J. Nig. Soc. Phys. Sci. 3 (2021) 340–343

Journal of the
Nigerian Society

of Physical
Sciences

Preparation and characterization of Zinc metal soap from shea
butter (Vitellaria paradoxa)

O. Amosa,∗, T. E. Odetoyeb, D. S. Ogunniyib

aDepartment of Industrial Chemistry, Federal University, Lokoja, P.M.B 1154, Lokoja, Nigeria
bDepartment of Chemical Engineering, University of Ilorin, P.M.B 1515, Ilorin, Nigeria

Abstract

Zinc metal soaps are of great importance in the manufacture of personal care products and other industrial applications. Variations in the soaps and
their properties are usually due to the type of oil used in the synthesis. Shea butter (Vitellaria paradoxa), being a valuable industrial raw material,
was investigated for the synthesis of zinc metal soap. Locally obtained shea butter was characterized, refined and used to synthesize metal soap of
zinc which was characterized. The zinc soap produced exhibited an off-white appearance, pH of 7.8, non-foaming, and no free alkalinity present.
The functional groups in the soap were confirmed by FTIR.

DOI:10.46481/jnsps.2021.238

Keywords: Zinc metal soap, Shea butter, FITR.

Article History :
Received: 2 August 2021
Received in revised form: 7 October 2021
Accepted for publication: 8 October 2021
Published: 29 November 2021

c©2021 Journal of the Nigerian Society of Physical Sciences. All rights reserved.
Communicated by: E. A. Emile

1. Introduction

Shea butter is a common fat extracted from the fruit of the
African shea tree (Vitellaria paradoxa) that is available in many
African countries in the wild [1]. The shea tree fruits are usually
collected by women while the fat is obtained traditionally by
boiling the kernel in water and scooping the solidified fats after
cooling. The butter is also obtained by screw press. It usually
presents yellow colour in the raw form with the processed ver-
sions being ivory or whitish in colour [2]. Shea butter consists
mainly of stearic and oleic fatty acid moieties in its triglycerides
[3]. It is widely used in cosmetics as a moisturizer, salve, lotion
and medicinal purposes [4]. Shea butter is edible and used as an

∗Corresponding author tel. no: +2348030677482
Email address: olusegun.amos@fulokja.edu.ng,

olusegunamos@yahoo.com (O. Amos )

alternate butter in chocolate production [1]. It has been reported
as a potential raw material for biodiesel production [5].

Metal soaps are alkaline-earth or heavy-metal long chain
carboxylates that are insoluble in water but soluble in non-aqueous
solvents. Metal soaps can be represented with a general for-
mula (RCO 2 ) 2 M where M can be Zn, Cd, Pb, Ba, Ca, Co,
Cu, Al, Fe, e.t.c. and R is a linear or branched alkyl group [6].
Metal soaps can be produced using any of the following pro-
cesses: double decomposition (metathesis), direct reaction of
carboxylic acid with metal oxides, hydroxides and carbonates
and direct reaction of metals with molten fatty acid [7].

Metallic soaps have been found to be useful in various fields.
Metal carboxylates are significant in the production of poly
vinyl chloride (PVC) which is a vital commercial polymer [8].
Metal carboxylates have been used as green initiators for the
ring-opening alternating copolymerization of cyclic anhydride
and epoxide [9]. Metal soaps from non- conventional oils can

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O. Amos et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 340–343 341

also be utilized as paint additives, stabilizer for PVC and for in-
hibition of corrosion in metals and used as grease [8],colouring
vanishes, cosmetics and textiles [10]. The metal soaps of lead,
manganese, cobalt and zinc soaps are used in paints to accel-
erate drying while copper soaps exhibit fungicidal properties.
Soaps of zinc, iron, nickel, cobalt and chromium have been ap-
plied for the production of water proofing leather and canvas.

