total solubility parameter, 6:. The solubility parameter values along

with the e ~ ~ o r s in these components are given in Table.6.4. The variation of the HSP

with temperature is linear in the temperature range studied. The values of the

Hansen solubility parameters are in the following order:

5;1> &>sf The & values are higher compared to ;Sp which may be due to the presence

of relatively higher hydrogen bonding interactions than dipolar interactions between

the polar solutes and the CAP surface. The emrs in the&, fif and hj are

decreasing with increase of temperature. The values of correlation coefficients, r,

given in Table 6.4 indicate that the regression model was better at lower

temperatures than at higher temperatures. The relative errors in HSP are calculated

using the relation.

S Relative error = = .

X

where s is standard deviation and ;is mean value. For example the relative errors

in&, and & at 333.15 K are, 0.37,0.51 and 0.19 respectively. A comparison of

relative errors in the three components indicates that they are in the following order.

s:<bi </-Sf

Further, the variation of the solubility parameters with temperature is shown

in Fig.6.22. & and 6 values are decreasing with increase of temperature where as

Jf values are nearly constant. The plots are used to extrapolate the data to 298.1 5K

and the extrapolated results are also included in Table.h.4. During the application of

CAP in the design of membrane materials? in the coating formulations and as

adsorbent materials, the Hansen solubility paran~et!xs are often required at room

temperature or at 298.15 K. Therefore the extrapolated HSP given in Table.6.4 can

be utilized to understand the behaviour of CAP in presence of different materials at

ambient temperatures.

Table 6.4. The Hansen solubility parameters for solid surface of CAP calculated

using Eq.6.13, at seven temperatures: dispersive, &, polar, &g , hydrogen bonding,

& and 6: total solubility parameter. r, is the correlation coefficient, of the

regression.

T(K) 8 WPa) In 85 (MPa) In 6: (MPa) 'I2 ,$ (MPa) '" r

*Extrapolated data

Fig.6.22 The variation of Hansen solubility permeters with temperature:

dispersive, 8 (a),, polar, 6{ (o), hydrogen bonding & (m), total solubility

parameters 6; (x) the plots are drawn using the data given in Tables 6.4 and

extrapolated to 298.15K.

6.5. Enthalpy of adsorption

The enthalpy of adsorption,LW,, for the adsorption of a solute on solid

surface of CAP has been calculated using the following relation. '

The values of enthalpy of adsorption, AHa, are given in Tnble.6.5. AH,,

values are negative for the 21 solutes. The AH, values are more negative for polar

solutes compared to n-alkanes. This may be attributed to the fact that in n- alkanes

only dispersive interactions present where as in polar solutes in addition to

dispersive interactions the CAP surface also exhibit the dipolar and hydrogen

bonding interactions. The variation of Mi, with hydrocarbon chain length in n-

alkanes and 1-alcohols has been shown in Fig. 6.23. The AH, vnlues are linearly

increasing with increase of chain length in n-alkanes where as in I-alcohols the

variation is not linear.

Table 6.5 Enthalpy of adsorption, -AH,, (k.Jmol-') of solutes on the cellulose

acetate phthalate (CAP) surface in the temperature range 343.1 5 - 403.1 5 K.

Solutes - AH, (ldmof')

Acetone 16.1 1k0.32

Diethyl ether 19.4kk0.53

Methanol

Ethanol

I -Propano1

1 -Butan01

Benzene

Toluene

2-Propanol

2-Butnaol

Dichloromethane

Trichloromethane

Methyl acetate

Ethyl acetate

1-Butyl acetate

1 ,4 -~ ioxhe

THF

Fig. 6.23 The variation of enthalpy of adsorption, -AH, with carbon number, Z, of

n-alkanes (m), and alcohols (I) on CAP solid surface.

Conclusion

The surface thermodynamic properties for cellulose acetate phthalate are

determined by IGC. The surface thermodynamic p r o w = for cellulose acetate

phthalate are determined by IGC technique. The partial molar weight fraction

activity coefficients, Innm, and Flory-Huggins interaction parameter, x i , are

calculated using the measured specific retention volumes, V: .The Inn" and X:

values have been used to understand the interactive ability of CAP surface with

different solutes and as function of temperature. The specific retention volumes of

21 solute probes have been used to calculate the three components of Hansen

solubility parameters. The dispersive component, b j , and hydrogen bonding

component, s:, are decreasing with increase of tempernturc where as polar

component, 84, is nearly constant in the temperature range studied.

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