Method of adhering substrates

A method of adhering two substrates comprising covalently modifying a smooth surface of at least one of the substrates is disclosed. A smooth surface of a first substrate is covalently modified to present groups, which are one member of an acid-base pair (MABP groups). The covalently modified surface is contacted with a smooth surface of a second substrate. The smooth surface of the second substrate presents groups are the acid-base complement of the MABP groups (MABP complements). The interaction of the MABP groups with the MABP complements results in the formation of an adhesive bond between the smooth surface of the second substrate and the covalently modified surface of the first substrate. The MABP complements may inherently be present on the surface of the second substrate or the surface of the second substrate may be modified to present the MABP complements.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a method of adhering substrates and to an 
article formed by the method. In particular, the method of adhering 
substrates includes covalently modifying a surface of at least one of the 
substrates. 
BACKGROUND OF THE INVENTION 
In many applications, particularly those involving electronic or optical 
materials, it is frequently desirable to adhere two substrates together in 
a manner that avoids substantially altering the properties of the 
substrates at their interface. 
There are a variety of ways of adhering two substrates together. One common 
method involves coating a surface of at least one of the substrates with a 
relatively thick layer of an adhesive prior to bringing the substrates 
into contact. While this method may produce a joint with good mechanical 
strength, the presence of the layer of adhesive may alter, sometimes 
dramatically, the physical characteristics of the substrates or the 
substrate-substrate interface. This can be a particular problem in 
electronic applications which have exacting requirements and often entail 
bonding very thin sheets of material. 
Van der Waals forces have been utilized to bond two substrates together 
without the addition of an adhesive. Van der Waals forces, however, are 
relatively weak and are easily disrupted by environmental contaminants. To 
achieve any significant bonding by dispersion forces alone requires 
exceedingly clean, exceedingly smooth surfaces. In addition, van der Waals 
forces drop off rapidly with separation (e.g., decrease as 1/D.sup.3 for 
two flat planes, where D is distance). These factors make the range of 
applications and materials to which bonding based on van der Waals forces 
can be applied, extremely limited. 
Acid-base interactions offer the potential to form a stronger, less 
environmentally sensitive adhesive bond, i.e., an adhesive bond which is 
less sensitive to the presence of contaminants. Thus far, this approach 
has been confined to the application of relatively thick interaction 
layers onto substrates, to interdiffused polymers, and to polymer-surface 
adhesion (e.g., the enhancement of the adhesion of paints or coatings to a 
metal substrate). All of these techniques can substantially alter the 
physical characteristics of the adhered substrates and/or the article 
produced. In particular, these methods of adhering substrates may 
substantially alter the dielectric, optical, thermal or mechanical 
properties of the substrates or of the substrate-substrate interface. 
It is therefore an object of the invention to provide a method of adhering 
substrates to form an article without substantially altering the physical 
characteristics of the substrates. More particularly, it is an object of 
the invention to provide a method of adhering substrates which does not 
substantially alter the dielectric, optical, thermal or mechanical 
properties of the substrates in the vicinity of the substrate-substrate 
interface. 
It is a further object of the invention to provide an article including two 
substrates, which are strongly bonded together despite the lack of a 
discrete, substantial adhesive layer between the substrates. 
It is another object of the invention to provide a method of adhering two 
substrates, which have substantially identical physical properties, to 
construct an article. For example, the method permits the adhering 
substrates, which are formed from identical material, to form an article 
which has very similar physical properties to that of a single substrate 
formed from the material. 
It is yet another object of the invention to provide a method of adhering 
substrates in such a manner that the substrates may be separated and 
readhered without any further application of an adhesive. Further, another 
object is to provide a method of adhering substrates that forms an 
adhesive bond between the substrates that is capable of reforming after 
rupture due to shear. 
These and other objects and advantages of the present invention will be 
apparent from the description of the invention which follows. 
SUMMARY OF THE INVENTION 
The present invention provides a method of adhering substrates which 
includes covalently modifying a smooth surface of a first substrate to 
present groups, which are one member of an acid-base pair (MABP groups), 
and contacting the covalently modified surface with a smooth surface of a 
second substrate. The smooth surface of the second substrate presents 
groups, which are the acid-base complement of the MABP groups (MABP 
complements), i.e., the other member of the acid-base pair. When the 
smooth surface of the second substrate is brought into contact with the 
covalently modified surface, the two surfaces adhere. 