Metal soaps are also important in the studies of degradation
in paints, their formation resulting in chemo-mechanical dam-
age in historical oil paintings have been reported [11]. The sizes
of the metal soap nucleus have been found to influence unde-
sirable metal soap crystallization and growth in historical oil
paintings [11, 12]. Hence, there is a need for more research to
be directed towards the production and properties of more metal
soaps from various oil sources for more suitable applications.
Moreso, there is a growing demand for metal soaps as polymer
additives [13]. In pharmaceutical field, zinc stearate is a com-
ponent of the familiar facial or dusting powder and serves as an
antiseptic emollient when incorporated with petrolatum. Zinc
stearate is also useful as an activator in the accelerated vulcan-
ization of rubber. Utilization of shea butter in the production of
metallic soap is a potential means of promoting agro-industrial
economy of the African countries where the shea trees abound.
The aim of this study is to investigate the preparation of zinc
metal soaps from shea butter as a potential industrial raw ma-
terial for metal soap production. Based on the review of earlier
literature, works on zinc soap of shea butter is scarcely reported.

2. Materials and Methods

2.1. Source of material

The Shea butter (Vitellaria paradoxa) was purchased from
Kabba Central Market in Kogi state, Nigeria.

2.2. Refining of shea butter

A portion of the shea butter was refined via sequential steps
of degumming, neutralization and bleaching in accordance to
the methods described in literature [14, 15] with little modifi-
cation of washing the oil several times with hot water during
neutralization process to ensure complete removal of the foot.

2.3. Characterization of the butter

The physical and chemical properties of the shea butter sam-
ple were analyzed according to ASTM standards [16] for the
determination of density, specific gravity, acid value, iodine
value and saponification value. The functional groups in the
shea butter were determined by the Infrared spectrophotome-
ter while the fatty acid constituents of shea butter were also
recorded on the Agilent Gas Chromatograph-Mass Spectrome-
ter (Model 7890A GC system,5675C inert MSD with triple axis
detector).

Figure 1: Saponification of oil to form sodium soap

Figure 2: Precipitation of zinc soap

2.4. Synthesis of Zinc Metal Soap

Metathesis, which is the mostly used method involves two
reactions. Firstly, the aqueous caustic is reacted with fatty acid
and subsequently, the reaction of the formed sodium salt with
the inorganic salt of the desired metal as shown in Figures 1 and
2.

The saponification reaction is usually carried out at high
temperature and in the presence of excess water to dissolve
the glycerol formed in the reaction mixture. Consequently, this
disallowed the reactions between glycerol (by-product) and the
product of metal soap.

The zinc metal soap was prepared by metathesis in aque-
ous alcoholic solution as summarized in Figure 3, where R in
Figures 1 and 2 is stearic acid.

Figure 3: Summary of zinc metal soap formation

9.2 g of refined shea butter was dissolved in 10 ml of ethanol
at a temperature of 70◦C followed by the addition of 5 ml of 5M
NaOH solution. To this mixture, 20 ml of 1.86M ZnSO4 solu-
tion was slowly added with continuous stirring. Upon addition
of the solution, the precipitation of the metal soap commenced
[8] with little modification). The metal soap that was precip-
itated was then filtered off and washed with 30 ml hot water
(80◦C) and air dried at room temperature for 48 h. The colour
of the metal soap was noted and the yield was calculated.

2.5. Characterization of the Zinc metal soap synthesized

The properties of the zinc metal soap prepared were de-
termined for pH, free caustic alkalinity, foaming power and
stability[8]. As stated by Osmond [17] that FTIR are highly ef-
fective for the characterization of metal soaps, the spectra were

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O. Amos et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 340–343 342

also recorded on Fourier Transform Infra-Red spectrophotome-
ter.

3. Results and Discussions

3.1. Properties of shea butter

The melting point of the crude shea butter was 32◦C while
its cloud point was found to be 24◦C. Table 1 shows the results
obtained for the properties of both the crude and refined shea
butter.

