Arcillas

C lay Minerals (1966) Ii, 143.

UREA

COMPLEXES
AN INFRARED

WITH MONTMORILLONITE:
ABSORPTION

STUDY*

M . M. M O R T L A N D t
D epartment of Soil Science, Michigan State University,
E ast Lansing, U.S.A.
( Received 22 O ctober 1965)

A B S T R A C T : Urea was observed to become protonated in H-, Fe-, and Al-montm orillonite films and to form hemisalts when urea was present inexcess of the
n umber of available protons. The fully protouated urea disappeared upon dehydrat ion, then the original protonated condition was re-established by rehydration.
T his completely reversible reaction indicates the importance of water in the prot onated form. Indications were that urea will coordinate through the carbonyl
g roup to the metal ion in Cu (II), Mn (II), and Ni (II)montmorillonite. Urea
m ay be bonded to the metal ion in Mg-, Ca-, Li-, Na-, and K-montmorillonite
b y coordination and possibly by ionization of the N - H bond. The importance of
i on-dipole interaction in urea complexes with montmorillonite is suggested.
D ecomposition of urea to ammonium ions was observed mainly in the Cum ontmorillonite system.
T h e m e c h a n i s m s of interaction betweenclays a n d organic c o m p o u n d s have been
r eviewed recently b y G r e e n l a n d (1965). T h e n a t u r e of the interaction between u r e a
a n d silicate m i n e r a l surfaces has been j u d g e d b y M i t s u i & T a k a t o h (1963) to be
p r i m a r i l y t h r o u g h h y d r o g e n b o n d s arising b e t w e e n the c a r b o n y l a n d a m i n o g r o u p s
o f the u r ea a n d e x p o s e d h y d r o x y l groups a n d oxygen a t o m s of the silicate surface.
H o w e v e r , since minerals such as m o n t m o r i l l o n i t e possess a high c a t i o n exchange
c a p a c i t y which m a y be n e u t r a l i z e d b y h y d r o g e n o r m e t a l ions, p r o t o n a t i o n o f the
u r e a might be expected in the first case a n d i o n - d i p o l einteraction in the second.
T h e w o r k r e p o r t e d here concerns the n a t u r e of the interaction between u r e a a n d
m o n t m o r i l l o n i t e p a r t i c u l a r l y as related to the k i n d of the e x c h a n g e a b l e cation.
METHODS
T h e m o n t m o r i l l o n i t e used in this study was H-25 f r o m U p t o n , W y o m i n g , supplied
b y W a r d s N a t u r a l Science Es t a b l i s h m e n t . H o m o i o n i c clays were p r e p a r e d b y the
* A uthorized for publication by the Director as Journal Article No. 3695 of the Michigan
A gricultural Experiment Station, East Lansing, Michigan.
t P rofessor of Soil Science. This work was supported by National Science Foundation
P roject GP 1978.
B

144

M . M. Mortland

s ame techniques employed byFarmer & Mortland (1965). Fresh H-montmoriUonite
w as prepared by passing suspension of the Na-clay through a colunm of H-IR120
r esin.
F resh, reagent grade urea was used to prepare a standard solution and aliquots
o f this were added to 50 mg clay suspensions. Three levels of urea were used in
p reparing urea-clay complexes--one-half the cation exchange capacity (C.E.C.) (50
m -moles urea per100 g of clay), amount equal to the C.E.C. (100 m-moles urea
p er 100 g of clay), and twice the C.E.C. (200 m-moles urea per 100 g of clay).
A fter the urea-clay systems were equilibrated overnight, the suspensions were
p oured into small dishes of aluminium foil and the water evaporated away. The
r esulting thin clay films (2-5 m g / c m ~) could be readily stripped from the aluminium.
Infrared spectra were obtained by mounting the clay films at right angles to the
b eam of a Beckman IR-7 spectrophotometer. Thus infrared spectra were obtained
o n self-supporting clay films treated with urea.
REACTIONS

OF UREA

WITH ACID MONTMORILLONITE
SYSTEMS

F igs. 1 and 2 show the spectra of urea absorbed on H-, Fe-, and Al-saturated
m ontmorillonite. The features of these spectra...