According to another aspect, the present invention includes a method of 
adhering substrates comprising modifying a smooth surface of a first 
substrate by covalently bonding a monolayer of a modifying agent to the 
surface and contacting the covalently modified surface with a smooth 
surface of a second substrate. The modifying agent includes a MABP group 
and the smooth surface of the second substrate presents MABP complements. 
The present invention also provides an article which comprises a first 
substrate including a smooth surface, which is covalently modified to 
present MABP groups, and a second substrate, which includes a smooth 
surface which presents MABP complements. The smooth surface of the second 
substrate is adhered to the covalently modified surface of the first 
substrate. 
DETAILED DESCRIPTION OF THE INVENTION 
The present invention provides a method of adhering substrates. One 
embodiment of the method comprises covalently modifying a smooth surface 
of a first substrate to present MABP groups. The covalently modified 
surface is contacted with a smooth surface of a second substrate, which 
presents MABP complements. When the two smooth surfaces are brought into 
contact, the surfaces adhere to each other. The mechanism of this adhesion 
is not completely understood, but it is believed that when the two 
surfaces are brought into contact, charge transfer between the two 
surfaces occurs due to proton transfer between the MABP groups and the 
MABP complements. 
The substrates in the method of the present invention may be configured in 
any one of a large variety of shapes, including sheets, blocks or more 
complex shapes. Each substrate is configured to include at least one 
smooth surface. The precise configuration of the substrates for any given 
application will obviously depend on the particular requirements of that 
application. The method is especially useful for adhering substrates where 
at least one of the substrates is a thin sheet. 
The substrates may be formed from any material that is capable of being 
fashioned to include a smooth surface. At least one of the substrates is 
formed from a material that is capable of being covalently modified to 
present MABP groups or to bond a monolayer of a modifying agent to a 
surface of the substrate. Typically, the substrates are formed from 
nonmetallic materials and preferably from nonmetallic, inorganic 
materials. Preferably, at least ore of the substrates is formed from a 
material which is resistant to electronic breakdown. Electronic breakdown 
could give rise to the formation of a conductive path through the 
substrate, which may lead to a dissipation of surface charge and hinder 
the formation of an adhesive bond to another substrate. More preferably, 
at least one of the substrates is formed from an insulating material. 
Exemplary materials which may be used to form the substrates include 
silica, oxidized silicon (e.g., a silicon wafer with a surface layer of 
native oxide), alumina, titanium oxide, chromium oxide, tin oxide, 
germanium oxide, and silicate-containing materials (e.g., calcium 
silicate, borosilicate or aluminosilicate). Preferably, the substrates 
comprise silica or oxidized silicon. 
In a preferred embodiment, the first substrate, which includes the smooth 
surface to be covalently modified to present MABP groups, is formed from 
silica. Other materials, which the first substrate may preferably 
comprise, include oxidized silicon and germanium oxide. In another 
preferred embodiment, the first and second substrates both comprise a 
silicate-containing material, and preferably both comprise silica. 
The substrates to be adhered in accordance with the present invention each 
have at least one smooth surface. For the purposes of the present 
invention, a smooth surface is one which, when brought into contact with a 
second smooth surface, permits a sufficient number of contact points 
between the two surfaces to allow an adhesive interaction to occur. It 
will be appreciated that, when two surfaces are brought into contact, the 
actual contact points occur where the asperities of the surfaces touch. 
Smoother surfaces are capable of entering into a greater number of contact 
points per unit area than comparatively rougher surfaces. The number of 
contact points at an interface, which includes a comparatively rough 
surface, may be increased by compressively loading the interface, thereby 
compacting the asperities and permitting the transfer of additional 
charge. The compressive loading of the interface may result in a reduction 
of the roughness of the surface. 
Some materials may be naturally available with a surface having the desired 
smoothness. With other materials, a surface having the desired surface 
smoothness may be prepared by one of a number of known processsing 
methods, including machining processes, such as mechanical polishing; 
chemical processes, such as chemical polishing; low energy ion 
bombardment; and processes involving heat treatment. Heat treatment 
processes typically include heating a substrate to a temperature at which 
the substrate becomes somewhat plastic and subsequently cooling the 
substrate to a temperature at which the substrate is substantially rigid. 
Substrates, which have a smooth surface formed by heat treatment, are 
preferably used in applications at a temperature where they remain 
substantially rigid. 