The values obtained for shea butter, both refined and un-
refined shown in Table 1. When compared with the values
obtained by Chiboret al [18],a high acid values of 4.48 and
5.32 for the crude and refined butters respectively indicated that
the oil sample contained more free fatty acids thus indicating
its exposure to rancidity. The values depend on the method
of extraction as the acid values obtained by solvent extraction
and cold press methods were 3.50 and 9.54 mg KOH/g respec-
tively [19]. The saponification values obtained for the crude
and refined samples were 122 mg/KOH and 150 mg/KOH, re-
spectively. These values were smaller to the one reported by
Abdul-Hammed for the solvent extracted (172.2mg/KOH) and
cold press extracted (185.7 mg/KOH) methods [20, 21, 22, 23].
A high saponification value shows that the oil is suitable for in-
dustrial use, especially for soap making [24]. The desirably low
iodine number of shea butter is indicative of the rich saturated
fatty acids contents of the oil, which ensure stability against
oxidation and rancidity of food prepared from the oil. It also
promotes desirable properties for shortening and margarine pro-
duction.

3.1.1. Physicochemical properties of prepared zinc metal soap
Figure 4 shows the metal soap produced.
The properties of the prepared zinc metal soap are as shown

in the Table 2.
The appearance of the zinc metal soap prepared is as shown

in Figure 4. The metal soap was milky/off-white in colour as
a characteristic of zinc salts. Zinc salts are not expected to be
coloured since it is not a transition metal. Zinc metal soap being
white can be applicable in white paints as pigments and paint
driers [8].

The pH of the metal soap was 7.8 which is neutral in nature
and thereby making it safe to handle. The yield of 6.38 38
g (98%) is a good one and also a signal of high profitability if

Table 1: Physicochemical properties of crude and refined shea butter sample

Properties Crude Oil Refined Oil
Density (g/cm3) 0.69 0.69
Specific gravity 0.90 n.d

Acid value 4.48 5.32
Iodine value 3.04 4.568

Saponification value 122 150
n.d – not determined

Figure 4: Prepared Zinc metal soap sample.

Table 2: The properties of the zinc metal soap prepared

Metal
Soap

Colour Weight
(g)

pH Foam
Stability

Free
Caustic
Alkalinity

Zinc
metal
soap

Milky 6.38 7.8 No foam Nil

industrially exploited. . The metal soap produced was a mixture
of fatty acid carboxylates of zinc, mainly zinc stearate since the
major fatty acid components of the shea butter was stearic acid
There was no free caustic alkalinity present in the synthesized
d metal soap which indicated that there are no alkalinities left
in the soap. The zinc metal soap prepared has poor detergency
properties due to its hydrophobic property.

Figure 5: FTIR of refined shea butter.

Figures 5 and 6 showed the infrared spectra of the refined
shea butter and that of Zn metal soap in region 4000 to 500 cm−1

for both samples respectively. From the spectra, the absorption
band at 2926 cm−1 was the asymmetry vibration of CH2 stretch,

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O. Amos et al. / J. Nig. Soc. Phys. Sci. 3 (2021) 340–343 343

Figure 6: FTIR spectra of zinc metal soap.

while that at 2850 cm−1 was due to the symmetry vibration of
CH2 stretch. The C = O stretch of triglyceride linkage was seen
at 1742 cm−1. The small absorption band observed at 1669
cm−1 was the characteristic carbonyl stretching band of ester
linkage (C = O). It was observed that the absorption at 1710
cm−1 due to C = O of free fatty acids in the shea butter (Figure
5) has disappeared in the metal soap (Figure 6). The C − H
absorption of bending vibration of CH2 and CH3 bands was
observed at 1470 cm−1. The bands of CH2 bending vibrations
were also observed at 1381 and 1109 cm−1. The absorption due
to C − O ester bonds were observed at 992, 830 and 1097 cm−1

The CH2 rocking vibration can be clearly seen at 722 cm−1.
The FTIR spectra confirmed the functional groups fatty acid
carboxylates of zinc produced.

4. Conclusion

In this study, the preparation of zinc metal soap from shea
butter was successfully done using the double decomposition
(metathesis) method. The shea butter metal soap produced ex-
hibited desirable characteristic metal soap colour, hydrophobic
and free caustic alkalinity properties. FTIR spectra confirmed
the functional groups of the soap. Successful production of
metallic soap from shea butter establishes its potential use as
an industrial raw material in addition to the traditional uses.
More attention is to be paid to industrial application of shea
butter in African communities towards the development of the
agro-industrial economy. Further work can be done on the pro-
duction of high purity zinc stearate from shea butter.

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