Typically, the smooth surfaces of the present invention have a surface 
roughness of no more than about 5.0 nm RMS roughness and preferably of no 
more than about 2.0 nm RMS roughness (as measured by atomic force 
microscopy). Most preferably, the smooth surfaces of the present invention 
have a surface roughness of no more than about 0.5 nm RMS roughness. 
The smooth surface of the first substrate is covalently modified to present 
MABP groups. The MABP groups are one member of an acid-base pair, where 
acid and base are defined in terms of the Bronsted-Lowry theory of 
acid-base interaction. Under the Bronsted-Lowry theory, an acid is defined 
as a proton donor and a base is defined as a proton acceptor. The base has 
a pair of electrons available to share with the proton. This pair of 
electrons is usually present as an unshared pair but may be present in a 
.pi. orbital of the base. 
The MABP groups may be either basic groups, i.e., proton acceptor groups, 
or acidic groups, i.e., proton donor groups. In a preferred embodiment, 
the MABP groups include basic groups (i.e., proton acceptor groups) and 
more preferably amino groups. 
For the purposes of the present invention, the phrase "basic groups" is 
defined to include at least one type of proton acceptor group but may 
include more than one type of proton acceptor group. Examples of proton 
acceptor groups include amino groups, (e.g., aliphatic amino groups, 
cycloaliphatic amino groups, aromatic amino groups), amidine groups and 
nitrogen-containing heterocyclic aromatic groups (e.g., pyridine groups, 
imidazole groups, quinoline groups). The amino groups may include but are 
not limited to primary amino groups, secondary amino groups, tertiary 
amino groups or a mixture thereof. 
Similarly, the phrase "acidic groups" is defined to include at least one 
type of proton donor group but may include more than one type of proton 
donor group. Examples of proton donor groups include but are not limited 
to carboxy groups, phenolic groups and groups which include --SiOH 
(silanol groups), --SH (e.g., thiol or thiophenol groups), --SO.sub.3 H, 
--PO.sub.3 H.sub.2, --BOH or --GeOH. Other examples of surfaces which 
present groups which may act as acidic groups include hydrated metal 
oxides having amphoteric hydroxyl groups (such as --FeOH, --SnOH or 
--AlOH). 
The MABP complements are the acid-base complement (in the Bronsted-Lowry 
proton donor-proton acceptor sense) of the MABP groups. In other words, if 
the MABP groups are proton acceptor groups (basic groups), the MABP 
complements are proton donor groups (acidic groups). Alternatively, if the 
MABP groups are proton donor groups (acidic groups), the MABP complements 
are proton acceptor groups (basic groups). In a preferred embodiment of 
the present invention, the smooth surface of the first substrate is 
covalently modified to present proton acceptor groups and the smooth 
surface of the second substrate presents proton donor groups. Preferably, 
the covalently modified surface presents amino groups and the smooth 
surface of the second substrate presents amphoteric hydroxyl groups, more 
preferably silanol groups. 
The smooth surface of the first substrate may be covalently modified to 
present MABP groups by reacting a modifying reagent with the surface. The 
modifying reagent typically includes a reactive functional group, a spacer 
group and at least one MABP group. For the purposes of this invention, a 
reactive functional group is a group which is capable of reacting with a 
reactive site (e.g., a surface --OH group) on the surface of the 
substrate. Exemplary compounds which may be used as a modifying reagent 
include aminoalkyldialkylalkoxysilanes, such as 
3-aminopropyldimethylethoxysilane or 
N-(2-aminoethyl)-3-aminopropyldimethylethoxysilane. 
The modifying reagent may include more than one reactive functional group. 
Such reagents, however, are more prone to polymerization on the surface, 
which may lead to an increase in surface roughness or to a buildup of a 
thicker reagent layer, i.e., a layer having more than a monolayer of 
reagent. Either of these effects may alter the physical characteristics of 
the interface or may weaken adhesion between the modified surface and 
another surface. Preferably, the modifying reagent is a compound which 
only has a single reactive functional group. One suitable modifying 
reagent, having a single reactive functional group, is an 
aminoalkyldialkylalkoxysilane (the alkoxy group). Modifying reagents, 
which have only a single reactive functional group, are capable of 
reacting with substantially all of the reactive sites present on the 
substrate surface while avoiding polymerization. In addition to reacting 
with the reactive sites on the substrate surface, a modifying reagent may 
react with a second molecule of modifying reagent to form a dimer. Such 
dimers, which are generally only adsorbed and not covalently bonded to the 
surface, typically may easily be removed from the modified surface, e.g., 
by rinsing the surface with an appropriate solvent. 
The modifying reagent may be reacted with the surface by any suitable 
method, including, for example, treating the surface with a solution of 
the modifying reagent or vapor phase treating the surface with the 
modifying reagent. Vapor phase treatment is preferred, since the potential 
for side reaction or polymerization of the modifying reagent is lessened. 
Vapor phase treatment also reduces the chance of particulate contamination 
of the surface and permits greater control over the fraction of reactive 
sites modified. 
The method of the present invention includes reacting a sufficient number 
of reactive sites on the surface with a modifying reagent, which includes 
a MABP group, to permit the modified surface to adhere to a surface which 
presents MABP complements. The reaction of the modifying reagent may 
result in the surface being substantially covered with a modifying agent, 
which is covalently bonded to the surface. In one embodiment of the 
invention, substantially all of the reactive sites on the surface are 
reacted with the modifying reagent. In another embodiment of the 
invention, the reaction of the modifying reagent with the surface produces 
monolayer of the modifying agent covalently bonded to the surface. 
Typically, adhesion between the two surfaces occurs if their interaction 
leads to the transfer of charge and produces a charge density on each of 
the surfaces of at least about 1.0 mC/m.sup.2, preferably of at least 
about 2.0 mC/m.sup.2, and more preferably of at least about 3.5 
mC/m.sup.2. 
The smooth surface of the second substrate may inherently present MABP 
complements. For example, many metal oxides and other nonmetallic 
inorganic materials present amphoteric hydroxyl (--OH) groups at their 
surface. The amphoteric hydroxyl groups are proton exchange groups which 
may serve either as proton donor groups (acidic groups) or as proton 
acceptor groups (basic groups) depending on the nature of the group or 
groups with which the amphoteric hydroxyl groups interact. In other words, 
the amphoteric hydroxyl groups may serve as the acid-base complement of 
either an acidic (proton donor) group or a basic (proton acceptor) group. 
A surface which presents amphoteric hydroxyl groups may be adhered to a 
substrate surface which has been covalently modified to present proton 
acceptor groups (basic groups), in which case the amphoteric hydroxyl 
groups serve as proton donor groups. A surface presenting amphoteric 
hydroxyl groups may also be adhered to a substrate surface which has been 
covalently modified to present proton donor groups (acidic groups), in 
which case the amphoteric hydroxyl groups serve as proton acceptor groups. 
Alternatively, the surface of the second substrate may be modified to 
present the MABP complements. In another embodiment, the smooth surface of 
the second substrate may be modified by covalently bonding a monolayer of 
a modifying agent, which includes a MABP complement, to the surface. These 
modifications may be carried out by the methods, described above, used to 
covalently modify a surface to present MABP groups (since both MABP groups 
and MABP complements may be either an acidic or a basic group in a given 
situation). 
When the covalently modified surface, which presents MABP groups, is 
brought into contact with the smooth surface of the second substrate, 
which presents MABP complements, the two surfaces adhere. Typically, prior 
to contact neither surface carries any appreciable amount of charge. Once 
the two surfaces are brought into contact, charge transfer between the 
surfaces occurs. It is believed that proton transfer between the MABP 
groups and the MABP complements occurs as a result of the transfer of a 
proton from the acid to the base of the complementary Bronsted-Lowry pair. 
Such a process would generate two charged species, the conjugate base of 
the proton donor (i.e., the deprotonated acid) and the conjugate acid of 
the proton acceptor (i.e., the protonated base). The net result would be 
to build up a charge on each of the surfaces that is proportional to the 
number of MABP group/MABP complement pairs that come into contact. 
When two surfaces adhered by the present method are separated, a partial 
electric discharge across the gap between the separating surfaces may 
occur as some of the transferred surface charge recombines during the 
separation process. Because the two surfaces continue to retain the MABP 
groups and the MABP complements, however, if the surfaces are brought back 
into contact, charge transfer occurs again, thereby readhering the 
surfaces. This allows the surfaces to be separated and readhered 
repeatedly without damage or reduction in their adhesion. The adhesion of 
two surfaces, grounded in an interaction of acidic and basic groups, is 
also capable of reforming after rupture due to shear. 
In another embodiment of the invention, the smooth surface of the first 
substrate may be modified by covalently bonding a monolayer of a modifying 
agent, which includes a MABP group, to the surface. The covalent 
modification of the surface may be carried out, for example, by allowing a 
controlled amount of a modifying reagent to react with the surface. The 
modifying reagent includes a spacer group, at least one MABP group and at 
least one covalently reactive functional group. Preferably, the modifying 
reagent has only one covalently reactive functional group. For the 
purposes of the invention, monolayer is defined as a layer which has the 
thickness of a single molecule or group and which substantially covers 
and, preferably, entirely covers a surface. The monolayer of the modifying 
agent is preferably configured to present the MABP groups. In a preferred 
embodiment, the smooth surface of the first substrate is modified by 
covalently bonding a monolayer of a modifying agent, which includes a 
proton acceptor group, more preferably an amino group. 
Typically, the spacer group of a modifying reagent is quite small, 
containing no more than six or seven carbon atoms and preferably no more 
than two to four carbon atoms. If the spacer group is too large it may be 
possible for the MABP group to wrap around and become buried, such that 
the MABP group is no longer exposed. This would decrease the number of 
groups available to interact with the second surface, thus weakening 
adhesion. 
In addition, if the size of the spacer group is kept quite small, the 
thickness of the monolayer may be minimized. The covalently bonded 
monolayers of the present invention are quite thin, preferably no more 
than about 2.0 nm thick and more preferably no more than about 1.0 nm 
thick. Substrate-substrate interfaces which include such thin monolayers 
exhibit very little alteration of their physical properties (relative to 
the original substrate materials). 
The method of the present invention is particularly useful for adhering 
substrates without substantially altering the dielectric, optical, 
thermal, or mechanical properties of the substrates in the vicinity of the 
substrate-substrate interface. For those applications where the thermal 
properties of the substrate-substrate interface are important, the 
thickness of the monolayer at the substrate-substrate interface may be 
critical. Heat transfer through a layer is inversely proportional to the 
thickness of the layer, i.e., the presence of a relatively thick adhesive 
layer with poor thermal conductivity could dramatically alter the thermal 
properties of a substrate-substrate interface. The method of the present 
invention provides a means of adhering substrates with a minimal effect on 
the thermal properties of the substrates or the substrate-substrate 
interface. 
Where a discrete, substantial adhesive layer is present between two 
substrates, the layer may expand or contract during curing or may have a 
different thermal expansion coefficient from one of the substrates. Either 
of these effects may result in the creation of internal mechanical 
stresses within the substrate(s) and/or adhesive layer, thereby weakening 
the adhesive bond. In contrast, the adhesive bond formed between two 
substrates by the present method is capable of reforming after rupture due 
to shear and does not introduce mechanical stresses at the 
substrate-substrate interface. 
The method may be useful in forming articles to be employed in a number of 
different applications, and in particular, in applications involving 
electronic or optical materials. For instance, the method of the present 
invention may be used to bond a dielectric cover to a semiconductor die, 
to bond a thin semiconductor die to a smooth dielectric substrate or to 
bond a semiconductor to the waveguide of an optical device. Other 
applications where the method of the present invention may be employed 
include the construction of optical devices. The present method also 
provides a method of adhering two substrates, which are formed from 
identical material, without requiring the presence of an additional 
substantial layer of adhesive material between the two substrates. 
The present method may also be effective for bonding two different 
materials which have different thermal expansion coefficients but must be 
able to survive thermal cycling. During thermal cycling, adhered materials 
with differing thermal expansion coefficients may expand or contract at 
different rates, thereby producing a shearing force which may rupture the 
adhesive bonds between the two materials. Because the two surfaces 
continue to retain the MABP groups and the MABP complements, the adhesive 
bond between the substrates is capable of reforming after rupture due to 
shear. 
The present invention also provides an article which comprises a first 
substrate which includes a smooth surface covalently modified to present 
MABP groups, and a second substrate, which includes a smooth surface which 
presents MABP complements. The smooth surface of the second substrate is 
adhered to the covalently modified surface of the first substrate. 
Typically, the substrates comprise a nonmetallic, inorganic material. 
Preferably the substrates comprise silica, oxidized silicon, alumina, 
titanium oxide, chromium oxide, tin oxide, germanium oxide, wollastonite, 
and other silicate-containing materials, and more preferably the 
substrates are formed from silicon having a surface layer of native oxide 
or from silica. The substrates may also comprise a metal having a surface 
metal oxide layer. 
The smooth surface of the first substrate is preferably modified to present 
proton acceptor groups, which are covalently bonded to the surface. More 
preferably, the smooth surface of the first substrate is covalently 
modified to present proton acceptor groups, which comprise at least one of 
an amino group, an amidine group or a basic nitrogen-containing 
heterocyclic aromatic group. Most preferably, the smooth surface of the 
first substrate is covalently modified to present amino groups. 
In a preferred embodiment of the invention, the article comprises 
silicate-containing substrates and, more preferably, comprises silica 
substrates. Preferably, the smooth surface of the first substrate is 
covalently modified to present aminoalkyldialkylsilane groups and the 
smooth surface of the second substrate presents silanol groups. 
A number of specific embodiments of the present invention are described in 
the examples set forth below. These examples are offered by way of 
illustration and not by way of limitation.

EXAMPLE 1 
Preparation of Silica Sheets 
Silica sheets were prepared according to the following procedure. A silica 
tube, having a 14 mm inner diameter and 1 mm thickness, was placed in a 
laminar flow hood and the end of the tube was sealed with an oxy-hydrogen 
flame. After heating the closed end for one minute to evaporate surface 
impurities, the end was removed from the flame and rapidly blown into a 
large bubble. The bubble was placed near a flame of reduced intensity, and 
portions were flattened by exposure to radiant heat. The advancing contact 
angle of water on the freshly prepared silica surface was about 
45.degree.. The flattened portions of the silica bubble had a thickness of 
about 1.0 .mu.m. 
Prepared mica patches of about 4 .mu.m thickness were cleaved from a larger 
sheet of mica for use as a backing sheet. Patches were broken from the 
flattened portions of the silica bubble and the exterior side 
(corresponding to the outer surface of the bubble) of each patch was 
attached to a mica backing sheet. The placement of a mica sheet onto the 
silica patches was necessary to maintain the cleanliness of the silica 
surface prior to use. Since charge transfer occurs when a clean silica 
surface is brought into contact with a clean mica surface, adhesion 
between the surfaces of the silica patch and the mica backing sheet was 
spontaneous. The samples were then placed in a vacuum chamber where about 
50 nm of silver was evaporated onto the interior sides of the silica 
patches to form silver-silica-mica structures. The structures were then 
removed from the vacuum chamber and stored in a desiccator. 
Prior to start of an experiment, a sample was prepared for mounting in a 
Surface Force Apparatus (SFA) by being glued, mica side up, onto a 
cylindrical silica lens using a low melting point epoxy resin. The mica 
backing sheet was removed just prior to experimentation. 
EXAMPLE 2 
Modification of the Surface of a Silica Sheet 
The protective mica backing sheet was removed from a silver-silica-mica 
structure and the silver-silica structure was placed in a sealed glass 
container, which was partitioned into two compartments by a 
poly(tetrafluoroethylene) (PTFE) connector. A modifying reagent, 
3-aminopropyldimethylethoxysilane, was introduced into a compartment at 
one end and allowed to react with the silica sheet in the second 
compartment at the other end of the container via the vapor phase for 24 
hours. The PTFE connector prevented the silane liquid from creeping and 
wetting the silica sheet directly, ensured that the silica was only in 
contact with the silane through the vapor phase. At the end of the 24 hour 
period, the modified silica sheet (silylated silica sheet) was removed and 
mounted in the Surface Force Apparatus. 
In addition to reacting with silanol groups on the silica surface, the 
aminopropylsilane modifying reagent reacts with a second molecule of 
modifying reagent to form a dimer. An absorbed layer of about 3 nm of 
silane dimer, which was present on the silica surface, was dissolved by 
exposing the surface to aqueous solution. To avoid any contamination of 
the silica surface by the dimer during experimentation, the apparatus was 
filled, drained, and refilled before liquid phase measurements were taken. 
The contact angle of water with the surface of the silylated silica sheet 
was not measured, but was estimated to be: 45.degree. advancing, 
35.degree. receding, .+-.5.degree.. 
In this example and all of the other examples described infra, fluids were 
filtered through an alumina membrane having a 0.02 .mu.m pore diameter 
prior to being permitted to come into contact with the silica sheet. 
EXAMPLE 3 
Preparation of a Clean Silylated Silica Sheet for Experiments with 
Unsilylated Materials 
A silylated silica sheet was prepared according to the procedure described 
in Example 2 but was not allowed to come into contact with aqueous 
solution. Instead, prior to using the silylated silica sheet in 
experiments with unsilylated material, the silylated sheet was rinsed in 
filtered ethanol and blown dry with filtered nitrogen gas prior to 
mounting in the Surface Force Apparatus. This procedure was sufficient to 
prevent any possible silane contamination of an opposing, untreated 
surface. 
EXAMPLE 4 
Measurement of the Interaction Between a Clean Silylated Silica Sheet and 
an Untreated Silica Surface Under Dry Conditions 
The force between an untreated silica surface (prepared according to 
Example 1) and a surface of a clean silylated silica sheet, modified to 
present the amino groups according to the procedure of Example 3, was 
measured under a dry nitrogen gas atmosphere. The experiment was performed 
using a modified Surface Force Apparatus (SFA), which measures the forces 
between two surfaces using the deflection of a cantilever spring. The use 
of the SFA to measure forces and the technique for measuring the amount of 
charge transferred between two surfaces has been previously reported (see 
descriptions in D. J. Smith, J. Electrostat., 26, 291 (1991) and in R. G. 
Horn and D. T. Smith, Science, 256, 362 (1992)). Unlike the system 
presented here, the interfaces described in these earlier publications are 
between macroscopically dissimilar materials (such as between unmodified 
silica and mica). 
The silica substrates were mounted in a crossed-cylinder geometry to avoid 
edge effects in the force measurements. Silver layers about 50 nm thick 
were deposited on the back side of each silica sheet, creating an optical 
interferometer that was used to measure surface separation. The silvered 
sheets were glued to cylindrical lenses and the silver layers were 
connected to an electrometer circuit which permitted the measurement of 
the amount of charge transferred from one surface to the other on contact. 
Prior to contact between the two surfaces, no appreciable charge was 
measured on either of the surfaces. After the surfaces had been brought 
into contact measurable amount of charge was observed to transfer 
spontaneously from one surface to the other, the untreated silica surface 
becoming negatively charged and the amine-modified surface positively 
charged. In addition, an attractive electrostatic force was measured 
between the two surfaces after they had been placed in contact, i.e., a 
significant amount of work per unit of area in contact was required to 
separate the two surfaces (3.3 J/m.sup.2). The surface charge density on 
each of the surfaces was high enough (about 6.9 mC/m.sup.2) that, upon 
separation of the surfaces, electric discharges were observed. These 
discharges were evident in the attractive electrostatic force measured 
between the two surfaces and also in the measured surface charge 
densities. 
The range and magnitude of the attractive force between the amine-modified 
silica surface and the untreated silica surface is much greater than the 
attractive forces due to van der Waals bonding. Because of the range and 
magnitude of the attractive force generated between the amine-modified 
silica surface and the untreated silica surface, the adhesive energy of 
the bond between the two surfaces is of the same order of magnitude as the 
cohesive energy of the silica substrates, i.e., the same order of 
magnitude as the energy required to fracture the bulk material of the 
silica substrates. 
EXAMPLE 5 
Measurement of the Interaction Between a Clean Silylated Silica Sheet and 
an Untreated Silica Surface Under Aqueous Conditions 
The same SFA system described in Example 4 was used except that an aqueous 
sodium chloride solution rather than nitrogen gas was employed as the 
intervening fluid. The silica surfaces were prepared as described in 
Examples 1 and 2. An attractive electrostatic double layer interaction was 
measured, confirming that the two surfaces carried charge of opposite 
sign. The silica surface and amine-modified silica surface therefore 
adhere and also demonstrate opposite signs of charge when the surfaces are 
immersed in water. While the strength and range of the interaction is 
considerably less than observed when dry nitrogen gas is utilized as the 
intervening fluid, the interaction is attractive. In contrast, where van 
der Waals bonding is the sole basis for attraction between two surfaces, 
such as between two unmodified silica surfaces, no adhesion is seen if the 
surfaces are immersed in water. 
Although the present invention has been described in terms of an exemplary 
embodiments, it is not limited to these embodiments. Alternative 
embodiments, examples, and modifications which would still be encompassed 
by the invention may be made by those skilled in the art, particularly in 
light of the foregoing teachings. Therefore, the following claims are 
intended to cover any alternative embodiments, examples, modifications, or 
equivalents which may be included within the spirit and scope of the 
invention as defined by the claims.